D = gas diffusivity De = effective diffusivity of gas in packed-bed d = diameter of element = forced convection heat transfer coefficient h k = gas thermal conductivity k , = bulk gas mass transfer coefficient k, = effective thermal conductivity of packed-bed Npr = Prandtl number ( p C p / k ) N x , = Reynolds number (2Rup/p or dup/p) Ns, = Schmidt number ( p / p D ) q = heat flux R = radius of packed-bed = radial distance from element r T, = cavity wall temperature T, = .element temperature Tg = bulk stream gas temperature
t = time u = bulk stream gas velocity A X = distance between element and cavity wall y = mole fraction of sample component y , = mole fraction a t center of packed-bed y o = mole fraction in bulk carrier gas stream a = thermal diffusivity of packed-bed p = gas density p e = effective density of packed-bed p = gas viscosity TD = time constant for diffusion rT = time constant for heat transfer
RECEIVED for review January 4, 1968. Accepted May 23, 1968.
Gas Chromatographic Detector for Simultaneous Sensing of Phosphorus- and SulfurContaining Compounds by Flame Photometry Malcolm C. Bowman Entomology Research Diuision, Agricultural Research Service, U.S. Department of Agriculture, Tifton, Ga. 31794 Morton Beroza Entomology Research Division, Agricultural Research Service, U . S. Department of Agriculture, Beltscille, Md. 20705 A dual flame photometric detector that monitors gas chromatographic effluents with high sensitivity and specificity responds simultaneously to phosphorus compounds on one channel and to sulfur compounds on the other. The dual detection system consists of two light-sensing assemblies fastened to a central metal block that contains the burner for the column effluent. The assemblies are identical except that one includes a 526-mp interference filter to sense phosphorus compounds and the other a 394-mp filter to sense sulfur compounds. With the dual detector, the responses to 21 pesticides at three concentrations were determined. The phosphorus response is linear with concentration; the sulfur response is not. The response ratio, defined as the phosphorus response divided by the square root of the sulfur response, provides a means of estimating the atomic ratio of phosphorus to sulfur in a molecule. This response ratio (x 103) ranged from 5.2 to 6 for PS compounds, 2.8 to 3.3 for PS2 compounds, and 1.7 to 2.3 for PS, com pounds. ADVANTAGES OF USING two detectors instead of one to monitor
a gas chromatographic effluent have been demonstrated many times, and combinations such as electron capture-flame ionization and flame ionization-thermal conductivity are frequently used. Under fixed conditions, the two detectors give for each compound a characteristic relative response, which may provide an identification more certain than can be derived from the retention time only, or which may allow an evaluation of the purity of an eluting substance. A flame photometric detector devised by Brody and Chaney ( I ) for the analysis of either phosphorus- or sulfur-containing (1) S . S . Brody and J. E. Chaney, J. Gas Chromatogr., 4, 42 (1966).
1448
ANALYTICAL CHEMISTRY
compounds has been modified to accomplish both of these detections simultaneously. Brody and Chaney utilized a 526mp interference filter to sense phosphorus compounds and a 394-mp filter to sense sulfur compounds. Although they mentioned the desirability of having a detector for recording both responses, such a device has not yet been described. We have now combined the two detectors into a single unit. Because the two detectors monitor the same burning effluent from the gas chromatographic column, precise determinations of their relative response to various compounds may be made. Data obtained with 21 pesticides containing phosphorus and/or sulfur are presented to demonstrate the usefulness of the detector and to characterize its performance. Our major interest in the dual P/S detector is for pesticide residue analysis. The persistent insecticides, mainly chlorinated hydrocarbons, are being replaced by nonpersistent insecticides, and many of these contain phosphorus and sulfur. However, these insecticides may form metabolites having a different P/S content. By providing the relative P/S response, the new dual detector verifies the presence of phosphorus or sulfur in the molecule and can give an indication of the relative P/S content of a molecule as it emerges from a gas chromatographic column. The single flame photometric detector of Brody and Chaney has been invaluable in pesticide residue analysis (2-15). It (2) M. Beroza and M. C. Bowman, J . Agr. Food Chem., 14, 625 (1966). (3) M. C. Bowman and M. Beroza, J. Ass. Ofic.Anal. Chem., 49, 1046 (1966). (4) Ibid.,p 1154. ( 5 ) M. C. Bowman and M. Beroza, J . Agr. Food Chem., 15, 465 (1967). (6) Ibid.,p 651.
can detect as little as 0.1 ng of a phosphorus compound and several nanograms of a compound containing sulfur. The response is sufficiently specific to allow the analysis of most plant or animal tissues with little or no cleanup. As a n example of the detector's selective response, the response of parathion, which contains one phosphorus atom per molecule, was more than 130,000 times greater than that of aldrin, which contains no phosphorus (16). There appears to be no sacrifice in this selectivity with the dual detector. Furthermore, the dual detector may be used in temperature-programmed analyses as easily as the single detector (Y), thus permitting analyses to be conducted on environmental samples containing many pesticides or metabolites having widely differing volatilities. EXPERIMEWAL Dual Deteetor. A photograph of the dual detector is shown in Figure 1. It consists of two photomultiplier tube assemblies (available from Micro Tek Instruments, Inc., Baton Rouge, La.) attached to a central metal block. The detector differs from that of Brody and Chaney in that the mirror and metal plate opposite their photomultiplier tube assembly is replaced by a second window and photomultiplier tube assembly. One photomultiplier tube assembly contains a 526-mp interference filter and the other a 394-mp filter. Each photomultiplier tube has its own power supply, electrometer, and recorder, which will be referred to hereafter as a channel. Since the detectors sense the light emission of the Same burning effluent, splitting of the effluent stream, which is required in many dual systems and may be a source of error or instability, is neither required nor desirable. Pesticides. The 21 pesticides testeo, with one exception, were pure materials (usually analytical standards) obtained from their manufacturers. Table I gives their chemical names and compositions. Gas Chromatography. An F & M Model 700 instrument was equipped with a 240sm, 6-mm 0.d. (4 mm i d . ) glass column packed with 5 % DC-200 on 80/100 mesh GasChrom Q (Applied Science Labs., State College, Pa.). The flow rates of the nitrogen carrier gas, oxygen, and hydrogen were 160.40, and 200 mllminute, respectively. The temperature of the injection port was 240 "C, and that of the detector was 210 "C (external). The column was operated both isothermally and with temperature programming; temperature was programmed to rise 10 T j m i n u t e from 140 to 240 "C and was then maintained at 240 "C. With the dual detectors and associated apparatus connected, pesticides were injected in 5 p1 of benzene in the usual way. Unless otherwise stated, response was based on peak height. Balancing the Response of the Two Detectors. With the column operating isothermally at 190 "C, the response of parathion (retention time 3.00 minutes) was determined with the interference filters in one position and then reversed.
(7) M. C. Bowman and M. Beroza, 3. Ass. Ofic.Am/. Chem., 50, 926 (1967). (8) /bid., p 940. (9) /bid.,p 1228. (10) M. C. Bowman and M. Beroza, 3. Agr. F w d Chem., 16, 280 (1 968). (11) Ibid., p 399. (12) M. C. Bowman, M. Beroza, C. H. Gordon, R. W. Miller, and N.0. Morgan, 3. Econ. Enromol., 61,358 (1968). (13) C. Corley and M. Beroza, 3. Agr. Food Chem., 16,361 (1968). (14) J. C. Maitlen, L. M. McDonough, and M. Beroza, 3. Agr. FoodChem., 16, 549 (1968). (15) R. K. Stevens, 3. Ass. Ofic.Am/. Chem., 50, 1236(1967). (16) M. Beroza and M. C. Bowman, Enoiron. Sci. Tech., 2, 450 (1968).
I
Figure 1. Dual flame photometric detector Central metal block in which column effluentis burned holds two photomultiplier tube assemblies: one is equipped with a 526-mr interference filter and the other with a 394-mufilter
By this means it was determined that one channel produced a response that was 1.30 times the other. Because the sulfur response (Rs) is less than that of phosphorus ( R p ) , the more sensitive channel was assigned to the detection of sulfur, and the response to phosphorus was corrected in all subsequent analyses by multiplying it by 1.30. RESULTS AND DISCUSSION Brody and Chaney showed that the response of their phosphorus detector was linear with concentration; in our analyses, we found the peak height of at least 20 pesticides proportional to the concentration of pesticide to at least the 250-ng level. The response of their sulfur detector to parathion and malathion increased approximately as the square of the concentration. However, Brody and Chaney did not evaluate the relative response (RpjRs) characteristics of the detectors to the same compounds or the effect of column temperature or retention time on response. Separate injections of compound, which one tries to make equal, are needed to determine the relative response with single detectors. The dual detector overcomes this possible source of error by monitoring the light emission of the same flaming effluent with both detectors simultaneously. The ability to temperature-program the column (9) greatly facilitated the acquisition of data because a number of compounds with widely differing retention times could be evaluated in a single analysis. As an initial probe, it was desirable to demonstrate that the relative response of a compound containing phosphorus and sulfur would not remain constant if the amount of compound injected were the same but the retention time were changed. Thus, three concentrations (25, 100, and 250 ng) of parathion, which contains one phosphorus and one sulfur atom per molecule, were injected with the gas chromatographic column maintained isothermally at successive intervals of about 10 "C to secure the appropriate spread of retention times. The relative response to the three concentrations of compound on a log scale is plotted against retention time on a linear scale in Figure 2 (left). The relative response (RpjRs) varied both with retention time and the amount injected. When the relative response was similarly plotted against column temperature instead of retention time (Figure 2, right), the relationship was linear. The 21 pesticides were then chromatographed in several groups (all pesticides are not resolved on the same column) by using temperature programming with the dual channel detector. One pesticide with phosphorus and no sulfur (Gardona) and another with sulfur and no phosphorus VOL 40, NO. 10, AUGUST 1968
1449
Table I. Chemical Names and Compositions of Insecticides
Azinphosmethyl Bay 30911 Carbophenothion Ciba C-8874 Ciba C-9491 Coumaphos Diazinon Dursban EPN Ethion Fenthion Gardona Imidan Malathion Methyl parathion Methyl Trithion Parathion Phorate Stauffer N-2788 Sulphenone Zinophos
Molecular formula
Chemical name
Insecticide
0,O-dimethyl phosphorodithioate S-ester with 3-(mercaptomethyl)- 1,2,3-benzotriazin-4(3H)-one 0-2,4-dichlorophenyl 0-methyl methylphosphonothioate S-([(p-chlorophenyl)thio]methyl ) 0,O-diethyl phosphorodithioate 0-(2,5-dichloro-4-iodophenyl)0,O-diethyl phosphorothioate 0-(2,5-dichloro-4-iodophenyl) 0,O-dimethyl phosphorothioate 0,O-diethyl O-(3-chloro-4-methyl-2-oxo-2H-l-benzopyran7-yl) phosphorothioate 0,O-diethyl 0-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate 0,O-diethyl 0-3,5,6-trichlor0-2-pyridylphosphorothioate 0-ethyl 0-p-nitrophenyl phenylphosphonothioate 0,0,0’,0’-tetraethyl S,S’-methylenebisphosphorodithioate 0,O-dimethyl 0-[4-(methylthio)-rn-tolyl] phosphorothioate 2-chloro-1-(2,4,5-trichlorophenyl)vinyl dimethyl phosphate 0,O-dimethyl S-phthalimidomethyl phosphorodithioate S-[l,2-bis(ethoxycarbonyl)ethyl]0,O-dimethyl phosphorodithioate 0,O-dimethyl 0-p-nitrophenyl phosphorothioate S-{[(p-chlorophenyl)thio]methyl) 0,O-dimethyl phosphorodithioate 0,O-diethyl 0-p-nitrophenyl phosphorothioate 0,O-diethyl S-[(ethylthio)methyl] phosphorodithioate 0-ethyl S-p-tolyl ethylphosphonodithioate p-chlorophenyl phenyl sulfone 0,O-diethyl 0-2-pyrazinyl phosphorothioate
318 271
9.7 11.4
20.2 11.8
343
9.0
28.0
441
7.0
7.3
413
7.5
7.7
363
8.8
8.2
304 351 323 384 278 366 318
10.2 8.8 9.6 16.1 11.2 8.5 9.7
10.5 9.1 9.9 33.3 23.0 20.2
330 263
9.4 11.8
19.4 12.2
315 29 1 260 260 252 248
9.8 10.7 11.9 11.9 ... 12.5
30.5 11.0 37.0 24.6 12.7 12.9
...
Table 11. Data from Temperature-Programmed Gas Chromatography of Insecticides with the Dual-Channel (Phosphorus and Sulfur) Detection Systema Retention time Relative Iden100 ng injected 50 ng injected 25 ng injected to paratifithion RP X Rs X RP X Rs X RP X Rs X cation 10-9 10-8 10-9 RplRs 10-8 10-9 No. Insecticide Minutes (1.OO) RplRs RplRs 1 Zinophos
2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20 21
3.00 Phorate 3.70 Bay 30911b 4.05 Diazinon 4.60 Stauffer N-2788 5.30 Methyl parathion 5.30 Malathion 5.85 Dursban 6.15 Parathion 6.15 Fenthion 6.45 Sulphenone 7.15 Gardona 7.30 Ciba C-9491 7.50 Methyl Trithion 8.20 Ethion 8.55 Ciba C-8874 8.60 Carbophenothion 8.90 Imidan 9.80 EPN 9.90 Azinphosmethyl 10.35 Coumaphos 12.00
0.49 0.60 0.66 0.75 0.86 0.86 0.95 1 .oo 1 .oo 1.05 1.16 1.19 1.22 1.33 1.39 1.40 1.45 1.59 1.61 1.68 1.95
350 306 174 295 306 244 161 224 230 224 c
115 122 108 207 110 106 43.0 72.8 24.7 31.9
400 2450 95.0 286 1180 181 297 160 170 566 131 d
54.0 313 536 41.5 315 17.2 14.9 6.07 2.93
8.8
1.2 18 10 2.6 14 5.4 14 14 4.0 c
d
23 3.4 3.9 26 3.4 25 49 41 109
178
150 88.4 146 149 123 80.0
110 120 115 c
52.8 61 .O 52.5 109 55.4 50.1 21.2 36.1 12.2 15.6
93 505 26.0 66.4 256 46.5 63.2 38.3 47.0 150 33.9 d
13.6 69.2 146 11.2 64.5 4.52 3.90 1.37
“
19 3.0 34 22 5.8 26 13 29 26 7.7 c
d
45 7.6 7.5 49 7.8 47 93 89 e
88.8 74.2 44.0 75.4 71.2 58.6 40.2 54.5 58.0 55.6 C
26.5 30.3 25.8 49.4 27.5 24.8 10.4 17.9 6.00 7.60
22.0 118 7.00 19.0 64.4 11.1 16.7 10.0 11.o 36.3 10.3 d
3.41 19.2 30.5 2.69 18.9 1.0
40 6.3 63 40 11 53 24 54 53 15 c
d
89 13 16 102 13 104
e
e
e
(I
6
6
R p = response (amp) of phosphorus channel; RS = response (amp) of sulfur channel. Known to be impure. c Contains no phosphorus. No response detected. d Contains no sulfur. No response detected. e Insufficiently sensitive under these GC conditions. A short column, operated isothermally, to produce a short retention time is recommended for these concentrations. 5
b
1450
ANALYTICAL CHEMISTRY
1 1
'
I
PHOSPHORUS CHANNEL
SULFUR CHANNEL
""
I
\I
'I
240 1 2 ~
140
Ib'
0
c
2 rh
0 I t-5XPD"MP
4 8" Retention Tlme(minutes) I
'
1'2 "230' ' ' 200 ' ' ' " 1'70 Column Temperature ('C)
Figure 2. Ratio of phosphorus to sulfur response (&I&) of three concentrations of parathion on log scale us. retention time and column temperature on linear scale (Sulphenone) were included. Three quantities (100, 50, and 25 ng) of each were injected in 5 p1 of benzene. The responses and relative responses (RPIRs)are given in Table I1 with the compounds listed in order of increasing retention time. Absolute retention times and those relative to parathion are included. Composite chromatograms of the 50-ng quantities are shown in Figures 3 and 4. The phosphorus chromatograms are given as they were recorded-i.e., without applying the 1.30 correction factor. Peaks were attenuated as indicated to keep them on scale. I n reexamining the parathion data of Figure 2, it was observed that roughly straight-line relationships were obtained when the phosphorus and sulfur individual responses were plotted against retention time on a log-log scale. The slope of the sulfur line was twice that of the phosphorus line, suggesting that the square of the phosphorus response was proportional to the sulfur response, or the phosphorus response was proportional to the square root of the sulfur response. The latter relationship (Rp/Z/Rs),hereafter referred to as the response ratio, seemed to be the more convenient one to use, because the phosphorus response is linear with concentration and would provide a direct measure of concentration. When the response ratios of the compounds (containing both P and S) in Table I were compared with the phosphorussulfur content of their molecules, an interesting relationship appeared (see Table 111). The ratios of PS compounds ( X lo3) were within 5.2 to 6.0, ratios of PS2 compounds were within 2.8 to 3.3 (about I/*), and ratios of PS3 compounds were within 1.7 to 2.3 (about of the PS response ratio) regardless oj' concentration injected. Thus, the response ratio of a given compound is approximately equal to the response ratio of a PS compound multiplied by the atomic ratio of phosphorus to sulfur in the given molecule. The converse is also true; the response ratio may be used to provide a measure of the phosphorus-sulfur content of the molecule. Furthermore, for different concentrations of an individual compound, the range of the response ratios is much less than the overall range for all compounds having the same phosphorussulfur content. For example, the response ratios ( X 103) of parathion for the 3 concentrations tested were 5.5 to 5.6, those of Zinophos 5.5 to 6.0, and those of Ciba C-9491 5.2 to 5.3 (all are PS compounds). Because the data obtained with the different types of compounds studied (phosphates, phosphonates, thionophos-
-
2 04 FULL SCALE I X IO'AMP RECORDER RESPONSE -f
02
0
FULL SCALE -j
Figure 3. Dual-channel recording of sulfur and phosphorus response to 50-ng amounts of 12 pesticides in 5 pl of benzene in temperature-programmed gas chromatography
PHOSPHORUS GHANNFI
240 12
SULFUR CHANNEL
coumaphos EPN
x2X4-Othlan
-
Gardona
x4
P
methyl porathlon
180
4
X4-
XlO
140
Boy
-
30911
-
~
Zinophos
0
I
b 5 X IO'AMP
2
0.4
FULL SCALE -+ +-I X IO'AMP RECORDER RESPONSE
02
0
FULL SCALE 4
Figure 4. Dual-channel recording of sulfur and phosphorus response to 9 pesticides as described in Figure 3
Table 111. Response Ratios (Rp/dR> of Compounds Containing Phosphorus and Sulfur Response ratio X lo3 at concentration Identification PS (ng) indicated Pesticide number content 100 50 25 1 Zinophos PS 5.5 5.8 6.0 2 Phorate 2.0 2.1 2.2 ps 3 3 Bay 30911 PS 5.7 5.5 5.3 4 Diazinon PS 5.5 5.6 5.3 5 Stauffer N-2788 2.8 2.9 2.8 ps* 6 Methyl parathion PS 5.8 5.7 5.6 7 Malathion 3.0 3.2 3.1 ps2 8 Dursban PS 5.6 5.6 5.5 Parathion PS 9 5.6 5.5 5.5 Fenthion 10 PSp 3.0 3.0 2.9 13 PS 5.3 5.2 5.2 Ciba C-9491 14 Methyl Trithion 1.9 2.0 1.9 PSI 15 Ethion 2.8 2.8 2.9 P*Sa 16 PS Ciba C-8874 5.4 5.2 5.3 17 Carbophenothion 1.9 2.0 1.8 ps3 Imidan 18 3.3 3.2 3.3 ps2 EPN 19 PS 6.0 5.8 ... 20 Azinphosmethyl 3.2 3.3 , , , ps2 21 Coumaphos PS 5.9 ... ...
VOL. 40, NO. 10, AUGUST 1968
1451
Table IV. Data on Gas Chromatography of Phorate and Five of Its Metabolites with Phosphorus-SulfurDetectora Rs X lo-'
rE x 103
562
2.27
RP
Retention time (min)
PS content
(GH60)nP-SCHzSCHzCHa (phorate) S 0
1.90
PSS
(CZH~O)~P-SCHZSCHZCHS S 0
4.90
50.4
5.00
66.0
Compound
R p
X IO-*
8
S
II
II
II
/I
I1
(GH~O)ZP-SCH~SCH~CHS
I1
170
87.5
1.70
101
2.08
402
3.33
0
'0
11
(CZH~O)ZP-SCH~SCH~CH~ 0
0
I/
11
/I
I/ /I
(GH~O)ZP-SCHZSCHZCHS 0 0 (C~H~O)~P-SCHZSCHZCH~
1.40
P8
21 1
3.65
PSI
42.4
22.5
2.83
3.65
PS2
56.6
30.0
3.27
0 Analyzed by gas chromatography with a 90-cm, 10% DC-200 column operated isothermally at 180 "C. Other parameters were those given in text.
phates, thiolphosphates, and dithiophosphates) showed no observable relationship between response and chemical structure, we analyzed a set of closely related compounds for special effects. The compounds were phorate and five of its metabolites, which differ only at two points; they have either a P=S or P=O group and either a sulfide (S), sulfoxide (SO), or sulfone (SOn)group. The results obtained from separate injections of these compounds are given in Table IV. The R p / l / R s ratios were within the range expected for PS2 and PSa compounds; however, the response ratios of the two sulfoxides were much lower than those of the corresponding (P=O or PES) sulfides and sulfones, and the relative responses of the sulfides were slightly greater than those of the sulfones. The sulfoxide differences, which were significantly large, could prove useful for identification, but these results must be regarded as no more than preliminary at this stage. Experiments were conducted to determine whether reliable R p / d & r a t i o s may be determined with a single flame photometric detector by changing the filters, and the results were good. This procedure requires twice as much time, and care must be exercised to assure the reproducibility of injection and gas chromatographic parameters. Cross interference between the channels was checked by
145 2
ANALYTICAL CHEMISTRY
injecting 50 ng each of Gardona (has P and no S) and Sulphenone (has S and no P) and noting the response on both channels. With Gardona, the response of the phosphorus channel was 800 times that of the sulfur channel. With sulphenone, the response of the sulfur channel was 22 times that of the phosphorus channel. If we consider the amount of sulfur in Sulphenone roughly comparable to the amount of phosphorus in Gardona, the phosphorus channel responds to sulfur about 36 times more readily than the sulfur channel does to phosphorus. The response to phosphorus on the sulfur channel was linear with concentration and the response to sulfur on the phosphorus channel was nonlinear. The performance of the detector in pesticide residue analysis was demonstrated by injecting the equivalent of 25 mg of extract of corn silage fortified with 50-ng amounts of eight of the insecticides. As has been previously reported with the single flame photometric detectors (9), no significant interference from the extract was observed.
RECEIVED for review February 15, 1968. Accepted May 20, 1968. Mention of proprietary products is for identification only and does not imply endorsement of these products by the U.S. Department of Agriculture.
