Anal. Chem. 1998, 70, 1223-1227
Technical Notes
Real-Time Atom Residence Time Compensation in Chemical Vapor Generation Atomic Spectrometry. A More Precise Analytical Signal Alex J. Blois, Ryan G. Keddy, and R. Scott Daniels*
Department of Chemistry, Acadia University, Wolfville, Nova Scotia, B0P 1X0, Canada Richard Dams
Laboratory of Analytical Chemistry, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium
Batch chemical vapor generation techniques, using flowthrough optical cells for the spectrochemical determination of mercury, produce transient signals whose peak area generally varies inversely as a function of the carrier gas flow rate. One significant source of within- and between-day signal variability is the vacillation of the carrier gas flow rate. Methods are presented to compensate for this variability by separately performing signal compensation in postacquisition and real-time modes. These methods involve processing the transient absorbance or fluorescence and the carrier gas flow rate data to produce new analytical signals. Results are presented demonstrating improved signal precision (reproducibility), from 31 to 4.4% (CV, n ) 10), for the cold-vapor atomic absorption and atomic fluorescence determination of mercury. Chemical vapor generation (CVG) techniques for atomic spectrometry involve the production and removal of the analyte from the sample matrix by the generation of a gaseous species as a result of a chemical reaction. CVG is best known for the cold-vapor atomic absorption1 and cold-vapor atomic fluorescence2 determination of mercury, and the generation of hydrides of metals and metalloids.3-6 Other volatile metal/metalloid derivatives include fluorides,7 β-diketonates,7 dithiocarbamates,7 chlorides,7 oxides,8 ethyls,9-15 and carbonyls.16,17 (1) Daniels, R. S.; Wigfield, D. C. Sci. Total Environ. 1989, 89, 325-329. (2) Stockwell, P. B.; Godden, R. G. J. Anal. At. Spectrosc. 1989, 4, 301-309. (3) Sturgeon, R. E.; Liu, J.; Boyko, V. J.; Luong, V. T. Anal. Chem. 1996, 68, 1883-1887. (4) Dedina, J. Prog. Anal. Spectrosc. 1988, 11, 251-360. (5) Liao, Y.; Li, A. J. Anal. At. Spectrom. 1993, 8, 633-636. (6) Puk, R.; Weber, J. H. Anal. Chim. Acta 1994, 292, 175-183. (7) Yan, P.-X.; Ni, Z.-M. Anal. Chim. Acta 1994, 291, 89-105. (8) Lopez-Molinero, A.; Castillo, J. R.; Mermet, J. M. Talanta 1990, 37, 895899. (9) Sturgeon, R. E.; Willie, S. N.; Berman, S. S. Anal. Chem. 1989, 61, 18671869. (10) D’Ulivo, A.; Chen, Y. J. Anal. At. Spectrom. 1989, 4, 319-322. (11) Rapsomanikis, S.; Donard. O. F. X.; Weber, J. H. Anal. Chem. 1986, 58, 35-38. S0003-2700(97)01047-0 CCC: $15.00 Published on Web 02/14/1998
© 1998 American Chemical Society
Chemical vapor generation has long had the advantage of analyte separation from the sample matrix, usually resulting in improved sensitivity and selectivity of measurement. Improved sensitivity has been observed by preconcentration18,19 of the chemical vapor before thermal desorption into a spectrometer. Selectivity may be improved by the use of atomic fluorescence where fewer false positive signals may occur,20 and is additionally improved by the use of chromatographic techniques,21 which provide chemical speciation. The “peak height versus peak area” signal question in atomic spectrometry has been studied, discussed,22 and continues to be of interest.23,24 The basis for deciding on a better analytical signal depends to a certain extent on the intended use of that signal. Instantaneous peak height absorbance signals may be better suited for kinetic studies while a peak area absorbance may be better suited for quantitation in the presence of significant and variable sample matrix effects. Where quantitation is of interest, the better signal is that maintaining the highest signal-to-noise ratio and, therefore, yields a lower limit of detection. The peak area signal has come to replace the peak height signal, for quantitation, primarily because of improved data acquisition systems and its improved independence from the rate of atomization. A new, more precise (less variable) quantitative signal is proposed for chemical vapor generation employing atomic absorption and/or atomic fluorescence. This new signal will be of greatest benefit to specific applications of chemical vapor genera(12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
Ashby, J. R.; Craig, P. J. Sci. Total Environ. 1989, 78, 291-232. Bloom, N. Can. J. Fish. Aquat. Sci. 1989, 46, 1131-1140. Thaler, E. G.; Caulton, K. G. Organometallics 1990, 9, 1871-1876. Rapsomanikis, S. Analyst 1994, 119, 1429-1439. Vijan, P. N. At. Spectrosc. 1980, 1, 143-144. Lee, D. S. Anal. Chem. 1982, 54, 1182-1184. Dumarey, R.; Dams, R.; Hoste, J. Anal. Chem. 1985, 57, 2638-2643. Bayens, W.; Leermakers, M. J. Anal. At. Spectrom. 1989, 4, 635-640. Daniels, R. S., unpublished work, University of Gent, 1993, and Acadia University 1996. Falter, R.; Scho ¨ler, H. F. J. Chromatogr. A 1994, 675, 253-254. Sturgeon, R. E.; Chakrabarti, C. L.; Bertels, P. C. Anal. Chem. 1975, 47, 1250-1257. Welz, B. Spectrochim. Acta 1992, 47B, 1043-1044. Doidge, P. S. Spectrochim. Acta 1993, 48B, 473-474.
Analytical Chemistry, Vol. 70, No. 6, March 15, 1998 1223
tion, particularly where analyte concentration steps are involved and where high imprecision is observed. THEORY In chemical vapor generation techniques the following may represent the peak area absorbance in a closed, flow-through optical cell:4
QA ) )
∑ A ∆t
(1)
t
∑ (log e)(N σ /S)∆t
(2)
∑N ∆t t
(3)
) (log e)(Nσoτ/S)
(4)
t o
) (log e)(σo/S)
QA is the peak area (s), At is the instantaneous absorbance at the time t, ∆t is the acquisition increment (1/acquisition frequency), log e is the decadic absorbance constant, Nt is the number of absorbing centers (atoms) in the optical path at some time t, N is the total number of absorbing centers (atoms), σo is the effective atomic photon capture cross-sectional area (cm2 atom-1), otherwise called the Naperian atomic absorption coefficient, S is the cross-sectional area of the cylindrical optical analysis volume (cm2), and τ is the mean atom residence time in the optical path. Where the atom residence time is determined by the flow rate of the carrier gas, and where the optical analysis volume is cylindrical, eq 4 may be rewritten as
QA ) (log e)(Nσol/F)
(5)
where l is the optical analysis volume path length (cm) and F is the carrier gas flow rate (STP cm3/s). Equation 5 defines the inverse relationship of the carrier gas flow rate and the peak area signal for closed, flow-through optical cells. To remove the dependence of the peak area signal on the flow rate, two options are proposed: (1) postacquisition flow atom residence time compensation and (2) real-time flow atom residence time compensation. These signals compensate for neither atomization inefficiencies, nor atomic spectral changes due to temperature fluctuations. Temperature considerations are described elsewhere.25 1. Postacquisition Flow Atom Residence Time Compensation. Postacquisition flow atom residence time compensation should be performed when the flow rate variability during analyte atomization is minimal and when a good estimate of the atomization flow rate, F, can be made. The signal should be of greater universal utility if it was independent of experimental conditions: (1) the carrier gas flow rate and (2) the optical analysis cell dimensions. This can be obtained by multiplying the peak area QA (eq 5) by F/l. Further, in order that the signal represent a physically meaningful quantity, Nσo, i.e., the total number of analyte atoms in the sample multiplied by their effective atomic photon capture cross-sectional area, QA in eq 1 should be multiplied by F/(l log e) as shown in (25) Dedina, J.; Welz, B. Spectrochim. Acta 1993, 48B, 301-314.
1224 Analytical Chemistry, Vol. 70, No. 6, March 15, 1998
Nσo (cm2) )
F F Q ) l log e A l log e
∑ A ∆t t
(6)
The notion of combining a flow rate with the absorbance signal has been reported12 for the purpose of creating a convenient criterion of sensitivity termed “normalized absorbance”, which should not be confused with the work presented here. 2. Real-Time Flow Atom Residence Time Compensation. A real-time flow atom residence time compensation should be performed when the flow rate variability during analyte atomization is large and when a good estimate of the atomization flow rate, F, cannot be made. Again, as in postacquisition flow compensation, a signal should be of greater universal utility if it was independent of carrier gas flow rate and optical analysis cell dimensions. This is achieved by multiplying the peak area QA by F/l log e as performed in eq 6 to produce the signal equal to Nσo. To account for the carrier gas flow rate varying with time, the instantaneous flow rate, Ft, is moved into the sum as shown in
Nσo (cm2) )
1 l log e
∑A F ∆t t t
(7)
An advantage of processing the signal in such a manner is the ease with which simplified, absolute methods of analysis can be used. Where an experimental signal is obtained for a given standard, it can be checked against the theoretical expectation, Nσo. For example, in the case of atomic absorption for mercury, for a sample containing 1.0 ng of Hg, and given an effective atomic photon capture cross-sectional area26 of 1.5 × 10-14 cm2 atom-1, an atomic absorption peak area signal of 0.045 cm2 would be expected and is calculated using eq 7. The above two signal compensation options may also be applied to atomic fluorescence. This should be most applicable where the detection is in a closed, flow through cell. The analogous fluorescence signals for eqs 6 and 7 are given in
∑f ∆t
N ∝ FQf ) F N∝
t
∑F f ∆t
(8) (9)
tt
Qf is the peak area fluorescence and ft is the instantaneous fluorescence. Some atomic fluorescence detectors are designed with an openflow design rather than a closed-flow cell. For instances where the fluorescence signal is not inversely proportional to the carrier gas flow rate, but is inversely proportional to Fn (where n < 1), the peak area should be multiplied by Fn or Fnt (eqs 10 and 11).
∑f ∆t
N ∝ FnQf ) Fn
t
(10)
The value of n is determined experimentally.
N∝
∑F f ∆t n t t
(11)
EXPERIMENTAL SECTION Apparatus. A schematic of the apparatus and setup is shown in Figure 1. Activated charcoal-filtered air flow was produced by (26) Gilmutdinov, A. K.; Abdullina, T. M.; Gorbachev, S. F.; Makarov, V. L. Spectrochim. Acta 1992, 47B, 1075-1095.
Figure 3. Carrier gas flow rate variation with absorbance for a 4.5µL injection (75 pg) with high-pressure flow. Figure 1. Schematic of tandem AAS-AFS instrumentation with installation of digital mass flow meter (DMFM) and optional goldimpregnated silica gel adsorber; (- - -) electronic or electrical connections and (s) argon gas flow.
Figure 4. Variation of (a) peak area atomic absorption and (b) peak area atomic fluorescence signals for 75 pg of mercury as a function of carrier gas flow rate. Absorbance power factor n is 0.991 and r2 is 0.976. With a fluorescence, shield gas flow rate of 400 mL/min, the power factor n is 0.563 and r2 is 0.989. Figure 2. Carrier gas flow rate vacillation for the following: (1) highpressure flow during direct sample injection using three-way crossover injection valve, (2) high-pressure flow during thermal desorption of sample, (3) low-pressure flow during direct sample injection, and (4) low-pressure flow during thermal desorption of sample.
a Cole-Parmer Instruments Co. model 3 Dyna-Vac membrane pumpslow-pressure flow. Where fluorescence data are reported, the carrier gas was supplied from a cylinder of argonshighpressure flow. The flow was regulated using an Arco miniature airline regulator, model 127132-320, using a 1-15 psi no. 29622 spring. The flow rate was monitored using an Omega FMA1812 0-500 SCCM (0-5-V output) digital mass flow meter. Atomic fluorescence measurements were made using a PS Analytical atomic fluorescence spectrometer (Merlin Mercury Fluorescence System, model 10023). A Laboratory Data Control UVD monitor was the single-wavelength (253.7-nm) absorption spectrometer used in this study. The modifications adapting this absorption spectrometer for a 30-cm-path length cell are described elsewhere.27 A 254-nm interference filter was used. Elemental mercury vapor was collected on 120-µm gold treated silica (part no. 883001; Phase Separations). The mercury vapor was released by the heating of a 0.8-cm diameter, 1.0-m length of coiled 26gauge (AWG) Chromel A resistive heating wire. Electrical heating was provided using 20 V ac. Mercury Vapor and Injection Port. Approximately 5 mL of elemental mercury was placed in a 50-mL glass bottle having a septum and screw top through which elemental mercury vapor was sampled. The mass of mercury sampled was calculated using vapor pressure data28 and the ideal gas law. A 10-µL 1701-RN Hamilton syringe was used for sampling the calibrant mercury vapor. Alternatively, a 4.5-µL single end capped four-way over/ (27) Daniels, R. S. Ph.D. Thesis, Carleton University, Ottawa, Canada, 1991. (28) Handbook of Chemistry and Physics, 56th ed.; Weast, R. C., Ed.; Chemical Rubber Co., Cleveland, OH, 1976; p D-182.
under valve (Upchurch Scientific, Inc., V-101C) was used as a microliter flow injection volume for elemental mercury vapor. All tubing was Teflon PFA (i.d. 0.063 in. and o.d. 0.125 in.), and the bottom reservoir was heat-sealed to contain ∼0.5 g of elemental mercury. The 4.5-µL calibration volume was given 2 min to reach equilibrium with the saturated elemental mercury vapor contained in the sampling tube beneath. Data Reduction. All signals were acquired at 10 Hz using a two-channel, 12-bit computer acquisition system. The data were stored in ASCII files that were later processed using the described equations and an Excel spreadsheet. Peak integration was also performed in the spreadsheet. RESULTS AND DISCUSSION Peak area signal variability in chemical vapor generation techniques is a function of all variables in eq 3, i.e., number of analyte atoms N, the effective photon capture cross-sectional area σo, absorption cell path length l, and the carrier gas flow rate F. Matrix variability and sources of signal variability expected from uncertainty in σo and l will not be considered here. The intended number of analyte atoms, N, may vary: (1) from a calibrant sampling point of view and (2) from poor instrument design resulting in variable analyte loss or poor and variable atomization efficiency. Carrier gas flow rate vacillation is a likely candidate for significantly contributing to signal variability and is the focus of this study. The carrier gas flow rate variability is not only a repeatability concern but may contribute more significantly to poor signal reproducibility, i.e., between days or between instruments. Chemical vapor generation systems have many modes of operation: (1) detection systems can be absorption or fluorescence based, or both; (2) carrier gas flow can be generated from high pressure (gas cylinder) or low pressure (vacuum pump); (3) sample/calibrant introduction can be by syringe injection or flow Analytical Chemistry, Vol. 70, No. 6, March 15, 1998
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Table 1. Summary of Absorbance and Fluorescence Signals with a Comparison of Relative Errora absorbance
X h 10 s CV
fluorescence
QA/s
postacquisition Nσo/cm2
real-time Nσo/cm2
Qf/s
postacquisitionb FnQf
real-timeb ∑Fnt ft∆t
0.0137 0.0043 31
0.0048 0.00021 4.4
0.0050 0.00032 6.4
3308 429 13
6114 330 5.4
6130 352 5.7
a Replicate 4.5-µL injections of saturated mercury vapor: flow rate, 298 ( 84 (28%) mL/min, X h 10 ( s (CV); temperature, 24.1 ( 0.19 (0.8%) °C, X h 10 ( s (CV); Clausius-Clayperon mass, 83.1 ( 1.3 (1.5%) pg, X10 ( s (CV); absolute mass based on Nσo (eq 7), 110 ( 7 (6%) pg, X h 10 ( s (CV).b n ) 0.397.
injection; and (4) injection can be performed directly or with an analyte concentration step. All of the above conditions have been studied, however, to demonstrate the utility of a flow-compensated signal; it is not necessary to demonstrate flow compensation for all these conditions. Carrier Gas Flow Rate Variability. Figure 2 shows typical flow rate variability for high-pressure and low-pressure flow under conditions of direct injection of mercury vapor and thermal desorption. Random flow rate variability was observed to be highest during thermal desorption of analyte under low-pressure flow conditions. This was found to be due to backpressure generated by thermal expansion of the gold-impregnated, silica gel, mercury adsorber and the concomitant action of both the mass flow controller and the membrane pump. In the case of thermal desorption from other materials, e.g., gold-coated quartz sand,29 carrier gas flow rates can change as much as from 800 to 200 mL/min during 60 s of heating. The differences in flow rate, illustrated by the smaller decrease in flow rate while switching the three-way over/under flow injection valve, are believed to be due to small differences in the two paths of the three-way crossover valve. Carrier gas flow rate changes of this magnitude would normally go unnoticed with rotameters ubiquitously used with chemical vapor generation instruments. The mass flow controller used in this work results in higher short-term noise with low-pressure flow compared to high-pressure flow. The relative position of a typical absorbance transient relative to a changing flow rate is shown in Figure 3. A large change in flow rate is observed during the absorbance transient, and variability in this flow rate will ultimately result in signal variability. Absorbance and Fluorescence Peak Area Signal Dependence on Carrier Gas Flow Rate. To correct for flow rate variability, it is necessary to know the functional dependence of the peak area signals with the carrier gas flow rate; i.e., the power factor n must be determined in eq 10. Approximately 75 pg of mercury, delivered using a 4.5-µL flow injection valve, was repeatedly delivered to tandem atomic absorption and atomic fluorescence spectrometers. The experimental validation of the inverse relationship between carrier argon flow rate and peak area signals for atomic absorption and atomic fluorescence is shown in Figure 4. The low power factor n ) 0.563 for atomic fluorescence is caused by the nonflow through design of the atomic fluorescence spectrometer. Studied at other times, this power factor has been (29) Dumarey, R.; Heindryckx, R.; Dams, R.; Hoste, J. Anal. Chim. Acta 1979, 107, 159-167.
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observed to be as low as 0.39 and as high as 0.54. It is the chimney, sheath argon flow rate that is responsible for n being less than unity. Comparison of Conventional Peak Area Absorbance and Fluorescence with Flow-Compensated Signals. To demonstrate the utility of flow compensation, in both postacquisition and real-time mode, 10 replicate 4.5-µL vapor injections of saturated mercury vapor were made. The flow rate was intentionally varied between runs, and the results of calculations are summarized in Table 1. Postacquisition and real-time flow compensation significantly improved the precision of measurement. This was the case for both absorbance and fluorescence measurements. Before signal compensation, the fluorescence spectrometer signal precision was always significantly better than the absorbance precision. This is a result of the auxiliary argon sheath gas flow in the fluorescence spectrometer which makes the fluorescence peak area signal less dependent on the analyte carrier gas flow rate, i.e., power factors less than unity. Power factors (eqs 10 and 11) for fluorescence were observed to vary between 0.39 and 0.54 while the corresponding power factor for the absorbance spectrometer ranged between 0.9 and 1.1. Nonunity power factors were not applied to the absorbance data presented. Although the fluorescence signal, Qf, varied less with betweenrun variations in the flow rate, the absorbance signals consistently showed a relatively larger improvement in signal precision from the described flow compensations. This larger relative improvement for the absorbance signal compensation is a result of the less variable, near-unity power factor. Significant differences are not observed between real-time and postacquisition flow-compensated signals for both absorbance and fluorescence. Although real-time flow compensation should provide a greater improvement in signal precision over postacquisition flow compensation, this is not observed, possibly owing to poorly matched response times between the mass flow meter and the spectrometers. CONCLUSIONS A novel method for enhancing atomic absorption and fluorescence signal precision has been developed where chemical vapor generation is used. Given that many chemical vapor generation techniques frequently suffer large within-run and between-run fluctuations in the carrier gas flow rate, especially where analyte concentration steps are used, this flow compensation method should be of use to improve analytical precision.
ACKNOWLEDGMENT The authors express their gratitude to Dr. Peter Stockwell (P.S. Analytical, U.K.) for the use of atomic fluorescence equipment and Mr. Rob Bongers (Interscience, BE) for the use of atomic absorption instrumentation. We also thank Dr. David Stiles (Acadia University) for editing comments. Grants from the Acadia University Faculty Association and NSERC General Grants in part funded this work. R.S.D. is particularly grateful to the Laboratory
for Analytical Chemistry for hosting a generous and creative environment for a postdoctoral study where ideas for this work were spawned.
Received for review September 17, 1997. January 8, 1998.
Accepted
AC971047E
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