The application of laser-induced breakdown spectroscopy (LIBS) for the analysis of aerosol systems is a challenging problem that entails a wide range of physical phenomena that are coupled to the ultimate analyte response. While the analysis of aerosol particles dates back to some of the earliest LIBS studies, the evolution of understanding of the many processes involved in transforming a solid particulate into a collection of dissociated atoms and ions necessary for atomic emission spectroscopy has largely occurred over the last decade. During this time, a number of studies have attempted to elucidate the physics of particle vaporization, dissociation and ionization, diffusion of heat and mass, and ultimately, the resulting atomic emission. This review seeks to summarize the evolution of thought with regard to LIBS-based analysis of aerosol systems and provide insight into future research directions.

The technique of laser-induced breakdown spectroscopy (LIBS) makes use of a laser-induced plasma to vaporize and dissociate a targeted material. Subsequent atomic emission from the analyte species within the laser-induced plasma forms the basis of LIBS as an analytical technique. One might trace the beginning of quantitative LIBS-based aerosol analysis to the pioneering work of Radziemski, Cremers, and colleagues (1). In their study, beryllium-rich aerosols were used for generation of calibration curves and subsequent calculation of detection limits. They demonstrated the viability of LIBS for detection and quantitative analysis of aerosol samples. In a following study, in which particle sizes were extended to the submicrometer-sized range, Radziemski and colleagues (2) generated calibration curves for three analytes — namely cadmium, lead, and zinc — which were characterized by initial linearity followed by various degrees of saturation at higher concentrations. The saturation effects were attributed to incomplete vaporization of the analyte-containing particles. Over the following two decades, the LIBS community often cited an upper size limit for complete particle vaporization of about 10 μm, some directly referring to the extensive work of Radziemski and Cremers (3,4), while others referred to such a limit as a more general guideline. Radziemski and Cremers (2) did report that the LIBS technique is useful as long as the aerosol particles are below about 10 μm in diameter and somewhat monodisperse, although no systematic study was ever undertaken to specifically quantify this issue. Only in more contemporary LIBS studies was the issue of an upper size limit for quantitative analysis of aerosol particles addressed directly (5,6), as discussed in more detail later.  

An issue closely related to complete particle vaporization is the issue of the independence of the analyte response on the analyte source. In the larger analytical community, such a topic would be considered to fall under the label of matrix effects, although such terminology was not largely used in the LIBS community with regard to aerosol analysis. Semantics notwithstanding, the issue was addressed from the beginning, including in the early work by Radziemski and colleagues (2). An important finding of their study was the general agreement (within 10%) of lead atomic emission signals of comparable atomic lead concentrations when nebulizing either lead acetate, lead chloride, or lead nitrate. In addition to analysis of particle-derived analyte signals, researchers also have looked at the gas phase of the aerosol system, where the relative independence of analyte signals on the molecular source was reported in several studies (7,8). Specifically, Dudragne and colleagues (7) demonstrated that analyte signals for fluorine, chlorine, sulfur, and carbon scaled with the number of respective analyte atoms in the constituent molecules for a wide range of compounds, concluding that the parent molecules were fully dissociated in the laser-induced plasma. Tran and colleagues (8) verified that SF

 and HF yielded identical fluorine atomic emission signals when the gas composition was adjusted to contain the same atomic fluorine concentration.  

This discussion is by no means a comprehensive review of the early evolution of LIBS for aerosol analysis, but rather, is intended to set the stage for a more in-depth discussion of the physics of LIBS-based analysis of aerosol particles. However, the general concepts outlined here, namely, complete particle vaporization and independence of the LIBS signal on analyte source (that is, lack of significant matrix effects) — do provide the basis and motivation for a wide range of LIBS-based aerosol studies. In a number of applied studies, LIBS-based sensing has been implemented successfully for continuous on-line monitoring of emissions and industrial processes (9–24), for analysis of ambient air particulate matter (25–28), and for general aerosol systems and nanoparticles (29–33). LIBS also was used in conjunction with single-shot analysis to effectively sample and analyze aerosol populations using discrete particle analysis (12,25,34,35). Other studies have addressed the feasibility of LIBS for analysis of biological materials and bioaerosols (36–44). In view of the wide array of potential applications, important research issues regarding LIBS-based analysis of aerosol systems can be identified for further analysis, as discussed in the remainder of this review.  

Plasma–Particle Interactions: The Limiting Cases of Finite and Infinite Rates of Relevant Transport Properties

 implies an interaction between the analyte particle and the laser-induced plasma rather than between the particle and the laser beam. This is an important distinction that arises from consideration of the laser focal volume and the resulting laser-induced plasma volume. The latter is typically several orders of magnitude greater, which directly translates to a dominance of the probability of plasma–particle interactions as compared with direct laser-particle sampling. This is verified in the context of plasma volume measurements, particle sampling rate considerations, and direct imaging studies (45–47). The plasma–particle processes also must be framed in the context of the overall plasma lifetime (~100 μs for ~100 mJ/pulse air breakdown) and the analytical time-scales (that is, detector delays and gates), which typically range from ~5 μs to 50 μs following breakdown (48). 

With this framework in mind, the introductory comments speak directly to widely proposed assumptions within the LIBS community, namely, that complete dissociation of constituent species within the highly energetic laser-induced plasma results in analyte signal linearity with increased analyte concentration, and on the independence of the analyte atomic emission signal from the analyte source. From a physics point of view, these concepts can be extrapolated to the context of process rates. If the time scale for complete analyte vaporization and dissociation is much less (that is, an order of magnitude faster) than the analytical time scale, it becomes essentially instantaneous, and the atomic concentration might be expected to scale linearly with analyte mass. If the time scale for diffusion of heat through the plasma and the time scale for diffusion of analyte mass through the plasma are also much less than the analytical time scale, the analyte spatial distribution, excitation, and ensuing atomic emission should correspond to the overall plasma conditions (bulk or spatially averaged), with no localized perturbations due to the presence of a vaporizing analyte particle. 

This scenario corresponds to the ideal situation, in which aerosol particle-derived analyte species interact uniformly with an unperturbed analytical laser-induced plasma. In contrast, the opposite scenario might be envisioned, in which the time scale of particle vaporization and dissociation, and the time scales for heat and mass transfer, are all very slow compared to typical analytical time scales of plasma evolution (~5–50 μs). Such a scenario will result in a nonlinear analyte response, as a significant portion of analyte atoms will remain in a nonemitting phase bound within the solid particulate. Furthermore, the local absorption of plasma energy required for particle vaporization will not be sufficiently replaced by the diffusion of heat from the larger, surrounding plasma, resulting in localized suppression of plasma temperature or electron density, which can subsequently affect the atomic emission of the analyte species that are also confined to this spatial region due to finite scales of mass transfer. Figure 1 depicts the two scenarios outlined earlier, which might be considered the two limiting cases for plasma–particle interactions pursuant to quantitative LIBS-based particle analysis for a given analytical time scale.  

Clearly the roles of heat and mass transfer processes in combination with the rates of particle vaporization and dissociation are of critical importance to understanding and optimizing the analyte response for LIBS-based aerosol particle analysis. Along these lines of thought, contemporary research studies have sought to quantify the plasma–analyte interactions, and will be the focus of the remainder of this review. 

The relative simplicity and analyte sensitivity of LIBS, combined with the ability to analyze individual particles from an aerosol system, have motivated a wide range of applications, as briefly described earlier. Figure 2 nicely illustrates the promise of LIBS for quantitative aerosol analysis, depicting a spectrum obtained from a single aerosolized 

 spore of about 1 μm in diameter, as adopted from work of Dixon and Hahn (41). The two calcium atomic emission lines correspond to about 4 fg of calcium, thereby demonstrating the femtogram limits of detection available with LIBS for select analyte species. However, the process of transforming from a solid, micrometer-to submicrometer-sized particle to individual neutral atoms and ions involves a complex set of physical processes, as summarized in Figure 3. 

As a starting point to understand the relevant issues, researchers have taken a more critical look at the upper particle size limits for quantitative analysis of aerosol particles. Carranza and Hahn (5) explored the vaporization and analyte response of individual silica microspheres in an aerosolized air stream. The upper size limit for complete particle vaporization was found to correspond to a silica particle diameter of 2.1 μm for a laser pulse energy of 320 mJ, as determined by the deviation from a linear mass response of the silicon atomic emission signal for progressively increasing diameters of silica microspheres. Their findings were discussed in concert with factors that may influence the vaporization dynamics of individual aerosol particles, and it was considered that the larger particles might not have sufficient plasma residence time to completely vaporize on the analytical time scale that the emission measurements were recorded, namely, 35 μs following plasma initiation. 

A more recent study examined the complete vaporization of carbon-rich particles (specifically glucose particles and sodium hydrogenocarbonate particles) in a laser-induced plasma, and reported an upper size limit of 5 μm for complete vaporization (6). A larger limiting size with the carbon-rich particles as compared with the silicon particles (2.1 μm versus 5 μm) most likely reflects the marked difference in melting points and volatility when comparing the more refractory silicon particles to the more volatile carbon-rich particles. 

While the exact upper size limits for quantitative analysis will always depend upon the nature of the analyte and the specific experimental configuration (for example, laser energy, wavelength, and configuration), these studies have helped to answer the important question of the limiting particle size for quantitative analysis, although additional studies are necessary to elucidate the underlying physics of this 2–5 μm size limit.  

To gain a deeper knowledge of the laser-induced plasma–particle interactions as related to aerosol particle analysis, researchers have turned to spatially resolved spectral measurements and direct plasma imaging. Hohreiter and Hahn (45) were the first to directly image analyte species as they vaporized and dissociated from an individual glass microsphere, and subsequently diffused into the surrounding plasma. As shown in Figure 4, their study provided direct evidence of the actual time scales of particle vaporization and analyte diffusion, revealing that the plasma–particle interaction is initially limited to a spatial region about the particle. Analysis yielded diffusion coefficients of the calcium atoms ranging from 0.06 to 0.02 m

 /s over the delay times of 2–30 μs, respectively, following plasma initiation, and a time scale of 15–20 μs for complete dissociation of the 2-μm (mean diameter) glass microspheres. In several related studies, Lithgow and Buckley (49–51) explored the influence of the spatial location on the atomic emission from aerosol particles in laser-induced plasmas. The spatially resolved spectral measurements revealed a variability in analyte signal with the location of particles within the plasma. Additionally, they proposed the use of spatially resolved measurements to maximize the particle detection efficiency. 

The finding of spatially localized analyte species following particle vaporization with LIBS-based aerosol analysis raises the question of the importance of localized plasma perturbations due to loss of plasma energy to vaporization and dissociation processes. Previous studies revealed no changes to the overall plasma (spatially averaged bulk plasma) properties of temperature and electron density due to the presence of aerosol particles (52). However, the same study revealed a dependence of the analyte signal (carbon in the study) on the source of the analyte when comparing particulate-derived carbon to gaseous-derived carbon (namely CO

). Together, such findings suggest the examination of the problem from the other point of view, namely, changes to the localized plasma properties about the particle-derived analyte species, rather than changes to the bulk plasma. Diwakar and colleagues (53) examined the issue of localized plasma perturbations by examining the analyte signal from a range of multicomponent submicron-sized aerosol particles. Analyte species examined included sodium, magnesium, and cobalt with the addition of the elements copper, zinc, or tungsten at mass ratios from 1:9 to 1:19 (analyte-to-concomitant species). Their measurements revealed a perturbation in localized plasma temperature, which was attributed to the loss of plasma energy required to vaporize and ionize the aerosol particle mass. The measurements provide direct evidence of a matrix effect for aerosol particles, as the resulting analyte signals were affected by additional particle mass, as attributed to localized plasma perturbations in the vicinity of the particle. The resulting perturbations to analyte response were minimized at longer plasma delay times (~60 μs), which is attributed to the allowance of sufficient time for complete vaporization of the analyte particles in combination with sufficient time for the analyte atoms to thoroughly diffuse throughout much of the plasma, thereby equilibrating with the overall bulk plasma conditions. The authors concluded that quantitative LIBS analysis of aerosol particles should be performed with careful attention given to the temporal plasma evolution and analytical time scales, and noted that the finite time-scales of particle dissociation and heat and mass transfer are equally important.  

Much of the LIBS research has been driven by experimental studies, although numerical simulations and modeling studies can provide valuable insight into the plasma dynamics (54–71). A recent study by Dalyander and coworkers (71) specifically addressed the time scales of heat and mass transfer with laser-induced plasmas for the application of aerosol particle analysis. The model included simultaneous solution of the heat and mass transfer equations using physical models for the plasma thermal conductivity and for the mass diffusivity of the analyte species (calcium and magnesium in the study). The analysis revealed significant species gradients due to the finite scales of mass transfer, and localized temperature perturbations due to energy requirements of particle dissociation and ionization. A very useful parameter for directly comparing the time scales of heat and mass transfer is the dimensionless Lewis number, defined as the ratio of the thermal diffusivity to the mass diffusivity. Figure 5 presents the volume-averaged Lewis number from the numerical simulations for the species calcium and magnesium as a function of time following plasma initiation. The Lewis number ranges from about 4 to 1, with a Lewis number of essentially unity for plasma delay times beyond 5 μs following breakdown. The findings are in excellent agreement with the experimental results discussed earlier and indicate that the time scales of heat and mass diffusion are comparable and finite. In aggregate, such a near-unity Lewis number is indicative of a finite rate of diffusion of heat into the region of particle dissociation, and of a comparable, finite rate of diffusion of analyte species away from the particle region. Such a scenario, therefore, suggests initial suppression of local plasma temperature, as energy that is utilized for particle dissociation can only be replaced over a finite time scale. 

The above narrative is intended to make two key points about the evolution of laser-induced breakdown spectroscopy for the specific application of aerosol analysis. First, a new technique can move into the analytical community with an initial set of key assumptions (for example, complete and rapid particle dissociation, or independence of the analyte signal on analyte source) that have not yet been evaluated rigorously. What often follows is a parallel path of fundamental investigations in concert with many applied studies examining the applicability and performance of the analyte scheme. Second, only through fundamental scientific studies can the underlying physics behind the key assumptions be evaluated critically, after which the optimal paths forward can be identified with the ultimate goal of a widely accepted analytical technique. One can propose that the last decade has seen LIBS-based aerosol analysis in this second phase, in which the key physical processes are being unraveled. It is also noted that beyond LIBS, plasmas are a central component of many of today's leading analytical methods for chemical analysis, microanalysis, and materials characterization, including laser-ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) and inductively coupled plasma–atomic emission spectroscopy (ICP-AES). Hence, related studies of plasma–analyte interactions in inductively coupled plasmas remain relevant to the problems studied in the context of LIBS (72,73). In all of these analytical techniques, the complex plasma–analyte interactions are directly related to the ultimate analyte response, and to the ultimate quality of the results provided by the techniques. It is envisioned that by understanding and overcoming the problems associated with the plasma–particle interface, LIBS will successfully move forward in the analytical community.

The author would like to acknowledge the significant contributions made by his current and former graduate students to the topic of LIBS-based aerosol analysis and offer his gratitude for their fine efforts: Jorge Carranza, Vincent Hohreiter, Ken Iida, P. Soupy Dalyander, Michael Asgill, Philip Jackson, Bret Windom, and Prasoon Diwakar. This work was supported in part by the National Science Foundation through grant CHE-0822469, as part of the Plasma-Analyte Interaction Working Group (PAIWG), a collaborative effort of the University of Florida, Federal Institute of Materials Research and Testing (BAM) in Berlin, and the Institute for Analytical Sciences (ISAS) in Dortmund, jointly funded by the NSF and DFG.  

 is with the Department of Mechanical & Aerospace Engineering, Plasma Analyte Interaction Working Group, University of Florida, Gainesville, Florida. 

(1) L.J. Radziemski, T.R. Loree, D.A. Cremers, and T.M. Hoffman, 

(2) M. Essien, L.J. Radziemski, and J. Sneddon, 

(3) D.K.Ottesen, J.C.F. Wang, and L.J. Radziemski, 

(4) S. Yalcin, D.R. Crosley, G.P. Smith, and G.W. Faris, 

(7) L. Dudragne, Ph. Adam, and J. Amouroux, 

(8) M. Tran, B.W. Smith, D.W. Hahn, and J.D. Winefordner, 

(11) R.E. Neuhauser, U. Panne, R. Niessner, G.A. Petrucci, P. Cavalli, and N. Omenetto, 

(12) D.W. Hahn, W.L. Flower, and K.R. Hencken, 

(14) R.E. Neuhauser, U. Panne, R. Niessner, and P. Wilbring, 

(15) H.S. Zhang, F.Y. Yueh, and J.P. Singh, 

(16) M.H. Nunez, P. Cavalli, G. Petrucci, and N. Omenetto, 

(17) S.G. Buckley, H.A. Johnsen, K.R. Hencken, and D.W. Hahn, 

(18) H.S. Zhang, F.Y. Yueh, and J.P. Singh, 

(19) L.G. Blevins, C.R. Shaddix, S.M. Sickafoose, and P.M. Walsh, 

Articles

The author discusses the evolution of thought with regard to LIBS-based analysis of aerosol systems and provides insight into future research directions.

Laser-Induced Breakdown Spectroscopy for Analysis of Aerosol Particles: The Path Toward Quantitative Analysis

In the third installment of this tutorial, the authors explain the determination of the isotopic composition of a sample from a mass spectrometric measurement, and the methods of calculation as well as the units used. This tutorial outlines the practices in common usage so that researchers new to this field can obtain a good understanding of the fundamentals involved.

The reporting of isotopic compositions from mass spectrometric (MS) measurements is not a straightforward matter because in most cases, some degree of correction is necessary to extract the true isotopic composition from the measured ion currents. Also, there is no consensus on the units in which to make the report (1). As the measurement technology and its role in the application progressed, both the methods of correction and the units have developed independently for each isotopic species under investigation. Originally, isotope practice was developed to determine small changes in natural abundance, and the notations and conventions adopted are considered by some to be unsuitable for modern high-throughput, high-enrichment work, as is practiced in tracer techniques (2). 

The rationalization of stable isotopic composition reporting by the adoption of the International System of Units (SI) has been considered recently (3). In fact, the "natural" way of reporting isotopic composition is to use 

 defined as the quantity of a particular isotope divided by the total quantity of that element in the sample. Fractional abundance is therefore a dimensionless quantity, closely analogous to mole fraction. Unfortunately, in cases of polyisotopic species, it is often not possible to measure all of the isotopes present, and recourse is made to normalizing the quantity of the particular isotope to that of the major (most abundant) isotope present. On this basis, the 

defined in terms of the ratio to the major isotope, are related to fractional abundance via  

 specifies the particular isotopic species, and the summation runs over all the isotopes present. 

Many researchers prefer to express abundance as the deviation from that observed in some standard material, and this gives rise to the concept of excess abundance Δ

. This scheme is favored by scientists working with nitrogen isotopes, who often quote abundance in terms of atom percent excess. 

Scientists working with other light isotopes such as hydrogen, carbon, and oxygen, in the geochemical field, took this one step further and adopted a scale known as the delta value, defined in the next section; this is currently the most adopted way of reporting isotope determinations using commercial instrumentation. 

A mass spectrometer can never provide an absolute measurement of ion beam intensity. For a given partial pressure of gas in the inlet, the signal recorded at the detector is influenced by the ionization efficiency of the source for that particular species, the transmission of the ions through the mass analyzer, and the gain of the various amplifiers used in the ion detection (active devices such as the electron multiplier as well as the electrometers used for final stage amplification). Not only are all of these factors governed by the actual experimental conditions at the time of measurement (such as ambient temperature, ion source cleanliness, and state of instrument tuning), but they all are also subject to drift throughout the working day, and often within the timescale of the experiment being performed. Isotope ratio mass spectrometers are designed to minimize these effects. For example, they usually are operated in flat-topped peak mode (4,5), which minimizes the effect of mass drift. Secondly, they use Faraday buckets rather than electron multipliers as detectors because of the greater stability of the former. Finally, arrangements usually are made for simultaneous measurement of all of the ion beams required and the results reported as the ratios of the ion currents relative to the major beam. Even with these precautions, the ratios obtained are not absolute (principally because of uncertainty in the values of the feedback resistors of the electrometer amplifiers), and still are subject to drift because of sensitivity to ambient temperatures. 

The effects of drift can be eliminated by the inclusion of a standard sample in the experiment. In the case of continuous flow instruments, this is achieved by admitting a pulse (or pulses) of reference (cylinder) gas to the instrument as part of the measurement sequence. In dual inlet systems, where the sample and reference gases are each held in bellows connected to a microvalve, a series of alternate measurements of the ratios obtained from the two gases are made. In either case, the reported measurement of isotope ratios is a comparison between the measurements made on the sample and the reference materials. The conventional way of expressing enrichment is on the delta scale, defined as, intercomparison, working or transfer standard, and internal or reference standard (6).  

 indicates the delta value of the sample with respect to the reference, and 

 are the ratios of the ion currents measured for the sample and reference gases, respectively. Delta values often are multiplied by 1000, and therefore expressed as "parts per mill" (or just "per mill," in accordance with the English edition of the International Standard ISO 31-0, Quantities and Units – Part 0: General Principals, sub-clause 2.3.3), symbol ‰†. As an example of how the delta value is obtained from the raw ion currents, we take the output from a dual inlet machine. For simplicity, we assume that it is measuring 

 44 and 45. The measurement sequence comprises four determinations of the reference gas alternated with four measurements of the sample gas, giving the results shown in Table I.  

Note how the delta value is calculated for each sample measurement by comparison with the average ratios of the reference measurements made immediately before and immediately after it, with a similar strategy being applied to each reference measurement. 

In passing, it is worth discussing to what extent the strategy of using a reference gas can eliminate drift during the course of a series of measurements. For correcting short- to medium-term fluctuations due to such factors as ion source conditioning, it is highly successful. However, not all sources of drift can be corrected in this way, and care must be taken that the process itself does not introduce errors. For example, in dual-inlet systems the reference gas is stored at subatmospheric pressure in a set of bellows. Each time this gas is admitted to the mass spectrometer, there is likely to be some fractionation due to the pumping characteristics of the vacuum system. In other words, as the experimental run progresses, the isotopic composition of the reference gas is likely to change gradually, becoming more enriched in the heavier species. Although instruments are designed to minimize this problem, it can be noticeable in the most precise work. The introduction of a drift of this nature is much less likely in continuous flow systems, where the reference gas is stored and injected at much higher pressures. 

The mathematical manipulation of delta values must be undertaken with care. For example, consider the delta value of some material A, expressed with respect to another material B  

The isotopic composition of any gas can be expressed relative to that of another gas by means of a delta value. For example, suppose we also have a standard gas and we conduct an experiment in which our reference gas is used as the sample and measured with respect to the standard. We would obtain the following delta value:  

Simple rearrangement allows us to express 

which can be substituted in equation 2 to obtain  

As an example, suppose we know that the reference gas has a delta value of –17.9‰ with respect to an international standard. We can express the enrichment in our sample by  

or 25.2‰ with respect to the standard. 

The generally accepted agency for the definition and supply of stable isotope standards for light elements is the International Atomic Energy Establishment (IAEA)]. The IAEA distinguishes six types of reference material that it denotes primary reference, calibration, reference, intercomparison, working or transfer standard, and internal or reference standard (6), although some of these terms are used in different contexts by other authors. They can be summarized as follows: 

Primary reference material — a material that is generally agreed to be a standard against which isotopic abundances of other materials are expressed. It is used to define a scale for reporting variations of stable isotopic composition. 

Calibration material — a material that has been carefully calibrated against the primary reference material and for which the calibration values have been internationally agreed. It is used as a proxy for the primary reference if the primary reference is not available. 

Reference material — a material that has been calibrated against the primary reference and that is believed to be sufficiently homogenous for the isotopic composition to be stated with well-defined uncertainties. 

Intercomparison material — a material of proven homogeneity that allows interlaboratory comparisons of measurements to be made. 

Working standard — a gas used as the reference gas in the MS measurement. This is not a standard in the usual sense, as it merely acts as a means of indirect comparison between prepared samples and reference materials. 

Internal standard — a material routinely run as a sample as quality controls in the MS analysis. 

For hydrogen and oxygen analysis, it has been accepted for more than 50 years that ocean water should be used as the primary reference material, as this represents the largest available reservoir on earth. Unfortunately, ocean water itself is not sufficiently homogenous, and so a hypothetical mixture of waters taken from various sampling sites was proposed and given the title Standard Mean Ocean Water (SMOW) (7). For carbon analysis, CO

 generated from a geological formation in the U.S. (PeeDee Belemnite, PDB) was adopted (8). The nitrogen standard is taken as atmospheric N

, while for sulfur work, a material from a meteorite, Cañon Diablo Troilite (CDT), was adopted. 

Because SMOW is a purely hypothetical material and the stocks of PDB and CDT are exhausted, these primary references are unavailable and have been replaced by synthetically prepared analogues named VSMOW, VPDB, and VCDT, respectively (Table II). In each case, the 

 stands for Vienna, where the headquarters of the IAEA is situated. To clarify the relationships between these primary references and the other types of standards defined earlier, the particular case of the water standards used for hydrogen and oxygen work will be examined.  

Table II: Primary references for selected light elements 

The primary reference VSMOW was prepared to have as close an isotopic composition to the purely conceptual SMOW as possible. At the same time as VSMOW was prepared, two reference materials also were obtained. The first, derived by melting Antarctic ice, was named SLAP, for Standard Light Antarctic Precipitation (originally this water was known as SNOW, but this was changed in order to avoid confusion with SMOW). It is substantially depleted in heavy isotopes of both hydrogen and oxygen with respect to VSMOW. The second reference material, again obtained by melting ice, this time from Greenland, has isotopic composition intermediate between VSMOW and SLAP, and is called GISP (Greenland Ice Sheet Precipitation). In addition, the opportunity was taken to recalibrate two existing U.S. water standards, NBS-1 and NBS-1A, onto the VSMOW scale. This gave a grand total of five reference materials, as indicated in Table III. Because the isotopic composition of these waters is internationally agreed, they now can be regarded as calibration materials if so required.  

Table III: Reference materials for 2H and 18O analysis 

In addition to supplying small samples of these calibration materials, the IAEA offers the reference materials IAEA-302A & B (ä

  = 508.4‰ and 996‰) and IAEA-304A & B (ä

  = 251.7‰ and 502.5‰), respectively. Some laboratories choose to prepare their own reference materials and to have them calibrated against V_SMOW independently. For example, in Cambridge, we currently use local tap water (–49.0‰) and a series of gravimetrically prepared standards in the range –49 to 874‰. Our reference materials are incorporated into every analytical run and are used not only for two-point calibration of the mass spectrometer (see SMOW/SLAP correction below), but also to check linearity, thereby adopting the roles of internal standards.  

When these measurements are made, each sample or standard is compared directly with a reference gas, as described earlier. This working standard is used solely for the purpose of allowing samples to be compared with each other, and therefore can be of arbitrary source. We use standard cylinder (tank) hydrogen for this purpose. The isotopic composition of the working standard is unimportant in this case because both reference materials and samples are incorporated in the same run; therefore, it is assigned the value of 0‰ on the reference scale, and all measurements are made with respect to this. Once all these results have been obtained, they are converted to the VSMOW scale using the data from the reference materials. 

Calculations Peculiar to Specific Isotopes and Compounds

There is a complication peculiar to gaseous hydrogen when measuring isotope ratios by mass spectrometry. The isotopologues of interest are 

 3). There is an ion-molecular reaction, however, which occurs in the ion source, 

, which gives rise to a second species at 

 3. Consideration of the kinetics shows that if the ion current at the major collector is denoted I

 then the ion current at the minor collector will  

In the third installment of this tutorial, the authors discuss the determination of the isotopic composition of a sample from a mass spectrometric measurement.

The Role of Naturally Occurring Stable Isotopes in Mass Spectrometry, Part III: Small Gas Molecule Calculations

 will know, the feature article in this year's August Buyers' Guide provided an in-depth analysis of the current state of the materials analysis market, and the conclusion (unfortunately) was that no, the spectroscopy market is not immune to the economic downturn that the U.S. economy is currently experiencing ("The Spectroscopy Market Hits a Bump in the Road," Lawrence S. Schmid).

However, amid the generally muted tone of the report were some bright spots and a definitively positive outlook for the future of the industry. And one thing that appears certain is that lasers and laser-based techniques will play an integral role in the eventual recovery of the spectroscopy market, with IR, NIR, and others trending upward. As always, 

 aims to lead the way in bringing readers the latest research on such cutting-edge techniques and methods, and this month is no exception, as evidenced by the feature, "Laser-Induced Breakdown Spectroscopy for Analysis of Aerosol Particles: The Path Toward Quantitative Analysis" by David W. Hahn. Here, further applications of LIBS are explored, with a novel application in the area of aerosol particle analysis. LIBS is associated to a large extent with areas such as homeland security research, itself a rapidly growing market segment, as seen in the June issue of 

 ("Current Status of Standoff LIBS Security Applications at the U.S. ARL," by DeLucia and colleagues), and this latest addition to the scientific literature only reaffirms the importance of this technique in the materials analysis community.

Look for more research on LIBS and other laser-based techniques in future issues of 

, as upcoming issues will feature further installments of our "Lasers and Optics Interface" column, as well as additional laser-related content (see our October issue for LIBS analysis of complex samples such as paint, petroleum, and motor oil).

If there is a trend emerging in the field of materials analysis, you can bet our columnists and regular authors will be covering it, and LIBS research is just one example.

Deparments: From the Editor

Introduction to this month's issue by David Walsh.

From the Editor

My first (and so far, only) wife recently gave me a gift: a black scarf that she had crocheted. The scarf wasn't all black — positioned at appropriate positions were colored yarn so that the scarf mimicked the visible emission spectrum of hydrogen! (OK, so both my wife and I are geeks . . . .) She said that she let each row stand for 1 nmr of wavelength so that the scarf represented the 300-nm range that is commonly used to define the approximate limits of visible light. I now look forward to the cold weather so I can wear it. I wondered what inspiration a Balmer-series scarf might provide me for a column for "The Baseline." Once again, my wife gave me a gift — an idea. Why not a column on color?

Color is the perception of relative different-wavelength light intensities by the human eye — in doing so, the eye is acting like a spectroscope, an analogy pointed out in a previous column (1). Typically, when the word 

 is used, it is used in exclusive reference to variations in visible light. This is rather anthropocentric, as other animals may see different regions of light. Bees and other insects, for example, have different chromophores in their eyes that are insensitive to red light but are sensitive to ultraviolet light. (Bug zappers take advantage of this physiological difference.) 

A discussion of the history of the discovery of color is best found elsewhere. However, probably the single largest advance in the understanding of color was when Isaac Newton demonstrated in the early 1670s that white light was a combination of all colors of light. This makes color perception an issue of variable absorption (or, equivalently, variable reflection) of different components of white light. Objects do not emit color; rather, they absorb and reflect colors in different amounts, allowing them to be perceived as colored based upon what colors they do and do not absorb. 

The spectral colors are those colors that can be defined by a single wavelength or frequency or energy of light. Figure 1 shows a printed version of a continuous visible spectrum. As good as Figure 1 may be, it is not a perfect representation because the figure is being reproduced using the four-color printing process, rather than using a different colored ink for every possible color on the continuous spectrum — an impossibility in any case. (More discussion of this is found later.) 

Spectral colors are also called monochromatic and are sometimes referred to as pure colors. The colors are commonly separated into six or seven main regions: red, orange, yellow, green, blue, and violet (the familiar ROY G. BV). Some people add indigo as a separate color, making ROY's last name BIV. However, there is still debate as to whether indigo deserves to be a separate color. It should be noted, though, that the identification of these six (or seven) specific colors as "the" colors of the rainbow is completely arbitrary! However, they are so ingrained in our society that they seem to be the "natural" colors. 

As mentioned earlier, objects have a color caused by what colors of light are reflected from them. However, objects may not have a spectral color, because more than one wavelength (or more correctly, range of wavelengths) can be reflected simultaneously. Also, the relative amount of light reflected will influence the apparent color of the object. An object that reflects both blue and red may look bright pink or burgundy, depending upon the relative amounts of red and blue light that are reflected. Note that neither "bright pink" nor "burgundy" is in our list of spectral colors. The spectral colors may themselves be complete as a set, but their number of possible combinations is literally infinite. 

In addition to the absorption–reflection process, different colors can be generated another way: as the emission of lights of certain colors. These colors are referred to as additive colors. Different combinations of colored lights of various intensities will yield different perceived colors. Again, we cannot have an infinite number of differently colored lights, each one representing a different spectral color. Instead, we rely upon various standard combinations that can be varied to generate an overall light color of different types. 

"Standard" combinations of lights are, technically, anything but: there are an infinite number of base colors that can be used to reproduce all possible colors. Over time, standards have been developed for various reasons (including the availability of phosphors of the right colors), and currently, most colored-light generation uses the combination of red, green, and blue lights to generate colors (the so-called RGB color model; see Figure 2). Those three colors are known as the primary colors for this particular model. Many devices that emit or detect light — televisions of the various forms, digital cameras and projectors, and color scanners — use an RGB color model, arguably the most natural choice because it uses the specific colors the cones in the retina are sensitive to. 

An RGB color model is probably most visible if you take a very close look at your television or computer monitor: the entire screen is full of tiny red, green, and blue dots called pixels. The intensity of each pixel color is varied to produce the desired color. Because of different manufacturing processes and materials, however, a color image may look different from one output device to another, leading to some variation in perception of color in many consumer products. Thus, there is no single "RGB" color model; several specific RGB color models have been defined industrially, by Microsoft and Adobe for example. Specifying an exact color also requires that you define which system, or color space, you are using. 

The RGB color model might make biological sense, but it is not the only possible model. Any set of colors — usually but not required to be three — that can stimulate the three different cones in the retina can be used to generate colored light. 

If you want to generate color in print, another tactic is necessary — you have to use colored pigments that, when combined, yield the desired color. Again, you can use either an infinite number of pigments, one for each possible color, or select a standard set of a few colors that can be combined in infinite ratios. Solid objects have a color based upon the wavelengths of light they are reflecting, not producing. Thus, generating color by mixing pigments is known as a subtractive color model. As with additive color models, there are many possibilities, but some standards have been agreed upon for simplicity. 

Red, yellow, and blue make up a model called the RYB model that is still used today in art (Figure 3). However, this system has limits — it is not possible to generate bright colors like magenta, since a subtractive color scheme is based upon absorbed light; colors that are produced are invariably darker than the starting pigments. 

The most common color model for printed color is the one based upon cyan, magenta, and yellow: the CMY, or CMYK (which includes black; the letter K stands for "key," from the black key line used in printing to align the different colors), color model (Figure 4). Although the combination of cyan, magenta, and yellow does yield black, many printing processes use black pigment in addition to the three primary colors: it saves pigment and money; the combination of three pigments can overload the paper it's printing on; or the combination of the three primary pigments may not turn out a satisfactory black color. These colors, including black, make up the "four color" printing process often spoken of in the publishing industry. 

The CMYK color model also has its limitations. Light colors are hard to represent unless an inkjet printer can produce a smaller dot of pigment, so that the (usually) white paper can blend with the smaller dot of pigment. Some color printing processes use lighter inks of the base colors, so that lighter hues can be produced (abbreviated with the lowercase letter for the color(s) used; hence, CcMmYK for six-color printing, and so forth). Gray ink or pigment is also available. Some color printers add additional colors to the fundamental four, including orange and green. Specifications for color inkjet and laser printers should list what colors of ink or toner are used; the wide variety of possible colors explains why it can be so confusing to buy replacement inks or toners for your color printer.

Translating color accurately from one medium to another is a challenge because different display or printing devices have slightly different color models — one computer monitor's "red" may be slightly different than a magazine printer's "red." Lighting also affects the look of the printed color, a fact not lost on marketers and art museum curators.

How well did this publication reproduce colors? Generate yourself a spectrum from a slit of sunlight (you can do it — you're a spectroscopist!) and compare it to Figure 1. The difference will illustrate how difficult it can be to represent color.

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

, available through the SPIE Web Bookstore at 

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

Columns: The Baseline

Columnist David W. Ball discusses how a scarf featuring the visible emission spectrum of hydrogen inspired his latest column, on color.

Color

Efficient creation, transport, containment, and detection of ions are issues at the core of mass spectrometry. Improvement in any of these areas leads to improved overall performance of the mass spectrometer. As in most sequences of operations, maximum performance benefits usually are obtained with improvements at the beginning of the sequence. As a result, improvements in the efficiency of ionization often result in large rewards in instrument sensitivity. As electrospray ionization (ESI) progressed through its first few generations of source design, the instrument performance increases were substantial. Once the ions are created, it is the role of ion lenses to extract ions from the source efficiently and focus them as they pass through the mass spectrometer. As one might expect, ion lenses of diverse designs have evolved to match ionization sources of different types. Lens designs also vary with the masses, charges, and velocities of the ions to be focused. Advanced texts and computer programs are available for a broad overview (1—4). The tutorial of Wollnick (5), the review by Burgoyne and Hieftje for mass spectrographs (6), and the detailed study of ion deceleration lenses by O'Connor and colleagues (7) are especially useful for mass spectrometrists interested in instrument design.

In "beam" mass spectrometers, ions are first created in the ionization source, then moved elsewhere in the instrument for sorting by mass (the mass analyzer), and are finally sent off to the detector. Thus, beam instruments physically transport either a continuous stream, or a series of packets of ions, through the components of the instrument, proceeding from ionization source through the mass analyzer to the detector. In contrast, "in time" instruments can store ions within a cell or a trap, but only after the ions are transported there from the separate ionization source. Mass analysis occurs as the instrument monitors the behavior of this trapped ion population with time. However, transport of ions out of the cell might still be required for ion detection. If the detection is done in situ, then the elements of the holding cell also function as ion lenses. The trapped ions never cease their movement, and scattering collisions with neutral molecules reduce the trapping efficiency. 

The set of coordinates used to chart ion movement within a mass spectrometer is shown in Figure 1, which shows that "

" usually is chosen to correspond to the direction of ion movement from the source toward the detector, and "

" represent the mutually orthogonal axes. In a magnetic sector instrument, "

" traditionally was set to be the axis orthogonal to the magnet pole faces; in Figure 1, this is set as the vertical axis from the point of view of the instrument operator. In a beam instrument, therefore, ions move along the 

-axis from source to detector, motivated by the accelerating potential. Additionally, of course, the ion path can be curved (as through a magnet), or oscillating (as through a quadrupole mass filter), and therefore, the ions move in both directions on the 

-axes. In a reflecting time-of-flight mass spectrometer, the ion path turns back on itself, and ions therefore move in both +

directions to reach the detector. The goal of ion lenses is to maximize transmission and shape the ion beam in (

) as it passes from the source to the detector. Ion lenses can include both potential or magnetic elements, reflecting the fact that moving ions will respond with a change in direction to either an applied potential field or an applied magnetic field, following the fundamental laws of physics. 

We concentrate here exclusively on potential-driven lenses. Mass spectrometers usually contain multiple sets of ion lenses, and more complex lens structures often are assembled from combinations of simpler elements. In this overview, we will describe four simple types of ion lenses, starting with source lenses, which act primarily to extract ions from the ionization source. Source lenses influence the divergence of an ion beam in the "

" directions, and pften are designated by that coordinate name (

-lens) when they are used immediately after the ionization source. Next, we will describe the chicane lens, which acts to remove the line-of-sight from an ion beam. Removal of a line-of-sight is often important in the reduction of background signals. We'll provide a short description of the einzel lens, commonly used to restrict the energy width of an ion beam. Finally, we will describe a multicomponent ion funnel used to transport ions from a relatively high-pressure electrospray ionization (ESI) source into the lower-pressure mass analyzer of an instrument. 

The potentials applied to potential lenses can be static or dynamic, or even a combination of both. Static operation means that the device potentials are fixed; a dynamic mode means that the applied potentials can vary with time. One reason to adopt a dynamic mode of operation is to maintain lens efficiency across the full range of ion masses in the beam that we wish to focus. For example, in an inductively coupled plasma–mass spectrometry (ICP-MS) system, the analytical range of atomic ion masses varies over about 1.3 orders of magnitude. Therefore, a dynamic ion lens maximizes ion transmission in the full ion mass range. An organic mass spectrometer deals with an even broader mass range (encompassing about 2.5 orders of magnitude), and the need for dynamic operation might be even more pronounced. Ionization sources might also produce multiply charged ions, and the ion lens system can even be designed and operated to discriminate between various charge states. Finally, remember that there is a fundamental limit to how tightly an ion beam can be focused, as the Coulombic repulsion between same-charged ions will always tend to cause spatial divergence in the ion beam. 

Focusing lenses can be placed anywhere along the ion path in a beam mass spectrometer. They are commonly placed immediately after the ion source (and are therefore termed 

), but also are sited between components in a multicomponent instrument and immediately before the detection system. As source lenses, their primary purpose is to aid in the extraction of ions from the source and to focus and shape the ion beam as it passes into the mass analyzer. The repeller fitted to most electron ionization (EI) and chemical ionization (CI) sources is a crude ion lens. A flat plate repeller can be used to nudge the ions toward the mass analyzer through the ion source exit aperture. As Park and Ahn (8) describe, the repeller can be shaped to direct the ions out of the source more efficiently. Traces for equipotential lines in even a simple EI source can be complicated, with field penetration through each and every conductance aperture. The electrons from the source filament in an EI source can be made to spiral in a constrained area with the use of a source magnet. But gas-phase sample molecules ionized by those electrons within that volume maintain their initial distribution of velocities, and the ions depart from that volume in all directions. A thorough understanding of the potential gradients inside the source underlies good lens design.  

The repeller is usually only the first stage in a series of ion lenses that work together to extract the ions from the source, accelerate them to a potential commensurate with the operation of the mass analyzer, and shape the ion beam. For example, a quadrupole mass filter will accept ions of relatively low kinetic energy (up to 50 V, for example, depending upon the quadrupole size and electronics). A circular ion beam shape created through circular conductance and lens apertures matches the acceptance function of the quadrupole (although ions that proceed down the exact center of a quadrupole rod structure are not mass-analyzed). Therefore, the ion lens in such a quadrupole-based system usually consists of a series of plates set orthogonal to the 

-axis, with central circular apertures, and operating focus potentials of ± tens of volts. More extensive shaping of the ion beam usually is not needed, and the assumption is often made that the apertures are illuminated uniformly by the ion beam.  

If the mass analyzer consists of electric or magnetic sectors, the ion beam is accelerated through the source potential to a higher velocity than in a quadrupole-based instrument, and the ion beam usually must be transformed to a rectangular profile. The physical slits often found along the ion path of a sector instrument accomplish the beam shaping by acting as stops for the ions with off-axis directions. Of course, discarding ions lowers instrument performance, and 

-lenses (and other forms of beam-shaping lenses) usually are used first used to "squash" the ion beam into a proper profile shape. 

In the simplest configuration, a pair of plates with faces orthogonal to the 

-axes can be installed. The plates can be paired electrically, or can be decoupled, and also can be linked dynamically to the mass-analyzing function of the instrument. As the need to shape the ion beam becomes more stringent, the plates themselves can be formed into complex shapes, or a series of multiple smaller plates can be used. Simple focusing plates can be joined with an einzel lens or can be conjoined into an ion zoom lens. An electrostatic quadrupole ion guide can be added. All such permutations and combinations are designed to preserve ion transmission across the mass range of interest. Because the source lens is early in the sequence, improvement in ion transmission into the mass analyzer generates considerable benefits in overall instrument performance. Therefore, the variety of patented configurations and devices for all types of mass spectrometers is considerable. There is always a balance to be achieved in ion transmission that balances ion lens complexity with such factors as ion path length, operating pressures along that length, and ease of instrument operations. For example, those of us who have reached a certain age might recall a lengthy tuning process to coax high mass-resolving powers from a sector instrument while maintaining the maximum possible sensitivity.  

Many ionization sources produce not only ions, but also neutral species and photons. If there is a line-of-sight from source to detector, the arrival of fast neutral species and photons at the detector can contribute to a background signal that limits instrument performance. The background signal is of course independent of any mass analysis that is completed, and is instead dependent upon the operation of the ionization source. 

Several approaches to reduce this noise have been developed. The detector can be placed off-axis, and a lens system used to deflect a mass-analyzed ion beam to the detector. A "bent" quadrupole removes the line-of-sight from source to detector. In an ICP-MS instrument, the flux of photons from the source can contribute to a significant background noise at very high sensitivity operation. A photon stop (literally a shadowing barrier) can be used to block photons from the direct line of sight, but such a barrier also blocks the ions following the same path.  

Iizuka and Hirata (9) described the use of a chicane lens (Figure 2) fitted to the front end of an ICP-MS system. The bend in the ion path is accomplished with a combination of simple lens elements and results in a 3–4× increase in the measured sensitivity of the instrument. Chicane lenses of the same general design have also been used in organic mass spectrometers. For example, Boyd and colleagues (10) incorporate a chicane lens into a hybrid instrument, and the central stop of the chicane lens is an important component of the neutralization reionization mass spectrometer described by Turecek and colleagues (11). 

Many websites cite only the early work of Aberth and colleagues in 1974 (12) as describing the design and operation of an einzel lens, but in fact the characterization dates back to Rangel in 1942 (13) and Adams and Read in 1972 (14). Aberth's work was significant in describing the use of the einzel lens in a mass spectrometer used for chemical analysis. A simplistic diagram of a three-element ion lens is given in Figure 3, conveying the essential elements of operation, which are: the first and third elements of the lens are coupled, and the central element is held at a different potential; and a focal point for the ion beam moving along the 

-axis is achieved at some point after the lens. If the ions are decelerated to a very low kinetic energy at the central electrode, the einzel lens can be operated in such a way that it becomes an ion energy selector. The details for this operation are beyond this introduction, but we may revisit them in the future. The SIMION simulation for an einzel lens is found at: 

Ryba and Mains (15) described an einzel lens as part of an improved Mattauch–Herzog mass spectrometer. The performance evaluation for this lens using the computer code of the time can be compared with the capabilities of the modern SIMION program. Einzel lenses are also commonly used in sector–quadrupole mass spectrometers as part of the deceleration lens stack, as described by Suter and colleagues (16). Li and coworkers (17) have described the role of the einzel lens as part of the interface between an ion mobility spectrometer and a time-of-flight mass spectrometer. Miniature mass spectrometers require miniaturized lens systems, and an einzel lens can be micromachined to match the scale of the entire instrument. For example, Syms and colleagues (18) describe a micromachined einzel lens used in conjunction with a quadrupole electrostatic lens to couple an ion source to a mass analyzer in a miniaturized instrument.  

While the issue of source lenses has been explored thoroughly during the development of the EI source over the past 60 years, and the CI source over the past 40 years, the source in both instances operates at reduced pressure relative to atmosphere, and the ion paths are not excessively scattered by collisions with neutral gas molecules. The typical EI source operates at 10

  torr, and the CI source operates at about 1 torr. Conductance apertures limit the pressure within the mass analyzer, and the changes for operation of the source and focusing lenses for CI compared to EI usually were limited to increasing the absolute magnitude of the applied potentials. With ESI, the ionization occurs at atmospheric pressure. Not only does the conductance problem become more acute, but the lensing system requires special attention. An advantage of the ESI source is that the ions are formed anisotropically, with some initial velocity directed away from the tip of the sprayer. 

At first, ESI sources were coupled with the usual and customary lens elements. But it soon became evident that new designs were needed. As an example, in 2001, Schneider and coworkers (19) discussed lenses that could work at atmospheric pressure and with a variety of sprayer positions. A two-to fourfold increase in the absolute level of the ion signal could be achieved with proper selection of the lens potentials. Interestingly, the authors report that the relative distributions of the multiply charged ions from the ESI source could be varied with different operation of the lens. This signature observation should alert ESI users to the influence of interfacial spray chemistry, along with bulk solution chemistry, on the detailed distribution of charged states in an ESI mass spectrum. 

Maximum ionization efficiency in ESI usually occurs at flow rates different from those required in usual practice. Therefore, to maximize the creation of ions from an ESI source, multiple emitters have been used, creating ions in an array of initial positions, all at atmospheric pressure. The ion lens "problem" of collecting these ions and directing them efficiently to the mass analyzer is significant. Ibrahim and coworkers (20) describe the use of sequential ion funnels (Figure 4) to accomplish this task. A leading high-pressure ion funnel is followed by a "conventional" ion funnel with a conductance limiting aperture between them. The conventional ion funnel  comprises 100 brass plates, each 0.5 mm thick and separated by 0.5-mm-thick PTFE sheets. The central aperture in each of these plates decreases in diameter for the series of plates along the 

-axis. A 180° out-of-phase RF field is applied to adjacent electrodes to create an effective potential well that confines the ions while a DC gradient of 16 V/cm pushes the ions through the ion funnel. 

The importance of the ion lens system in a mass spectrometer often is underestimated, and the chance to explore and redesign within the modern generation of instruments is far too limited. While attention may be focused (pun intended) on applications, our understanding of ion optics as applied to mass spectrometry continues to be explored. Consider this brief synopsis of "ion optics"–related presentations available only on Monday at the 2009 annual meeting of the American Society for Mass Spectrometry in Philadelphia. Poster 211 (21) described curved radio-frequency ion guides. Poster 311 (22) explored the concept of "rim losses" in mass spectrometers. Poster 473 (23) described an interesting lens system on a mass spectrometer that could be tuned to offer some discrimination between singly and doubly charged ions. Given the underlying simplicity of the physics that governs ion motion, one might be tempted to think that approaches to efficient ion optics and ion lenses were issues relegated to the past. The conclusion that "everything useful has already been discovered" has proven the undoing of many a scientist.  

 never could properly remember out the right-hand rule, and it was not fun to watch him trying to do so during the final examination for Physics 201. In later years, his limerick "On the Quad Rods" was a daring investigative exposé of the origin of ion burn. That piquant rhyme cemented his reputation, one way or the other. He can be reached at 

(2) K.J. Hanszen and R. Lauer, "Electrostatic Lenses," in A. Septier (Ed.), 

 (CRC Press, Boca Raton, Florida, 2008).  

(7) P.J. O'Connor, G.E. Leroi, and J. Allison, 

(10) R.K. Boyd, E.W. Dyer, and R. Guevremont, 

(11) F. Turecek, M. Gu, and S.A. Shaffer, 

(12) W.H. Aberth, R. Schnitzer, and F.C. Engesser, 

(16) J.-F. Suter, R.M. Stepnowski, and U.P. Schlunegger, 

(17) F. Li, T. Horvath, H. Schmidt, S. Sielemann, and J.I. Baumbach, 

(18) R.R.A. Syms, L. Michelutti, and M.M. Ahmad, 

Columns: Mass Spectrometry Forum

Columnist Kenneth L. Busch looks at the impact ion lenses can have on the performance of the mass spectrometer.

Mass Spectrometry Forum: Ion Lenses

The Exoscan FT-IR spectrometer from A2 Technologies is designed for laboratory and field use. Sampling interfaces for the instrument include internal reflectance, grazing angle reflectance, diffuse reflectance, and specular reflectance. Applications include liquids, powders, pastes, and reflective and nonreflective solids. The spectrometer can be used as a handheld, battery-operated instrument or, with an optional docking station, as a laboratory bench-top system. 

The 7700 Series ICP-MS system from Agilent Technologies is designed to be simple to operate and to provide data integrity in trace element analysis. According to the company, the system occupies 73 cm of bench space. The instrument features the company's ICP-MS MassHunter software. 

Avantes' AvaSpere-50-LS-HAL integrating sphere with an internal halogen light source can be used for reflection measurements with diffuse halogen light as illumination. It is designed to be used with the company's AvaSpec spectrometers. According to the company, the internal light source provides 160 times more light on the sample than with an integrating sphere with an external fiber-coupled light source. The device has an internal diameter of 50 mm, a sample port diameter of 10 mm, and an SMA terminated reference port. The 5-W halogen lamp is stabilized and cooled with forced air flow. A 24-V, 1.25-A power supply is included. 

The Visible-NIR Raman spectral engine from BaySpec has a wavelength range of 300–1100 nm. The instrument can be used for Raman illuminated analyses at various wavelengths from 300 nm to 800 nm for Raman spectroscopy. According to the company, the instrument's 2048 × 64 pixel CCD array achieves a typical resolution of 5 cm

 . The instrument uses a VPG transmission grating and reportedly is calibrated for life. 

The MB3000 FT-IR analyzer from ABB is designed for ease of use, reliability, and constant results. The instrument is available in two versions: the MB3000 for mid-IR analyses and the MB3600 for near-IR applications. The instrument can be combined with the company's Horizon MB software for data acquisition, processing, and analysis. 

The Nicolet iN10 MX FT-IR microscope from Thermo Fisher Scientific is designed for high-speed imaging, increased accuracy, and ease of use. The instrument incorporates the infrared optics in a single, integrated system, which reportedly improves sampling and enables the use of a room-temperature IR detector and reduces bench space. According to the company, the microscope can be used to analyze samples as small as 3–4 μm for single-point analyses and mapping or chemical imaging analyses. 

SeqLab de-novo sequencing software from Shimadzu Scientific Instruments is designed for use with the company's AXIMA MALDI mass spectrometers. According to the company, researchers can use MS-MS to find de-novo sequences and tags for peptides and related MS-MS-MS data to help decipher peptide sequences. 

The S1 TURBOSD handheld XRF analyzer from Bruker AXS is designed for mining applications ranging from exploration and survey to final processing. The analyzer can be used for on-site geochemical analysis of mine face, drill core, or prepared samples. The instrument's mining calibration reportedly compensates for interelement issues that can affect analysis results. According to the company, it can be used with sample concentrations ranging from 100 ppm to greater than 50%. 

Econic CCD spectrometers from B&W Tek are designed for academic and government teaching laboratories. The spectrometers include a 2000-element linear CCD array, a fiber-coupled spectrograph, a built-in 16-bit digitizer, and a USB 2.0 interface. Standard wavelength ranges are 250–950 nm and 350–1050 nm with a standard slit size of 25 mm, reportedly delivering a resolution of 0.8–10 nm depending on configuration. The instruments include external event synchronization, external trigger input, and the company's BWSpec software. 

Quasi-continuous-wave (QCW) high-power vertical diode laser stacked arrays from DILAS reportedly are available in wavelengths ranging from 808 nm to 9xx nm with power ranges up to 300 W QCW per bar with optional fast-axis collimation. The AuSn soldered vertical stacks are designed for operation at high temperatures in applications such as diode pump solid-state laser and defense requiring QCW lasers in the kilowatt power range. The conduction-cooled stacks are available in one-bar, seven-bar, and eight-bar configurations. 

Fiberguide Industries' fiber-optic vacuum feedthroughs are available in unlimited cell, flange, and port designs. The feedthroughs reportedly can be used at temperatures as high as 175 °C (250 °C for pigtail) using the company's gold-coated fiber. Fiber selection includes Multimode, Single Mode, and Polarization Maintaining with core diameters ranging from 50–1500 μm and connector interface selections of SMA (standard), ST, FC, and ferrule style. According to the company, the fibers are helium leak-tested and certified to 10

The TrayCell cuvette from Hellma is designed to be used with a sample volume of 0.7 μL. According to the company, the cuvette uses the surface tension of the sample to form a repeatable optical path of 0.2 mm or 1 mm. The cuvette reportedly can be inserted in any standard spectrophotometer's cuvette holder. The lower part contains fiber optics and mirrors that interface to an upper part that contains collimating optics. 

The HIS Examiner 100 QD hyperspectral imaging system from ChemImage is designed for the examination of questioned documents. The optical-based forensic document imaging system can be used to detect forgeries and alterations, visualize hidden security features, and examine travel documents. The system reportedly is based on liquid crystal filter technology and uses reflected, transmitted, or emitted light from each point in a sample image to create chemical-based contrast within the image. 

Edmund Optics' Biotech Optics catalog includes more than 16,900 products for researchers and developers in biotech research, development, and production. The catalog lists products such as ultrasharp longpass filters for Raman spectroscopy, UV-NIR triplets for two-photon excitation microscopy, and aspherized achromats for retinal photocoagulation. 

Glass Expansion's VeeSpray ceramic nebulizer is designed for use in ICP emission and mass spectrometers. According to the company, the nebulizer has easy-to-change sample and gas lines and can tolerate samples containing particles, high concentrations of dissolved solids, and hydrofluoric acid. 

McPherson's Vacuum UV Analytical Spectrophotometer (VUVAS) system is designed to measure transmission and reflectance from 120 nm to 400 nm to within 0.3% in vacuum or with inert gas purge. Applications include thin-film or material characterization and, with an optional polarizer, the spectrophotometer can check S and P response as well. The instrument reportedly can accommodate samples smaller than 350 mm in diameter in solid, liquid, or gas phase with optional holders or sample chamber configurations. According to the company, the vacuum-compatible system simultaneously collects sample and reference spectra. 

Pyroelectric detectors from Microwatt Applications include detectors for use with FT-IR and with NDIR. The MWA 5500 series detectors, for use with FT-IR, are single element DLATGS detectors with spectral sensitivity ranging from the UV to 1000 μm. The MWA 4000 series detectors, for use with NDIR, are single-element permanently poled lithium tantalate detectors that can be operated from 1 Hz to 100 Hz. 

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