Spectroscopy previews the upcoming IFPAC Annual Meeting, to be held January 25–28, 2009, in Baltimore, Maryland.

The 23rd International Forum for Process Analytical Technology (IFPAC-09) will be held at the Baltimore Marriott Waterfront Hotel, located within Baltimore's Inner Harbor, from January 25 through January 28, 2009. IFPAC is the leading conference in process analysis and technology, and this will be an exciting forum in which to learn about recent developments in the field of process analytical chemistry, technology, and control applications in many different industries.

In addition to the conference events, attendees can enjoy various shops and harbor restaurants, the Baltimore National Aquarium, the Maryland Science Center, The Peabody Conservatory, and more.

On Sunday afternoon, January 25, Lawrence Yu, Steven Kozlowski, and Christine Moore, of the FDA, will present a special workshop entitled 

Quality by Design in Pharmaceutical Development and Manufacturing: An FDA Update.

 This in-depth look at the FDA's viewpoint of their Quality Initiative will be presented from the perspective of their three review offices: biotech products, generic drugs, and new drugs. The purpose of this workshop is to discuss implementation progress of Quality by Design (QbD) and process analytical technology within the pharmaceutical and biopharmaceutical industries with the goal of supporting overall operational excellence. The program will end with an interactive roundtable discussion that encourages audience participation and networking among attendees.

Operational Excellence Through Process Understanding and Control ... Facing Today's Challenges

, will be chaired by Stephen Jacobs, Duncan Low, and Christine M.V. Moore on Monday morning, January 26.

Daily lectures, workshops, and short courses focusing on the latest developments in process analytical technology will be conducted by leading experts in their respective fields. Topics include separation techniques, Raman spectroscopy, chemical imaging, process control and quality, electrochemical techniques, chemometrics, pharmaceutical design space, PAT for biologics manufacturing, handheld (portable) instrumentation, standards in process analytical chemistry, and future trends.

The three-day exhibit hall will be open from January 26–28, 2009, at the conference. The exhibit floor is considered an integral part of the annual conference, and showcases the new products and services most desired by this specialized audience.

All-inclusive registration includes three days lodging and meals. For more information and to register online, visit the IFPAC web page at 

Articles

IFPAC

A higher resolving power in ICP-MS is used primarily to remove interferences from the measured ion signal, in contrast to the usage of higher resolving power in organic and biological mass spectrometry to establish ion empirical formulas. A practical attainable resolving power of 10,000  suffices for most such purposes in ICP-MS, and is achieved in commercial instruments through proven instrumental means. The ability to "dial in" resolving power as needed in newer trap mass analyzers shows promise for speciation analysis in ICP-MS with chromatographic separation of sample components.

Over the past 10 years, the practical attainable mass resolution in organic and biomolecular mass spectrometry (MS) has increased by approximately a factor of 10. Many instruments continue to operate with essentially unit mass resolution at ion masses of a few thousand. Other instruments can reach a resolution of several tens of thousands in that same mass range, and benchmark experiments showed that resolutions exceeding 1,000,000 could be reached (although perhaps only at lower ion masses, and clean and simple samples). What counts most is what mass resolution can be achieved on a daily basis. The term 

 is that level of performance that can be reached without extraordinary measures and without disrupting the workflow in the laboratory. Ideally, higher resolution could be dialed into an analysis as the situation requires, in an analogy to the focus on an optical camera. However, the classical processes for achieving higher resolution in sector-based instruments involve controlling the direction and energy dispersion of a beam of ions emitted from the source, often with the assistance of physical limiters such as ion slits. The physical reconfiguration of the instrument, and confirmation of the desired consequential effect, are procedures that take several minutes even for experienced operators. Peak matching, which involves the creation of an overlap of peak signals corresponding to ions of two different masses on a scope, and measurement of a ratio of accelerating voltages needed to do so, takes even more time and effort. Practical attainable resolution often is below peak instrument performance. 

The recent increase in attainable resolution has been achieved with the use of Fourier transform–ion cyclotron resonance (FT-ICR) mass spectrometers, and even more recently, orbitrap instruments. In both of these instruments, the mass resolution is established by the length of the observation period for the frequency signal generated by an orbiting cohort of ions trapped within the instrument. The observation period for even very high mass resolution is still only on the order of a few seconds, which is a short enough period that higher resolution mass measurement can be achieved for samples derived from a chromatographic separation. Indeed, with a high enough mass resolution, some of the need for chromatographic separation can be relaxed, and impressive results can be obtained directly for mixtures introduced into the source of the mass spectrometer.

Before we begin a discussion of why higher "mass resolution" is valuable, we should review basic definitions to buttress our understanding of the difference in the terms 

. It is unrealistic to expect that this repetition of the definitions will alter the extended history of casual misuse of these terms, and their not-quite-correct usage in the preceding paragraphs. However, clear and concise language is an integral part of accurate and precise analysis. 

 is a measure of the ability of an instrument to differentiate between ions of similar masses. Resolving power is the large number often misidentified as "resolution." Resolving power is defined as 

 is the mass of the lower mass ion, and Δ

 and the next higher mass ion from which differentiation is sought. Figure 1 is reproduced from Sparkman's 

, which provides a more thorough discussion of the use and misuse of these two terms (1). The actual numerical value of resolving power depends upon the degree of differentiation sought, and can be specified at a 10% valley, or at full width half maximum (FWHM). The situation represented in the figure is a theoretical ideal in which two symmetrical peaks are of equal height. The real world usually is not so tidy, but both unequal peak heights and asymmetry can be accommodated within mathematical models to provide a meaningful value for resolving power. Note that resolving power is a unitless number. 

), and is a measure of the observed separation between two adjacent mass peaks. The description of the ion peak separation as resolution comes with units. For example, a resolution can be reported as some parts per million for ions of a specified mass. Resolution can be a small number if the separation of the ion masses is "just barely," or it can be a larger value if the instrumental resolving power is high. Sparkman notes that the MS usage of "resolving power" and "resolution" differs from IUPAC recommendations. The issue eventually will need to be revisited officially as more analyses at higher resolving powers are reported, and the need to compare results grows. Practically, in refereed publications, and even in the title of this article, the tendency is to use "resolution" in place of "resolving power," reflecting the preponderance of the term 

 in other venues. In the remainder of this column installment, we will strive to use the terms correctly. 

Figure 1: Schematic of peaks representing ions of two adjacent masses, and the values used for derivation of the resolving power (m/Îm at a specified valley). 

For molecular MS, a higher resolving power can provide an accurate mass measurement for an ion through proper calibration. If the empirical formula of the ion is predictable from other information, then it is a necessary condition that the measured mass corresponds to the calculated expected mass. Incrementally higher resolving power provides greater analytical certainty in the confirmation of the ion identity. Conversely, if the ion mass is known with high accuracy, the empirical formula of an ion can be determined from that value alone for lower masses, or the number of possible combinations of atoms can be narrowed to a few for higher masses. A balance must be struck between the mass measurement accuracy required and the loss of sensitivity that usually accompanies adjustment of the instrument for higher resolving power. Transience of ion signals also can preclude anything but a lower resolving power measurement. The number of possible atomic compositions (empirical formula) for any measured mass increases rapidly as the mass of the ion increases. The accurate mass measurement is therefore most often used for confirmation rather than derivation of an ion empirical formula. It is worth remembering that valuable information can be derived by measurement of the exact mass difference between two ions in a mass spectrum. In this situation, the accurate mass difference establishes the empirical formula of the neutral atomic fragment.

In inductively coupled plasma coupled with mass spectrometry (ICP-MS), the high temperature of the plasma source ensures that molecular species introduced into the source are reduced to simple atomic species, or sometimes diatomic species such as elemental oxides or chlorides. Atomic ion masses are usually distinct, but the isobaric interferences for the diatomic species can be problematic and have been tabulated (2–4). Numerous strategies have been developed to reduce the contribution of these interferences, including modified sample preparation and cleanup, reactive collision cells (5), and spectral peak deconvolution methods (6) that also can be used for other forms of MS. However, the most direct means for reducing isobaric interferences is to increase the resolving power of the mass analyzer. Therefore, in ICP-MS, higher resolving power has a fundamentally different purpose than in organic MS. The purpose is to remove interferences rather than determine empirical formulas. Once isobaric interferences are removed through mass resolution, the isotopic signature of an elemental ion can be measured directly and accurately. This measurement provides direct confirmation of the accuracy and precision of the intensity measurement, and is therefore a basic metric of instrument performance. Additionally, it is only once these accuracy and precision metrics are known that variations in isotopic distributions can be tracked. Such work is distinct from the highly sophisticated analyses completed in isotope ratio MS, which uses dispersed ion beams and separate detectors, and which can track isotopic variations at the part-per-billion level. New applications can be found within the performance envelope lying between unit mass resolution ICP-MS and isotope ratio instruments. For example, higher mass resolving power in ICP-MS will also be needed to provide the requisite accuracy in developing clinical applications (7).

Several manufacturers offer high-resolving power ICP-MS instruments. As an example, the Finnigan Element2 is a high resolving-power (high-resolution) ICP-MS instrument available from Thermo Fisher Scientific (Waltham, Massachusetts). The instrument uses a fast-scanning magnetic sector in a double-focusing BE geometry to achieve a high resolving power. Fast-scanning sector magnets have been used in mass spectrometers used for organic analysis, scanning rapidly over a mass range from about 

 2500, covering the molecular mass range of interest. For ICP-MS, the mass range of interest is much narrower, corresponding to the mass range of atomic ions. The specifications for the Element2 instrument list a scan speed from 

 240 and return in under 150 ms. This scan speed would provide about seven full scans across a 1-s sample bolus, whether it originated in a flow injection analysis or in a chromatographic peak. Although spectral averaging across a narrow peak provides an approximation of the steady-state mass spectrum, in general, the more scans across a peak, the better. The goal is to provide a scan speed sufficiently fast for an accurate measurement of mass spectra as samples are eluted into the ICP source from a chromatographic inlet such as gas chromatography or liquid chromatography, as separation of species via chromatography is used with ICP-MS. It is interesting to remember that early generation sector magnets used in organic mass spectrometers could not scan rapidly, and accelerating voltage scanning could be used across a smaller mass range with a faster effective scan speed. The company literature for the Element2 states that the instrument uses a combination of accelerating voltage and magnetic field strength scanning in its routine scan function.

The topic of interest for this column is the higher resolving power available with the sector instrument for ICP-MS. As in sector instruments for organic MS, a higher resolving power is achieved in the Element2 with a physical slit that narrows the dispersion of the ion beam from the source. The Element2 uses a three-position slit, as shown in Figure 2 (figure adapted directly from the company literature) to provide specified resolving powers of 300, 4000, or 10,000 (described as low, medium, or high resolution). The ion mass for which these resolving powers is achieved is not noted. The slit for resolving power 300 is the widest, and provides the best instrument sensitivity. This low resolving power should provide at least unit mass resolution across the entire mass range of interest, and would be used for relatively clean samples expected to be free from interferences. The medium resolving power is sufficient to prevent most common interferences, and the resolution of 10,000 is suggested for use with complex samples in which the matrix may be unknown or uncharacterized. The highest resolving power is insurance against an unexpected interfering ion signal. The payment for this insurance is a decreased sensitivity. Sensitivity is about an order of magnitude less at resolution 4000 compared with resolving power 300, and almost an order of magnitude lower again at the highest resolving power of 10,000.

Figure 2: Source slit assembly used in a commercial high resolving-power ICP-MS instrument (Finnigan Element2, as described in the company literature). 

With freedom from isobaric interferences, and the ability to certify instrument performance through observation of the correct elemental isotopic envelope, the confidence limits (error bars) on a set of replicate elemental quantitation values can be determined. Then, elemental distributions in samples can be ascertained reliably, and the distributions of all elements can be queried. This is a complex multidimensional data set that can be used in interesting new applications. For example, Norris and colleagues (8) used the trace elemental profiles of feathers from the Western sandpiper to distinguish bird populations that inhabit different winter sites. High resolving-power ICP-MS was used to determine the concentrations of 41 trace elements in the feathers sampled from known discrete populations of these birds. Statistical analyses using linear discriminant analysis identified 15 elements for which the measured concentrations evidence one of the five distinct wintering sites. Those five sites are indicated in Figure 3a. Three sites are quite a distance from one another; the last two sites are distant from the other three but only 3 km from each other. Still, even these two sites could be distinguished based upon the elemental distributions. The distributions for six elements at each of the five sites is depicted in Figure 3b. Note that the differences between the sites exceed the confidence limits established for each measured value. Without the confidence limits, the comparisons would lose meaning. This analytical work complements the genetic information that can be indicative of the differences in breeding populations of the birds. The ICP-MS analysis can be automated easily and can handle large numbers of easily collected samples; additional applications in similar areas can be expected to appear in the future. Compilations of high resolving-power ICP-MS publications, talks, and posters through 2004 are available on the web (9).

Figure 3: (a) Map of the winter-resident locations of five separate populations of Western sandpiper; these grounds are designated by letter abbreviations S, L, B, R, and A. (b) Plots of the concentrations of six elements in the feathers of these populations, showing that the populations can be distinguished. 

In organic MS, if moderate resolving power is good, then higher resolving power is thought to be still better, as long as the loss in sensitivity is not too severe. The situation is somewhat different in ICP-MS, where resolving power sufficient to remove the isobaric interferences is all that is required. The ICP itself acts to reduce other potential interferences to a well-known population such that a resolving power of 10,000 is all that is needed. Higher resolving powers in research instruments for ICP-MS have been demonstrated, but more as a demonstration of feasibility. FT-ICR instruments and orbitrap instruments do offer considerable promise in ICP-MS because of the manner in which higher resolving power is achieved — that is, by longer observation of the frequency signal from an ion population in stable orbit inside the instrument. The Orbitrap in particular represents an instrument in which the requisite resolving power can simply be accessed as the analysis requires it. Consider an experiment in which distributions for a variety of elements are to be determined, as in the example given earlier for the analysis of feathers drawn from migratory bird populations. An exploratory scan at a moderate resolution establishes what elements are present, and whether there is any indication of an isobaric interference based upon deviations from expected isotopic intensities. If there is no indication of interference, a random higher-resolution scan can be programmed into the analysis series to act as a quality control check. If there is an indication of an isobaric interference, the higher-resolution scan is selected automatically. This decision can be made in real time even as samples are introduced over a few seconds, and eventually might be fast enough so that speciation through chromatography, with higher resolving power confirmation of results, is achieved. Applications of hybrid trap–Orbitrap instruments to ICP-MS also can be foreseen, if the need for the analytical results justifies the relatively high initial cost of such instruments. A similar argument that invoked the high cost of MS as a detector for ICP was invoked as the first ICP-MS instruments were constructed. The informing power of the results soon rendered that argument obsolete.

is often reminded that new significant accomplishments in analytical sciences are usually the result of dedicated problem-solvers diligently searching for solutions, rather than developers of solutions waiting patiently for problems to arrive at their laboratory door. An "open-door" policy is best matched to an open mind and a fearless willingness to explore. Knock on KLB's electronic door at 

, Second Edition (Global View Publishing, Pittsburgh, Pennsylvania, 2006). 

Inductively Coupled Plasma Mass Spectrometry Handbook

 (Blackwell Publishing, Ames, Iowa, 2005). 

(4) M. Plantz, "Common Molecular Ion Interferences in ICP-MS," found at 

https://www.varianinc.com/media/sci/apps/icpms06.pdf

(5) D.R. Bandura, V.I. Baranov, and S.D. Tanner, 

(7) E.E.M. Brouwers, M. Tibben, H. Rosing, J.H.M. Schellens, and J.H. Beijnen, 

(8) D.R. Norris, D.B. Lank, J. Pither, D. Chipley, R.C. Ydenberg, and T.K. Kyser, 

http://www.hricpms.com/pdfs/2004Jan_journals.pdf

http://www.hricpms.com/pdfs/MCICPMSTalksPosters.pdf

Columns: Mass Spectrometry Forum

Columnist Ken Busch discusses the ability to "dial in" resolving power as needed in newer trap mass analyzers, which shows promise for speciation analysis in ICP-MS with chromatographic separation of sample components.

Higher Resolution Mass Analysis in Inductively Coupled Plasma–Mass Spectrometry

Spectrometer discovers signs of active cryovolcanoes on Saturn's largest moon, Titan.

Molten lava, magma, and hot volcanic eruptions all come to mind when the phrase "active volcano" is mentioned, but this is not the case for Saturn's largest moon, Titan, where scientists have found evidence of ice volcanoes and ammonia frost.     

Figure 1: These radar images show the outlines of regions "1" and "2," which have been hypothesized to be due to cryovolcanic activity. Image credit: NASA/JPL. Full image and caption. 

, according to data collected during several fly-bys of Titan by NASA's Cassini spacecraft, scientists think the moon contains active cryovolcanoes, which spew super-chilled liquid into its atmosphere. Rather than erupting molten rock, it is theorized that the cryovolcanoes on Titan would erupt volatiles such as water, ammonia, and methane.  

The international Cassini spacecraft is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. It has been orbiting Saturn and studying its rings and moons since 2004, with a projected end date of September 2010. 

"Cryovolcanoes are some of the most intriguing features in the solar system," said Rosaly Lopes, a Cassini radar team investigation scientist from NASA's Jet Propulsion Laboratory in Pasadena, California. "To put them in perspective – if Mount Vesuvius had been a cryovolcano, its lava would have frozen the residents of Pompeii." 

Changes in brightness and reflectance in two distinct regions were documented by visible and infrared mapping spectrometer data collected on Titan fly-bys from July 2004 to March 2006 has led scientists to believe the volcanoes are active. In one region, the reflectance of the surface surged upward and remained higher than expected. In the other region, the reflectance shot up but then trended downward. There was also evidence of ammonia frost at one of the two sites, which was only evident at times when the region was inferred to be active.  

"Ammonia is widely believed to be present only beneath the surface of Titan," said Robert M. Nelson of JPL, a scientist for Cassini's Visual and Infrared Mapping Spectrometer team. "The fact that we found it appearing at times when the surface brightened strongly suggests that material was being transported from Titan's interior to its surface." 

Figure 2: In this infrared map, the location of two regions that changed in brightness are labeled. These regions are hypothesized by some to be areas of cryovolcanic activity on Titan. Image credit: NASA/JPL/University of Arizona. Full image and caption. 

Despite this evidence, there are some scientists, familiar with spectrometer data, who argue that the ammonia identification is not certain and the brightness changes might not be associated with changes on Titan's surface. Another possible explanation could be the transient appearance of ground "fogs" of ethane droplets very near Titan's surface, driven by atmospheric processes rather than geophysical. "There remains the possibility that the effect is caused by a local fog, but if so, we would expect to see it change in size over time due to wind activity, which is not what we see," said Nelson.  

Another hypothesis offered by NASA scientists is that the Saturnian moon is taking its landform evolution cues from a moon of Jupiter.  

"Like Callisto, Titan may have formed as a relatively cold body, and may have never undergone enough tidal heating for volcanism to occur," said Jeffrey Moore, a planetary geologist at the NASA Ames Research Center, Moffett Field, California. "The flow-like features we see on the surface may just be icy debris that has been lubricated by methane rain and transported down slope into sinuous piles like mudflows." 

All of these hypotheses will be researched further, as scientists are still analyzing the data from Cassini's most recent fly-bys of Titan on December 5, 2008, and December 21, 2008.  

Departments

Active Volcanoes in Space?

Improvements in engineering and manufacturing processes and tools have rapidly lowered the cost to make products, and to distribute them. The next generation of optical sensing technologies will offer a framework for the creation and testing of new business models based upon the distribution of knowledge and service. In fact, this concept of "distributed sensing" already has emerged in networked systems monitoring various aspects of the environment.

The key to all this will be to lower the cost of trying new marketing ideas. This is accomplished primarily by being speedy to market; by using technologies that are already proven, tested, and functional — spectroscopy certainly fits the bill; and by developing products that are configurable in many different formats. Also, once a new business model is developed, it might be desirable to replace certain technologies with lower-cost versions. That's where distributed sensing networks can help: by making marketing acceptance simpler to demonstrate and to qualify, thus mitigating engineering costs.

If this all sounds suspiciously familiar, that's because it is — thanks to Apple's reinvention of the marketing model for music, which has managed to distribute more than 5 billion songs since 2003 (and a bunch of videos as well). What technologies will emerge as the iTunes Store of the optical sensing world? And what should designers, manufacturers, and specifiers of optical sensing technologies consider as they build and deploy systems and components for a world that is more "connected" than at any other time in history? Let's consider some scenarios.

Until 1995, I spent most of my schooling and all of my career path as far from math and science as possible. Not because of a particular dislike for those things, but simply because my interests were elsewhere. 

Figure 1: Data collection. Networks of distributed sensors could be used to correlate data on coastal environments around the world. 

But something ironic happened as my career progressed — I ended up at a company selling spectrometers and optical components. Talk about a baptism of fire! After about three months of dancing around the most head-scratching questions and discovering that my wit and charm could only take me so far, I discovered an important truth: if I could understand just a handful of key concepts, I could sell optical sensing systems to folks with many more years of scientific training and experience.

Selling is one thing; understanding is quite another. I suspect it would be much harder for a layman like me to break in to the business today, in part because optical sensing systems and components have become more powerful and complex, as well as more integrated with other systems. For example, your spectrometer has to be able to work with various other devices, like light sources and sampling optics, each with its own operating profile. What's more, there are simply more types of technology today than ever before; knowing just a few key concepts is not enough.

But what if the devices themselves were smarter? What if devices had built-in intelligence and could recognize how each device works with the others — understood their roles, if you will — and could share that understanding with users? That would be quite powerful, for layman and expert alike.

In fact, smart technologies already exist all around us. They are part of what Chris Anderson of 

 magazine ("The End of Theory," July 2008) calls "The Petabyte Age" — an era defined by the ubiquity of sensors and the capacity to extract, store, and interpret the data they produce.

Consider adaptive optics, which came out in the 1990s and which have been used to correct optical aberrations in devices such as telescopes and laser communication systems. Indeed, instruments are constantly experiencing changes in temperature and pressure and undergoing mechanical stress and optical decay, resulting in the gradual degradation of the designed performance parameters.

In a spectrometer optical bench, for example, optical components could become fully adaptive by adding micro-motors to control focus, slit width, and grating rotation. The bench would have the ability to configure itself, adapting to changing conditions like temperature shifts, or accommodating different needs, such as widening the slit for fluorescence and then narrowing the slit for high-resolution work. Or the bench would refocus if it's been stressed by thermal shifts, or change wavelength range to optimize readings, or even insert or remove filters to recalibrate itself. Indeed, because spectroscopic analytical instruments in particular require careful mechanical, electronic, and optical design to assure their long-term service reliability and performance, making them "smart" seems like, well, a smart idea. 

Using adaptive optics and other smart technologies could make preventive maintenance nearly transparent to the user (similar, in a way, to how antivirus protection software does its thing without us giving it too much thought) and provide a valuable edge in the marketplace to designers and manufacturers of optical sensing products. Competition has never been more robust, and industries — and the researchers who collaborate with them — are constantly looking for ways to better manage their supply chain, to reduce waste or off-spec product and increase yields in manufacturing, and to build value in the brand or selling of their products. Small savings or improvements can drive the difference between being successful versus merely surviving; instrumentation that is rugged and reliable and built around solutions to give better operating decision data can help drive success.

In one "smart instrument" scenario, the intelligence in the system directs the user to perform maintenance checks. For example, in a spectrometer setup, the spectra of a mercury–argon calibration source could be used to determine if wavelength calibration has drifted, if the spectrometer is out of focus, and so on. Linearity testing, dark noise analysis, and other tests could be performed. For light sources, bulb intensity and lifetime could be monitored.

If that sounds like a bit of a stretch, consider that smart technology exists today in optical sensing devices such as detectors, imaging systems, and materials stress monitors.

Expertise in Components, Instruments, and Applications

Making systems and components intelligent and collecting that intelligence are important parts of the distributed sensing model; they make the sensing network worthwhile. After you collect the data, you can use it to make decisions on how to best obtain answers about an application. In the case of a smart spectrometer system, for example, simple tasks like changing integration time or slit width to optimize signal-to-noise performance are straightforward.  

Smart devices sense other devices in the community, predict their performance as a system, optimize performance, make recommendations for new devices that will enhance performance, and compare performance among similar systems in the broader network. More complex issues can be addressed through artificial intelligence, neural nets, pattern recognition, and data mining — in short, any analytical technique that is appropriate and useful. The goal is to capture all the expertise accumulated in the design, manufacture, and use of optical sensing systems and components, and put that knowledge to use for the researcher, developer, or technician.

Also, this model could serve as the structure for a sort of expert system database. Using a spectrometer system as an example, we can use keywords in our database that describe certain conditions and report recommendations for dealing with them (that the recommendations are relevant presumes a certain level of expertise on the user's part). The user must supply some idea about the "why" of the application, so the software can make narrower choices. For example, if the user picks the Beer's Law applications feature, the software makes certain inferences: that samples and standards are being used, that maintaining absorbance linearity might require sample dilution, that stray light correction is needed, and so on.

Ultimately, the hardware becomes appliance-like and self-sufficient; the value resides in the network itself, not the components. The distribution of knowledge becomes the new currency.

Back in 1995, when I was answering my first questions about optical resolution, grating efficiency, and the like, there were really only three ways to share information with customers: by phone, by fax, or by mail.

The Internet soon changed all that. With the World Wide Web, information sharing is no longer limited by time or distance; the only barrier is access. So it's no surprise that researchers have already begun to harness the power of the Internet to share data via cabled and wireless networks of sensors. The Sensor Web, a concept developed at NASA/Jet Propulsion Laboratory in the late 1990s, is one such example. There's a great description at 

 of a Sensor Web that uses remote sensing, satellite imagery, and surface measurements to monitor volcano activity near Antarctica.

The implications for instrument design seem readily apparent: We need to think of new products beyond the confines of the laboratory. The ability to connect to the Internet wirelessly or via the Ethernet should be as much a part of instrument design as consideration of processing speed and power requirements. What's more, there are implications for how instruments communicate with each other, how they share information, and how the user interface should be structured.

The Sensor Web module uses a good analogy to describe what comes next. Because each node on the Sensor Web represents the location of an imaging station, those nodes are likened to the pixels of a camera. Only by collecting data from all the "pixels" does an image begin to emerge.

The same idea applies to other collections of instruments. Instruments become a community of independent modules that communicate. The communication can occur within the instrument and its subordinates — secondary channels in a multichannel spectrometer setup, for example — or among other instrument modules that are plugged in anywhere on the World Wide Web. The communities can be a few modules, as in a typical spectrophotometric experiment like measuring absorbance of a sample in a cuvette, or the communities can be large and very distributed. Imagine mapping atmospheric ozone using a network of spectrometers equipped with sampling devices looking directly up through the atmosphere. Obtaining measurements from a thousand locations around the world would take only hours to set up. Imagine measuring UV absorbance of air pollutants across a highway. Just find two office buildings with Internet access and put a deuterium source on one roof and a spectrometer on the other.

There's a slightly more mercenary side to what this model offers, which is the ability for both the instrument and the knowledge supplier to communicate directly with users through online chat, video, voice, text, or more. The network of users becomes at once a delivery mechanism for marketing information and for the distribution of other intellectual products like operating manuals or repair and service reports.  

The smart instrument modules of the network form an infrastructure upon which information can be shared and businesses can be built. The value resides in the network itself, not the pieces (think of all the folks developing applications for the iPhone and iPod touch). Forward-thinking companies in all sorts of technology industries won't be focused solely on designing and distributing better MP3 players or cell phones, for instance, but on creating methods for distributing the information those devices capture, display, and store.

None of this will happen overnight. However, driven by an increasing focus on the technology focus of the day — think of areas such as energy self-sufficiency and environmental sustainability — researchers and developers are likely to consider the impact of distributed sensing models applied to applications such as ozone monitoring, agriculture yields, and environmental analysis.

The good news is, the technologies already exist to make all this happen, and the Apple model certainly proves that distributed information has value to many. Consider that 20 years ago, it was hard to imagine a spectrometer portable enough to take to the sample, but today many thousands of spectrometers are in use in situ in even the most inhospitable places. Advances in lasers and optics have been no less dramatic.

So, perhaps the question for distributed sensing is not if, but when?

is Senior Director, Brand for miniature fiber optic spectrometer pioneer Ocean Optics, Inc. He joined Ocean Optics in 1995 and has held various positions within the company in sales, marketing, and customer relations. Morris has a bachelor's degree in journalism from Pennsylvania State University and nearly 25 years of experience in marketing, public relations, and publishing.

Laser and Optics Interface

The author discusses what the future holds for the field of optics.

How Will Distributed Sensing Inspire Changes in Optical-Sensing R&D?

Thermo Fisher Scientific's DRA-EV-300 diffuse reflectance accessory is designed for material science and general QC measurements on solid materials. The accessory has a wavelength range of 220–1100 nm and is designed with a sintered PTFE internal coating. According to the company, the accessory's prealigned configuration on a kinematic mount simplifies installation. The instrument's horizontal reflectance sample mounting holds samples in place with gravity. 

Mobile laboratory benches from OMNI Enclosures are designed for mass spectrometers. The benches reportedly optimize laboratory space and provide a quieter environment. The benches feature integrated accessories designed for comfort, organization, and safety. According to the company, the bench systems are available in various sizes and will support mass spectrometers and related equipment from all manufacturers. 

A 52-page catalog of IR–FTIR supplies and accessories is available from New Era Enterprises. The catalog includes application notes and lists products such as infrared optics windows, lenses, ATR elements, and optical coatings; liquid, solid, and gas transmission accessories; HATR crystal plate mount and liquid transmission cell reconditioning. 

Photon Engineering's FRED Optimum software is designed for modeling all types of spectroscopic systems. The 3D CAD optical engineering software can simulate the propagation of light through any optical–illumination system modeling source radiative phenomena and arbitrary optomechanical geometry with algorithms to model fluorescence, polarization, interference, scatter, diffraction, transmission, reflection, and absorption at any surface. 

The ALPXS aerosol-to-liquid particle extraction system from Meinhard Glass Products collects and concentrates airborne metals into a liquid sample for on-site or laboratory analysis. Particles in air are drawn through the device at rates as high as 200 L/min, are charged at 8000 V, and are collected in a liquid volume of 10–20 mL. Applications include real-time sampling of welding fumes, workplace exposure measurement, and environmental metal particle monitoring. 

Sensors Unlimited's model SU1024LDH SWIR digital line scan camera is designed for high-speed NIR spectroscopy and machine vision. The camera includes an uncooled linear InGaAs image sensor system with 1024 25 μm × 25 μm pixels. Quantum efficiency reportedly is greater than 70% from 1.0 to 1.6 μm. The camera reportedly sees beyond the visible range and outputs 14-bit Camera Link at frame rates greater than 46,000 lines/s. 

Sensors Unlimited, part of Goodrich Corporation

Glass Expansion's TruFlo device measures and digitally displays the sample flow rate to an ICP-OES system or ICP-MS nebulizer. According to the company, the device can be programmed to sound an alarm when a deviation from specified flow rates occurs. It reportedly also can provide an electronic record of sample flow rate for regulatory compliance. 

The CryoMill from Retsch is designed for cryogenic grinding. The mill is continually cooled with liquid nitrogen from an integrated cooling system before and during the grinding process. Grinding parameters can be stored, and LEDs in the display indicate the current state of operation. The mill has a vibrational frequency of 25 Hz. Samples mainly are ground by impact but are also ground by friction, which reportedly provides finer grind sizes. The mill includes one grinding station for grinding jar volumes of 25, 35, and 50 mL. An optional adapter holds as many as four 5-mL grinding jars. 

Xflash 5000 liquid-nitrogen-free silicon drift detectors from Bruker AXS are designed for use with the company's QUANTAX EDS microanalysis systems. The detectors reportedly provide energy resolutions as low as 123 eV at Mn-Ka and a 100,000-cps input count rate. According to the company, the detectors are equipped with electron traps that support interference-free X-ray detection. 

Hellma's model 668.006-NIR reflection probe is designed for diffuse reflection measurements in the visible and NIR ranges. The probe reportedly was developed for online analysis of scattering goods. The probe includes seven circularly arranged illumination fibers with a core diameter of 200 μm each for homogeneous lighting of samples. The probe's central reception fiber has a diameter of 600 μm and directs the diffuse reflected light back to the spectrophotometer's detector. Applications include determining the composition of bulk goods or pastes and monitoring the moisture content of powders or granulates. 

Nunavut Raman systems from BaySpec integrate volume phase grating–based spectrographs, deep-cooled cameras, light sources, and a fiber-optic probe in a portable enclosure. Available systems include 785-nm and 1064-nm options. According to the company, the systems are designed for high throughput and long life. 

The GladiATR diamond ATR accessory from Pike Technologies can be used for analysis of hard objects such as coated metal wires, polymer pellets, and geological samples. Crystal plate options include germanium and choices for heating and cooling. The accessory is available with a liquid-crystal display. According to the company, the video display enables users to position the sample to desired areas for analyzing visible sample features. Illumination and camera image are integrated below the diamond crystal. 

The Axios FAST simultaneous X-ray fluorescence system from PANalytical is designed for high speed and throughput for routine analysis and dedicated real-time process control in applications such as mining exploration and steelmaking. The instrument reportedly allows simultaneous fixed-channel measurements of as many as 28 elements. Features include a 4-kW X-ray tube, a high-throughput sample loading turret mechanism, externally mounted channels, self-monitoring facilities for remote diagnostics, and dedicated software and hardware interfaces. 

Custom inorganic standards from Inorganic Ventures can be used in applications such as ICP, ICP-MS, atomic absorption, and ion chromatography. Each standard is supplied with a certificate of analysis that details NIST traceability, certified values, and trace impurities. The standards reportedly are manufactured in a "green" manufacturing facility. 

Departments: Products

Products

The coupling of HPLC with ICP-MS is a relatively new technique that combines two very well established analytical methods. The combination of these analytical methods provides what could prove to be a very useful technique in clinical analysis. The total LC-ICP-MS market amounts to only a few percent of the overall ICP-MS market, but it is rapidly developing into a significant niche market. 

LC-ICP-MS is proving to be useful for the speciation and quantitation of those species in relatively complex matrices, such as those in biology. The connection of HPLC instruments to ICP-MS systems is relatively simple, and does not require an extensive amount of additional hardware or service to perform. This fact allows laboratories that already have both HPLC and ICP-MS to utilize the technique easily. 

LC-ICP-MS is valuable for evaluating environmental exposure of humans and animals; for detection of various, often very rare, diseases and disorders; and for other clinical analyses. In the long run, this will result in the largest demand coming from clinical analysis laboratories. However, for the time being, academic laboratories, and to a lesser extent government laboratories, are where the action is in exploring the full potential of the technique. 

The global market for LC-ICP-MS will be less than $15 million in 2008, but this does not include those ICP-MS and HPLC systems that were installed separately. This market estimate includes only those HPLC and ICP-MS systems specifically installed together, as well as connection kits, software, and service related to all LC-ICP-MS operation. As a relatively new and small segment of the ICP-MS market, LC-ICP-MS demand will easily top 20% annual growth in the near-term as the technique begins to mature. 

The foregoing data were based on SDi's market analysis and perspectives report entitled Global Assessment Report, 10th Edition: The Laboratory Life Science and Analytical Instrument Industry, September 2008. For more information, contact Stuart Press, Vice President — Strategic Analysis, Strategic Directions International, Inc., 6242 Westchester Parkway, Suite 100, Los Angeles, CA 90045, (310) 641-4982, fax: (310) 641-8851, 

Market Profile Column

The coupling of HPLC with ICP-MS is a relatively new technique that combines two very well established analytical methods. The combination of these analytical methods provides what could prove to be a very useful technique in clinical analysis. The total LC-ICP-MS market amounts to only a few percent of the overall ICP-MS market, but it is rapidly developing into a significant niche market.

Market Profile: LC-ICP-MS

As a branch of science, spectroscopy is inherently quantitative. Quantities have two parts: numerical values and units. I will assume that everyone knows what numbers are (although a future column might devote space to the conventions for expressing numbers ...). The units system that science (including spectroscopy) uses, however, might not be as obvious. All scientific units are based upon seven fundamental, so-called base units. In this first part, we will review the history of these units and several of the base units themselves. We will complete our review in the next rendition of "The Baseline."

The units system currently used in almost all science has been, in most cases, rigidly defined. However, this wasn't always the case. 

Historically, units systems sprang up all over the world as commerce developed. Some became obsolete, while vestiges of others remain. One of the oldest units of length, for example, is the cubit, for which there is evidence of use in Egypt in 2700 BCE. Many civilizations — Egyptian, Roman, Hebrew, Babylonian, Arabic — used the cubit, but its defined length varied from just over 300 mm (in today's units) to over 650 mm. In part because of the imprecision of its definition, the cubit is not used as a unit today except in historical discussions.

There is evidence from ancient Sumer that the length of a human foot was adopted as a unit of measure: the foot. The ultimate question, however, was "Whose foot?" The length of a foot eventually was standardized in England, and finally was supplanted by the SI system. Units for other types of quantities — volumes, energies, even currencies — were developed and ultimately were based upon some physical object or objective. An acre, for example, is apparently the area of earth that can be tilled by a single ox in a single day. Although most units systems of measurement have been standardized, the units systems of currencies remain stubbornly self-defined by individual governments (the introduction of the euro [€] being a well-known exception of governments working together for a common unit of exchange).

As part of the exchange of new ideas that was part of the European Renaissance, scientists of the time began to understand the need for a unified system of units. After the development of the decimal system for representing fractional parts gained acceptance in the late 1500s and early 1600s, some scientists — like Leibniz in the 1670s — saw an obvious system of units based upon factors of 10.

It took the French Revolution to finally bring a codified system of units, based upon powers of 10, into existence. In 1791, the French National Assembly requested that the French Academy of Sciences establish a logical system of units. A Commission appointed by the Academy (including famed mathematicians–scientists Lagrange, Coulomb, and Laplace) spent months considering how such a system would be defined, mostly based upon physically measurable objects like the earth and the day. They then spent years making measurements or constructing prototypes, in the midst of almost constant political turmoil. A "metric system" was adopted by France in April of 1795. In 1798, the French invited representatives from some neighboring countries (not including Great Britain or the fledgling United States) to hear presentations of results and consider adoption of a standard system. In 1840, the system was made compulsory in France, and other countries slowly adopted the new system. (Currently, almost all countries have adopted such a system for everyday use, not just for commerce. Holdouts include Liberia, Burma [Myanmar], and the United States.)

In 1875, an international treaty called the Convention du Mètre (the Meter Convention, or the Treaty of the Meter) established three organizations to oversee an international system of units; to date the treaty has been signed by 51 countries, with several dozen others having associate status. One of the organizations, the Bureau International des Poids et Mesures (BIPM; the International Bureau of Weights and Measures), is charged with ensuring consistency of the units around the world, so that any one country's meter is the same as any other country's meter. Another organization, Conférence Générale des Poids et Mesures (CGPM, the General Conference on Weights and Measures), meets every four to six years to determine what the standards are. At their 11th meeting in 1960, the Conference dropped the phrase "metric system" and adopted the phrase "Système International d'Unités", or "International System of Units" (abbreviated SI) as the name for the worldwide system of units. At the same time, the Conference also updated the list of approved numerical prefixes. The Conference works on important but seemingly trivial conventions. For example, in 2003, the Conference passed a resolution that the decimal point could be represented by a comma or a period, but that commas used to separate left-of-decimal digits into groups of threes were not appropriate.

To date, the CGPM has recommended the adoption of seven fundamental, or base, units. With the exception of one unit, all are based upon physical phenomena that can be measured in the laboratory, allowing them to be replicated around the world. Units for all other quantities are derived units, which are product or division combinations of the various base units.

The base units describe the following quantities: length, mass, time, electric current, temperature, amount of substance, and luminous intensity. Table I summarizes the base units for the various quantities. Interestingly for readers of this publication, several of the units have a spectroscopy connection.

Table I: The base units of fundamental quantities 

The base unit of length is the meter (also spelled 

), whose abbreviation is "m". A 1-m length is a little more than a yard. Its original definition was one-ten millionth of the distance between the Earth's equator and the North Pole on a meridian that passed through Paris, France. Once this distance was determined (after a multiyear surveying project), this distance was represented by the spacing of two marks on a 90:10 platinum-iridium bar kept in Paris. Because the bar's length varied with temperature, it was specified that the bar be at the melting point of ice to yield the correct distance between the marks.

In 1960, the 11th CGPM redefined the meter as 1 650 763.73 wavelengths of the orange-red light (λ ≈ 605.8 nm) emitted by xenon-86 in a vacuum (Figure 1). This tied the definition of the meter to a laboratory measurement that could be reproduced worldwide. In 1983, the 17th CGPM redefined the meter as being the distance that light travels in a vacuum in 1/299 792 458 of a second, and in 2002, there was a codicil that this definition be restricted to lengths for which relativistic issues were negligible. This is its current definition.

Figure 1: A portion of the emission spectrum of Xe. The arrows indicate the particular emission line that, for 23 years, used to be the basis of the definition of the meter. 

Note that area and volume are not part of the base units. Instead, they are derived units because they are the products of two lengths or three lengths, respectively. A short discussion of derived units, including a common unit of volume, will be found in the next column. 

The base unit of mass is the kilogram. It is the only base unit that is not the fundamental unit of its type, and is instead a combination of the fundamental unit and a numerical prefix (to be discussed in the next column). The kilogram was defined originally as the mass of one cubic decimeter (0.1 m) of water at 0 °C in 1795, which was changed to 4 °C in 1799 because water is at its greatest density at that temperature. Later in 1799, a platinum cylinder was machined that, given the abilities of the time, was as close as possible to the mass of 1 cubic decimeter of water at 4 °C. 

In 1879, a new cylinder was machined. Made this time of a 90% platinum and 10% iridium alloy, it is a right-circular cylinder 39.17 mm wide and high. This cylinder, called the international prototype kilogram (or IPK), was declared the official kilogram by the first CGPM in 1889. Six copies also were constructed; currently, all of the prototype kilograms reside in an environmentally controlled vault at the International Bureau of Weights and Measures outside of Paris. Other copies of the IPK exist and reside in countries around the world.

The kilogram is the only base unit that is based upon a physical object, rather than some laboratory-produced phenomenon. Its definition as such is subject to criticism. For example, in periodic reevaluations of the masses of the IPK and its replicas, the exact masses of the cylinders change very slightly over time — on the order of a few dozen micrograms. This is likely due to the absorption and desorption of contaminants on the surfaces of the spheres, as well as the inevitable production of scratches and other imperfections upon handling. Because of this, there are some modern attempts to reconsider the definition of a kilogram in terms of other quantities. Suggestions to date include defining a kilogram as a specific number of carbon or silicon atoms; measuring a certain number of heavy metal ions by the amount of current they can generate; using special electronic balances that measure the electrical power needed to determine the mass of an object; or defining the amount of force needed to accelerate a kilogram of mass. Some of these methods do not currently have the precision needed to define a kilogram any better than the current definition. Ultimately, a CGPM conference will have to consider and approve any new definition.

The base unit of time is the second (which has the occasionally troublesome abbreviation "s", not sec). It was originally defined as 1/86 400 of a day (with the denominator coming from the product of 60 × 60 × 24). However, it was recognized quickly that the length of the day is inconstant and, in fact, slowly increasing. It is estimated that the day lengthens about 2.3 s every 100 000 years, or by about 23 μs per year. This might not seem like much, but it's enough time for light to travel over 6000 m — which means, by the way, that if you're using a GPS system for navigation, your predicted position could be off by 6 km! Clearly, a more accurate definition of the second is necessary.

At the 11th CGPM in 1960, the second was redefined in terms of a fraction of a particular year; specifically, 1/31 556 925.974 7 of the tropical year starting at 1900 January 0 at 12 h ephemeris time. (In calendar-speak, January 0 is the day before January 1. There are some logical reasons for defining a "day 0" for a month, but don't ask me the reasons . . .) This definition lasted only seven years, as the accuracy of atomic clocks (for a review of such, see a previous column [1]) improved. In 1967, at the 13th CGPM, the second was redefined as the duration of exactly 9 192 631 770 periods of the radiation involved in the transition between the two hyperfine levels of the cesium-133 atom, provided that the atoms had an altitude equal to the average ocean surface of the earth (this part being added in 1980) and were at 0 K (this part was added in 1997). Hence, both the meter and second units are intimately tied to spectroscopic principles.

Base Units of Temperature and Electric Current

Temperature is a property of a sample of matter that is a measure of the average kinetic energy of the particles (atoms, molecules) in the sample. The first temperature measurement device apparently was constructed at about 1000 BCE, when Avicenna, a Persian–Arabic philosopher, built an air thermometer whose gas expanded and contracted with changing temperature. In 1592, Galileo Galilei built what is now called a Galilean thermometer (Figure 2). Acting on the principle of buoyancy, the thermometer was composed of a larger-diameter glass tube with objects that sank or floated depending on the varying density of the water as temperature changed. Galilean thermometers can be purchased today as novelty items in stores.

Figure 2: A Galilean thermometer. Photo by Alistair Riddell. 

The first liquid–alcohol thermometer was constructed in 1654. In 1724, Daniel Fahrenheit published a numerical scale for expressing temperature. Originally, the temperature of a mixture of salt and ice was set arbitrarily at 0, the freezing point of water was set at 32, and 96 was set at core body temperature. Later, the interval between freezing and boiling of water was set to 180 increments, slightly moving average human body temperature to 98.6. This scale defined the Fahrenheit temperature scale, symbolized as "°F". Currently, the U.S. is the only major country that continues to use the Fahrenheit scale for everyday purposes.

In 1742, Anders Celsius, a Swedish astronomer, published a scale based solely upon the freezing point and boiling point of water. Originally called the centigrade scale, Celsius originally put 0 at the boiling point of water and 100 at the freezing point of water! (This was reversed by others three years later, after Celsius's untimely death in 1744.) The centigrade scale was more logical than Fahrenheit's scale, and likely appealed more to scientists who were developing a unit system based upon powers of 10. The ninth CGPM officially renamed it the Celsius scale in 1948. 

As useful as these earlier scales were, they were both relative — zero and any other benchmarks were based upon arbitrary chemical or physical processes. As early as 1702, however, it was noted by Gillaume Amontons (of Amontons' gas law fame), a French physicist, that there should be some sort of temperature scale for which there should be some absolute minimum — absolute zero, as it were. In 1787 and 1802, respectively, French scientists Jacque Charles and Joseph Louis Gay-Lussac noted how the volume of a gas varied with temperature under constant pressure; Gay-Lussac was the first to note how a factor of 273 was involved. Finally, in 1848, William Thomson, Lord Kelvin, published a paper titled "On an Absolute Thermometric Scale" (2), in which he argued for an absolute scale having the same degree size as the Celsius degree and having zero degrees at the minimum possible temperature, which he argued was about 273 degrees below 0 °C. The Kelvin absolute temperature scale is named in Lord Kelvin's honor.* 

In 1954, the 10th CGPM formally defined the temperature of the triple point of water (the point in the phase diagram of water at which all three phases can exist in equilibrium) as having the temperature of 273.16 degrees in the Kelvin scale. In 1968, the 13th CGPM defined the size of the degree as 1/273.16th of the thermodynamic temperature of water at its triple point, and recommended that the unit be called "kelvin," with the abbreviation K, rather than "degrees Kelvin" or "°K". Note that the temperature of the triple point is a defined temperature, not an experimental one, and the size of the kelvin unit is based upon it. 

Electrical current is caused by the movement of electric charges, either electrons or ions. The unit of current is the ampere (sometimes shortened to amp but whose sanctioned abbreviation is A), named after André-Marie Ampère, a French physicist who proposed some fundamental ideas in electromagnetism. In 1893, the ampere was defined as the current needed to deposit 0.001 118 g of silver from a silver nitrate aqueous solution. However, at the ninth CGPM in 1948, a new definition was adopted: 1 A is the current that, running in two straight parallel conductors of infinite length that are 1 m apart in vacuum, would produce a force of 2 × 10

  newtons per meter of the conductors. This only changed the value of the coulomb by about 0.01%. 

In the next column, we will consider the rest of the base units and a few issues that concern us about all of these units. 

* Although William Thomson was knighted in 1866 for his plethora of scientific achievements — including work in thermodynamics, laying the first transatlantic cable, oceanography, electrodynamics, and the age of the Earth — he was not given his baronage until 1892. The title Lord Kelvin derives from the River Kelvin, a river that passes by the University of Glasgow, Scotland, UK, where Thomson was on the faculty. (See Figure 3.) Thus, the name of a river is now permanently ensconced in the fundamental units of science. Kelvin is also rather famous for some major misstatements of future applications. He is said to have stated: "Heavier-than-air flying machines are impossible" and "Radio has no future." Such blunders are perfect examples of Clarke's law (after science fiction author Arthur C. Clarke): "When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong."

Figure 3: The River Kelvin in Glasgow, Scotland, UK. The river's name is now irreversibly attached to the standard unit of temperature. Photo by Finlay McWalter. 

Thomson died in 1907; his remains were interred in the nave of Westminster Abbey, right next to Isaac Newton. He had no heirs or close relatives, so the title of Lord Kelvin died with him and reverted to the Crown. Although in theory it is possible that the Crown might bestow the title of Lord Kelvin on someone else, those who understand the peerage system better than I think that the chances are very remote. Hence, we might have seen the first and last Lord Kelvin.

Thanks to my colleague Stephen Benn of the Royal Society of Chemistry for details concerning hereditary baronies in the UK peerage system.

 is a professor of chemistry at Cleveland State University in Ohio. Many of his "Baseline" columns have been reprinted in book form by SPIE Press as 

, available through the SPIE Web Bookstore at 

 (published in May 2006), is available from SPIE Press. He can be reached at 

Columns: The Baseline

Columnist David ball discusses the history of the units system used by scientists and several of the base units themselves.

The Seven Base Units, Part I

A femtosecond optical parametric oscillator (OPO) has been used to characterize the transmission of a number of different all-silica photonic bandgap (PBG) fibers specifically developed for gas sensing in the mid-infrared region. One of these fibers was chosen for its optimum transmission band (bandgap) overlap with methane fundamental absorption spectrum around 3.2 μm. Fourier transform–infrared (FT-IR) spectroscopy of methane has been demonstrated using the air-core of the PBG fiber as a gas cell and the OPO as a light source for concentrations down to 1000 ppm. A comparison with theoretical predicted values is presented alongside the measurements. An evaluation of the signal noise suggests that a measurement resolution better than 100 ppm concentration is achievable using this system.

Gas sensing is becoming increasingly important for process control, environmental studies, and safety monitoring. Typically, it is necessary to determine the concentration and composition of a gas sample. Gas species selectivity, which can be an issue for many different types of gas sensors, can be addressed in the case of optical sensing by the well-proven technique of absorption spectroscopy. The use of optical fibers in spectroscopy offers a number of potential advantages over conventional gas cell configurations: they can easily provide remote access to hazardous locations, and distributed sensing topologies become feasible. Moreover, due to their high mechanical flexibility and their small dimension, they present a potential for miniaturization of the system, which is an important advantage to access confined sites. Unlike optical fibers, typical gas cells can be relatively large and not suited to remote measurement or use in small spaces. Furthermore, the size of the cell determines the minimum quantity of gas required, which can be a limiting factor if only small gas samples are available. 

Examples of optical fiber gas sensor systems already exist, but these are limited due to potential contamination of the sensing area (for example, D-shaped fibers [1], in which the cladding thickness is reduced greatly on one side of the fiber, to allow interaction with the light in the core), or the fiber is used only to access conventional gas cells and thus is not directly involved in the sensing aspect (2). Moreover, optical fiber-based systems demonstrated to date typically operate below 2 μm due to the large silica absorption in the mid-infrared (mid-IR) region. Below 2 μm, gas absorption features are weak overtone resonances, thereby requiring long interaction lengths and highly sensitive techniques to make useful gas concentration measurements. In the 3–6 μm region, it is possible to probe stronger fundamental resonances; therefore, fiber delivery in this wavelength band could potentially enable more sensitive detection, contingent upon the availability of suitable fiber-compatible mid-IR sources. The recent development of new light sources operating in this region, such as lead-salt lasers (3), quantum cascade lasers (4), or difference frequency generation lasers (5,6), means that there is potential for enhanced sensitivity for mid-IR spectroscopy. 

Recently, silica photonic bandgap (PBG) fibers, similar to the one illustrated in Figure 1, have attracted particular interest for their ability to guide light in an air-core unlike the solid core of other microstructured fiber and conventional fiber designs. Unlike conventional silica fibers, these PBG fibers rely upon the so-called bandgap effect (7) for guidance rather than total internal reflection. This means that they can transmit only over a narrow range of wavelengths, often referred to as the fiber's bandgap, typically 100–200 nm wide in the IR. PBG fibers can guide over 98% of the light outside the fused silica in the hollow core and air holes around the core (8), and thus are ideally suited to gas sensing. The low fraction of light guided in the glass means that the material absorption plays a smaller role in overall fiber attenuation characteristics, and therefore, these fibers have the potential to transmit light in the mid-IR region beyond the usual silica transmission window (below 2 μm). We have previously demonstrated an all-silica PBG fiber for mid-IR transmission and its potential for methane gas sensing (9,10). In this article, we present a number of PBG fibers designed and fabricated for operation around 3.2 μm, thereby making them suitable for methane sensing. Mid-IR absorption measurements for methane are also presented, in which we have used one of these fibers as a gas cell and a femtosecond optical parametric oscillator (OPO) as a light source suitable for use with a Fourier-transform spectrometer. 

 Figure 1: Scanning electron micrograph of a PBG fiber. 

Highly sensitive gas absorption spectroscopy requires a light source that combines a broad wavelength range, good spatial beam quality, and appropriate output power. Commonly used thermal sources allow easy access to a wide range of spectral regions but cause severe problems when it comes to tight focusing and guiding in an optical setup and offer limited optical power. Laser sources overcome this problem, but it is only relatively recently that laser sources in the 3-μm region have become available (3–6). Where a laser exists at a suitable wavelength, it is usually necessary for it to be either tunable or broadband to record absorption spectra across an appropriate spectral region to allow for species identification and intensity normalization. For wavelength regions in which direct laser emission is not available, parametric oscillation in a nonlinear crystal can be used to produce light beams with spectra centered on wavelengths longer than the laser source itself. If both the pump laser and the parametric conversion process are broadband, then parametric oscillation can be used to realize a broadband tunable laser in any wavelength range lying within the transmission window of the nonlinear crystal used. 

In our experiments, we used 100-fs pulses from a Ti:sapphire laser as the pump and a periodically poled lithium niobate (PPLN) crystal as the nonlinear medium for converting 800-nm light into tunable mid-IR light covering the 2.7–3.4 μm region. These wavelengths correspond to the nonresonant idler output of the oscillator, but a complementary near-IR output (the signal) also exists from 1–1.2 μm, which is resonant in the OPO cavity. The oscillator emission range is shown in Figure 2, in which different spectra were obtained by slight adjustments of the OPO cavity length; 2.5 μm of cavity length adjustment was sufficient to tune the center wavelength of the idler pulses by several hundreds of nanometers. 

Figure 2: Tuning abilities of the OPO used in the experiment. Alternate spectra are shown in different colors for clarity. 

Transmission Measurements of Photonic Bandgap Fibers

To identify the PBG with the transmission profile that best matched the methane absorption band, we measured the transmission spectra of a number of HC-PBG fiber samples. The experimental arrangement for measuring the spectra is shown in Figure 3. The OPO idler beam was sent through a cavity mirror to an FT-IR spectrometer consisting of a scanning Michelson interferometer. Light leaving the interferometer was coupled into an 80-cm-long PBG fiber using a 25-mm focal length ZnSe lens. The average power after the lens was ~500 μW, but it was still below the saturation level of the detector, which was a PbSe photodiode. The detector could be positioned to measure the idler with or without the fiber, and a Ge filter was placed between the interferometer and the fiber-coupling lens to remove wavelengths other than those of interest. The interferometer scanning mirror was mounted on a linear actuator running at a frequency of 0.5 Hz over a usable range of 3.6 mm, corresponding to a postprocessed wavelength resolution of about 3 nm in the measured spectrum. The spectra were calculated by a Fourier transform of the interferogram acquired in a single period scan of the interferometer mirror. To produce a calibration interferogram and hence compensate for any nonlinearity in the optical path scan, a He-Ne laser also was directed into the interferometer along a parallel path to the main beam. 

Figure 3: Experimental setup for PBG fibers transmission and methane sensing. Intensity (arbitrary units) 

Several fibers were characterized. They were fabricated using a stack-and-draw technique (11): silica capillary tubes were stacked in an array with a triangular lattice configuration to manufacture a preform (Figure 4). A total of 19 capillaries were omitted at the center of the stack to form the core. The whole stack was then jacketed with a silica cladding and drawn down to the final fiber dimensions. The optical properties of the PBG fibers can be tailored mainly by changing the air-filling fraction (fraction of air in the fiber cross-section). The fibers had a core diameter of ~40 μm and a pitch, which is the distance between the center of two neighboring cladding holes, varying between 6 and 7 μm due to natural variations in the drawing process (in this case, the variation was introduced deliberately by varying the different pressures applied in the core and cladding during the fabrication process). 

Figure 4: Preform stack of silica capillaries with 19 tubes removed from the center in order to form the core. 

For transmission measurements, the idler spectrum was measured sequentially directly from the oscillator and then after being transmitted through a fiber. The spectral bandwidth of the OPO light was about 100 nm, so to cover the entire PBG transmission band, it was necessary to measure several interferograms by tuning the OPO to several different central wavelengths covering most of the region of interest. Starting from one end of the idler spectrum, the He-Ne signal and idler with and without the fiber were recorded. The idler was then tuned in steps of 80 nm and spectral measurements were repeated so that a total of four interferograms were recorded for each tuning position to obtain spectra for both the reference idler and the idler via the fiber after Fourier transforming. Therefore, the fiber transmission spectrum over the 3.2–3.4 μm region could be calculated. 

Figure 5: Transmission of the optimum PBG fiber chosen for methane sensing. 

All four fibers tested revealed a bandgap providing guidance of light at wavelengths above 3 μm. However, for different reasons, such as the position of the bandgap in terms of wavelength, the optical source wavelength limitation, or the presence of strong spectral oscillations in the transmission bandgap, only one fiber was chosen for methane measurements. As can be seen in Figure 5, the spectrum of this fiber displays a relatively broad transmission window around 3.32 μm, where the strongest methane spectral features can be observed. This fiber was thus considered to be the optimum one for methane detection. Figure 6 shows the overlap between the methane spectral lines (from the HITRAN database [12]) and the bandgap of the selected fiber. 

Figure 6: Spectral overlap between methane fundamental absorption lines (in red â from Hitran) and fiber 2 bandgap (in black). 

To determine the filling dynamics of the methane–nitrogen gas mix within the fiber core, a pressure sensor was attached to one end of an 80-cm-long PBG fiber. At the other end of the fiber, a vacuum pump, a methane–nitrogen supply, and another pressure sensor were attached. Pressures at both sensors were monitored simultaneously, showing that the entire fiber was evacuated and then filled with a methane–nitrogen gas mix at 2 bar. This filling process was completed in <10 min, and this time was used as a typical filling time for our system. 

To evaluate the system for spectroscopic gas detection, both ends of the fiber were mounted inside connectors that allowed light to be coupled into and out of the fiber through CaF

 windows while providing a means to fill the fiber core with a precisely controlled mixture of methane and nitrogen. The absorption spectrum of methane was measured over the 3.19–3.36 μm range. Results are shown in Figure 7a, and the calculations of absorption derived from the HITRAN database taking into account the pressure, temperature, fiber length, and the system spectral resolution are presented in the same figure (dashed lines). Measurements were repeated for 1%, 0.5%, and 0.1% methane concentrations and are shown in Figures 7b, 7c, and 7d, respectively. The main absorption band (3.32 μm) is still clearly visible (30% absorption) for a volume concentration of 0.1% (1000 ppm). 

Figure 7: Measured (solid lines) and theoretical (dashed lines) absorption lines of methane for different concentrations. 

The measured absorption lines show a good general agreement with those predicted from the HITRAN database. In particular, the principal feature, which is the broadest absorption band, between 3.31 and 3.32 μm, is located at the expected wavelength. Discrepancies between theoretical and experimental results might be explained by the small signal-to-noise ratio at the edge of the OPO signal and data-processing error during the concatenation required due to the limited bandwidth of the measured idler. Indeed, as different OPO pulses over the spectral region of interest were used, concatenating the data was necessary, which could introduce some error. However, we note that measurements over such a wide range of wavelengths are not indispensable. In some gas sensing schemes, one absorption line and an appropriate reference measurement (for example a gas-free reference) might be sufficient. 

In our experiments, the OPO instability also made measurements difficult. A slight change in temperature can cause a cavity tuning, resulting in wavelength and power shifts. The comparison between the methane-free and the methane-full fiber output signal required care to ensure that there was no significant change in the OPO spectrum between signal and reference measurements. This is not a significant issue for high gas concentrations (>0.1%), in which the absorption was still strong enough. However, it is a limiting factor for lower concentrations because the intensity of the absorption lines decreases significantly for concentrations below 0.05%. Improved source stability and simultaneous measurement of the light spectrum before and after the fiber would bring substantial improvements. 

Optimizing the tradeoff between the requirements for maximum gas interaction length with the increasing fiber losses associated with longer fibers might increase the system performance further. 

We report the characterization and potential sensing application of fused-silica PBG fibers for use in the mid-IR region. These fibers, in which the majority of the light is confined in a bandgap-guiding air-core, provide excellent interaction between the guided light and the gas in the core. Furthermore, as the light is guided in a gas-filled core, the glass material absorption is less significant and we are able to operate at longer wavelengths (up to 3.45 μm), which is impossible using conventional silica glass fibers. For the fibers described here, there exists a good spectral overlap, with methane fundamental absorption lines indicating an interesting potential for gas sensing. Using one of these fibers as a gas cell, we have demonstrated methane sensing for concentrations down to 0.1% by FT-IR with a femtosecond OPO as a mid-IR optical source. The system shows that PBG fibers have a great potential for mid-IR gas sensing. 

(1)   F.A. Muhammad, H.S. Al-Raweshidy, and J.M. Senior, 

(2)   G. Stewart, C. Tandy, D. Moodie, M.A. Morante, and F. Dong, 

(3)   Secdilas IR series of PbSe laser diodes, Laser Components (UK) Ltd., Goldlay House, Chelmsford, Essex, UK. 

(4)   J. Devenson, D. Barate, O. Cathabard, R. Teissier, and A.N. Baranov, 

(5)   C. Erny, K. Moutzouris, J. Biegert, D. Kühlke, F. Adler, A. Leitenstorfer, and U. Keller, 

(6)   IRIS1000 DFG-based mid-infrared laser system, Novawave Technologies, Redwood City, California. 

(8)   P.J. Roberts, F. Couny, H. Sabert, B.J. Mangan, D.P. Williams, L. Farr, M.W. Mason, A. Tomlinson, T.A. Birks, J.C. Knight, and P. St. J. Russell, 

(9)   N. Gayraud, L.W. Kornaszewski, J.M. Stone, J.C. Knight, D.T. Reid, D.P. Hand, and W.N. MacPherson, 

(10) L. Kornaszewski, N. Gayraud, J.M. Stone, W.N. MacPherson, A.K. George, J.C. Knight, D.P. Hand, and D.T. Reid, 

(12) L.S. Rothman, D. Jacquemart, A. Barbe, D. Chris Benner, M. Birk, L.R. Brown, M.R. Carleer, C. Chackerian Jr., K. Chance, L.H. Coudert, V. Dana, V.M. Devi, J.-M. Flaud, R.R. Gamache, A. Goldman, J.-M. Hartmann, K.W. Jucks, A.G. Maki, J.-Y. Mandin, S.T. Massie, J. Orphal, A. Perrin, C.P. Rinsland, M.A.H. Smith, J. Tennyson, R.N. Tolchenov, R. A. Toth, J. Vander Auwera, P. Varanasi, and G. Wagner, 

Gas sensing in the mid-infrared region is examined.

Spectroscopy-01-01-2009