The Role of Argon Metastable Atoms in the Ionization of Organic Molecules M. M. SHAHIN and S. R. LIPSKY Department of Internal Medicine, Yale Universify School of Medicine, New Haven 7 I , Conn.
b A microionization chamber of coaxial design, previously described as a detection system for gas chromatography, has been used to investigate rhe mechanisms b y which argon carrier gas provides high sensitivities to organic molecules with ionization potential below 1 1.6 e.v. It i s shown that for vapor concentrations (ethylene) up to 3 to 4 pap. 104 in the carrier gas the Penning Effect i s the sole reaction mechanism for the response of the detector. Beyond this concentration, the collisions o f the secondary electrons with sample molecules become important and the response i s either increased or decreased depending on the ionization potential of the test molecule and the field strength which determines the electron energy. The decrease i s affected b y quenching of the electron energy, while the increase i s brought about b y ion multiplication of the test molecule. Molecules of I.P. >1 1.6 e.v. have been used to further demonstrate these latter processes. The detailed analysis of the mechanisms has also revealed conditions under which such an ionization system could operate. Vapor concentrations as high as 5 to 1 O%of the carriergasmayexistwithout any overloading phenomena, thus extending the range of this detector b y two orders of magnitude beyond that of conventional argon detectors.
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O F THE outstanding developments in the field of gas chromatography in recent years has been the introduction of sensitive ionization de-
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Table 1.
vices which have radically simplified the detection of minute quantities of organic compounds. Among the systems widely in use today is the argon detector which was initially introduced by Lovelock, James, and Piper (8, 9) and later developed (7‘) as a sensitive detector for almost all organic molecules. Until recently this device had certain relative limitations which involved lack of response to inorganic vapors and fixed gases and t o those few organic molecules whose ionization potential ( 1 2 . ) are greater than 11.6 e.v., and overloading phenomenon which is usually observed when the sample concentration exceeds 1 pap.lo3of the carrier gas. The first of these deficiencies has now been removed (4, 12) by the design of a new ionization system which provides the highest known response of any detector for fixed gases. In a later study (6),it was found that for the analysis of permanent gases the mechanism responsible for the operation of this particular detector system is different from that which operates for organic compounds. For the latter, detection with this device appears to be dependent upon a mode of operation similar to that noted for the conventional argon detectors in that both argon carrier gas and high field strengths are prerequisite to their operation. From the original work on argon detectors (8) it has been maintained that because only organic molecules of I.P. below 11.6 e.v. could be detected with such high sensitivity, then the dominant reaction responsible for the operation of these devices must be due
to the “Penning Effect.” In this reaction the ionization of the sample molecules is brought about not by direct electron impact but through collisions with energetic metastable argon atoms. Recently, however, the predominance of this mechanism has been questioned ( I S ) . It has been further suggested (IS) that in addition to the Penning Effect, direct ionization of the sample molecules by energetic electrons may contribute to the high sensitivity of these detectors. It is the purpose of this paper to report on an investigation which was subsequently carried out to study the details of the mechanisms of ionization, particularly as applicable to the new and more versatile detector, and also to present data on the conditions which delay overloading phenomena when large samples are eluted from the column. The availability of Kr and Xe as carrier gases also made it possible to observe distinct changes in response patterns of certain compounds. Since the metastable energies of Kr and Xe (8.3 e.v. and 9.9 e.v., respectively) are considerably lower in energy than those for argon (11.6 e.v.), it was hoped that the estent of the contribution of the various mechanisms to the response of the organic compounds could be clarified by the comparison of their respective performances. EXPERIMENTAL
The ionization detector used in this work has been previously described (4, 1%’). This system which is basically an ionization chamber of coaxial design with 1-mm. electrode spacing, uses a
Applied Voltage Required for On-set of High Sensitivity Region of Detector for Various Carrier Gases and Test Samples” Sample gasesb
ButaEthyl- AcetHydroEnergy of Xylene Toluene diene Benzene Butene-1 ene ylene Ethane Xenon Krypton Methane gen Carrier metastable Z.P. Z.P. Z.P. Z.P. Z.P. Z.P. Z.P. Z.P. I.P. Z.P. Z.P. I.P gas states in e.v. S.9 e.v. 9.2 e.v. 9.2 e.v. 9.4 e.v. 9.7 e.v. 10.6e.v. 11.4e.v. 11.6e.v. 12.1e.v. 13.1e.v. 13.1e.v. 15.4e.v. 35 35 35 35 45 so 130 220 90 Argon 11.5, 11.7 volts volts volts volts volts volts volts volts volts 35 35 35 90 230 >400 Krypton 9.9, 10.5 volts volts volts volts volts volts Xenon 8.3, 9 . 4 110 160 >>300 volts volts volts Concn. of H2 and. CH, maintained at 1 p.p. lo8. All others wcre approxiniatcly lxtwecn 1 anti 3 11.p. lo4. Ionization potentials were taken from reference (If ).
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ANALYTICAL CHEMISTRY
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150 APPLIED
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20 VOLTAGE
Figure 1. Comparative response gases at different voltciges
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300
VOLTAGE
Figure 2. Response of detector to ethylene at 1 pap. l o 3 concn. at various voltages, using pure argon(III), argon 0.27% hydrogen(ll), and argon 2.85% methane(1) as carrier gases (25 ml./min.)
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of detector to various
Concns. of CzH4, Hz, CH4, arid He are accurately maintained a t 1 p.p. 1 03; that of Xe is approximately 1 p.p. 1 O S Argon carrier gas (25 ml./min.). Temp. 165' C.
200-me. tritiated titanium foil as the source of radiation. The total volume of the detector is 280 pl. The ancillary apparittus used for this work was a conventiorial gas chromatographic unit (Model 10, Barber-Colman Co., Rockford, 111.) equipped with a high sensitivity elechometer (Model E-302, Gyra Co., Chicago, Ill.) which was operated a t ranges of 3 X 10-l0 to 1 X lo-" amp. A Keithley Model 240 variable high vo.'tage supply was used as the precise source of voltage. Three different chromatographic columns were used in the course of this study. They were: (1) a n 8-ft. X l/rinch i.d. column packed with Linde Molecular Sieve 5A, 613 to 80 mesh, and maintained a t 100" C.; ( 2 ) a 4-ft. X '//*-inch i.d. column packed with silica gel, 40 to 60 mesh, and maintained a t 100' C.; and (3) .a 4-ft. x l/s-inch i.d. column packed with 15% ethylene glycol succinate on 80 t o 90 mesh Anakrom ABS (Arialabs, Hamden, Conn.), and maintained a t 165' C. Reproducible microliter quantities of sample gases were injected into the column through a silicone rubber stopper by means of a gastight, precision calibrated microsyringe (Yo. 263 M.L.S. Starrett Co., Athol, Mass.). Liquids were introduced by means of a IO-pl. Hamilton syringe. High purity krypton and xenon used as carrier gases were supplied by the Linde Co. (The Rare Gas Division, New York, N. Y,), Mixtures of argon with known quantities of impurity used. in this work were made up by Matlieson Co. (East Itutherford, IC. J.) and subsequently analyzed by mass spectrometry. In
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experiments where a precise level of contamination of the carrier gas by the test molecule was required, a secondary flow containing the contaminant was allowed to mix with the effluent stream emerging from the column. The combined flow was then allowed to enter the detector. RESULTS AND DISCUSSION
Using argon as a carrier gas a number of reprebentative test gases were analyzed a t a constant concentration in an effort to compare their relative responses in the detector a t different field strengths. Figure 1depicts the response patterns of concentration of 1 p.p. IO3 of ethylene, methane, hydrogen, helium, and xenon a t various voltages. The application of the low-voltage (0 to 2.5 volts) region of this ionization system for the analysis of diatomic and certain polyatomic gases has been already described (4, 12), and its mechanism of operation has been attributed to the changes in the drift velocity of the electrons in the bystem (6). After the maximum, where this effect has diminished and saturation current is reached (10 volts), the effective response of the system to ethylene still appears to be significantly different from that of hydrogen and methane. Thus, a t approximately 30 volts (field strength 330 volts per cm.) the sensitivity of the detector to ethylene is about 8 times greater than that of hydrogen and methane for the same level of concentration. This phenomenon which is char-
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acteristic of all molecules with I.P.
