Anal. Chem. 1988, 60,600-605
600
selectively has great potential for analytical applications. The limits of detection of the compounds investigated are in the subnanograms range and the linear range of the analytical curve covered approximately 2 orders of magnitude, which is often observed for a surface emission technique ( I O ) . The SECD method can be combined with other selective enhancement approaches such as the selective heavy-atom perturbation method ( 4 ) in order to improve the specificity of the simple but powerful R T P technique for the analysis of complex mixtures. Registry No. Benzo[ghl]perylene,191-24-2;benzo[a]pyrene, 50-32-8;chyrsene, 218-01-9; coronene, 191-07-1;dibenzoanthracene, 53-70-3;fluoroanthracene,206-44-0; phenanthrene, 85-01-8;pyrene, 129-00-0;triphenylene, 217-59-4; 2,6-dimethylquinoline,877-43-0; acridine, 260-94-6;7,&benzoquinoline, 230-27-3;4-bromobiphenyl, 92-66-0;dibenzofuran, 132-64-9;2,6-dibenzoquinoline,257-89-6; dibenzothiophene, 132-65-0;indole, 120-72-9;isoquinoline, 11965-3; 1-naphthol, 90-15-3; phenazine, 92-82-0;quinoline, 91-22-5; a-cyclodextrin, 10016-20-3;i3-cyclodextrin, 7585-39-9; y-cyclodextrin, 17465-86-0.
LITERATURE CITED Roth, M. S.J. Chromatogr. 1967, 30, 276. Parker, R. T.; Freedlander, R. S.;Dunlap, R. B. Anal. Chim. Acta 1980, 120, 1. Vc-Dinh, T.; Lue-Yen, E.; Winefordner, J. D. Anal. Chem. 1976, 4 8 , 1186. Vo-Dinh, T.; Hooyman, J. R. Anal. Chem. 1979, 51, 1915. Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1982, 739, 315. Wellon, S.L.; Paynther. R. A,; Winefordner, J. D. Spectrochim. Acta, Part A 1974. 30, 2133. Paynther, R. A.; Wellon, S.L.; Winefordner, J. D. Anal. Chem. 1976, 4 6 , 736. Aaron, J. J.: Kaleel, E. M.: Winefordner, J. D. J. Agric. Food Chem 1979, 27, 1233.
DeLima, C. G.; de M. Nicoia, E. M. Anal. Chem. 1978, 50, 1658. Vo-Dinh, T. Room Temperature Phosphorimetry for Chemical Analysis; Wiley: New York, 1984. Donkerbroek, J. J.; Gooljer, C.; Velthorst, N. H.; Frei, R. W. Anal. Chem. 1982, 54, 891. Cline Love, I. J.; Skrilec, M.; Habarta, J. G. Anal. Chem. 1980, 52, 754. Seypinski, S.;Cline Love, L. J. Anal. Chem. 1984, 56, 322. Seypinski, S.;Cllne Love, L. J. Anal. Chem. 1984, 56, 331. DeLuccia, F. L.; Cline Love, L. J. Anal. Chem. 1984, 56, 2811. Bello, J.; Hurtubise, R. J. Appl. Spectrosc. 1986, 40, 790. Bello, J.; Hurtubise, R. J. Anal. Lett. 1986, 79(798) 775. Hoshino, M.; Imamura, M.; Ikehara, K.; Hama, Y, J. Phys. Chem. 1981, 85, 1620. Alak, A.; Heilweil, E.;Hinze, W. L.; Oh, H.; Armstrong, D. W. J. Liq. Chromatogr. 1984, 7, 1273. Szejfli, J. Cycldextrin and Their Inclusion Complexes; Academias Kiado: Budapest, Hungary, 1982. Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-Verlag: New York, 1978. Hinze, W. L. Sep. Purif. Methods 1981, 70, 2. Vo-Dinh, T.; Alak, A. M. Appl. Spectrosc. 1987, 41(6), 963. Vo-Dinh, T.; Martinez, P. R. Anal. Chim. Acta 1981, 13, 125. Vo-Dinh, T.; Gammage, R. B. Anal. Chem. 1978, 50, 2054. Vo-Dinh, T.; Gammage, R. B. Anal. Chim Acta 1979, 107, 261. Armstrong, D. W.; Stine, G. Y. J. Am. Chem. SOC.1983, 705.2962. Armstrong, D. W.; Nome, F.; Spino, L.; Golden, T. J. A m . Chem. SOC. 1986, 108, 1418.
RECEIVED for review June 25,1987. Resubmitted October 28, 1987. Accepted November 19,1987. Research was sponsored by the Office of Health and Environmental Research, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. This research was also supported in part by an appointment of A.M.A. to the postgraduate Research Training Program under Contract No. DE-AC05-760R00033between the U.S. Department of Energy and the Oak Ridge Associated Universities.
Vaporization Kinetics for Solids Analysis with Electrothermal Atomic Absorption Spectrometry: Determination of Lead in Metal Samples Thomas M. Rettberg’ and James A. Holcombe*
Department of Chemistry, University of Texas at Austin, Austin, Texas 78712
A method Is proposed for the direct analysis of species evolved from metaHurglcai samples that Involves extrapolation of a function describing the evolution rate of the analyte. The second surface atomlrer was employed to determlne the tlme-dependent vaporization rate of an analyte from an lndMual sample over several heatlng cycles. These data were fit by using a flrst-order klnetic expression and were related to the total amount of analyte In the sample by uslng a callbratlon curve constructed wHh slmpie, aqueous standards. Results are shown for the determlnatlon of Pb In Sn, Cu, and steel.
The direct analysis by electrothermal atomization (ETA) of solid metal samples containing trace concentrations of other metals has been performed by numerous researchers (1-15). Present address: Varian Atomic Absorption Resource Center, 20 W. Touhy, Park Ridge, IL 60068.
Many of these previous studies elucidated some of the important factors relating to the direct analysis of these sample types, some of which are often an impediment to a successful, straightforward analysis. Items mentioned have included the slow vaporization rates of the analyte from the host metal (2, 8,14); problems caused by the host metal, such as background or the buildup of residue (2, 4, 8, 9, 11); and/or problems related to graphite interactions, such as carbide formation. Despite the unique nature of solid metal samples, the analyses mentioned above typically employed a protocol for heating and signal measurement similar to that used for aqueous samples. Direct sample heating using rapid heating rates to high furnace temperatures was employed to produce an absorbance “spike”, yet broad, tailed peaks were often still observed. While high atomization temperatures [one source listed 3200 “C (3)]maximize the partial pressure and evolution rate of the analyte, they also occasionally resulted in rapid vaporization of the bulk sample and high background levels. Complete volatilization of the analyte within a reasonable integration period w8s a prerequisite for accurate quantitative analysis, yet this often was difficult to attain ( 2 , 8, 12, 14).
0003-2700/88/0360-0600$01.50/0 0 1988 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
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where k is a temperature-dependent rate constant. The change in M , with time (Le., rate) is seen to be linearly dependent on the value of k. The amounts remaining at various times t,, t 2 ,and t 3 are denoted by points A, B, and C in this figure. The amount vaporized between t = 0 and t = tl is given by
AM,,, = Meo - McoeTkt
(2)
Likewise, the amount vaporized between times tl and t 2 is
AM,,,= Mcoe-kt,- Mcoe-kt2
0
t2
and similarly for t3, etc. A general expression for the amount vaporized between time intervals t - At and t can be expressed as
' 3
AM,,,= Mcoe[-W-At)l- M
time
Flgure 1. Schematic representation of the fractional amount of the original mass of a component, M", in the condensed phase of a mixture while an exponential loss process is occurring. Two rates (0.1 and 1.0 s-I)are shown. Points A, B, and C are the fractional amount
remaining at times t
t,, and t,.
In all cases, "matrix-matched" solid metal standards were recommended for calibration. A previous paper (13) discussed the use of the second surface atomizer for the direct analysis of several solids for various trace metals. In a river sediment sample which contained about 10% Fe (NBS Standard Reference Material 16451, it was found that only slow and incomplete evolution of Cu could be achieved without total vaporization of the bulk sample, even though temperatures well in excess of the typical ETA appearance temperature €or Cu were employed. The peak areas of the absorbance signals obtained over several repetitive cycles with the same sample was plotted with respect to the "cycle number" (i.e., vaporization time) and the resulting curve approximated an exponential decay. The present studies evaluate a method to associate the rate of analyte evolution (vaporization) with the total amount of analyte initially present in the sample. Atomic absorption peak area measurements were used to define a curve describing the rate of release of P b vaporized from a "host" Sn sample. The empirical rate data for the other samples studied followed a similar relationship. A mathematical expression was derived to describe this relationship and relate the experimental data to the total amount of analyte in the sample. This unique extrapolative approach required only a knowledge of the rate of vaporization at a given time; hence only partial vaporization of the analyte from the sample was necessary for quantification. Since the second surface atomizer was used, minimal interferences due to effects of the host metal were encountered; hence aqueous standards could be used for calibration.
THEORY Rettberg and Holcombe (13) previously mentioned a case where exponentially decreasing peak area absorbances were observed for a series of furnace heating cycles of an individual solid sample. Their absorbance data appeared to comply with a first-order decay function. Since peak area is proportional to the mass of analyte released into the furnace volume (16), the data could be used to evaluate not only the vaporization kinetics but also the total quantity of analyte in the solid. Figure 1 is a schematic representation of an exponential decrease in mass as a function of time. This figure may represent the specific case of mass loss due to vaporization and is the basis of the derivation to follow. The analyte mass remaining in the condensed phase, M,, at time t is related to the original amount present, Meo,by
M , = Mco(e-kt)
(3)
(1)
oe-kt
(4)
where At is the time interval during which vaporization of the analyte is occurring. If At remains constant over several discrete vaporization steps, eq 4 can be rewritten as
AM,,,= (e-knAt)Mco(ekAt - 1)
(5)
where nAt = t. The logarithm of eq 5 yields In (AM,,J = -kt
+ In [MCo(ekAt - 111
(6)
Thus, a plot of In (AM,,,) versus t should produce a straight line with a slope of -k and intercept of In [M,"(ekAt- l)]. These terms can be exploited to determine Meo,the original amount of analyte in the sample. Equation 6 represents the basis for the analytical approach discussed here. It is of particular interest that M," may be accurately determined without the necessity of significantly depleting the sample of the analyte, i.e., M," - M,,t may be small as long as the slope and intercept can be accurately determined. In these studies with the second surface atomizer, the amount vaporized during each time interval corresponds to the amount of analyte transported to the second surface during the transfer cycle. Studies with wall vaporization of aqueous standards showed approximately 80% transfer efficiency. The effect of the cup on this efficiency is not known but the improved geometry would predict at least an 80% efficiency. No analyte was detectable on the walls after the transfer cycle in this study. The duration of the transfer cycle is given by At while n corresponds to the cycle or "shot" number for a given sample. Thus, when plotted, experimental data points were separated on a time axis by time periods spaced according to the transfer time chosen (Figure 2). Conversion from the experimentally obtained absorbance values to mass units (Le., A, to M,,J was performed by assuming that the peak area was proportional to mass and correlating the absorbance data with a standard calibration curve prepared from aqueous standards. Once the experimentally determined absorbances were converted to mass, the slope and intercept were used to solve eq 6 for the determination of M,". Division of M," by the initial sample mass, m, gave the concentration of the analyte in the original sample. The first-order decay function represented by eq 1 is analogous to radioactive decay, e.g. 14Cdating. As with 14C dating, the extrapolation of the rate expression is valid as long as the rate expression does not change. A more specific evaluation of the factor(s) responsible for limiting the rate of P b vaporization from these samples is presently under study.
EXPERIMENTAL SECTION Apparatus. Details of the system specifically related to the second surface atomizer can be found elsewhere (13,17-19). All absorbance data were recorded as peak areas on a Varian AA-875
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
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Table I. Parameters and Results for Pb Determinations host metal
approx sample mass, mg
Cu (SRM 393) Cu (SRM 1252) steel (SRM 1289)
1.5-5 21-27 5-7 1-2
Sn
a
nm
261.4
217.0 261.4
283.3
transfer temp, “C
transfer hold time, s
1500 1500 1450 1450
15 10 10 6
actual concn, p g / g 85“
0.039 f 0.002* 60 f Z b 5 flb
concn ( I g / g ) determined in this study 58 f 7 0.027 f 0.003 41 f 7 2.2 f 0.3
Determined by graphite furnace atomic absorption standard additions. NBS certified value.
0
b
A,
I
2
3
4
I5
30
45
60
cycle t me ( s )
-1.5,
\ ‘ \
\
-3.5;
-4.Cl
J
0
I
2
3
4
15
30
45
60
cycle time ( 5 )
Figure 2. (a) Plot of Pb peak area absorbances of four atomization cycles for a 2.60-mg Sn sample. A transfer temperature of 1500 OC for 15 s was used. (b) In (absorbance)vs time for these same data.
spectrometer. Absorbance, time, and temperature data were displayed on a Varian GTA-95 video display screen and a separate strip chart recorder was used to obtain a permanent record. A Pb hollow cathode lamp was used for all determinations. Less sensitive atomic lines were employed when necessary to provide the sensitivity appropriate for the analyte concentration being determined. The background correction system was operational during the analyses. The GTA-95 provided up to 3.1 L/min of an Ar sheath gas through the furnace ends. Typically, a flow between 1.5 and 3.0 L/min was used during all heating cycles excluding the transfer and atomize cycles, when a minimal flow of
