tion to band-broadening appeared to be very small, separations with capillary columns were also attempted. The resolution attainable on an oil of marjoram sample with programmed temperature and a capillary column using the thermal conductivity detector was equivalent to that achieved with a flame ionization detector but a t a 100-fold decrease in sensitivity. With this type of performance, nu-
merous applications previously restricted to the flame ionization or helium ionization detectors can now be successfully analyzed utilizing the reduced volume thermal conductivity detector. Received for review January 16, 1973. Accepted June 4, 1973.
Enhancement of the Sensitivity and Selectivity of the Coulson Electrolytic Conductivity Detector to Chlorinated Hydrocarbon Pesticides John W. Dolan’ and Randall C. Hall2 Department of Entomology Purdue University, West Lafayette, lnd 47907
Factors which influence the sensitivity and selectivity of the Coulson electrolytic conductivity detector to chlorinated hydrocarbon pesticides were determined and optimized. The most influential factors which affect sensitivity are absorptive surfaces, electrode polarization, system stability. and furnace temperature. Replacement of the standard 4-mm i.d. quartz reaction tube with one of 0.5m m i d . , replacement of the silicone rubber septum at the furnace exit with a Teflon fitting, and increasing the maximum cell voltage to 44 V dc resulted in a minimum detectability of 0.1 ng for heptachlor and a useable sensitivity of 0.4 ng a s compared to 2 ng and 5 ng, respectively, for the unmodified detector. The most influential factors which affect selectivity are furnace temperature, reaction gas composition, and reaction gas flow rate. Optimization of these parameters enables most chlorinated hydrocarbon pesticides to be selectively determined in the presence of other halogenated materials such as PCB with selectivities >103:1.
Gas chromatographic detectors such as the Coulson electrolytic conductivity ( I , 2 ) and microcoulometric (3, 4 ) , which rely upon thermal degradation of the analyte for the creation of detectable compounds, are a t present the only detectors which can be used routinely for the selective detection between nanogram levels of compounds containing the same elements. Specificity between compounds containing the same elements can be achieved from differences in thermal stability, whereas specificity between groups of compounds containing different elements can be achieved by scrubbing interfering degradation products and/or selective titration. The authors ( 5 ) recently reported the application of controlled thermal degradation for the selective determi1 Present address, Department of Environmental Toxicology, University of California, Davis, Calif. 95616. 2 To whom all correspondance should be sent.
(1) D. M Coulson. J. GasChrornatogr.. 3, 134 (1965) (2) D. M . Coulson, Advan. Chrornatogr, 3, 197 (1966). (3) D. M Coulson and L A . Cavanagh. Anal. Chern.. 32, 1245 (1960). ( 4 ) D hil Coulson. L. A Cavanagh. J. E de Vries, and B . Walther. J . A g r FoodChem.. 8, 399 (1960) ( 5 ) J. W. Dolan. R C. Hall. and T M . Todd, J. Ass. Oftic. Ana/. Chern.. 55. 537 (1972).
2198
nation of several chlorinated hydrocarbon pesticides in the presence of polychlorinated biphenyls (PCB), The method employed the Coulson electrolytic conductivity detector and was based on the greater ease of thermal degradation of the pesticides to form HC1, which was monitored. Depending upon conditions, detector response to chlorinated pesticides such as aldrin, dieldrin, heptachlor, and heptachlor epoxide can exceed the response to PCB by 105. Controlled thermal degradation has also been utilized by Rhoades and Johnson (6) for the selective determination of N-nitrosamines in the presence of other nitrogen-containing compounds. The selective detection of chlorinated hydrocarbon pesticides in the presence of PCB greatly facilitates their determination in environmental samples. The lengthy column cleanup procedures which were required by previous methods (7-10) can be eliminated. This decreases analysis time, expense, and the possibility of introducing interfering substances from absorbents and reagents. Realization of the full potential of controlled thermal degradation for the selective detection of pesticides in the presence of chemically similar substances is presently precluded by the decreased response of the Coulson electrolytic conductivity detector under selective conditions (response is decreased to approximately 20% of its original value) and lack of knowledge of the applicability of this method to other compounds. In an effort to enhance the utility of this method, the factors which influence the response of the Coulson electrolytic conductivity detector were investigated with two objectives in mind-enhancement of sensitivity and determination of what types of chlorinated compounds can or cannot be detected under a given set of operating conditions. The influence of such factors as reaction gas composition, furnace temperature, residence time, and conductivity solvent on detector response to 13 pesticides, several PCB formulations, and a variety of model compounds is discussed. (6) J . W . Rhoades and D E. Johnson, J . Chromatogr. Sci. 8, 616 (1970) (7) 0 . W. Berg, P. L. Diosady, and G . A V. Rees, Bull. Enwron. Contarn. roxicoi., 7 . 338 (1972). ( 8 ) V. E. McClure,J. Chrornatogr.. 70. 168 (1972) (9) C. E. Collins. D. C Holmes, and F J. Jackson. J . Chromatogr.. 71. 443 (1972). (10) D . Snyder and R. Reinert. Buli. Environ. Contarn. Toxicol.. 6. 385 (1971)
A N A L Y T I C A L C H E M I S T R Y , V O L . 45, NO. 13, N O V E M B E R 1973
EXPERIMENTAL Apparatus. A Coulson electrolytic conductivity detector mounted on a Tracor MT-220 gas chromatograph was modified to allow alteration of reaction gas composition with maintenance of constant total gas flow rate as previously described ( 5 ) . The gas composition entering the furnace was comprised of 1) helium carrier, 2) hydrogen, and 3 ) helium make up gas. The reaction gas which was 2 and 3 combined was regulated to give a constant total flow of gas and was added to the carrier just prior to the furnace. Separations were performed on a 6-ft by 1/4-in. glass column packed with 3% OV-1 on acid washed 80/100 mesh Chromosorb W. Oven temperatures were used which would give a 3- to 6-minUte retention time. A 4-port valve was used for solvent venting to prevent carbon deposition in the reaction tube. Quartz reaction tubes were cleaned prior to use by heating at 900 “C for 0.5 hr under a continuous flow of 0 2 , and were used empty. Influence of Reaction Parameters. The influence of reaction parameters on detector response t o a given compound was determined by comparing peak heights t o those observed for “standard operating conditions” (810 “C furnace temperature and 80 ml/ min Hz reaction gas, unless otherwise specified). The response at standard conditions was arbitrarily assigned a value of 100, and the response a t other conditions was expressed as a percentage of this value. Detector selectivities for the various compounds were determined from the absolute detector response. This was calcu-
lated from peak areas and expressed as the strip chart response in cm2 per gram of HC1 which could theoretically be produced from the analyte. Reagents. The pesticides used were analytical standard qualit y and were procured from their manufacturers. PCB used were commercial formulations and were donated by Monsanto Co. The model compounds used were reagent grade and were purchased from Aldrich Chemical Co.
RESULTS AND DISCUSSION Enhancement of Sensitivity. Reduction of Surfaces Available for HC1 Absorption. The silicone rubber septum and stainless steel fitting a t the exit of the pyrolysis furnace were replaced with a piece of Teflon (Du Pont) which fit tightly on the end of the quartz reaction tube. The Teflon transfer tube from the end of the furnace to the cell was replaced with smaller inside diameter Teflon tubing (0.063-in. 0.d. x 0.031-in. i.d.) and made to extend as far as possible into the cell. This eliminated the absorptive surfaces of the rubber septum and stainless steel fitting housing it, as well as reducing exposed glass surfaces within the cell. From the change in peak area and the chlorine content of the injected sample, the amount of HC1 absorbed by the septum, stainless steel fitting, and exposed glass was calculated. These absorbed the HC1 produced from 0.463 ng of heptachlor (containing 66.53% chlorine by weight) and 0.517 ng of dieldrin (55.91% chlorine), or the HC1 from an average of 0.49 ng of chlorinated pesticides. If this were the only limiting factor, the lower limit of detectability for the detector with the silicone septum, stainless steel fitting, and with partially exposed glass in the transfer line would be about 0.5 ng for a chlorinated hydrocarbon pesticide with about 60% chlorine content. The limit of minimum detectability for heptachlor with these parts in place was about 2 ng. The modifications mentioned above were used throughout the rest of the study unless otherwise indicated. The standard Coulson electrolytic conductivity detector uses a 4-mm i.d. x 280-mm quartz reaction tube (35.19 cmz surface area) in the furnace, which is a possible source for HC1 absorption. To determine the quantity of HC1 lost in the reaction tube. the 4-mm tube was replaced by the smallest bore tube available, a 0.5-mm i.d. X 280mm quartz tube (4.40 cm2 surface area). A Teflon adaptor was made to line the stainless steel fitting before the furnace so the tube would fit tightly. The smaller tube improved detector performance in several ways.
The variation in peak height was reduced by using the smaller tube, which results in the capability for greater precision in determining pesticide residue levels. Based on 20 identical injections of heptachlor with and without hydrogen reaction gas flow, peak height for injections using the small tube had a standard deviation of approximately 20% of that obtained with the large tube. The concentration of heptachlor and the output attenuation were adjusted for the two tubes so the peaks were approximately the same height. The detector response in terms of peak area per gram of sample was doubled by substitution of the small bore tube for the large one. Response for heptachlor for the large tube was 1.96 x 108 cm2/g, whereas the small bore tube gave a response of 3.84 X 108 cm2/g (cell voltage 23.5 V dc, attenuation ~ 2 ) . The amount of tailing was also reduced with the small tube. The cause of tailing is believed to be due to two factors. As the HCl absorbed on exposed surfaces in the detector gradually bleeds off, detector response continues, even though the major part of the peak has passed. Polarization of the cell electrodes is also believed to account for part of the tailing phenomena. When the peak passes through the cell, H+ and C1- ions migrate toward and aggregate about the electrodes. As these ions diffuse off the electrodes and pass out of the cell, the response slowly diminishes. Since both these phenomena do not occur before the peak passes through the cell, tailing is observed only on the back side of the peak. For this study, tailing was expressed as:
where w o 0 5 is the width of the peak at 5% of the height, h. Most of the tailing occurred in the bottom half of the peak. Since the width a t the base line was rather arbitrary, width a t 5% was selected for tailing measurements. Tailing for the large tube represented 18% of the peak height, whereas only 8% was observed for the small tube. As the proportion of the area in the peak tail was reduced, the proportion of area in the height increased in order to maintain the same area response per gram of sample. This is a definite advantage since in many residue labs, pesticides are quantitated by measuring the peak height. not area. In this case the detector with greater response in height per gram has a lower minimum level of detectability even if two detectors give the same response in area per gram of sample. The response in height per gram for the detector fitted with the small bore tube was about 2.5 times that of the large tube (10.7 x 107.mm/g us 4.37 x l o 7 mm/g for heptachlor; cell voltage 23.5 V dc, attenuation x2). Remaining Absorptive Surfaces. After the standard reaction tube and silicone septum were replaced by a 0.5mm i.d. reaction tube and Teflon septum, some peak tailing was observed to remain. To determine whether the furnace or cell was responsible for this tailing, a series of valving experiments was conducted. A 3-port plastic valve was placed between the furnace exit and the detector cell, so the furnace effluent could be directed through the cell or vented to the atmosphere. An additional supply of carrier gas was run through the cell so that gas flow in the cell did not stop entirely when the furnace was vented. The 4-port valve normally used for venting solvent was also used. Three valve combinations were used: in position 1, both valves were open so that the column effluent passed directly into the cell; in position 2, the 4-port valve was in vent position so column effluent was vented before enter-
A N A L Y T I C A L C H E M I S T R Y , V O L . 45, NO. 13, N O V E M B E R 1973
* 2199
3 0
n
t
-
L
B
A
C
Time
Figure 1. Effect of 3-port and 4-port valve on peak tailing for heptachlor (first peak) and dieldrin (second peak). See text for explanation
I4O1 I20
2 1001 .? 80 0
I x
60-
-
a
40 -
20
-
i
10
20 30 40 50 Cell V o l t a g e ( v dcl
0
Figure 2. Peak height vs. detector cell voltage for 10 ng of hep-
tachlor Detector conditions furnace temperature, 810 " C , reaction gas, 80 ml min H Z
e
1
e
2.4 -
-
22-
0
i
e
e 10 20 30 40 Cell V o l t a g e ( v dc)
Figure 3. Per cent tailing vs. detector cell voltage for 40 ng hep-
tachlor Detector conditions furnace, 825 "C, reaction gas. 80 ml/min H2
ing the furnace; and in position 3, the 3-port valve was in vent position to allow column effluent to pass through the furnace and then be vented by the 3-port valve. Figure 1A shows normal peak tailing for heptachlor and dieldrin (valve position 1). In Figure l B , the valves were changed from position 1 to 2 a t the top of each peak 2200
(valve returned to position 1 between peaks). Peak tailing is only slightly diminished for heptachlor and unchanged for dieldrin, indicating that the tailing is produced after the 4-port valve. The valve position was changed from 1 to 3 a t the top of each peak in Figure C (valve returned to position 1 between peaks). If the cell were not responsible for the tailing, the peak should drop vertically to the base line as soon as position 3 was set. If the cell were responsible for the tailing, a fairly normal tailing peak should be observed under the same conditions. The peak, however, dropped off rapidly and then tailed. This indicates that both the quartz reaction tube and the cell contributed to tailing. A furnace of smaller dimensions with a quartz reaction tube of small internal surface area, a cell of smaller time constant, and an ac conductivity bridge should theoretically reduce tailing.
Detector Response as a Function of Voltage. The Coulson conductivity detector uses a dc voltage and Wheatstone bridge circuit to measure changes in conductivity in the detector cell and thus measure detector response. The commercial bridge circuit uses a 0-30 V dc supply variable in 3-V increments. The polarity of the cell electrodes had little effect on detector response. Initial observations indicated that detector response increased with increased cell voltage. To further study the voltage-response relationship, the 30-V Zener diode in the power supply was replaced by one with a value of 54 V dc. Increased voltage results in an increased detector response in terms of peak height (Figure 2). Since the data fall on a pseudoparabolic curve with a slope greater than one, the highest possible cell voltage would be favored. The increase in response, however, is not without disadvantages. As is seen in Figure 3, peak tailing increases with increased cell voltage. This has the main disadvantage of decreasing resolution of two closely eluted peaks, since the increase in peak height due to voltage more than compensates for the loss of height due to increased area in the peak tail. Another disadvantage of increased cell voltage is seen as increased base-line noise with increased voltage. The signal-to-noise ratio did not vary predictably with voltage. On some days it increased with increasing voltage, but on others increased voltage caused little change in signal-to-noise ratio. The other factors entering into the variation need to be controlled before a conclusive study of the signal-to-noise ratio as a function of voltage may be made. The net result for the best operating voltage is to use the highest possible voltage which produces a base line within the desired limits. Day-to-day changes in detector conditions and gradual saturation of the ion exchange resin cause the maximum usable operating voltage to vary from day to day. Residence Time. The length of time the sample remains in the furnace (residence time) is a factor in the efficiency of thermal fragmentation. The flow of carrier gas from the column and the reaction gas mixture are the factors controlling residence time in the Coulson detector. The flow controllers available were not reproducible enough to obtain quantitative data concerning residence time. Variation in precision of the flow controller would prevent flow measurements unless the cell were disconnected for a flow check using a soap bubble flow meter. It was felt that disconnecting the cell between each reading would place more variables in the system, so only qualitative observations were made concerning the effect of residence time on detector response. The flow remained constant over a day's time if the controller was not readjusted during the period.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 13, N O V E M B E R 1973
I t was noted that increased residence time resulted in increased detector response whether the flow adjustment was in either the carrier or reaction gas. Response decreased if the hydrogen reaction gas was eliminated entirely. Hydrogen for the reaction forming HC1 was probably the limiting factor in this case. A hydrogen flow of about 1-10 ml/min seemed to be adequate to supply excess hydrogen for the reaction. At times the addition of hydrogen to the system created increased background noise. In this case, it was more advantageous to operate with no hydrogen, since the loss of detectability through base-line noise was greater than the loss of deleting hydrogen. The cause of this phenomenon was not determined. The standard Coulson conductivity zed water for a solvent system. A brief investigation of ethanol (100, 95, and 75%), dimethylsulfoxide (DMSO), and formamide indicated water is the solvent of choice. Formamide was too viscous to flow well even through a short ion exchange bed. DMSO performed about the same as water in detector response, but gradually dissolved the ion exchange resin. Ethanol performed as well as water, but evaporation and changes in concentration presented problems. Since none of the solvents outperformed water and since none were as inexpensive or as easy to use, water was used as the solvent system throughout the rest of the study. Noise and Stability. 'The unmodified Coulson detector produced a great deal of system noise. This noise, expressed as an unstable base line on the strip-chart recorder, was a limiting factor in minimum detectability. Replacement of the standard two-conductor cables to the cell and recorder with two-conductor cable with a grounded shield improved the base line considerably. The purity of the water in the system is also a critical factor. Periodic replacement of the water and ion exchange resin was necessary to keep noise a t an acceptable level. The quartz reaction tube should be burned out with oxygen regularly to minimize stability problems arising from carbon buildup in the tube. Burning out the tube was conveniently performed by letting oxygen flow through the furnace overnight. Hydrogen reaction gas flow produced a considerable amount of background noise as noted earlier. Increased dc cell voltage also increased base-line noise, but this was offset by a greater increase in peak height. Often a t a cell voltage of 44 V dc, the xl output attenuation setting could not be used due to base-line instability, but a t other times no drift was shown a t xl. Enhancement of Selectivity. Selectiue Detection of Chlorinated Pesticides i n the Presence of Polychlorinated Biphenyls. Preliminary results ( 5 ) indicated chlorinated hydrocarbon pesticides could be selectively detected in the presence of PCB. These investigations were done with the unmodified Coulson electrolytic conductivity detector. The reported most advantageous detector conditions of 600 "C for selectivity lacked the desired minimum detectability needed for routine trace pesticide residue analysis. Three further studies were made to determine the usefulness of this technique. First, a temperature-response study for PCB, chlorinated pesticides, and selected reference compounds was made to determine the behavior of these compounds as a function of temperature. Second, a group of 13 chlorinated pesticides was chromatographed under selective and nonselective detector conditions to determine their relative response and to superficially study possible dechlorination reactions. The last part of the study was to determine the most advantageous conditions for quantitative selective
I60 '*OI
140
1 t.
I
I
i 4
% 1200
-
Q
g 100 a
-
>
% €30al
L
T
-
40 20 -
60
0
500
600 7 0 0 8 0 0 900 1000 Furnace Temperature ("C)
Temperature vs. peak height for 0.3 pg of Aroclor 1254 ( A )and seven pesticides (e) (average for 30 ng each of lindane, heptachlor, heptachlor epoxide, dieldrin, DDT, N chlordane, and o@'-DDT)
Figure 4.
Detector conditions: cell voltage, 26 V dc: attenuation X 4 for Aroclor 1254. X 6 for pesticides; 80 ml/min H2 reaction gas
detection of chlorinated hydrocarbon pesticides in the presence of PCB. Detector response us. furnace temperature for seven chlorinated hydrocarbon pesticides and Aroclor 1254 is shown in Figure 4. At 815 "C, the pesticides give a nearmaximum response for the temperatures studied. As the temperature was decreased, the detector response for the pesticides also decreased. This indicates that the mechanism for formation of HC1 from the pesticides is near its maximum efficiency a t the recommended operating temperature of 820 "C. PCB, however, shows an increased response with increased temperature over the range of temperatures studied. From Figure 4, it can be deduced that the mechanism for HCl formation from PCB is only about half as efficient a t 815 "C as a t 940 "C. The curve appears to begin flattening out near 940 "C, indicating a plateau will be reached where the response will no longer increase with increased temperature. At any given temperature, the efficiency of the dechlorination reaction which forms HC1 from both PCB and chlorinated pesticides will depend on the bond dissociation energy of the C-C1 bond. Selective detection of a compound with weaker C-C1 bonds should be possible in the presence of a compound with stronger C-C1 bonds if a thermally dependent dechlorination mechanism is in operation. The aromatic C-C1 bond (the only C-C1 bond in PCB) has a bond dissociation energy of 4-13 kcal/mole more than the aliphatic C-C1 bond (the main C-C1 bond of chlorinated pesticides). In Figure 4, the pesticides reach a maximum efficiency plateau a t a much lower reactor temperature than PCB. I t is also seen that although no detector response for PCB is shown below 625 "C, the pesticides give a response about half their response a t 815 "C (with 80 ml/min hydrogen reaction gas flow). If the difference in bond energy of the aromatic and aliphatic C-C1 bond were responsible for the relative differences in response of PCB and chlorinated pesticides, it follows that other organochlorine compounds having these bond types should show similar response in the Coulson
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 13, N O V E M B E R 1973
2201
100
I " " " " " ' 1
1
Table I I . Influence of Reaction Conditions on Detector Response to Model Compounds Relative responsea
0
00
800 "C
700 "C
0
Compounds
Chlorobenzene 1,2-Dichlorobenzene 1,3-Dichlorobenzene
1,4-DichIorobenzene 1,2,3-Trichloro-
ae
0
t
benzene
Furnace Temperature ('C)
Figure 5. Relative peak heights vs. furnace temperature for PCB, chlorinated pesticides, and reference compounds. 0 = chlorocyclohexane, = pesticide average (same as Figure 4 ) , A = Aroclor 1254 (same as Figure 4 ) , V = pentachloroanisole, = cu,3,4-trichlorotoluene, X = average of 11 isomers of chlorobenzene, 0 = common to V , + , and X
+
Detector operating conditions 80 m l / m i n H2 reaction gas
Table I. Relative Peak Height as a Function of Furnace Temperature for Several Pesticides Compound
Heptachlor Heptachlor epoxide Aldrin Dieldrin Endrin
a-Chlordane ?-Chlordane Lindane Pentachloronitrobenzene o,p'-DDT p,p'-DDT p,p'-DDE
Methoxychlor
810°C"
100 100 100 100 100 100
100 100 100 100 100 100 100
710°Ca
610°Ca
35.8
23.1 13.5
41.6
27.5 38.9
23.9 34.2 34.6 63.3 32.7 16.0 20.6 1.4
24.7
18.0 7.2 1.3 16.6
Hzb
NOH2?
1.000
0.701
0.078
0.038
2.520
0.982
0.166
0.028
1.642
0.607
0.082
0.01 7
2.180
0.794
0.132
0.021
3.080
0.904
0.115
0.006
1.842
0.537
0.073
0.002
2.861
0.738
0.062
0.013
2.082
0.572
0.052
0.009
1.495
0.861
0.026
0.012
0.296
0.398 27.815
0.009
0.009
22.168
21.806
26.449
16.074
17.188
10.420
17.750
17.168 14.392
27.516
7.824
14.963
1.221
21.798 4.514
5.907
8.631
0.344
0.372
a Response expressed relative to chlorobenzene. 80 ml/rnin of hydrogen reaction gas. 80 ml/min of helium substituted to maintain constant residence time.
18.0 1.7 10.8
15.8 19.7 0.0 14.3
a All heights expressed as per cent of height at 810 "C (no H2 reaction qas).
conductivity detector. Detector response to a variety of chlorinated compounds is shown in Figure 5 . Two distinct curve types can be seen. Simple chlorinated aliphatics which can be readily dehydrochlorinated, such as chlorocyclohexane, give a convex curve. Compounds which can not readily undergo dehydrochlorination, such as chlorinated benzenes, give a concave curve. All but one of the pesticides (lindane) in Figure 5 have chlorine bound to a n unsaturated carbon, which would give them less aliphatic character. As might be expected, the average of the pesticides falls between compounds having only aliphatic and those having only aromatic C-C1 bonds. Pentachloroanisole follows the same degradation curve as the chlorobenzene series, since it contains only aromatic C-C1 bonds. The fact that trichlorotoluene follows the chlorobenzene degradation curve provides evidence for a dehydrochlorination mechanism for alkyl chlorines, since dehydrochlorination is an unlikely reaction for chlorine a to the aromatic ring in a,3,4-trichlorotoluene. Although the determination of the exact mechanism of selective detection was not the goal of this study, this suggests that the mechanism for selective detection of 2202
1,3,5-Trichlorobenzene 1,2,3,4-Tetrachlorobenzene 1,2,4,5-Tetrachlorobenzene Pentachlorobenzene Hexachlorobenzene Chlorocyclohexane 1,2,3,4,5,6-Hexachlorocyclohexane 1 -Chlorotetradecane Pentachloroanisole a,3,4-Trichlorotoluene
Hzb
NO H2?
chlorinated hydrocarbon pesticides in the presence of PCB is related to the bonding character of the C-C1 bond. The factors influencing the C-C1 bond character, such as steric and electronic effects, were not dealt with in this study. Thermal Degradation of Chlorinated Hydrocarbon Pesticides. If controlled thermal degradation is to be used as a means of selective detection, it should apply to a variety of chlorinated hydrocarbon pesticides. T o determine if this method was applicable to the common chlorinated pesticides, 12 pesticides and one metabolite were studied a t furnace temperatures of 810, 710 and 610 "C (no hydrogen reaction gas flow). Although mechanisms were not determined for certain, the stability of the different pesticides a t each temperature appears to depend on the mechanism of dechlorination. It can be seen in Table I that the DDT group ( p , p ' DDT, o,p'-DDE, and methoxychlor) changed very little in peak height from 610 to 710 "C, but increased significantly in peak height from 710 to 810 "C. One mechanism, perhaps a dehydrochlorination, takes place a t 610 "C (or below), producing a relatively stable compound which is further dechlorinated between 710 and 810 "C. The cyclodienes (heptachlor, heptachlor expoxide, a chlordane, y-chlordane, aldrin, and dieldrin), however, probably cannot readily undergo dehydrochlorination because of the ring strain produced in the resultant molecule. In Table I, it is seen that all the cyclodienes follow the same general pattern. At 710 "C, they give on the average of 34% of their peak height a t 810 "C. At 610 "C they drop to 14%. This suggests that for the cyclodienes, response a t 710 "C or below is due to the removal of only
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 13, NOVEMBER 1973
V
Table I I I. Selectivity of Chlorinated Hydrocarbon Pesticides over PCB as a Function of Reactor Temperature Minimum usable detectability
Temperature 610 " C 71 0 " C
810 "C
Chlorinated pesticides
5-10 ng 1-5 ng 0.2-0.5 ng
PCB
>50 1.19
> 5 Pg 10-20 ng
Selectivity >104:1 >103:1 500: 1
aliphatic chlorines. The thermal energy above 710 "C is sufficient for the removal of chlorine bound to unsaturated carbons as well. Response to Model Compounds. The chlorinated pesticides (with the exception of lindane) each contained chlorine bound to a variety of carbon atoms, such as aromatic, vinyl, allylic, and aliphatic. Although PCB contained chlorines bound only to aromatic carbons, each chromatograph peak contained more than one of the PCB isomers and each isomer represented a wide range of multiple substitutions and steric effects. Thus, it is extremely difficult to determine the influence of reaction conditions on a given type of C-C1 bond from the information derived from these compounds. Consequently, model aromatic and aliphatic chlorinated compounds were chosen for further study. Detector response to these compounds a t 700 and 800 "C with and without hydrogen reaction gas is shown in Table 11. When no hydrogen reaction gas was used, helium makeup gas was substituted a t the same flow rate. This circumvents any effects due to residence time. However, in view of the continuously deteriorating reaction tube surface and the difficulty in accurately reproducing gas flow rates (described previously), the data should be treated only qualitatively. Detector response for the compounds in this table is reported relative to the response for chlorobenzene a t a furnace temperature of 800 "C and 80 ml/min hydrogen reaction gas. Corrections were made for the quantity of chlorine contained in the compounds. Thus, a value of 1.000 for all compounds would indicate that they were all degraded to the same extent. Several interesting trends are shown by the data in Table 11. The chlorobenzenes are more resistant to thermal dechlorination than the chlorinated aliphatics. The formation of HC1 from the chlorobenzenes is significantly retarded by a decrease in temperature and the elimination of hydrogen reaction gas. In contrast, production of HC1 from the chlorinated aliphatics is enhanced by the elimination of hydrogen reaction gas and is little influenced by a temperature decrease of 100 "C. The substituted chlorobenzenes, pentachloroanisole, and cu,3,4-trichlorotoluene, exhibit characteristics of both the halogenated aromatics and the halogenated aliphatics. The dechlorination of these compounds is reduced by a decrease in temperature, but is slightly enhanced by the elimination of hydrogen reaction gas. It is also shown by these data that ortho substitution enhances thermal degradation ( i e . , 1,2-dichlorobenzene us 1,3-dichlorobenzene and 1,2,3-trichlorobenzene us. 1,3.5-trichlorobenzene),which is possibly due to steric strain. Quantitative Detection of Chlorinated Pesticides i n the Prerence of Polychlorinated Biphenyls. A reactor temperature of 710 "C appeared most advantageous for quantitative selective detection of chlorinated pesticides in the presence of PCB. At 710 "C (no hydrogen reaction gas), the minimum usable detectability for heptachlor was between l ng and 5 ng. At 610 "C, this dropped to between 5 ng and 10 ng. Response for all other chlorinated pesticides
0) u)
0
n v)
(z
Time Figure 6. Coulson conductivity detector response to 10 ng dteldrin ( A ) , 5 pg Aroclor 1254 ( B ) , and 10 ng dieldrin and 5 p g
Aroclor 1254 ( C ) :non-selective conditions Detector conditions: furnace temperature. 810 " C : cell voltage, 44 V dc: attenuation X8, no H? reaction gas. V = valve peak
l
v
l
v
I Time
Figure 7. Coulson conductivity detector response for 5 pg Aroclor 1254 ( A ) , 10 ng dieldrin and 5 pg Aroclor 1254 ( B ) , and 5 ng dieldrin and 5 pg Aroclor 1254 (C): selective conditions Detector conditions. furnace temperature. 710 "C, cell voltage. 44 V d c . attenuation X 4 , no H 2 reaction gas. V = valve peak
except lindane was less than for heptachlor. Under the same conditions a t 710 "C, none of the Aroclors (1221, 1254, or 1260) gave a measurable response for 5 ng. Although a furnace temperature of 600 "C was most advan-
ANALYTICAL CHEMISTRY, V O L . 45, NO. 13, N O V E M B E R 1973
2203
tageous for selectivity, 710 "C was most advantageous for quantitation of pesticides in the presence of PCB. The minimum detectability necessary for routine trace pesticide residue analysis was not attainable a t 600 "C (Table 111). The selectivity a t 710 "C of more than lO3:l for chlorinated pesticides in the presence of PCB is sufficient selectivity for normal residue levels found in environmental samples ( 11 ) . At a furnace temperature of 810 "C (0.5-mm i.d. quartz reaction tube), the Coulson conductivity detector gives the same response for 5 pg of Aroclor 1254 and 10 ng of dieldrin, a selectivity of 500:l. I t should be noted that a selectivity of approximately 50:l is obtained with the standard 4-mm i.d. reaction tube. At these levels, dieldrin cannot be determined in the presence of PCB (Figure 6). (11) L
M Reynolds, Residue R e v . 34, 27 (1971)
At 710 "C, however, 5 pg of PCB gives no response, while dieldrin response is decreased slightly (Figure 7). Determination of 5 ng of dieldrin in the presence of 5 pg of PCB (a selectivity of more than l O 3 : l ) offers few problems. I t should be noted that the chlorinated pesticides give a more slightly elevated response in the presence of PCB a t 710 "C than they do alone. This is believed to be due to PCB providing HC1 for saturation of the absorptive sites in the detector even though levels of PCB are too small to give a detector response. For routine analysis, this problem would probably not be encountered, since PCB levels should be much less than those used here Received for review March 26, 1973. Accepted June 1, 1973. Journal Paper No. 5072, Purdue University Agricultural Experiment Station, West Lafayette, Ind. 47907.
Effect of Carbonyl Structure and Reaction Temperature on Determination of Trace Carbonyls in Aliphatic Hydrocarbons M. W . Scoggins Research and Development D/vision. Phillips Petroleum Company. Bartlesville. Okla. 74004
Methyl ketones and aldehydes without branching at the a-carbon through C9 and most branched-chain isomers through C7 in aliphatic hydrocarbons can be determined using the heterogeneous 2,4-dinitrophenylhydrazone-e~traction method. Quantitative results are achieved at 25 "C by proper choice of reaction time and ultraviolet absorbance measurement at 345 nm. Carbonyls react with 2,4-dinitrophenylhydrazine according to their carbon skeleton structure as follows:
0 C-C-C-C-C-C-C-H
II
0
>
0 c
It
C-C-C-C-C-C-C
I1 I
C-C-c-c-c-c
>
0
II c-c-c-c-c-c-c
> 0
>
II
c-c-C-c-C-c-c
Extraction of carbonyl hydrazones by cyclohexane from aqueous 1M HCI solution saturated with 2,4-dinitrophenylhydrazine is rapid and quantitative in a single step except for formaldehyde, acetaldehyde, and acetone.
The determination of ketones and aldehydes is of major importance in the petroleum, food, biochemical, and related fields for the control of odor, corrosion, catalyst poisoning, and similar problems. While a number of titrimetric methods ( 2 ) are available to determine high concentrations of aldehydes and ketones, they are not applicable to the determination of trace concentrations in aliphatic hydrocarbons. Currently, the most widely used technique ( 1 ) Sidney
Siggia, "Quantitative Organic Analygis via Functional Groups," John Wiiey and Sons, New York, N.Y., 1963, Chap. 2 .
2204
for trace determinations of carbonyls is some modification of the 2,4-dinitrophenylhydrazine(DNPH) reaction. Early versions of the DNPH method (2, 3) consisted of' reaction of carbonyls with DNPH to form the 2,4-dinitrophenylhydrazones followed by addition of alcoholic alkali to form a red complex. The addition of alkali removed the spectral interference caused by unreacted DNPH by shifting the absorbance of the 2,4-dinitrophenylhydrazones into the visible region. However, the intensity of the color complex faded rapidly and its sensitivity in the visible region was only one-half that in the ultraviolet region. Toren and Heinrich ( 4 ) eliminated the spectral interference of unreacted DNPH in the ultraviolet region by the selective in situ extraction of the 2,4-dinitrophenylhydrazone product into isooctane. This technique was successfully used to determine a butadiene-furfural condensation product by room temperature reaction with DNPH in an aqueous phosphoric acid-ethanol-isooctane system. Continuous mixing was necessary in this heterogeneous reaction system because of solubility differences of the reactants in the two phases. At the suggestion of these authors, the method was modified in this laboratory by eliminating ethanol from the system and replacing the isooctane with cyclohexane to obtain a general method for the determination of trace concentrations of low molecular weight carbonyls, particularly acetone, in aliphatic hydrocarbons. Heistand ( 5 ) carried out a determination similar to our modified Toren-Heinrich analysis except for reaction temperature. Since heat was thought to be necessary to drive the DNPH coupling reaction to completion in a reasonable length of time, a reaction temperature of 60 "C was (2) G. R . Lappin and L. C. Clark. Anal. Chem.. 23,541 (1951). (3) M. F. Pooie and A. A. Klose, J . Amer. Oil Chem Soc.. 28, 215 (1951), (4) P. E. Toren and 6 . J . Heinrich, Ana/. Chem.. 27, 1986 (1955). (5) R . N. Heistand, Anal. Chim. Acta. 39,258 (1957).
A N A L Y T I C A L C H E M I S T R Y , V O L . 45, N O . 13, N O V E M B E R 1973
