Anal. Chem. m
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Mechanistic Characterization of Chloride Interferences in Electrothermal Atomization Systems J. M. Shekiro, Jr.,l a n d R. K . Skogerboe* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Howard E. Taylor
U.S. Geological Survey, Box 25046, M S 407, Denver Federal Center, Denver, Colorado 80225
A computer-controlled spectrometer wlth a photodiode array detector has been used for wavelength and temperature resolved characterlzatlon of the vapor produced by an electrothermal atomlzer. The system has been used to study the chlorlde matrlx Interference on the atomlc absorptlon spectrometrlc determlnatlon of manganese and copper. The suppresslon of manganese and copper atom populatlons by matrix chlorides such as those of calcium and magneslum is due to the gas-phase formatlon of an analyte chlorlde species followed by the dHfuslon of slgnlflcant fractlons of these specles from the atom cell prlor to completion of the atomlzation process. The analyte chlorlde specles cannot be formed when matrlx chlorides wlth metal-chloride bond dlssoclatlon energies above those of the analyte chlorides are the prhrclpal entlties present. The results lmllcate that multiple wavelength spectrometry used to obtaln temperature-recpdved spectra Is a viable tool In the mechanlstlc characterlzatlon of Interference effects observed wlth electrothermal atomization systems.
Matrix interferences for atomic absorption analyses using electrothermal vaporization atomizers (ETAAS) have been extensively studied (1-10). While such interferences are derived from chemical or physical interactions between the sample matrix and the analyte, rigorous characterization of the actual controlling phenomena has remained difficult (9). The suppression effects of common matrix halide salts, particularly alkali and alkaline-earth chlorides, on the determination of transition metals have been widely reported ( 2 , 5 ,7 , 8 ) . The suggested mechanism(s) by which this suppression effect is manifested include: (1)vapor-phase formation of analyte halide molecules that diffuse from the optical path prior to dissociation to atoms ( 2 , 8 ) ;( 2 ) condensed phase formation of analyte halide molecules followed by vapor diffusion loss prior to atomization (11);(3) thermal expulsion of solid or liquid phases containing analyte from the electrothermal systems prior to reaching the atomization temperature (12,13);and (4) changes in the atomization processes that affect analyte populations (9,14). While the formation of volatile chlorides with diffusion loss seems to be most widely supported, it is noted that direct observation of vapor-phase analyte halide molecules has not been reported. Rather, the mechanism has been implied exclusively by atomic absorption measurements. The shortcomings of conclusions derived from indirect inference and the resultant need for simultaneous observation of both atomic and molecular forms of analyte have been recognized (15). The application of mass spectrometry to the study of interferences in ETAAS has clearly
* To whom correspondence should be addressed. 'Present address: E. I. du Pont de Nemours & Co., Inc., Jackson Laboratory, Deepwater, N J 08023. 0003-2700/88/0380-2578$01.50/0
shown the advantages of making supplemental molecular measurements (15, 16). Vapor-phase ultraviolet absorption spectra for alkali (17-20), alkaline-earth ( 5 , 1 9 , 2 0 ) and , transition-metal (21, 22) halides have been recorded in ETAAS systems to establish the nature of spectral interferences. The reported spectra collectively represent laborious wavelength-by-wavelength accumulations of absorbance data for the direct vaporization of the respective salts. We have been unable to find reports in which the atomic and molecular absorption spectra of the transient populations were simultaneously monitored for solutions containing both analyte and a matrix salt. Since temporally defined spectra for such systems offer significant promise for elucidating the mechanisms of ETAAS matrix interferences, we have used this approach to study the effects of common matrix element chlorides on the analytical determination of Cu and Mn. The results presented in this report show that the vaporphase formation of analyte halides is the predominant mechanism resulting in suppression of analyte atom populations. The analyte halide formation process occurs when matrix halides are present having metal-chloride bond dissociation energies below that of the analyte halide; if the matrix halide has a bond dissociation energy above that for the analyte halide, the formation of the latter is not observed. Data are also presented indicative of the probable diffusion of significant fractions of the analyte-halide from the optical path before atom formation is complete. The potential diagnostic value of using multiple wavelength spectrometry for simultaneously monitoring atomic and molecular spectra in the characterization of other matrix interference effects is demonstrated. EXPERIMENTAL SECTION Instrumentation. A block diagram of the instrumentation utilized herein is presented in Figure 1. A photodiode array (Model RL 1024C/17, EG and G Reticon, Sunnyvale, CA) with 1024 diodes spaced at intervals of 2.5 X 10" cm was mounted in the existing slit plane of the spectrometer. The optical characteristics of the spectrometer system (Model 303, Perkin-Elmer Corp., Norwalk, CT) were designed so a wavelength interval of 150 nm could be interrogated offering a diode-bdiode resolution of 0.15 nm in the absence of other resolution limitations. The entrance-slit cam drive of the spectrometer was replaced by a screw-drive mechanism, the mechanical wavelength drive was converted to stepper motor (Model M0061-FC08, Superior Electric Co., Bristol, CT) control, and all spectrometer electronics were replaced to facilitate automation of instrument control and data acquisition. Details of the design, construction, and evaluation of this unit for multiple wavelength and/or multielement absorption analysis have been presented (23). The present work relied on a deuterium arc source (Model D-102-S, Heath Co., Benton Harbor, MI) to cover the 200-350 nm wavelength range. The Model CRA-90 graphite furnace workhead (VarianTechtron, Ltd., Springvale, Australia), was mounted on a stage with X-Y manipulators to facilitate optical alignment. Light masks with 0.8- and 0.2-cm orifice diameters were placed before 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 23, DECEMBER 1. 1988
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Flgure 1. Block diagram of the electrothermal atomizer muniple wavelength spectrometer system (Mi-M4 are mirrors. L i s a bos, and G is the gratiig).
and after the furnace, respectively,and the image of the source was focused on the exit mask. The furnace was purged with 10% methane in argon to preserve the pyrolytic coatings of the graphite atomizer and support electrodes. The system is computer controlled by a Model LSI-11/02 (Digital Equipment Carp., Hudson, NH) with execution through software for the adjustment of experimental variables aswell as data acquisition, presentation, and analysis (23).For the present investigation, the spectrometer-limited optical resolution was 1.5 nm; so the data collection involved averaging the signals recorded by sequences of 10 adjacent diodes to obtain 100 points per spectrum. The intensity of the light source required single PDA integration times of 500 ms to accumulate signals equivalent to 95% of the diode saturation level at the mast intense wavelengths of the source. Thus, all diodes were interrogated at 500-ms intervals; the 20-30 ,IS required to interrogate a diode was considered negligible compared to the integration time. The required integration time, however, imposed the limit on the timetemperature resolution of the system (see following discussion). Reagents. Stock solutionsof Mn and Cu were prepared from better than 99.99% pure metals dissolved in 0.2 M HN02to obtain working solutions with metal concentrations of 3 g/L. Reagent grade ascorbic acid, NaC1, KCI, CaCIy2H2O, MgCIy6H,0, MnClr4H20, and CuCl&H20 were dissolved in distilled-deionized water to prepare stock solutions that were 10% (w/v) of the anhydrous salt. The diluted working solutions were prepared fresh' daily from these stock solutions in glassware that was precleaned with "0,. Procedure. A 5.0-,tL sample aliquot was dispensed into the furnace with an Eppendorf pipet. Each sample was dried at 95 'C for 60 s followed by a 400 'C/s ramp to an atomization temperature of 2500 O C , which was maintained for 2 8. The ash cycle was not used to avoid possible sample losses at the temperatures typically used for ashing. Data acquisition was initiated 55 s into the dry cycle, to establish base-line data prior to the atomization process, and continued at 500-ms intervals throughout the atomization ramp. Thus, 10 spectral scans were obtained at the conclusion of the dry cycle and 16 scans were obtained during the atomization cycle. This provided spectral scans representing integrations over 200 "C temperature increments during the ramp period. Absorbance versus wavelength versus temperature data obtained from these experiments were used for evaluation.
RESULTS AND DISCUSSION The temperature-resolved absorption spectrum obtained during the atomization cycle for a solution of MnCI2 is presented in Figure 2. The oeeurrence of two distinct events is apparent in this figure. First, the vaporization and absorbance of a molecular entity is evident over the 1100-1500 "C temperature range. The spectrum is characterized by an intense hand below 200 nm and a secondary hand at 242 nm (22,22). The fact that it also appears in the expected temperature range based on the MnClz boiling point of 1100 "C, supports the assignment of the spectrum to MnC1. The second feature is the appearance of atomic absorption at 279 nm because of the three unresolved Mn spectral absorption lines
II*PERhTYPI.
0
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Figure 2. Spectra obtained with a 1% MnCI, solution: (A) the wavelength-temperature profile:(B) absorption spectrum of MnCl at 1500 "C: (C) absorbance versus temperature profilesfor MnCl at 242 nm (solid) and Mn at 279 nm (hatched).
Flgure 3. Threedimensional profiles obtained with 1 % solutions of matrix chloraes: (A) KCI, (B) NaCI. (C) CaCI,. (0) MgCI,.
at this position. The atomic absorption signal appears at a nominal temperature of 1800 "C, passes through a maximum, and diminishes to an unmeasurable level at ahont 2300 "C. The disappearance of the halide absorbance prior to the appearance of atoms indicates that direct dissociation of the vapor-phase halide does not contribute exclusively to the atom population. According to Sturgeon et al. (14),the production of Mn atoms originates from the dissociation of MnO. Hence, it is pasible that the gas-phase halide is converted to the oxide in the lower temperature range and then to atoms at the higher temperature. It is noted, however, that the diffusion coefficients of atoms and molecules approximate 1cm2/s or more (24) at these temperature levels so such species could readily be transported the 1-2 cm necessary to escape the furnace and the optical path in 1-2 s. This time estimate is consistent with the observation periods for the halide and the atoms and implies that much of the halide could be removed from the optical path prior to the appearance of the atoms. Although the present studies did not allow estimation of the amount lost, the lass possibility suggests that the direct vaporization and atomization of MnO(s) formed on the interior surfaces of the furnace could account for the atom population. To characterize the vaporization behavior and spectra of the matrix halides, 1% (w/v) solutions of sodium, potassium, calcium, and magnesium chloride were examined under the same conditions. The profiles presented in Figure 3 show
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ANALYTICAL CHEMISTRY, VOL. 60. NO, 23. DECEMBER 1,
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Table I. Summary of Data for Chloride Matrices with and without Manganese Present M-CI bond dissociation energy, metal salt system* MnC1, CaCI,
CaCI, + Mn C"C1, KCI KCI i Mn MgCL MgC& + Mn NaCl NaCl + Mn
bp, 'C (27)
1100 1600
kcal/mol (8, 26)
85 81
*3 2
temp rangec of observation: OC
absorbance maximum, nm lit. (ref) observedl 240-250 (21) 214 (26)
242 218 224 237
993 1500
84+2 101 + 0.5
223 235 (26)
1412
75 i 3
200 (26)
1413
98 i 2
246 (26)
%
mangamatrix halide
nese
chloride
atomic Mn
11W1700 1500-2300 1300-2300 1500-1700 800-1400 11~1900 1100-1900 Noa
1700-2300
1100-1900 900-1700 900-1700
suppression of Mn atom absorbanced
1300-2300
26
1300-2300
15
1300-1700
1500-2300
4
No'
1300-2300
15
1100-1900 247
'Mn added to matrix salt solutions as the nitrate to obtain a manganese concentration of 2100 mg/L unless otherwise noted. 'Values measured herein have an uncertainty of +2 nm. 'Since the measured signals were integrated over a temperature range of 200 OC, the temperature resolution is limited to this increment. dSuppression estimated by integrating the absorbance at 279 nm over the range of observation usina an eauivalent solution of Mn(N0.). as the reference. 'No indicates that MnCl absorotion was not observed.
Table 11. Solubilities of the Relevant Salts (26)
cation
solubility, a / L at 100 'c-vs nitrate chloride
Ca
3760
K Mg Mn Na
2470
cu
V.S.' V.S.' 1800 2437
1560 567 727 1238 391 1079
mncn corrected re1 salubilitf 0.62
0.22 0.28 1.0 0.15 0.76'
'Calculated for the chloride salts relative to the MnCI, concentration of 4.8 g/L (2100 mg of Mn/L) with each matrix salt at 1% (w/v). lVery soluble. 'Calculated relative to the Mn for a Cu concentration of 1500 me/L.
Figure 4. Three-dlmenslonalpmfiles obtained by addition of 2100 mg of MnIL to the matrix sail solutions: Key is the same as in Figure 3.
distinct differences in the spectra and their appearance temperatures (see summary in Table I). The spectra obtained are in agreement with thme reported in the literature and the temperature ranges of observation are consistent with what one would predict from boiling point data. The sharp peak observed at 285 nm for MgC1, (Figure 3D) is from atomic absorption of Mg at this wavelength. T o characterize the effects of the respective matrix salts on Mn absorption, it was added to each of the 1%salt matrix solutions as the nitrate to obtain a final Mn concentration of 2100 mg/L. The results obtained are summarized in Figure 4 and Table I. These include estimates of the degree of matrix interference on the atomic absorption of Mn obtained by using an equivalent concentration solution of Mn(N0J2, analyzed under the same conditions, as the reference standard. The MnCl absorption maximum at 242 nm was not observed for the KC1 or NaCl matrices and the Mn atomic absorption signals obtained for these matrices were in agreement with the absorption for the reference solution within experimental error (i5%). The lack of interference for these matrices has been reported by others (6,lO). The presence of there matrices also resulted in the appearance of the atomic popultion at temperatures nominally 400 "C lower than observed in their absence. Although congruent vaporization of the Mn and the K and Na salts is evident, clearly the majority of the Mn atoms appeared 1-2 s after the alkali halide spectra diminished to unmeasurable levels. Again this
implies that the matrix halides may have diffused from the atom cell prior to the appearance of the Mn atoms. Under such circumstances the gas-phase formation of MnCl species would not be anticipated. The MnCl absorption maximum at 242 nm was obgerved for the CaCI, and M&12 matrices and its maximum population was coincident with their respective population maxima The temporal delay between the maxima of the Mn molecular and atomic populations was
