RESULTS AND DISCUSSION
1
0 TO 6.3V FILAMENT TRANS.
-l CHOPPER 8 PIN SOCKET CHOPPER PLUG
1
I
Figure 4.
3
Our interests have been directed primarily toward the use of polarographic techniques in the study of pesticides and their residues. Considerable data have been accumulated on the use of this technique in the pesticide field (I, 4-6). I n our laboratory, great value and versatility was found in the interchangeable use of the d.c. and a.c. methods, an outstanding instance being the development of an analytical procedure involving the use of a polarograph for the determination of total bromide on crop materials (2).
Recorder 60-cycle chopper modification
Figure 4 shows the changes that are made at the S p i n socket of the recorder 60-cycle chopper to enable it to accept either ax. or d.c. signal input. The chopper itself is a plug-in component of the circuit and requires no modification. The circuit is connected to the instrument transfer switch of Figure 1 by means of a k o n t a c t plug as shown. The d.c. calibration and operation of the modiied Model X V polsrograph is identical to that given in the instrument manual only when the control panel transfer switch is in the d.c.
LITERATURE CITED
position. With the switch in t,he ax. position the instrument voltage and current calibrating control and the displacement control are both inoperable. Therefore, the d.c. calibration is made only in the d.c. mode. The function of the d.c. displacement control is replaced with the previously described base line control on theax. power supply. The a.c. calibration is made by adjusting potentiometer R2 (Fi re 2) while a.c. power measuring the output of supply with a high impedance measuring device such as a precision oscilloscope.
tg
(1) Allen, P. T., Beckman, H., “Residue Reviews,” Vol. V, p. 91, SpringerVerlag, Berlin, 1964. (2) Beckman, H., Crosby, D. G., Allen,
P. T., Mourer, C., unpublished data, University of California, Davis, Calif.,
1966. (3) Caudill, P. R., University of Ken-
tucky, Lexington, Ky., pnvate communication, 1963. (4) Gajan, R. J., “Residue Reviews,” Vol. V, p. 7, Springer-Verlag, Berlin, 1om
-I-_.
( 5 ) Martens, P. H., Nangniot, P., Ibid., Vol. 11, p. 26, 1963. (6) Miller, D. M., Can. J . Chem. 34, 942 (1956). (7) ZM., 35, 1589 (1957).
Mass-Spectromelric Determination of Traces of Carbon Dioxide in the Presence of Oxygen P. J. Ross, Division of Soils, C.S.I.R.O.,
Cunningham Laboratory, St. Lucia, Brisbane, Australia
conditions, most Ucommercial mass spectrometers can NDER FAVORABLE
just detect 10-50 v.pm. (p.p.m. by volume) of a component in a 1-ml. sample of a gas mixture. Oxygen in mass spectrometers may produce a beam of C02+ ions (1,3, 4) which is probably formed by reaction of the oxygen with carbide in the filament (4). Brain and Evans ( I ) , using a Metropolitan-Vickers MS2 mass spectrometer, reported that this beam increased in size over a period of 50 minutes. Under such unfavorable conditions the 10-50 v.p.m. limit of detection for traces of COS in a sample cannot be reached by normal methods of analysis. One solution is to concentrate the CO, by freezing with liquid nitrogen; this also allows the detection of as little as 0.5 v.p.m. if sufticient sample is available (2). Another solution is to remove the oxygen chemically. However, both these methods are inconvenient and unnecessary for mess\Ilp ments on samples containing more than 100 v.p.m. COS,as will be shown. The method to be described allows the detection of 10-20 v.p.m. CO, in a 1-ml. 1436
ANALYTICAL C H E M I ~ Y
sample containing oxygen with a single analysis. Although a fast pumpout double-inlet mass spectrometer was used in these investigations, the principle of the method should be applicable to other instruinents. EXPERIMENTAL
Apparatus. The instrument used in the analyses was an MS3 doubleinlet maw spectrometer (Associated Electrical Industries Ltd.) modified in accordance with the manufacturer’s instructions to include a Mullard ME1403 electrometer pentode in the detection system. The inlet system of this instrument allowed the delivery of reproducible doses of approximately 1 ml. of sample to two expansion vessels, whence they were admitted to the ion source. The instrument is supplied with a tungsten filament; this was operated at a current of 4.5 amperes and an electron emission of 100 pa. The filament was not carburized and had been used only for the analysis of the permanent g-Procedure. The principle of the method was to estimate the effect of oxygen in the sample by admitting
pure oxygen at the same pressure as the sample oxygen. The mass spectrometer was conditioned for 15 minutes with a sample of air on days when analyses were to be carried out. This was found to be desirable to stabilize the peak at mass 44 induced by the presence of oxygen. This peak was larger than without conditioning, but its increase with time was smaller. To analyze a sample, the expansion vessels were first evacuated thoroughly (i a sample containing 1% C02 or more had just been analyzed, the expansion vessel concerned was first flushed with air). A dose of sample was expanded into one of the vessels and the displayed on the peak at mass 32 (02+) electrometer output meter. Sufficient pure oxygen to give the same peak height (to within 0.2y0) ww then admitted to the other expansion vessel. The instrument was focused on mass 44 and the amplifier output displayed on a chart recorder. The oxygen was admitted to the analyzer tube for 30 seconds; it was then shut off, and the sample admitted for 30 seconds. Oxygen was then admitted for 10 seconds, sample for 10 seconds, oxygen for 10 seconds, and so on to give seven peak heights (three for the sample, four for oxygen). Analysis for other components
Table 1. Accuracy and Precision of Determination of COSin Mixtures Containing 2% OS
COI
Actual
COI
2
found (means of five
content, analyses), v.p.m. v.p.m. 0 -4.7 87.0 89.2 392.2 390.7 719.7 717.3
Std. dev., v.p.m. 9.8 8.9 4.7 13.8
Mean error, v.p.m. -4.7 2.2 -1.5 -2.4
being equal to that of the oxygen in the sample, was less than the total pressure of the sample and caused a smaller rise in the background peak at m w 44. Experiments with pure argon indicated that the increase in the background peak at msss 44 was proportional to tube pressure and did not change significantly from day to day. The correction was estimated from the relation between background at msss 44 and tube pressure so obtained. For a sample containing 20% oxygen, the correction was equivalent to 14 v,p.m. COS,which was near the limit of detection. The corrected peak was used to calculate the proportion of COz in the sample, assuming there was no contribution to mass 44 from other components. Known volumea of GOn at atmospheric pressure were expanded into calibrated vessels, which were then 6lled with air which had been passed through a long tube of Ascarih (asbestos impregnated with NaOH) to remove GO,. Ascarite-treated air alone was included as a 0 v.p.m. CO, mixture. These mixtures were d y d in random order on five diflerent days. The particular expansion vessel used was, for each sample, randomly selected from the pair. A mixture containiig 1% GOz was included for the calculation of gas analysis constants.
v)
w
I-
3
$ 1
Sample
WSULTS AND DISCUSSION
I
1
C
IO”*A ION
Au:
CURRENT
Figure 1. lon current at m/e 44 during analysis of sample containing 392.2 v.p.m. COC 20% Or
of the mixture was then carried out if required. Ten seconds was a conveniently short time interval for admissions, yet was long enou the sample GO, to be removel? from for the all analyzer tube (the 44 ak from a sample containing 99.8 N,, 0.1% GOz fell to the background level in 5 seconds on closing the inlet valve). The two 3O-second peaks were not measured, their purpose being to sta-
b h the 44 peak induced by oxygen. The heights of the seven r e m a i n i i peaks relative to the recorder zero were measured at a fixed time interval after admission and the mean difference between the sample and oxygen peaks was calculated. When the sample contained gases other than 0%and Cot, it was necess to apply a small correction to this?fierence; for the preasure of the pure oxygen sample in the tube,
A typical record from one analysis is shown in F’igure 1. The results of the analyses are given in Table I. It is clear from the mean errors that the results are unbiased. It is also clear from the standard deviations that the limit of detection for a single analysis is 10-20 v.p.m. COz in a 1-ml. sample. LmRATURE CITED
(1) Brain, D. H., E v e , P. B. F., U.
IC.
At. Energy Authonty, Ind. Group 8095 (1958) (Declassified reprint); C.A. 53,20942f (1959). (2) Parkinson, R. T., Toft, L., A n d @ 90,220 (1965). (3) Roberteon, A. J. B., ‘‘M- Spectmmetry,” p. 93, Methuen and Co., London, 1954. (4) Young, J. R., J . A w l . Phys. 30, 1671 (1959).
VOL 38, NO. 10, SEPTEMBER 1966
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