I n the case of detection of trace amounts of \vater, thr coated quartz crystal is now a commercial reality. Probably many other types of detectors will be developed in the future. LITERATURE CITED
D. E., “Improved Cryogenic Thermometer,” paper F-2, 1962 Cryogenic Engineering Conference, Los Angeles, Calif 1962 .. ~~. ., ~ - Inem lYJl.
( 5 ) King, W. H., Jr., paper C 19.11, International Symposium on Humidity and JIoisture. JIav“~ 20-23. 1963. ~
(1) Bevan, 8. C., Thorburn, Samuel, J . Chronlatog. 11, 301-6 [ 1963).
s., U. s. Patent 257,171 (July 4, 1945). ( 3 ) Flynn, T. >I., Hinriah, H., Newell, ( 2 ) Dyke, K.
-
(4) Keulemans, A. I. AI., “Gas Chromatography,” Reinhold, Xew York,
~~
~
~
and Their Application to Cltrasonics,” Tan Nostrand, Princeton, X. J., 1950. (8) Oberg, P., Longensjo, J., Rev. Sci. Instr. 30, N o . 11, 10.53 (1959). ( 9 ) Sauerbrey, G., 2. Physik. 155, 206 (1959). (IO) Slutsky, L. J., Wade, W.H., J . Chem. Phys. 36, N o . 10, 2688-92 (1962).
~~~~
Washington, D: C. (6) Landee, 12. W., Davis, D. C., Albrecht, A . P., “Electronic Designer‘s Handbook,” JIcGraw-Hill, Kew York, 1951. ( 7 ) JIason, W. P., “Piezoelectric Crystals
RECEIVEDfor review April 3, 1964. Accepted JIay 18, 1964. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1963.
Electron Drift-Velocity Detector for Gas Chromatography V. N. SMITH and J. F. FlDlAM Shell Development Co., Emeryville, Calif
b The effect of trace impurities on the drift-velocity of electrons in pure argon has been exploited for the detection of trace concentrations of permanent gases in gas chromatography. The detector is a smallvolume, parallel-plate ionization chamber in which a tritium source is used to ionize the argon in a region near one electrode. Negative voltage pulses of short duration are applied to this electrode to drive electrons from the ionized region toward the opposite electrode. The pulse duration is selected so that not all electrons are collected in pure argon carrier gas. Trace concentrations of components eluted from a chromatographic column and passing through the detector increase the electron drift-velocity and thus increase the electron current to the collector electrode. Sensitivity has been measured for a iiumber of permanent gases and a few light hydrocarbons. The lower detectable limit for nitrogen is about 10-10 gram/second.
I
X ~ N Y applications of gas chromatography a detector is required for trace concentrations cd the so-called permanent gases-e.g., He, Hz, N,, 02, and CO. Most of the available detectors, including the flame and argon ionization detectors, as well as ionization cross-section, thermal conductivity, or gas density balance detectors, are inadequate in this respect. Lovelock ( 2 >3 ) has described the application of both direct and indirect’ electron mobility t,echniques to the problem of detecting traces of the permanent gases, and, more recently, Shahin and Lipsky ( 4 ) have described a mode of operat,ion of a n ionization detector, using a low S I
d.c. potential and high operating temperature (150” to 200” C , ) j which exhibits high sensitivity to the permanent gases as a result of changes in electron drift-velocity . The authors have investigated the direct electron mobility or drift-velocity technique in some det’ail and have developed a detector system (cell, pulse generator, and commercial electrometer) which provides high sensitivity to the permanent gases and excellent base line stability. The ultimate sensitivity of t’he detector system has been determined for several permanent gases and light hydrocarbons. I n t’he course of this development the drift-velocity of electrons in pure argon was measured as a function of electric field strength. These measurements are discussed and the data are presented here to illustrate the characteristics of the cell and the effects of changing operat,ing parameters. THEORETICAL BASIS
When a uniform electric field is applied to a gas containing free electrons, the electrons are rapidly accelerated to a terminal drift-velocity toward the positive electrode. In general, the electric field increases the thermal agitation energy or temperature of the electrons to a value above that of the gas, and the directed drift toward the positive electrode is a diffusion phenomenon superimposed upon the thermal motions of the electrons. In a pure monatomic gas such as argon the drift-velocity is relatively low and is increased by the presence of traces of more complex gases .such as nitrogen, carbon monoxide, or hydrocarbons. This effect can be understood by considering the diffusion equations for electrons in a gas.
Healey and Reed ( I ) have used Maxwell’s diffusion equations to derive the following expression for the driftvelocity of electrons in a gas under the influence of an electric field.
w = -K S e E T*P where
K
= =
.I’
=
ZLI
e
=
E T*
= =
P
=
drift-velocity of electrons coefficient of diffusion for electrons in the gas number of molecules of gas in 1 cc. a t 760 mm. of Hg pressure and 15” C. electronic charge electric field strength ratio of mean agitation energy of.electrons to that of the gas a t 15” C. pressure of 760 mm. of H g expressed in dynes per sq. em.
The factor T* in Equation 1 is a measure of the electron temperature, which can be quite high in a monatomic gas because the electrons do not lose appreciable energy in collisions with these atoms. However, in gases having more complex molecules the electrons can lose energy in collisions with the molecules by exciting vibrational or rotational states. These inelastic collisions reduce the electron temperature, and since T* appears in the denominator of Equation 1, the drift-velocity is increased. Intuit’ively,this result may seem paradoxical, but the physical picture can be clarified by considering two extreme casesperfectly elastic collisions and completely inelastic collisions. I n the case of perfectly elastic collisions, the electrons bounce off the gas molecules lvith negligible energy loss because of the large ratio of molecular mass to elect,ronic mass. Thus, the direction of the electron’s travel is changed a t VOL. 36, NO. 9, A U G U S T 1964
e
1739
b
-a
= W T ~
(3)
Now if the pulse width is increased further, more electrons will be collected until a second critical value T~ is reached such that d = b and all electrons are collected. Then by substitution in Equation 2, we get for the drift-velocity
1
1
Figure 1 .
Schematic diagram of two-electrode cell
every collision and the energy imparted by the electric field is almost completely randomized by the large number of collisions. On the other hand, if the collisions are completely inelastic, the electrons lose all their energy at every collision and are accelerated between collisions only in the direction of the electric field. Thus, the energy imparted by the electric field results in a completely directed motion of the electrons. Techniques. Two techniques of measurement of electron drift-velocity have been investigated by the authors. I n the first technique the threeelectrode cell described by Smith and Merritt (6) for the detection of electronegative gases was employed. I n the second technique a two-electrode cell and pulsed voltage, similar to the system described by Lovelock (2, S), was employed. The three-electrode cell performs well, a t a frequency of 150 kc., as a detector for traces of permanent gases in pure argon carrier; but this cell, in its original form, requires a strontium-90 or other high-energy beta source to ionize the gas uniformly throughout the cell. The two-electrode cell, on the other hand, uses a tritium source, which may be more intense and yet less hazardous than a strontium-90 source. For this reason and because its construction is simpler the two-electrode cell was used for the present work. I n applications where the cell must operate a t a temperature above the limit of tritiated foil (>200° C.) or where chemical reactions may release the tritium, the three-electrode cell with a strontium-90 source would be used. Theory of Two-Electrode Cell. A schematic diagram of the two-electrode cell and associated circuitry is shown in Figure 1. The gas adjacent to the radioactive source is ionized 1740
ANALYTICAL CHEMISTRY
out to a distance a b y the beta particles emitted by the tritium. Ionization is heaviest close t o the source but extends to a distance of about 5 mm. for t h e 18-KEV tritium beta particles. When a negative voltage pulse is applied to the source electrode, the electrons in this slab of ionized gas move toward the opposite electrode with a velocity given by Equation 1. If the pulse duration is 7, the electrons will move a distance
d
= WT
(2)
If the pulse duration is very short, so that d < b - a ( b = electrode spacing), no electrons will reach the collector. If the pulse width is increased, a critical value 71 will be found such that d = b a, and some electrons on the outer edge of the ionized region will just be collected. Then from Equation 2 we have
The pulse generator must be coupled to the cell through a capacitor in order to make the average value of the applied voltage exactly equal to zero. Cnder these conditions any electrons that do not reach the collector during a given pulse move in the opposite direction during the positive portion of the cycle and eventually recombine with positive ions and do not contribute to the measured current. If direct coupling is used, the electrons not collected during one pulse remain stationary between pulses and continue to move toward the collector on successive pulses until they are finally collected and contribute to the measured current. The measured current is the average value over many repetitive pulses, and the electron current must be balanced by an equivalent positive ion flow to the source electrode. Because of the fact that no direct voltage is applied to the cell it is necessary to have a t least a small gas flow toward the source electrode in order to collect the positive ions, I n the absence of gas flow the positive ion density builds up until space-charge and ion-electron recombination cause the current to drop essentially to zero. Another consequence of the capacitive coupling is a variation in effective amplitude of the pulse with changing duty-cycle (ratio of pulse duration to
Tritium Disc
Connection for
Anode
1
Connection
Electronics. A versatile commercial pulse generator assembly (Tektronix 160 Series) was used for the measurement of cell characteristics. This 7+300v pulse generator was entirely satisfactory for measurements where the I I II pulse duration and amplitude were varied. However, for low noise and Cell good base-line stability in the gas Cry sta I MultiAmplifier OSC. Amplifier Clipper v i brator Clipper chromatographic applicat'ion, the pulse 85 k c . width and amplitude must be constant to + O . l ~ o or better. The commercial assembly does not meet this requirement because of its wide-range adjustability and the resulting coarseness of the controls. A special pulse generator was designed and built which provides the Bias Supply stability demanded by the electron drift-velocity detect'or. A block diaFigure 3. Block diagram of pulse generator gram of the unit is shown in Figure 3 . The signal is derived from an 85-kc. total activity of t h e source is a p crystal oscillator for good frequency interval between initiation of pulses). proximately 250 mc. T h e disk is stability. The out,put signal is amplified This effect is readily compensated for, and clipped and is then used to trigger clamped to a threaded brass holder, and with a stable oscillator and pulse which permits easy removal of the a mono-stable multivibrator. The posiforming circuit, no problem arises. tive output pulse of the multivibrator A 3/3?source when cleaning the cell. When the drift-velocity cell is used as drives the final amplifier-clipper stage inch diameter hole through the foil a. detector for gas chromatography, the and source holder serves as the gas to provide a negative pulse of about 190 pulse duration and amplitude are vent. The rim of the source holder is volts, peak. The pulse width is adjustchosen so that only some fraction sealed with an O-ring to confine the able from about 1 to 7 pseconds by (usually less than one half) of the means of a 10-turn potentiometer, gas flow to the central vent. available electrons are collected when pure argon carrier is floning through the cell. This background current is canceled by a bucking-current supply. Table I. Analysis of Argon Carrier Gas When gas components eluted from the Impurities in P.P.M. chromatographic column appear in the Cylinder No. 9225 6885 8810 3617 8103 cell, the cell current increases and the reCompound sulting difference-current is amplified Hz ...
