Selective Emission Spectrometric Determination of Nanogram Quantities of Organic Bromine, Chlorine, Iodine, Phosphorus, and Sulfur Compounds in a Helium Plasma Carl A. Bache and Donald J. Lisk Pesticide Residue Laboratory, Cornell University, Ithaca, N . Y. A microwave-powered helium plasma has been used to fragment and excite organic bromine, chlorine, iodine, phosphorus, and sulfur compounds eluting from a gas chromatograph. This intense source results in production of many atomic lines of these elements. The most sensitive and selective line of each element has been determined. Elemental emission response is quantitative for each element in a variety of organic compounds. The detection system has been applied to the selective analysis of drugs and pesticide residues. THEGAS CHROMATOGRAPHIC DETECTION of organic compounds containing sulfur, phosphorus, and halogens by emission spectrometry in a microwave-powered argon plasma was originally reported by McCormack, Tong, and Cooke (I). The application of this detection system to analysis of phosphorus-(2, 3) and iodine containing ( 4 ) pesticide residues has also been described. Although band emission occurred, the production of atomic lines for sulfur, chlorine, and bromine in an argon plasma was not observed ( I ) . The possibility of inducing atomic emission in these elements using helium (with its higher metastable energy) in place of argon was considered. Tong ( 5 ) experimented with a microwaveinduced helium plasma but finally chose argon owing to the ease of plasma initiation and operation at atmospheric pressure. In the work reported, the use of a helium plasma was found to initiate effectively atomic emission in sulfur, chlorine, and bromine as well as iodine and phosphorus with the production of sensitive lines for their selective quantitative determination in eluted organic compounds.
Identification of Atomic Spectral Lines in Figure 1
Table I.
Phosphorus lines in Phosdrin 1 2 3 4 5 6 7 8 9
Sulfur lines in dimethylsulfoxide
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ANALYTICAL CHEMISTRY
5432.8 5453.8 5606.1 5640.0
Chlorine lines in Lindane 14 15 16 17 18
4794.5 4810.1 4819.5 5423.3 5423.5
Bromine lines in carbon tetrabromide 19 20 21 22 23
4678.7 4704.9 4785 5 4816.7 5182.4 ~
Iodine lines in diiodomethane 24 25 26 27 28 29 30 31 32
4666.7 5161.2 5338.2 5345.1 5407.4 5435.8 5464.6 5496.9 5625.7
Table II. Sensitivity and Selectivity of Most Intense Elemental Atomic Lines
Element (1) . , A. J. McCorrnack. S. S. S. Tong, -. and W. D. Cooke, ANAL. CHEM., 37, 1470 (1965). (2) C. A. Bache. and D. J. Lisk. Ibid.. . 37., 1477 (1965). . , i3j Ibid.,38, 1757 (1966). (4) Ibid.,p. 783. (5) S. S. C. Tong, Ph.D. Thesis, Cornel1 University, Ithaca, N.Y., ( 1966).
2136.2 2149.1 2478.57 (atomic carbon) 2534.0 2535. 7 2553.3 2554.9 4587.9 4602.0
10 11 12 13
EXPERIMENTAL Apparatus. The equipment used was identical to that described earlier (3) except that a 1.5-mm bore, 8-mm 0.d. quartz discharge tube and helium carrier gas was used. The thick tube wall provided sufficient heat capacity to prevent its deterioration by the intensely hot plasma. The plasma was operated at a pressure of 5 to 10 mm (as measured at the exit of the discharge tube) and 70% power. The helium plasma could only be initiated at pressures in this range. Although the plasma can be sustained at higher pressures, the narrowbore quartz discharge tube rapidly deteriorated at pressures above 50 rnm. A slit width of 50 microns was used consistently except for spectral scanning (10 microns). A column 6 feet long consisting of 10% DC-200 on 80-100 mesh GasChrom Q operated at various isothermal temperatures between 130" and 210" C was used throughout this study.
Wavelength, A
Line number
Bromire Chlorine Iodine Phosphorus Sulfur
Wavelength, 4785.5 4794.5 5338.2 2535.7 5453.8
A
g element/sec
Selectivity ratio us. phenanthrene
2 x lo-" 6 X 10-11 5 x 10-11 9 x lo-'* 5 x lo-"
38 44 38 lo00 22
Sensitivity,
Figure 1. Spectra showing designated elemental atomic lines when scanning Phosdrin (P), dimethylsulfoxide (S), Lindane (Cl), carbon tetrabromide (Br), diiodomethane (I), and Dibrom (Br, C1, P)
RESULT!; AND DISCUSSION
lengths. Line 3 was attributed to atomic carbon (6)and was also observed in an argon plasma (1). Lines 17 and 18 (chlorine) were not resolved. No atomic lines for fluorine were observed when organofluorine compounds were scanned. All of the above compounds were scanned from 2000 to 7000 A. Only those portions of the spectra shown above were presented, as these contained the greatest number of atomic lines (including those most sensitive and analytically usable) and the least background emission. Many other
Figure 1 illustrates the recorded spectra of 2-carbomethoxy1-methyl-vinyl dimethyl phosphate (Phosdrin) showing atomic phosphorus lines (numhered) in the ultraviolet and visible regions. Similarly, atomic lines of sulfur, chlorine, bromine, and iodine are shown when scanning, respectively, dimethylsulfoxide, gamma-hexachloro-cyclohexane (Lindane), carbon tetrabromide and diioclomethane. The bottom spectrum shows production of bromine, chlorine, and phosphorus lines when scanning O,O-dimethyl-O-(l,2-dibromo-2,2-dichloroethyl) phosphate (Dibrom). This spectrum was scanned more slowly to separate closely occurring bromine and ( 6 ) R. W. B. Pearse and A. G. Gaydon, “Identification of Molecuchlorine lines. Table I lists all numbered lines and wavelar Spectra,” Wiley, New York, 1963. VOL 39. NO. 7 , JUNE 1967
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NANOGRAMS ( QUIV) SULFUR INJECTED
2
8
40
7
$5
120
A60
Figure 2. Area response curves for halogen, phosphorus, and sulfur compounds
Y
known atomic lines (7), particularly iodine, of these elements were observed at lower signal attenuations. Identification of the numbered lines was made by reference to literature values (7). These lines do not appear in the spectra of compounds in which the particular element is absent. The unnumbered lines result from excitation of helium, its impurities, hydrocarbon, and other hetero element emission, and recombination products of all of the above. Table I1 lists the most sensitive usable line for each element and its selectivity ratio when compared to phenanthrene. These lines were chosen on the basis of their intensity and the absence of intense background emission (as judged by inspection and selectivity ratio) and were used consistently for obtaining the data presented for these elements in the remainder of the study. The sensitivity was determined a t a
3&-!e d -1B
40
I5 2b 23 --30 35 WANOGRAMS (EQUIVALENT) ELEMENT INJECTED
4L
(7) W. R. Brode, “Chemcal Spectroscopy,” Wiley, New York, N. Y . , 1949.
Table 111. Coordinate Data for Area Response Curves (Figure 2) Equivalent Peak area, Equivalent Peak area, Ng element arbitrary Ng element arbitrary Compound injected units Compound injected units Phosphorus (2535.7 A) Bromine (4785.5 A) O,O-Diethyl 0-(2-isopropyl-Cmethyl1-Bromodecane 3.5 1.3 dpyrimidinyl) phosphorothioate 1,2-DibromoethyIbenzene 3.6 1.5 (Diazinon) 2.5 C1Methoxy-3,5-dibromobenzonitrile 1.7 0,O-Diethyl S-(ethylthiomethyl) (Bromoxynil methyl ether) 6.6 3.0 phosphorodithioate (Thimet) 2.1 2.0 3.1 1,2,4,5-Tetrabromobenzene 8.1 O,O-Diethyl S-Z(ethy1thio)-ethyl 3,5 Dibromo-Cmethoxymethylbenzoate 9.9 4.9 phosphorodithioate (Disyston) 3.2 3.3 13.7 CBromobiphen yl 6.5 S-[l,2-Bis(ethoxycarbony1)ethyl] 0,O13.9 2,CDibromonaphthol methyl ether 30.4 dimethyl phosphorodithioate 4,4’-Dibromobiphenyl 41.0 19.6 5.3 (Malathion) 8.3 Chlorine (4794.5 A) O,O-Diethyl 0,pnitrophenyl thio2-Chloro-isopropylamino-6-ethylaminophosphate (Parathion) 7.4 5.5 s-triazine (atrazine) 6.6 4.1 O,O-Diethyl S-p-chlorophenylthiomethyl Hexachlorocyclohexane gamma isomer phosphorodithioate (Trithion) 16.7 11.4 4.3 (Lindane) 7.3 0,0,0 ’,O ’-Tetraethyl S,S‘-methylene 1,4,5,6,7,8,8a-HeptachIoro-3a,4,7,7a16.0 bisphosphorodithioate (Ethion) 21.3 tetrahydro-4,7-endomethanoindane 25.5 20.0 (Heptachlor) 10.0 5.5 O,O-Dirnethyl-0-(2,4,5-trichlorophenyl) 9.9 phosphorothioate (Ronnel) 5.7 Sulfur (5453.8 A) 1,3,4,5,6,7,8,8-0ctachloro-l,3,3a,4,7,7ahexahydro-4,7-methanoisobenzofuran 0,O-Diethyl S-2(ethylthio)-ethyl (Telodrin) 13.8 7.7, 8.2 8.3 phosphorodithioate (Disyston) 1.3 6,7,8,9,10,10-Hexachloro-l,5,5a,6,9,9a0,O-Diethyl S-(ethylthiomethyl) hexahydro-6,9-methano-2,4,3-benzo6.8 phosphorodithioate (Thimet) 1.4 dioxathiepin-3-oxide (Thiodan) 20.9 10.6 O,O-Dirnethyl-O-(2,4,5trichlorophenyl) CChlorophenyl-4chlorobenzene9.6 phosphorothioate (Ronnel) 1.6 18.7 sulfonate (Ovex) 11.8 O,O-Diethyl Opnitrophenylthio1,l-Dichloro-2,2-bis(4-chlorophenyl) 1.6 10.4 phosphate (Parathion) ethylene (p,p‘-DDE) 26.8 1.59 5-Ethyl-5-(l-methylbutyl)-2-thio1,2,3,4,10-10-Hexachloro-6,7-epoxy3.2 23.2 barbituric acid (Thiopental) 1,4,4a,5,6,7,8,8a-octahydro-l,4,5,86,7,8,9,10,10-Hexachloro-l,5,5a,6,9,9adimethanonaphthalene (Dieldrin) 27.9 16.5 hexahydro-6,9-methano-2,4,3-benzoIodine (5338.2 A) 37.7 dioxathiepin-3-oxide (Thiodan) 3.4 2-Iodochlbrobenzene 3.8 O,O,O’,O’-Tetraethyl S,S’-methylene 1.0 28.0 3.5 3-Iodomethylbenzoate 4.8 bisphosphorodithioate (Ethion) 1.1 1-Idodecane 6.0 1.5 2 42-Dimethylaminoethyl)-2-thenyl36.8 4.3 7.5 amino] pyridine (Methapyrilene) 1.9 2-Iodobiphenyl 17.0 0.0-Diethyl S-pchlorophenylthiomethyl 4.2 61.1 7.7 6.2 2,5-Diiodomethylbenzoate 32.7 phosphorodithioate (Trithion) 2,3,5-Triiodomethylbenzoate (TIBA 2-Chloro-10-(3-dimethylarninopropyl) 5 1. O 18.1 methyl ester) 44.5 phenothiazine (Chlorpromazine) 8.5
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Table IV.
Recovery of Pesticides from Agricultural Samples ReAdded, Elemental covery, Compound Sample ppm emission % Bromoxynil Wheat 0.20 4785.5 A (Br) 77 Disyston Soil 0.17 5453.8 A ( S ) 78 Thimet Sugar beets 0.15 5453.8 A (S) 91 Thiodan Potato 0.20 4794.5 A(C1) 74 CONTROL
4
8
12
i s 2 0
MIg2TEs2i
32
36
4b
44
48
4 8 MINUTES
Figure 3. Gas chromatograms of pesticides added to agricultural samples
signal to noise ratio of :1 to 1. Because the helium plasma could only be initiated a i d maintained at pressures below 1 cm, it was not possible tD study the effect of higher discharge pressure on sensitivity (3). Figure 2 illustrates the linearity of response of the detector for different organic compounds containing the particular element in varying weight percentages and organic linkages. Table I11 identifies the compounds used and their coordinates. This quantitative responsi: should prove valuable, for instance, in identifying and quantitating the main product and side products in a reaction rnixture when each contains one or more of the atoms of the s,bove elements. Figure 3 shows gas chromatograms representing the recovery of pesticides added to various agricultural samples.
Table IV presents the per cent recoveries and the respective emission lines which were measured. Thiodan was extracted from potatoes by blending with acetonitrile and partitioning the filtered aqueous-acetonitrile solution with hexane according to the official method of Mills (8). The hexane solution was concentrated and directly injected for analysis. The chromatogram shows the typical principal thiodan peak (31 minutes) and the second minor peak (44 minutes). Recovery of the thiodan was based solely on the area of the principal peak. Bromoxynil was extracted from wheat and isolated prior to gas chromatography by the procedure used for ioxynil in wheat ( 4 ) . Disyston was extracted from soil with chloroform (9) and the filtrate was evaporated. The residue was redissolved in ethyl acetate and injected for analysis. Thimet was extracted from sugar beets by blending with acetone and partitioning the filtered, aqueous extract with chloroform. The chloroform solution was concentrated and injected. By allowing about a minute between sample injection and discharge ignition, the solvent escaped, thus preventing it from extinguishing the discharge. No visible carbon deposits formed in the discharge tube even after three months of continuous use. The production of sensitive selective atomic lines for analysis of the elements studied should prove very valuable for quantitative and qualitative analysis of most pesticides as residues or metabolites and in drug research. Work is now under way to apply this detection system to analysis of many pesticides and metabolites in a variety of biological samples. RECEIVED for review January 30, 1967. Accepted March 27,1967. (8) P. A. Mills, J. Onley, and R. Gaither, J. Assoc. Ofic.Agr. Chemists, 46, 186 (1963). (9) C. A. Bache and D. J. Lisk, J. Assoc. O@c. Anal. Chemists, 49, 647, (1966).
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