The Teflon system is more difficult to construct and operate properly than the glass system. However, it is not fragile and the separation yield is fairly high. There are five disadvantages with the Teflon separator: the helium pressure in the ion source cannot be reduced to less than 1-2 X torr without restricting the flow in the chromatograph, the enrichments are relatively low, a lag time of 20 to 30 sec is observed between the appearance of a peak in the chromato-
graph and the mass spectrometer, the maximum usable flow rate is approximately 15 rnl/min, and the operating temperature is limited to the range 280" to 330" C. The operating characteristics of both separators are summarized in Table 111. RECEIVED for review April 7, 1967. Accepted August 11,
1967.
An Improved Microwave Emission Gas Chromatography Detector for Pesticide Residue Analysis H. A. Moye Pesticide Research Laboratory, Agricultural Experiment Station, University of Florida, Gainesville, Fla. 32601 The sensitivity and selectivity of the microwave emission gas chromatography detector have been improved for several types of pesticides by utilizing argonhelium mixtures for the chromatography carrier and microwave discharge gases. The detector was evaluated and optimized for the type of cavity, discharge tube, microwawe power, discharge pressure, slit width, carrier and discharge gas, and carrier gas flow rate. Limits of detection were determined for a number of organic phosphate pesticides plus D.D.T., lindane, and 2-iodobutane. I t was possible to determine 0.5 ppb parathion in orange essence, and a direct comparison is made with electron capture detection for parathion in celery.
A MICROWAVE EMISSION gas chromatography detector, in which the effluent from a gas chromatographic column was fed into a microwave discharge, was first reported by McCormack, Tong, and Cooke ( I ) . This detector was used by Bache and Lisk for organic phosphate residues (2) and organic iodide residues (3) utilizing argon as the carrier gas at atmospheric pressure. Subsequently, they reported a sensitivity enhancement by operation at reduced pressure (4). The present study demonstrates an enhancement of sensitivity and selectivity for phosphates using argon-helium mixtures over that obtained with argon or helium. This enhancement is also apparent for the chlorine emission of p,p'-D.D.T. and lindane, and also for the iodine emission of 2-iodobutane. Using parathion as a representative organic phosphate the detector was optimized for carrier gas composition, monochromator slit width, carrier gas now rate, discharge tube diameter, discharge pressure, microwave power, and emission wavelength. Sensitivities are reported for a number of organic phosphate pesticides plus p,p'D.D.T., lindane, and 2-iodobutane at the optimum values for parathion. Chromatograms showing the determination of parathion in celery and in oranges are presented, with a direct comparison to electron capture detection.
(1) A. J. McCormack, S. S. C. Tong, and W. D. Cooke, ANAL. CHEM.,37, 1470 (1965). (2) C. A. Bache, and D. J. Lisk, Ibid.,37, 1477 (1965). (3) C. A. Bache, and D. J. Lisk, Ibid.,38,783 (1966). (4) Ibid.,p. 1757.
EXPERIMENTAL
Apparatus. Warner-Chilcott Models 601-1 oven, 660 injection and output housing, and a 607-4 proportional temperature controller were used. A 1P28 photomultiplier tube was used in conjunction with an Eldorado Model 211 photometer. The microwave power was generated by a Raytheon PGM 10 X 1 generator. An American Instrument Co. scanning monochromator, with 1200 lines/mm grating and bilaterally adjustable slits, Model 4-8401, was used. A second monochromator of the same type was used in the scattered light tests. The Evenson cavity tested was obtained from Opthos Instrument Co., Rockville, Md. Standing waves were minimized with a Micro-match 225.3 R.F. power meter. The tapered cavity was obtained from Raytheon. This cavity was made up of a tapered matching section, No. 7097-1001 G1 and a coaxial adapter section no. 7097-5002 G1. Carrier gas flow measurements were made using calibrated Matheson Model 620PBV flowmeters. Nupro valves, Model 2M, were used for flow control. A heavy duty Gast type vacuum pump, Model 10336, was obtained from A. H. Thomas Co. Quartz tubing for discharge tubes was obtained from Thermal American Fused Quartz Co. A Sargent SR recorder was used. Stainless steel Swagelok fittings were used throughout. All pesticides were analytical grade (96-100 purity) obtained from their respective manufacturers. The 2-iodobutane was Eastman reagent grade. RESULTS
Cavity Selection. Operation of the detector was first begun using the Evenson cavity. This cavity has been reported to give less sensitivity than the tapered cavity (1). It was found to be somewhat less sensitive in this laboratory; however, its major difficulties were high noise and low discharge tube life. These were caused by hot spots appearing within the discharge tube that etched the tube, giving high background emission and consequent decomposition of the tube. Efforts to cool the tube using forced air only resulted in a complete loss of sensitivity, apparently because of deposition of the effluent onto the walls of the discharge tube. This cavity also suffered from difficulty in tuning, difficulty in mounting, and a softening of the internal Teflon insulators. It was abandoned in preference to the tapered cavity. Tuning of the tapered cavity by adjusting the stub was unnecessary, as it gave extremely low reflected power when reVOL. 39, NO. 12, OCTOBER 1967
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10
20
$ETil
30sL,T
p50
60
f
I
70
80
Figure 1. Effect of monochromator slit width on parathion signal to noise ratio SE30 8OjlOO G.C.Q., 4' X lj8" glass, total flow: 27 cc/min., Col.: 180" C, inj: 210" C, gas: 85% He-l5%A, power: SO%, pressure: 25 mm Hg, tube: 0.5 mm., 10 ng parathion. 5
ceived from the manufacturer. None of the previously mentioned difficulties were experienced with the tapered cavity. Discharge Tube Selection. Although no comparative studies are presented here it became immediately obvious that the smaller the diameter of the discharge tube the greater the sensitivity of the detector. Five-, 2-, 1-, and 0.5mm i.d. tubes were evaluated with the 0.5-mm tube giving highest sensitivity. This tube was of 6-mm o.d., it fit excellently into 0.25-inch Swageloks, and showed no bending or deformation a t the discharge area as did some of the thin-walled tubes tested. A 0.5-mm tube lasted for more than 3 months of continuous use when care was taken to allow solvents sufficient time to exit the discharge tube before initiation of the discharge. Microwave Power. For all compounds reported here microwave power did not appear to affect signal to noise ratios. Fifty percent power was chosen for good discharge stability (except where otherwise noted) and to minimize magnetron aging due to excessive reflected energy. Discharge Pressure. Best results were obtained a t minimum pressure. The vacuum pump was capable of producing a vacuum of only 25 mm of H g a t the flow rates of gas used through the chromatography column. Significant reductions in retention times were noted when operating a t this pressure as compared to atmospheric pressure. This difficulty could be eliminated by introducing a capillary restriction a t the entrance of the discharge tube. It was felt, however, that the resulting low flow rates would interfere with efficient column operation, All measurements in this study were made with the 25-mm discharge pressure. Slit Width. The monochromator was adjusted for optimum slit width. This was determined by plotting the slit width ES. the signal to noise ratio for 10 ng of parathion measured a t the phosphorous atomic emission of 253.57 mp. This line has been utilized earlier for phosphorus determination ( I , 2, 4). It was found that to a close approximation the peak to peak noise present was directly proportional to the background intensity upon which the atomic line (molecular band) was superimposed. The peak to peak noise 1442
ANALYTICAL CHEMISTRY
25 -
50
L
n
10
n
20
I
I
~
I
30 40 50 60 70 S L I T WIDTH, p
l
1
1
8 0 90 100
I
Figure 2. Effect of slit width on parathion/lindane selectivity ratio 5 % SE30 80/lOO G.C.Q., 4' X 1/8" glass, col.: 180" C,
inj.: 215' C, total Row: 27 cc/min., gas: 85 He-15 %A, power: 50%, pressure: 25 mm Hg, tube: 0.5 mm, 10 ng parathion, 1 fig lindane.
x
x)
signal was 3 (i0.5 of the background radiation. Consequently, in the signal to noise ratio measurements described here, the noise amplitude was calculated from the background value. The signal (from pesticide emission) was determined by measuring the peak area, the peak width a t the half height times the peak height. A 6-micron slit gave optimum signal to noise ratio, as seen by Figure 1. This setting was used throughout the subsequent studies. Figure 2 shows the dependence of the selectivity ratio of parathion to lindane on the monochromator slit width. The selectivity ratio is defined here as previously defined (2). It is defined as the ratio of peak areas of the two compounds of interest measured at one optical wavelength. Lindane has been used previously as a representative hydrocarbon for selectivity comparisons (4). It was chosen here because of its stability and ease in chromatography. Only a decrease in oven temperature from 180" to 150" C was necessary with the SE30 columns used for the parathion measurements to give excellent chromatography. Carrier Gas. After unsatisfactory results were obtained using argon as a carrier gas, argon mixtures were used to study phosphorous emission from a discharge. Of those gases mixed with argon (N2, C o n , 02,and He), all but H e gave very high continuum radiation even when the gases were present in trace quantities. All but helium also gave very unstable and noisy discharges. Measurements were taken to determine the effect of He included in (or substituted for) the argon carrier gas. The effect of carrier gas composition on detector sensitivity is shown in Figure 3 where per cent He in the A-He
140-
720 680 -
130 -
640-
760
160-
150-
A
120-
100 110
a
n
600 -
90r
180-
_I
Q
c3
L?
> 400t 2 360-i
7060 -
30r
5
320-
L?
280 -
w
4 50[ 0
20
10
20 3 0 40
50 60 70 ' l o He
80 90 100
Figure 3. Effect of carrier gas composition on parathion signal to noise ratio
40 1
5 % SE 3080/100 C.C.Q., 4' X l/S" glass, total flow:
27 cc/min., col.: 180' C, inj.: 220" C, slit: 6 w, power: SO%, pressure: 25 mm Hg, tube: 0.5 mm, 10 ng para-
thion.
carrier gas is plotted against signal to noise ratio. Any amount of He in the discharge gas gave higher sensitivities than did pure A. The highest sensitivity occurred at 85% He. It was impossible to sustain the discharge for 100% He with the 50 power that was used for the other points. One hundred per cent power was necessary for this point, which was permissible because of the independence of microwave power on signal to noise ratio. At this power and pressure setting the 100% He discharge caused rapid decomposition of the thick walled capillary discharge tube. The selectivity ratio of parathion to lindane cs. gas composition is given in Figure 4. This curve maintains the same shape as that of parathion signal to noise OS. He percentage. This is because the parathion signal to noise is a maximum at 85% He while the lindane signal to noise is constant, independent of gas composition. consequently, maximum sensitivity coincides with maximum selectivity at 85 % He. Flow Rate. The signal to noise ratio of parathion as a function of flow rate was studied. Results were erratic, even after several repeats. A slight increase in sensitivity was obtained, however, by operation at relatively high flow rates. A 85% He-15z A mixture was used with a constant discharge pressure of 25 mm of Hg. Sensitivities. Numerous organic phosphates were evaluated for their limits of detection at the optimum parameter for parathion (with the exception of carrier gas flow rate and column temperature). The limits of detection were set at a signal to noise ratio of two to one, using peak heights for measurement. All peak heights were measured with a compound retention time of 2 to 3 minutes. The atomic line of phosphorus at 253.57
1
1
1
1
1
1
1
,
10 2 0 30 40 20 60 70 80 90 100 io He
Figure 4. Effect of carrier gas composition on parathion/lindane selectivity ratio 5 SE30 80/100 G.C.Q., 4' X 118" glass, total flow : 27 cc/min., col.: 150" C CL-180" P, inj.: 215" C, slit: 6 p, power: SO%, pressure: 25 mm Hg, tube: 0.5 mm, 10 ng parathion, l p , g lindane.
L
210
220
240 250 millimicrons
230
260
I
270
Figure 5. Emission spectrum of chloroform
mp was used. Lindane, p,p'-D.D.T. and 2-iodobutane were also evaluated at these parameters. The 206.2-mp iodine atomic line previously reported was used for 2-iodobutane (I, 3). After attempts to use the 278.8-mp C-CL band emission were unsuccessful for lindane and p,p'-D.D.T., an ernisVOL. 39, NO. 12, OCTOBER 1967
1443
-
I -
0.5 p.p.b. PA RAT HION IN ORANGE ESSENCE
Figure 6. Comparison of electron capture and microwave emission responses for celery extract containing 10 ppb parathion
I
minutes sion spectrum was run from 200 mp to 700 mp on chloroform, with a scan speed of 1 mp per second. A band head at 221.0 ml.c appeared, the only emission which differentiated chloroform from hexane (Figure 5 ) . This band did not correspond to any reported in the literature. Table I gives these limits of detection. Analytical curves were linear for several decades, as reported previously (2). Residue Analysis. Figure 6 compares the chromatograms resulting from electron capture and microwave emission analysis of the same celery extract containing parathion. This extract was prepared by spiking celery leaves with parathion, blending with hexane :isopropanol, filtering, washing the extract with water, and concentrating. Ten times as much parathion-i.e., 10 ng-was necessary to give the same peak height with the microwave instrument as did 1 ng with the electron capture instrument. Ten microliters and 1 pl of the extract were injected respectively. However, there was a complete absence of extraneous peaks with the microwave detector. This celery contained 10 ppb of parathion before extraction. Figure 7 shows 0.5 ppb parathion in orange essence that has received no cleanup. Orange juice was extracted with methylene chloride, the methylene chloride extract was washed several times with water, filtered, concentrated, and analyzed.
Figure 7. Microwave emission response to extract of orange juice containing 0.5 ppb parathion
Similar responses were obtained with unclean trithion extracts of soybeans, although rapid deterioration of the gas chromatography column was evident. DISCUSSION
Difficulties were initially experienced in the operation of the detector and accompanying gas chromatography system even after switching to A-He mixtures. Erratic sensitivities and high background radiation were corrected by the installation of stainless steel Swagelok fittings throughout the gas system and by properly heating the discharge tube along its length before it reached the cavity. This was accomplished by wrapping a glass sleeve with heating tape, fitting it over the discharge tube and insulating it with two lengths of glass tubing of large and increasing diameter. The heating tape was supplied power by a Variac. It was initially thought that most of the background radiation measurement at 253.57 mp was scattered light. To
Table I. Limits of Detection (Based on Peak Heights)
Calculated on signal/noise = 2/1 Pesticide Parathion Trithion Ronnel Methyl trithion VC-13
Aspon Dicapthon Ethion Thimet p,p'-D.D.T. Lindane 2-Iodobutane
1444
Limit of detection (ng) 0.15 0.07 0.19 0.34 0.43 0.48 0.65 0.48 0.11 11.5 1.2 0.05
ANALYTICAL CHEMISTRY
Enlission (mp) 253.51 253.57 253.51 253.57 253.51 253.57 253.57 253.51 253.57 221 .oo 221.00 206.20
N NUTES
Figure 8. Microwave emission response to 10 ng of parathion at carrier gas composition of 0 He, 85 He, and 100 % He
verify this a second monochromator was connected to the exit slit of the monochromator normally used. With the first monochromator set at 253.57 mp the second monochromator was scanned from 200 mp to 700 mp. No scattered light could be detected. Both monochromators were then set at 253.57 mp and the slits were varied to optimize the signal to noise ratio for parathion. No improvement could be realized over the single monochromator arrangement. Optimum sensitivity of the detector was realized with an 85 % He carrier gas. Figure 8 illustrates the superior response obtained for parathion over either pure A carrier gas or pure He carrier gas. An optimum monochromator slit of 6 mp was determined. These two parameters most affected the sensitivity and selectivity of the detector. As has been reported (4) for 100% A, the detector here shows an enhancement in sensitivity for reduced pressure over that obtained at atmospheric pressure. Maximum sensitivity was obtained at the lowest pressure, 25 mm, that was possible with the type of vacuum pump used.
The detector, when evaluated in terms of signal to noise ratio, is relatively insensitive to microwave power and flow rate of carrier gas. Column temperature had no effect on background radiation or signal to noise ratios. These independencies allow the detector to be operated over a wide range of chromatographic conditions without loss of efficiency. Temperature programming would have no effect on the detector. The A-He microwave emission chromatograph has been used in this laboratory on a routine basis, giving progressively less operating difficulties than those experienced with a variety of electron capture instruments. Studies aimed at extending the usefulness to chlorinated hydrocarbons and other types of compounds are underway.
RECEIVED for review May 12, 1967. Accepted July 19, 1967. This investigation was supported in part by U.S. Public Health Service Grant EF 00203-06 from the Division of Environmental Engineering and Food Protection.
Repetitive Gas Chromatographic Analysis of Thermal Decomposition Products Paul D. Garn and Gaylord D. Anthony The University of Akron, Akron, Ohio 44304 An apparatus for studying the kinetics and mechanisms for thermal decompositions is described. The sample holder is connected to the sample loop of a gas chromatograph by a long narrow diffusion path. This arrangement causes the decomposition to take place in an atmosphere of its own decomposition product gases, while only a negligible amount i s retained within the sample holder and the diffusion path. The decomposition product gases are repetitively sampled into the gas chromatography by means of a solenoidoperated sampling valve controlled by an adjustable timer. The adjustable timer also controls the integration of the chromatographic peaks through a set of time-delay relays. The type of data obtainable is shown for a lanthanum oxalate hydrate.
SEVERAL METHODS, including thermogravimetry and differential thermal analysis, have been used to study thermal decompositions ( I ) . Reactions may also be followed by continuous evolved gas analysis or repetitive time-of-flight mass spectrometry. Differential thermal analysis and thermogravimetry both have limitations as well as capabilities for the study of systems which decompose to yield one or more gaseous products. Differential thermal analysis has an advantage in studying reversible reactions in that pressure change will yield heats of reaction and equilibrium data, at least for rapid reactions. It does not give good numbers, however, except for total heat effects. The hopeful assumption that the instantaneous value of AT measures the rate of reaction at that moment does not make it so. Nor does the arbitrary assignment of temperature homogeneity to a sample being heated from the edge cause the temperature to become uniform. So we must look elsewhere for kinetic data. ~~
Thermogravimetry has been used with an assortment of mathematical treatments to obtain such kinetic data as the kinetic order followed and the activation energy. The treatments are, in general, restricted to some limited portion of the weight loss curve. In most cases they assume uniform temperature even at heating rates common for DTA. With the current fascination for small samples, a substantial fraction of the weight must be lost to permit detection, and this early weight loss is measured as a small difference between two large numbers. For any but simple reactions its use is impaired because the nature of the weight loss must be inferred. This latter difficulty has caused experimenters to send the gaseous products through some other measuring device to determine the products, but if this is done the thermobalance is superfluous. Continuous measurement of evolved gases by most detectors suffers the same flaw, lack of identification. However, simple reactions can be followed well because the sample holder can be designed to assure temperature homogeneity, and the sensitivity can be varied to measure small initial reaction as well as rapid reaction. Repetitive mass spectrometry at high repeat rates can be performed by allowing the evolved gases to enter the ionizing chamber of a time-of-flight mass spectrometer (Z), but this technique suffers three disadvantages : quantitative measurement is poor; atmosphere conditions in the sample holder are limited by the need to make this type of measurement; few experimenters wishing to study complex thermal decompositions can acquire a time-of-flight mass spectrometer and the equipment would be wasted on most studies wherein numbers are more important than identification.
~~
(1) Paul D. Garn, “Therrnoanalytical Methods of Investigation,” Academic Press. New York, 1965.
(2) Henry L. Friedman, J . Appl. Polymer Sci., 9, 651-62 (1965). VOL. 3 9 , NO. 12, OCTOBER 1967
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