Chronologies sequence significant developments in mass spectrometry (MS), although there is always some imprecision in determining the "proper" date for the conceptualization or adoption of an idea. Because hindsight offers a better focus, significant advances and the publications that describe them are sometimes more easily recognized years after they first appear. But it's not necessarily a singular date that is important. Instead, a clear description and execution of a new approach, and its subsequent adoption, often in an unexpected arena, can result in a transformative development in the practice of MS. This column describes some central transformative themes and their impact in modern analytical MS.

A comprehensive history of mass spectrometry (MS) would delineate, inter alia, the use of MS to identify isotopes, confirm the structure identity of both inorganic and organic samples, support geochemical applications through isotopic analysis, present a flexible platform for mixture analysis specifically with environmental applications, and describe its use in studies delineating fundamental ion reactivity, biomolecular identification and characterization, forensic chemistry, process control, and numerous "-omics" applications. Certainly, there are research and applications areas that have been missed in this list, but such is the nature of MS. Also clearly, this is not a list of sequential developments. Instead, it is a compilation of independent active application venues in which MS plays a supporting, if not a central, role. An overview of these areas from the broadest possible perspective allows underlying themes to be identified. Some of these are common across many applications and can be considered transformative. This column describes those transformative themes and links them to the modern day applications of MS. Figure 1 separates transformative themes into a central group of three and a surrounding cohort. The central themes are isotopic analysis, exact measurements, and information processing. 

Figure 1: Three central themes (in red) supporting transformative modern mass spectrometry, surrounded by concept areas representing distinct transformative research and experimentation.			

We have discovered all of the stable isotopes and measured their masses and abundances. In MS, the distribution of isotopes in atomic and molecular ions, in fragment ions resulting from dissociations, or in supramolecular ions, can be predicted from natural occurrence and has been mapped as a function of differential reaction rates, or specifically designed artificial manipulation. When we measure the mass of an ion, we use that mass to deduce atomic and empirical formula composition, using our knowledge about the mass and abundance distribution of isotopes. We can also use our knowledge about isotopes to serve as a quality check on our measured data, both in terms of the measured mass and the measured abundance. For example, for a given number of 

C atoms in an ion, there is a known and predictable number of 

C atoms. Ions with distinctive isotopic envelopes (such as chlorine or bromine) are visually evident in a mass spectrum. Isotopic analysis, and our knowledge of isotopic distribution, underlies every facet of modern MS. If the isotopic distribution varies from what we expect, either our measurement is inaccurate or we are looking at the end result of a differential phenomenon. 

To repeat (with a slightly different emphasis), we have discovered all of the stable isotopes, and we have measured their exact masses and their exact abundances. Those measurements are accurate (true) and precise (reproducible) to extraordinary tolerances. They are "exact," and ongoing research is focused on making them more so. Exact measurements at the limit requires sophisticated and specialized instruments, and "exacting" protocols for sample collection, preparation, and analysis. Measurement performance has steadily improved with each generation of MS instrumentation. Accurate mass measurement has become more accessible across the board. Dynamic ranges are extended with better electronics. As a consequence, instrumental sensitivity increases, and consequently, limits of detection and quantitation trend lower and lower. Better analytical performance places new demands on sample collection and sample storage protocols, with the result that the overall error is often determined by noninstrumental factors.  

MS is inherently a multidimensional and data-rich analysis method. Even when light-beam oscillographs were used to record the data, the three concurrent traces (for example, 1×, 10×, and 100×) provided an automated means of scaling intensity measurements. When film was used as the detection method, as in a focal plane spark-source mass spectrometer, a scanning densitometer (and subsequent processing) converted film exposure as a function of plate position into intensity as a function of mass. Digitization of the analog output of a detector allowed for data system recording of data, comparison of data, and archiving and sharing of data. Higher digitization rates and higher-bit transformation meant more and more data. But importantly, after the data was in a digital format, it could be archived, shared, processed with statistical methods, assessed with information theory, and displayed in more meaningful ways. Effective information storage and processing is rapidly becoming a concern across-the-board in modern MS applications. 

Surrounding themes incorporate the analytical capabilities derived from each of the central themes described above. Each theme represents a separable and distinctive premise in the use of MS, which the following vignettes distill independently of specific applications. Some transformative developments have been recognized with prizes and honors from the community, and others are overdue for such acknowledgement.

Basic MS had been in place for some 30 years, typically used for the discovery of elemental isotopes, before its potential applicability to organic analysis became apparent. Gases were readily introduced into the mass spectrometers of the times and certain elements could be ionized directly from filaments. Other elemental samples had no significant vapor pressure and could not be analyzed by MS until suitable methods of sample introduction and ionization were devised. Such methods were promptly devised. A few organic molecules were sufficiently volatile that their vapors could be swept into the mass spectrometer. As long as their partial pressures exceeded that of residual gases in the system, ions derived from these compounds could be identified. But the processes through which the ions were formed were less clear. Sample introduction systems, such as the direct insertion probe, provided a means to introduce many organic samples as gas-phase molecules into the source of the mass spectrometer. But it was the transformative development of electron ionization as a reproducible and generally applicable ionization method that sparked the revolution. In the electron ionization (EI) source, electrons from the filament interact with the gas-phase molecule to cause the ejection of an electron, leaving an odd-electron molecular ion M

. Two factors make electron ionization a transformative theme. The first is that the pressure in the EI source as it was ultimately developed was sufficiently low that the creation of the molecular ion and its subsequent dissociations were unimolecular events. Neither the molecule nor the ion created from it collide or interact with any other species. Therefore, all of the fragment ions in the mass spectrum must originate from the molecular ion in simple or successive rational dissociative reactions. Ultimately, these chemical reactions could be interpreted, collected, modeled, and, to an extent, predicted. The second factor is that the ionization itself is a Franck-Condon transition. The removal of the electron is so fast that the molecular structure does not change during ionization. Although dissociations may result in or require rearrangement, the structural starting point is set, and interpretation of the spectrum can evidence that structure. 

Nothing in MS stays simple for long. Electron ionization provided a theme in which we could consider the isolated unimolecular reactions of ions. But there were hints of ion or molecule reactions, such as surface reactions with the metal walls of the EI source. The development of chemical ionization (CI) MS in the mid 1960s was the direct result of a generalized theme of induced ion reactions in MS. In CI, neutral sample molecules react with reagent ions. Such reactions had been studied for years in other venues, such as those concerned with reactions of charged and neutral species present in the atmosphere. The broad impact of CI was that (using methane as the reagent gas, for example) it brought the predictability of acid or base chemistry into the gas phase and into MS. As an ionization method, CI could be energetically tuned. Further, after the idea of a selected reagent ion took hold, all manner of reagent ions could be used, for both positive and negative ion analysis, and with a designed selectivity. The choice of the reagent ion became part of rational selectivity in analysis. The concept that reactions in the rarefied vacuum of the mass spectrometer could be designed and exploited became a theme in MS that continues to the present day. A second part of this theme encompasses the idea that ions, after they are formed, can be prompted to react, again by an induced reaction. Collision-induced dissociation is the classic example of such an activation, and surface-induced and photon-induced dissociation, and more recently electron-capture and electron transfer ion activation methods are also included. Where in the mass spectrometer we choose to make such reactions occur, whether in the ionization source or elsewhere, is linked to several other themes described here. 

In EI, ionization takes place in an isolated, well-controlled gaseous environment. In CI, the reagent ion is present in great excess of the sample molecules, and so ionization of sample molecules again takes place in a controlled and characterized gaseous mixture. Such environments may seem far preferable to a situation in which a matrix effect might be observed, such as ionization in the presence of a liquid or solid matrix. However, the term "matrix effect" has commonly been burdened with a negative connotation. Matrix effects were those that decreased sensitivity, skewed selectivity, or added noise and uncertainty to the measurement. Central to these deleterious issues was the fact that the composition of the matrix was unknown or not controlled. On the other hand, matrix-assisted ionization methods, including electrospray ionization (ESI), matrix-assisted laser desorption–ionization (MALDI), and their precursors in matrix-assisted secondary ion MS and fast atom bombardment, have as a central tenet the control of the physical and chemical properties of the matrix. The matrix augments and mediates the ionization process. It is worth noting that matrix effects that increase the efficiency of ionization have an even longer history in isotopic geochemistry. In the more modern applications of MALDI and ESI, the matrix is a bridge to assist desorption of sample molecules and ions, provide a route to ionization through protonation or cationization, control the flux and form of ionization energy, and physically separate sample molecules and potentially reactive species. Rather than eschewing a matrix, informed selection of an ionization matrix of known properties became a tool to aid in the analysis. Then, as we became more adept at assessing the mass spectra produced, novel interactions between the same molecules or ions and the matrix could be explored, such as the unfolding of complex biomolecular ions in liquid matrices of various properties. 

In its classical application of MS to the identification of compounds, and the discernment of molecular structure, a basic assumption is that one compound is present in the ionization source at any given time. The ions observed in the mass spectrum therefore should all be correlated to that compound, and that structure, through rational fragmentation reactions. Real-world samples are almost always mixtures. Consider an environmental sample; it may contain tens of primary components at similar concentrations, and hundreds of secondary components at 10–100× lower levels. Column chromatography, independently of the use of MS as a detection method, was developed as a means to separate such mixtures. Analytical gas chromatography (GC) was developed first in the 1950s, and liquid chromatography (LC) in the 1970s, followed by other column-based methods (xC). These were transformative developments in and of themselves. Consider that in a benchtop distillation, a physical separation of mixture components is achieved, with separated components collected into separate flasks, for example. In analytical GC, no such physical separation of components occurs. Instead, different sample components require a different transit time through the column; a detector at the column exit provides a signal for each eluted compound. The signal output can provide some information about the component (as in an electron-capture detector contrasted with a thermal conductivity detector). However, the end result of this analysis is not physically separated compounds; instead it is information that characterizes the mixture, much of which may remain undisturbed. Information is the transformative end result. But the use of a mass spectrometer as a detector provides the ultimate unique information signature for each component in the mixture — its mass spectrum. On-line analytical separations combined with MS detection transformed specialized MS into a widely adopted general analytical tool. The informing power of the so-called hyphenated methods, so self-evident now, was a salient topic of research in the early 1980s.  

As sector-based mass spectrometers (such as a Nier-Johnson geometry instrument consisting of an electric sector followed by a magnetic sector) were used for organic compound analysis, broad signals were noticed at the spectral baseline. These signals were interspersed with the normal sharp peaks. Ultimately, they were identified as signals arising from metastable ions. Metastable ions are those ions which dissociate in field-free regions (such as between the source and the first sector) as they traverse the mass spectrometer. Passing the ions through a localized region of higher pressure, reversing the order of the sectors, and decoupling the sectors led to the development of mass-analyzed ion kinetic energy spectrometry (MIKES). The transformative factor in MIKES was the espousal of a process that induced purposeful reaction of a mass-analyzed ion as it passed through two independently operated sections of the instrument, and the assessment of the ionic products of that reaction. The same process is exemplified in the triple-quadrupole mass spectrometer; the first quadrupole selects an ion by mass. The ion is activated in the central rf-only quadrupole, and the ionic products of the activated reaction are mass-analyzed by the third, independently operated, quadrupole. By analogy to xC–MS, the first mass analysis is analogous to a separation, here accomplished through separation by mass rather than differential retention on a column. The transformative aspect was the speed with which ions can be selected, activated, and analyzed, and then the selected factors changed, resulted in a multiplexed analytical instrument that could rapidly produce large volumes of data, and could be reconfigured in real time for complex mixture analysis. Hours became minutes, and sometimes seconds and milliseconds. MS-MS could be extended to MS

. With ion-trap instrumentation, the sequence of processes could be extended almost to the limits of ion detection (a few hundred ions), with the flexible use of the MS-MS process as a series of analytical filters designed to preserve signal over noise. 

Because we know the exact masses of all the isotopes, and the distribution of these isotopes, we can predict the appearance of the molecular isotopic envelope for a molecular ion of known empirical formula. We can compare the measured envelope to the predicted envelope as an assessment of the measurement quality. Conversely, when confronted with an ion of unknown formula, we can use the measured mass to narrow the realm of possibilities, or to establish the formula with a given degree of certainty. The latter was an early classical application of higher-resolving-power instruments in organic analysis; determination of the ion empirical formula helped to link the ions and their fragmentation reactions. An accurate mass measurement can be used as a difference measurement to establish the formula of the fragment species lost. The usefulness of an accurate mass measurement has evolved, such as its use for tracing the incorporation of stable isotopes or mass defect labels as part of a sample derivatization strategy. In each of these applications, the transformative value of an accurate mass measurement comes from the interplay between necessary and sufficient conditions. Conventional understanding cautioned that as the mass of ions increases, the number of possible formulas increase, and the ability to definitively establish a formula diminishes. But a 

scale in a mass spectrum has two factors. Increasing the charge "

" on an ion increases the effective mass range of an instrument. But another transformative facet of accurate mass measurement is revealed in the calculation of an accurate molecular mass from repetitive measurements of multiple charged molecular ions formed in electrospray ionization. In these measurements, random errors are mitigated by the greater number of independent measurements, a basic statistical tool with pervasive application in MS. 

With specialized instrumentation, the abundance ratios between certain stable isotopes present in a sample can be determined with astoundingly high accuracy and precision. For example, the Vienna Standard Mean Ocean Water (VSMOW) is a suggested standard for the isotopic composition of freshwater, and is given as 

H = 155.76 ± 0.1 ppm. Variations from this composition, within that stated limit of error, are used to discern differential processes in hydrological systems, predicated on that fact that an exact measurement can be achieved. Other stable environmental isotopes include those of carbon, nitrogen, oxygen, and sulfur. Measurements of these isotopes in various terrestrial samples track the carbon cycle (especially through plant photosynthesis), and can track the passage of nutrients in the biosphere. Sample dating measurements depend on these accurate abundance ratio measurements. These measurements are, of course, not limited to Earth's biosphere, or the terrestrial realm. Isotopic studies in cosmochemistry help to discern some of the earliest fundamental processes, even those that predate the creation of our own solar system. The importance of sample collection and sample provenance in all of these studies should be self-evident.  

Reactions of ions such as dissociation to lead to fragment ions can be described in the terms used to describe any other reaction. There is an activation energy related to the observed rate of reaction, a distribution of energy in an activated transition state, and a structure associated with that transition state. Progress along a unimolecular reaction coordinate from reactants to products can be described by energy curves derived from experimental observation, but also from computational models. The calculational model (Rice-Ramsperger-Kassel-Marcus [RRKM]) could be used to produce a theoretical mass spectrum, if the internal energy distribution of the isolated ion was known (1). Calculational models, however, have a much broader impact on MS. Polyatomic ions of moderate structural complexity can be modeled and the probable structure of lowest energy established. Detailed space-filling models can provide insights into reactions such as the folding or unfolding of complex biomolecular structures. On an instrumental platform, expansive improvements in computational performance and data storage have transformed the archival use of data systems in the compilation and searching of mass spectral libraries. Libraries have been supplanted with databases and data mining searches the databases to establish new correlations and patterns in the data. Improved computational capabilities are used to compile and compare the terabytes of data measured in "-omics" research. Computational models are used to track stable isotope incorporation through analysis of the isotopic profile of ions formed from products of metabolic processes, and hydrogen or deuterium exchange reactions, through multiple generations of processes. Simply stated, modern MS research would not be tenable without underlying computational support.  

It was with some trepidation that this collection was compiled. The themes are broad enough so that the danger of a blatant omission might be avoided, but the choice of themes is sure to leave some dissatisfied that their field of application or research did not receive adequate attention. Still, on reflection, the centrality of themes, and the organization of the related developments coalesced into the coherent group presented here. If there is to be debate, let it commence. As quoted in Hughes (5), Frederick Soddy wrote: "So easy is it to fall into the error of thinking that things which look obvious after a discovery were just as obvious before." As a science, MS has routinely amazed its adopters, and still often confounds its practitioners, because it evolves so quickly, and spreads rapidly through new and unexpected vectors of application. MS is founded on the simplest and most basic of scientific principles, and yet makes 

 contributions to the most complex modern analytical issues, and reflects the ongoing scientific challenge to reach analytical certainty, as reflected in this quote from Aston's Nobel prize celebration "No probable, possible, shadow of doubt, no possible doubt whatever" (5). 

http://masspec.scripps.edu/mshistory/mshistory.php

A History of European Mass Spectrometry, 

K.R. Jennings, Ed. (IM Publications, 2012), ISBN: 9781906715045.  

 premiered the first "Mass Spectrometry Forum" in February, 1994. Two decades and about 75 columns later, this adventure draws to a conclusion with this last column. Thank you to the readers who have perused the columns over the years, and who have responded with comments and ideas. I am grateful to the editors at 

 who made this opportunity available. I will someday convince them that it is properly MS/MS and not MS-MS, but the world will survive regardless. This column is the sole responsibility of the author, who can be reached at 

Articles

Mass Spectrometry Forum

Some central transformative themes and their impact in modern analytical mass spectrometry are discussed, such as isotopic analysis, exact measurements, information processing, and more.

Transformative Mass Spectrometry

Laser-induced breakdown spectroscopy (LIBS) is an emerging analytical method that has been the focus of substantial research over the last 25 years. The recent emergence of commercially available LIBS systems from major manufacturers is a sign that the technology is maturing. This column is the first of a three-part series focusing on major aspects of LIBS. This installment concentrates on the basics of the measurement and typical implementations. Part II will discuss the choices for LIBS hardware in detail, in particular lasers and spectrometers, and illustrate the trade-offs between cost, size, and performance. Part III will discuss LIBS analysis in some depth, exploring the various ways to go from a LIBS spectrum to a solution. Overall, this column series is intended to provide an overview for those considering implementation of LIBS to solve a particular analytical problem, and an introduction for those interested in learning more about LIBS.

Laser-induced breakdown spectroscopy (LIBS) has been the subject of a number of recent books (1,2) and numerous review papers (3–5). Widely referenced charts in the introduction of the Cremers and Radziemski book (1) and the first chapter of Noll's book (2) illustrate the dramatic increase in the annual number of LIBS papers from near zero through the 1970s to an annual rate of approximately 300–400 today. As an example, Figure 1 shows the annual rate of papers and patents containing "LIBS" or "laser-induced breakdown" in their titles from Google Scholar (6). Interestingly, the past few years may indicate a leveling in the publication rate, and the data can be closely modeled by the following logistic function:  

Although it is too early to determine whether the leveling in the rate will be continued, if this trend holds the annual publication rate would peak around 500 per year, which is the equivalent of about three full monthly journals on an annual basis. 

Figure 1: Annual rate of publications and patents containing "LIBS" or "laser-induced breakdown," from Google Scholar. Accessed August 29, 2013.			

Clearly, there has been much progress in the science and application of LIBS. Today we increasingly see LIBS units in corporate laboratories and even controlling industrial processes. LIBS is deployed to take advantage of a number of positive attributes, which include minimal sample preparation, particularly compared with methods requiring digestion; speed of analysis; sensitivity to light elements, particularly compared with X-ray fluorescence (XRF); and "stand-off" ability in which the LIBS system is removed by distances of anywhere from millimeters to meters from the sample being measured. Because of these advantages there has been an explosion of interest in LIBS (see Figure 1) over a wide range of applications. However, LIBS is a complicated measurement, with special considerations related to the sample matrix and working hardware folded into the ablation process and the analytical laser plasma. Understanding the trade-offs between various component choices in a LIBS system, as well as the basic hardware arrangements, is key for a successful implementation of LIBS.  

One of the benefits and drawbacks of LIBS with a single laser pulse is that the same laser pulse is used for sample preparation (ablation) and analysis (laser plasma). A high-energy pulsed laser is focused into a small spot on a sample. For a typical nanosecond-class laser, the initial portion of the laser pulse penetrates into the sample and causes ablation of the solid sample. The duration of such a laser pulse is several orders of magnitude more than the characteristic lattice vibration time in a solid, meaning that local heat transfer is relatively fast compared with the characteristic time of the laser pulse. Hence, particularly for more conductive materials, nanosecond-class laser ablation results in a substantial amount of heating and melting of the sample. The rapid heating because of the high fluence of a nanosecond-class LIBS pulse typically leads to an explosive boiling process that can eject liquid and even solid-phase particles from the ablation target. The resulting crater, viewed under high magnification, often closely resembles a crater on the moon, with high walls created during "impact" and surrounded by ejected material. 

As the laser pulse progresses in time, the high electric field in the focal region of the laser begins to accelerate naturally present ions, plus ions formed via multiphoton interactions in the atmosphere above the sample, and from the sample itself. These free electrons increase the absorption of the laser in the atmosphere above the sample, leading to greater multiphoton ionization and further absorption of the laser pulse. Electron impact ionization and dissociation can also become important. Longer wavelengths are more readily absorbed in the forming plasma, with the implication that infrared (IR) laser wavelengths form a more robust plasma than do ultraviolet (UV) laser wavelengths in general. Simultaneously, the relatively dense cloud of ejected and vaporized material in the ablation plume above the sample may also be absorbing laser photons in the region tens of micrometers above the surface. In time, depending on the wavelength of the laser, much of the trailing part of a 4–8 ns laser pulse may be absorbed in the atmosphere above the sample. This forms the analytical plasma.   

Lasers with picosecond pulse durations follow generally the same process as described above for nanosecond-class lasers because the laser pulse is still long with respect to solid lattice vibrations, and hence heat transfer in the sample is still important in the ablation mechanics. However, LIBS with femtosecond lasers is quite different. Femtosecond lasers have pulse durations short enough that heat transfer in the sample is not important and sample melting is minimized; thus, ablation is much more of a mechanical process, with direct photochemical bond-breaking and Coulomb explosion among the primary ablation mechanisms. Femtosecond ablation craters are much more precise than craters from thermally dominated ablation of nanosecond-class lasers, with a minimal residual heat affected zone. Because of the nonthermal nature of the ablation, with femtosecond lasers there is much less of a problem of preferential desorption and nonstochiometric ablation than with nanosecond lasers. However, given the substantial cost and complexity increase of femtosecond lasers over nanosecond lasers, they remain a research tool. In large part, this is because of the fact that there is little or no evidence that overall detection limits are improved with femtosecond lasers over nanosecond lasers in LIBS. 

The resulting laser plasma forms the analytical engine for LIBS. Interestingly, LIBS plasmas formed over a wide range of total energies follow a roughly self-similar time-temperature history as they cool, parameterized by the energy deposited in the plasma. Initially in a severe state of disequilibrium, with "temperatures" in excess of 50,000 K, analyte particles are vaporized and the analyte is dissociated, with resulting atoms and ions electronically excited. "Temperature" is in quotation marks as temperature is ill-defined when the plasma is in a high degree of disequilibrium. In this condition, electron temperatures, ionization temperatures, and electronic, vibrational, or rotational excitation temperatures may be significantly different as energy cascades through the plasma. The plasma cools rapidly after the laser energy source is removed. The highly-excited free electrons in the plasma slow, and due to conservation of energy emit a broadband continuum of light referred to as Bremsstrahlung emission. During this phase, "temperature" is high enough that only very high electronic states of atoms relax in sufficient numbers to emit, but this emission is typically overpowered by the continuum. 

LIBS emission is typically collected after the plasma approaches local thermodynamic equilibrium (LTE), which may be 0.5–1 µs after formation of a 100–400 mJ plasma, but may be much less time if lower laser energies are used. When the plasma approximates LTE, temperature can be clearly defined, as there is a match between molecular temperatures and electron temperatures. For this reason, higher-energy (>5–10 mJ) LIBS plasmas typically use detectors that can be triggered "on" at a point in time after the continuum emission has reduced and the plasma is in LTE. Plasmas with smaller deposited energy may be able to use detectors that are not triggerable because the continuum emission is much less intense. In addition, some detectors such as intensified charge-coupled devices (CCDs) and electron-multiplying CCDs can gate both "on" and "off," allowing a precise window of emission to be collected. Photomultiplier tubes with integrating electronics can also time-gate in this manner. The optimum emission window for each element is a function of the upper and lower energy levels of the emission line being monitored (7). Typically the average plasma temperature during the time of the emission measurement is in the range of 7500–10,000 K. Figure 2 is a conceptual illustration of the plasma emission and collection time line. 

Figure 2: Conceptual illustration of plasma emission and collection time line for various moieties in the plasma.			

An example of the resulting LIBS spectrum is shown in Figure 3. Much like in inductively coupled plasma–optical emission spectrometry (ICP-OES) or other emission methods, multiple emission lines are shown from most of the elements present in the sample. Individual emission lines can be used for quantification over fixed ranges of concentration, or individual lines can be combined either in a piecewise fashion or in a multivariate calibration to yield a potentially more robust calibration over a wider range of concentrations. As indicated above, because of the coupling of the laser pulse in the ablation and the plasma formation, the resulting emission (and calibration) is dependent on both the experimental parameters (laser wavelength, energy, and spot size, to start) and the material. A deeper dive into analysis of LIBS spectra will be covered in part III of this series. 

Figure 3: LIBS spectrum of a gemstone, showing multiple elements and multiple lines in a typical spectrum. This spectrum was recorded using a single 10-mJ, 266-nm laser pulse and an echelle spectrometer with an intensified camera, using a gate delay of 1 Âµs.			

Methods of separating the laser ablation process and the LIBS diagnostic plasma have proven to be profitable both in improving detection limits (double-pulse LIBS) (8), and in moving toward matrix- or sample-independence (laser ablation-LIBS) (9). A review of these additions to the method is beyond the scope of the article, and insofar as commercial systems are presently based on "single-pulse" LIBS, can be left as the subject of continued research. 

The basic LIBS setup is shown in Figure 4. It consists of a pulsed laser, possibly a video camera or other range-finder, beam-shaping and steering optics, collection optics, and a wavelength-sensitive detector (most commonly the combination of a dispersive element and a detector).  

Figure 4: "Basic" LIBS setup including major hardware components.			

The choice of laser becomes one of the first decisions when considering a LIBS system. Typically, but not always, the choice is an Nd:YAG laser operating at the fundamental wavelength of 1064 nm or one of the harmonics (532, 355, or 266 nm being the most common). More than anything, this choice is driven by cost, as the Nd:YAG is able to deliver the lowest cost per joule of laser energy. Flashlamp-pumped Nd:YAG lasers are available in a range of energies in the fundamental from 10–500+ mJ, and for between $10,000 and approximately $35,000+ USD. Nd:YAG is also available with lower pulse energies, but not typically in commercial quantities. In commercial systems, flashlamps typically last between 20 million and 50 million shots, the latter being "only" about 2 months if the laser is run at 10 Hz, 24/7, but several years under common laboratory conditions. Diode-pumped Nd:YAG lasers have lifetimes that are easily 50–100 times longer than their typical flashlamp-pumped counterparts. Several older studies have used excimer (typically ArF 193 nm) or pulsed CO

 lasers, among a variety of pulsed lasers. 

Beyond laser pulse energy, the wavelength of the laser is important in determining the efficiency of the ablation and the ratio of laser energy absorbed in the material to that absorbed in the plasma, which for a solid material sits above the sample surface. In brief, shorter wavelengths are more effective at ablation, because the more energetic photons more likely exceed the bandgap of the material and can cause molecular bond-breaking. Shorter wavelengths can also make smaller spots on the material, resulting in a higher fluence than an equivalent-energy pulse from a longer-wavelength laser. However, the trade-off comes from the fact that longer-wavelength photons are better absorbed in the evolving plasma, so a more robust plasma is created with longer-wavelength lasers. In fact, this increased absorption in the plasma also shields the sample from a portion of the laser beam, which is referred to as "plasma shielding." A detailed discussion of this trade-off between ablation efficiency and plasma formation will be covered in part II of this column series, which covers hardware choices in more detail. 

At the most basic, detection of a LIBS single emission line can be accomplished with a bandpass filter and a single-point optical detector such as a photodiode or a photomultiplier tube (PMT). In fact, such an arrangement, with the addition of a second detector for a background or a baseline, can work well for simple systems in which one or only a few known elements are being measured. Otherwise, a variety of combinations of spectrometers (Czerny-Turner, echelle, or Paschen-Runge) and detectors (PMT, CCD, intensified CCD, or electron-multiplying CCD) are used for detection. Key variables in the selection of the dispersion or detection pair include required wavelength resolution, sensitivity, and detector gating and time resolution, as well as a match between the spectral range of the detector and the emission lines of the elements to be measured. Most LIBS emission lines are in the 180–500 nm range, with a few key elements having lines in the 500–850 nm range.  

The basic LIBS setup was shown in Figure 4. There are several offerings in the marketplace that are essentially desktop devices designed as a tool for laboratory analysts to make routine and exploratory measurements, potentially in conjunction with other instruments. Typically, these instruments have movable sample stages monitored by video, Class 1 sample chambers, and analytical software that allows familiar functions for analysts such as library building, calibration curves, elemental ID, and spectral analysis. Such laboratory systems are used for a variety of functions in material identification, qualitative analysis, and quantitative analysis. Surface mapping and depth profiling are also possible. These laboratory systems excel at analysis of solid materials. 

The analysis of liquid phases is somewhat more complicated than solids. There is clearly the propensity of liquid samples to splash when interacting with the laser plasma. As with solid samples, it is key to maintain a consistent lens-to-sample distance to achieve a constant fluence on the sample. These two facts are at odds with each other if a high-repetition rate laser is used. Approaches to measuring liquids on a free surface interface include measurements over a weir or measurements in a laminar jet (10). In both cases, the laminar nature of the flow results in a very predictable location for the free surface of the fluid. LIBS combined with laser-induced fluorescence (LIF) on a free surface has been shown to improve detection limits (11). Other approaches include single- or double-pulse LIBS within the liquid volume, with the latter case forming a second plasma inside the cavity formed by the first. Based on work by Professor Mike Angel of the University of South Carolina and others, double-pulse LIBS has been shown to have an advantage at typical pressures, but at the substantially elevated pressures typical of the sea floor, single-pulse LIBS has an advantage (12). Overall, detection limits for particular elements in liquids compared with solid materials are not as good, typically by an order of magnitude or more, because of the interaction with the liquid. Commercial adaptations of methods for LIBS measurement of liquids are available on the market. 

Gas composition is similarly easy to measure, and is much less sensitive to matrix effects because of the fact that there is no ablation event. Our group and others have applied this to measurement of mixing ratio between gases in turbulent fluid flows, and to air and fuel ratio measurements in combustion systems. In conjunction with Professor Paul Puzinauskas' group at the University of Alabama, this was even achieved through a fiber optic in a working engine (13). From the measurement of gases, it is also a small step to measure bulk aerosol suspended in the gas. Substantial work on measurement of individual aerosols has been accomplished by Professor David Hahn's group at the University of Florida (14), as well as work by our own group and several others. Bulk aerosol suspended in air or another carrier gas tends to yield repeatable and relatively simple calibration curves, while individual aerosol quantification remains a challenge. 

Finally, LIBS configurations that employ the standoff detection of LIBS have been used, particularly on solid samples. Examples of this can be found in industrial scenarios, in which LIBS has been used for measurement of coal, phosphate ore, and the composition of scrap metal. Truly "extreme" standoff LIBS measurements have been accomplished by the Laserna group at the University of Málaga, who have recently surveyed major and trace elements in the materials on the Cathedral of Málaga, Spain, from an average distance of 35 m (15). Previous research by that group has included work at even greater distances to identify traces of explosives. The LIBS work at extreme standoff distances remains experimental or prototype in nature, but the industrial applications mentioned above all have had commercial prototypes and (in a few cases) commercial sales. 

This column, part I in a three-part series, was written with the aim of explaining the basic considerations surrounding the physics and hardware choices that are important for making successful LIBS measurements. We've covered these topics, as well as the main modes in which LIBS is deployed for measurements on solids, liquids, and gases. As we've seen, the ablation event and the plasma are strongly dependent on both the sample matrix and the choice of LIBS hardware. For this reason, it is difficult to imagine a "universal" LIBS system or calibration; rather, the systems and calibrations are tailored for particular applications. In parts II and III of this series, we will focus in more depth on specific hardware components that might be considered for particular applications and on analysis methods that are required to translate a LIBS spectrum into an actionable answer — either qualitative or quantitative. 

Handbook of Laser-Induced Breakdown Spectroscopy

Laser-Induced Breakdown Spectroscopy: Fundamentals and Applications

(3) F.J. Fortes, J. Moros, P. Lucena, L.M. Cabalin, and J.J. Laserna, 

(6) Google Scholar (n.d.). Retrieved August 29, 2013, from 

(7) B.T. Fisher, H.A. Johnsen, S.G. Buckley, and D.W. Hahn, 

(8) J. Scaffidi, S. Angel, and D.A. Cremers,

(10) M. Taschuck et al., "Laser-Induced Breakdown Spectroscopy of Water Samples with Millijoule Pulse Energies," Plasma Science /ICOPS, The 29th IEEE International Conference on IEEE, 2002, p. 140. 

(11) H. Loudyi, R. Kheireddine, S. Laville, F., Vidal, M. Chaker, and M. Sabsabi, 

(12) J. Bonvallet and S. Angel, "Deep Ocean LIBS: Calibration Issues," presented at The Great Scientific Exchange (SciX 2013), Milwaukee, Wisconsin, 2013. 

(13) F. Ferioli, P.V. Puzinauskas, and S.G. Buckley, 

(14) P.K. Diwakar, K.H. Loper, A.-M. Matiaske, and D.W. Hahn, 

Columns: Lasers and Optics Interface

An overview for those considering implementation of LIBS to solve a particular analytical problem, and an introduction for those interested in learning more about LIBS. Part I concentrates on the basics of the measurement and typical implementation.

LIBS Basics, Part I: Measurement Physics and Implementation

The authors have been characterizing an electrothermal, near-torch vaporization (NTV) micro- and nano-sample introduction system for inductively coupled plasma (ICP) spectrometry. In this article they demonstrate that portable coiled-filament assemblies can be used for taking part of the laboratory to the sample and concentration determination in a laboratory using NTV-ICP–atomic emission spectroscopy (AES).

Typical inductively coupled plasma–atomic emission spectrometry (ICP-AES) systems have been developed and characterized, and they are frequently used with a pneumatic nebulizer for the introduction of liquid samples into the ICP. Although solid samples are usually acid-digested to a liquid before their introduction into an ICP (also typically using a nebulizer), the focus of this report is on naturally occurring aqueous samples. But before such samples can be introduced into an ICP, they must first be collected in the field. 

Conceptually, field sampling involves on-site collection of anywhere between 100 mL to 1 L of various samples followed by their shipment to a laboratory for analysis. There are costs associated with sample transport because of the relatively large volume (and weight) of samples that must be transported from a sampling site in the field to a laboratory for analysis by ICP spectrometry. Extra costs can be incurred if sample storage (sometimes in a cooled or refrigerated unit) is also required before analysis. 

Practically, in addition to the costs involved and depending on the analyte of interest, there are chemistry-related issues that must be considered as well. For instance, aqueous samples most likely must be filtered (ideally on-site), typically using a coarse filter. They also must be stabilized (often using a strong acid) before transportation. Sample stabilization is required to minimize analyte loss resulting primarily from adsorption on the walls of the sample container or to prevent precipitation. Safety precautions should also be considered when dealing with the transportation of strong or corrosive reagents to the field (for sample stabilization). Field-stabilized samples are then shipped to a laboratory and are usually stored before analysis. 

The elapsed time interval between sample collection and analysis is often critical, especially if there is natural species interconversion — that is, if the species of interest is labile. Let's consider chromium (Cr) as an example: A Cr

  interconversion over time has been reported (1). Furthermore, depending on the analyte of interest, acidification may also change the oxidation state of the species of interest. Oxidation-state information (that is, speciation) is important because toxicity is often oxidation-state dependent. Again, chromium is a  typical case here: Cr

  is reported as carcinogenic (1). In addition, redox equilibrium is, among others, a function of the final pH of an acidified aqueous sample, the oxygen concentration, and the final ionic strength of the sample. And, it depends on the amount of colloidal and fine particulate material that may still be present in a sample (even after coarse filtering). As already alluded to, the type of acid used to stabilize the sample and the final pH of an acidified sample may shift naturally existing equilibria. It may also help to release particle-bound species or dissociate ligand-bound species (likely both), thus altering the naturally occurring species distribution in the sample. In natural waters, for instance, acidification has been reported to induce a reduction of Cr

  (2). Because such chemical transformations begin to take place immediately after sampling, species concentrations determined in a laboratory (after unavoidable delays because of transport and possibly storage) may not necessarily reflect actual species concentration in a sample at the time and conditions (for example, temperature) of sampling. Thus, a species concentration "bias" will likely develop. The United States Environmental Protection Agency (US EPA) recommended that unless stabilized with reagents, Cr-containing water samples "must be analyzed within 24 hours of collection" (2). Additional sample handling steps are required if longer transport and storage times are necessary. 

Two conclusions can be drawn from the discussion above. First, elemental analysis can be made cheaper if smaller amounts of sample are brought to the laboratory for analysis. Second, concentration information will more accurately reflect species distribution at the time and conditions of sampling if species are collected separately at their naturally occurring oxidation state and are stored by deposition on an appropriate support. Because it is difficult to bring an ICPA-ES system to the field for on-site measurements, concentration determination of the species deposited in the field must be done in a laboratory. 

This article describes an approach that uses removable and interchangeable coiled filament assemblies onto which samples are deposited (for example, by pipetting or electrodepositing samples) on-site. The coils with dried sample residues are subsequently transported to a laboratory where they are used with an electrothermal, near-torch vaporization (NTV) sample introduction system and ICP-AES (1). 

Electrothermal, Near-Torch Vaporization Sample Introduction for ICP Spectrometry

A schematic illustration of the instrumentation used for this work is shown in Figure 1a and a photograph of the vaporization chamber and the portable coiled-filament assembly is shown in Figure 1b. The relatively small volume vaporization chamber of the NTV sample introduction system directly replaces the spray chamber of a typical pneumatic nebulizer. In essence, NTV can clip onto any ICP torch with a standard ball joint (Figures 1a and 1b). A coil made from rhenium (Re) filament serves as the sample holder (Figure 1c). The coils were fabricated by manually winding a rhenium filament around a screw with an appropriate diameter. Of the possible elements with high melting points that could be used to make filaments (Table I), Re was selected because unlike tungsten (W) (5,6), Re does not have a known ductile temperature. Thus, unlike W (7), Re does not become brittle after several heating (and cooling) cycles.  

Figure 1: (a) Schematic illustration of the near-torch vaporization (NTV) sample introduction system developed for use with removable, interchangeable, and portable rhenium coiled filament assemblies and micro- or nano-volume samples. (b) Photograph of the vaporization chamber showing the removable coiled filament assembly and the ceramic support rod (coin included for size). (c) Detail of the removable and interchangeable coiled filament assembly. (Adapted with permission from reference 3.)			

Unlike previous work with fixed coils and cups (3) or other sample holders of the type encountered in traditional electrothermal vaporization (ETV) sample introduction for ICP spectrometry (5,6), the small coiled-filament assemblies were fabricated to be removable (Figures 1c and 2a). These removable coiled-filament assemblies, with a total mass of 1.4 g each, had identical coupling mechanisms and dimensions so that they could be used interchangeably. Furthermore, removable and interchangeable assemblies with different coil sizes were fabricated, so that volumes ranging from <100 nL to 100 μL could be accommodated. A photograph of the removable and interchangeable coiled-filament assemblies is shown in Figure 2b.  

Table I: Melting point of some of the elements considered for filament materials			

To interchange coiled-filament assemblies (or to simply deposit a sample by pipetting a micro- or nano-sample into the loops of the coil, Figure 2), the ceramic supporting the coiled-filament assembly was retracted from the vaporization chamber (Figures 1a and 1b). If required, a coiled-filament assembly already installed on the ceramic support was manually removed and a new assembly (for example, of the type shown in Figure 2b) was interchanged on the ceramic. This operation was completed in a few seconds. The ability to easily and rapidly interchange coiled-filament assemblies proved to be beneficial, as we'll discuss later. It is worth noting that for sample pipetting or for coil interchange, the ICP torch operated continuously without interruption. In other words, the ICP was not extinguished.  

Figure 2: (a) Removable coiled filament assembly sample holder as installed on the ceramic support rod. For clarity, a red-color food-additive was added and it acted as the sample. (b) Photograph of interchangeable coiled filament assemblies with different capacities shown on a substrate support (coin included for size). (i) coil with capacity â¤1 Î¼L; (ii) coil with 1â10 Î¼L volume capacity; and (iii) coil with 10â100 Î¼L capacity. (Adapted with permission from reference 3.)			

The sample introduction sequence into an ICP was as follows: The ceramic support rod with an interchangeable coiled-filament assembly (Figures 1c and 2) attached to one of its ends was removed from the vaporization chamber. A micro- or nano-volume sample was pipetted on the coil and the coiled-filament assembly was reinserted into the vaporization chamber. A seal was formed at the end of the vaporization chamber. Cables running through the ceramic support connected the coiled filament to an external power supply (Figure 1a). The sample on the coil (Figure 2a) was then dried by applying low electrical power from the external power supply (for example, ~100 °C corresponding to ~1 W) to the coil. After drying, the residue that remained on the coil was vaporized into an ICP by applying higher electrical power to the coil (for example, 45 W, corresponding to 2610 °C, as measured using an optical, infrared [IR] thermometer). The vaporized, dried sample residue that remained on the coil was carried to the ICP by an Ar-H

) carrier gas. Hydrogen was used to prevent oxidation of the Re coil (3).  

Some Analytical Performance Characteristics of NTV-ICP-AES

To provide reference points for comparison purposes, the following brief narrative provides some analytical performance characteristics of the NTV-ICP-AES system. Examples of typical analytical signals are shown in Figure 3a, and their reproducibility is shown in Figure 3b. The detection limits obtained using NTV sample introduction and an ICP optical emission spectrometer equipped with photomultiplier tube (PMT) detectors are included in Table II. The improved precision obtained by NTV-ICP-AES compared to other electrothermal vaporization–based approaches (5,6) is noteworthy.  

Figure 3: (a) Typical analyte signals and (b) typical precision.			

Taking Part of the Laboratory to the Sample

Instead of depositing samples on a coil by pipetting them in the laboratory (3), samples can be pipetted on removable coiled-filament assemblies (of the type shown in Figures 1c and 2) in the field. Samples on coils can also be dried in the field. Thus, portable coiled-filament assemblies with dried sample residues on them can be shipped to the laboratory rather than shipping large volumes of sample to a laboratory, which reduces shipping, handling, and storage costs.  

Table II: Precision, and absolute-unit (picogram) and relative-unit (picogram per milliter or parts per trillion) detection limits			

Furthermore, instead of pipetting samples on coiled-filament assemblies, samples can be electrodeposited on them followed by a drying step on-site. Electrodeposition also offers added benefits such as preconcentration and matrix cleanup, which can also be performed on-site. Thus, coiled-filament assemblies with matrix-free, preconcentrated dried residues of samples can be shipped to a laboratory where concentrations can be determined by NTV-ICP-AES. In this work, such capability is demonstrated using a single electrodeposition potential and lead (Pb) as the test analyte (4). 

By electrodepositing analytes at two distinct potentials (and using different coiled-filament assemblies), different species of the same element can be electrodeposited on coils in the field. Chromium is a typical case where Cr

  can be electrodeposited at -0.3 V (versus Ag/AgCl) and both Cr

  can be electrodeposited at -1.6 V (versus Ag/AgCl). Shipping two coiled-filament assemblies to the laboratory, one loaded with a dried residue electrodeposited at -0.3 V and both Cr

  at -1.6 V allowed Cr speciation in the laboratory by NTV-ICP-AES (1). 

It is worth emphasizing that the simplicity of an electrodeposition setup (that is, a sample container, electrodes, and a handheld potentiostat) would make the instrumentation lightweight, small, and portable, and thus suitable for field use. 

On-Site Sample Deposition on Coiled Filaments and In-Laboratory Analysis

Previous work has shown that coils with different capacities (ranging from 100 nL to 100 μL as shown in Figure 2b) can be used interchangeably. An example of the use of nanoliter volumes is shown in Figure 4. Subsequently, it was shown that emission signals obtained using interchangeable coiled-filament assemblies with different capacities could be used for constructing calibration curves (3). These results indicate that pipetting different volumes of sample on coils with different volume capacities can be substituted for sample dilution. Although the approach described above was demonstrated in the laboratory, it opens up the possibility for shipping coiled-filament assemblies with dried sample residues on them to the laboratory, thus reducing shipping, handling, and storage costs.  

Figure 4: Example signal obtained using 100 nL of diluted Pb standard.			

But would dried residues of samples pipetted on coiled filaments and dried on-site remain on the coil during transport, handling, and possibly storage before analysis by NTV-ICP-AES in the laboratory? This question was addressed by placing loaded coil assemblies in a dustproof box, simulating transport by driving the box around, and storing the box in the laboratory for several days before analysis by an NTV-ICP-AES system (Figure 1a). From the results obtained it was concluded that there was no sample loss during transport and storage. Why was this the case? 

In the absence of specific adsorption sites, when a metal such as Re and an aqueous sample come into contact, "a metallic surface in contact with water forms a thin layer of a (hydrous) oxide" (8), most probably a monolayer (1). Such monolayers are thermodynamically favorable because they reduce the surface free energy; in fact, such layers are subject to the same equilibria as bulk metal oxides (8). Based on these conclusions, it can be speculated that -Re-OH

 is formed. If a metal ion is also present in the sample (and depending on the pH of the sample, the potential of zero charge of the Re surface, and ignoring multivalent ions), it can be speculated that the following reaction takes place (1):  

From this reaction, it can be concluded that cations are attached to the surface by a phenomenon characterized by a strong binding energy (such as chemisorption) rather than one with comparatively weaker binding energy (such as physisorption). In fact, it has been reported that in some cases chemisorbed species on surfaces require strong ion exchangers (such as acids) to be eluted from a surface they are attached to (1). Alternatively, the application of high temperature is required to vaporize chemisorpted species (as is the case here). It is worth noting that chemisorption on metal surfaces in contact with aqueous solutions is a common phenomenon (9–12), and that it occurs spontaneously and with rapid kinetics (10). Based on the above, and because of the bond formation between Re and M

  (as suggested by equation 1), it is not surprising that dried solution residues remain on the coil during transport and storage. 

These ideas were confirmed by determining copper (Cu) in tap water. Copper in tap water originates from plumbing materials. Tap water (obtained from a local tap) was diluted on-site using 18.2-MΩ·cm water and was air dried on-site. Coiled filaments with dried, Cu-containing residues on them were then transported to the laboratory where Cu concentration was determined using NTV-ICP-AES. The Cu concentration was found to be below the action level required to trigger the EPA's lead–copper rule (13). The rule stipulates that "if Pb concentration exceeds an action level of 15 ppb or if the Cu concentration exceeds an action level of 1.3 ppm in more than 10% of the customers taps sampled," some kind of corrective action must be taken. Thus far, to test for this rule, only Cu has been dealt with. Lead concentration determination is described below. 

On-Site Pb Electrodeposition on Coiled Filaments

Rather than simply pipetting samples on coiled filament assemblies, is it possible to electrodeposit analytes on such filaments? In addition to preconcentration, electrodeposition offers some analyte selectivity and provides preconcentration and matrix cleanup. Pb was selected as the analyte of choice because of its environmental significance and because it has one of the poorest detection limits by ICP-AES. The experimental set up for Pb electrodeposition is shown in Figure 5. A coiled-filament assembly served as the cathode and a platinum (Pt) wire was used as the anode. To simplify instrumentation and further enhance portability, a reference electrode was not used. To facilitate on-site operation, a homemade, handheld, battery-operated power supply was used. The interelectrode distance was kept constant, the sample volume was arbitrarily selected to be 20 mL, and the electrodeposition time was 3 min. Because of its simplicity and light weight, the electrodeposition instrumentation was portable.  

Figure 5: Experimental set up and sample processing steps used for Pb electrodeposition. (Adapted with permission from reference 4.) See text for discussion.			

For electrodeposition, metal ions such as Pb

  can be deposited anodically or cathodically (4). In this case, cathodic deposition (by reducing M

, according to equation 2) was selected.  

  (because of its high hydrogen over-potential), can be electrodeposited from acidic solutions with efficiency close to 100%. To facilitate portability and because of the minor effect of temperature (4), the effect of temperature was not considered. 

Applicability of Pb electrodeposition on coiled-filament assemblies in the field and analysis in the laboratory by NTV-ICP-AES was demonstrated using the determination of Pb in hard tap water. Typical hard water has a mineral content greater than 180 mg/mL (expressed in CaCO

 equivalent concentration [14]). The water sample was taken from a local residential tap that was fed with one of the "hardest" groundwaters in Canada (15) (with water hardness reported to be as high as 960 mg/L in CaCO

 equivalent concentration [15]). For comparison, soft waters have a hardness of <60 mg/L (16). 

In many parts of the world, Pb in drinking water comes from aging Pb water pipes as well as from solder and flux used for plumbing purposes. According to the US EPA (13), an action level for Pb in tap water is set at 15 ng/mL. Guidelines for Canada and those published by the World Health Organization (WHO) (17) are set at 10 ng/mL (18). 

The approach described above was tested in the field by placing the instrumentation shown in Figure 5 in a Plexiglas enclosure (thus preventing contamination from airborne particulate matter). Subsequently, Pb electrodeposition using tap water took place for 3 min. After electrodeposition and air-drying, a salt deposit was visibly noticeable on the coil. When the dried electrodeposited sample including the salt deposit were vaporized into an ICP (as described by path A in Figure 5), a bright yellow-orange emission was visually noticeable. This intense yellow-orange emission caused stray light and a double peak (for example, one for Pb and the other from stray light) at the Pb wavelength, with the peak caused by stray light interfering with the Pb emission signal. To eliminate the peak because of stray light, a rinsing step with 18-MΩ·cm water was added following electrodeposition (as shown in path B in Figure 5), thus rinsing away the salt deposit. This step eliminated the peak caused by stray light. Rinsing did not cause any noticeable Pb loss from the coil because during cathodic deposition a metal ion is reduced to the metallic form (equation 2). As discussed earlier, it can be suggested that Pb was chemisorbed on the coil, thus with-standing mechanical vibrations during transportation. Additional evidence has been provided by others who also reported the use of electrode rinsing (4). In one case, it was reported that electrode rinsing is a "standard practice" (4). The portable coiled-filament assemblies with Pb electrodeposited on them were rinsed and dried on-site, and were subsequently transported to the laboratory where the Pb concentration was determined by NTV-ICP-AES. The Pb concentration was found to be below the action level for Pb set by the EPA's lead-copper rule (13). Additional details such as validation and precision information can be found in reference 4. An example signal obtained after Pb electrodeposition is shown in Figure 6. The signal shown in Figure 6 is in the parts-per-trillion (pictogram per milliliter) level, which is clearly below the current detection limit of ICP-AES with pneumatic nebulization sample introduction. The Pb detection limit following a 3-min electrodeposition was 25 pg/mL (or 25 ppt); according to the data included in Table II, this limit is >30 times better than that obtained without electrodeposition from 5-μL samples.  

Figure 6: Example signal obtained after electrodeposition of Pb. See text for discussion.			

As briefly described earlier, the Cu concentration from the same tap water was determined by direct pipetting on coiled-filament assemblies. Thus, direct pipetting of Cu and electrodeposition of Pb on portable coiled filaments can be used for compliance testing of the EPA's lead-copper rule. 

On-Site Electrodeposition for Cr Speciation

Because of the success of the electrodeposition approach described above for Pb, the following question arose: Is it possible to perform controlled-potential electrodeposition using two different coiled-filament assemblies and two distinct electrode potentials to deposit two different species of the same element? In other words, can electrodeposition be used for Cr speciation? 

The electrochemical cell used for Cr speciation is shown in Figure 7. It consists of a polyethylene beaker and a removable cap that supports the electrodes (1). The cap was supported by an adjustable arm, thus also controlling the immersion depth of the electrodes. The coil served as the working electrode and a coiled platinum wire served as the counter electrode. A compact Ag/AgCl electrode was used as the reference electrode. The coils were preconditioned by applying high electrical power to them (for example, 45 W corresponding to 2610 °C) for 5 s before their use. For selective electrodeposition of Cr species, a voltage of -0.3 V (versus Ag/AgCl) was used for electrodeposition of Cr

  and a voltage of -1.6 V was used for electrodeposition of both Cr

  concentration was determined by difference. At the end of a 60-s electrodeposition time, the cell cap was raised, the coiled filament assembly was removed from its holder in the cup, and the assembly was carefully rinsed with distilled, deionized water (18 MΩ), thus removing any salt deposit on the coil.  

Figure 7: Experimental setup used for Cr speciation by electrodeposition.			

For method development, calibration, and validation, artificial seawater was used (1). An acetate buffer was added immediately before electrodeposition (to buffer the sample to pH 4.7). The buffered artificial seawater was immediately spiked with known amounts of Cr

, and the spiked solutions were used right away. Chromium emission signals from buffer blanks were negligible. Natural surface seawater samples were collected off-shore from Halifax Harbor (Nova Scotia, Canada). Then, they were filtered through a prewashed 0.45-μm filter and were immediately buffered to pH 4.7 with 15 mL of the buffered sample placed into a PTFE beaker that served as the electrochemical cell (Figure 7). Chromium species in seawater were immediately electrodeposited for 60 s on coiled-filament assemblies. The coils were washed with deionized water, air dried, and subsequently transported to the laboratory where the Cr species were determined by NTV-ICP-AES. The Cr species concentrations were found to be well below the maximum levels set by the EPA for saltwater and continuous exposure. 

The emission signals shown in Figure 8 have been included for indirect (for example, Cr

  versus Cr) comparisons of the signal enhancements obtained because of 60 s of electrodeposition. For more-direct comparisons, the detection limit (3σ) for Cr was found to be 0.5 ng/mL (or 2.5 pg in absolute amount). This detection limit is 200-fold superior to that reported for Cr using graphite-based ETV-ICP-AES without the use of chemical modifiers and ~sixfold better than ETV-based methods using chemical modification (19). When gains because of electrodeposition are factored in, rough improvements range from 250- to almost 13,000-fold (1). Additional details (such as validation and precision) can be found in reference 1. These results compare favorably with those reported by others for Cr speciation of river water using microcolumns (20).  

Figure 8: (a) and (b) Cr signals obtained after a 60-s electrodeposition; (c) Cr signal obtained without electrodeposition. The differences in concentration between (a), (b), and (c) are noteworthy (Adapted with permission from reference 1.)			

Peer Reviewed Article

The authors have been characterizing an electrothermal, near-torch vaporization (NTV) micro- and nano-sample introduction system for inductively coupled plasma (ICP) spectrometry.

Rhenium Coil Field Sampling for Determinations by ICP-AES with Electrothermal Vaporization Introduction

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