1654
Anal. Chem. 1983, 5 5 , 1654-1660
Metal Speciation by Evolved Gas/ Inductively Coupled Plasma Atomic Emission Spectrometry E d w a r d R. Prack a n d Glenn J. Bastiaans*
Department of Chemistry, Texas A&M University, College Station, Texas 77843
An evolved gas analysls/emission spectrometer system capable of speciatlng lnorganlc compounds in solld samples Is described. Samples are gradually heated to 2300 OC In a graphlte sample probe that Is moved In a controlled manner into an inductively coupled plasma (ICP) dlscharge. As the sample Is heated, Its components vaporize at characterlstlc temperatures into the supporting Ar of the ICP which acts as an atomlc emlsslon source from whlch the characterlstlc emlssion of the evolved vapors can be detected. Identlflcatlon of a glven compound Is based on the posltlon of the sample at the time of evolutlon of the metal. Problems which can complicate Identification and speclatlon Include chemlcai reactions In the sample and between the sample and its surrounding.
Speciation is becoming an important concept in the characterization of solid samples. The determination of the chemical or oxidation state of low and trace level elements can be applied, for example, in the evaluation of the environmental impact of inorganic pollutants and in the study of catalyst systems. In the analysis of inorganic pollutants, speciation can be very useful since the toxicity of many metals varies widely with chemical form. In catalyst systems it is important to know in what chemical forms the catalyst exists under active and passive conditions. Present analytical techniques fall short of the challenge of speciation of solid samples. Evolved gas analysis (EGA) is a technique with promise in the area of solid sample speciation. EGA is a thermal analytical method in which gases released from a sample are monitored as a function of temperature. Several types of analyzers have been used in combination with evolved gas analysis, with mass spectrometry being the most important. Recent applications of EGA/MS include minerals ( 1 , 2 ) ,oil shale (3), polymers (4), plated films (5), and lunar soil (3). High-resolution mass spectrometry is used primarily to study organic compounds (6). EGA has been used with conductivity detectors for evolved COz and SOz in order to analyze urban dust for inorganic carbonates and sulfates (7,B). EGA with an MIP-AES detection system has been used to demonstrate characteristic identification of compounds through coevolution of gases (9). Studies of Cd, Hg, Pb, and Zn halides, sulfates, and sulfides were done. Previous efforts have shown that while EGA with MS or AES may have the potential for metal speciation, little progress has been made in these areas for several reasons. Two major disadvantages of existing EGA techniques for inorganic compounds are inadequate upper operating temperatures and/or insufficient temperature control. As a result, the evolution of compounds with similar boiling points cannot be resolved, and high boiling solids cannot be vaporized. Physical and chemical factors such as adsorption and chemical reactions can affect the evolution temperature of the compound. Another problem is that no detailed theoretical or experimental study has been performed on the basic dynamics of the vol-
Table I. Summary of Ar ICP Emission Spectrometer Operating Conditions forward rf power, kW argon plasma flow, L/min argon auxiliary flow, L/min plasma observation zone, mm above top of coil
1.25 7.32 0.14
30
atilization process of inorganic compounds. In this work an evolved gas analysis method has been implemented by using an Ar inductively coupled plasma (ICP) both as a source of heat and as an element detector. Heating is achieved by slowly moving the sample along the vertical central axis of the torch. The sample is originally placed below the luminous portion of the ICP discharge and then raised into it. Detection is based upon observation of atomic emission of the elements of interest from the evolved gases. For speciation to be achieved via this method, one must be able to detect the differences in vaporization temperatures of different compounds of the same element and to relate the vapor composition as a function of temperature to the components in the original sample. In this study, the dynamics of several systems were studied, and speciation is demonstrated on mixtures of compounds of the same metal. EXPERIMENTAL SECTION Apparatus. The evolved gas/ICP spectrometer system consisted of two parts: a solid sample introduction and heating section and an atomic emission ICP spectrometer. The direct sample insertion system is shown in Figure 1. A demountable ICP torch (10) was modified so that an electrode in a mechanical insertion device replaced the normal central aerosol tube of the torch. An undercut graphite cup electrode (Ultra Carbon no. 781) served as a sample cup (11)and was placed on top of a Pyrex tube which we call the probe. The probe was mounted on a T bar connected to a translator which was used to move the probe vertically in the torch and thereby control the temperature that the sample experienced. Temperature programming was accomplished by varying the rate of movement of the translator. Ideally the sample cup should be able to withstand high temperatures and should be inert toward the sample as well as being reusable. The graphite electrode offered the best compromise in meeting these conditions. Emission was observed by using the optics and instrumentation described previously (12). Operating conditions of the system are summarized in Table I. Reagents. Reagent grade solids were used to study compounds of Pb, Cd, Hg, and V. The compounds and mixtures of compounds of the same element were diluted by mixing them with graphite powder (Union Carbide Grade 48). Procedure. Samples on the order of 5 X lo-’ to 2.5 X g total weight were used with the sample cup. The samples consisted of approximately 1%(w/w) of the compound or mixture of interest in a graphite matrix. After the addition of solid, the sample cup was placed on the Pyrex tube and the probe was inserted into the torch. With the probe resting at the desired starting height, the ICP was ignited. Emission intensity from the plasma was monitored as the probe was moved slowly upward. Both translator movement and data acquisition were controlled by a laboratory computer (DEC PDP 11/34). Such control allowed convenient programming of the rate of probe movement. Emission was monitored at the wavelengths listed in Table 11.
0003-2700/83/0355-1854$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
k
To Translator
Flgure 1. Samplet insertloii system: (A) sample cup; (B) torch.
Table 11. Emission Wavelengths Monitored species
wavelengths, A
Cd
3 26 1.. 1 194 2.3
Hg Pb V
4057.8 29 24.0
THEORY Thermal methods are concerned with. the interaction of a material with its surroundings and itself upon the application of thermal energy (13). The temperature which the sample experiences causes measurable changes in chemical and external physical properties. In this study we are observing the vaporization of different solids within a sample. It is important to understand ithe effect of varying temperature on the sample because it is used here as a separation technique prior to elemental detection via ICP-AES. The vaporization behavior of inorganic salts as a function of temperature can be classified into three categories (9): 1. Congruent vaporization MaJcb(c) = MaXb(g)
(1)
2. Congruent vaporization with dissociation
3. Incongruent vaporization
After sample has entered the vapor phase via one of these mechanisms, the element of interest is contained in the evolved gas and a peak appears in the emission signal as atoms of the selected element are swept into the plasma by a stream of argon gas. Speciation information can be obtained by measuring emission intensity of a specific element as a function of sample temperature. Compounds can be identified in two ways: simply by the appearance temperature of the metal evolution peaks as done in this work or by the coincidence of metal and nonmetal peaks evolving from the same compound. For reactions of type 1 and 2, the metal and nonmetal peaks should have identical locations. For these compounds, identification can often be satisfactorily done by observing only the metal evolution peaks. For reactions of type 3, the metal and nonmetal peaks may not appear at the same temperature. Also several salts of a metal may decompose to a common product which will ultimately produce a single peak of the metal's emission. For compounds of this later type, accurate identification can be achieved by observation of both metal and nonmetal evolution peaks. Vaporization in real cases may well involve more individual stepwise processes than those indicated above. One possible complication to vaporization under the conditions of this work is the interaction of sample material with the graphite walls of the probe. In this case, evolution of the element of interest can be treated as incongruent vaporization. In general, halides and simple oxides should belong to type 1,other oxides and sulfides should belong to type 2, and most oxyanion compounds should belong to type 3. The rate of appearance of atomic emission due to the vaporization of a compound can be theoretically predicted if the rate limiting step of the sample transfer process is known. In cases of congruent vaporization (eq 1and 21, the rate limiting process may be the transfer of vaporized sample from the surface (14) or the vaporization process itself. For incongruent vaporization the rate of the decomposition reaction may also be the rate-limiting step. One can describe the rate of a decomposition reaction by an Arrhenius type expression, and it has also been found that the rate of vaporization of sample from graphite surfaces can be treated in the same manner under conditions where mass transport is not the rate limiting process (15). Rogers et al. have suggested and theoretically treated the case of mass transfer limited sample vapor supply in work involving the pyrolysis of both organic and inorganic compounds (14). For sample volatilization mechanisms that can be described by an Arrhenius rate expression, the rate of disappearance of the sample (-dn/dt) is -dn/dt = nA exp(-E/RT)
(4)
where n is the amount of sample unreacted or un9aporized in the condensed phase, A is the frequency factor, E is an activation energy, R is the gas constant, and T i s temperature. The emission intensity observed from the vaporized atoms will be directly proportional to the number of atoms flowing through the volume of observation at any given point in time. The relationship describing the emission intensity becomes
I = C,(l/F) Vo(-dn/dt) where Ma symhlolizes the metal cations, Xb symbolizes the nonmetal anions, and (c) refers to a coindensed phase. For congruent vaporizations the metah and nonmetals are evolved at the same temperature, whereas for the incongruent vaporizations a nonmetal species is evolveld a t a different temperature than tlhe metal. Actual vaporization behavior may be more complicated than these cases indicate.
1655
(5)
where C1 is a constant, F is the volumetric flow rate, and V,, is the observation volume. Substituting eq 4 into eq 5 one obtains
I = C,n exp(-E/RT) where Cz = C,VJ/F, and In I = In n
+ In C2 - E / R T
(6)
(7) From the treatment of Rogers (14),differentiation of eq 7 with
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
respect to temperature yields d(ln O / d T = E / R P - (C2/r) exp(-E/RT)
2400
(8)
rt
assuming temperature is a linear function of time, i.e., r = dT/dt. Integrating one obtains In ( I / I o )= E/R(l/TO - 1/T) - (CZ/r) s ' e x p ( - E / R T ) To
dT (9)
where Io is the intensity a t some reference temperature To. The integral on the right-hand side of eq 9 can be evaluated by asymptotic expansion, yielding In ( I / I o )= E / R ( l / T o - 1 / T ) C,R/rE(TL exp(-E/RT) - To2exp(-E/RTo)) + 2(C2R2)/rE2(T3exp(-E/RT) - To3exp(-E/RTo)) + .., (10) Higher order terms are negligible, as RT/E is generally on the order of (14). Equation 10 describes the shape of emission profiles (emission vs. time or temperature) resulting from a sample supply process limited by the rate of vaporization or the rate of a decomposition reaction. A different emission profile will be obtained if one assumes mass transport is the limiting supply process. In this case, Rodgers assumed that the carrier gas becomes saturated a t the sample and the rate of sample disappearance will be (14) -dn/dt = PF/RT
(11)
where P is the vapor pressure of the volatilizing component and F is the volumetric flow rate. Equation 11 is obtained from the ideal gas law by taking the first derivative of n with respect to time. By substituting eq 11 into eq 5 and assuming that the temperature dependence of P is given by the Clausius-Clapeyron equation, one obtains
I = (CZ/T)PO exp(m/RTo) exp(-m/RT)
(12)
where Cz = CIVo/R and AH is the heat of vaporization. Differentiation of eq 12 with respect to temperature yields dI/dT = [C2Po/T exp(AH/RTo) exp(AH/RT)] X [ m / R P - 1/Tl (13) where Po and To are the vapor pressure and temperature, respectively, at the onset of vaporization. Profiles of this type are distinguished by a comparatively gradual rise with temperature followed by a sharp drop off at a temperature designated T, where vaporization of the sample component of interest is complete (14). T, is related to no as follows no =
(-P$ exp(AH/RTo)J/Rr
ST' To
(1/T) exp(-AH/RT) d T
(14) The difference in the emission profile resulting from mass transport controlled sample vapor production should allow this type of behavior to be distinguished from the other processes discussed above. The fitting of experimental profiles to these models may also be done in order to obtain estimates of A and E or T,.
RESULTS AND DISCUSSION To characterize this method for the speciation of solid samples, the vaporization behavior of mixtures of V, Pb, Cd, and Hg salts was observed. The temperatures experienced by the sample at different heights with respect to the plasma discharge were measured in order to establish a relationship between temperature and probe movement. Finally, emission profiles predicted by the models of the theory section were fitted to experimental data in order to establish the parameters
-60
-50
-40
-30
-20
-10
0
10
HEIGHT (mm)
Flgure 2. Calibration of temperature vs. probe height: experimental polnts and second-order fit. which control the shapes and positions of the emission peaks resulting from the vaporization of different salts. Temperature. The temperature that the samples experienced was measured in two ways. At temperatures lower than 1000 OC, a W/5% Re thermocouple was used for temperature calibration. The thermocouple was mounted in a quartz tube with the thermocouple junction sealed in quartz. This apparatus was used as a temperature probe in the ICP torch from 60 to 23 mm below the top of the load coils, at which point the quartz seal would melt. Higher temperatures were calibrated by observing the initial points of vaporization of pure (undiluted) P b metal, PbO, PbS04, and CdC12. These solids are types 1 and 2 compounds with well-characterized boiling points (16). The probe heights where these compounds first produced emission were assigned the boiling points of the salt. Only 50 K separated the maximum thermocouple temperature and the minimum boiling point. Since no discontinuity exists in the temperature height relationship, it can be argued that the boiling points are a good estimate of sample temperature a t the onset of emission. The relationship between height of the sample relative to the load coils of the ICP and temperature is best characterized by a second-order polynomial fit of height vs. temperature. The plot of height vs. temperature is shown in Figure 2 where height is taken as zero at the top of the load coil and negative below the load coil. Vanadium Compounds. For discussion purposes the behavior of the vanadium compounds can be divided into qualitative and quantitative aspects. Pure samples of NH4V03, VzO5, Vz04, and VzO3, mixed with graphite, as well as several binary and ternary mixtures of the salts with graphite were heated. The compositions of these mixtures are given in Table 111. Figure 3 shows profiles of V emission vs. probe height for the pure vanadium compounds. The order of evolution is NH4VO3,Vz05,VzO4, and Vz03. The NH4V03sample yielded a single peak at a relatively low height (temperature) indicating apparent congruent vaporization. Although heating of NH4V03is known to produce VzOs via decomposition (17),this reaction does not produce significant amounts of Vz05under the conditions found here, since a second peak does not appear. Similarly, VzOsproduces a single peak at a greater probe height. Again congruent vaporization appears to occur for the single salt sample.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
1657
I
Tc
tt-
I
I
I
1095
1140
1185
E
W
3
3
2
a,
e:
1050
1095
1140
1185
Temperature
1050
1230
Temperature
(" C)
Figure 3. Evolution profiles of single V compounds: (A) NH4V03;(B) vzo5; (c) vzo4; ([I)v 2 0 3 .
1230
(" C)
Figure 4. Evolution profiles of several V,05/V,0, 75%/25%; (B) 4 9 % / 5 1 % ; (C) 27%/73%.
mixtures: (A)
.~
Table 111. Solids Mixtures vanadium mixtures 1. NH,VO, .VZO5 2. NH,VO, V Z 0,
3. lV",VO, 2'3'
I
I
I
I
1095
1140
1185
1230
lead mixtures
c
1. PbCl,
PbCO, PbSO, 2. PbC1, Pb304 PbCO, 3. PbCO, Pb(NO,), P'bSO,
J C
Y
5
.3
v) v)
'i w 3
Two major peaks were observed in tlhe profile of V204indicating some form of incongruent vaporization was occurring. It is possible that the graphite present in the probe walls and sample matrix may reduce some of the Vz04to V203during heating. This mechanism is suggested by the fact that the V203sample started evolving atom vaplor a t the same probe height as observed for the second V204peak. The V203sample produced several emission peaks. This behavior indicates that several components of differing volatility were initially present or were produced during initial heating. V203 is known to be a solid with unstable stoichiometry ( 1 7 ) . Thus multiple oxidation states may have existed before heating. It is also possible that further reduction of the solid may have occurred due to the presence of graphite during heating. A limited ability to qualitatively and quantitatively speciate mixtures of the V salts is illustrated by Figures 4 and 5. The peaks observedl in the mixture profiles are assigned by using
2
1050
Temperature
(" C)
Figure 5. Evolution urofiles of V salt mixtures: (A) NH,VOJV,O,: (BI
the data in Figure 3 and by varying the relative percentages of the different chemical forms of the metal in the mixture. The order of evolution for pure compounds determined from Figure 3 also describes the order of evolution of the species present in the mixtures. The locations of peaks with respect
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
to temperature in mixtures representing specific chemical species do not always correspond to the location of the pure compound or that of the same species in a different mixture. This observation can be attributed to two factors: low practical temperature resolution and interactions occurring in the sample. To confirm peak assignments, the effect of adding more of one species to the mixture was observed. This standard addition procedure produced relative increases in the peak area of the added species and confirmed the peak's identity. The effect of such a variation is demonstrated in Figure 4 which contains the profiles of three Vz05+ Vz04 mixtures of differing proportions. Quantitative agreement between peak areas and known mixture composition can be obtained by assigning VzO, to the f i s t peak and the remainder of the peaks to Vz04. These data are compared in Table IV. All possible binary mixtures of the V salts examined were resolvable into their two components, as illustrated by Figure 5. The dotted lines separate the peaks assignable to the individual mixture components. The assignment of peaks is not straightforward and requires the analysis of several knowns of different proportions. The resolution of Vz04 and V203is only attainable with small sample sizes (less than 1 X lo4 9). In general, the emission vs. probe height profiles become simpler a t smaller sample sizes because fewer and narrower peaks are observed. This effect is probably due to the fact that vaporization of the original form of the salt occurs more quickly as size decreases. Thus, there is less time for secondary reactions to occur and all of the original salt enters the vapor phase more rapidly. Profile F of Figure 5 is a ternary mixture of NH4V03,Vz05, and Vz04. The first peak was assigned to NH4V03. The following unresolved series of peaks are attributed to Vz05 and Vz04. This failure to resolve the final two components must be due to the presence of NH4V03and may be caused by the evolving NH4V03catalyzing interconversion reactions between Vz05and V204. In order to evaluate this methods ability to provide reliable quantitative information, peak areas were measured and related to the masses of samples heated. Peaks were defined to be signal variations greater than three times the background noise. Areas were determined with a computer program which background corrects the data and then numerically integrates the area under the peaks. Background was taken as the average of signal observed before and after peak evolution. Corrections were made by subtracting the average value of the background from each point in a data file. A calibration curve for V from V205 mixed only with graphite was constructed and found to be linear over a V mass range of 0.3-14 pg. The relative percentages of the different chemical forms of vanadium in several binary mixtures are given in Table IV which lists the actual and observed percentages of vanadium with the corresponding standard deviation of the mean in the various vanadium mixtures. The percentages given in Table IV refer to the relative amount of the metal in a given chemical form as a percentage of the total amount of the metal in all the chemical forms in the sample. From three to seven duplicate runs were performed for each mixture, for which sample size and the total amount of vanadium present in the sample varied. The mixture data demonstrate the ability to quantitatively speciate binary mixtures by use of relatively simple data analysis. Pb, Cd, and Hg Salts. The decomposition behavior of Pb, Cd, and Hg compounds proved to be more problematic than that of the vanadium salts. PbC12, PbC03, Pb304,Pb(NO&, PbS04, CdClZ,CdO, CdC03, HgClZ,HgO, and HgSO, were characterized. Qualitatively the pure lead compounds show characteristic peaks with the lead chloride appearing first followed by an overlapped grouping of lead carbonate, oxide,
I
I
I
E
t
Wr 1090
1145
1200
Temperature
1255
1310
(" C)
Flgure 6. Evolution proflles of single Pb compounds: (A) PbCI,; (9) PbCO,; (C) Pb304; (D) Pb(N03),; (E) PbS04.
h
4
.3
VI
i
t
I
I
I
I
I
1050
1095
1140
1185
1230
Temperature
(" C)
Figure 7. Evolution proflles of Pb salt mixtures: (A) PbCI,/PbC03/ PbSO4; (B) PbCI,/Pb304/PbCO,; (C) PbCOJPbNO,/PbSO,.
and nitrate with lead sulfate appearing last, as illustrated in Figure 6. The behavior observed here can be explained by the fact that the lead carbonate, mixed oxide and nitrate decompose, within 100 deg of each other, to PbO (18, 19). The three lead ternary mixtures that were analyzed are listed in Table 111, and their profiles are shown in Figure 7.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
1659
~-
Table IV. Vanadium Mixture Data %
%
mixture
actual
observed
1. NH,VO, V,O,
58 42 64 36 49 51 75 25 27 73 42 58 47 55
60 i 4.5 40 i 4.5 63 i 2.4 37 i 2.4 49 i 5.0 51 i. 5.0 77 i 4.0 23 + 4.0 26 i 3.0 74 3.0 43 i. 1.4 57 i 1.4 47 i 1.1 53 i 1.1
2. NH,VO, V,O,
3. NH4V0, "2'3
4.
v,o, VZO,
v205 v204
5.
v,o, v20,
6. V,O, v20,
_____-.____
mixture
actual
observed
NH,VO,
36 64
37 i 5.5 63 + 5.5
49 51
48 i 3.0 52 i 3.0
23 77
21 i 2.1 79 i 2.1
v20,
v20,
v*o,
v204
v20,
Table V. Lead, Cadmium, and Mercury Mixture Dsta %
mixture lead data
cadmium data mercury data
PbCl, PbCO, PbSO, 2. PbC1, Pb,Oi PbCO, 3. PbCO, Pb(NO,), PbSO, 11.
ccic1,
CdO CdCO, HgC4
Ha HgSO,
actual observed 24 54 22 20 44 36 38 26 36 31 36 33 35 34 31
2 2 ? 4.3 51 ?. 3.5 27 i 4.3 22 i 4.2 78 i 4.2
1700
1800
f i
1900
2000
2100
2200
1900
2000
2100
2200
h
73 i 3.9 27 i 3.9 34 r 4.2 66 + 4.2 36 i. 3.3 64 i 3.3
PbC12 and Pbi304 were always resolvable components, but Pb(N0J2, PbC03, and Pb304could not be resolved from each other because of their decomposition to a common product. The salts of Cd and Hg exhibited belhavior similar to that of the lead compounds. CdC12 was found to vaporize early when heated, but CdO and CdC03 evollve at the same probe height indicating that the carbonate decomposes to the oxide before volatilization. Iri the case of Hg, l-IgC12 can be resolved, but HgSO, and HgO yield overlapping peaks. In these situations where decompo~itionsoccur, speciation may perhaps still be obtained if the atomic emission from products of the decomposition are observed. Suitable lines for the detection of S, C, N, and 0 in the ICP may be found in the NIR spectral region (20-23). Table V contains thie peak area and sample composition data for the Pb, Cd and Hg mixtures. The peak areas of the compounds that decompose to oxides are grouped together. Here again quantitation capabilities are limited by the decomposition problem. Volatilization Dynamics. In the theory section equations were developed which may be used to predict evolution peak shapes determined by the rates of different processes. Equations 10 and 14 were used to generate theoretical peak shapes corresponding to the kinetic and mass transfer cases, respectively. These peak shapes are traced in Figure 8. For comparison, this figure also contains the experimental evolution profile of a sample containing powdered P b metal diluted with graphite.
1700
500
1800
700
900
1100
1300
1500
1700
TEMPERATURE (OK)
Comparison of theoretical and experimental profiles: (A) mass transport limited model: (6)kinetic model; (C) experimental data from volatilization of Pb. Flgure 8.
From the profiles in Figure 8 it is apparent that the kinetic model fits the experimental data much better than the mass transfer model. The parameters of eq 10 which gave the best fit to experimental results for the kinetics model were E = 140 kcal/mol, C2 = 1.5 X and To= 1875 K. The activation energy is considerably greater than the sample's heat of vaporization of 42.36 kcal/mol. The excess activation energy found here may be attributed to other processes such as molecular dissociation and interactions with the graphite present (15). The experimental profile may also be compared to the analogous experimental and theoretical results of van de Broek et al. (15). Qualitative agreement was obtained between their data and ours. However, their profiles showed different values for the constant, C2,and the activation energy, E , of eq 10. Their value of E also varied with the rate of temperature change and furnace type. Thus the differences in C2 and E
1660
Anal. Chem. 1983, 55, 1660-1665
are understandable since they are dependent on the system used and operating conditions.
CONCLUSIONS We have examined the use of EGA-ICP system for the speciation of metal salts. Encouraging preliminary results were obtained for the speciation of solid samples, as evidenced by the resolution of binary and ternary mixtures of salts of a common element. Quantitation was also found to be possible within accuracies of 1 to 9%. The EGA-ICP system offers several advantages over other proposed solid sample speciation techniques. The system is very simple, with both the vaporization and excitation energy supplied by the plasma discharge. This arrangement prevents loss of vaporized material during transport to the detector and allows for minimal dispersion of the sample between vaporization and excitation, which minimizes peak broadening. The EGA-ICP system also has a higher maximum operating temperature, which increases the range of samples than can be examined. Several limitations were encountered with the EGA-ICP system. If different components in a solid mixture decompose to a common product then speciation cannot be obtained by observing the emission of a single element. The possibility of solving this problem by simultaneously observing the emission of several elements must still be investigated. Several experimental factors play a role in determining actual resolution, the most important of these are sample size and temperature ramp rate. As the sample size decreases, peak resolution increases. This was noted in the case of the V203 + V204mixture, where the peaks were separable only with samples containing less than 0.5 pg of material. Decreasing the temperature ramp rate was observed to increase peak resolution. Optimization of these parameters is essential for successful speciation of samples with chemical forms that evolve at similar temperatures. The current experimental system allows control of sample temperature with a practical resolution of only 50-100 OC. A finer control of sample temperature in future designs may enhance speciation capability. In addition, the current ICP torch design does not restrict sample introduction to the central plasma region. Correction of this feature should result in better sensitivity and detection limits although the moderate present values of these parameters did not present a problem in this work. Chemical reactions between sample components and the graphite probe walls of sample matrix may also limit the separation of sample. In many cases, the probes employed
had to be aged by heating with sample several times before reproducible results could be obtained. The use of pyrolytic or other specially treated graphite or metal cups may alleviate this problem. As in all high temperature applications, the graphite cups also tend to deteriorate with repeated use. Attempts to model the rate limiting process which controls the temperature and rate of sample evolution resulted in the conclusion that volatilization follows an Arrhenius type rate expression. Although the energy of activation of the volatilization process was greater than the heat of vaporization for the modeled system, it was not possible to identify specific chemical reactions which may be involved in volatilization. Since many reasonable solutions seem to exist for the problems and limitations discussed above, further studies will be undertaken in order to expand the speciation abilities of EGA/ICP toward mixtures of solids. The high sensitivity of ICP emission spectrometry makes this approach especially attractive for trace level analyses.
LITERATURE CITED Muller-Vonmoos, M.; Kahr, G.; Rub, A. Thermochim. Acta 1977, 2 0 , 307.
Morgan, D. J. J . Therm. Anal. 1977, 72,245. Gibson, E. K. Thermochim. Acta 1973, 5 , 243. Friedman, H. K. Thermochim. Acta 1970, 7 , 199. Gallagher, P. K. Thermochim.Acta 1980, 47, 323. Schuetzie, D.: Cronn, D.: Crlttenden, A. L: Charlson, R. J. Environ. Sci. Techno/. 1975, 9 , 838. Malissa, H.; Puxbaum, H.; Pell, E. 2.Anal. Chem. 1976, 282, 109. Gall, S . ; Paullk, F.; Pell, E.; Puxbaum, H. Z . Anal. Chem. 1976, 282, 291.
iauer, C. F.; Natusch, D. F. S. Anal. Chem. 1981, 53, 2020. Wlndsor, D. L.; Heine, D. R.; Denton, M. B. Appl. Spectrosc. 1979, 3 3 , 56. Salln, E. D.; Horlick, G. Anal. Chem. 1979, 57, 2284. Fernandez, M. A.; Bastiaans, G. J. Anal. Chem. 1979, 51, 1402. Buzas, I . "Thermal Analysis"; Heyden: New York, 1974; Vol. 1. Rodgers, R. N.; Yasuda, S. K.; Zlnn, J. Anal. Chem. 1960, 32, 672. Van Den Broek, W. M. G. T.;de Galan, L. Anal. Chem. 1977, 49, 2176.
"CRC Handbook of Chemistry and Physics", 52nd ed.; Chemical Rubber Co.: Cleveland, OH, 1972. Cotton, F. A.; Wiiklnson, G. W. "Advanced Inorganic Chemistry": Interscience: New York, 1972; pp 821-827. Duval, R.; Wadler, C. Anal. Chim. Acta 1960, 23, 257. "Merck Index", 9th ed.: Merck: Rahway, NJ, 1980; pp 709-711. Northway, S. J.: Fry, R. C. Appi. Spectrosc. 1980, 3 4 , 332. Northway, S. J.; Brown, R. M.; Fry, R. C. Appl. Spectrosc. 1980, 34, 338.
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RECEIVED for review April 28,1983. Accepted June 13,1983. This work was supported by the Welch Foundation, Grant A-866, and the National Science Foundation, Grant CHE 79-21584.
Optimization of Response of Chemiluminescence Analyzers A, A. Mehrabzadeh, R. J. O'Brien,* and T. M. Hard Chemistry Department and Environmental Sciences Doctoral Program, Portland State University, Portland, Oregon 97207 The behavior of chemllumlnescence analyzers Is discussed in terms of the response equatlons for exponentlaldllution and plug-flow reactors. Three operational modes are dlstingulshed In each case, and their characterlstlcs and advantages are treated. The response equations are tested experlrnentally with the ozone/ethyiene chemiluminescent reactlon. Appllcation to the popular NO/O, analyzer is dlscussed.
Chemiluminescence (CL) as an analytical technique in both
the gas and liquid phases has been the subject of increasing study. A recent review (1) lists 71 references for a 2-year period. CL analyzers are widely used for atmospheric measurements of trace gases because of their sensitivity and relative simplicity. In particular, ozone and oxides of nitrogen (NO,) are routinely measured via CL, and commercial instruments are available for this purpose. The detection limits of these instruments range to the parts-per-billion level a t 1 atm total pressure, and modified or laboratory-constructed instruments have been made several orders of magnitude more
0003-2700/83/0355-1660$01.50/00 1983 American Chemical Soclety
