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date; a previously reported value of lo-’‘ M was obtained by extrapolation from higher concentrations ( 2 ) . T h e linear range (on a log-log plot) extends from 2 x M to 2 X M. T h e onset of leveling off can be seen below 2X M; however, significant slope still remains to push the detection limit below lo-’’ M (17). T h e authors believe t h a t the linear range can be extended to include a wider concentration range through knowledge of, and control of, reagent and analyk concentrations and residence times in the reaction zone.
reagents, which respond to different analytes (or categories of analytes). Such an approach would extend CL to simultaneous multicomponent analysis. Finally, pressurizing a microporous membrane system using the energy stored in an inflatable elastomeric reservoir (18) could provide a portable reagent delivery system.
LITERATURE CITED (1) S. Stieg and T. A. Nieman, Anal. Chem., 50, 401 (1978). (2) W. R. Seitz and D. M. Hercules in “Chemiluminescence and Bioluminescence”,M. J. Cormier, D. M. Hercules, and J. Lee, Ed., Plenum Press, New York, 1973, pp 427-49. (3) D. Siawinska and J. Siawinski, Anal. Chem., 47, 2101 (1975). (4) U. Isacsson and G. Wettermark, Anal. Chim. Acta, 83,227 (1976). (5) T. L. Sheehan and D. M. Hercules, Anal. Chem., 49, 446 (1977). (6) S. N. Lowery, P. W. Carr, and W. R. Seitz, Anal. Lett., 10, 931 (1977). (7) 6.M. Strom, M.S. Thesis, Iowa State University, Ames, Iowa, 1974. (8) M. P. Neary, R. Seitz. and D. M. Hercules, Anal. Lett., 7, 583 (1974). (9) R. Delumyea and A. V. Hartkopf, Anal. Chem., 48, 1402 (1976). (10) R. L. Veazey and T. A. Nieman, 29th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1978, paper 99. (1 1) K. S. Subramanian, C. L. Chakrabarti, J. E. Sueiras, and I. S. Maines. Anal. Chem., 50, 444 (1978). (12) S.Stieg and T. A. Nieman, Anal. Chem., 49, 1322 (1977). (13) Poroplastic Data Sheets, Moleculon Research Gorp., Cambridge, Mass. (14) W. R. Seitz. R o c . Conf. Anal. Appl. Biolum. Chemilurn.,Brussels, 1978, to be published. (15) T. G. Burdo and W. R. Seitz, Anal. Chem., 47, 1639 (1975). (16) R. G. Delumyea, Ph.D. Thesis, Wayne State University, Detroit, Mich.,
CONCLUSIONS T h e reported method of reagent addition has been shown to provide the anticipated advantages, with reagent economy and good precision important in general, and localization of emission and high sensitivity of particular utility in CL analysis. T h e membrane system offers certain present advantages and possibilities for future applications t h a t are not readily feasible with conventional use of a peristaltic pump for reagent delivery (although a peristaltic pump certainly has the advantages of wider availability, applicability, and general acceptance). Flow rates through the membrane are below the normal flow rates available with peristaltic pumps. Also, the membrane system is able to allow a reaction to take place under conditions where the “effective” reagent concentration is orders of magnitude higher than in the bulk. Future studies will examine storage and/or immobilization of reagents within the membrane pores. In addition, because of the localization of emission, it should be possible to have a sample stream containing several analytes flow along a row of membrane strips, where each membrane contains different
1974
(17) J,-D. Ingle, Jr., and R. L. Wilson, Anal. Chem., 48, 1641 (1976). (18) Chem. Eng. News, 55, 30 (July 11, 1977).
RECEIVED for review July 28, 19’78. Accepted December 8, 1978. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to Research Corporation for support of this research.
Trace Element Laser Microanalyzer with Freedom from Chemical Matrix Effect H. S. Kwong and R. M. Measures* Institute for Aerospace Studies, University of Toronto, 4925 Dufferin Street, Downsview, Ontario
Canada
Evaluation of trace concentrations of heavy metals in biological, medical, industrial and environmental materials has become a major analytical task ( I ) . The impact of heavy metal contamination on the biosphere is slowly being recognized as a potential health hazard of growing proportions (2). Consequently, there is an increasing demand for a trace element microprobe t h a t can be used to rapidly monitor the concentration of a select group of elements in a wide variety of material samples. T h e ideal characteristics of such a trace element microprobe are: (1) Selective microsampling capability. ( 2 ) No sample preparation-in situ measurement. (3) Real time analysis. (4) High relative and absolute concentration sensitivity. ( 5 ) Freedom from matrix effects (chemical or physical) so that calibration is independent of substrate containing element. (6) Highly selective for element of interest. (7) Linearity of response over wide dynamic range of concentrations. (8) Minimum variation in sensitivity between elements. (9) Depth profiling capability. (10)Simultaneous multielement measurement possible. (11) Capable of distinguishing between isotopes. (12) Insensitive to the nature of substrate material.
A study on the susceptibility of TABLASER-Trace (element) Analyzer Based on Laser Ablation and Selectivity Excited Radiation to various chemical interference effects has been carried out. Tests were made on a diverse range of chromium doped samples to study source interference; anion-anion interference; cation-anion interference, and Ionization lnterference. The observed laser induced fluorescence signal was found to be relatively independent of the source materials used, even under conditions of doping with high concentrations of compounds such as copper sulfate and potassium sulfate. The calibration curves for chromium in three different matrix materials are linear with a 45’ slope. These observations provide evidence that our TABLASER is relatively free from chemical interference effects. The absence of these undesirable effects is attributed to (a) the extremely high temperature of the laser generated plasma (enabling complete dissociation of the compounds) and (b) the subsequent rapid expansion of the laser ablated plasma into the low vacuum region (which minimizes molecular association through interspecies collisions). This new technique has the potential for development into a new form of laser trace element microanalyzer that could be used for in situ high sensitivity measurements. 0003-2700/79/0351-0428$01 O O / O
M3H 5T6,
C
1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
429
Table I. Principal Microtrace Element Analysis Techniques technique
principle
potential and limitation
sensitivity limit
Ion Microprobe Mass Anal zer (1MMA) ( 3 v
high energy ion sputtering of secondary ions for analysis in mass spectrometer
surface, in situ element analysis; sample preparation required; contamination is possible through preparation; experiences severe matrix effects; isotope measurement feasible; operates a t high Torr); all elements are detectable; vacuum simultaneous multielement analysis is, in principle, possible.
sensitivity varies over several orders of magnitude for wide range of elements; background interference limits sensitivity; p p m for most elements.
Emission Laser Microprobe
laser ablation combined with spark excitation, subsequent line emission of specific element monitored
Laser Microprobe Mass Analyzer
laser ablation generating ions for analysis in time of flight mass spectrometer
surface, in situ element analysis; sample preparation not required; contamination is introduced through cross-excitation electrodes; experiences severe matrix effects; isotope measurement not possible; operates in atmospheric condition; all elements are, in principle, detectable; simultaneous multielement analysis is feasible. thin film element analysis; sample preparation required; contamination is introduced via sample preparation; experiences moderate matrix effects; isotope measurement feasible; operates a t high vacuum Torr); all elements are, in principle, detectable; simultaneous multielement analysis is feasible.
sensitivity varies slightly for all elements; background plasma emission limits sensitivity; from 1 0 t o 100 PPm. sensitivity varies over t w o orders of magnitude for most elements; background interference limits sensitivity and accuracy ; ppm range for most elements.
’’
LAMM MA^
.4lthough there are several sensitive trace element microprobes currently available for quantitative element analysis, each has its drawbacks and limitations, see Table I. We intend t o present evidence t h a t the combination of laser ablation and laser selective excitation spectroscopy represents a new form of laser ultra-trace element microprobe t h a t possesses the potential to closely approach the ideal analytical tool outlined above. W e have named our technique TABLASER-an acronym for “trace (element) analyzer based (on) laser ablation (and) selectively excited radiation”. Laser ablation has long been used as a method for microsampling materials ( 4 ) and is the preferred approach used in many microprobe systems in use today ( 5 ) . On the other hand laser selective excitation spectroscopy ( 7 ) (or laser induced resonance fluorescence) can achieve extremely high sensitivity ( 8 ) . In essence, a fan of vaporized target material is created by laser ablation within a vacuum chamber. T h e rapidly expanding vapor cloud is then probed by a pulse of dye laser radiation t h a t is tuned t o selectively saturate one of t h e resonance transitions within t h e element of interest. T h e subsequent enhanced spontaneous emission is monitored and found t o be proportional to the original concentration of t h e element within t h e target material. We have previously demonstrated t h a t this approach can be used to accurately a n d conveniently measure t h e radiative lifetime of atomic transitions (9). In the course of developing this new approach we have discovered that operation with a low background pressure (10-j to Torr) results in a n additional attractive feature; a freedom from chemical matrix effects. In the preliminary work reported herein, our TABLASER was used t o detect trace concentrations of chromium in several diverse materials ranging from metal alloys t o organic substances. Chromium was chosen in this initial investigation for two reasons: its resonance transitions are directly accessible by t h e nitrogen pumped dye laser available in our laboratory and chromium is one of the elements t h a t has recently been recognized t o be of importance ( 1 0 ) .
EXPERIMENTAL M a t e r i a l a n d S a m p l e P r e p a r a t i o n . Stock solution I (K2Cr20;)and stock solution I1 (Cr(hT0J3) were both prepared from reagent grade salts using 10 mg of chromium in 1 mL of
Table 11. Doped Chromium Content in Skim Milk Powder and Flour Samples
sample no.
substrate
skim milk s10 SlOO powder SlOOO skim milk SClOQ powder SClOO SCIOOOU s 1 0 0 . CuS0,b skim milk powder SC 10 0 .K ,S0 4c F10 flour FlOO FlOOO
calculated relative concen. tration, chromium source ppm potassium dichromate, K,Cr,O,
10 100 1000
chromic nitrate,
10 100 1000 99 99 10 100 1000
Cr(NO,),
K,Cr,O. Cr(NO,), Dotassium dichromate, K,Cr,O,
Not used in this measurement. Addition of 1%b y weight of copper sulfate (CuSO,). Addition of 1%by weight of potassium sulfate (K,SO,). solution. Dilutions of 10. 100. 1000 were made from each st.ock solution. Twenty milliliters of each diluted solution was mixed with 20 g (dry weight) of Carnation skim milk powder to generate samples: S10, S100, SlO00 containing K2Cr20iand SC10, SC100, and SCl000 containing Cr(N03)3. Their concentrations are outlined in Table 11. Diluted stock solution I, 20 mL, was also mixed with 20 g (dry weight) of flour powder to generate F10, F100, and F1000. In addition to the above samples, 2 samples of the 20-mL, 100 times diluted stock solution I and I1 were separately mixed with 20 g (dry weight) of Carnation skim milk powder; 200 mg of anhydrous reagent grade copper sulfate (CuSO,) was added to one and 200 mg of potassium sulfate (K2S0,) was added to the other generating two additional samples: S100.CuS04 (containing 100 times more by weight of CuSO, than its chromium content) and SC100.K2S04(containing 100 times more by weight of K2S04than its chromium content). In all cases, the mixtures were thoroughly blended into paste form. The samples were frozen before undergoing the freeze-drying process to avoid foaming. After the drying process. which took about 48 h. the solid samples were broken down and ground into a fine powder in an alumina mortar to ensure homogeneous mixing. Sample powder was then pelleted into small tablets and mounted onto the sample holder and kept dry for later use. In order to
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979 ~
Table 111. Comparison between Values of Chromium Concentrations in Skim Milk Powder Obtained by Calculation and Neutron Activation Analysis
sample no.
calculated relative concentration, ppm'
neutron activation analysis relative concentration, ppm
9.6 * 0.4b 103. t 2b SlOOO 1000. I 2 0 b a Uncertainty is expected t o be less than 1%. Stand. ard deviation. s10
SlOO
10 100 1000
5
I
sp
METASTABLE Sm-rE
,',I
N2 LPSER PUMPED TUNPBLE I1
W E LASERW
Figure 2. Partial energy diagram for chromium(1)
(8)
[email protected] D E M Y TRIGOEF;
m T I O NG+MER
(E)
(C 1
8 (A) L
RUBY LPSER
aa
WOTODETEC-
TION SYSTEM (D I
t DATA ACQUISITION
a DATA
DISPLAY
f '
-
'
SYSTEM ( F )
ensure the accuracy of this sample preparation, samples S10,S100, and S l O O O containing 10, 100, and 1000 ppm of chromium were checked by the neutron activation technique. The result confirmed the calculated value as presented in Table 111. This test, however, did nothing to evaluate the homogeneity of the sample. Steel samples obtained from NBS were also used to test our new technique. These were designated: SRM 665, SRM 664, SRM 663, SRM 662, and SRM 661 and contained 0.007%. 0.06%, 1.31%, 0.3%, and 0.69% of chromium by weight, respectively. Since these standards are designed for microanalysis such as electron and laser probe analysis, we assumed that the homogeneity of these standard samples was sufficient for our measurements ( 1 1 ) . Apparatus. A block diagram of our TABLASER system is presented as Figure 1. A detailed description of this facility is published elsewhere (12). Basically the TABLASER comprises six major components: A Q-switched ruby laser (A) served for ablation and delivered an energy pulse of 10 to 20 mJ at 694.3 nm with a duration of about 30 ns. A nitrogen laser pumped dye laser (B) was employed for the selective excitation of the ablated vapor cloud. The wavelength of the dye laser was tuned to the 428.9 nm ('P2'-'S3) resonance line of chromium (Figure 2 ) . Its peak power was about 1 kW its duration was close to 6 ns, and its spectral line width was better than 0.01 nm. An ablation chamber (C) was used in which the chromium doped samples were mounted onto a sample holder. The pressure within the chamber was kept between and Torr by a diffusion and rotary pump combination. The dye laser axis, the ruby laser axis, and the axis of the receiver optics were mutually orthogonal and intersected 5.5 mm in front of the target surface. Figure 3. A fraction of the laser induced resonance fluorescence emanating from the ablated vapor cloud was captured by a lens and focused onto the entrance slit of a Czerny-Turner Monochromator (SPES ~ 1 7 0 011). The resulting spectrally filtered radiation (2-nm linewidth) was focused onto the photocathode of a RCA-C31034 photomultiplier tube (D). The output of the P M T was subsequently displayed on a Tektronix 7704 oscilloscope (E) where it was photographed. A Tektronix 556 oscilloscope (F) was used for delaying the dye laser pulse to optimize the signal observed.
Figure 3. Relative positions of the observed volume, excitation column, and target sample
RESULTS AND DISCUSSION Ablation of the sample target generates a plume of dense, high temperature plasma that is contaminated with particulate debris. T h e ions within this plasma rapidly recombine as the plume expands and cools. T h e atomic species formed by plasma recombination and the remaining ionic species expand into the low pressure background region faster t h a n the heavier particulates. This separation of the ablated cloud creates a particulate free zone of atomized material suitable for dye laser probing. The Tektronix 556 oscilloscope provided t h e delay trigger necessary t o synchronize t h e arrival of the dye laser pulse with t h a t of this particulate-free window a t the region uf observation. Mie scattering was in this way essentially eliminated. A more detailed discussion of this problem is provided elsewhere ( 1 2 ) . It should be noted that the very short diiratioii of'the dye laser pulse, relative tu the radiative lifetime ut the laser pumped transition. made it possible t o Tely upon temporal discrimitlation t o distinguish between laser scartered sigilais
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
431
SKIM MILK
RELATIVE RLATIVE
CHROMIUM CONCENTRATION
lppm)
Figure 4. Resonance fluorescent signal vs. relative chromium concentration (ppm) for skim milk powder. Error bar indicates standard deviation, typical uncertainty is N 20%
:
/
I I
IO RELATIVE
IO0
1000
CHROMIUM CONCENTRATION
10,000
ippm I
Figure 6. Resonance fluorescent signal vs. relative chromium concentration (ppm) for flour. Error bar indicates standard deviation, typical uncertainty is N 30%
Table IV. Effect on Signal Intensity by Changing the Chemical State of Chromium and Adding Additional Chemical Compounds SC100. s100, SClOO K,SO, SlOO CuSO, signal intensity, 0 . 8 6 i 0.88 i 0.88 i 0.81 i V 0.24 0.18 0.14 0.24 r u b y laser 182 1 7 i 18.3. 16 r energy, mJ 3.6 2.6 4.2 2.4
/
8 1
CHROMIUM CONCENTRATION
100,000
lppm I
Figure 5. Resonance fluorescent signal vs. relative chromium concentration (ppm) for NBS-SRM steel standards Error bar indicates standard deviation, uncertainty ranges between 15% and 30%
and those arising as a result of laser enhanced emission (resonance fluorescence) (12). Laser saturation of the resonance transition was employed (ij, to maximize the enhanced emission signal and (iij to reduce its dependence upon intensity fluctuation of the dye laser pulse (13). A linear relation has been established between the laser induced resonance fluorescent signal and the relative concentration of chromium over several orders of magnitude Figure 4, 5 , and 6. T h e slope of the log-log plot is 45' for each of the three types of sample material studied, i.e., flour, skim milk powder, and NBS SRM steel. T h e typical uncertainty of the signal intensity is around 20% to 3070. For extremely powdery materials such as flour, it tends to be somewhat higher. There are several factors that influence the signal intensity on a shot to shot basis. (i) Piepmeier et al. (14) reported that the amount of material sampled is a function of ruby laser energy. T h e ruby laser output of our system was. unfortunately, somewhat irreproducible. The variation in its energy could be as high as 1 3 0 % over a given experimental run. (ii) There was also a jitter of =k20ns in the delay of firing of the thyratron of the N2 laser. This led to a spread in the time of probing the vapor a t a given location, which in turn created a spread in the amplitude of resonance fluorescence since the density in the vapor cloud is a function of time because of its rapid expansion. (iiij There may also have been some degree of inhomogeneity in the distribution of the doped element. This would also contribute to variations in the observed signal. Inhomogeneity of NBS steel samples has been discussed a t length by Yakowitz et al. (11). (iv) Variation of power density or t h e dye laser may have also contributed to the spread in the observed signal amplitude. However, this should have only
given rise to a second-order effect since the power density of the dye laser was high enough to approach saturation of the atomic transition (15). Chemical matrix effects (16) represent one of the major complicating factors t h a t plague the trace element atomic fluorescent technique. T o evaluate the susceptibility of our TABLASER to this type of matrix effects, several tests were undertaken. These included a comparison of the signal response from several samples with the same concentration of chromium, albeit t h a t the chromium was in two different chemical states Le., Cr3+ and Cr,0i2-. Furthermore, two of the samples were prepared with excess (100:l) amounts of copper sulfate or potassium sulfate to produce the four different skim milk powder samples indicated in Table 11. T h e use of two different chromium salts allows the source interference effect to be evaluated. Potassium and copper were chosen as added cation contaminants t o study the vulnerability of this technique to cation-cation interference. Furthermore, the use of potassium with its lower ionization potential (K, 4.34 eV; Cr, 6.76 eV) should reveal the severity, if any, of the change in the sampling efficiency of the refractory metals due to the change in the plasma temperature as suggested by Ahrens et al. (17). Sulfate and nitrate were chosen as the concomitant anion because of their severe cation-anion interference, reported widely in other trace element techniques (16, 18). In this way, we hoped to determine if the TABLASER was prone to major variations in the chemical composition of the target material. Table IV clearly indicates that there is no significant variation of the induced fluorescent signal, (a) when different chromium compounds were used, (b) when different chemical states of chromium were used, and (cj when 1% by weight of an additional compound, such as copper sulfate or potassium sulfate was added to the sample. The slightly lower signal intensity for S100CuS04 was mainly due to lower ruby laser energy used in t h a t series of measurements. These results suggest the following three major observations: (1) Complete atomization of the ablated material occurs. Consequently, the laser induced fluorescent signal is independent of the chemical state or the form of the chromium while within the target.
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ANALYTICAL CHEMISTRY, VOL. 51, NO 3, MARCH 1979
( 2 ) T h e degree of breakup of the chromium compound and the sampling efficiency of the ablation laser appears to be insensitivity to the presence of other elements having significantly different ionization potentials (concomitant concentrations higher than 1%by weight were not used because this might alter the physical properties of the substrate). (3) T h e added cation and anion did not induce chemical removal of the atomic chromium or introduce spectral interterence to the induced fluorescent signal. Furthermore, the immunity of this technique from cation-cation interference effect is reinforced by the linearity of the log-log plot for the NBS-SRM steel standards where the chemical composition varied over two orders of magnitude for individual impurity elements such as carbon, manganese, silicon, and molybdenum. Since the key to eliminate chemical matrix effects is high temperature (Kirkbright, 1974) (19), it is worth estimating the approximate temperature of the laser generated plasma. T o achieve this, the mean speed of the neutral chromium atoms was monitored and was estimated to be around 5 x lo5 cm s-l which corresponded to 6.5 eV. In fact, since a substantial amount of energy of the plasma was lost in the early phase of expansion through plasma emission, it is reasonable to assume t h a t the initial plasma temperature for the heavy species (such as atoms and ions) was estimated to be in excess of lo5 K. At such high temperatures, molecular association is highly unlikely. Furthermore, the streaming motion of the neutrals and ions (20) drastically reduces interaction between them as they expand and cool thus minimizing molecular asaociation between the neutral chromium and concomitant atoms, that constitute the major ingredients of the matrix materials used in the analysis. The sensitivity of our current facility, albeit rather crude, is in the case of flour around 1 ppm which corresponds to an absolute detection limit of g. The present sensitivity is limited by signal shot noise and stray dye laser light from the reaction chamber. T h e ultimate sensitivity limit of this technique, however, can be improved by several orders of magnitude if the system is carefully optimized. A detailed discussion of the expected limits of sensitivity to be achieved in practice is provided elsewhere (12). I t is worthy pointing out, however, that a relative sensitivity of close to 1 ppb should easily be attained for most elements. At present the spatial resolution is less than 'COO km. This, however, does not reflect the resolution capability of this technique. Spatial resolution down to 1 pm has been achieved by LAMMA (6) using essentially the same sampling mechanism. Monitoring of specific trace elements in water samples is also possible with our TABLASER approach. Sample treatment, in this instance, will be necessary. T h e sample preparation procedure would be similar to t h a t adopted for t h e skim milk and flour reference pellets. T h e carrier substrate should be chosen so a s to minimize the interference with regard to t h e elements of interest. Powdery material (such as flour) should, however, be avoided since we suspect that with such targets the ratio of particulate to atomized vapor
produced varies on a shot t o shot basis. T h e sensitivity for water analysis would be considerably greater than that for a solid sample because of the preconcentration that takes place during sample preparation. In view of the relative freedom of various interference effects associated with our TABLASER, the adoption of a common substrate might enable a single calibration curve t o be used for most elements under investigation. This would make monitoring of trace elements in water samples extremely simple. Extension of our TABLASER to multielement analysis could be achieved in one of two ways. Either one dye laser could be tuned to one resonance line of each element successively or several suitably tuned dye lasers (pumped by a common nitrogen laser) would be used simultaneously. The photodetection system would be correspondingly different in the two cases. T h e range of elements accessible by the TABLASER approach is limited by the availability of suitable laser radiation. In essence, almost all elements, that are likely to be of interest as a trace constituent, can be included if nonlinear optics, such as second harmonic generation, is employed. L I T E R A T U R E CITED Chem. Eng. News, 49, 29 (July 1971). W. R. Wolf, Anal. Chem., 5 0 , 190A (1978). K. F. J. Heinrich and D. E. Newbury, Ed., "Secondary Ion Mass Spectrometry", Natl. Bur. Stand. ( U . S . ) ,Spec. Publ., 427, 191 (1975); T. Hall et al., Eds., "Microprobe Analysis as Applied to Cells and Tissues", Academic Press, London, New York, 1974, p 23. F. Brech and L. Cross, Appl. Spectrosc., 16, 59 (1962); E. F. Runge et al., Spectrochim. Acta, 20, 733 (1964). K. W. Marich et al., J . Phys. E , 7 , 830 (1974). F. Hiilenkamp et al., Appl. Phys., 8 , 341 (1975). S. J. Weeks, H. Haraguchi, and J. D. Winefordner, Anal. Chem., 50, 360 (1978). W. F. Fairbank, Jr., et al., J , Opt. SOC.A m . , 65, 199 (1975). R. M. Measures, N. Drewell, and H. S. Kwong, Phys. Rev. A , 16, 1093 ( 1977). M. S. Black and R. E. Sievers, Anal. Chem., 48, 1872 (1976). H. A. Laitinen, Anal. Chem., 43, 809 (1971). H. Yakowitz et al., "Standard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards 11: Cartridge Brass and Low-Alloy Steel", Natl. Bur. Sfand. (U.S.),Mkc. Pub\. 260-10, (1965); R. F. Michaelis et al. "Standard Reference Materials: Metallographic Characterization of an NBS Spectrometric Low-Alloy Steel Standard", Natl. Bur. Stand. ( U . S . ) , Misc. Publ., 260-3, (1964). R. M. Measures and H. S. Kwong. "TABLASER-Trace (element) Analyser Based on Laser Ablation and Selectively Excited Radiation", Appl. Opt., in press. R. M. Measures, J . Appl. Phys., 39, 5232 (1968); J. W. Daily, Appl. Opt., 17 . . , 335 I197RI \.-. E. H. Piepmeier and D. E. Osten, Appl. Spectrosc.. 25, 642 (1971). J. W. Dailv. ADD/. O D ~ . 16. . 568 (1977). W.G. Sch;enk,'"Analytical Atomic Spectroscopy", Plenum Press, New York and London, 1975. L. A. Ahrens and S. R. Taylor, "Spectrochemical Analysis", 2nd ed., Addison-Wesley Publishing Co., Reading, Mass., 1961, p 119. J. Smeyers-Verbeke, Y. Michotte, and D. L. Massart, Anal. Chem., 5 0 , 10 (1976); E. J. Czobik and J. P. Matousek, Anal. Chem.. 5 0 , 2 (1978). G. F. Kirkbright et al., "Atomic Absorption and Fluorescence Spectroscopy". Academic Press, London, New York, 1974, p 507. H. S. Kwong. "Free expansion of atomic species in vacuum after laser ablation", in preparation.
___
-3.
RECEIVED for review August 11, 1978. Accepted November 6, 1978. T h e support of the National Research Council of Canada and Environment Canada (the Atmospheric Environment Services Branch) is greatly appreciated.
