Anal. Chem. 1985, 57,773-776
pure phthalate example (Table I, pure c) the reliability values of the incorrect answers are low, while the phthalate values have actually improved (highest 77%). Unknown Mass Spectra of Mixtures. For the phthalate example it is conceivable that this modified algorithm is less sensitive to the data causing the original retrieval of the octenes. To see if their data can be recognized when actually present, to the spectrum of the phthalate was added that of the best matching octene. The results from this mixture spectrum (Table I, mixture c) are striking; where the pure phthalate spectrum gave two octene spectra of average reliability 9%, with 25% 3-octene present 12 octenes were retrieved of average reliability 83%. This value was only 55% with the original algorithm (Table I, mixture a); thus the subtraction procedure not only gives improved discrimination against incorrect similar spectra but also gives better recognition of mixture components. This is also shown dramatically by the recall/reliability plot of results from the 110 mixture “unknowns” (Figure 3, lines 1vs. 2,3 vs. 4,5 vs. 6,7 vs. 8), with the performance for the 60% and 100% components now yearly equal. For the PBM algorithm incorporating weighted file ordering (12) on a commercial GC/MS system, these additions should increase the average search time per unknown by -20%; “real-time” identifications will still be possible during a 40-min GC/MS run for -100 components. The addition of forward search capabilities has reduced the number of class IV wrong answers for unknown mass spectra of both pure compounds and mixtures by approximately half (Figures 2 and 3).
ACKNOWLEDGMENT We thank B. L. Atwater (Fell), D. R. Bartholomew, R. G. Dromey, I. K. Mun, and J. W. Serum for stimulating discussions. LITERATURE CITED Pesyna, G. M.; McLafferty, F. W. “Determination of Organic Structures by Physical Methods”; Nachod, F. C., Zuckerman, J. J.; Randall, E. W.; Eds.; Academic Press: New York, 1976 Vol 6, pp 91-155. Chapman, J. R. “Computers in Mass Spectrometry”; Academic Press: London, 1978; Chapters 5-7. Hertz, H. S.;Hites, R. A.; Biemann, K. Anal. Chem. 1971, 4 3 , 681. McLafferty, F. W.; Hertei, R. H.; Villwock, R. D. Org. Mass Spectrom. 1974, 9 , 690.
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(5) Abramson, F. P. Anal. Chem. 1975, 47, 45. (6) Pesyna, G. M.; Venkataraghavan, R.; Dayringer, H. E.; McLafferty, F. W. Anal. Chem. 1978, 4 8 , 1362. (7) Dromey, R. G. Anal. Chem 1979, 5 1 , 229. (8) Atwater (Fell), 8 . L.; Venkataraghavan, R.; McLafferty, F. W. Anal. Chem. 1979, 5 1 , 1945. (9) Domokos, L.; Henneberg, D.; Weimann, B. Anal. Chlm. Acta 1983, 150, 37. (10) Cleil, P.; van’t Kiooster, H. A.; van Houweiingen, J. C. Anal. Chim. Acta 1983, 150, 23. (11) Shackelford, W. M.; Cline, D. M.; Faas, L.; Kurth, G. Anal. Chlm. Acta 1983. 146. 15. (12) MunY’I. K.: Bartholomew, D. R.; Stauffer, D. B.; McLafferty, F. W. Anal. Chem. 1981, 5 3 , 1938. (13) McLafferty, F. W.; Stauffer, D. B. I n t . J . Mass Spectrom. Ion Proc. 1984, 5 8 , 139. (14) Davis, J. M.; Giddings, J . C. Anal. Chem. 1983, 55, 418. (15) Atwater (Fell), B. L.; Stauffer, D. B.; Peterson, D. W.; McLafferty, F. W. Anal. Chem., in press. (16) Stauffer, D. B. Phi3 Thesis, Cornell University, 1984. (17) Stauffer, D. B.; Ellis, R. D.; Peterson, D. W.; McLafferty, F. W. submitted for publication in Anal. Chem. (18) Electronic Data Division, Wiley, New York. (19) Speck, D. D.; Venkataraghavan, R.; McLafferty, F. W. Org. Mass Spectrom. 1978, 73, 209. (20) McLafferty, F. W. Anal. Chem. 1977, 49, 1441.
Douglas B. Stauffer Fred W. McLafferty* Chemistry Department Cornell University Ithaca, New York 14853
Robert D. Ellis David W. Peterson Scientific Instrument Division Hewlett-Packard 1501 California Avenue Palo Alto, California 94304 RECEIVED for review July 9, 1984. Resubmitted November 26, 1984. Accepted December 3, 1984. This research was supported under the Industry/University Cooperative Research Projects Program of the National Science Foundation, Grant CHE-8303340.
Trace Element Determination in Flames by Laser Enhanced Ionization Spectrometry Sir: Laser-enhanced ionization (LEI) or optogalvanic spectroscopy (OGS) in flames has been developed during the last few years, particularly by a group at the National Bureau of Standards (NBS) in Washington as a technique for trace element analysis in water solutions (1-5). Substantial theoretical and experimental work in this field has demonstrated the applicability and potential of the method for this purpose (6-8). In particular, extremely low detection limits have been achieved-in many cases in the sub-part-per-billion (sub-ppb) region. The basic principle for laser-enhanced ionization is that atoms in a flame are excited by laser irradiation to higher energy levels, from which they are more easily ionized by collisions with the flame molecules. This process causes a charge increase, which is monitored by applying an electric field over the region of interaction and measuring the corresponding current increase. This work reports on detection limits for one-step laserenhanced ionization in flames for a number of elements, using a pulsed excimer laser system for excitation. Detection limits in the part-per-billion to part-per-trillion region are obtained
for 13 elements. These limits compare favorably with those reported previously in the literature, using a similar method of analysis, and they are in some cases the lowest values reported for any direct method of analysis, without any preceding sample preparation. Although the method can in principle detect only one element at a time, we want to stress that a considerable number of elements can be detected with high sensitivity with the same experimental setup without changing the dye of the laser system. This fact might be of great importance for practical applications of the method.
EXPERIMENTAL TECHNIQUE The experimental setup for laser-enhancedionization in flames, used in the present work, is shown in Figure 1. Since this setup has been described in more detail elsewhere (8),only its basic features are described here. The laser light is generated by an excimer pumped dye laser (Lambda Physik EMG 102 and EMG 2002), yielding 10-ns pulses at 20 mJ (in Rhodamine 6G),frequency doubled using a KDP crystal (efficiency - l % ) , and directed into the flame about 1 cm above the burner. The repetition rate for the laser is normally 10-30 Hz. The burner is a conventional air/acetylene single-slot
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ANALYTICAL CHEMISTRY, VOL.
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Table 11. A Comparison of Detection Limits (ng/mL) for Various Trace Element Analysis Techniques" element
LEI (NBS) (2-step)
LEI (this work) (1-step exc.)
Bi
(1-step)
Cr
Fe
Ga In Mg
0.02 0.003 2 0.001 1 2
Mn Na
Ni Pb
15
Sn T1
0.08 0.08
2 10 2 2 0.07 0.006 0.1 0.3 0.05 2 0.6
30 4 24 0.08 0.04
co
50 2 2 4
50 0.6
5 0.04 0.08 0.09 0.3
2
0.09
0.015
AAS (in flame)
30 0.1 0.8 0.8 5 10 100 20
AAS (flameless) 0.4 0.2
0.2 1 0.1 0.04 0.004
0.02 0.9
0.2 0.3 1
LIF
ICP
3
50 2 0.9
1000
1 30 0.9
0.2 0.2 0.4 0.1 2 10
25 4
0.2 14 30 0.7 0.1 0.2 4 10 30 200
AAS, atomic absorption spectroscopy; LIF, laser induced fluorescence; ICP, inductively coupled plasma; detection limits for AAS and ICP from ref 13 and for LIF from ref 14. flame SUPPlY laser
T
preamplifier sample
trigger
& I I oscilloscone I
recorder
amplifier boxcar
1
1
I
signal
Figure 1. Block diagram of the experimental arrangement for laser enhanced ionization spectroscopy in flames by one-step laser excitation.
burner, of the type used in atomic absorption spectrometry (AAS), and the sample, in the form of a water solution, is aspirated into the flame via a nebulizer as in AAS. A potential of -2000 V is applied between a pair of plates, located outside the flames, and a pair of stainless steel rods placed slightly above the grounded burner. The signal is collected on the rods, amplified by a video preamplifier, and averaged in a boxcar integrator. In order t o reduce electrical interference, the plates, the rods, and the amplifier are all shielded by a copper enclosure.
RESULTS With the experimental setup described above we have determined detection limits for 13 elements in water solution, and the results are collected in the tables below. All the measurements were performed with one-step laser excitation, using frequency-doubled light from the dye Rhodamine 6G with wavelengths ranging from 285 nm to 305 nm. For some elements, however, the laser transition used was not the optimal one with respect to the ion production but was the best available in this wavelength region. These elements are included in this investigation in order to demonstrate the above-mentioned ability of detecting a considerable number of elements with high sensitivity using frequency-doubled light from one single dye. In the tables we have compared our results with the corresponding results obtained a t NBS using LEI ( I b ) , as well as with the results found in the literature for other trace element analysis methods (atomic absorption spectroscopy, U S ; laser induced fluorescence, LIF; and inductively coupled plasma, ICP). A typical measurement was performed with a repetition rate of the laser of 10 Hz and a total time constant of the boxcar of 3 s. The detection limit is defined as the lowest concentration, which yields a signal clearly distinguable
from that of the blank. This was done by comparing the signal for a given concentration with the uncertainty in the blank level due to noise, drift, etc. when sampled for 1 min. This uncertainty represents two standard deviations in a number of such measurements.
DISCUSSION AND CONCLUSIONS As can be seen from Table I, our LEI detection limits compare' favorably with those reported by NBS, and we shall briefly comment on this comparison here. Obviously, a more direct comparison can be performed only in cases where the same wavelength region has been used by the two groups, i.e., for Cr, Fe, In, Mg, Na, Ni, and T1. For Fe, Mg, Na, and T1 the detection limits presented here are significantly lower than those reported by NBS, while for Cr and In we have not been able to reach the limits of NBS. A possible explanation for our noticeably low detection limits for Fe and Mg might be that, in addition to the collisional ionization, photoionization can take place from the excited state. In the present work we are using an excimer laser system (Lambda Physik), which generates very short, intense pulses (-10 ns, -200 kW), while the Fe and Mg data of NBS are obtained with a flash-lamp pumped laser system (Chromatix) with longer, less intense pulses (-1 ps, -1 kW). The high laser intensity increases the probability for photoionization, particularly when the laser photon energy is close to the ionization energy of the laser-excited state (9). The excess energy (Le., the energy exceeding the ionization limit) when two photons are absorbed (see Figure 2) is included in Table I. When this excess energy is small, the photoionization process may compete with, or even dominate over, the collisional ionization process. We believe this to be the most reasonable explanation for our extremely low detection limits for Fe and Mg. For Na our detection limit refers to a hypothetical solution without any sodium impurity. This limit is obtained by comparing the sodium signal (for a known concentration) with the noise level of the blank solution off the sodium resonance, while the corresponding NBS value is obtained by comparing with the noise level on resonance. The detection limit obtained for Cr in our work is more difficult to explain when compared with the corresponding NBS value. It is interesting to note, however, that our previously reported results, obtained with a Chromatix laser, gave comparable sensitivities for Cr and Fe, in agreement with the results reported by NBS (6). For the remaining elements, Bi, Co, Ga, Mn, Pb, and Sn, we have worked in a different and in most cases less optimal, wavelength region than that used by the NBS group. In some
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EION
Col Iis i o n a l ionization
limit
I
1
x”1
Photo ionization
I
Laser Xi2 excitation
Flgure 2. Excitation and ionization scheme for one-step laser enhanced ionization in flames. E, and E, denote the lower and upper laser level, respectively, while EIONrepresents the ionization limit. E, is the excess energy above the ionization limit when a second photon is absorbed in a photoionizationprocess.
of these cases (Co, Ga, and Pb) we still get comparable detection limits, while in other cases our limits are considerably higher, either due to unfavorable oscillator strength of available lines in our experiment or to high energy of the lower level of the laser transition (see Table I). In comparing the detection limits of LEI, for the elements studied here, with those of other standard methods of analysis, such as AAS in flame, flameless AAS, LIF, and ICP (see Table 11),we find that, when an optimal wavelength is used, one-step LEI has detection limits which are quite comparable with those of flameless AAS and usually superior to those of the other methods. In addition, LEI has in comparison with flameless AAS, the advantage of having the flame as an atomizer. Furthermore, it can be noted that even with rather unfavorable excitation lines, for instance, LEI can compete with AAS in flames. These observations, supporting those obtained earlier by the NBS group, demonstrate the power of the LEI method as a tool for trace-element analysis. The high sensitivity of the method has the consequence that preconcentration, frequently used in other methods, is here usually superfluous. Furthermore, we have shown elsewhere that LEI in flame can be conveniently combined with high pressure liquid chromatography (HPLC) (10). The work reported here is the first in a series of works we intend to perform in order to investigate the applicability of the LEI method for trace-element analysis under various experimental conditions. Besides using different dyes, which make other elements available for investigation as well as more optimal lines available for some of the elements studied here, our results can be improved particularly in the following two
ways. (a) The use of two-step laser excitation (5, 8) can increase the rate of ionization considerably and reduce the detection limits accordingly. In the case of Mg, for instance, we have found that the signal can be enhanced at least 2 orders of magnitude by using a second laser-excitation step. In addition, a second excitation makes it possible to reduce optical interferences substantially, which could be of great importance for investigating real samples. (b) For elements with low degrees of atomization in an ordinary air/acetylene flame, the number of free atoms in the flame, and consequently the LEI signal, can be considerably improved by means of a higher flame temperature, e.g., by using an N20/acetylene flame. It should be noted, however, that “improvements” of this kind may lead to a substantial increase in complexity of the analysis, and any of them should be used only when there is a definite need for it.
ACKNOWLEDGMENT The authors wish to thank Thomas Berglind for stimulating discussions. Regigtry No. Bi, 7440-69-9;Co, 7440-48-4; Cr, 7440-47-3;Fe, 7439-89-6; Ga, 7440-55-3; In, 7440-74-6; Mg, 7439-95-4; Mn, 7439-96-5; Na, 7440-23-5; Ni, 7440-02-0; Pb, 7439-92-1; Sn, 7440-31-5; T1, 7440-28-0;HzO, 7732-18-5. LITERATURE CITED (1) Green, R . 6.; Keiler, R. A,; Schenck. P. K.; Travis, J. C.; Luther, G. C J . Am. Chem. SOC.1978, 98, 8517. (2) Turk, G. C.; Travis, J. C.; DeVoe, J. R. Anal. Chem. 1979, 51, 1890. (3) Turk, G. C.; Travis, J. C.; DeVoe, J. R.; O’Haver, T. C. Anal. Chem. 1978, 50,817. (4) Travls, J. C.; Turk, G. C.; Green, R. Anal. Chem. 1982, 54, 1006A. (5) Turk, G. C.; Mallard, W. G.; Schenck, P. K.; Smyth, K C. Anal Chem. 1979, 51,2408. (6) Berglind, T.; Rublnsztein, H.;RosBn, A. Institute of Physics, Chalrners University of Technology, GIPR-216, 1980, unpublished results. (7) Travis, J. C.; Schenck, P. K.: Turk, G. C.; Mallard, W. G. Anal. Chem. 1979, 51, 1516. (8) Axner, 0.; Berglind, T.; Heully, J. L.: Lindgren, I.; Rubinsztein-Dunlop, H. J. Appl. Phys. 1984, 55,3215. (9) Curran, F. M.; Lin, K. C.; Leroi, G. E.; Hund, P. M.; Crouch, S. R. Anal. Chem. 1983, 55,2382. (IO) Berglind, T.; Nilsson, S.; Rubinsztein-Duniop, H., submitted for publication in Anal. Chem. (11) Reader, J.; Corliss, C. H.; Wiese, W. L.; Martin, G. A. Natl. Stand. Ref. Data Ser. ( U S . , Natl. Bur. Stand.)NSRDS-NBS 68. (12) Striganov, A. R.; Sventitskii, N. S. NBS Monograph ( U S . ) 1968, no.
145. (13) Weeks, S. J.; Haraguchi, H.; Winefordner, J. 0. Anal. Chem. 1978, 50,360. (14) Fassel, V. A. Science 1978, 202, 183.
Ove Axner Ingvar Lindgren Ingemar Magnusson Halina Rubinsztein-Dunlop* Department of Physics Chalmers University of Technology/University of Gothenburg S-412 96 Goteborg, Sweden RECEIVED for review November 30, 1983. Resubmitted July 23,1984. Accepted November 5,1984. This work has been supported by the Swedish Natural Science Research Council.
Differentiation of Cationic from Neutral Transition-Metal Complexes Using Wires as Field Desorption Emitters Sir: Numerous types of emitters have been described for field desorption (FD) mass spectrometry ( I ) . Carbonaceous
microneedles (2-4) or microneedles formed from metal carbonyls (5), silicone whiskers (6),and cathodically reduced
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