Behavior of Demeton in Electron Affinity Detectors SIR: Electron affinity detectors are being more widely used in gas chromatography for determining pesticides at the submicrogram level. Halogeri-containing pesticides have been most commonly determined (3, b, 16) because of the high capture cross section of these compounds for electrons; applications of these detectors to some phosphate pesticides have been reported ( 1 , 13, 15). The behavior of some organic compounds in electron affinity detectors have been studied and the results have been interpreted with regard to relative electron affinities of the compounds (2, 9-12). This communication describes the behavior of one phosphate pesticide, demeton (trade name Systox, Chemagro Corp., Kansas City, Mo.), in two electron affinity detectors: the behavior of other phosphate pesticides in several electron affinity detectors will be described in a subsequent paper. Demeton shows an anomalous response in one of the detectors compared to other halogenated and phosphate pesticides that we have studied. although similar behavior has previously been reported for one or two compounds (IO), its significance on the design of electron affinity detectors and the interpretation of chromatograms has been neglected. EXPERIMENTAL
Apparatus. The gas chromatograph was a Micro-Tek Model GC-2500 R, equipped with a Micro-Tek GC 2500074 electron affinity detector and d.c./ pulse variable voltage power supply. The pulse voltage was nominally one microsecond on and 50 microseconds off. The column was 0.6 meter of 6mm. 0.d. borosilicate glass tubing packed with 2y0 SE-30 and 0.2770 Versamid 900 on 100- t o 110-mesh -4nakrom -4BS. Conimcrcial grade nitro-
COLUMN EFFLUENT
ELECTROMETER 100 MESH STAINLESS STEEL SCREEN TRITIUM, STAINLESS STEEL BACKED NEGATIVE POLARITY VOLTAGE
EXIT Figure 1.
2206
e
Electron affinity detector ANALYTICAL CHEMISTRY
T -m w
W
CT
W 0
n tW z
CT 0 3 1
W
m z
2
v7 W
grade by Chemagro Corp., Kansas City, N o . ; it was used without further purification. Systox is a mixture of two isomers, thiono-Systox [O,Odiethyl OS(ethy1thio)ethyl phosphorothioate] and thiolo-Systox [0,0-diethyl 8-2-(ethy1thio)ethyl phosphorothioate]; the ratio of the thiono isomer to the tliiolo isomer is usually 60 :40. When Systos \\as analyzed by gas chromatography nith electron affinity detection, two major peaks were obtained as expected. To identify the isomer producing each peak, some Systox was heated a t 125OC. for 2l/2 hours to convert the thiono isomer to the thiolo isomer (4, and another chromatogram waj obtained. Both chromatograms are shown in Figure 2. The peak with the shorter retention time was produced by thiono-Systos; the other peak, by thiolo-Systox.
E
RESULTS AND DISCUSSION
When the electron affinity detector was operated in the normal mode, Systox yielded a chromatogram with two lnrge peaks for both the d.c. and Figure 2. Chromatograms of heated and unheated demeton (Systox) Sample: 100 ng. in 1 ~ l . of benzene. C.; detector, Temgerature: column, 130' 180 C.; inlet, 190' C. Flow: column, 50 ml./min.; detector, 200 ml./min. Detector: electron affinity, normal pulsed mode; polarizing voltage, 10 volts 1. Thiano isomer 2. Thiolo isomer
gen, purified by passage through a molecular sieve trap, was used as a carrier gas for the d.c. measurements; 90% argon-lO% methane (The Matheson Co., Inc.) was used as carrier gas for the pulsed studies. Electron Affinity Detector. The detector was of the Lovelock design (parallel plate), as shown in Figure 1. I n the normal mode of operation, the column effluent entered the detector through the 100-mesh gauze and flowed past the stainless steel-barked tritium foil to the esit tube. In the reversed mode, the gauze was removed, and the carrier gas flowed in the opposite direction. Hence, the column effluent flowed around the foil into the open chamber and out the tube a t the opposite end. In the reversed mode, the detector operated in a similar manner to some commercially available electron affinity detectors; also, the flow is the same as in the Lovelock direct electron mobility detector ( 7 ) . Demeton. The demeton (Systox) was furnished as analytical stnnd:ird
2 2
IOnp X ATTN
ATTN
I
'
xs";r;l
I O 0 1;: ATTN x
Figure 3. Chromatograms of Systox obtained with detector in normal d.c. mode Temperature: see Figure 2. Flow: see Figure 2. Detector: electron affinity, normal d.c. mode; polarizing voltage, 20 volts 1. Thiono isomer 2. Thiolo isomer
pulsed modes of detection as shown in Figures 3 and 4. A similar detector has been reported not to respond to Systox (1). While the signal-to-noise ratio produced by Systox was not as large as for other phosphate pesticides studied, good sensitivity was obtained. Although the small peak following the thiono-Systox peak was not positively identified, it may be produced by the sulfoxide. In all cases the detector was operated at the polarizing voltage which produced the maximum sensitivity on the voltage-response curve. Peak areas for both isomers, as measured with a polar planimeter are given in Table I.
10 19 CTTN
25 n g CTTN
/I ATTN
Figure 4. Chromatograms F Systox obtained with detector in normal pulsed mode Temperature: see Figure 2. Flow: see Figure 2. Detector: electron affinity, normal pulsed mode; polarizing voltage, 10 volts 1. Thiono isomer 2. Thiolo isomer
X
A'h
2 5 ng ATTN X I
-T m W W 4
IT 0 w
n
Table 1. Response of Electron Atfinity Detector to Systox Isomers
Peak area (sq. e m . )
Amount, Pulsed D.c. ng. Thiono Thiolo Thiono Thiolo 10 25 100
5.4 17.6 50.4
10.0 23.4 49.2
2.7 5.0 11.0
2.2 3.0 9.2
tion occurs when the amount is large. This reversal of behavior is clearly shown in Figures 5 and 6. The variation with polarizing voltage in the shape of the peak produced by each isomer when the detector was operated in the reversed mode, is shown in Figure 7. At and above 5 volts thiono-Systox produced a double peak; below 15 volts, electron capture predominates; above 15 volts, ionization predominates. Thiolo-Systox shows capture below 10 volts and ionization above 10 volts. Normal chromatograms, showing only peaks resulting from electron capture, were obtained for the voltage range up to 60 volts with the reversed mode detector for halogenated compounds
I-
z
W
CT CT 3
2
0
IOOng
ATTN X 4
1
I
v, W
z
2v, W
LL
2 IC nq ATTN X I
30 VOLTS
2 5 nq
I
T I
10 W
a W
tc
E 0 c w z
n
Figure 6. Chromatograms of Systox obtained with detector in reversed pulsed mode
I5 VOLTS
Temperature: see Figure 2. Flowr see Figure 2. Defector: electron affinity, reversed pulsed mode; polarizing voltage, 30 volts 1. Thiono isomer 2. Thiolo isomer
cc
0 3 1
m w
z 0 a v,
tc W
Figure 5. Chromatograms of Systox obtained with detectw in reversed d.c.mode Temperature: see Figure 2. Flow: see Figure 2. Detector: electron affinity, reversed d.c. mode; polarizing voltage, 30 volts 1. Thiono turner 2. Thiolo isomer
Because of the effect of the carrier gas ( 2 ) , the detector was more sensitive to Systox in the pulsed mode than in the d.c. mode. When the electron affinity detector was operated in the reversed mode, the thiono and the thiolo isomers produced a signal for both the d.c. and pulsed modes of detection, as shown in Figures 5 and 6, but the thiolo isomer gave an increase in current a t all amounts of the isomer. This increase must be produced by ionization instead of electron capture. The response of the detector to the thiono isomer is dependent upon the amount of the isomer in the detector. Electron capture occurs when the amount of isomer is small, while ioniza-
5 VOLTS
2
3 VOLTS
.
4 -
6 t T l M E (khk
jure 7. Variation of peak shape with polarizing voltage Sample; 15 ng. of Systox in 1 MI. of benzene. Temperature: column, 1 3 5 ' C.; detector, 170' C.J inlet, 180' C. Flow: see Figure 2 Detector: electron affinity, reversed d.c. mode 1. Thiono isomer 2. Thiolo isomer
VOL. 35, NO. 13, DECEMBER 1 9 6 3
2207
such as carbon tetrachloride and for parathion and methyl parathion. In gaseous electronic detectors employing radioactive sources to provide a source of free electrons, ionization and electron capture must be competing processes (6). The kind of interaction that predominates will be determined by the types and concentration of the compounds present in the gas stream and by the geometry of the detector. Ionization as well as electron capture may be produced a t low applied voltages a t near-atmospheric pressure with electrons. Otvos and Stevenson (14) observed ion production in a number of gaseous hydrocarbons upon irradiation with electrons from carbon-14 and strontium-90 while Lovelock and Lipsky (IO) observed an increase in current for cyclohexane in an electron affiity detector when radium was used as the source. The latter authors recognized that ionization can occur a t higher applied voltages. Low results obtained by Otvos and Stevenson (14) for the apparent ionization crosssections of ammonia and Freon-12 are readily explained by postulating that the electron-capture process was competing with ionization for these com-
pounds so as to produce a lower net relative ion production. The first peak in Figures 5 and 6 at 25 nanograms of Systox, and the first peak in Figure 7 at voltages above 10 volts show maximum reversal when the amount of the thiono-systox in the detector is a maximum. This change from electron capture to ionization M the concentration of the active compound in the effluentincreases, indicates that both electron capture and ionization can take place in the same detector under the same operating conditions. Detector design is an especially critical variable for those compounds which will both caQture and ioniae. These studies
against the flow of carrier gas, minimizes the ionization phenomenon. The pulsed mode of detector operation (8) will not, however, eliminate interferences from ionization in a poorly designed detector. LITERATURE CITED
(1) Clark, s. J., 140th National Meeting, ACS, Chicago, Ill., September 1961. (2) Clark, S. J., 144th National Meetmg, ACS, Los Angeles, Calif., April 1963. (3) Coulson D. M., Pesticide Research
Bulletin, Stanford Research Institute, 2, No. 1, I (1962); Ibid., No. 2, 1. (4) Fukuto, T. R., Metcalf, R. L., J . Am. Chem. SOC.76, 5103 (1954). (5) Goodwin, E. S., Goulden, R., Reynolds, J. G., Analyst 86, 697 (1961).
( 6 ) Littl:?ood,
A. B., "Gas Chromatography, p. 286, Academic Press, Wew York, 1962. (7) Lovelock, J. E., ANAL. CHEM.33, 162 (1961).
(8) Ibid., 35, 474 (1963). (9) Lovelock, J. E., Nature 89,729 (1961). (10) Lovelock, J. E., Lipsky, S. R., J . Am. Chem. SOC.82, 431 (1960). (11) Lovelock, J. E., Simmonds, P. G., Vandenheuvel, W. J. A., Nature 197,
249 (1963). (12) Lovelock, J. E., Zlatkis, A., Becker, R. S.. Ibid.. 193. 540 (1962). (13) Mbore, A. D.', J . @con. Entomol. 55,
.-Gas Chrbmbg. I, No. 2, 23'(1963).
'
(16) Watts, J. 0.1 K l e b A. K., J . Assoc. Ofic. Agr. Chemists 45, 102 (1962).
J. E. BARXEY I1 C. W. STAXLEY C. E. COOK
Midwest Research Institute
425 Volker Blvd. Kansas City 10, Mo.
RECEIVEDfor review August 29, 1963. Accepted October 15, 1963.
Latent Heats of Vaporization from Distillation Rate Data SIR: The observation that the temperature dependence of physical rate processes follows the same relationship as the temperature dependence of chemical processes (4) evolved from a demonstration that chemical kinetic principles apply when physical reactions such as distillation, gaseous effusion, and dialysis occur (6). The following discussion shows that, for Table I.
Compound Benzene Chloroform Methanol n-Butanol Ethyl acetate Methyl ethyl ketone n-But ylamine Water
E (kcal. per mole) 18.85 18.18 19.18 21.16 19.19 18.85 18.75 21.12
Av.
Formic acid Acetic acid Ref. (6). Ref. (a).
20.86 21.38
'' Ref. @).
* Ref. (1).
2208
ANALYTICAL CHEMISTRY
distillation rate data, a relationship exists between the energy of activation as determined from the Arrhenius equation and the latent heat of vaporization. The energies of activation were calculated from the slopes of the lines in the plots of log k US. 1/T. Examples of such plots for benzene and n-butanol have been shown (4). By using these
Results Obtained
H , (kcal. per mole) C = E - H, calcd. using A (mole (kcal. av. A per min.) per mole) and C 22.31 22.04 22.02 22.25 22.08 21.90 21.61 20.77 21.87 22.47 21.98
11.40 11.34 10.69 10.73 11.51 11.21 11.11 11.39 11.18 10.39 IO. 07
H. (kcal. per mole), literature values
7.37 6.89 7.89 9.67 7.84 7.65 7.78 10.88
7.375 6.84O 8.49" 10. 45a 7 . 636 7.64" 7.67c 9.73c
9.18 10.10
10.47d 11.31d
values for the energies of activation, the Arrhenius equation, k = A e - E / R T , was solved for A. These values for A of the compounds studied were in close numerical agreement. Also, the published values for the Iatent heats of vaporization, H,, a t the boiling points differed from the calculated energies of activation by a fairly constant value, C. It appears that the constants, A and C, depend on the geometry of the distillation set up and include the work involved in moving the vapor from the surface of the liquid to the receiver. Based on distillation rate constants and on the average values obtained for A and C, heats of vaporization were recalculated using the equation
All these data are presented in Table I. Calculations of the latent heats of vaporization for formic and acetic acids presented difficulties because of their associating vapors. The values obtained are based on a volatilization forming the equilibrium vapor and depend on the degree of association under the conditions used. Because of the uncertainties in the degrees of associa-
