Consider that a reference-quality mass spectrum for a compound is measured when a few "ideal conditions" are fulfilled, or we assume that they are. For instance, we assume that the instrument measures the spectrum across the entire mass range with no mass discrimination. Ideally, ions of all masses are analyzed and detected with equal probability and the recorded ion intensities accurately reflect the extent of the various fragmentation processes. We assume that the resolving power of the instrument is sufficient to distinguish ions of adjacent masses. There are other assumptions that often "go without saying." But we are going to say them here because when the assumptions are not true, our measurement data can be skewed. We assume that the sample for which the reference mass spectrum is recorded is a pure sample, and purity may include enantiomeric composition, or for larger molecules, higher-order structures. Finally, we assume that the amount of sample in the ionization source of the mass spectrometer remains constant during the time required to record the mass spectrum. Each of these desiderata is, of course, an "ideal condition" not usually fully met. Sometimes, the assumed condition is not even slightly achieved. To achieve at least a measure of sample purity, chromatography is often used to introduce samples into the mass spectrometer. Many forms of column chromatography, however, elute samples into the mass spectrometer within a varying concentration centered around a retention time. The elution profile defines the peak shape. Here, we explore the interaction of dynamic chromatography with the dynamic aspects of mass spectrometry (MS).   			

Let's start with a leading and hopefully thought-provoking question: If a pure sample is eluted from a gas chromatography (GC) column operated in its normal fashion, and there is no detector, what is the peak shape at the end of the column? The dynamic processes of GC are well understood, and we can model the processes of molecular diffusion in the mobile and stationary phases to arrive at an answer. If the sample is a pure compound and the diffusion processes are unperturbed, then the sample will be eluted within a symmetrical peak shape as shown in Figure 1, with the 

-axis representing time after sample injection at the front of the column and the 

-axis representing some parameter proportional to the amount of sample eluted from the column. The time point at which the mass flow rate from the column reaches a maximum is called the 

, and this is labeled on the figure. On the same column, using the same GC conditions, the retention time should be reproducible, and it can be used as a factor in identifying the sample. One analytical principle to keep in mind here is that of "necessary and sufficient." More precisely, if the retention time of a reference standard is known, then if the unknown sample is the same compound as the reference it will be eluted at the same retention time. However, if the unknown sample is eluted at the same retention time, it may be the same compound as the reference. To be sure of our identification we need more information, and that brings us to information provided by the detector other than just a determination of the retention time. We want a detector that provides additional and characteristic information, which brings us directly to MS. Although chromatography coupled with mass spectrometry seems like the marriage made in heaven, we should always be aware of the mutual give-and-take in the relationship. In this column, we discuss the dynamic characters of our techniques, and we will start very simply. 

Figure 1: This symmetrical shape is the idealized representation of a peak eluted from a GC column. The x-axis is time after injection of the sample onto the column, and the y-axis is a detector response proportional to the amount of sample eluted from the column. 			

The characteristics of the detector determine the nature of the 

-axis parameter in Figure 1. If the detector operates to produce a response that is directly and linearly proportional to the mass flow of the sample entering it, and if it operates to produce a measureable signal in a time frame much shorter than incremental changes in mass flow, then the measured peak shape with this detector is very close to the modeled peak shape. If the detector produces a response that is concentration-dependent, then the relevant proportionality factor will change the appearance of the peak, although the peak shape may still be symmetric overall. If the detector operates in such a fashion that it provides a signal from some types of eluted sample molecules and not others (such as with an electron-capture detector), then the proportionality factor for the 

-axis response changes from sample to sample and peak to peak. Most modern GC detectors produce a near-instantaneous response, so that the 

-axis in Figure 1 remains directly proportional to time. We can foresee that if the detector itself has dynamic characteristics that have a scale that is similar to the dynamics of the elution of the GC peak, then a proportional factor must be applied to the 

The quintessential GC detector that provides the wealth of information needed to identify compounds as they are eluted from the column is the mass spectrometer. Accordingly, we need to evaluate the role of MS as a GC detection method in terms of its response characteristics that may provide a proportionality factor that changes the 

-axis (time) in Figure 1. Many types of ionization methods, mass analyzers, and detectors used in MS have been previously described in this column. The definitive matrix for MS ionization methods and their mass- or concentration-dependent response factors across all types of compounds is a construct that will probably never be completed. It cannot be predicted with specificity, and therefore the calibration curve for the particular sample of interest remains a reliable method to establish the proportionality on the 

-axis. In the remainder of this column, we discuss the dynamic nature of mass analysis using various types of instruments and describe the effects of that analysis on the measured elution profile of a GC peak (the 

The time required for mass analysis, specifically the scanning of the mass analyzer, is usually the parameter of concern in assessing whether a representational GC peak elution profile can be measured. This was more of a concern in the past than it is now. However, contemporary technical notes available on the web sites of multiple instrument vendors describe the effect of scan rate on the measured GC peak profile and tout the fast scan rate of their instruments. Figure 1 represents the idealized result of a detector that instantaneously produces a signal. But every mass spectrometer needs at least some time to differentiate between ion masses and measure ion flux. Leaving aside dispersive instruments, in which ions within a specified mass range are considered to be simultaneously detected, instruments with a single-channel detector will use some means to differentiate ions by mass that involves a dynamic time parameter. Perhaps the simplest mass analyzer is the time-of-flight (TOF) instrument, in which the time required for ions of different masses to traverse the instrument from the source to the detector is measured and then converted to the ion mass through a simple mathematical relationship. 

It is interesting to recall that some of the first commercial TOF instruments were marketed as detectors for GC. At that time, the GC column technology provided relatively broad peak elution profiles. Then, as now, the ionization method had to be operated in a pulsed rather than a continuous fashion, and the TOF mass analysis was completed within a few microseconds for each individual "scan." This speed of spectral data acquisition could resolve even slightly overlapped peaks in the GC elution profile. The ultimate rate-determining step in these earlier TOF instruments was the speed at which the data could be collected and collated. But the speed at which this step could be accomplished was still rapid compared to the width of the GC peak. In modern GC, the peaks are far narrower, but concomitantly, the electronics of TOF instruments are also much faster. Thus, the dynamic character of the TOF mass analysis (microseconds) is still usually orders of magnitude shorter than the elution profile (seconds).   

Common scanning mass analyzers are magnetic sector instruments and quadrupole-based mass filters. In their GC–MS configurations, these analyzers are coupled to a continuous ionization source such as an electron ionization or chemical ionization source. Sample compounds are eluted from the column into the source, where a continuous flux of ions is created and transported into the mass analyzer. Scanning a magnetic sector mass analyzer is a complex process. The electromagnet may exhibit hysteresis and may require a settling time, and temperature control of a large mass of the magnet may also become important. The relationship between the mass of the ion passing through the instrument from the source through the analyzer to the detector is given by 

 is the radius of the electromagnetic sector, and 

 is the accelerating voltage. The linear scan of 

 is therefore not linear with mass, and for technical reasons, often proceeds from a higher mass to a lower mass. The data processing system automatically converts the signal from the detector from a nonlinear scan into the linear scale of 

 window in the spectrum is monitored for the same period of time is therefore not valid. For instance, a total scan time of 70 ms (quoted in recent vendor literature for a magnetic analyzer GC–MS instrument) must be apportioned across the selected mass range factoring in the actual scan of 

, the magnetic field strength. The instrument operator should not hesitate to dive into the details of what is going on and to understand the dynamic nature of instrument operation. 

Quadrupole mass filters were combined into commercial GC–MS instruments in the 1970s, and marketed as smaller, faster-scanning mass analyzers (1). Because scanning is accomplished electronically, the scan function can be linear, bidirectional, or even nonsequential in mass. The speed is limited by the stability of the driving electronics, the response time of the detector, and the speed of the data acquisition process. This simple scan function provides the opportunity to use this instrument as a platform to delineate the interaction of scan time for the mass analyzer with elution time of a peak from the GC system. Figure 2 overlays the scan function on the elution profile of a peak; only alternate scans are shown for clarity. The vertical lines shown for two scans intercept the peak profile and show that the amount of sample in the source is changing during the time required to scan the mass analyzer. In this example, the time required for the scan is purposefully chosen so that it is similar in scale to the time required for the elution of the peak. On the leading side of the peak, the flux of the sample ions increases during the time required for a scan of the mass analyzer. On the trailing side of the peak, the flux decreases during the scan time. If the 

-axis is assumed to be proportional to sample flux, then the change in flux over the course of any given scan can easily cover a 2–3-fold range. This far exceeds the expected error in accuracy in measuring ion intensities (which is ±10% in the most generous of situations). So, what is the expected change in the measured mass spectrum caused by this change in sample flux? Assume that the scan direction is from low-mass to high-mass. Mass spectra recorded on the leading edge of the peak will have peak intensities at higher masses that are elevated over reference spectra levels. Conversely, assuming the same scan direction, mass spectra recorded on the trailing edge will have peak intensities at higher masses that are depressed below reference spectra levels. 

Figure 2: Ideal signal for a peak shape with discrete scans of the detector shown across the width of the peak. Only every other scan is indicated in the scan line, for clarity. The mass flow varies significantly over the course of each scan of the analyzer, as shown by the intercept of the vertical lines on the peak profile. 			

So which of these mass spectra do we choose to search against the library to identify the compound that just was eluted from the GC column? If we choose mass spectra recorded on the leading or trailing edge of the peak, there will be a scanning bias in the recorded mass spectrum that could throw off the validity of the library search. Do we then hope to catch a mass spectrum recorded right at the top of the peak, when the relative change in the flux of the sample entering the mass spectrometer might be minimized? We do not. Instead, our normal practice is to add together and average all of the scans recorded across the width of the peak, so that any scanning bias on one side is counterbalanced by the reverse bias on the other edge. Given a symmetrical peak shape, the assumption is justified, and the data processing software performs this function automatically and in the background. A general rule of thumb is that 8–12 scans recorded across the width of the peak will provide an accurate representation of the peak after processing and an accurate peak area that can be used for quantitation. Fewer scans may still provide a mass spectrum sufficient for identification purposes, but the connect-the-dots peak profile generates errors in the peak area values. 

To reach our desired number of scans across GC peaks that have become narrower, we require ever-faster scan speeds. The achieved scan speed is a combination of how fast the analyzer can scan, how fast the detector signal can be digitized, how fast the data can be stored, and how wide the desired mass range is. But there is yet another assumption, and that is that the ions (of whatever mass) are moving at a velocity sufficient to transit the analyzer in the short mass window imposed by the scan rate. For example, an ion entering the front of a quadrupole mass filter has to make it all the way through to the detector before the analyzer conditions switch to the "next" mass. Furthermore, the ion's flight through the filter is not a linear one. Instead, ions in stable trajectories follow oscillating paths. More details are beyond the scope of this column, other than to note that ion transit times become a concern at the very fastest scan rates. How fast is this rate? One vendor describes the top scan speed as 12,500 Da/s across a mass range of 150 Da. The time listed as required for a full scan is 0.01 s, and the interscan settling time is 0.0006 s. At the quoted scan speed of 625 Da/s, a full scan requires 0.2 s, and the settling time is 0.01 s. There are several intermediate settings, of course. The scan rate chosen by the analyst will depend on the expected narrowness of the GC peaks. If most GC peaks are a few seconds wide, with the narrowest perhaps 1-s wide, then the slower scan speed will only provide about five scans across that narrowest peak, and that will probably not be enough for quantitative purposes. However, that set of conditions may suffice for the other GC peaks. If the desired mass range is 450 Da (say from 

 500), then the quoted times for a full scan at a given scan speed will be three times longer. In that case, the system may record only one or two scans across a 1-s wide peak, and the changes in flux of the sample during the elution profile will probably distort the mass spectrum excessively across many GC peaks. The prepared analyst selects a scan speed option fast enough to cover the expected chromatographic conditions. In analysis of unknowns, that may be a "guess" based on experience. 

This column closes with a brief discussion of two other situations in which the dynamic nature of chromatography interacts with the dynamic nature of the scanning mass spectrometer. The first of these is the selected ion monitoring (SIM) experiment. In SIM, the mass range is not scanned continuously from one end of the range to the other. Instead, the mass analyzer is set to sequentially pass selected ions of different masses. The mass analyzer reaches the first mass conditions, resides there for a specified time window, and then switches to the next selected mass. The effective scanning speed is therefore related to how quickly the analyzer can move from one mass window to the next, and by extension, to how quickly the analyzer can step through the sequence. A SIM experiment can monitor changes in the elution profile intensities for one ion, 10, or 20. The time variation in signals for each of the mass-selected ions should reflect the elution profile of the peak. It is also possible for the data system to transition from recording full-scan mass spectra to a SIM mode in real time, providing full mass spectra for library searching and SIM data for lower limits of detection.  The ability to switch between the two data acquisition modes for narrow peak widths emphasizes  the need for a high scan speed  and a stable scan function of the mass analyzer. 

I chose the second situation to highlight the importance of understanding dynamics as it begins in Figure 1, in which we assume that the profile represents the elution of a single pure compound A from the chromatograph. Now, moving to Figure 3, we add the elution profile for a smaller amount of a nearly coeluted peak B, and ask whether MS information can provide evidence for this second component and how scan rates matter in our assessment. A horizontal line for the limit of detection is also provided in Figure 3. A scan rate sufficient to characterize the peak A profile must be several times faster to characterize the profile of peak B for two reasons. The limit of detection effectively narrows the peak B elution profile. Furthermore, the peak B elution profile is a subset of the peak A elution profile that must be differentiated. A first approximation is that two nearly overlapped peaks narrow the effective elution profile by a factor of 2. Three nearly coeluted peaks narrow the effective profile for a single peak by a factor of three. The demand for faster scan speed increases concomitantly. Faster scan speeds that seem to provide an over-determination of peak profiles via processing of full mass spectra provide a large set of data that can be mined for any evidence of coeluted samples near the limit of detection. 

Figure 3: A smaller amount of a nearly coeluted peak may provide a detector response very near the limit of detection, and as a consequence, a narrower effective peak width.			

  enjoys the strange looks that he gets while explaining the cut-and-weigh method of determining peak areas in strip-chart record gas chromatography to the younger analytical chemists. However, he is forced to admit that sharp scissors were taken away from him in fourth grade, and he settled for folded colored construction paper until sophomore year in college, when integrating circuitry captured his attention. Back in the early years, GC peaks were still tens of seconds wide, GC columns were packed by hand, and mass spectrometers scanned at glacial speeds. Younger folk tell the story that things are different now, reaching levels characterized as instant gratification. This column is the sole responsibility of the author, who can be reached at 

Articles

Mass Spectrometry Forum

When we think about the mass spectrum of a compound, we assume that various ideal conditions are met, but often, many of them are not fully met. Here, we explain many considerations one should take into account, particularly when using GC to introduce samples into a mass spectrometer.

Dynamic Gas Chromatography Meets Dynamic Mass Spectrometry

The C-Flow 700d concentric nebulizer from Savillex is designed to be a universal nebulizer for inductively coupled plasma–optical emission spectrometry and inductively coupled plasma–mass spectrometry. According to the company, the nebulizer is chemically inert and has an uptake line with a zero-dead-volume connection. 				

WITec's modular alpha300/500 Raman microscope is designed for analysis of the chemical characteristics of a sample three-dimensionally, underneath and at the surface. According to the company, the microscope provides high spectral quality and spatial resolution for   minute sample structures. 

Shimadzu's EDX-LE energy dispersive X-ray fluorescence spectrometer is designed for screening elements regulated by RoHS/ELV directives. According to the company, the spectrometer is equipped with automated analysis functions and a detector that does not require liquid nitrogen. 

Diamond, Ge, and Si crystal options and a specular reflection plate from PIKE Technologies are designed for use with its MIRacle single-reflection attenuated total reflection (ATR) accessory for FT-IR applications. According to the company, an optional force sensor connected to a digital readout, a liquid retainer, a flow-through attachment, a selection of clamps, and dedicated pressure tips for use with various sample types are also available for the ATR accessory. 

Ondax's SureBlock notch filters and XLF-series filters are designed to enable capture of Raman spectra in the terahertz spectroscopy regime (10–  200 cm

 , or 300 GHz to   6 THz) with any single stage spectrometer. According to the company, the filter systems also capture simultaneous Stokes and anti-Stokes Raman signals up to greater than 5000 cm

  (150 THz) with throughput up to 80%.  				

Andor's Newton electron-multiplying CCD spectroscopy detector systems are designed with low noise electronics, cooling to   -100 °C, up to 95% peak quantum efficiency, multi-MHz readout, USB 2.0 connectivity, and various readout modes. According to the company, the instrument uses the company's electron multiplying CCD platform with sensor formats optimized for ultralow-light-level spectroscopy applications.  				

A full-spectrum fiber-coupled LED light source with an SMA connector for spectroscopy applications is available from Mightex. According to the company, the LED source covers the range from 380 nm to 780 nm continuously, and the spectral range can be expanded by adding more LEDs in both ends of the spectrum.  

Amtek's MCA8000D compact digital multichannel analyzer is designed to digitize an input signals to measure the pulse height and obtain the pulse height spectrum. Free display and acquisition software and a free software development kit are included.  				

The Exemplar Plus "smart" spectrometer from B&W Tek is designed with a low stray light unfolded Czerny-Turner spectrograph. According to the company, the spectrometer has a TE cooled back-thinned CCD detector that is linearly summed for high dynamic range and provides data over the 190–1100 nm spectral range. 

An ion mobility spectrometry (IMS) analyzer from Photonis is designed for simple integration with most spectrometers. According to the company, the analyzer combines the company's resistive glass products with an ion gate design to create a compact platform that can be scaled or customized to interface with a variety of instruments for IMS analysis that can be performed at room temperature. 

An application note from SPEX CertiPrep describes an examination of lipstick for lead contamination and to determine if any other potentially toxic metals were present. According to the company, 48 lip products were tested for the presence of toxic elements by ICP-MS.

Plus series nebulizers from Meinhard are designed with low internal volume and A-type nozzles. According to the company, the TR+ (borosilicate glass), the TQ+ (quartz), and the SilQ (high purity quartz) nebulizers include gas and liquid fittings in high purity PFA with a variety of options, and natural aspiration is 500 µL/min for 1 L/min argon at 50 psi (340 kPa), with a useful range of  10–700 µL/min. 

The AvaSpec-Mini spectrometer from Avantes is designed to provide a resolution of up to 0.2 nm. According to the company, the device weighs 155 g and is equipped with an industry-standard SMA-905 connector. 

An application note from Rigaku describes sulfur analysis in petroleum products using the company's ZXS Primus wavelength dispersive X-ray fluorescence spectrometer. According to the company, the report contains information about sample preparation, method calibration, and repeatability. 

Departments

Products

Tunable diode laser absorption spectroscopy (TDLAS) is a relatively new gas analysis technique that has become quite popular in a number of industries, including those focusing on environmental applications. It provides major advantages for selected gas analysis applications and should see demand continue to grow, even in a difficult economic environment. 

 Environmental gas analysis techniques - 2012 			

   TDLAS is a fairly recent development within the spectroscopy sector. The use of a laser source instead of an optical filter combined with a conventional lamp provides a much narrower linewidth and results in a more sensitive and selective instrument that can measure sub-part-per-million levels of many gases. The rapid development and improvement of inexpensive, lower power, and rugged lasers have allowed the TDLAS market to develop over the past decade. Although TDLAS instruments can measure only a single gas per sensor, they are more rugged and easier to maintain than conventional nondispersive infrared (NDIR) and Fourier transform infrared (FT-IR) gas analysis instruments. These advantages make TDLAS particularly attractive for monitoring emissions in industrial settings. 

   The global market for TDLAS-based analyzers in the environmental sector will total around $30 million in 2012. This consists primarily of instruments used for continuous emission monitoring systems (CEMS), while portable and fixed-site ambient systems also are included. Although this market cannot avoid being impacted by a weak global economy, demand should still experience some growth in 2013 before picking back up to a more robust pace. 

   The foregoing data were derived from research for SDi's upcoming market analysis and perspectives report titled 

Environmental Analysis Instrumentation: Monitoring the Health of our Planet, January 2013

. For more information, contact Stuart Press, Vice President, Strategic Directions International, Inc., 6242 Westchester Parkway, Suite 100, Los Angeles, CA 90045, (310) 641-4982, fax: (310) 641-8851, 

Market Profile: TDLAS Analyzers for Environmental Analysis

When contemplating how to perform atomic spectrochemical analysis for trace metals, the following key components immediately come to mind:

These five components represent assemblies made up of a complex array of mechanical, optical, and electronic parts that serve to make up a complete instrument. A very important subset between sample and excitation source (components 1 and 2 above) is the general category of sample introduction — two otherwise very simple words that represent research and development rivaling that devoted to the other four components combined. Proper sample introduction may mean the difference between a correct or incorrect answer when it comes to reporting analytical results, and the right kind of sample introduction, in this case, is some kind of nebulizer.   

To keep things a bit more focused, this article devotes itself to methods using an inductively coupled plasma (ICP) with an emphasis toward atomic emission spectroscopy, although the atomic absorption technique also uses nebulizers. References that describe nebulizer designs and their application to chemical analysis are available in the usual sources of information (1–4). 

To perform atomic spectroscopy, free atoms must be produced that can be excited to be able to spectrally observe their signature radiation profile. Producing free atoms requires a fair amount of energy. First, energy is needed to break up molecules in the sample matrix into free atoms. Second, enough energy needs to be left over to excite the free atoms sufficiently to the point where they will emit radiation, specifically radiation that is in the target wavelength region. The process of sample nebulization can significantly affect the efficiency whereby free analyte atoms are produced. 

Before any further discussion about nebulization, an understanding of energy is necessary, or more to the point, energy management. Unfortunately, energy is not a limitless commodity. To the spectroscopist, energy is to a large degree measured as temperature. Temperature is associated with everything from dissociation of molecules into free atoms, to the degree of excitation of free atoms to excited states. Harnessing energy to produce a high enough temperature is the purpose of the excitation source (component 2 above). A flame (that is, the chemical combustion resulting in the breaking of chemical bonds) was the historical excitation source for producing high temperatures. Now, much higher temperatures are provided by a general category of atmospheric pressure discharges termed 

. These sources, which supply seemingly limitless temperatures, are in fact by their nature, limited. It is this notion of energy limits that presents a boundary for the spectroscopist to consider when choosing a method of sample introduction. The argument is not complete without at least mentioning that standing between the seemingly endless supply of energy and production of free atoms is the concept of energy transfer. The efficiency of energy transfer during sample introduction is a huge part of what turns an average analytical method into an exceptional one. The nebulizer is once again the largest factor in maximizing that energy transfer efficiency.     

An excitation source by nature has to be hot. Just how hot is defined by how it is produced. Historically, the flame was the simplest source available to produce elevated temperatures. Besides ease of control, the flame quickly became a useful analytical tool because it was easy to introduce a liquid sample. Flame temperatures could be changed by using different gases for combustion, but eventually temperatures reached a plateau. The process of breaking chemical bonds through burning had reached its limit. Soon thereafter, researchers began using electricity to produce an excitation source with much higher temperatures. The only limit was how much electric current could be generated. Controlling the current and voltage across a gap, the spectroscopist could produce either a direct current arc or a momentary but repeated spark to heat the sample. Energy transfer occurred by directly coupling the electrical energy to the sample and creating very high temperatures that produced both free atoms and excited atoms nearly simultaneously. The two biggest problems with these types of sources were contamination from the electrodes and inconvenience of handling liquid samples. Electrodeless discharges, or plasmas, emerged as an alternative. With the concept of electromagnetic induction, electrical energy could be coupled to an inert gas to create a plasma by way of an induction coil carrying electrical current at radio frequencies (rf). As a result, ICP emerged as a commercial product in the early 1970s (5,6). 

An argon ICP is an ionized gas that contains argon ions at a concentration of about 1%, and about an equal number of free electrons. Argon turns out to be an economical gas extremely well suited for producing hot plasmas at atmospheric pressure with rf current. Additionally, this type of plasma brought back the advantage of handling liquid sample introduction much like the flame. Along with argon ions and electrons, argon ICPs also contain another valuable species that gives the argon ICP its amazing analytical sensitivity — argon metastables. These ions are a critical part of the energy transfer balance in the plasma, and their concentration is directly tied to the temperature in the plasma. Perturbing that temperature by, for example, introducing sample aerosol into the plasma affects the equilibrium concentration of the argon metastable ions, which thereby affects analytical performance. For this reason, it is vital that nebulizers produce the right properties of aerosol to minimize changes to the temperature in the plasma and, therefore, the argon metastable concentration.      

Figure 1 shows what happens to an aerosol particle as it passes through a plasma. 

The challenge with every analysis presented to an analytical spectroscopist starts with the sample. It can be a solid or a liquid, and sometimes a vapor or a gas. If the sample is a solid, conventional wisdom says to somehow render that solid into a liquid. If it is a liquid, it might be clear or have suspended or dissolved solids. If the sample is a vapor or a gas, the concern is how to maintain the integrity of that vapor or gas to preserve it as much as possible and prevent loss of the analyte. Remember, the final goal with any method is to produce free atoms in the plasma. Keeping in mind the motto "free atoms lead to good excitation," the charge of the analytical chemist is to prepare that sample into a form that makes it easiest to introduce into the plasma. With any of these aforementioned sample matrices, the atomic spectroscopist knows that the farther he or she can refine the sample, the more the excitation source will achieve efficient production of free atoms. Said another way, the more that the method helps increase the transfer of energy from the source to the sample, the more excited analyte atoms are produced, and consequently, the better the method. On a more practical note, every sample preparation method should yield as simple a matrix as possible before introduction to the plasma. Most often this matrix is defined as either aqueous or organic, with the common characteristic of being homogeneous. 			 

Figure 1: Diagram showing a single Ã¦rosol particle as it progresses from liquid droplet to free atoms. Adapted from reference 7.			

To perform metals analysis by ICP spectroscopy, free atoms need to be produced that in turn can become excited. The energy available from the plasma needs to be transferred to the sample as efficiently as possible. To make that happen, we must have a sample that is in a form that readily allows efficient transfer of that energy. That form is primarily produced with a nebulizer. 

This all sounds very straightforward, but in reality, nothing could be further from the truth. If it really were all that straightforward, we wouldn't have created such an extensive literature base describing everything from directly inserting the sample into the plasma on the tip of a graphite rod (8), to electrothermal vaporization of difficult-to-digest sample matrices (9). The difficult nature of samples submitted for metals analysis has led to the development of a wide variety of nebulizer types. Furthermore, nebulizers have to satisfy stringent analytical figures of merit including stability, piece-to-piece reproducibility, ruggedness, ease of operation and cleaning, and continued market availability. Nebulizers often define one manufacturer's performance edge over the competition, and today, where this performance edge is razor thin, nebulizer design is an aggressively protected art. 

Nebulizer types generally fall into either pneumatic or nonpneumatic categories (Table I). These are all available either through major instrument manufacturers or through independent distributors of analytical accessories.  													

 Table I: General categories of nebulizers used in inductively coupled plasma spectroscopy			

A new instrument user is usually presented with the nebulizer that comes with their instrument. The manufacturer typically provides one or two options for other difficult matrices. If the instrument was purchased for a specific purpose, one or two nebulizer types can be sufficient. Still, it is a good idea to select the best nebulizer for the matrix and analytical method at hand. Any discussion of which nebulizer is the best one for an application usually involves knowing the properties of the sample. The following are some criteria that become important when deciding on a particular nebulizer. 

 Nebulizers used to be made of borosilicate glass or quartz. Now nebulizers are available made from a variety of materials, including polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), or polyetheretherketone (PEEK), for better compatibility with the matrix chemistry. For example, matrices rich in hydrofluoric acid should be run with non-glass nebulizer bodies. Applications in the precious metals industry might benefit from the tight manufacturing specifications and reproducibility gained with a glass concentric design. 

 The discussion of percent dissolved total solids (%TS) in the sample can apply as much to describing sea water as a solution of monomer or polymer in an organic solvent. Both liquids are clear, but contain dissolved solids that can interfere with the nebulization process. Any of the high solids nebulizers available today are robust and maintain their performance despite being exposed to harsh chemicals. They do not readily plug, and are not easily damaged if cleaning becomes necessary. Users typically select one of the high solids nebulizers for samples with a %TS above 2%, and for levels below 1 %TS one of the low flow or high efficiency nebulizers is considered safe to use. Some concentric nebulizers are designed for moderate dissolved solids levels but care must be taken when there is particulate matter present that can clog the inner capillary.       

Nebulizers produce an aerosol that contains a wide range of particle sizes. The more a nebulizer produces an aerosol rich in particles approaching  1 µm or less, the better. Smaller aerosol particles are more easily transported and dissociate more quickly in the plasma, producing free analyte atoms more efficiently. Chromatographic methods interfaced with ICPs typically deliver only microliter amounts of sample. Selecting a high efficiency or low-flow nebulizer in this instance, with the added benefit that these nebulizers also produce a very fine mist, helps get the most performance from the small amount of sample available for analysis. 

 Concentric nebulizers have a natural tendency to draw liquid sample in the process of producing the aerosol. Negative pressure at the point of aspiration causes fluid to flow through the sample line without any external pumping action, albeit under a limited set of circumstances. These are termed 

. Other nebulizers, though they are considered to be pneumatic, fail to produce that zone of negative pressure, and thereby require an external pump to deliver sample to the nebulizer to produce an aerosol. A self-priming nebulizer might become necessary when a sample matrix is incompatible with the pump tubing used for delivery, as is the case with a harsh organic solvent. In another situation where the pump tubing produces high levels of contamination the only choice may be for the nebulizer to directly sip sample solution from its base to avoid high background signals.    

 Nebulizers have to, by nature, consume sample volume. When there is ample sample volume available for analysis, the amount of sample a nebulizer consumes is of little concern. However, in instances where sample volume is at a premium (for example, with precious metal digests or biological materials), sample consumption is of greater concern. (See the discussion above on primary aerosol production.) For these applications there are a variety of high efficiency and low-flow nebulizers available. Unfortunately, these types of nebulizers usually cannot tolerate high levels of dissolved solids before plugging or salting up. Babington and frit nebulizers use higher volumes of sample while being able to tolerate percent levels of dissolved solids. The reverse is true with low-flow nebulizers (low volume with low levels of dissolved solids). In addition, analysis time is affected by the lower sample consumption because of the time required for the sample to be transported between the sample vessel and the nebulizer. This can be enhanced by choosing capillary tubing, but has the added risk that the line can plug. 

 One direct consequence of pneumatics when it comes to producing an aerosol is the relationship between gas flow, back pressure, and sample flow on the quality of aerosol production. Generally speaking, the higher the back pressure the better the properties of the primary aerosol. While most ICP instrument manufacturers specify an inlet pressure for argon of as high as 100 psig, many high efficiency nebulizers require back pressures approaching 150 psig or higher. This level of pressure can easily be delivered from a compressed gas cylinder but not from the liquid argon Dewar containers typically used in ICP laboratories. For this reason, nebulizer designers targeted their performance metrics for the lower pressures even though better performance can be realized at the higher pressures. 

 In the past, nebulization was produced by passing a gas under high pressure through an orifice and relying on the rapid expansion of the gas at that orifice to break down the liquid into minute aerosol droplets. This created an effect whereby gas flow rate directly influenced the properties of the aerosol produced; that is, increasing the gas flow raised the pressure and consequently produced a better aerosol. This also meant that changing gas flows to optimize the ICP performance also changed the performance of the nebulizer posing difficulty to researchers. This spawned research into methods that could produce an aerosol independently of gas flow. Devices were developed with which a vibrating quartz membrane excited at microwave frequencies produced a primary aerosol rich in small particle sizes. With this method, a sample aerosol could be produced independently of gas flow without requiring elevated gas pressures. This turned out to be so efficient that the aerosol itself would overwhelm the energy balance in an ICP and necessitated desolvating that aerosol before introducing it to the plasma. Desolvating the aerosol ended up improving analytical performance by removing vapor from the aerosol and reducing the degree of sample loading in the plasma, which consequently prevented the temperature in the plasma from becoming cooler. Though nonpneumatic nebulizers solved many of the problems created with pneumatic nebulizers, they were not as compact as the typical glass or plastic nebulizer and required an extra layer of equipment maintenance.   

There is a slow, but deliberate progression toward introducing samples at 100% efficiency. The earliest nebulizers were 1–2% efficient. That meant that 98–99% of the sample was wasted while introducing only microliter volumes of sample into the ICP. Recent experiments have shown that by introducing sample into the plasma at extremely low flow rates (less than  50 µL/min), sample can be introduced at essentially 100% with little sacrifice to the overall detection limit. Amazingly, this 100% efficiency is achieved without overloading the plasma and avoids many of the problems associated with matrix effects in the plasma due to sample loading (10,11). This resulted in optimum energy transfer efficiency leading to maximum generation of free analyte atoms, with minimal reduction in overall plasma temperature — all factors discussed above as being important to good method development. The sample best suited to this nebulizer must be very clean and contain no suspended particles, and possibly also have a relatively low percentage of dissolved solids.   

Have we seen the last of new nebulizers for ICP? Perhaps not. Nebulization is not only for us to own. The world is full of nebulizers, whether they are used for fuel injection in cars, air brush painting of model airplanes, or controlling humidity in hospitals and laboratories. It remains to be seen whether a development in these other fields matches the needs of atomic spectrometric chemical analysis. 

There will continue to be those who seek the perfect nebulizer for each special application. Others are content with one or two types and have used those same nebulizers for many years. However, it's never good to remain too comfortable in a routine. New matrices and detection limit pressures will always challenge conventional wisdom and skill. While not touched on here, new designs in spray chambers (often referred to as aerosol chambers or filters) are showing properties of aerosols previously hidden that are challenging the theory of the purpose of the chamber in the first place (12,13). We still need nebulizers (or something else) that get us from the world of samples to the world of free analyte atoms with as much efficiency as scientifically possible. That time may be at our doorstep. 

Liquid Sample Introduction in ICP Spectrometry, A Practical Guide, First Edition

 (2) A. Montaser and D.W. Golightly, Eds., 

Inductively Coupled Plasmas in Analytical Atomic Spectrometry

 (5) S. Greenfield, I.L. Jones, and C.T. Berry, 

 (7) Meinhard An Elemental Scientific Company, used with permission. 

To choose the right type of nebulizer for atomic spectroscopy methods that use inductively coupled plasmas, start by understanding how plasmas are formed and what nebulizers do.

Nebulizers for Inductively Coupled Plasma Spectroscopy

Laser ablation for direct solid sample chemical analysis has advanced over the past 50 years with applications in many disciplines, including environmental, geological, medical, energy, security, and others. Although the choice of laser is still highly dependent on the application requirements, there are distinct fundamental effects attributed to the laser pulse duration that drive the ablation sampling process. An overview of nanosecond and femtosecond laser ablation is presented with respect to analysis based on the optically induced plasma at the sample surface (such as laser-induced breakdown spectroscopy or laser ablation molecular isotopic spectrometry) and transport of the ablated mass aerosol to an inductively coupled plasma.

Laser ablation is widely recognized as a powerful technology for direct solid sampling chemical analysis. Without requiring sample preparation in most cases, any solid sample can be analyzed for elemental and isotopic content within minutes. There are two primary approaches for detecting the ablated mass — either by measuring the photons emitted by the optical induced plasma at the sample surface or by entraining the ablated aerosol into a gas stream with delivery to a secondary source. Laser-induced breakdown spectroscopy (LIBS) and laser-ablation molecular isotopic spectroscopy (LAMIS) are based on measuring optical emission in the plasma (1–10). The inductively coupled plasma (ICP) with mass spectrometry (MS) or optical emission spectroscopy (OES) methods are based on the transport of the ablated aerosol (8,11–20). The sampling process is the same (laser ablation); detection is dictated by the application. 

Laser ablation actually dominates in applications other than chemical analysis, such as cutting, welding, micromachining, and laser-assisted in situ keratomileusis (LASIK). Although chemistry is not emphasized in these other applications, their requirements are similar: efficiently use laser photons to remove mass. Over the past 50 years, research has addressed almost every parameter influence on the ablation process. The basis of this column is to provide physical concepts on a critical parameter driving efficient mass removal — the laser pulse duration. This column describes observations from nanosecond and femtosecond ablation research with comments related to chemical analysis based on both plasma emission and particle detection. Picosecond lasers are not addressed mainly because they have not been that prevalent in this field; applications are primarily driven by the availability of commercial lasers. Until about 15 years ago, the majority of research emphasized nanosecond pulsed lasers, which had replaced microsecond and longer pulsed lasers. There are early references on long-pulse laser ablation (21) but for the most part, chemical analysis would be challenging if not for the development of short pulsed (nanosecond and femtosecond) lasers. Based on a series of concepts established from time-resolved measurements, we provide an overview of the processes under which ablation occur. Foremost, we admit upfront that much of the pulsed laser ablation research is empirical — there is no unified theory that predicts the quantity of mass ablated, the chemistry of the plasma, or the aerosol properties from an ablation event. The discussion herein does not delve into specific mechanisms, but instead presents concepts based on observations from reproducible time-resolved measurements.

The laser beam provides defined photon energy (wavelength) and power (number of photons per unit time) that is incident on a sample surface for a specific duration of time. For the most part, the wavelength of the laser is not resonant with any specific elemental transitions in the sample (although some research has looked into resonance excitation). In Figure 1, we show the first step in the process in an illustration of a generic sample and spatial representation of nanosecond and femtosecond pulses. In this case, both lasers have the same color or wavelength. This is a first order approximation, because the optical bandwidth of each laser is different. In Figure 1, the femtosecond laser pulse is essentially a delta function in time and space compared to the extended nanosecond pulse. Common values of the laser pulse duration for commercial Nd:YAG and Ti:sapphire lasers are 6 ns and 100 fs, respectively. At any instant, the 100-fs laser pulse is 30 µm long and the 6-ns laser pulse is 1.8 m long. The newer Yb-based fiber lasers have pulse durations of approximately 300 fs. In these cases, the nanosecond laser pulse is 60,000 times longer than the Ti:sapphire and 20,000 times longer than the Yb femtosecond laser pulses. The slight differences in the femtosecond pulse durations will be discussed later; for now, we assume they are similar although on a pure fundamental level there can be subtle differences in excitation and the ablation processes.  

 Figure 1: Initial general concept diagram showing spatial extent for red nanosecond and femtosecond pulsed laser beams aiming at the same sample from right to left.  			

Figure 2 shows where things get interesting for the laser-material interaction. First, the optical penetration depth of the laser is different even though the sample and laser wavelength are the same. From a linear one-photon Beer's law model, this would not be the case. However, high peak intensity (energy/area-time) of the laser drives nonlinear processes, in this case multiphoton (

 = 2,3, . . . ) absorption. Therefore, the optical penetration depth of the energy is less and the energy per unit volume is greater for femtosecond laser pulses compared to nanosecond laser pulses. The optical penetration depth depends on the sample and the laser irradiance (energy/area-time). In most publications, the parameter of interest is reported as the energy per unit area (fluence), defined by the laser beam spot size on the sample. Although studies show good correlations to fluence, energy per unit volume drives the ablation process (22). Energy is needed to break bonds and move atoms (and particles) from solid to vapor phases. Again, this is only part of the process, as we ignore changes in reflectivity and other processes that are time dependent. Energy imparts work on or in the sample — energy of the photons (laser wavelength), number of photons (pulse energy), and irradiance (power per unit area). Based on an effective optical penetration depth, there is a well-defined volume or number of atoms that have bonds, bond energies, latent processes, and so on, that will play a part in the explosion (ablation) of mass from the irradiated sample. The energy becomes dissipated into different processes, including electronic excitation, ionization, heating, shock, and vaporization. Exact mechanisms, from photons in to kinetic energy of vapor, photon emission (atomic, ionic, and molecular) to aerosol formation are not completely established, and are  the basis of many fundamental endeavors. The images in Figure 2 are an example of shock wave and nonlinear absorption induced optical index change for nanosecond and femtosecond laser pulse interactions in a  transparent sample. 

Figure 2: Concept diagram shows the laser pulses entering the same sample from right to left. Optical penetration depth of laser energy in the sample is different because of linear (nanosecond) and nonlinear multiphoton absorption (femtosecond) processes. Top image shows shockwaves induced by optical absorption of the nanosecond pulsed laser inside and outside a glass sample (60). Bottom image represents a transient change in the optical properties from the high intensity femtosecond pulse (26). The irregular vertical line is the glass surface in both images.			

A second concept presented in Figure 2 is that the femtosecond pulse is effectively finished on this time scale, say 300 fs after firing each laser, whereas the nanosecond pulse is still irradiating the surface. Ramifications of this behavior are discussed with reference to Figure 3. Keeping with a high-level overview, Figure 3 shows empirical processes distinct for femtosecond and nanosecond laser pulses. The first thing to notice is that the plasma (electrons, atoms, molecular, and particles) is propagating into the laser beam for the nanosecond pulsed laser; laser energy is coupled into the plasma. Even using femtosecond time-resolved imaging above the sample surface, no physical entity has been measured, leaving the surface on the femtosecond time scale; electrons have been measured picoseconds after the laser pulse (23–32). Optical photographs in Figure 3 of laser plasmas established from a copper sample with the same laser energy, spot size, and wavelength confirm the robustness of a nanosecond laser-heated plasma versus a more delicate femtosecond plasma, which is related to plasma heating. The proportion of laser energy absorbed or even reflected by this plasma depends on many conditions, including time, temperature, electron, and mass density. Laser plasma heating is a likely reason why nanosecond pulsed lasers are more common for LIBS applications because they offer a more robust, longer persistence plasma. Ideally, the ablated mass is a vapor, heated to excite atoms, ions, and molecules for optical emission by LIBS and LAMIS. Over a short time, as the plasma expands and cools, the vapor nucleates and condenses to form particles that are available for entrainment and transport to the ICP. 

Figure 3: Concept diagram shows that the nanosecond laser is still illuminating the sample while the plasma is expanding, whereas the femtosecond pulse is over before any mass can leave the surface. The heated region is deeper for the nanosecond versus the femtosecond pulse. Images are photographs of the laser plasma, showing the effect of laser-plasma heating from the nanosecond plasma (61).			

The concepts presented in Figure 4 illustrate that heating and the heat affected zone (HAZ), which is proportional to the square root of the pulse duration, are significantly larger for nanosecond pulses. For a laser pulse duration of 100 fs, the HAZ in an alloy is about 4 nm compared to approximately 1000 nm for a 6-ns pulse. Figure 4 shows a large heated and melted region as the energy in the nanosecond pulse has had time to dissipate into the sample; phonon modes initiate about 10 ps after the initial laser beam interaction with a sample, so a 100-fs laser pulse is over before a molecule can vibrate. The HAZ describes part of the mechanism because it is the amount of energy in this zone as a function of time; how the atoms and molecules respond to this energy per unit volume and time is an ongoing research program in many disciplines. Heating to a melt phase is considered detrimental in the ablation process because the sample can undergo preferential vaporization. Heating also drives elemental migration within the bulk, effectively changing the stoichiometry in the sample. Accuracy of analysis can be compromised by single and successive pulsed laser ablation if heating is sufficient to drive these processes. For accurate analytical chemistry, the ablated mass should be stoichiometric to the sample composition. (The word "should" is used here because quantitative analytical chemical analysis can be achieved under fractionation conditions if matrix-matched standards are available and ablate exactly the same as the sample, which is often not the case.) A final comment about Figure 4 is that this is the time period in which LIBS and LAMIS are performed. The exact time depends on the laser pulse duration and energy. Common times are approximately 1 µs for nanosecond pulsed LIBS, several microseconds for LAMIS, and several hundred nanoseconds for femtosecond pulsed LIBS, all after the laser pulse. The graph in Figure 4 shows the different plasma persistence from nanosecond and femtosecond ablation with the same wavelength, energy, and spot size; pulse duration was the only different parameter. 

Figure 4: Concept diagram shows the heat affected zone (HAZ) in the sample. HAZ is proportional to the square root of the pulse duration and therefore there is much greater heating for nanosecond than femtosecond pulses. The graph shows relative decay rates of measured optical emission from two plasmas initiated with lasers having all properties equal except the pulse duration (25).			

Figures 5 and 6 describe the particle (aerosol) character of the ablated mass. We believe there are at least two processes for producing particles: some are ejected from the surface as distinct particles (fracture, spallation, or melt flush) (Figure 5) and others undergo nucleation and condensation from vapor to form nanoparticles (Figure 6). Particles are important for laser-ablation ICP-MS, but deleterious for LIBS and LAMIS. Ideally, there would be no ejected particles, or if there were, they would not originate from melt flushing and would be stoichiometric. Ejected particles also can be too large to be entrained, transported, or digested in the ICP. Particles from the condensate will be of nanometer size and are better suited for ICP analysis. Ejected particles are observed shortly after the laser pulse; whereas condensed particles are formed later in time as the plasma expands and cools. 

Figure 5: Concept diagram representing direct ejection of particles. The particle sizes and size distribution are related to the heated volume. The images at right are examples of time-resolved shadowgraphs for ejected particles from a surface (on the left). The spatial resolution of the images can resolve particles greater than a few hundred nanometers. 			

We know from differential mobility analysis (DMA) and scanning electron microscopy (SEM) measurements that the particle size distribution is strongly dependent on the sample and laser properties. Sticking with this overview of only laser pulse duration, there are numerous studies showing that the particle size and size distribution are smaller and narrower, respectively, for femtosecond than for nanosecond laser pulses (33–35). This observation holds for both the ejected and nucleated particles. Some reasons for smaller particles from femtosecond ablation include less melting, higher energy per volume, smaller HAZ, and in many cases, less total mass ablated per pulse, so that the nucleated particles are smaller; nucleation and condensation are based on the vapor density. The higher expansion rate of the femtosecond plasma also supports a more rarified vapor. The shadowgraph images in Figure 5 show ejected particles from nanosecond laser ablation and only the shockwave from femtosecond ablation; ejected particles could not be observed in these types of measurements under the energy conditions generally used for femtosecond sampling into the ICP-MS system. SEM images in Figure 6 show particles from nanosecond and femtosecond ablation when all conditions, except the pulse duration, were constant. Again, these analyses are general cases; laser parameters can be adjusted to provide smaller or larger particles from any sample. We are addressing the parameter space that is optimized for ablation chemical analysis commonly used for LIBS, LAMIS, and laser-ablation ICP-MS applications. A caveat about particle size measurements using a DMA system: it does not distinguish agglomerated particles from primary particles. As particle sizes become smaller, agglomeration is enhanced. Therefore, DMA measurements should be supported by SEM images. 

Figure 6: Concept diagram showing the sample and plasma cooling stage. Plasma cooling leads to nucleation and condensation of nanoparticles. The scanning electron microscopy images (right) show particles collected from ablation with two lasers having the same parameters except for the pulse durations (61).			

Figure 7 shows the sample after the ablation process is finished and images of craters produced using nanosecond and femtosecond laser ablation with all parameters equal except the pulse duration. Again, these figures are only for discussion to demonstrate commonly observed effects. The HAZ is larger for the nanosecond case and shows a rim around the crater; a smaller area of the original sample was influenced by the femtosecond laser, and the ablated mass was removed by nonthermal photophysical processes without producing a rim due to melt flushing. The total amount of mass removed (ablated) will strongly depend on the energy in the laser pulse. Generally, microjoules or millijoules of energy per pulse are used for femtosecond or nanosecond laser ablation, respectively. Therefore, less mass will be removed for the femtosecond pulse. Nanosecond pulsed lasers in the deep UV can produce clean craters without a significant rim, and rim formation has been observed under nonoptimal femtosecond cases. Again, many parameters govern the ablation process and can be varied to produce a wide range of effects. Irrespective of the rim, the fundamentals of the HAZ remain intact; using femtosecond laser ablation less heating, more energy per bonds, and more efficient use of photon energy for ablation are obtained. 

Figure 7: Concept diagram shows the sample after all processes are finished. The affected region in the sample is related to the pulse duration. Melting leads to rim formation around the crater (ablated mass). The images are white-light interferograms of copper surfaces after ablation using nanosecond (top) and femtosecond (bottom) pulses having the same energy, wavelength, and spot size (14).			

Given these attributes of femtosecond laser pulses we ask the following question: How is analytical chemistry affected? Precision and accuracy have been reported to be improved in many ICP-MS studies (36–39). Improved precision using femtosecond laser ablation has been attributed to smaller particles and a narrower particle size distribution. In addition, the lower amount of mass does not overload the ICP. Onset of vaporization, ionization, the ion cloud, and chemical interference effects are highly dependent on the aerosol (dry, in this case). Accuracy is improved by using the femtosecond laser because of minimized heating of the sample. But there are other attributes beyond these performance metrics. Common criticisms of laser ablation are inhomogeneity of the sample, representative sampling, and sensitivity. However, all of these concerns can be mitigated by using high repetition rate femtosecond laser ablation. High repetition rate femtosecond laser ablation sampling has been shown to support bulk sample analysis by faster scanning over small sample areas (40–44). The amount of sample needed will be related to the inhomogeneity, and in most cases, will be much less than that required for sample dissolution in acid and subsequent liquid nebulization. It is more practical to remove smaller amounts of mass from each low-energy femtosecond laser pulse with high repetition rates to get the mass quantity sufficient for analysis, instead of relying on a single high-energy nanosecond laser pulse to remove the same amount of mass. The ICP benefits from the finer and more continuous high repetition rate femtosecond laser ablation process, which is similar to a liquid nebulizer. There are many corollaries for laser ablation with liquid nebulizers such as controlling the amount of mass and providing a continuous stream of sample to the ICP with the right particle distribution. Early nebulizers wasted sample because the droplet size distribution, which affects precision and accuracy, was not suitable for the ICP. The same goes for laser ablation: As we improve the particle size and distribution, performance improves.  

Laser ablation provides amazingly sensitive absolute detection — at the femtogram and attogram levels (37,45–52). The reason for the relative insensitivity (compared to liquid nebulization) is the small amount of mass that is sampled during ablation. By using high repetition rate femtosecond lasers, the amount of mass can be increased to provide limits of detection comparable to those obtained using liquid nebulization, without requiring as much mass as liquid nebulization. Laser ablation is better suited for solids analysis (Why use acid digestion and more sample than necessary?) As the laser continues to ablate to achieve sufficient mass, elemental migration and preferential vaporization need to be minimized. These processes are mitigated by using femtosecond laser pulses with the very small HAZ. 

Fundamental processes have been studied for years, and we ignored processes like shockwaves, spallation, Coulomb explosion, phase explosion, plasma chemistry, spatial gradients in the plasma, and more. Even if we do not fully understand the ablation processes, reproducible control of the interaction to produce an ideal aerosol for ICP-MS or an ideal plasma for LIBS or LAMIS can be achieved; understanding the fundamentals drives advancement. In spite of our inability to completely unravel underlying mechanisms, the beauty of this technology is that the process is reproducible using set parameters (just like those of any analytical technique) for laser energy, pulse duration, fluence, and wavelength. Analytical performance metrics are improved using femtosecond rather than nanosecond laser ablation. This does not mean that nanosecond pulsed laser ablation is not a viable solution for many applications. Good analytical chemistry can be performed with a nanosecond pulsed infrared (IR) laser if matrix-matched standards exist and if the ICP-MS is optimized for such ablation conditions. For most LIBS applications, nanosecond pulsed IR lasers are common to establish a robust long-persistence plasma (1,2). Femtosecond laser ablation provides advantages for applications that need excellent precision, high efficiency, and do not have matrix-matched standards (53). Finally, the femtosecond laser pulse duration is (to the best of our knowledge) the only way to ablate with nanometer spatial dimensions, both lateral and depth (45,54–59). Again, the HAZ defines the volume of sample affected by the energy, allowing imaging and depth profiling on the nanometer scale. 

The authors would like to thank Dr. Tim Suen for the design of the pulse-duration concept slides. R.E.R., X.L.M., and J.J.G. acknowledge support from the Chemical Science Division, Office of Basic Energy Sciences, and the Defense Nuclear Nonproliferation Research and Development Office of the US Department of Energy (DOE) under contract number DE-AC02-05CH11231 at the Lawrence Berkeley National Laboratory. J.Y. acknowledges support from the DOE SBIR Program. 

 (3) R. Fantoni, L. Caneve, F. Colao, L. Fornarini, V. Lazic, and V. Spizzichino, 

Spectrochimica Acta-B-Atomic Spectroscopy

 (4) R. Noll, V. Sturm, ü. Aydin, D. Eilers, C.D. Gehlen, M. HÖhne, A. Lamott, J. Makowe, and J. Vrenegor, 

 (5) D. Santos, L.C. Nunes, G.G.A. de Carvalho, M.S. Gomes, P.F. de Souza, F.O. Leme, L.G.C. dos Santos, and F.J. Krug, 

 (8) R.E. Russo, A.A. Bol'shakov, X. Mao, C.P. McKay, D.L. Perry, and O. Sorkhabi, 

 (9) X. Mao, A.A. Bol'shakov, I. Choi, C.P. McKay, D.L. Perry, O. Sorkhabi, and R.E. Russo, 

(10) X. Mao, A.A. Bol'shakov, D.L. Perry, O. Sorkhabi, and R.E. Russo, 

(12) R. Hergenroder, O. Samek, and V. Hommes, 

(14) R.E. Russo, J. Gonzalez, and  C. Liu, LabPLus International (2006).  

Columns: Lasers and Optics Interface

Although the choice of laser is still highly dependent on the application requirements, there are distinct fundamental effects attributed to the laser pulse duration that drive the ablation sampling process. Here, we explain the differences between nanosecond and femtosecond laser ablation.

Femtosecond vs. Nanosecond Laser Pulse Duration for Laser Ablation Chemical Analysis

Issue PDF

Click the title above to open the Spectroscopy January 2013 regular issue, Vol 28 No 1, in an interactive PDF format.

Spectroscopy-01-01-2013

Dynamic Gas Chromatography Meets Dynamic Mass Spectrometry

Products

Market Profile: TDLAS Analyzers for Environmental Analysis

Nebulizers for Inductively Coupled Plasma Spectroscopy

Femtosecond vs. Nanosecond Laser Pulse Duration for Laser Ablation Chemical Analysis

Vol 28 No 1 Spectroscopy January 2013 Regular Issue PDF