Analyte Matrix Excitation Investigations in the High-Voltage Spark Discharge Using an Echelle/CCD System

C H E R Y L A. BYE* and A L E X A N D E R S C H E E L I N E t School of Chemical Sciences, University of Illinois, Urbana-Champaign, Box 48, Roger Adams Laboratory, 600 S. Mathews St., Urbana, Illinois 61801

The matrix dependence of analyte excitation in the high-voltage spark discharge has been investigated. The analyte excitation distributions of Fe(I), Mo(I), and Cu(I) have been measured. The large wavelength range of a custom echelle/CCD system was exploited for these determinations. The analyte excitation within the high-voltage spark on a time-integrated basis was determined to be independent of both sample matrix and sampled species.

Index Headings: Boltzmann plot; CCD; Echelle spectrometer; Emission spectrochemical analysis; Spark, ac.

INTRODUCTION

The analytical high-voltage spark discharge is rou- tinely employed for the direct elemental analysis of con- ductive solids. Unlike its liquid analysis counterpart [the inductively coupled plasma (ICP) source], the spark is plagued with severe matrix effects which result in non- linear analyte working curves and complicated alloy-spe- cific analysis methods. 1,2 The origins of these complex and undesirable analytical characteristics have been the subject of intense investigation. 3-1s One aspect that dif- ferentiates spark analysis from its liquid counterpart is that, unlike the ICP, analyte sampling is convolved with analyte excitation. The plasma (spark) serves as both a method of sample introduction and an excitation res- ervoir. An added complication arises from the transient and dynamic plasma environment of the spark itself. To fully elucidate the origin of the severe spark matrix ef- fects, one must first deconvolve the plasma-based or an- alyte excitation-based matrix effects from sample-based matrix effects.

To separate the plasma-based matrix effects from those that are analyte induced, one can characterize the spark's plasma behavior in terms of reproducibility, state of equilibrium, spatial homogeneity, and temporal homo- geneity. A series of such measurements have been con- ducted, which include spatially resolved electron density mapping of single spark discharges, 19 temporally and spa- tially resolved electron density mapping, 2° and argon ex- citation temperature determinations. 21 These investiga- tions have focused on the major plasma species in the form of At(I) and Ar(II), and have demonstrated that the plasma is reproducible from spark to spark as well as spatially homogeneous. 19-21 The temporally resolved measurements have shown that the electron density ex- periences only minor relaxation throughout the dis- charge lifetime. 2° In addition, the argon excitation tern-

Received 18 June 1993. * Current address: Department of Chemistry, Indiana University, Bloo-

mlngton, IN 47405. t Author to whom correspondence should be sent.

perature measurement implies that the spark is in LTE for most atomic transitions and in partial LTE for ionic transitions. 21 All these results indicate that the plasma is not the major source of the observed spatial analyte emission heterogeneity. To further support this conclu- sion, we conducted a series of matrix analyte excitation distribution and temperature measurements for a few alloys and pure metals. The results of these matrix-de- pendent analyte excitation measurements are the subject of the current investigation.

HISTORICAL AND THEORETICAL CONSIDERATIONS

The Boltzmann plot is the most common method for viewing excitation distributions. 22-25 By making a series of emission radiance measurements for a number of tran- sitions, one can determine the excitation distribution and temperature of the emitting species using Eq. 1:

ln{giA,ic~= E, + l--![47rg°\ \ Iiih ] kTe, ln\hlNo]" (1)

The emission radiance (]ij) of a particular transition is measured and plotted as a function of the upper-level energy (Ei) for that transition. Additional constant fac- tors such as the transition wavelength (~), transition probability (Aij), and excited-state statistical weight (gi) are also required. These constant factors are available for some of the transitions, depending on the particular element. The statistical weight of the upper level can also be calculated from the excited-level total angular momentum, J (gi = 2J + 1). The ground-state statistical weight (go), the ground-state population density (No), and the plasma thickness (l) have been separated in the final log term, since they are common to all levels. The Boltzmann plot is constructed by plotting the emission- radiance-dependent log term against the excited-state energy. The excitation temperature (Te,) is calculated from the slope of Eq. 1. The intercept of the Boltzmann plot is rarely used since it requires an absolute intensity calibration in order to be meaningful. Accurate absolute intensity calibration is difficult to achieve experimen- tally, so in practice only relative intensity calibrations are performed.

The above formulation of the Boltzmann plot assumes that the plasma source is in local thermodynamic equi- librium (LTE). On the basis of previous calculations, the Griem criterion for LTE is satisfied for atomic emission in the spark discharge. 21,26 In addition, nonequilibrium correction factors (b factors) could be added to correct for any nonequilibriumY 1,23,25,27-31 Approximate function-

Volume 47, Number 12, 1993 0003-7028/93/4712-203152.00/0 APPLIED SPECTROSCOPY 2031 © 1993 Society for Applied Spectroscopy

al forms for the b-factor corrections are available2 ° At the L T E limit, the b factor converges to unity. The cal- culated correction factors in this investigation were uni ty within exper imental error. Therefore , the b-factor cor- rect ion has been omit ted in the functional form of Eq. 1.

E X P E R I M E N T A L C O N D I T I O N S

Spectroscopic observations were made on a custom echelle spect rometer which was optimized for charge- coupled array detect ion (CCD). The specific echelle spect rometer design and characteristics have been doc- umented. 32,~3 A list of the exper imental appara tus and conditions has been tabula ted in Table I. A thyra t ron- triggered, adjustable-waveform source was employed24 Spectral da ta covering a 143-nm segmented range from 344.6 to 544.1 nm were obtained. Spectral resolution was no worse than 0.5 ~ and varied with echelle diffraction order. Light collected from a 25-um spot at the cathode surface and direct ly on the discharge axis was imaged at uni ty magnification. Spectral da ta acquisit ion was per- formed in a temporal ly integrating mode. Spark emission echellograms were collected with the use of a 20-s ex- posure, which integrated the light emi t ted by 4000 spark events. Five replicate measurements were made for each cathode matrix. The cathode matrices consisted of a se- ries of copper-containing samples (electrolytic copper, 360 brass, nickel silver solder, and 2017 aluminum), two iron-containing samples (NBS 1265 electrolytic iron and 316 stainless steel), and molybdenum. Measurements were also per formed on magnesium and zinc electrodes, bu t the spectral da ta were insufficient for monitoring their excitat ion distributions. In tens i ty calibration was per formed with the use of the diffuse reflection from a MgO-frosted slide and a ter t iary s tandard cont inuum source. 35 Wavelength calibration, spectral extractions, intensi ty calibration, and general da ta reduct ion were per formed with software developed in-house) G

An example of a typical spark echellogram is presented in Fig. 1. This doubly dispersed spark emission spect rum was obta ined in the cathodic space charge adjacent to an electrolytic iron (NBS Fe 1265) cathode. A 15-shade grayscale is used in Fig. 1 to il lustrate spectral emission intensi ty contours, with the darker shades corresponding to the higher emission intensities. Spectral information is represented in bo th the vertical and horizontal di- mensions of the detector and figure. The vertical labels refer to the position of the diffraction orders off the echelle grating. The spark's cont inuum emission in Fig. 1 is also helpful in identifying the location of the various diffraction orders. Lines which trace the position of the echelle orders have been added for clarity. Within a given diffraction order, spectral informat ion is horizontal ly dis- persed, and wavelength increases from right to left, as indicated in the horizontal label of Fig. 1. A reference white cross hair has been placed on the Fe(I) 430.79-nm transi t ion in order 54, and the wavelength scale at the bo t tom of the figure refers only to the wavelength dis- persion within tha t specific order. Shor ter wavelengths are located at the top r ight-hand corner and the longer wavelengths are located at the bot tom lef t -hand corner. The characterist ic richness of the Fe(I) emission spec- t rum is clearly observable in Fig. 1.

TABLE I. Experimental parameters and conditions.

Source

Feed optics

Spectrometer

Detection

Electrodes

Gases Repetition rate Miscellaneous conditions

Adjustable-waveform, thyratron-triggered spark source

L1 = 101 ttH, L2 = 78.4 uH, L3 = 11.7 uH Cm,in = 0.0466 #F, unipolar waveform

Off-axis paraboloidal concave mirror pair f = 76.2 mm, D = 50.8 mm, 0 = 90 ° Source and spectrometer placed at focal

points Scheeline custom echelle spectrometeV 3

Predisperser setting = 133.5 Echelle setting = 4510

Charge-coupled array detector (Thomson 7882 chip)

384 x 576 array of 23-um-square detec- tion wells

Metachrome-II overcoat for UV sensi- tivity

Photometrics, Tucson, AZ Anode: Coaxial flowjet with 1/8 -in. (3.18-

mm) thoriated tungsten pin ground to 30 ° included angle

Cathode: Beveled disc rotated at 1 rpm Diameter = 2.54 cm Material: Electrolytic Cu, 360

brass, A1 2017, nickel silver, NBS 1265 Fe, pure Mo, 316 stain- less steel

Gap: 2 mm Ar (welding grade): 13.0 mL/s 200 Hz Peak current = 60.0 Amps Observation window = 0.00 mm from cath-

ode tip (cathode space charge region)

Exposure time = 20.0 s

R E S U L T S A N D D I S C U S S I O N

There were a total of 38 interference-free Fe(I) emis- sion lines present in the electrolytic iron and 316 stainless steel echellograms. The average results of the replicate intensi ty measurements for both the electrolytic iron and stainless steel have been used in the Bol tzmann plot construction. The Fe(I) intensities were first relative- intensi ty cal ibrated and then internally normalized with respect to the most intense Fe(I) emission line (4307.9 ~). This addit ional internal intensi ty normalizat ion was per formed so tha t comparisons could be made between the different metals. The unnormal ized intensities were not direct ly comparable due to the lower percent com- position of the iron in the stainless steel (~ 65 % ). All the internal ly normalized Fe(I) intensities in the two differ- ent matrices were equivalent within the uncertaint ies of the measurements. The Fe(I) transit ion probabilities used in Bol tzmann plot construct ion were obta ined from a computer ized database available from N I S T 37 or f rom O'Brian e t a l 2 s

Th e excitat ion distr ibutions of Fe(I) from the two ca- thodic materials are presented in Fig. 2. The Fe(I) ex- ci tat ion t empera tu re for the electrolytic Fe electrode is 7861 ± 328 K and 7864 ± 331 K for the stainless steel sample. The 3 K difference in the Fe(I) exci tat ion tem- peratures in the two different i ron-containing sample matr ices is statistically insignificant. These two metals have radically different composit ion, and their spark

2032 Volume 47, Number 12, 1993

64

6Z

60

58

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : : , - " - ' : ":;7~ ~

~ . ~ "' -:- : = . ~ . A , ~ , , : . . . . ,, ~ . . . . . . ~ . . . . . . ~ - - ..................... " . . . . . . . . . . . . . .

I J I I J I 4 3 3 4 3 Z 4 3 1 4 3 ~ 4 2 9 4 2 8

Wavelength (n~)

F m . 1 . Atyp ica lNBS1265e lec tro l~ i c i ronsparkeche l logram.

5 6

5 4

0 r d

5 Z e r

5 0

48

4 6

4 4

spectrochemical analysis procedures are very different. 39 In addition, the electrode sampling characteristics are also notably different. However, the analyte excitation is independent of the sample matrix in this particular case, as indicated by the small difference in the Fe(I) excitation temperatures.

Originally the molybdenum electrode Mo(I) excitation

results were intended to be used as a test for source and analyte excitation equilibrium. Molybdenum was chosen because high-precision, accurate, and internally consis- tent Mo(I) transition probabilities were available. 4° Mo- lybdenum is similar to both the electrolytic iron and stainless steel; all form oxide layers while sparking. If the spark discharge is in LTE, one would expect the

35.5

34..5

33.5

32.~. _5

32

31.5

31i

30.5

Fro. 2 .

l Cathode Matrix • NBS1265Fe7861_+328K • 316Ste~nlossToB4±331K

• t

t o

t i't

i i I

I I I I 35000 40000 45000 50000

Upper Level Energy (cm-~)

Results of Fe(I) matrix excitation investigations.

55000

37.5

37

36.5

36 -%

3 35.5

35

34.5

~oo

[ • PureMoS000¢6COK

0 0 •

• o

r i J i 37000 41000 45000 49000 53000

Upper Level Energy (cm- ,)

F i a . 3 . Mo(I) Boltzmann plot.

APPLIED SPECTROSCOPY 2033

TABLE II. Tabulation of Cn(I) data used in the construction of Cu(I) Boltzmann plots.

Normalized intensity Cu(I) 5153.2 ,~ transit ion

Nickel Pure Cu 360 Brass silver 2017 A1

(~ Ao 4' E i (99.95 % ) (61.5 % ) (26 % ) (4 % ) ) (MHz) gi (cm-1) 1082~C 899oc ~900oc 641oC a

5292.5 11 ± 3 8 62,403 17 ± 2 15.7 ± 0.8 15.2 ± 0.8 13 ± 2 5153.2 38 ± 9 8 49,935 100 100 100 100 4651.1 60 ± 15 4 62,403 50 ± 6 50 ± 1 46 ± 2 73 ± 11 4539.7 21 ± 5 4 63,585 3.2 ± 0.2 3.1 ± 0.4 2.1 ± 0.1 ND 4530.8 8.4 ± 2 2 52,849 3.0 ± 0.8 2.5 ± 0.2 3.1 ± 0.3 ND 4022.6 19 ± 5 4 55,388 5.9 ± 1.7 7.6 ± 0.6 6.8 ± 0.3 ND

ND = not detected.

excitation temperatures of the sampled species to be the same, and dependent only upon plasma conditions. Only 17 interference-free Mo(I) emission lines were available for Boltzmann plot construction. Internal intensity nor- malization was performed relative to the 4232.6 ~ Mo(I) transition to be consistent with the previous Fe(I) mea- surements. The Mo(I) excitation temperature calculated from the Boltzmann plot (Fig. 3) was 8000 ± 600 K. This result is in excellent agreement with the Fe(I) results. This observation indicates that the excitation behavior of both Fe(I) and Mo(I) is similar. More importantly, it implies that the analyte excitation mechanism is sample matrix independent.

The iron, stainless steel, and molybdenum electrodes are all relatively difficult to sample; i.e., they have high melting points. To further test the hypothesis that an- alyte excitation is sample matrix independent, we made excitation measurements on a series of copper-containing alloys: electrolytic copper (mp = 1082°C), brass (mp = 899°C), and nickel silver solder (mp ~ 900°C). The copper content of the samples ranged from 99.95 % in the elec- trolytic copper to 61.5 % and 62 % in the brass and nickel silver alloys, respectively. In addition, a low-copper-con- taining aluminum alloy was employed. The aluminum 2017 alloy had a 4% copper composition and a melting point of 641°C. However, due to the low copper content of the aluminum alloy, only three Cu(I) emission lines were detected. In the copper, brass, and nickel silver samples only six Cu(I) emission lines with measured transition probabilities were detected. Emission inten- sities and pertinent Boltzmann parameters have been tabulated for the Cu(I) emission in the four matrices (Table II). There was excellent agreement among the internally normalized electrolytic copper and brass Cu(I) emission intensities. The intense nickel silver Cu(I) in- tensities agreed within the uncertainty of the measure- ment to those of both the copper and brass, but dis- agreement was notable in the lower intensity transitions. This disagreement most likely originated from the in- creased background, continuum emission, and scattering from easily sampled lead and zinc in the nickel silver matrix. Since only three Cu(I) emission lines were ob- served in the aluminum alloy, the number of comparisons that could be made was limited. One of the observed Cu(I) lines (5153.2 A) was used for internal normalization and, therefore, was unavailable for comparison purposes. The two remaining detectable Cu(I) emission liners showed a mixed level of agreement. However, these two

Cu(I) emission lines (5292.5 A and 4651.1 ~) are from resonantly autoionizing levels, so the agreement ob- served in Table II is quite surprising. 42,4~

Cu(I) Boltzmann plots were constructed for each of the copper-containing samples and have been presented in Fig. 4. With the exception of the autoionizing levels, the Cu(I) excitation distributions were fairly linear. Since the autoionizing levels are particularly susceptible to nonequilibrium recombination enhancement, it was not surprising that they are about an order of magnitude too intense. 42,43 Due to their anomolously high intensity, these autoionizing levels were excluded from the Boltzmann Cu(I) excitation temperature calculations. The measured Cu(I) excitation temperatures agreed within the uncer- tainty in the temperature calculation. Both the Cu(I) excitation temperature of 8300 ± 2500 K for electrolytic copper and 8400 ± 2100 K in the brass matrix were in excellent agreement with the measured Fe(I) and Mo(I) excitation temperature (see Table III). The Cu(I) exci- tation temperature of 6900 ± 1100 K measured in the

TABLE III. Summary of matrix excitation temperature determina- tions.

Upper- level

Excitation energy Sample tempera ture Number of spread

composition Analyte Te. (K) lines used (cm- ' )

Electrolytic iron Fe(I) 7861 ± 300 38 19,460 Fe (99.99%) Corr. = 0.939

mp = 1535°C

316 Stainless steel Fe(I) 7864 ± 300 38 19,460 Fe (~65%) Corr. = 0.939

mp = ~ 1400°C

Molybdenum Mo(I) 8000 ± 600 17 16,978 Mo (99.95%) Corr. = 0.919

mp = 2617°C

Electrolytic copper Cu(I) 8300 ± 2500 4 13,650 Cu (99.95%) Corr. = 0.774

mp = 1083°C

360 Brass Cu(I) 8400 ± 2100 4 13,650 Cu (61.5%) Corr. = 0.839

mp = 899°C

Nickel silver Cu(I) 6900 ± 1100 4 13,650 Cu (62 %) Corr. = 0.925

mp = ~9000C

2017 Aluminum Cu(I) NA 3 NA Cu (4%)

mp = 6410C

° NA = not anlayzed.

2034 Volume 47, Number 12, 1993

34

33.5

33

32.5

g ~2 _5

3'1.5

31

l Cathode Matrix

Autoionizing $ Levels

t !

30.5

• Normalization Point

I I I I I I I 50000 52000 54000 56000 58000 60000 62000

Upper Level Energy ( c m - 1)

Fro. 4. Results of Cu(I) matrix excitation investigations.

640OO

nickel silver matrix was lower, but still agreed with the Fe(I) and Mo(I) excitation temperatures within the un- certainty of the measurement (Table III). With such sample diversity, the excitation temperature trends and matrix independence of analyte excitation observed are probably not coincidental.

CONCLUSIONS

It is clear from these studies that analyte excitation is independent of sample matrix. In addition, the overall excitation conditions of the plasma are reproducible and sample matrix independent. This consideration, coupled with the results of the previous spark plasma character- ization measurements, indicates that the plasma itself is fairly spatially homogeneous, reproducible, and close to equilibrium (LTE). 19-21 Once the analyte is introduced into the plasma, the plasma behaves as a nearly ideal (equilibrium) excitation source. This observation also suggests that the primary source of nonlinearity in the analyte working curves either originates from sample heterogeneity or is generated in the plasma/analyte sam- pling process. Therefore, further improvements in spark spectrochemical analysis will come from a better under- standing of the sampling process and the complex plasma sample chemistry occurring at the cathode/plasma in- terface.44. 45

ACKNOWLEDGMENTS

Support of the National Science Foundation (Grants NSF-CHE-85- 21584 and NSF-CHE-89-14401) is appreciated. Support in the form of teaching assistantships and fellowships from the School of Chemical Sciences at the University of Illinois is also appreciated. The authors thank D. L. Miller for suggestions on data extraction.

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