11). This may imply that the inclusion of a carbonyl in the C ring interferes n i t h the planarity of the A ring, which frequently introduces a hypsochromic shift (2). It is difficult to see how the 11-carbonyl electronic configuration of itself could directly affect the electronic nature of the A ring chromophore. A s in the above cited case of the 4-ene-3,20dione, the introduction of an 11-keto group into a 1,4-diene-3,20-dione iq hypsochromic (Figure 1, 4 and 5 , and item 5, Table 11). The wavelength of maximum absorbance is shifted toward the violet Ah = 3 mp. The effect of 11-keto on the A ring chromophore appears of the same nature for the 1,4-diene-3-one as for the 4-ene-3-one. Introduction of both a 1-ene and an 11-keto into 4-ene-3,ZO-dione shows both of the effects described above for their introduction separately. First. a hypochromic effect occurs at the wavelength of maximum absorbance : secondly, a bathochromic shift occurs which is due to the introduction of croqq conjugation, although it is tempered b y the 11-keto which tends to increase the absorbance toward the violet (hypsochromic) (item 7 , Table 11). The introduction of a n 11-hydroxyl group into a steroid structure con-
taining the 1,4-diene-3-one chromophore of the A ring affects the spectral absorbance. The effect varies with the Q or p configuration of the 11-hydroxy. Here an unusual relation exists. The exact amount of spectral absorption enhancement given the unsubstituted chromophore by the lla-hydroxy group is diminished by the 116-hydroxy group. This is readily seen by the log e1/e2 plot of Figure 1, 8 and 9, where the one is the mirror image of the other about log E J E ? = 0 (items 9 and 10, Table 11). The 116-hydroxy substituent is hypsochromic and hypochromic with reqpect to the lla-hydroxy substituent in pregna-1,4-diene-3,2O-dione (item 11, Table 11,and Figure 1, 10). Introduction of a Go-methyl substituent also does not affect the chromophore. This lack of effect of a G-methyl on the spectral absorbance is consistent with that shown in the simple 4-ene-3-one series (8) (item 8, Table 11). The analog computer permits such comparisons as discussed above t o be effected efficiently and easily. It offers a valid method of observing spectral shifts in the entire absorption band due to the effects of substituents and their spatial orientations that modify chromophores. It may serve
as a method to observe spectrophotometrically absorbing minor impurities which may not contribute t o the absorbance at, or to the position of the wavelength of maximum absorbance by comparing a spectrum of a possibly impure compound with a reference standard. LITERATURE CITED
(1) Applied Physics Corp., Monrovia, Calif., Bull. 114 (1961). (2) Braude, E . A,, Nachod, F. C., “De-
termination of Organic Structures by Physical Methods,” Chap. 4, pp. 147, 169, Academic Pre:s, New York, 1955. (3) Broie, W. R., Chemical Spectroscopy, 2nd ed., p. 191, Wiley, New York. 1946. (4) Fieser, L. F., Fieser, M., “Steroids,” Reinhold, Yew York, 1959. ( 5 ) Gillam, A. E., Stern, E. S., “Electronic Absorption Spectroscopy,” 2nd ed., Edward Arnold, London, 1957. 16) Mellon, 11. G., “Analytical Absorption Spectroscopy,” TTiley, New York, 19.50 _._.
(7) Miller, F. A., Gilman, H , “Organic Chemistry,” Vol. 111, Chap. 2, Wiley, New York, 1953. (8) Ringold, H. J., Bowers, A., Ezperientia 17. 65 119611. (9) dasseur, E., Acta Chem. Scand. 2, 693 (1948). RECEIVEDfor review May 10, 1962. Accepted July 5 , 1962.
Negative Ion Gas Analysis Technique V. N. SMITH and E. Shell Developmenf
J.
MERRITT
Co., Emeryville,
Calif.
b A gas ionization detector has been constructed to measure the concentration of molecules which will attach free electrons to form negative ions. Negative ions and electrons can be easily separated by utilizing the large difference between their mobilities in an electric field. The separation is achieved in a parallel plate ionization chamber with a central grid. An a.c. field is applied to the outer plates to separate electrons from negative ions, and a d.c. potential may b e applied to the grid to modify the cell characteristics or to cancel background current. Auxiliary equipment includes an electrometer, transformer, d.c. bias battery, and Sr90 source to provide free electrons by ionization. Performance data for oxygen and water in nitrogen, butane, and butadiene are given. A sensitivity of a few parts per million of oxygen has been achieved. Problems in variable gas mixtures are discussed. 1476
0
ANALYTICAL CHEMISTRY
T
use of ionization chambers to detect trace amounts of gas molecules with a n electron affinity is well known. The application of this technique for sensitive GLC detectors has been described by Lovelock ( g ) . The operation of these detectors depends upon several factors. I n the first place, the mobility of an electron in an electric field is about 100 times greater than the mobility of a negative ion. An electron which is captured by a molecule to form a negative ion takes 100 times longer to be swept to the positive electrode of the ionization chamber than i t would have taken as a free electron. Thus, the formation of negative ions enhances the density of the negative charge carriers, thereby increasing the recombination rate of negative and positive charge carriers. The final result is a decrease in observed ionization current. The operation of the detector described in this report depends primarily HE
upon the difference in mobility between electrons and negative ions but only to a minor extent upon recombination. I t s operation is similar to that of the Loeb electron filter ( I ) , in which a combination of alternating and direct voltages separates electron and negative ion currents. GENERAL DESCRIPTION
The experimental negative ion gas analyzer is shown schematically in Figure 1. The detector chamber consists of 8 glass envelope containing two parallel plate electrodes with a grid mounted between them. The two outer plates are connected to a source of alternating potential with the output balanced t o ground-e.g., by center-tapped secondary transformer. An electrometer amplifier is connected between the grid electrode and the center tap to measure direct ion current flowing to the grid. ii variable direct current potential
source is also connected in the grid electrode circuit to permit bucking out of background currents or study of effects of d.c. potential on cell characteristics. The gas in the cell is more or less uniformly ionized by beta particles from a strontium-90 source mounted outside the chamber. The gas filling may be changed either by flow or by evacuation and refill using a gashandling system. When the cell is filled with a gas mixture free of components t h a t form negative ions, electrons will be collected by the outer plates, if the ax. frequency is low enough to permit most electrons to drift the entire distance t o the positive electrode during each half cycle of the ax. voltage. This condition is easily met, because the electron mobility is high (see Calculation of Frequency and A.C. Voltage Levels). While the electrons are being collected alternately on one plate and then the other, the positive ions move a short distance first in one direction and then in the other. If the grid potential is set slightly positive, the positive ions will drift away from the grid toward one of the plates, where they neutralize the electronic charge. Under these conditions, the electrometer current is zero. S o w if oxygen (or other electronegative gas) is introduced as a contaminant in the gas, some of the eiectrons will attach to the O2 to form negative ions. Like the positive ions, the negative ions jitter because of the ax. field. I n addition to their jittering motion, they drift slowly to the positive grid. I n this case, a net current will flow through the electrometer. The actual operation of the cell is af-rected by several other factors: recombination, space charge, and a valve-type action by the grid. These factors are elaborated in the discussion of cell operation.
Integration yields (4)
To collect all ions of a particular kind in the entire volume, we must have X 2 d . This condition gives us a maximum value for the ratio of frequency to applied voltage expressed by the inequality: (5)
To collect all of the electrons in the cell, we must have
i A 4C Poafr
SLlpply
Figure 1.
Negative ion gas analyzer
where velocity, cm. per second mobility, sq. cm./volts see. = electric field, volts per cm.
u
=
k E
=
f = - ki _ Vo
The mobility depends upon the size and mass of the particle and on gas pressure. It will be assumed that the drift velocity is reached in a time very short compared with the period of the ax. field, so that in a sinusoidal electric field, we have : v =
.
kiV, - sin ut d
(2)
where
V o = voltage amplitude of applied DETAILS
OF
CELL CONSTRUCTION
sine wave separation between electrodes, em. w = angular frequency = 2 ~ f f = frequency, cycles per second k , = ionmobility d
-1photograph of the cell is shown in The circular plates are Figure 2. 11/2 inches in diameter. Their separation is 3/8 inch. The grid is constructed of 0.008-inch Nichrome wires l/8 inch apart and spot-welded together by a condenser discharge. Figure 2 shows a n open port. '17Then the cell is being used, this port is sealed by a thin-walled metallic sheet, held in place by a vacuum-type wax. The beta-rays from the source pass through this window. The Sr9o source has a strength of about 25 millicuries. The saturation ionization current was of the order of 10-9 ampere for pure butane in the cell.
=
During one-half cycle of the sinewave, the ion will travel a distance
kE
(1)
To meet both conditions 7 and 8, we must have :
k , 5 ck,
(8)
The approximate values of the ion and electron mobilities in nitrogen at atmospheric pressure are k, E 2 and k, E 1.75 X lo4. Hence c 2 acd Ii-e c3n choose a frequency for which e
