description and characterization of a linear-flow outer gas flow torch for inductively coupled...

Description and Characterization of a Linear-Flow Outer Gas Flow Torch for Inductively Coupled Plasma Emission Spectroscopy

G A R Y D. R A Y S O N * and D A N I E L Y A N G S H E N t Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003

An inductively coupled plasma torch has been developed which utilizes linear coolant gas flows and is operated at reduced applied power levels and argon gas consumption rates. The linear flow torch (LIFT) is con- structed by the addition of a machined insert between the outer and intermediate tubing of a conventional tangential flow torch (TAFT). This LiFT configuration has been demonstrated to be capable of providing improved detection limits, in comparison to a TAFT, at conditions of lower power and gas flow. Under those conditions the LiFT was also demonstrated to produce less severe interferences to analyte emission in the presence of an easily ionizable element.

Index Headings: ICP torch; Emission spectroscopy; ICP.

I N T R O D U C T I O N

Many efforts have been undertaken to improve the economy of operation and analytical performance of in- ductively coupled plasma (ICP) torches operated at lower argon gas consumption rates and lower applied radio-frequency (rf) powers. These efforts have included reduction of the overall dimensions of the torches, ~ mod- ification of current torch dimensions, 1-3 enhancement of the torch cooling efficiency, 4,5 and use of alternative cool- ant media (e.g., air, water, or radiative cooling). 6-s

Typically, ICP torches have incorporated tangential flows for vortex stabilization of the discharge. Optimi- zation studies have indicated the constriction of the inner diameter of the gas inlet tubes of the torch to be a de- sirable feature in the construction of a low-power, low- flow torch. ~,9,1° Constriction of the inlet tubing was pro- posed to result in a higher gas velocity of the gas vortex used to stabilize the plasma within the torch. Davies and Snook 1~,12 have described a demountable "laminar flow torch" (LaFT) design, which was reported to demon- strate increased linear dynamic range and a reduction in the measured noise by incorporating laminar flow intro- duction of the coolant gas at the base of the torch. The demountable LaFT designed by Davies and Snook had 21 holes, 2 mm in diameter, around the torch base be- tween the outer and intermediate tubes, in an at tempt to convert the tangential coolant gas flow into a laminar flow near the point of coolant gas introduction. A 10-fold improvement in the detection limits was reported in their studies. However, Montaser e t al. 13 have not been suc- cessful in duplicating these results.

Winge e t al. ~4 and Montaser e t al. 13 have described extensive noise studies involving laminar flow gas intro-

Received 16 March 1992. * Author to whom correspondence should be sent. t Present address: Certified Alloy Products, 3245 Cherry Ave., Long

Beach, CA 90801.

duction systems similar to that designed earlier by Da- vies and Snook. Winge and co-workers employed 3-mm- diameter thin-walled polytetrafluoroethylene (PTFE) tubes 30 mm in length. These were placed into the region between the outer and the intermediate tubes near the base of their demountable torch. The top of PTFE tubes was about 4.7 mm below the top of the intermediate tube. In addition to using the torch design of Davies and Snook, Montaser e t al. also used Teflon ® gas sheathing devices for both coolant gas flow and plasma gas flow in their demountable ICP torch design. A reduction of the mag- nitude of the noise power spectrum for the LaFT in comparison to that for a tangential flow torch (TAFT) was demonstrated by Montaser e t al. These researchers also reported no significant improvement in detection limits with the use of outer quartz tubes with extended lengths. 13 Winge and co-workers also demonstrated that the magnitude of the noise power spectrum was reduced as the applied rf power was decreased for the standard TAFT. These later two studies involved a LaFT operated at a forward rf power and coolant gas flow rate compa- rable to those of a regular TAFT; an improvement in the economy of the operation of those LaFT designs was not demonstrated.

An alternative design of a linear flow torch (LIFT) has been developed in our laboratory. 15 The design incor- porates several features which have been determined to improve the economy of operation as well as the analyt- ical performance of the torch, with reductions in the rate of argon gas consumption and the required rf power level. The torch designed in this laboratory employed a ma- chined insert with rectangular channels between the out- er and the intermediate tubes; the insert is placed near the fireball of the plasma discharge. A thin, ordered, coolant gas layer is formed between the plasma fireball and the inner wall of outer tubing. This gas layer is close to the inner wall of the outer tubing because of the con- figuration of the insert and the torch. This configuration enables the operation of the plasma. The advantage af- forded by the use of rectangular grooves, in comparison to circular channels, 18,~4 lies in the ability to construct inserts with smaller, well-defined, cross-sectional areas of each channel, thus offering the capability of higher- velocity outer (coolant) gas flow streams.

EXPERIMENTAL

Torch Design and Structure. As is depicted in Fig. 1, the linear-flow outer (coolant) gas torch design consists of a normal, commercially available TaFT (RF Plasma Products, Inc., Kresson, NJ) and an insert to convert the

Volume 46, Number 8, 1992 0003-7028/92/4608-124552.00/0 APPLIED SPECTROSCOPY 1245 © 1992 Society for Applied Spectroscopy

,,

i ",, - " % o1~ A

FIa . 1. C o n s t r u c t i o n a n d p a r a m e t e r s of l i nea r flow t o r c h ( L I F T ) . A = 17.85 r a m ; B = 16.0 m m ; C = 0.92 r am; D = 0.46 r a m ; E = 0.92 r am; F = 26.0 r am; H = 6.5 r a m .

initially tangential inlet gas flow to a linear laminar flow prior to its introduction to the discharge. The insert was constructed from a cylinder of Delrin with 30 equally spaced machined high-precision rectangular channels. High precision of the machining of the Delrin insert en- sures the even flow pattern of the coolant gas around the inner wall of the outer tubing and consequently avoids any distortion of the plasma fire ball. The thickness of the insert was dictated by the spacing between the outer and intermediate tubes of the torch. Machining of the gas inlet channels enabled the attainment of an effective spacing of 0.46 mm, as compared to a spacing of 0.5 mm, which was indicated experimentally to be optimal for a tangential-flow torch. 1 Although channels with a smaller depth were not used in these studies, the potential exists for the construction of an insert with the effective spacing which was calculated by Angleys and Mermet 16 to be the theoretical optimum.

The top of the insert was recessed from the top of the intermediate tube to minimize radiative heating of the insert which would result in the distortion of the top of the channels because of partial melting of the Delrin. The position of the insert within the torch was main- tained by placing three small drops of epoxy at equally spaced locations around the top of the insert between channels. Parameters of the insert and position of the insert inside the torch are indicated in Fig. 1. Delrin was selected out of convenience. The insert could be con- structed of alternative materials (e.g., boron nitride).

Reynold's Number Calculations. As indicated above, several designs of a LaFT have been described in the literature. H-~4 The implication of this designation is that the conventional TaFT does not display laminar gas flows. One indicator of the type of flow present in the systems (i.e., molecular, laminar, or turbulent flow) is the mag- nitude of the corresponding Reynold's number.

The Reynold's number for the annulus cooling gas flow region between the outer wall of the intermediate tubing and the inner wall of outer tubing is given by: ~2

Re = 2 V p (ro - ri) (1) au

where V is the volume flow rate, a is the area of annulus, u is the viscosity of the gas, p is the density of the gas and ro, ri are the radius of the outer and inner tube, respectively. Because of the temperature dependence of gas viscosity and density, Reynold's numbers were cal- culated for those conditions within a plausible temper- ature range which may exist in the vicinity of the fire- ball of the discharge. Similar calculations were also undertaken for a range of coolant gas flow rates.

With respect to the flow pattern inside the insert itself, Reynold's numbers can be calculated by the more general formula 17

Re = s d / v (2)

where s is the velocity of the coolant gas flow; d is di- ameter of tube or channel; and an intrinsic viscosity, v, of the coolant gas is described by the viscosity divided by the density of the gas. The gas velocity can be esti- mated by

V s = - - ( 3 )

n A

where n is number of channels, and A = l . w is the area of each channel (l and w are the length and width of each channel, respectively). Assuming that the equivalent di- ameter for each channel is d,

~rd 2 - - ~ I ' W .

4

Thus,

d = 2 (4)

If we substitute Eqs. 3 and 4 into Eq. 2:

2yp Re - (5)

nu( lwTr) 1/2"

The results of those Reynold's numbers calculations us- ing Eqs. I and 5 are graphically depicted in Fig. 2. Those Reynold's values for the coolant gas flow region are well within the criterion for laminar flow (i.e., below the ref- erence value, 2300). The flow pattern, within this region, is well within the laminar flow regime. The calculation resulted in even lower Reynold's numbers, about 77 % of that for the annulus cooling region, for the gas flow inside the insert channels, which are less than 10 mm from the plasma discharge. These calculations indicate the flow pattern to be within this laminar-flow region for both the TaFT and the LIFT. Since the cooling region of the torch is immediately above the insert, the actual Reynold's number at the cooling region for the LiFT designed in this laboratory may be more accurately char- acterized by the Reynold's number of the channels in- stead of the annulus.

Experimental Procedure. A 27.12-MHz quartz-con- trol led radio-f requency generator and impedance- matching network (RF Plasma Products, Inc.) was used with a three-turn load coil to sustain the discharge. Wavelength isolation was achieved by a 0.85-m-focal- length cross-dispersion Echelle monochromator typically used with a Spectrospan V plasma emission spectrometer (Applied Research Laboratories, Valencia, CA). T h e

1246 Volume 46, Number 8, 1992

2 5 0 0

2000

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-- 1 0 0 0 O

o

500

2500

LLLLLLLI 7 8 9 1 0 11 12 13 14

F l o w R a t e / L / r a i n

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(b) FIO. 2. Reynold's number for the annulus between the intermediate and outer tubing walls and Reynold's number for the LiFT insert channels: (I)293 K; ([:])473 K; (@)674 K; ([])857 K; (@)1100 K.

plasma, the torch box, and the impedance-matching net- work were located on a three-dimensional translation stage constructed in our laboratory to enable adjustment of the plasma position with respect to the entrance slits of the monochromator for maximum sensitivity.

Image transfer was accomplished by two precision spherical l l4-cm-focal-length mirrors with diameters of 11.4 cm placed in a over-and-under symmetrical arm pair with off-axis illumination for coma correction, as de- scribed elsewhere./s-2° The sagittal image of the plasma was located at the entrance slit of the monochromator to correct for astigmatism aberrations at the focal plane of the monochromator. The exit slits used throughout this study were 25 #m (verticle) and 100 ~m (horizontal).

The output signal from the photomultiplier tube (PMT) was amplified by a current amplifier (Model 427, Keithley, Cleveland, OH). The analog signal from the current amplifier was processed and digitized with the use of a data acquisition system (Models SR245 and SR235, Stanford Research System, Palo Alto, CA). The resulting signal was further processed and analyzed with a dedicated microcomputer system (Model 158, Zenith data systems, St. Joseph, MI).

TABLE I. Signal-to-noise ratios."

Wave- length Applied

Element (nm) Torch power S/N

Ca (I) 422.67 LiFT 750 50.1 TaFT 750 24.1 TaFT 1000 30.O

Ca (II) 393.4 LiFT 750 817.5 TaFT 750 111.2 TaFT 1000 356.6

Mg (I) 285.21 LiFT 750 23.1 TaFT 750 15.8 TaFT 1000 7.0

Mg (II) 279.6 LiFT 750 79.7 TaFT 750 2.8 TaFT 1000 52.7

. All solution concentrations were 10 mg L -1.

Stock solutions of 1000 mg Ca, Ba, Cu, Cr, Fe, Ni, Pb, Zn, and Mg L -1 were prepared by dissolution of reagent- grade nitrate salts in doubly distilled, deionized water. All sample solutions were prepared daily by serial dilu- tion with distilled, deionized water. A stock solution of 10,000 mg Na L -1 was prepared with reagent-grade NaC1 for all easily ionizable element (EIE) studies. The phos- phate solution was prepared by dissolution of NH4H2P04 for a stock solution concentration of 10,000 mg L -~.

Samples were introduced into the ICP with a concen- tric-glass nebulizer (RF Plasma Products) with a Scott- type, double-pass spray chamber. All solutions were de- livered to the nebulizer by using a peristaltic pump with a flow rate of 1.33 mL min -~.

The LiFT was operated at a lower applied power level and a lower outer (or coolant) gas flow rate. The LiFT was operated at 750 W of incident rf power with an outer argon flow rate of 10 L min -~. These conditions were significantly different from the rf power and outer cool- ant gas flow conditions at which the conventional T a F T was operated (i.e., 750 and 1000 W with 15 L min-~). Attempts at operation of the conventional torch at the power and flow levels of the LiFT resulted in either the extinguishing of the plasma or the melting of the outer quartz tubing, regardless of efforts to minimize reflected power levels. All measured intensities were corrected for variations in amplifier gain settings. The operating con- ditions of gas flow and applied power were not optimal for the operation of each torch. However, it is the purpose of this paper to present a comparison of the relative analytical performance of the LiFT with that of a TAFT.

RESULTS AND DISCUSSION

D y n a m i c Range. The intensity of the measured emis- sion signal was investigated as a function of concentra- tion. The results indicate that the LIFT, operated at 750 W , displays improved figures of merit [e.g., signal-to- noise (S/N)] over those measured with the TaFT oper- ated with an applied forward rf power of either 750 or 1000 W (Table I). Specifically, the LiFT was observed to demonstrate an increase in relative intensity for cal- cium ion (393.4 nm) by as much as one order of mag- nitude and an S/N of 817.5, as compared to that observed with a T a F T operated at the same applied power level (S/N = 111.2).

APPLIED SPECTROSCOPY 1247

TABLE II. Compilation of characteristic signal intensities for 10 mg L -t.

W a v e - S i g n a l S i g n a l no i se N e t b a c k g r o u n d B a c k g r o u n d noise l e n g t h

E l e m e n t (nm) L i F T T a F T L i F T T a F T L i F T T a F T L i F T T a F T

B a (I) 553.6 0,398 0.430 1.64 x 10 -3 4.28 x 10 -3 2.49 x 10 -2 7.32 x 10 -4 3.98 x 10 -4 9.80 x 10 -4 Ca (I) 422.7 6.54 3.981.38 1.01 7.33 x 10 -3 1.01 x 10 -1 0.128 0.161 C a (II) 393.8 204.6 21.1 "-- 3.08 1.33 × 10 -1 9.73 x 10 -2 0.171 C u (I) 324.8 0.102 0,682 1.15 x 10 -3 9,12 x 10 -3 2.31 × 10 -3 8.53 x 10 -3 8.51 x 10 -5 B a (I) 553.6 0.102 0.682 1.15 x 10 -3 9,12 x 10 -3 2.31 x 10 -3 8.53 x 10 -2 8.51 x 10 -3 Fe (I) 372.0 9.25 x 10 -2 0.123 5.90 x 10 -4 1,40 x 10 -3 2.31 x 10 -a 3.60 x 10 -2 1.38 x 10 -4 M g (I) 285.2 3.62 2.68 1.15 0.977 0.101 9.18 x 10 -2 0.155 M g (II) 279.6 0.236 1.37 1.38 x 10 -2 2.20 x 10 -3 7.60 x 10 -3 1.69 x 10 -a 3.99 x 10 -4 N i (I) 341.5 0.162 0.348 1.18 x 10 -3 1,65 x 10 -8 1.75 x 10 -a 4.82 x 10 -2 3.29 x 10 -4 P b (I) 405.8 1.80 x 10 -2 1.65 x 10 -8 3.49 x 10 -4 1.18 x 10 -~ 3.04 x 10 -3 ND" 1.55 x 10 -4 Zn (I) 213.8 9.02 × 10 -2 8.33 x 10- ' 1.83 x 10 -3 1.15 x 10 -2 1.05 x 10 -8 3.03 x 10 -8 9.49 x 10 -4

0.146 4.67 x 10 -3 4.67 x 10 -3 9.60 x 10 -4

0.171 1.68 x 10 -3 7.94 x 10 -4 5.13 x 10 -4 3.92 x 10 -3

N D = n o t d e t e c t e d .

It should be noted that, because of the different power and flow conditions which were required for the opera- tion of either torch, viewing position was found to be critical, and the use of the top of the load coil could not be used as a reference point to define the location of the observed emission within the discharge. In an effort to minimize those differences, all comparative measure- ments were undertaken with the use of the yttrium initial radiation zone (IRZ) internal reference point, as was first suggested by Koirtyohann e t al . 21

The results of these studies show the LiFT to produce a linear range of at least four and a half orders of mag- nitude. This result was found to be comparable to that of the TaFT tested under the same experimental con- figuration. The smaller linear dynamic range observed in those studies may be a result of the less efficient light- gathering capabilities of this optical system, which was designed for high spatial fidelity rather than for high optical throughput. Again, these studies are not intended to demonstrate the absolute capabilities of the analytical performance of this torch design, but rather to illustrate its performance in comparison to that of a conventional torch design.

Careful comparison indicates that the use of the LiFT provides greater enhancement for ion emission than for atom emission. It was also noticed that the plasma dis- charge with the use of the LiFT was visibly more diffuse in comparison to the fire ball sustained in a TaFT op- erated at the same applied power level.

TABLE IIL Comparison of detection limits determined with the use of the LiFT and TAFT. a

V i e w i n g W a v e - S o u r c e p o s i t i o n l e n g t h of

e l e m e n t r e l a t i v e d e t e c t i o n L i F T T a F T e m i s s i o n ( ram) (nm) ( p p m ) ( p p m )

B a (I) - 0 . 5 553.6 0.01 0.47 Ca (I) - 0 . 5 422.7 0.594 1.22 Ca ( I I ) + 4 . 0 393.4 0.031 0.24 C u (I) - 0 . 5 324.8 0.018 0.23 Cr (I) - 0 . 5 425.4 0.022 0.095 Fe (I) - 0 . 5 372.0 0.04 0.32 M g (I) - 0 . 5 285.2 1.32 1.98 M g (II) + 4 . 0 279.6 0.01 0.011 N i (I) - 0 . 5 341.5 0.043 0.057 P b (I) - 0 . 5 405.8 0.22 2.7 Zn (I) - 0 . 5 213.8 0.125 0.13

° D e t e c t s l i m i t s c a l c u l a t e d f r o m r e g r e s s i o n a n a l y s i s o f a p l o t of n e t s i g n a l - t o - b a c k g r o u n d no i se as a f u n c t i o n of c o n c e n t r a t i o n a t S / N = 3.

1248 Volume 46, Number 8, 1992

Signal-to-Noise Ratios. Table I shows the measured values of the signal-to-noise ratios for the magnesium atom, magnesium ion, calcium atom, and calcium ion with samples containing 10 mg L -1 for each of the ele- ments operated at conditions listed. These variations were observed to be consistent throughout the analyte concentration range investigated for all four emitting species. Operation of the TaFT at 1000 W power yielded a larger ratio than when it was operated at 750 W. How- ever, operation of the LiFT at 750 W yielded signal-to- noise ratios which were consistently larger than those observed with the TAFT.

Higher S/N values for LiFT may be attributed to en- hancement in emission signals and reduction in magni- tude of noise power, since the LiFT was operated at lower forward rf power and the magnitude of noise power should be lower, as predicted by Montaser 13 and Winge e t a l . 14

It should be emphasized that the LiFT operating gas flow parameters were considerably lower (20 % ) than those used by TAFT. Thus, the observed improvement in the analytical performance of the ICP torch with the added linear flow insert is even more significant since an im- provement in economy of operation has been achieved. Table II contains a compilation of representative signal and background measurements. These data have been included in an at tempt to clarify discussions of the an- alytical performance of the LiFT compared to that of a TAFT. 22 A complete compilation of detection limits (S/N = 3) using the LiFT and a TaFT is given in Table III.

A comparison of the long-term stability of the analyt- ical emission signal from the use of a laminar-flow con- verted torch with the emission signal from the use of a conventional tangential torch was undertaken. The re- sults of that comparison indicated that the incorporation of the linear-flow insert yielded results with a relative standard deviation (RSD) of 0.025 in the emission signal from a solution of 10 mg Ca L -1. This was a significant improvement in the long-term stability of the analytical emission signal from that for a TaFT (RSD = 0.119).

Interference Studies. In order to characterize the an- alytical performance of this outer coolant gas LiFT de- sign with any potential vaporization interferences, the susceptibility of the resulting plasma to sample-depen- dent interferences was investigated. Because of the ob- served more diffuse appearance of the discharge sus- tained in the LIFT, it was postulated that more severe interferences might be observed resulting from the for-

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C o n c e n t r a t i o n o f P h o s p h a t e / p p m

Fro. 3. Interference of phosphate on normalized calcium emission intensities. LiFT intensities. LiFT at Ca(I) (A); LiFT at Ca(II) ( , ) ; TaFT at Ca(I) (1); TaFT at Ca(II) (0). RF power: 750 W for LiFT and 1000 W for TaFT; cooling gas flow: 10 L rain -~ for LiFT and 15 L rain -~ for TAFT.

C o n c e n t r a t i o n of Sodium / p p m

FIG. 4. Interference of sodium on normalized calcium emission inten- sities. LiFT at Ca(I) (4); LiFT at Ca(II) ( . ) ; TaFT at Ca(I) (1); TaFT at Ca(II) (Q). RF power: 750 W; cooling gas 12 L min -1 for LiFT and 15 L min -1 for TaFT.

ma t ion of more re f rac tory species t han typical ly en- counte red with ICP opt ical emission spec t romet ry .

I n c r e a s i n g a m o u n t s of p h o s p h a t e ( p r e s e n t as NH4H2P04) were added to solutions of 10 mg Ca L -1, and Ca a tom and ion emission intensi t ies were measured as a funct ion of added phospha te concent ra t ion with the use of the T a F T and L i F T configurations. The resul ts of this s tudy are shown in Fig. 3. All intensi t ies were normal ized to the signal in tens i ty measured with no add- ed phospha te . Ca a tom emission for bo th T a F T and L i F T configurat ions exper ienced depressions in intensi t ies when the added phospha t e concent ra t ion reached 100 mg L -~ and the depressions become greater with increas- ing phospha t e concentra t ion. Ca ion emission intensi t ies for L i F T exper ienced a depress ion similar to those of the Ca a tom emission. Ca ion emission intensi t ies for T a F T were enhanced with low phospha t e concent ra t ion (up to 1000 mg phospha t e L -~) and displayed a depression in emission intensi t ies a t higher phospha t e concentrat ion. Resul ts of this s tudy indicated t ha t the Ca a tom emis- sions for bo th T a F T and L i F T were s imilar ly affected by the addi t ion of phospha te , bu t the L i F T displayed improved per formance , in compar i son to the TAFT, in resist ing the interferences resul t ing f rom the addi t ion of phospha t e for Ca ion emission in tens i ty measurements .

In a s imilar manner , increasing amoun t s of N a were added to solutions of 10 mg Ca L -~, and the a tomic and ionic emission signals f rom bo th the L i F T and the T a F T were recorded. T h e resul t ing normal ized signal intensi- t ies as a funct ion of added N a concent ra t ion are shown in Fig. 4. T h e normal ized a tom and Ca ion emission intensi t ies were s imilar ly affected with the use of e i ther torch configuration. T h e added N a caused a slight de- pression for the a tomic emission signal and significant e n h a n c e m e n t for the ionic emission signal wi th the use of bo th L i F T and T a F T configurations. However , the resul ts with the use of L i F T demons t r a t ed t ha t L i F T had improved resis tance to the effect of added N a and hence displayed less depress ion of the a tomic emission signal and less e n h a n c e m e n t of the ionic emission signal in compar ison to t h a t observed with the use of TAFT,

and the overall analyt ical pe r fo rmance for L i F T was less severely affected by the presence of Na.

C O N C L U S I O N S

T h e above discussion describes a design for the con- version of a convent ional T a F T to a low-flow, low-power, l inear-flow configuration. As previously repor ted , 13,14 low- er appl ied r f power inpu t will resul t in a reduct ion of noise level and, consequent ly , resul ts in higher signal- to-noise ra t io or lower detect ion limits. Bo th higher emis- sion in tens i ty and be t t e r s ignal- to-noise rat io a t lower outer coolant gas flow rates and lower forward r f power have been achieved. Addit ional ly, the use of this l inear flow outer coolant gas geomet ry has demons t r a t ed a de- creased susceptabi l i ty of Ca emission signals to the pres- ence of an easily ionizable e l emen t and a vapor iza t ion in te r fe ren t within the sample matr ix . T h e L i F T dem- ons t ra t ed significant i m p r o v e m e n t in analyt ical perfor- mance as well as in economy of operat ion.

ACKNOWLEDGMENTS The authors wish to acknowledge the financial support of Sandia

National Laboratories (Contracts 06-5630 and 05-9859) and the New Mexico State University Chemistry Department.

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APPLIED SPECTROSCOPY 1249

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1250 Volume 46, Number 8, 1992