Anal. Chem. 1988, 60, 1629-1631
Nevertheless, the achievement of low micromole sensitivity with a detector designed for qualitative use is very promising, and a suitably configured instrument is currently being assembled. It was anticipated that the selectivity of the method would be excellent, since the tracer in the eluent can only be diluted with a species that is chromatographically identical with it. To confirm this expectation, a solution of gasoline was spiked with B, dissolved in 50% aqueous methanol, and injected into the B*/toluene system used above. Except for a weak signal at the void volume corresponding to the injection solvent, the only signal observed was that of benzene. Similarly, a solution of rn-xylene spiked with B also gave rise to only the benzene signal. In summary, a difference between the retention of injected An and that of equilibrium An* on the column is used to induce isotope dilution. In this study, solubility was used as the basis for modifying retention. In principle, other means could be used; e.g. if the column element containing the An band is kept at a different temperature than the rest of the column, a similar effect should occur. The method is preliminary in its present form, but it appears to capture the important attributes of IDA and HPLC. While it is known to be based on fine differences in solute transport, the details of the interaction between analyte, additive, and the stationary phase are not fully understood. The technique is very selective, but the full sensitivity potentially available from it has not been realized, since a truly compatible detector has yet to be constructed.
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LITERATURE CITED (1) Wang, C. H.; Willie, D. L.;Loveland, W. D. Radiotracer Methodology in the Siological, Envifonmental, and Physical Sciences ; Prentice-Hall: Englewood Cliffs, NJ, 1975; pp 381-388. (2) Banerjee, S.Anal. Chem. 1985, 57, 2590-2592. (3) Banerjee, S.;Castrogivanni, M. A. J. Chromatogr. 1987, 396, 169-1 75. (4) Banerjee, S. US. patent 4 734 377. (5) Martire, D. E.; Boehm, R. E. J . fhys. Chem. 1983, 87, 1045-1062. (6) Jaroniec, M.; Martire, D. E. J . Chromatogr. 1986, 351, 1-16. (7) Tewari, Y. B.; Martire, D. E.; Wasik, S. P.; Miller, M. M. J. Solution Chem. 1982, 1 1 , 435-445. (8) Burris, D. R.; MacIntyre, W. G. Environ. Toxicol. Chem. 1985, 4, 371-377. (9) Banerjee, S. Environ. Sci. Techno/. 1984, 18, 587-591. (10) Yalkowsky, S.H.; Banerjee, S.Estimation of the Water Solubilities of Organic Compounds; Marcel Dekker, in press. (11) Eganhouse, R. P.; Calder, J. A. Geochim. Cosmochim. Acta 1976, 555-561. (12) Denkert, M.; Hackzell, M. L.; Schili, G.; Sjogren, E. J. Chromatogr. 1981, 218, 31-43. (13) Parkin, J. E. J. Chromatogr. 1984, 267,457-461. (14) Vigh, G.; Leitold, A. J. Chromatogr. 1984, 312, 345-356. (15) Takeuchi, T.; Ishii, D. J. Chromatogr. 1987, 393, 419-425. (16) Banerjee, S.;Steimers, J. R. Anal. Chem. 1985, 57, 1476-1477.
Sujit Banerjee Safety and Environmental Protection Division Brookhaven National Laboratory Upton, New York 11973 RECEIVED for review August 7,1987. Accepted March 25,1988. Research was carried out under the auspices of the U S . Department of Energy under Contract No. DE-ACOZ76CH00016.
Laser-Enhanced Ionization Spectrometry in a T-Furnace Sir: This communication reports on results obtained by laser-enhanced ionization (LEI) spectrometry with a modified electrothermal atomizer a t atmospheric pressure. In this T-shaped furnace the atomization of an analyte is performed inside the graphite tube, while the excitation of the produced atoms, subsequent ionization, and detection are carried out in an external cavity connected to the tube. Two elements, Mn and Sr, have been detected by using LEI spectroscopy with this new arrangement. Extremely high sensitivities were obtained for both elements. Laser-enhanced ionization spectrometry is an ultrasensitive method for detecting atoms or molecules in an ionizing medium. Two-color LEI is based on the enhanced rate of ionization which follows when the analyte element is selectively excited by light from two dye lasers tuned to appropriate transitions. This enhanced rate of ionization is detected as a current increase by applying a voltage across the interaction region. LEI spectroscopy in flames has been extensively used for detection of a wide range of elements and an impressive potential of this method for trace element analysis has been demonstrated (1-3). In atomic absorption spectrometry (US)-today the most widely used analytical method for trace element analysis-the transition from a flame to an electrothermal atomizer led to highly improved detection limits for many elements. The main advantages of a graphite furnace as an atomizer compared to a flame are the longer residence time for the atoms in the interaction region, possibilities for microanalysis, and a lower molecular background. For the same reasons the graphite furnace was introduced to LEI. Earlier reports on 0003-2700/88/0360-1629$0 1.50/0
LEI in graphite furnace (4-7), where both atomization and detection of an analyte were performed inside the graphite tube, have shown extremely high sensitivities for a number of elements. However, severe interferences occurred which originated from electrons thermally emitted from the heated graphite tube and the electrode, making it impossible to measure, for example, Sr, which atomizes rather slowly (7). The basic idea of the new construction of an electrothermal atomizer presented here is to spatially separate the atomization volume from the detection volume and thus overcome some of the problems occurring in earlier constructions.
EXPERIMENTAL SECTION A schematic view of the experimental equipment is shown in Figure 1. An excimer pump laser (Lambda Physik EMG 102) simultaneously pumped two dye lasers (Lambda Physik EMG 2002). The dyes used were Coumarin 153, producing light in the 522-600 nm region, and Coumarin 47, producing light in the 440-484 nm region. For Mn the output from Coumarin 153 was frequency doubled by a KDP crystal. The laser pulse duration was 20 ns and the repetition rate was 25 Hz. The graphite furnace, in which the samples of 50-wL water solutions were inserted, was a commercial Perkin-Elmer (HGA-72) Model. The graphite furnace has an inner diameter of 9 mm and is 50 mm in length. The current corresponding to the maximum temperature of 2900 K was 500 A. Argon at atmospheric pressure was used as a protective gas. It was partly flowing around the graphite tube and partly fed through one end of the tube to drive the atoms toward the interaction region. The total flow of Ar was about 6 L/min. The temperatures and times used for drying and charring were those recommended for AAS measurements in a commercial graphite furnace. Maximum atomization tem0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 15, AUGUST 1, 1988
f Oscilloscope
Boxcar
Figure 1. Experimental setup for two-color laser-enhanced ionization spectrometry in a T-furnace. Component values in the present work were C = 1 nF, R , = 22 k n , and R , = 16 kn.
0
10 s
Flgure 3. LEI signal obtained from 50 pL of deionized water. The signal correspondsto a contamhation level of 1 pg of Mn. The graphite tube was held at maximal temperature for 9 s.
t
0 10 s
0
Flgure 2. LEI signal obtained from 50-pL aqueous solution containing 50 pg of Mn. The graphite tube was held at maximal temperature for 9 s.
perature was used for both Mn and Sr. The interaction chamber consisted of two stainless steel plates 25 X 80 mm mounted outside the graphite furnace 8 mm apart. Two bars of Teflon were used to create a channel for the atoms to flow in and to insulate the plates from each other. The signal was measured on the outer plate, which was normally held at a negative potential of -500 V relative to the earthed inner plate. The system for analyzing the electrical signal consisted of a fast amplifier and a boxcar integrator (EG&G 4402 and 4420). The signal from each laser pulse lasted for slightly less than 100 ns. The boxcar integrator made it possible to register the signal from each laser pulse separately. Both the peak signal and the integrated signal during the time of atomization could be recorded by use of internal software.
RESULTS Two elements, Sr and Mn, have been investigated by use of two-color LEI in the T-furnace. In Mn measurements the enhancement of the signal due to two-color excitation compared to one-color excitation was large. The enhancement of the LEI signal when two-color excitation is used instead of one-color excitation is dependent upon the laser powers employed in the respective steps. This dependence was not investigated in detail but the enhancement was roughly about 3 orders of magnitude. This enhancement can be compared with the enhancement of less than 1order of magnitude for Mn in the furnace with an inner electrode. The difference in enhancement can probably be explained by the colder detection region in the T-furnace, making it more necessary to excite the atoms to high-lying states in order to obtain an
0
20 5
Flgure 4. LEI signal obtained from 50 pL of aqueous solution containing 50 pg of Sr. This signal was obtained wlth strongly attenuated lasers. The graphite tube was held at maximal temperature for 15 s (different compared to Mn).
efficient ionization. In the analysis of Sr the signal enhancement due to the second laser excitation was 1order of magnitude. The signals from 50 pg of Mn are shown in Figure 2. The graphite tube was held at maximal temperature for 9 s. Figure 3 displays the signal from deionized water which correponds to about 1pg of Mn. The signal at the end of the atomization time in Figure 1and Figure 2 originates from atoms that got stuck to the colder walls in the furnace outside the graphite tube. However, when a longer atomization time, i.e. time of maximal temperature, is used repetatively, this delayed signal is considerably lowered. The detection limit for Mn in these measurements was 1 pg. The signal from 50 pg of Sr can be seen in Figure 4. In these measurements the output from the lasers had to be attenuated 50000 and 1000 times in the respective laser steps in order to avoid saturation of the amplifier. The graphite tube was in this case held at maximal temperature for 15 s (note the different time scale between Figures 2 and 4). The detection limit for Sr in these measurements was 2 pg. The detection limits for Mn and Sr were totally determined by contamination. The extreme sensitivity of Sr indicates that the detection limit could be si&icantly lower with clean room facilities. It is interesting to compare the sensitivities obtained in the T-furnace (charge collected per unit mass sampled) with
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Table I. Detection Limits and Sensitivities for Two-Color LEI in the T-Furnace with Corresponding Spectroscopic Data
det limit, pg ultimat det limit, pg cont blank, pg sensitivity, fC/pg XI, nm XZ, nm laser energy/pulse XI, pJ laser energy/pulse X p , p J
Mn
Sr
1 0.001
2 0.001 2
1 50 279.48 456.4* 5 5000
500" 460.73 554.34 0.1 1
Sensitivity obtained with strongly attenuated lasers. second step cannot be identified.
The
sensitivities for LEI in flames (3). The sensitivities for flame LEI are given as charge collected per concentration. Assuming an aspiration rate of 6 mL/min and a laser repetition rate of 50 Hz, we equate the 5000 aC/ppb for Mn to 2.5 fC/pg, and the 4000 aC/ppb value for Sr to 2 fC/pg. This comparison gives the sensitivities for the T-furnace measurement to be 20 times more sensitive for Mn and 250 times more sensitive for Sr than flame LEI. Detection limits and sensitivities for two-color LEI in a T-furnace with corresponding spectroscopic data are compiled in Table I. The sensitivity is given as the total amount of charge collected at the plates during the whole time of atomization per picogram of analyte (femtocoulombs per picogram). I t should be pointed out that in the present construction of the T-furnace there is a high temperature gradient between the center of the graphite tube and the detection region. It is also noteworthy that the graphite in the furnace is not pyrolytically coated. These two facts imply that only a very low fraction of the atoms in the sample will reach the detection region because of the atoms sticking to the walls and formation of molecules. Comparing the sensitivity figures obtained here with the ideal case, i.e. 100% ionization of all atoms in the sample and unit charge collection efficiency, gives absolute efficiencies of about for Sr and for Mn. An improvement of the above-mentioned factors (which is now in progress) will increase the number of atoms in the detection region by several orders of magnitude. This will probably give detection limits for the method in the order of 1 fg. The linearity was only tested over 1 order of magnitude, where it was good. Also the reproducibility was good according to these preliminary measurements. Disturbances from electrons emitted from the heated graphite tube were not observed with a negative potential on the outer plate. A positive voltage, however, resulted in a very high noise level which was due to a large dc current carried by thermionically emitted electrons. Blocking of the laser light did not result in any detectable decrease of the noise level.
This strongly proves that the earlier statement considering the thermionic emission of electrons inside the heated graphite tube is correct and shows that this emission is very large. No effects from photoelectric emission due to scattering of laser light were observed in the present measurements. Thermionic electrons caused the main problem when an electrode inside the graphite tube was used for signal collection since the dc current caused by them made it impossible to retain a voltage between the electrode and the graphite tube. Due to this dc current it was only possible to measure a LEI signal during the first few seconds of atomization. This was a sufficiently long time to measure Mn but not long enough for Sr determination because of the slow atomization of that element. No disturbances from the large heating current (500 A) were observed in these experiments and it was not necessary to use the special trigger unit that was used in our earlier works (5-7). In conclusion, we have demonstrated in this work a very high sensitivity of the T-furnace in combination with insensitivity to disturbances from the heating current as well as to the thermally emitted electrons. This suggests that LEI in the T-furnace can become a very attractive method for ultrasensitive trace element analysis of microsamples when the problems in the present construction are overcome, i.e. the temperature gradient between the atomization and detection region is lowered and pyrolytic graphite is used for the construction of the furnace.
ACKNOWLEDGMENT The support of Ingvar Lindgren is greatfully acknowledged as are discussions with Ove Axner. Registry No. Mn, 7439-96-5; Sr, 7440-24-6;graphite, 7782-42-5. LITERATURE CITED (1) Travis, J. C.; Turk, G. C.; Green, R. B. Anal. Chem . 1982, 5 4 , 1006A. (2) Camus, P. Euroanal. V, Rev. Anal. Chem. 1987, 107-115. ( 3 ) Axner, 0.; Magnusson, I.; Petersson, J.; Sjostrom, S. Appl. Spectrosc. 1987, 4 1 , 19. (4) Gonchakov, A. S.;Zorov, N. B.; Kuzyakov, Yu. Ya.; Matveev, 0. I. Anal. Left. 1979, 72, 1037. (5) Magnusson, I.; Axner, 0.; Lindgren, I.; Rubinsztein-Dunlop, H. Appl. Spectfosc. 1986, 4 1 , 968-971. (6) Magnusson, I.; Sjostrom, S.;Lejon, M.; Rubinsztein-Dunlop, H. Spectrochim. Acta, Part B 1987, 428, 713-718. (7) Magnusson, I. Spectrochim. Acta, Part 8 , in press.
Sten Sjostrom* Ingemar Magnusson Mats Lejon Halina Rubinsztein-Dunlop Department of Physics Chalmers University of Technology S-412 96 Goteborg, Sweden RECEIVED for review October 6, 1987. Accepted March 19, 1988. This work has been financed by the Swedish Natural Science Research Council.
Presence of Hydroxylamine in the Phosphoric Acid/Nitric Acid/Hydrogen Peroxide Digestion Procedure for Selenium Determination Sir: In a recent paper in Analytical Chemistry (1) an interesting method for digestion of soil and plant materials for selenium determination was presented. The method included an addition of Mn(I1) to a mixture of phosphoric
acid/nitric acid/hydrogen peroxide, Mn(I1) serving as a covenient indicator of completeness of the digestion as it was oxidized to the purple MnO, ion when oxidation of the sample was complete.
0003-2700/88/0380-1631$01.50/0 0 1988 American Chemical Society
