Anal. Chem. 1985, 57, 2625-2628
graphic resolution can be relieved in favor of mathematically extracting considerably more information from the data. Studies are under way to evaluate the TOC approach to fused-peak sets containing more than two components.
ACKNOWLEDGMENT D~~~ ~ ~Department l of Environmental ~ ~ ~ Health, University of Washington, for the use of the Finnigan mass spectrometer and Gil Rohrback, Rohrback Technology Corp., Seattle, for helpful discussions. Registry No. p,p’-DDE, 72-55-9; Arochlor 1260,11096-82-5; dieldrin, 60-57-1.
we thank
LITERATURE CITED (1) Giddings, J. C. “Dynamics of Chromatography. Part 1. Principles and Theory”; Marcel Dekker: New York, 1965. (2) Jennings, W. G. HRC CC,J . H/gh Resolut. Chromatogr. Chromatogr. Commun. 1980, 3 , 601. (3) Dandeneau, R. D.; Zerenner, E. H. HRC Cc, J . High Resolut. Chromatogr. Chromatogr. Commun. 1979, 2 , 35. (4) Bertsch, W., Jennings, W. G., Kaiser, R. E., Eds. ”Recent Advances in Capillary Gas Chromatography”; Verlag: Heldelberg, 1981. (5) Scott, R. P. w. Adv. Chromatogr. 1983, 22, 247. (6) Jorgensen, J. W.; Lukacs, K. D. Science 1983, 222, 266. (7) Grob, K Chromatographia, 1975, 8 , 423.
2625
(8) Poland, A,; Glover, E.; Kende, A. S.J . Biol. Chem. 1978, 251, 4936. (9) Davis, J. M.; Giddings, J. C. Anal. Chem. 1983, 55, 418. (10) Woodruff, H. B.; Tway, P. C.; Cline Love, L. J. Anal. Chem. 1981, 53, 81. (11) Sharaf, M. A.; Kowalski, B. R. Anal. Chem. 1981, 53, 518. (12) Chen, J.-H.; Hwang, L.-P. Anal. Chim. Acta 1981, 733, 271. (13) Sharaf, M. A.; Kowalski, B. R. Anal. Chem. 1982, 54, 1291. (14) Appellof, C. J.; Davidson, E. R. Anal. Chim. Acta 1983, 146, 9. (15) McCue, M.; Mallnowski, E. R. J . Chromatogr. Sci. 1983, 21, 229. , (16) McCue, M.; Malinowski, E. R. Appl. Spectrosc. 1983, 37, 463. (17) Osten, D. W.; Kowalski, B. R. Anal. Chem. 1984, 56, 991. (18) Vaidya, R. A.; Hester R. D. J . Chromatogr. 1984, 287, 231. (19) Vandeginste, B.; Essers, R; Bosman, T; Reijnen, J; Kateman, G. Anal. Chem. 1985, 57, 971. (20) Bertsch, W. HRC CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1978, 1 , 85. (21) Lawton, W. H.; Sylvestre, E. A. Technometrics 1971, 73, 617. (22) Duewer, D. L.; Kowalski, B. R.; Fasching, J. L. Anal. Chem. 1978, 48, 2002. (23) Wold, S. Technometrics 1978, 20, 397. (24) Eastment, H. T.; Krzanowskl, W. J. Technometrics 1982, 24, 73. (25) Malinowski, E. R.; Howery, D. G. “Factor Analysis in Chemistry”; Wiley: New York, 1980. (26) Spjotvoll, E.; Martens, H.; Volden, R. Technometrics 1982, 2 4 , 173. (27) Borgen, 0.;Kowalski, B. R. Anal. Chim. Acta, in press.
Received for review April 8, 1985. Accepted June 14, 1985. This work was supported, in part, by a grant from the Department of Energy (DE-AT06-83ER60108).
New Sensitive and Selective Detector for Gas Chromatography: Surface Ionization Detector with a Hot Platinum Emitter Toshihiro Fujii* National Institute for Environmental Studies, Division of Chemistry and Physics, Tsukuba, Ibaraki 305, Japan
Hiromi Arimoto Analytical Application Department, Shimadzu Corporation, Nishinokyo, Nakagyo, Kyoto 604, J a p a n
A new selective detector for gas chromatography Is described whlch uses an electrlcaliy heated Pt fllament as an emitter surface. The Ionization mechanism is the posltive surface Ionization of chemical species on the hot surface. With the Pt emltter in an air environment, this detector provides extremely sensitive and specific response to organlc compounds, Which form their dlssoclative species at a low ionization potentlai.
In 1963, Folmer et al. (I), in a study of the Characteristics of the catalytic combustion detector (2),reported a new mode of detection which measures the charge released during the reaction on a Pt surface after gas chromatography. They constructed a modified detector to detect changes in electrical conductivity within the combustion chamber, Le., production of ions during combustion. This detector was composed of a catalytic platinum filament enclosed by a cylindrical electrode and the electric circuit to measure ion current flow between the filament and the cylinder They found that other operation modes (ion detection mode) of the catalytic combustion detector were possible which could immediately extend the scope and usefulness of this detector and increase its sensitivity without any explanation of its theory of oper-
ation. But these observations would indicate that they attached significance to the surface ionization phenomenon on the hot surface (3). Such a phenomenon could be exploited more usefully and extended if the explanation of the detection mechanism is made thoroughly. On the other hand, it is well-known that some classes of organic molecules ionize on the surface of hot solids, with the desorption of positive ions in the presence of weak electric fields. Mass spectrometric studies on the surface ionization of organic compounds have been made recently. The possibility of the thermionic emission of organic ions was first indicated in the observations of the mass spectra obtained by the ionization of the residual gases in the mass spectrometer (4). Systematic studies on the surface ionization of individual organics were begun in 1960s by a Russian group (5). It is now established (6-9) that compounds with heteroatoms ionize on the metal emitter, especially on refractory metal oxide emitters. Nitrogen-containing compounds are most effectively ionized; the extremely high ionization efficiency of amines on tungsten or rhenium oxide emitters has been demonstrated, These experimental results led to the aspect that the possibility of a sensitive and selective detection of specific compounds eluted from a gas chromatograph (GC),such as amine compounds, can be explored by use of the characteristics of the surface ionization of the organics.
0003-2700/85/0357-2625$01.50/0 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
Table 1. Performance Comparison of the SID with Other Ionization Detectors
I 3
FID sensitivity: C / g of X detection limit, g of X/s selectivity,*g of C / g of X linear dynamic range
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time(min) Figure 2. Typical gas chromatogram of amine compounds by the SID gas chromatography. Peaks are (1) trlmethylamine, (2) trlethylamine, (3) acetone (solvent), and (4) tripropylamlne. Sample size was 140 pg each.
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corresponds to n-propane for FID, azobenzene for TID, and tributylamine for SID. bC stands for acetone in the case of TID and dodecane in the case of SID.
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2627
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Figure 3. Emmer current dependence of tribufylamine response (solld
line), noise current (dotted lines), and background current (dashed line) under a 40 mL/min helium carrier gas mixed with addilonal gas of the dried alr at 20 mL/min. Sample size was 16 ng of TBA in acetone.
electrode bias voltage were investigated. The results of voltage-current characteristics lead to the conclusion that the ion currents level off at a voltage of more than 200 V. Therefore all the remaining data reported were obtained at the ring electrode bias voltage of +200 V with respect to the collector electrode. Surface Temperature Effect. Figure 3 illustrates the response effect caused by varying the emitter surface temperature. Emitter temperature is varied by changing the
emitter heating current. This figure shows that both the signal and background currents increase with the heating current. This behavior may be partially explained by the fact that from the Saha-Langmuir equation thermionic emission processes depend on the surface temperature. The large background current is almost probably attributable to the appearance of Na+ and K+ ions from Na and K impurity atoms in the Pt material of emitter. This assumption is consistent with the experimental fact that background levels gradually decrease with continuing operation time. Perhaps the decay is associated with the outgassing of alkali impurities from the hot Pt emitter. Besides, the emission of these positive ions from hot filaments is a well-known phenomenon, discussed at some length by Richardson (13) as long ago as 1916. However, no significant sensitivity drift has not been found for 1 month, indicating that the signal response is not related to the alkali impurity content of the emitter. A variation of emitter temperature results in a change in the ion signal (i). The i vs. T curves were bell-shaped in all cases but they showed different forms. These characteristics are not surprising, as many mass spectrometric studies reported similar characters for the surface ionization of organic compounds on a hot filament in the vacuum environment (7). The actual operating temperature of the emitter will be a thermal balance between heating current input and thermal conduction losses through the gas stream. Since the thermal conductivity of the predominant gas, He, is about 10 times greater than the thermal conductivity of air or most organic molecules, there is an especially strong thermal coupling of the emitter temperature with the surrounding detector components. Hence, the emitter temperature at a given heating current input will depend strongly on the He/air mixture ratio. Also, the evolution of large quantities of organic materials from the GC might be expected to cause a momentary thermal decoupling between the emitter and surrounding wall and an apparent signal correlated with the accompanying momentary increase in emitter temperature. Such thermal conductivity effects can probably occur in the present system. At the higher emitter current a decrease in the ratio of sample peaks to the detector noise level occurs. This result indicates that an optimum emitter current has to be found for the detection limit. Generally favorable operation is achieved with emitter current at 2.2-2.5 A, that generates a noise level of 1 X A in the present case. Under the optimum conditions, the minimum detectable amount at the signal to noise ratio of 2 was measured using TBA. The result is listed in the Table I. It is very important to choose exactly the optimal emitter material which gives the best response at the desired operational condition, as well as to search for the optimum gaseous atmosphere in which the emitter is placed. A structure and a positioning of the emitter should be studied. These will be reported elsewhere. Stability of Base Line. Figure 4 shows a practical and realistic example of the performance of the SID. It is of considerable interest to note the short time of about 60 min from the switch-on of the emitter until a stable base line is
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cr-
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13. NOVEMBER 1985
0
210
--time,min
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Figure 4. SID behavior in the initialization process: emitter current, 2.2 A; sample size, 80 pg of TBA in acetone. A. reached even at the high current level of 2 X Linearity. The measurement of the linear range has been made with TBA as a test sample, showing that the straight portion of the working curve covers nearly 4 orders of magnitude for the TBA with the uncertainty of +3.0%. Sensitivity a n d Selectivity. As already mentioned, sensitivity of the detector is strongly dependent on the IP of the species as well as on the yield of the species generated through the chemical reaction on the surface. Presumably the relative sensitivity for different organics, which is easily determined by a comparison of the signal current, varies in a wide range from sample to sample. A experiment revealed that a considerably strong signal was observed for m-xylene (M). This response is presumably due to the ionization of (M - H) radical (7) whose I P is 7.56 eV. Thus, it can be concluded that the present experimental setup of the Pt emitter for use in the surface ionization detector at least allows the detection of chemical species whose IP is less than 7.56 eV. For a given organic, the sensitivity of the detector can be also varied, depending on the operating conditions such as the emitter temperature. As can be seen from the Figure 3, the sensitivity increases with the higher noise level and therefore the optimum has to be found for the detection limit. The sensitivity(s) of the detector can be expressed as coulomb per gram of sample. Under the optimum conditions for the detection limit, sensitivity was 1.58 C/g for TBA and 2.98 X 10P C/g for dodecane. Consequently selectivity of this detector, which is defined as the ratio of sensitivity, was S(TRA)/S(dodecane) = 5.3 X
lo5
Reproducibility. The reproducibility of the detector was determined from replicate analysis (20 times) of the standard
sample a t 260 pg of TBA when the standard deviation was 204 and the coefficient of variation 1.9% using peak area (mm2). These figures are close to the generally accepted best performance of a microliter syringe in the hands of an experienced operator. Comparison of the Performance with Other Ionization Detectors. Table I gives a comparison of performance characteristic of SID with other ionization detectors of the FID and TID used in gas chromatography in terms of sensitivity, selectivity, detection limit, and linear dynamic range. All values of FID and TID are from the literature (14, 15). This result demonstrates that the SID provides extremely high sensitivity, compared with the well-established FID and TID. In conclusion, the principle of the detector described is consistent with a process of positive surface ionization on the hot surface. It is interesting contrast to the TID whose mechanism is negative surface ionization on low work function surface (15, 16). This work reveals that the excellent performance for amine compounds, together with easy fabrication and operation, makes this detector promising for all areas of GC applications. Further experiments would be desirable to obtain the relative sensitivity for other kinds of organics; this could reveal how sensitive and how specific the analysis of individual compound can be done in the many fields of analytical chemistry by this detector. ACKNOWLEDGMENT We are grateful to D. S. Linton a t Yokota Air Base for manuscript preparation. Registry No. Pt, 7440-06-4; tributylamine, 102-82-9; trimethylamine, 75-50-3; triethylamine, 121-44-8;tripropylamine, 102-69-2. LITERATURE CITED Folmer, 0. F.; Yang, K.; Perkins, G. Anal. Chem. 1983, 3 5 , 454. Schay, G., Jr.; Szekely, G.; Traply, G. Preprints of papers, Fourth International Gas Chromatography Symposium, June 13-16, 1962, Hamburg, Germany, p 1. Zandbert, E. Ya.; Ionov, N. I . Surface Ionization, Israel Program for Scientific Translations, Jerusalem, 1971. Palmer, G. H. J . Nud. Energy 1958, 7, 1. Zandberg, E, Ya.; Rasulev, U. Kh.; Schstrov, B. N. Dokl. Akad. Nauk SSSR 1967, 172, 885. Davis, W. E. Environ. Sci. Techno/. 1977, 1 1 , 587. Fujii, T. Int. J . Mass Spectrom. I o n Proc. 1984, 5 7 , 63. Fujii, T. J . Phys. Chem. 1984, 88, 5228. Zandberg, E. Ya.; Rasulev, U. Kh. Russ. Chem. Rev. 1982, 5 1 , 819. Kolb, B.; Bishoff, J. J . Chromatogr. Sci. 1974, 12, 625. Langmuir, I.; Kingdom, K. H. Proc. R . SOC. London, Ser. A 1925, 107, 61. Eastman, D. E. Phys. Rev., Sect. E 1970, 2 , 1. Richardson, 0. W. "The Emission of Electricity from Hot Bodies", 2nd ed.; Longmans Green and Co.: London, 1921; Chapter 8. Kolb, B.; Auer, M.; Pospisil, P. J . Chromatogr. Sci. 1977, 15, 53. Patterson, P. L. J . Chromatogr. 1978, 167, 381. Fujii, T.; Arimoto, H. Anal. Chem. 1985, 5 7 , 490.
RECEIVEDfor review April 22,1985. Accepted June 26,1985.
