Laser Solid Sampling for Inductively Coupled Plasma Mass

Laser Solid Sampling for Innductively Coupled Plasma Mas

Eric R. Denoyer and Kenneth J. Fredeen The Perkin-Elmer Corporation 761 Main Ave. Norwalk, CT 06859

James W. Hager Sciex 55 Glen Cameron Rd. Thornhill, Ontario L3T 1P2 Canada

Samples submitted for trace element analysis are rarely in a form suitable for direct introduction into an analytical spectrometer. Generally they are dissolved to form solutions for aspiration into the spectrometer excitation source. In many cases, it would be desirable to analyze solid samples directly, without lengthy sample preparation procedures. Many analytical methods have been developed for direct solids analysis, each with unique strengths and weaknesses. Most recently, laser sampling, used in conjunction with inductively coupled 0003-2700/91 /0363-445A/$02.50/0 © 1991 American Chemical Society

Spectrometry

INSTRUMENTATION plasma mass spectrometry (ICPMS), has been shown to be promising for direct trace element determinations in solids (1-4). In this article we provide an overview of laser sampling and describe its application to ICPMS analysis. After an introduction to t h e experimental and instrumental basis for laser sampling, analytical considerations involving its application to ICPMS analysis are discussed. This is followed by a survey of practical applications of the technique that are representative of the current technology. Finally, current and future research directions of laser sampling are summarized.

Laser sampling Since the early 1960s, high-powered lasers have been used to vaporize solid materials for chemical analysis.

Pulsed lasers can be focused onto a few square micrometers of a solid surface, achieving power densities a s high as 10 12 W/cm 2 (5). Light energy of such high intensity interacts with most any solid material, converting photon energy into thermal (kinetic) energy. This interaction results in the vaporization and removal of material from the exposed solid surface. Early analytical applications of laser sampling involved t h e use of a ruby laser to vaporize a solid specimen. An optical spectrometer w a s used to collect and analyze the light emitted from the laser-induced plasma located just above the sample (6). Although this approach proved useful for certain applications, the technique often suffered from poor sens i t i v i t y , poor precision, a n d considerable matrix effects. These problems stemmed primarily from

ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991 · 445 A

INSTRUMENTATION

Video camera

ICP torch Mass \ ooq_^ spectrometer \ ~—a

HJI

Argon - - — · — —

rf Supply

Figure 1. Schematic of instrumentation used in laser sampling for ICPMS.

the poor and irreproducible excitation c h a r a c t e r i s t i c s of t h e laserinduced plasma. Improvements were made by incorporating secondary spark excitation with carbon electrodes and maximizing optical throughput with the addition of collection mirrors around the sampling stage. Additional improvements in sensitivity were achieved by coupling a time-of-flight mass spectrometer to a laser sampling microscope stage (7). This combination also allowed organic mass spectra to be recorded in addition to inorganic spectra, broadening the applicability of the technique, especially in t h e biomedical area. However, the sampling and ionization processes were still intimately connected and highly interactive, and had to be carried out under vacuum. The use of an ICP in conjunction with laser sampling has the advantage of separating the ionization step from the sampling step. Therefore, both steps can be independently controlled and optimized. Also, because the ICP is an atmospheric pressure source, no vacuum is required for sampling. Figure 1 shows a schematic of the instrumental components used in laser sampling for ICPMS. A high-powered, pulsed laser beam is focused onto a solid sample that is placed in an enclosed sampling cell. The vaporized material resulting from the interaction of the laser beam with the sample is swept with a flow of argon into the plasma of an ICP mass spectrometer. The sample and the ablation event can be viewed safely, in real time, by using a remote electron-

ic video camera and monitor. The Nd:YAG laser has proven extremely useful for laser sampling I C P M S , and is used in most inhouse-built and commercial systems. Nd:YAG laser technology has evolved to the point where rugged, affordable, safe, and reliable systems are commercially available. Most can be operated u n d e r computer control, thus allowing integration into highly automated, easy-to-use laser sampling systems. Pulsed Nd:YAG lasers can be operated in either the single or the continuous pulse mode (10-20 Hz). Outp u t from t h e s e l a s e r s is s t a b l e , typically better than 2% rms, and the beam energy available is more than sufficient to sample most solid materials for ICPMS analysis. In fact, a typical Nd:YAG laser used at high energy (i.e., 500 mj) can easily overload an ICPMS system. Therefore, mass transport efficiency from the sampling cell to the ICP is not usually a signal-limiting factor. Many designs for sampling cells have been described (1-4, 8). Most rely on a flow of argon tangential, orthogonal, or concentric to the vapor plume to sweep vaporized material out of an enclosed sampling cell, through a transfer tube, and into the ICP. Because sample transport efficiencies are < 1.0, some sample redeposition can occur in the sampling cell and the transfer tube. Generally, this does not cause memory effects, except when material is deposited on the cell window through which the laser beam passes. Then, subsequent laser pulses can revaporize material previously deposited on the window,

446 A · ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991

thus interfering with the analysis. Systems designed with removable windows, which can be either replaced or cleaned, avoid this problem. Most laser sampling systems use electronic stepper motors to move the stage on which the sampling cell is mounted. Using a computer for remote control of the stage movement allows the sample to be moved precisely under the laser beam in various well-defined and reproducible p a t t e r n s . Rastering over a line or across a wide area provides an alternative to single spot analysis and is especially useful for analyzing lateral compositional trends and for bulk characterization of inhomogeneous materials. In today's industrial environment, where much of the laser sampling experimental work is being carried out, it is important not to neglect the safety aspects of working with highe n e r g y l a s e r s . Most c o m m e r c i a l Nd:YAG lasers and laser sampling systems are fully interlocked to protect the operator from exposure to laser light. As a result, laser samplers are being installed and used in atomic spectroscopy laboratories without the need for any modification or additional safety equipment beyond that normally used. This will undoubtedly promote the growth and development of the technique. The solid sampling toolbox

Like o t h e r c r a f t s m e n , a n a l y t i c a l chemists have a wide selection of tools a t t h e i r disposal. Although many of us, at one time or another, have probably used a wrench to hammer a nail, no one tool can be expected to perform all possible tasks. Ever since Bunsen and Kirchhoff (9) reported their results, atomic spectroscopists have been developing methods for analyzing solids directly. Today, laser sampling augments a diverse toolbox of solid sampling techniques and brings some unique capabilities into the atomic spectroscopy laboratory. Some of the more common solids analysis techniques are compared in Table I. Figures of merit typically used to judge the effectiveness of an analytical technique provide a useful basis for comparison. However, it should be emphasized that the importance of the strengths and limitations associated w i t h t h e s e techniques depends on the sample to be analyzed and the requirements of the given application. One of the primary benefits associated with laser sampling is that the sample is analyzed directly, often

INSTRUMENTATION Table 1. Comparison of solids analysis techniques3 Technique

Excitation and/or sampling species

Measured species

Detection limit (jig/g)

Probing techniques LS-ICPMS

ΕΡΜΑ

SIMS

Photons

Electrons

Ions

Ions

X-rays

Ions

Some key attributes commonly considered Strengths Limitations High sensitivity Semiquantitative screening Isotopic information

Moderate mapping/profiling Not fully characterized

High spatial resolution Excellent imaging Excellent mapping

Moderate sensitivity Sample charging Sample in vacuum

0.001-1000

High sensitivity Surface information Depth profiling

Variable sensitivity Sample charging Sample in vacuum

1000-10,000

Surface information Depth profiling Excellent mapping Good imaging Chemical speciation

Poor sensitivity Sample charging Sample in vacuum

0.005-0.5

10-300

Electrons

Auger electrons

Photons

Ions

0.2-20

Inorganic and organic information

Poor reproducibility

XRF

X-rays

X-rays

0.1-100

Excellent precision Well characterized

Moderate sensitivity Poor lateral resolution

ESCA

X-rays

Photoelectrons

Chemical information Surface information

Poor sensitivity Sample in vacuum

GDMS

Plasma

Ions

High sensitivity High precision

Conductive samples Complex spectra

Qualitative screening

Quantification Complex spectra Conductive samples

High sensitivity

Complex spectra Conductive samples

Auger electron spectroscopy LAMMA Bulk techniques

Arc/Spark OES SSMS

Plasma

Photons

Spark

Ions

1000-10,000 0.005-0.1

1-100 0.001-0.1

* ΕΡΜΑ: electron probe microanalysis, SIMS: secondary ion mass spectrometry, LAMMA: laser microprobe mass spectrometry, XRF: X-ray fluorescence, ESCA: electron spectroscopy for chemical analysis, GDMS: glow discharç e mass spectrometry, OES: optical emission spectroscopy, SSMS: spark source mass spectrometry.

without the need for any sample preparation. This reduces or eliminates the time associated with this process as well as the chance for sample contamination, and it can translate into a lower cost per analysis. As a result, laser sampling (LS) ICPMS has already found its way into service analytical laboratories. Probing the sample with a laser beam provides a route for studying the lateral distribution of elements in the solid sample. In some cases, depth gradients can be profiled for comparing surface and bulk constituents. The correlation of morphology and composition is a key advantage common to the probing techniques listed in Table I. LS-ICPMS has a lateral sampling spatial resolution on the order of 20-50 pm. This lies between the resolution that can be achieved with electron spectroscopy for chemical analysis (ESCA) (-1000 pm) and t h a t achieved with Auger electron spectroscopy ( 0 . 1 - 1 pm). Sample imaging is similar to optical microscopy—superior to ESCA but inferior to the electron imaging avail-

able with Auger spectroscopy and electron probe microanalysis. Sampling depth resolution is on the order of 1-10 pm per laser pulse; therefore, LS-ICPMS cannot provide the high depth resolution information t h a t is available with surface analysis techniques such as ESCA, secondary ion MS (SIMS), and Auger spectroscopy. Nonetheless, concentration gradients of interest are often on the micrometer scale and would be accessible with LS-ICPMS. However, as with any depth-profiling t e c h n i q u e , considerable a t t e n t i o n should be paid to concentration gradient calibration, which can be difficult to establish. Unlike many other probing techniques, laser sampling uses light r a t h e r t h a n ions or electrons for probing. Charged particles can electrically charge the sample (especially nonconductors), thereby disturbing the sampling event. Consequently, nonconducting samples can be analyzed directly with LS-ICPMS. In fact, most sample materials—whether rough, smooth, t r a n s p a r e n t ,


opaque, or reflective—can be analyzed. Because the sample is held at atmospheric pressure, no vacuum is required and sample changing takes only about 15-30 s. Although the sample craters tend to be very small, LS-ICPMS cannot be considered, in the strict sense, a nondestructive technique. Perhaps the most commonly cited benefit of ICPMS is its high detection sensitivity. This is an advantage for laser sampling. Detection limits for LS-ICPMS generally lie in the 0.0050.5 μg/g range. As an example, detec tion limits for several elements in a glass matrix are given in Table II. This high sensitivity allows charac terization of ultrahigh purity materi als used, for example, in the semicon ductor industry. However, many of the other probing techniques sample a much smaller a r e a (or volume) than is typical with LS-ICPMS, and their detection limits may be better in absolute terms. In practice, both major and tracelevel sample constituents can be of interest analytically. With the ex-

Table II. Typical detection limits for LS-ICPMS3 Element

Mass

Detection limit (ppm)°

Β Co Sr Pb Th U

11 59 88 208 232 238

0.3 0.02 0.006 0.02 0.006 0.006

a

Measured in a soda-lime glass matrix. "3σ.

tended measurement range capabili ties of most commercial ICPMS sys tems, constituents at the percent level can be measured in addition to trace-level species, providing a broader characterization of the sam ple composition. However, unlike ESCA, A u g e r s p e c t r o s c o p y , a n d SIMS, information available with LS-ICPMS is exclusively atomic in nature and no direct chemical speciation is possible. Sensitivity factors for LS-ICPMS are relatively uniform across the pe riodic table when compared with many other techniques (2-4). Fur thermore, LS-ICPMS spectra are rel atively simple, and interelement in terferences are easily identified and corrected as compared with many other techniques. Although spectra can be interpreted manually by the analyst, they lend themselves to rap id, efficient interpretation by a com puter (10, 11). The uniform sensitivi ty, combined with a u t o m a t e d spectral interpretation, opens the door to rapid, surprisingly accurate semiquantitative analyses. Sophisti cated algorithms are currently avail able that can interpret ICPMS spec t r a and provide a full e l e m e n t a l semiquantitative characterization of a sample within minutes. This devel oping area is perhaps one of the most promising for LS-ICPMS. Another exciting attribute of LSICPMS is the fact t h a t isotopic infor mation is inherent in the measure ment process itself. This capability allows the examination of isotopic signatures of constituent elements and can provide valuable insight into their origin and distribution. Isotopic analysis also makes the powerful iso tope tracer and isotope dilution tech niques possible (12). Preparing samples for analysis Most samples, whether they are gels, suspensions, or solids, are normally

prepared for trace element analysis by a combination of acid digestion, ashing, and/or fusion t r e a t m e n t s . The resulting solution, which is as sumed to be representative of the original sample, is then aspirated into the excitation source of the spec trometer. Thus the accuracy of the determination depends not only on the ability to make an accurate mea surement in the sample solution but also on the ability to prepare a solu tion t h a t is truly representative of the original sample. Over the years, numerous sample preparation procedures have been developed and successfully applied to a wide variety of sample types. Nev ertheless, difficulties are sometimes encountered during the dissolution and/or digestion steps, which can re duce the effectiveness of such proce dures. Potential problems include in complete dissolution of the sample, precipitation of insoluble analyte ele ments, loss of volatile analytes dur ing heating, and contamination of the sample during preparation. In many cases, the extra effort re quired to overcome these difficulties can significantly increase sample preparation time. The increased ana lyst time required per determination reduces sample throughput. In prac tical t e r m s , t h i s r e d u c t i o n in t h r o u g h p u t t r a n s l a t e s into a n in creased cost per analysis. All of these potential difficulties are compounded when the list of elements to be deter mined grows and the concentration of the elements to be determined de creases. These are especially impor t a n t considerations for extremely sensitive multielement techniques such as ICPMS. Clearly, avoiding sample handling and preparation by analyzing the sol id directly would be beneficial in t e r m s of reducing or e l i m i n a t i n g sample preparation, and avoiding the chance of contaminating the sample. Furthermore, for those techniques using focused probing beams (see Ta ble I), there is the added benefit of obtaining spatially resolved analyses. With laser sampling, many sam ples can be analyzed directly, with out the need for any preparation. However, this is not the case for every sample type. The extent to which the sample needs preparation for laser sampling depends on the na ture of the sample and the informa tion required. Valuable experience has been gained and reported in the literature for preparing solid samples for other solids analysis techniques, and many of these procedures prove extremely useful and convenient for

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INSTRUMENTATION laser sampling as well. In general, these procedures tend to be less labor intensive than those required for so lution sample preparation. For powdered samples, it can be helpful to stabilize the sample either by compacting it into a pellet, with or without a binding agent, or by fusing it to a borate glass. For samples that are nonhomogeneous, it may be ad visable to grind and mix the sample before stabilizing. In some cases, it is advantageous either to polish or etch the surface of a solid sample, unless correlation of surface topography and composition is important. Small samples, such as pins or crystals, can be imbedded in an epoxy cast and analyzed either di rectly or after polishing the cast sur face. Otherwise, small samples can simply be stuck into a piece of putty or onto a piece of tape or adhesive pad. Because the sampling cell is not in a vacuum, outgassing of the sup port material is not a problem with this approach. Analytical considerations of laser sampling E s t a b l i s h i n g s a m p l i n g strategies. Normally, samples are subjected to trace element analysis in an effort to solve a specific problem. Therefore, the analyst must clearly define and understand the problem so that real istic goals for the analysis can be de lineated. Only then can one establish a useful strategy for analyzing the sample that reflects those goals and best addresses the original problem at hand. As in any analytical task, the scope of the laser sampling task will reflect the need for accuracy, precision, detection sensitivity, ele mental coverage, and spatial distri bution. The success of the analysis will ultimately reflect how well these requirements are met. With laser sampling, several oper ating parameters are at the analyst's disposal to control t h e s a m p l i n g e v e n t . These include l a s e r pulse mode, laser energy, degree of focus, pulse repetition rate, laser exposure duration, sampling pattern, and la ser wavelength. Each provides a cer tain degree of freedom to fine-tune the laser sampling to best meet the analysis goals. H o m o g e n e i t y a n d sampling. If bulk or average composition is to be determined, sampling must be com prehensive. One approach is to sam ple many points on the specimen; an other is to sample across a line or over a broad area of the specimen. Most commercially available laser sampling devices provide the capabil

ity of moving the sample underneath the laser beam during analysis, thereby rastering the beam over a wide sample area. Where depth gra dients occur, a preanalysis exposure of 30-90 s normally can be used to expose the bulk sample beneath the surface, resulting in a steady-state signal. For most m a t e r i a l s , t h e major source of variability in LS-ICPMS analysis is sample heterogeneity. This problem is not unique to LSICPMS but is an important consider ation in all direct solid sampling techniques. Heterogeneity implies the presence of concentration gradi ents within the analytical specimen that can occur along the surface or as a function of depth in the sample. However, identification and charac terization of lateral or depth concen tration gradients can, in fact, provide valuable information about the na ture of the sample. This is a powerful capability of LS-ICPMS and has di rect application to the study of com posite natural and manufactured ma terials. L a s e r p u l s e m o d e . Stimulated emission is the inverse process of light absorption. Both processes de pend directly on the incident flux of resonant radiation, on the relative populations of the two energy levels involved, and on the transition cross section. The absorption process in volves a depletion of the incident ra diation by inducing a transition from the lower to the higher of the two energy levels involved. Stimulated emission, on the other hand, results in the amplification of the incident radiation by inducing a transition from the higher to the lower level, re sulting in the emission of a resonant photon. Absorption is more likely when the population of the lower state exceeds that of the higher state (as is usually the case in nature), whereas stimulated emission is the more likely process when the popula tion of the higher state exceeds that of the lower state. Lasing is achieved by affecting a population inversion in an active me dium, usually by exciting the active species (Nd 3+ in the case of the Nd: YAG laser) with a light source such as a xenon flashlamp. Under inver sion conditions a single resonant pho ton can produce a rapid cascade from the upper to lower energy states, re sulting in the emission of an intense pulse of light from the medium. For laser sampling with a Nd:YAG laser, it is important that the laser be capable of Q-switched operation and of achieving a repetition rate of


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Figure 2. Photomicrographs of laser-induced craters for (left) free-running and (right) Q-switched laser pulses (Nd:YAG laser, 1064 nm). > 1 pulse/s to establish steady-state analytical signals. In Q-switching, the efficiency or quality factor, Q, of the laser cavity is initially main tained at a very low level for typical ly a few hundred microseconds. This inhibits laser amplification, allowing the population inversion of the Nd ! + to be maximized. At this point the ef ficiency of the cavity is "switched" from low to high, thereby tapping the maximum energy available in the la ser medium in a very short period of time. The result is a much shorter la ser pulse, on the order of 10 ns, con taining nearly all of the energy. In t e r m s of o u t p u t power delivered within the laser pulse, an increase of approximately 10 4 is achieved by Q-switching (13). In the absence of Q-switching, oth erwise referred to as "free-running" or "fixed Q" operation, the envelope of the laser pulse is about 120-150 μβ in duration. Detailed examination of this long pulse has shown the pres ence of an i r r e g u l a r sequence of sharp, narrow pulses, each less than a microsecond wide, that dampen out with time {13). Because the energy contained in the pulse is delivered over a much longer time period than that produced by Q-switched opera tion, the power is considerably less. These two pulse modes of a Nd: YAG laser can lead to significantly different analytical outcomes. The physical characteristics of the sample "crater," or spot, of Q-switched and free-running lasers are significantly different. C o n s e q u e n t l y , t h e two pulse modes can be used to achieve very different sampling objectives.

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Both modes can be executed as ei ther a single pulse or a series of pulses. A single laser pulse provides a transient signal typically having a full width at half maximum (fwhm) of - 5 - 1 0 s (i, 2), although shorter times are possible (8). On the other hand, multiple pulsing can be used to produce a steady-state signal analo gous to that obtained with continu ous solution aspiration (2, 3). The pulse mode and the repetition rate both affect not only the intensity and duration of the analytical signal, but also the size and shape of the sample crater. A single free-running pulse produces a deep, narrow crater, whereas the single Q-switched pulse creates a wider, more shallow crater. Photomicrographs of such craters are shown in Figure 2. Several factors may contribute to t h e differences observed. In t h e Q-switched mode, the intense plasma generated early in the pulse (1-2 ns) can absorb some of the laser energy itself. This laser-induced p l a s m a then transfers energy to the sample (5). As a result of this secondary en ergy transfer, the dimensions of the sample crater are determined to a larger extent by the dimensions of the plasma rather than by the diam eter of the focused laser beam. Thus effective spot size may be several times greater than the diameter of the laser beam itself. In the free-running mode, little or no plasma is generated (5). The freerunning laser pulse is, in fact, an en velope of many pulses interacting with the sample for a much longer time than the Q-switched pulse.

140

105

1 4

» °

ce

^-~zz.'~ -,—.

100

200

i io3 ο

/

_*

Intensity

8. 102

0.1 0

300

400

Laser pulse energy (mj)

Figure 3. Dependence of LS-ICPMS signal of Ce and Zr in steel on laser energy (Nd:YAG laser, 1064 nm).

δ

10 II

X

Intens ty jverage integrated counts/s

Thus, energy can be conducted more deeply into the sample. As a result, the free-running pulse mode can provide a higher degree of microsampling for characterizing dis crete spots within a specimen, where as the Q-switched pulse mode can provide a wider, more representative s a m p l i n g of t h e b u l k s p e c i m e n . Therefore, the free-running mode may be better for discrete microsam pling of inclusions and localized fea tures, but Q-switching is generally better suited for larger area or bulk sampling. Another important difference be tween the two pulse modes involves vaporization efficiency. This relates the relative concentration of the analyte element present in the sample to that ablated from the sample. Vapor ization efficiencies of the analyte ele m e n t s in t h e f r e e - r u n n i n g pulse mode depend on each analyte's heat of vaporization to a greater degree than with the Q-switched pulse mode (3). This dependence results in a se lective fractionation of more volatile species in the vapor phase compared with the solid phase and has a direct effect on the relative response factors observed in the analytical measure ment. R e s p o n s e factors for e l e m e n t s across the periodic table are found to be more uniform for Q-switched oper ation than for free-running operation (3, 14, 15). This results in a smoother response function measured at the instrument detector and greatly fa cilitates semiquantitative analysis. Most LS-ICPMS analyses are car ried out in the Q-switched mode. However, there is currently consider able research activity in investigat ing the mechanisms of laser-solid in teractions and their implications for trace metals analysis (16-18). These

8

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6 4

-

2

^^ 0 | r 10

• · •

• 20 ·

30

40

50

60

70

80

Crater radius (μηι)

Figure 4. Dependence of ICPMS ion signal on laser crater radius (Co in NIST SRM 661 AISI 4340 steel; Nd:YAG laser, 1064 nm, free-running pulse mode). efforts are directed not only at deci phering the effect of parameters such as laser wavelength, but also at de termining the mechanisms (e.g., va porization and ablation) involved in laser sampling. A better understand ing of these phenomena will provide direction for improved laser sampling methods development and advances in instrumentation. L a s e r e n e r g y . Laser energy is also an important parameter control ling the sampling event and is typi cally reported along with experimen tal data. Each pulse mode can be operated within a wide range of out put energies. Therefore, sampling c h a r a c t e r i s t i c s of t h e two p u l s e modes, such as crater depth, lateral width, and volume sampled, can be controlled. This flexibility provides a broad range of sampling conditions for each mode. Laser energy can be easily calibrated using a disk calo rimeter. Two means are commonly used to control laser output energy. The first method controls the energy delivered (voltage applied) to the laser flashlamps. This control can be varied c o n t i n u o u s l y over t h e o p e r a t i n g range of the laser. However, because there is a threshold energy required for lasing, a lower limit of about 2 0 25 mJ/pulse is set on the energy that can be used with this method. The second method, available only in the Q-switching mode, is to control the delay between flashlamp pulse i n i t i a t i o n a n d a c t i v a t i o n of t h e Q-switch gate. A delay corresponding to the time to reach maximum inver sion provides the m a x i m u m laser output, whereas other values result

in less energy transmitted from the laser cavity. In fact, with very short or very long delays, none of the laser energy produced is transmitted out of the cavity. With this approach, ener gy emitted from the laser can be con trolled by passing only a selected por tion of the pulse. Most commercial Nd:YAG lasers allow Q-switch delays ranging from 100 to 600 μβ to be used in combination with any flashlamp voltage. This provides continuous control of laser energy from zero to the laser maximum output. Energies normally used in laser sampling range between 10 and 500 mJ/pulse. As shown in Figure 3, the analytical signal depends on the la ser e n e r g y u s e d , b u t a l i m i t is reached with the Nd:YAG laser at about 200 m j where increasing laser energy provides little or no gain in signal (14, 16). There are several pos sible reasons for this. The laser plas ma produced may set a limit on the amount of energy t h a t can be trans ferred to the sample, especially at the very high peak powers found in the Q-switched pulse (5). In addition, nonspectroscopic matrix effects re sulting from sample loading of the ICPMS interface and ion optics can limit the amount of sample that can be introduced into the ICP mass spectrometer without interference (19), although this effect is not likely to be the only factor producing the signal-energy plateau observed. Fi nally, the plateau observed at higher laser energies may reflect changes in particle size distribution with laser energy, which can affect particle transport and excitation efficiencies (4).


INSTRUMENTATION There are other reasons that favor the use of laser energies only high enough to provide the sensitivity re quired for the analysis. High laser energies can, in some cases, damage the sample. High-purity quartz and lithium borate fusion disks, for exam ple, can shatter under high (> 300 mJ) laser energies, although this problem can sometimes be reduced by annealing. Another reason for op erating the laser at lower energies is to reduce the sample loading on the ICPMS system. Besides the nonspectroscopic interferences arising from space charge effects (19), the sam pling cones of the ICPMS interface can become clogged as a result of sample overloading. S a m p l i n g c r a t e r size a n d i o n signal. Laser energy also affects the size of the crater sampled. However, the analytical signal is not a linear function of the radius of the sampled crater. In fact, in the free-running mode, the analytical signal can be fit to a cubic function of the crater radi us, reflecting the dependence on the volume sampled (Figure 4). In the Q-switched mode, the analytical sig nal follows more closely the square of the crater radius, reflecting a depen dence on area r a t h e r t h a n volume (20). This is consistent with the fact that single-pulse Q-switched craters are wide and shallow, whereas sin gle-pulse free-running craters are deeper and more hemispherical in nature (Figure 2). In the case of repetitive continuous pulsing, craters resulting from either mode can be made to be high-aspect ratio holes with rounded bottoms (16, 21). However, on a per-pulse compar ison, considering comparable crater diameters, the free-running pulse will remove a much larger mass of sample t h a n will t h e Q-switched pulse. Therefore, ICPMS interface overloading can sometimes be a big ger problem with free-running than with Q-switched operation. The effective size of the spot sam pled depends not only on the pulse mode and beam energy, but also on the degree of focus and thermal prop erties of the sample material. In gen eral, spot sizes on the order of 2 0 30 μπι are the minimum possible un der normal conditions, and spot sizes of 100-500 pm are often used in situ ations where lateral spatial resolu tion is not critical. Defocusing the laser beam is often used to reduce the energy density of the beam impinging on the sample, and it provides a more representative sampling of the bulk sample. In fact, the laser focusing lens can be re-

\z) Laser exposure (50 pul: 0 Transient

Continuous 10-Hz pulsing

Single pulse

ί

Time

Figure 5. Comparison of signal profiles for various pulsing strategies for LS-ICPMS (Nd:YAG laser, 1064 nm).

moved altogether to achieve sam pling spots that are the full diameter of the laser beam, approximately 6 mm in diameter. P u l s i n g strategies. Another ap proach to controlling the sensitivity, lateral sampling resolution, and ICPMS interface loading is to select a discrete number of laser pulses for ir radiating the sample, r a t h e r t h a n continuously pulsing the laser (e.g., 10 Hz). A comparison of the ICPMS signals for discrete and continuous pulsing is shown in Figure 5. With discrete sampling, spot sizes are generally smaller, and the duty cycle of the ICPMS interface is re duced over that found for continuous sampling. However, a single pulse of the laser produces a narrow tran sient signal of relatively low intensi ty compared with t h a t obtained by continuous pulsing. Therefore, it is often advantageous to use 50-100 pulses per irradiation rather than a single laser pulse. This way, the ad vantages of smaller sample spots and r e d u c e d s a m p l e l o a d i n g on t h e ICPMS can still be realized, but the signal intensity is increased by sever al orders of magnitude over singlepulse sampling. Sample homogeneity and preci sion. Measurement precision achieved for homogeneous materials is typically in the range of 2-10% RSD. Ultimate ly, reproducibility of the analysis will depend on variability resulting from analytical imprecision and sample heterogeneity. But the two sources of variability must be clearly distin


guished. Although the analytical pre cision will be affected by ion counting precision as well as the stability of sample transport and ionization, one of the most important sources of ana lytical variability is the laser sampling process. Because laser sampling is a dynamic process, the specimen being sampled is constantly changing with time. This can be a small effect for many materials, but it can be signifi cant if the sample removal rate is very high (as is the case with plastic and polymeric materials). Internal standardization is a pow erful technique for improving preci sion and accuracy, and its use is recommended wherever possible. However, it may be difficult to iden tify an element in the specimen that is appropriate as an internal stan dard and that exists in the sample and standards alike. Where the sam ple is ground, mixed, and pelletized, an internal standard of choice can be added to the sample. Otherwise, the matrix element can, in some cases, be used as an internal standard—es pecially with discrete, or pulsed, sampling. The distribution of the in ternal standard element in the sam ple must be the same as that of the analyte element. The internal stan dard should also be measured at the same time (i.e., same laser transient signal) and by the same detection system to achieve the truest internal standardization possible as well as the best improvements in precision and accuracy. Several alternative approaches for

Table III. LS-ICPMS quantitative analysis of NIST 612 glass using matrix-matched standards3 Element

Measured (μ9'9)

Certificate (μ9'9)

Β Co

25.8 ±1.9 33.8 + 1.1

(32) 35.5 ±1.2

Sr Ce Nd Sm Eu Gd Dy Pb Th U

74.1 ±3.3 35.4 ±1.7 33.3 ±1.5 35.8 ± 1.3 33.4 ±1.4 37.0 ± 2 33.1 ±1.2 42.8 ±1.1 36.9 ±2.1 31.8 ±4.1

78.4 ±0.2 (39) (36) (39) (36) (39) (35) 38.57 ±0.2 37.79 ± 0.08 37.38 ±0.08

"Values in parentheses are informational values only. Errors given for measured re sults are the standard deviation; errors given for certificate values are either the full range of values found or the standard deviation, whichever is larger.

signal normalization have been pro posed for laser sampling. One ap proach involves measurement of the acoustic signal in the sampling cell generated by the laser pulse (22). An other involves measuring the degree of light scattered by the ablated ma terial in the sampling cell (23). These techniques provide a measurement of t h e a m o u n t of m a t e r i a l a b l a t e d , which is independent of the ICPMS measurement. Although an indepen dent measurement can be advanta geous, both these approaches depend on the physical properties of the ab lated material, such as particle size distribution and expansion distance (24). Because the physical properties of the ablated sample plume can be affected by the sample matrix type, these normalization approaches may be vulnerable to matrix-related ef fects. Although further development may be necessary, these approaches offer promise for improved precision in laser sampling. Applications Gray (1) first reported the develop ment and application of a laser sam pling system for ICPMS analysis of geological materials. His paper clear ly demonstrated the feasibility of la ser sampling for ICPMS. Because a low-repetition-rate (1 Hz) ruby laser was used, most of the results report ed were limited to discrete single pulses. Arrowsmith (2) reported the first application of LS-ICPMS using a high-repetition-rate (10 Hz) laser, which opened the door to practical

steady-state signal measurement. A significant improvement in reproduc ibility and precision was achieved, providing improved calibration and internal standardization for trace el ements in steel and copper. LS-ICPMS can be used for quanti tative analysis and for semiquantita tive screening applications. The accu r a c y t h a t c a n b e a c h i e v e d in quantitation depends, to a large ex tent, on the availability of calibration standards and on the ability to per form internal standardization. Cali bration curves can be established for either continuous or discrete pulse sampling. Where good calibration standards are available, accuracy is often limited only by measurement precision and sample homogeneity, and is typically within 5-20%. For example, Table III shows results for quantitative analysis of NIST 612 glass using m a t r i x - m a t c h e d stan dards to establish calibration. On av erage, results are within 7% of the known value, and all results fall within 15% of the known value. Gray (25) performed quantitative determi nation of trace elements in leaf mate rials prepared for analysis by p e p t i zation. Results for the analysis of NIST SRM 1573 tomato leaves and SRM 1571 orchard leaves were with in 20% for most elements analyzed. Although it is always desirable to match the matrix of the standards and samples, it is also possible to use n o n m a t r i x - m a t c h e d s t a n d a r d s for calibration. For example, Hager (14) studied the need for matrix matching for quantification. He found that low er laser energy, with the attendant reduction of the ICPMS plasma and interface loading, results in reduced dependence on m a t r i x m a t c h i n g . These findings are consistent with those of Gillson et al. (19), who corre lated matrix effects in ICPMS with sample loading and the high ion cur rents that result. Figure 6 shows re sults from our laboratories of the quantitative determination of trace elements in steel using glass stan dards for calibration. Similar results were obtained for the analysis of lith ium borate fusions of uranium oxide using glass standards for calibration and of nickel alloys using steel stan dards. One of the most powerful capabili ties of LS-ICPMS is the so-called semiquantitative analysis. This is d i s t i n g u i s h e d from q u a n t i t a t i v e analysis where calibration curves are established using standards contain ing the specific analyte elements to be quantified. In semiquantitative analysis, instrument response factors

(calibration slopes) for all elements are stored on the computer disk. Be cause these response factors are rela tive factors, they can be u p d a t e d (resloped) using surrogate elements (i.e., elements other than those to be determined). Typically, one to four el ements provide sufficient calibration information for all remaining deter minable elements in the periodic ta ble. Often, the elements used to up date the instrument response function are contained in or are spiked into the sample itself. Normally, the total ICP mass spec trum (4-240 amu) is measured for semiquantitative analysis. This re sults in a spectral fingerprint that quickly provides a snapshot of the sample of unknown composition. Re sults for semiquantitative analysis using a single internal standard for calibration are typically within a fac tor of 2 of the true value, but accura cy can usually be improved by updat ing the instrument response factors using additional elements. Broadhead et al. (26) and Hager in our lab oratory i n d e p e n d e n t l y performed s e m i q u a n t i t a t i v e analyses on the same USGS GXR-6 reference soil, up-

^ 105 § 10" EÏ103 i f 102 Φ

5 5

c io Φ

I 1

8 0.1 1 1 10 102 ί ο 3 104 10s 0 cCertified concentration ^g/g)

Figure 6. LS-ICPMS quantitative analysis of steel (NIST low-alloy SRM 661) using glass (NIST SRM 612) calibration standards (Nd:YAG laser, 1064 nm).

0.01 0.1 1 10 102 104 Reference concentration (μρ/ς)

Figure 7. LS-ICPMS semiquantitative analysis of USGS GXR-6 soil reference material (Nd:YAG laser, 1064 nm).


INSTRUMENTATION

6

10

ο ϋ

Cd Cu Pb Mn Ni ΤΙ Th U Zn Fe Sb As Co La Sc Η Certified • Found Figure 8. LS-ICPMS semiquantitative analysis of NIST SRM 1645 river sediment (Nd:YAG laser, 1064 nm).

d a t i n g t h e i r i n s t r u m e n t response functions with three response ele ments. Results were typically within 30% of t h e reference v a l u e s , as shown in Figure 7. Gray (25) analyzed tomato leaves semiquantitatively for 17 elements, and most results were well within a factor of 2 of the known values. Denoyer (27) analyzed a variety of pow dered materials by pelletizing with a polymeric binder. Elements in ma rine, river, and estuarine sediments; plant leaf materials; urban particu lates; and various soils and ores were determined semiquantitatively using a single element for internal stan dardization. As seen in Figure 8, re sults for NIST SRM 1645 river sedi ment were generally within 30% of the certificate values for elements covering 5 orders of magnitude in concentration. The ability to measure isotope ra tios by ICPMS opens the door to iso topic studies by LS-ICPMS. For ex ample, three of the four isotopes of lead result from the radiochemical decay of thorium ( 208 Pb) and urani um ( 206 Pb and 2 0 7 Pb); the fourth iso tope ( 204 Pb) is nonradiogenic. As a re sult, the relative abundance of the radiogenic isotopes compared with that of the 2 0 4 Pb isotope depends, to a large degree, on the nature of the ore body from which the lead was de rived. Lead extracted from different ore bodies can show significantly dif ferent isotopic signatures. Study of the lead isotope ratios can provide in formation about the chemical envi ronment and source of lead materi als, which is of great importance in areas such as toxicology, contamina tion control, geochemistry, and min

eral exploration. In Figure 9, the LS-ICPMS spec trum of a lead impurity found in a h i g h - p u r i t y zirconia is compared with that found in a low-alloy steel. Note that the two isotopic signatures are very different. That of the zirco nia is highly enriched in the 2 0 6 Pb, 207 Pb, and 2 0 8 Pb isotopes. Virtually all of the lead impurity in the zirco nia sample arises from the radio chemical decay of uranium and thori um. In fact, these two unexpected impurities were identified in the zir conia sample by s e m i q u a n t i t a t i v e analysis. Tye et al. (28) also used LSICPMS to identify radiogenic lead in gypsum. As we mentioned in the beginning of this article, the ability to correlate morphology with composition (i.e., to analyze a discrete feature or to study compositional v a r i a t i o n s across a phase, defect, or zone) holds great po tential for LS-ICPMS. For example, Denoyer and Wallace (29) reported the LS-ICPMS analysis of impurity inclu sions in high-purity quartz used for the semiconductor industry. Black in clusions, originally thought to be car bonaceous in nature, were shown in stead to be composed of Fe, Al, Si, Mn, and Ni, elements typically found in ores from which the silica is derived. The impurity inclusions were also shown to contain elevated levels of thorium and uranium compared with the quartz. This discovery was impor tant because thorium and uranium, which are alpha emitters, are known to degrade the performance of semi conductor materials in which they are found at elevated levels. A most interesting example of LSICPMS microsampling was present


ed by Batel (21), who studied the spa tial distribution of metals in Pacific manganese nodules. Cross sections of the nodules were mapped with the laser beam, revealing preferential concentration of the rare earth and platinum group elements at the core/ shell interface of the nodule. Jackson et al. (30) studied crystal line silicate and sulfide ores by LSICPMS. Using a custom-built laser microscope stage, they were able to record high-quality pétrographie images of 30-μιη-ίηίη sections in both reflected and transmitted light, and sampling spot sizes of 25-30 μπι in diameter were achieved. Elements in discrete phases of the crystals were quantified using glass and synthetic sulfide standards. The typical detec tion l i m i t s of 0.1 ppm r o u t i n e l y achieved were especially of interest because the discrete spots sampled were so small. Future directions LS-ICPMS complements a diverse toolbox of techniques available to the analytical chemist. Its combination of key attributes, which sets it apart from other techniques, includes its ability to provide isotopic informa tion, its high sensitivity, and the po tential for rapid semiquantitative analysis of materials of completely unknown composition. One advan tage is that it is an accessory tech nique of ICPMS. Thus, the analyst is not limited to the laser alone as a means of sample introduction but

Figure 9. Comparison of the LS-ICPMS spectra of (a) enriched and (b) natural lead.

can correlate information with other sampling approaches. The development of procedures for calibration, along with a fundamental understanding of the processes in volved in the laser sampling step, will undoubtedly promote applications of the technique beyond those currently reported. The availability of well-char acterized, certified reference materials is clearly lacking, especially for the low levels typically encountered in LSICPMS. New laser sources that are less expensive and smaller would cer tainly be desirable, especially those with selectable wavelengths. To be widely applicable, those lasers not only must be rugged and reliable, but must also be able to be operated safely and routinely. Finally, based on work in progress in our laboratories, we believe that with the large amount of information available in a single LS-ICPMS spec trum, data handling and interpreta tion represent one of the most exciting growth areas for the technique. Spec tral interpretation and semiquantita tive a n a l y s i s p r o g r a m s c u r r e n t l y available leverage this spectral infor mation content, but progress in this area must still be made. Besides the obvious extension of semiquantitative procedures, the invocation of multi variate analysis techniques using the information-rich spectra available with LS-ICPMS represents an excit ing opportunity for a significant con tribution to the field.

ence and Exposition on Analytical Chemistry and Applied Spectroscopy, New York, NY; Abstract 85. (12) Janghorbani, M.; Ting, B.T.G.; Lynch, Ν. Ε. Mikrochim. Acta (Vienna) 1989, ///, 315-28. (13) Siegman, A. E. leasers; University Sci ences Books: Mill Valley, CA, 1986; Chapter 26. (14) Hager, J. W. Abstracts of Papers, 1990 Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, New York, NY; Abstract 342. (15) Arrowsmith, P.; Wade, C. Presented at the 17th Annual Meeting of the Fed eration of Analytical Chemistry and Spectroscopy Societies, Cleveland, OH, 1990; Paper 297. (16) Briand, Α.; Mauchian, P.; Mermet, J. M. Presented at the 17th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Cleveland, OH, 1990; Paper 289. (17) Russo, R. E.; Chan, W. T. Presented at the 17th Annual Meeting of the Fed eration of Analytical Chemistry and Spectroscopy Societies, Cleveland, OH, 1990' Paper 290 (18) Pang, H. M.; Yeung, E. S. Appl. Spec trosc. 1990, 44, 871-75. (19) Gillson, G. R.; Douglas, D. J.; Fulford, J. E.; Halligan, K. W.; Tanner, S. D. Anal. Chem. 1988, 60, 1472-74. (20) Hager, J. W. Presented at the 17th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Cleveland, OH, 1990; Paper 294. (21) Batel, A. Presented at the Second In

ternational Conference on Plasma Source Mass Spectrometry, University of Dur ham, September 1990; Keynote lecture. (22) Houk, R. S. Presented at the Second International Conference on Plasma Source Mass Spectrometry, University of Durham, September 1990; Plenary lecture. (23) Richner, P.; Borer, M. W.; Brushwyler, K. R.; Hieftje, G. M. Appl. Spec trosc. 1990, 44, 1290-96. (24) Moenke-Blankenburg, L.; Gackle, M.; Gunther, D. Presented at the 1990 Win ter Conference on Plasma Spectrochem istry, St. Petersburg, FL, January 1990; Paper M12. (25) Gray, A. L. Presented at the 1990 Winter Conference on Plasma Spectro chemistry, St. Petersburg, FL, January 1990. Laser ablation short course. (26) Broadhead, M.; Broadhead, R.; Hager, J. W. At. Spectrosc. 1990, 11, 205. (27) Denoyer, E. R. Presented at the 17th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Cleveland, OH, 1990; Paper 442. (28) Tye, C. T ; Henry, R.; Abell, I. D.; Gregson, D. Research and Development, April 1989, 76-79. (29) Denoyer, E. R.; Wallace, G. Presented at the 16th Annual Meeting of the Fed eration of Analytical Chemistry and Spectroscopy Societies, Chicago, IL, 1989; Paper 742. (30) Jackson, S. E.; Longerich, H. P.; Fry er, B. J. Presented at the 17th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Cleveland, OH, 1990; Paper 296.

The authors thank P. Krause, K. Naumann, and W. Dannecker of the University of Hamburg, Germany, for the photomicrographs of the laser craters. References (1) Gray, A. L. Analyst 1985, 110, 551-56. (2) Arrowsmith, P. A. Anal. Cltem. 1987, 59, 1437-44. (3) Hager, J. W. Anal. Chem. 1989, 61, 1243-48. (4) Arrowsmith, P. A. In Lasers and Mass Spectrometry; Lubman, D. M., Ed.; Oxford University Press: New York, 1990. (5) Ready, J. F. Effects of High Power leaser Radiation; Academic Press: New York, 1972; Chapters 3-4. (6) Moenke-Blankenburg, L. Laser Mi croanalysis; Wiley: New York, 1989. (7) Denoyer, E. R.; Van Grieken, R.; Adams, F.; Natusch, D.F.S. Anal. Chem. 1982 54 26 A (8) Arrowsmith, P. Α.; Hughes, S. K. Appl. Spectrosc. 1988, 42, 1231-39. (9) Bunsen, R.; Kirchhoff, G. Ann. Chim. Phys. 1861, 62. (10) Ekimoff, D.; Van Nostrand, A. M.; Mowers, D. A. Appl. Spectrosc. 1989, 43, 1252—57 (11) Polk, D.; Zarycky, J.; Ediger, R. Ab stracts of Papers, 1990 Pittsburgh Confer

EricR. Denoyer (left) received his B.S. degree in chemistry from the University of Massa chusetts (1977) and his Ph.D. from Colorado State University (1982). After a year at the MS laboratory of the University of Antwerp, he joined American Cyanamid's Stamford, CT, research laboratory where he worked in atomic spectroscopy and electronics chemi cals. In 1987 he joined Perkin Elmer's product department, where he has been develop ing instrumentation and applications oflCPMS, laser sampling, and most recently, flow injection analysis. Kenneth J. Fredeen (center) received his B.A. degree in chemistry from Theil College (Greenville, PA) in 1980 and his Ph.D. from Texas A&M University in 1985. Since then, he has worked in the ICP business unit at Perkin Elmer. He is currently product champi on for laser sampling at PE, where he heads the laser sampling development laboratory. His research interests include solid sampling devices for atomic spectroscopy, plasma di agnostics, and the use of artificial intelligence in analytical chemistry. James W. Hager (right) is a senior research scientist at Sciex. He received his B.S. degree from the University of Colorado at Boulder (1980) and his Ph.D. in physical chemistry from the University of Toronto (1985). His research interests include laser vaporization and ionization techniques, plasma source MS, and MS of molecules of biological interest. ANALYTICAL CHEMISTRY, VOL 63, NO. 8, APRIL 15, 1991 · 457 A

Laser Solid Sampling for Inductively Coupled Plasma Mass

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