2020
Anal. Chem. 1981, 53, 2020-2027
(2) Shaw, G. J.; Qulrke, J. M. E.; Eglinton, G. J. Chem. SOC., Perkin Trans. 7 1978, 1655-1659. (3) Budziklewlcz, H.; Pesch, R. Org. Mass Specfrom. 1976, 1 7 , 821-830. (4) Budziklewlcz, H.; Drewes, S. E. Justus Llebigs Ann. Chem. 1968, 776, 222-223. (5) Leung, H.-W.; Ichikawa, H.; LI, Y.-H.; Harrison, A. G. J. Am. Chem. SOC. 1978, 700, 2479-2484. (6) Mauzerall, D. J. Am. Chem. SOC. 1960, 82, 2601-2605. (7) Jackson, A. H.; Kenner, G. W.; Sach, G. S. J. Chem. SOC. C 1967, 2045-2059. (6) Hoppe, W.; WIII, G.; Gassmann, J.; Weichselgartner, H. 2.Krlsfallogr. Krisfallgeom., Kristallphys., Krlstallchem. 1967, 728. 18-35. (9) Jackson, A. H.; Kenner, G. W.; Smlth, K. M.; Aplin, R. T.; Budzlklewicz, H.; Djerassl, C. Tetrahedron 1965, 27, 2913-2924. (IO) Quirke, J. M. E.; Shaw, 0.J.; Soper, P. D.; Maxwell, J. R. Tetrahedron 1980, 36, 3261-3267. (11) Morley, H. V.; Holt, A. S. Can. J. Chem. 1961, 39, 755-760. (12) Ellsworth, R. K.; Aronoff, S. Arch. Biochem. Biophys. 1968, 724, 358-364. (13) Nlcolaus, R. A. Rass. Med. Sper., Suppl. 21980, 7 , 1-23. (14) Chapman, R. A.; Roomi, M. W.; Morton, T. C.; Krajcarski, P. T.; MacDonald, S. F. Can. J. Cheh. 1971, 49, 3544-3564. (15) Boybn, D. B. Org. Mass Spectrom. 1970, 3 , 339-351.
(16) Alexander, R.; Eglinton, G.; GIII, J. P.; Volkman, J. K. J. Hlgh Res. Chromatogr. Chromatogr. Commun. 1980, 3 , 521-522. (17) Scott, A. I. Acc. Chem. Res. 1978, 7 7 , 29-36. (18) Holt, A. S.; Purdle, J. W.; Wasley, J. W. F. Can. J. Chem. 1966, 44, 88-93. (19) Scott, A. I.; Lee, E.; Townsend, C. A. Bioorg. Chem. 1974, 3, 229-237. ~. . (20) Quirke, J. M. E.; Maxwell, J. R.; Eglinton, 0.; Sanders, J. M. K. Tetrahedron Left. 1980. 21. 2987-2990. (21) Baker, E. W.; Yen; T. F.; Dickk,J. P.; Rhodes, R. E.; Clark, L. F. J. Am. Chem. Soc.1967, 89, 3631-3639.
RECEIVED for review January 21,1981. Accepted July 21,1981. This work was supported by the Natural Environment Research Council (Grant No. GR3/2951 and GR3/3758) for instrumentation. We thank the Science Research Council for a studentship (G.J.S.) and the National Aeronautics and Space Administration for support through a subcontract from the University of California a t Berkeley (Contract No. NGL 05003-003).
Speciation at Trace Levels by Helium Microwave-Induced Plasma Emission Spectrometry Chrlstopher
F. Bauer‘
and David
F. S.
Natusch”
Department of Chemistty, Colorado State University, Fort Colllns, Colorado 80523
An evolved gas analysls/emisslon spectrometer system capable of Identifying Inorganic compounds at trace levels In solid samples, especially environmental particulates, Is described. As a sample Is heated from 25 to 1000 OC at 140 ‘C/min, Its components vaporize at characterlstlc temperatures Into an atmospheric pressure, hellum, mlcrowave-lnduced plasma, which acts as an atomic emission source by whlch the chemlcal composltlon of the evolved vapors Is determlned. Unique excltatlon condltlons In He allow trace amounts of both metals and nonmetals to be determlned. Identlflcatlon of a glven compound Is based on the colncldent observation of Its metal and nonmetal components at Its temperature of vaporization. This coincidence exists for the pure halide, sulflde, and sulfate salts of Cd, Hg, Pb, and Zn. A number of obsfacles may hamper Identlflcatlon, most Importantly, chemical reactlons In the sample whlch alter the Identity of compounds before vaporlzatlon.
“Chemical form” is becoming recognized as an important concept in evaluating the environmental impact of inorganic pollutants. At present, analytical chemistry falls short of the challenge of compound identification in solids because the elements typically of interest to environmental chemists, namely, toxic metals such as Cd, Hg, and Pb, exist at concentrations too small for established methods of speciation, such as X-ray diffraction and infrared spectrometry, to be applicable. Consequently, the development of new methods capable of distinguishing the forms of trace-level metals in the solid state is highly desirable. Current address: Department of Chemistry, University of New Hampshire, Durham, NH 03824.
One approach that shows significant promise is evolved gas analysis (EGA)-a thermal analytical method which monitors gases released from a sample as a function of temperature. Of the several types of analyzers used for EGA, the mass spectrometer has become the most important. Recent applications of EGA/MS have included minerals (1,2),oil shales (31, polymers (41, and lunar soils (3). Environmental studies using evolved gas analysis are few (5). High-resolution mass spectrometry, used primarily to study organic compounds in urban airborne particulates (61, identified several inorganic species as well, namely, HzS04, NH4HS04, (NH&S04, NH4N03,NH4C1,NaHS04, NaN03, S8,As406,Cd, a Zn compound, and 12. In settled urban dust, evolved gas analysis using conductivity detectors for evolved C02 and SO2showed the presence of organic carbon, inorganic carbonates, and inorganic sulfates (7, 8). Despite the obvious cost advantages of atomic spectrometry over mass spectrometry as a selective detector, the former has barely been investigated. Geochemists were first to recognize that speciation of metal compounds by EGA/atomic spectrometry may be possible and profitable. The natural association of Hg with lead and zinc sulfides allows low-level mercury to be used as a tracer for locating potential areas of lead and zinc sulfide mineralization. By means of EGA, HgCl2 was distinguished clearly from HgS by the difference in the Hg vaporization temperature (9, 10). Since only HgS was indicative of sulfide ore deposits, EGA was superior to total Hg determinationsas a prospecting tool. Less detailed studies of Cd and P b minerals by EGA have also been reported (11). Hanamura (12) vaporized pure arsenic oxides, mercury chlorides, and Si and SiOz into an Ar microwave-induced plasma to demonstrate that the different species did indeed volatilize at different temperatures and that speciation based on this phenomenon was feasible. Robinson and Rhodes (13) gave a similar prognosis by observing Pb and Cd compound
0003-2700/81/0353-2020$01.25/00 1981 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981 TMOlO CAVITY
2021
ITz5!
-i
I
I
TUNING ELEMENT
2 4 5 0 MHZ POWER SUPPLY
Figure 1. Block diagram of the evolved gas analysis/emission spectrometer system.
Flgure 2. Furnace and transfer line components for evolved gas analysis system (all tubing is quartz): (A) furnace, showing induction coil above and clamp for furnace tube below: (B) furnace tube, set down through coil and clamped; (C) discharge tube, 0.15 cm i.d., 0.65 0.d.: (D) crucible pedestal; (E) crucible, graphite cup packed with C black, quartz cup rests In top; dlmensions in cm.
Table I. Summary of Instrumentation and Operating Conditions for Helium Plasma Emission Spectrometer Scintillonics, Inc., Fort Collins, CO; power supply Model HV-l5A, 2450 MHz, 120 W maximum output, 80-100 W used, reflected power less than 3 W
microwave cavity
TM,,,, brass, design of Beenakker (15)
volatilization temperatures in a slowly heated carbon rod atomic absorption spectrometer. Thorpe (14)attempted to identify Cd, Hg, and P b compounds in roadside dust and coal fly ash using an atmospheric pressure, He dc discharge as the analyzer. His results could not prove the existence of any particular compound; however, certain compounds were assigned a probability for occurrence. Indeed, previous efforta have shown that EGA with emission spectrometry or mass spectrometry may have the potential for metal speciation at trace levels; however, little progress has been made for several reasons. First, all of the atomic spectrometric methods have relied solely on a match between the EGA profile of the sample and that of a known material. This correspondence is sufficient as long as the compound volatilizes identically in both cases. Physical and chemical factors, for example, adsorption, inclusion (9),or chemical reaction, can affect the evolution temperature and may destroy this correspondence. Secondly, no detailed theoretical or experimental investigation has been performed on the basic phenomena. of dynamic volatilization of inorganic salts and the encoding of these processes by the evolved gas analysis experiment. Laying this foundation is a prerequisite for real-sample analysis. An evolved gas analysis method based on emission spectrometry with a He microwave plasma is presented in this report. The He plasma exposes to analytical scrutiny the nonmetal as well as metal components of the evolved vapors (15,16). Vaporization temperature is supplemented by the coincident evolution of metal and nonmetal components as evidence for compound identity. Realization of this potential for speciation requires that the method exhibit three essential features. First, some separation in vaporization temperature between different compounds of the same element must be provided. Secondly, the analysis of the vapor composition as a function of temperature must be interpretable in terms of the compounds in the original sample. Thirdly, the gas analyzer must provide detection limits compatible with the trace level stipulation. These three characteristics were the criteria by which evolved gas analysis was evaluated as a speciation tool. EXPERIMENTAL SECTION Apparatus. The evolved gas analysis instrument is illustrated in Figure 1. The furnace (Leco induction furnace, Model 521) was adapted to allow temperature programming by means of a motor-driven variable transformer (14). Figure 2 shows the
impedance-matching Hewlett-Packard slotted line (Model device 805A) modified to perform as their slide-screw tuner (Model 872A) by adding XY probe as a variable reactance monochromator GCA/McPherson, Model EU700-56 reciprocal linear 20 AImm dispersion spectral band-pass 0.5 A photomultiplier tube EM1 9781B (S-5 response) PDP 8/L using floppy disk storage computer crucible design used for sample containment. Ideally, the crucible should be inert toward the sample and reusable. Quartz inserts fulfilled this requirement; simple graphite cups were inadequate because they induced reduction of some metal ions and could not be cleaned adequately for reuse. The temperature in the sample is a function of crucible design and position, carrier gas, and temperature program rate. After optimization of these four factors, which will be described later, the temperature scale was calibrated by observing the variable transformer setting at which certain pure materials melted. The sample crucibles were supported within the furnace coils by a quartz pedestal (Figure 2), whose position was fixed by that of the furnace tube. The latter was held rigidly in place by means of a clamp mounted on the front wall of the furnace. The quartz transfer line and the discharge tube were joined by a quartz ball and socket joint. By wrapping the line with a Briskeat hightemperature heating tape (Brisco Manufacturing Co., Columbus, OH) and asbestos insultion, the temperature attained a maximum of 650 OC. This was varied by means of a variable transformer and monitored by a thermocouple. A thin piece of asbestos board was placed around the discharge tube where the tube enters the cavity to insulate the cavity from the heating tape coils, which became exposed after prolonged use. A quartz side arm intersecting the transfer line just upstream from the cavity expedited sample changes by providing a He flow to the dischargewhen the pedestal was removed. Upon replacing the pedestal and directing the He back through the furnace, air was swept into the discharge region. By tuning the circuit, the discharge could be maintained until the air was replaced by He. This bypass obviated reignition of the plasma between samples. However, repetition of this process over a full day’s use accelerated erosion of the discharge tube and caused silica deposits to build up at its exit, thus cutting down emission intensity. Components of the emission spectrometric detector and its operating conditions are summarized in Table I. Correction for background emission was performed by means of a programmable quartz plate mounted before the monochromator exit slit. During the course of an EGA experiment, one observation (three sam-
2022
a
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
plings of signal and background) was recorded every 2.6 s up to a designated number of points. Materials. Research grade helium gas and reagent grade solids were used without further purification. EGA Experiments. Microgram amounts of pure compounds were weighed on a microbalance and transferred to the sample crucible, which was then set into the top of the furnace pedestal. A few minutes were necessary for He to purge air from around the sample. After the computer was directed to obtain a given number of data points, data acquisition was begun simultaneously with the motion of the mechanical programmer. Data were still acquired beyond the maximum temperature (990 OC) until the specified number of points was reached. THEORY Inorganic salts can be classified according to three types of vaporization behaviors: (1) Congruent evaporation of a molecular entity, e.g. PbClz(1) PbClz(g)
-
at Tl (2) Congruent evaporation with dissociation, e.g. 2CdO(s) 2Cd(g) Oz(g) +
a t T2 (3) Incongruent evaporation, e.g. CdCO,(s) CdO(s) +
+
+ COz(g)
a t T3 CdO(s) 2Cd(g) + Oz(g) at T4where T35 T4. Recall that EGA data are in the form of emission intensity of a specific element as a function of sample temperature. When vaporization occurs a t T I ,T2,T3, and T4,a peak will appear in the emission signal. These temperatures, called “appearance” or “evolution” temperatures, are determined first by thermodynamic properties, namely, vapor pressure, condensed-to-vaporphase transition temperature, and decomposition temperature, and secondly by the kinetics of vaporization and mass transfer. For each type of reaction, compound identity can be inferred in two ways: by the evolution temperature and by the coincidence of metal and nonmetal peak temperatures. Previous investigations into EGA for trace metals have relied solely on the former criterion, but, as indicated above, this can be complicated by intersample variability in thermodynamic properties and kinetic behavior. The correlation between the EGA profiles of the metal and the nonmetal is a more reliable diagnostic. For reactions of type (l), the Pb-C1 chemical bond forces both elements to enter the gas phase and the plasma together. The individual EGA profiles for P b and C1 emissions will be identical, with peaks a t temperature T1.Despite dissociation in type (2) reactions, Cd and 0 are entrained in the carrier gas simultaneously, resulting in Cd and 0 profiles both having a peak at temperature T2 In both processes the conjunction of the metal and nonmetal peaks at the same temperature is indicative that they left the solid state at the same time and, therefore, were associated there as a compound. For processes like type (3), the first step may precede the second such that the metal and nonmetal components, in this case Cd and C, are evolved at different temperatures. Under these circumstances the only means of identification is by knowing temperature T3from a previous EGA of CdCOs. In some cases, T3and T4are in fact indistinguishable, and the decomposition can be considered a type (2) process. In actuality vaporization may be more complicated than the examples indicate (17). For instance, in process (1)dimers and higher order gaseous molecules may occur, and in (3) the condensed phase may be a liquid. In general, however, halide salts belong to category (1) along with a few oxides, notably +
PbO. The other oxides and sulfides behave as case (2). Most oxyanion compounds-carbonates, nitrates, and sulfatesbelong to category (3). RESULTS AND DISCUSSION Optimization a n d Standardization of Procedures. Observation of both metals and nonmetals in the same source raised the possibility of wavelength interferences between the two groups. The most sensitive nonmetal lines not subject to spectral overlap, as determined from wavelength tabulations and cross-checkingspectra of metals and nonmetals, are F I 6856.2 8, C1 I1 4794.5 8, Br I1 4785.5 A, C I 1931 A, and S I1 5453.9 A. Nitrogen (7442 and 7468 A) and oxygen (7772, 7774, and 7775 A) were barely detectable because of the insensitivity of the photomultiplier tube above 7000 A; in practice, these elements would be important to monitor. Metal lines selected were those usually chosen for emission spectrometry; Cd I 2288.0 .&,Hg I 2536.5 A, Pb I 2170.0 A, and Zn 12138.6 8. Signal-to-noise ratio (S/N) was optimized independently for He flow rate, microwave power, slit width, and photomultiplier gain for each element. The elements were introduced into the plasma at a constant rate from evaporating organic compounds or heated salts. The relative signal-tonoise ratio tends to increase with greater power and in some cases reaches a maximum (17). Although improvements indetection limits for some of these lines may have resulted from use of maximum power (120 W), increasing power caused exponentially enhanced erosion of the discharge tube, as indicated by the intensity of the Si 2528.5 A line, and considerable heating of the coaxial connectors closest to the cavity. In all experiments 80 W was used consistently and gave adequate results; only for F and S was 100 W necessary because of the decreased response of the photomultiplier tube at their wavelengths. Mass Transfer of Evolved Vapors. When vapors are released from the sample, transport to the plasma with a minimum of band-broadening is desirable so that different vapor releases might be separated in time. Since this work was investigative, no special precautions were taken with the design of the transfer line except to make it as short as possible. The remaining factors affecting broadening are the carrier gas flow rate and the temperature program rate. Lead chloride was analyzed repeatedly at different helium flow rates and temperature programs. Increased transport efficiency was indicated by a decreasing width-at-half-maximum for the EGA peak at 700 “C, where PbClz evaporates. The minimum peak width was located around flow rates of 420 mL/min and program rates at 140 “C/min. Transfer Line Temperature. If the transfer line is not maintained at a high temperature, it might act as an adsorption or condensation surface for vapors. Several problems could arise. First, the EGA peak shape may be distorted and broadened. Analyses of HgC12,monitored as Hg, show that decreasing temperature or flow rate causes peak broadening or tailing (17). Apparently part of the HgC12condenses under these conditions and is released only when the carrier gas was heated further by the increasing crucible temperature. Secondly, irreversible losses can occur. Lead chloride, monitored as Pb, showed a significant decrease in peak area per unit mass as flow rate and transfer line temperature were decreased. Lastly, since the maximum temperature of the transfer line was 650 “C, material that volatilized above this temperature unavoidably condensed in part, creating a memory effect. This problem was circumvented by analyzing a blank crucible before each sample at the wavelength of interest. Eventually, the line required disassembly and cleaning. Temperature Program Reproducibility. The mechanical reproducibility of the motor-driven variable transformer
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981 Cd'
P J
J CdO
I
n
CdS04
,
?'i
TEMPERATURE
_i 500
500 TEMPERATURE
990 ('C)
2023
990
TEMPERATURE
?C)
Flgure 3. EGA profiles of CI (left)and Cd (right) compounds, using CI and Cd emission line, respectively. was equivalent to 10 "C. With PbClz an analysis of variance on five replicates of five crucibles showed that the Pb evolution temperature variance between crucibles was negligible with respect to that of one crucible. The measured variance was about 60 "C, which exceeded the variability of the programmer alone. To parallel actual procedures, the crucibles had been removed from the pedestal after each trial; apparently the reproducibility of positioning the crucible in the furnace was the major factor in the temperature variance. Standard Procedures. For all subsequent EGA runs the following conditions were standardized: transfer line at maximum temperature of 650 "C, flow rate of He 420 mL/min, program rate 140 "C/min. These define minimum transfer line holdup and minimum peak width. The given flow rate is such that it is not far from that of optimum S / N for all elements. To account for uncontrolled memory effects, we analyzed an empty crucible a t the desired wavelength immediately before each sample. The resulting EGA profile defined the blank for that element at all temperatures. Pure Compound VaporizationBehavior. To determine whether metal salts would vaporize as predicted by the literature and whether identity could be established, we performed evolved gas analyses of pure materials. Comparison of the EGA data for compounds of a given element demonstrates the ability to distinguish between different compounds of that element, i.e., thermal selectivity. Figure 3 illustrates this characteristic for C1- and Cd-containing compounds, Figure 4 for 8- and Hg-containing compounds. It is apparent that the evolution temperatures of different compounds of a given element can be substantially different; however, overlap does occur in some cases. Significant overlap also existed while observing the metal emission line for the oxide, sulfide, and sulfate salts of P b and Zn (not shown) (17). Several general observations can be made. First, the observed peak temperature corresponds with, but is not necessarily equal to, the boiling, sublimation, or decomposition temperature for each compound. For example, in Figure 3 the EGA peak temperatures for the C1-containing compounds are 250, 260, 650, 720, 750, and 990 "C, for HgClZthrough NaC1, respectively. The correspondingboiling or sublimation points are 302,400,732,950,960, and 1413 "C (18). It appears
(+C)
pn' 500 990 TEMPERATURE ('C)
Flgure 4. EGA profiles of S (left)and Hg (right) compounds, Using S and Hg emission line, respectively. that, at least for the halide salts, as the temperature rises the vapor pressure increases sufficiently to exhaust the supply of solid before the boiling point is reached. Secondly, some compounds have low intensity peaks around 200 "C; intensities were found to vary irreproducibly. This temperature range coincides with that for release of adsorbed water and waters of crystallization from the sample or crucible. None of the nonmetals nor Hg exhibited this phenomenon, only Cd, Pb, and Zn. The absence of nonmetal peaks ruled out physical ejection of a portion of the sample. Experience has shown that Cd, Pb, and Zn exhibit a significant memory effect because they deposit in the transfer line and in the discharge tube during previous analyses of their salts. The steady-state emission signal arising from the release of these residual metals from hot surfaces may have been enhanced by the presence of this water vapor. Water vapor at low levels (17,19) does change the excitation characteristics of the plasma. Further examples of such interferences are presented in a later section. Finally, the narrowness of a peak depended on its position with respect to the maximum temperature of the furnace. Note again that in all EGA profiles the furnace temperature becomes isothermal at 990 "C (tothe right of the 990 "C mark in the figures). Those compounds evolving below the limit were narrow, whereas those at the limit were drawn out because the temperature was insufficient for driving the vaporization process quickly to completion. Most metal peaks arising from the less volatile Zn compounds, some P b compounds, and CdFz were broadened for this reason. Based on at least three analyses, run-to-run reproducibility of the profiles was excellent for all except the S-containing Cd and Zn compounds, whose peak temperatures and shapes shifted somewhat. Apparently, these shifts resulted because these compounds were vaporizing, at least part of the time, under isothermal conditions at the maximum temperature of the furnace. Thus, the shape and location of the peak on the horizontal scale is subject only to kinetic factors, such as the crucible shape and sample amount and shape, which varied from run to run. Only PbS04 and PbS showed poor profile reproducibility. The reasons for this behavior, exemplified in Figure 5, may include intersample heterogeneity,the noted memory problem for Pb, or chemical reaction with the quartz container. There is also a possibility that the background correction a t the Pb line used was not complete. Further investigation is required.
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
2024
ZnC12 on Z n
b
ZnS on S
/ o n Pb
PbS
on S
h
PbS04 on S
f
PbS
3-c
u . .
CdC120n Cd
C d S 0 4 on Cd
CdS on S
2 500
500 990 TEMPERATURE (*C I
75
I
CdC12 on C I
990
TEMPERATURE
(DC)
Flgure 5. Comparison of metal and nonmetal EGA profilesfor S" and SO-: compounds.
h
N a C l on C I
J!
500 990 T E M PER ATUR E P Cl
Figure 7. Comparison of metal and nonmetal EGA profiles for CI compounds.
25
HgsO,
on
s
590
500
TEMPERATURE
i"C 1
Figure 8. Comparison of metal and nonmetal EGA profiles for C0-: 25
300
600
compounds.
900
TEMPERATURE
(*C)
Figure 6. Comparison of metal and nonmetal EGA profiles for Hg compounds.
The means by which species may be identified is illustrated by comparison of metal and nonmetal data for the same compound, as illustrated by Figures 5-8 for sulfide and sulfate, mercury, chloride, and carbonate compounds. Most of the compounds have metal and nonmetal peak temperatures that are the same within the 60 "C reproducibility of the temperature scale. This correlation indicates that the metal and nonmetal left the sample simultaneouslyand were chemically associated therein. Even the sulfates, which vaporize incongruently, display this trend. Most salts behave as predicted by the literature (17). The few that do not apparently are subject to chemical reaction with the quartz container; these are dmcussed briefly. Metallic Cd (Figure 3) shows two peaks, whereas one peak, that of vaporizing CdO, was expected. It is suspected that the peak -500 "C represents vaporization of CdO because at this temperature the metal is molten (melting point 321 "C, boiling point 765 "C) and its vapor pressure is relatively high, as was the case for all the halide salts. The 800 "C peak matches the behavior of CdS and CdO (Figure 3) both of which are likely surface contaminanta of Cd metal. Another explanation
is that a partial reaction of the hot Cd metal with the quartz container occurs forming some CdO. The Cd halides tend to have three peaks in a characteristic pattern (Figure 3). The first peak in both the chloride and bromide profiies (600 "C) has been amplified by use of a wider slit width. Figure 7 shows the peak in the chloride channel appears between the first and second peaks in the Cd channel. Apparently, CdClzbegins to react with the quartz container starting -600 "C and continuing to 750 "C with the concomitant release of chlorine and production of CdO. This is reasonable since the 800 "C peak in the Cd channel matches the temperature of CdO (Figure 3). A similar phenomenon may occur for ZnC1, as well (Figure 7): ZnClzevolving at -600 "C but also producing some ZnO, which volatilizes at a higher temperature. The 600 "C peak is flat-topped because the signal saturated the output electronics. Lead sulfide (Figure 5) appears to show reactivity with the container-although the Pb and S signals both peak at 800 "C, which is within the temperature range reported for PbS vaporization (17),Pb is released at higher temperature as well. Actually, this observation is in accordance with theory, which predicts both type 1and 2 behavior. Thus, the 800 "C peak represents loss of S as PbS(g) and S2(g);the second process leaves Pb(l), which is not above its boiling point (1744 "C). As the temperature
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
Table 11. Comparison of Detection Limits Obtained byEGA System with Those by Other Techniques Using the Same Plasma detection limits ~~~~
element
'Line, A
F
6856.2 4794.5 4785.5 11931 15453.9 12288.0 12536.5 !2170.0 2138.6
c1 Br
C S Cd Hg Pb Zn
2025
GROUND 4 5 MIN
~
this work (ng) 1500 150 550 5 1200 0.5 1 10
ref 1 5 a ref 21 (ng) (ng)
0.28 0.20 0.016 1.0
ref 1 6 (pg)
0.34 0.64 0.40 2.5 0.04 0.02
0.56 0.35
Assumed 40 s release time to equate with EGA. Assumed 40 s release time to equate with EGA and flow rate of 50 mL/min.
I
L
25
rises, the remainder of the P b vaporizes. The carbonates vaporize incongruently. Therefore, identification must be based on the temperature of COZ release. Figure 8 shows that there is only a slight difference between the three metals, which would make it difficult to distinguish between theim. On the other hand, most of the major carbonate minerals (Ca, Fe, Mg) decompose above 450 OC so that the toxic metal species may be distinguishableas a group (20). It is apparent from Figures 3-8 that even for simple materials temperature alone may not uniquely indicate a given compound because of the overlap among compound appearance temperatures of a given element. However, since the coincidence between metal and nonmetal has been discovered for many of the compounds of interest, this additional observation becomes stronger evidence for identity. In several cases interfering phenomena such as transfer line memory and container reactivity have complicated interpretation. Instrumental modifications that might surmount these obstacles will be discussed later. Some confusion may yet arise because of overlap among appearance temperatures, as with P b and Cd chlorides or the sulfides and sulfates. Consequently, establishing evidence for or against a compound's presence will require a survey to determine complicity of all volatile metals and nonmetals in significant concentrations. It has been established, then, that there is a reasonable basis for claiming that EGA/ES has the ability to identify inorganic compounds. The next question is: Is this information available for trace level constituents? Detection limits were determined for each element by analyzing one of its compounds in a mixture with silica; it had been established beforehand that no reaction with the silica occurred, as detailed in the following section. Optimum S/N conditions were used. The limits were chosen to be three times the standard deviation of the noise of the EGA profiles. Table I1 demonstrates that the method can reach the part-per-million level, given a sample size of 10 mg, and thus qualitative information at trace levels is accessible. The fact that the metals have better limits than the nonmetals is expected because the photomultiplier tube sensitivity is poor at the longer wavelengths of the nonmetals. Since no special consideration was given to the optics and photomultipliertube, minimum detectability should be improved considerably by upgrading these components as demonstrated by the values measured by the other workers for the same type of plasma. Interferences. Some of the complications introduced by instrumental artifacts and container reactivity have already been discussed. Problems engendered by the sample itself are just as important. These are reactions between components within the sample, the physical size of the sample, and
A
SHAKEN
100
400
7EMPERATURE
rji
A 600
800
('C)
Flgure 9. Effect of grinding duration on decomposition of HgS0, In silica. EGA on Hg channel.
the upsetting of the plasma by other volatiles released from the sample. To determine whether reactions between a given compound and its matrix might limit the ability to identify that compound, it was necessary to prepare dilute mixtures of the compound in the matrix. Two means of preparation were available: dry mixing of the solids, and evaporation of a metal solution mixed with the matrix. The former method can be quick and easy as long as the desired concentration is no less than part per thousand by weight. Homogeneity is difficult to guarantee, and the removal of aliquots for study is subject to a sampling error. Solutions would perhaps overcome the homogeneity problem but there is no assurance that .the dissolved metal would recrystallize with its original anion instead of adsorbing onto the matrix. For example, vaporization of PbClz deposited by solution onto silica shows two peaks in the Pb channel. All the C1 evolves with the first Pb peak, but the second Pb peak suggests the formation of PbO in an amount more substantial for the dissolution procedure than for solid mixing. Consequently,dry mixing was selected because it involved less effort, was not limited to soluble salts, and did not alter the form of the compound of interest. The matrix investigated most eqtensively was silica, a common component of environmental samples. Although the sample container itself is silica, intimate mixing induces a number of reactions not observed from the container alone. Three to five grams of silica was combined with one to ten milligrams of the compound of interest. At these part-perthousand concentrations, removal of a 10-mg aliquot represents elemental amounh in the tens of micrograms, the same amounts used for the pure compound experiments. Homogeneity was promoted by gentle grinding of the pure compound to decrease the particle size, if necessary, before mixing with the silica, which was
c
in S102
Y)
z
Lu I-
E
In C I S102
25
Figure 10.
A 9 . 9 rng
500 99 0 T E M P E R A T U RE (’ C)
Behavior of PbCI2 volatilized from sillca.
instance, without a sulfur analysis to supplement the mercury analysis, it would be impossible to determine whether the ground sample contained two distinct Hg compounds, or one which decomposed. Experimental procedure for the vaporization of inorganic salts from silica was identical with that of the pure compounds except that silica blanks were analyzed. Most of the compounds showed no reactivity with silica as far as the shapes of the EGA profiles were concerned. Some obvious changes occur for PbClz (Figure 10). There are three distinct regions, which correspond between P b and C1 channels. The large peak at 650 OC is probably unadulterated PbC12. The lower temperature peak may be from the formation of a volatile lead oxychloride. This suggestion is supported by the fact that other reaction produds could not produce a more volatile form of P b than PbC12. The tailing at higher temperature may be diffusional delay in the solid state, although it is shown below that for the sample masses used (-3 mg) HgC12 exhibits no delay. For the sulfates of Cd, Pb, and Zn, SO3release occurs at a much lower temperature in SiOzthan in the pure material. Formation of a metal silicate structure at a lower temperature than for decomposition may release SO3, the metal being volatilized at a higher temperature. It must be pointed out that the reactions attributed to the EGA profiles are speculative. Incorrect assessment does not detract from the observation of primary importance for identification purposes-that the metal/nonmetal conjunction still exists. Consequently, this coincidence is still indicative of the association in the sample between that metal and nonmetal despite interfering reactions. Hence, thermodynamic intereferences will not necessarily prevent correct identification. To demonstrate how sample size-specifically, the thickness of the sample layer-affects appearance temperature, mixtures with Si02, were made with HgClz such that the mass of HgClZ was constant but that of SiOz increased. This increased the depth of the SiOz layer through which HgClz vapors had to diffuse (from 2-15 mm). Since HgCl2 has been shown to be nonreactive with silica, any changes in the EGA profile for Hg would be sample size induced. Figure 11 shows that increased sample size (depth) shifted the Hg signal to higher temperatures. Therefore, the shift is an artifact solely of the thicker sample layer. A very thin sample layer would minimize diffusional delay, but with the given instrumentationthis could not always be attained. Molecular gases, such as H20, COz, and organics are quenching agents for microwave plasmas and He afterglows;
TEMPERATURE
(‘C)
Effect of sample thickness on evolution temperature of HgCI,. EGA on Hg channel. Figure 11.
25
Flgure 12. Of CdCOp
600
300 TEMPERATURE
(‘CI
Enhancement of Hg Intensity by COPfrom given amounts
therefore, a change in the plasma excitation characteristics can be expected if these materials are expelled from a sample. In environmental samples, inorganic carbonates and organic materials would be common sources for such molecular gases. For this reason, both CaC03 and humic acid were subjected to EGA. The intensity of the He 4471-A line, used as an indicator of the plasma steady state, decreased when COZ was released from CaC03 and when HzO and organic materials were released from humic acid (17). The amount of volatiles causing significant quenching was about 0.1% by volume; this is similar to the threshold of quenching observed by others (15, 23). Although He emission was obviously decreased by molecular gases this does not indicate how excitation of analyte atoms is affected. To investigate this a capillary containing a drop of Hg was taped to the bottom of the furnace pedestal away from the hot zone. Then, varying amounts of CdCO3 were heated and decomposed to determine the effect of COZon the Hg signal at 2537 A (Figure 12). The profile observed when no CdC03was present probably resulted from desorption of Hg from the surfaces near the sample crucible when the temperature was raised. Increased amounts of COZ causes an additional “peak” to rise at -450 OC, the temperature of CdC03 decompositiorr (see Figure 8). This COzenhancement
Anal. Chem. 1981, 53,2027-2029
becomes notable above 132 g, which is approximately the amount which causes reduction in the He line intensity. These studies suggest that for real samples the amounts of COP,organic vapors, and H20 must be determined by EGA and compared with the EGAs of other elements to eliminate false peaks from consideration. Changing the experimental conditions might help minimize this problem, although no benefit was noted by narrow changes in the plasma input power (65-95 W) or the He flow rate (165-420 mL/min). It is important to realize that interpretation of the EGA data may be a difficult task. A number of interferences have been demonstrated, some of which can certainly be minimized by instrumental modifications. In particular: A furnace with a higher temperature limit and more reproducible temperature control is required. Programming the transfer line heating element in parallel with the furnace would help eliminate several artifacts discussed above. The sample crucible and transfer line should be designed for optimum mass transfer characteristics to improve thermal selectivity. Multielement detection would eliminate multiple sample variability and would accelerate data acquisition. Means of eliminating interfering chemical reactions should be investigated. In this regard, matrix isolation or reduced pressure operation may be fruitful alternatives. Identification is limited ultimately by the chemistry of the sample. Consequently, as much as possible should be discovered about the sample by additional analysis. At present, the reproducibility of this EGA/ES system is adequate for semiquantitative purposes. Twenty-five individual experiments run on 10-30 cLg of PbClz while monitoring P b resulted in a mass-normalized, peak area standard deviation of 45%. An analysis of variance comparing a single sample container with several containers clearly showed that the between-container variance was no greater than that for one cbntainer. Other factors contributing to the observed variance may be volatilization completeness, transfer line losses, plasma stability, and perhaps weighing error. An additional example (24)indicates average deviations -25 to 35% for evolution of COz. It is expected that closer control of the factors mentioned above would improve reproducibility considerably.
2027
As for applications, EGA/ES has already aided mineralogical explorations (9-11) and has helped identify trace carbonate minerals in coal fly ash (24). In addition, the technique would be useful for studies such as the volatilization of trace elements during coal pyrolysis (25). LITERATURE CITED (1) Muller-Vonmoos, M.; Kahr, 0.; Rub, A. Thermochim. Acta 1977, 2 0 , 387-393. Morgan, D. J. J. Therm. Anal. 1977, 72, 245-283. Gibson, E. K., Jr. Thermochlm. Acta 1973, 5, 243-255. Friedman, H. L. Thermochlm. Acta 1970, 1 , 199-227. McAdie, H. G. Thermochlm. Acta 1977, 18, 3-13. Schuetzle, D.; Cronn, D.; Crlttenden, A. L.; Charlson, R. J. Envltun. Scl. Technol. 1975, 9 , 838-845. (7) Malissa, H.; Puxbaum, H.; Peli, E. 2. Anal. Chem. 1976, 282, 109-1 13. ( 8 ) Gal, S.; Paulik, F.; Pel, E.; Puxbaum, H. 2. Anal. Chem. 1976, 282, 291-295. (9) Watllng, R. J.; Davis, 0. R.; Meyer, W. T. Geochem. Expkx., Roc. Int. Geochem. Explor. Symp., 4th 1973, 59. (IO) Meyer, W. T.; Evans, D. S. “Prospecting in Areas of Glaclal Terrain”; Jones, M. J., Ed.; The Instltute of Mining and Metallurgy: London, 1973; pp 127-138. (11) Meyer, W. T.; Lam Shang Leen, K. C. Y. Geochem. fxplor., Roc Int. Geochem. Expbr. Symp., 4th 1972, 325. (12) Hanamura, S. NBS Spec. Publ. (U.S.) 422, 1976, No. 422, 621-624. (13) Robinson, J. W.; Rhodes, L. J. Spectrosc. Leff. 1980, 73, 253-281. (14) Thorpe, Thomas M. Ph.D. Dissertation, Unlverslty of Illinois, Urbana, IL, 1975. (15) Beenakker, C. I.M. Spectrochlm. Acta, Part8 1977, 328, 173-187 (16) Zander, A. T.; Hiefje, G. M. Anal. Chem. 1976, 50, 1257-1260. (17) Bauer, C. F. Ph.D. Dissertation, Colorado State Unlversity, Fort Colllns CO, 1979. (18) “CRC Handbook of Chemistry and Physics”, 52nd ed.; Chemical Rub. ber Company: Cleveland, OH, 1972. (19) Skogerboe, R. K.; Coleman, G. N. Anal. Chem. 1976, 48. 611A622A. (20) Mlldowski, A. E.; Morgan, D. J. Nature (London) 1960, 286, 248-249. (21) Quimby, B. D.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1976, 50, 2112. (22) Harris, W. E.; Kratoctnvil, E. Anal. Chem. 1974, 46, 313-315. (23) Brassem, P.; Maessen, F. J. M. J. Spectrochim. Acta, Part 8 1976, 318. 537-545. (24) Bauer, C. F.; Natusch, D. F. S. Envlron. Scl. Technol. IS81, 75, 783-788. (25) Ting, E. T. G.; Manahan, S. E. fnvlron. Scl. Technol. 1979, 73, 1537- 1540. (2) (3) (4) (5) (6)
RECEIVED for review February 23, 1981. Accepted June 26, 1981. This material represents part of a doctoral thesis submitted to the Graduate School of Colorado State University.
Determination of Arsenic and Selenium in Water Fish, and Sediments by Inductively Coupled Argon Plasma Emission Spectrometry Peter D. Goulden,” Donald H. J. Anthony, and Kelth D. Austen Mtional Water Research Institute, Canada Centre for Inland Waters, Burlington, Ontario, Canada
A semlautomated system for the determinatlon of As and Se by hydrlde generation Is described. By use of a preconcentratlon step (X4) for water, detectlon limlts are 0.02 and 0.03 pg L-’ for As and Se, respectlveiy. Fish are dlgested with nitric, perchlorlc, and sulfuric aclds; sediments are brought Into solution by fusion with sodium hydroxide. The automated measurement system uses conventional contlnuous flow equipment connected to a “larger-diameter-than-usual” torch In the Inductively coupled argon plasma (ICAP) instrument.
Arsenic and selenium levels in environmental samples (i.e., water, fish, and sediment) are currently determined in this
laboratory using a continuous flow hydride generating system and atomic absorption spectrometry (1). The w e of atomic emission spectrometry with inductively coupled argon plasma excitation (ICAP) offers an alternative route to these determinations. In addition, ICAP provides the opportunity to simultaneouslydetermine the other hydride-forming elemenb of interest to us, namely, tin, antimony, and bismuth. There have been many analytical systems developed to carry out these determinations, in particular the system described by Thompson et al. (2) will simultaneouslydetermine As, Sb, Bi, Se, and Te in an automated system. However, this system is not completely suitable for our needs in that it does not use a conventionalcontinuous flow system, it uses a higher power
0003-2700/81/0353-2027$01.25/00 1981 American Chemical Soclety
