Use Of Florasil As An Adsorbent In Chromatography

Notes

Sw = within batch standard deviation. Sb = between batch standard deviation. St = total standard deviation.

Notes

Sw = within batch standard deviation. Sb = between batch standard deviation. St = total standard deviation.

applied to a column (100mm x 4.2mm) of silica gel (Grace 950, 60- 200 mesh, dried at 30°C for 2h then deactivated with 5% of water). The column was then percolated (at 1mL min-1) in turn with hexane (20mL), benzene-hexane (3:2) (8mL), benzene (8mL) and ethyl acetate-benzene (1:1) (14mL). Each eluate was evaporated and the residue dissolved in 1mL of hexane and subjected to gas chromatography on OV-17 as stationary phase with electron-capture detection. Quantitative recovery was achieved for all insecticides except Malathion, Disulfoton, Dimethoate and Phorate.

9.2.1.2 Supercritical fluid chromatography

Bavel et al. [153] have developed a solid-phase carbon trap for the simultaneous determination of polychloro insecticides, PCBs, PCDDs and PCDFs in natural waters using supercritical fluid extraction liquid chromatography. Recoveries were in the range 57134%.

9.2.1.3 Voltammetry

Farwell et al. [154] used voltammetry to identify organochlorine insecticides, PCBs, polychlorinated naphthalenes, and polychlorinated benzenes. They list tables of reduction potentials for over 100 organochlorine compounds.

9.2.1.4 Miscellaneous

Millar et al. [17] carried out experiments to study a method for the recovery of 18 organochlorine pesticides and seven PCBs from water. Extractions with dichloromethane, and 15% dichloromethane in hexane, at pH2, 7 and 10 and liquid-solid column chromatography using columns of Florasil or alumina produced excellent results. An investigation was also made into the effects of pH, temperature, and residual chlorine on the preservation of spiked samples and recommendations are made for the most suitable storage conditions.

9.2.1.5 Preconcentration

Leoni et al. [155, 156] observed that in the extraction of organochlorine insecticides and PCBs from surface and coastal waters in the presence of other pollutants such as oil, surface active substances etc., the results obtained with an absorption column of Tenax-Celite (a porous polymer; trademark registered by Enka NV; developed by AKZO Research Labs, Arnhem, Netherlands) are equivalent to those obtained with continuous liquid-liquid extraction technique. For natural waters that contain solids in suspension that absorb pesticides, it may be necessary to filter the water before extraction with Tenax and then to extract the suspended solids separately. Analyses of river and estuarine sea waters, filtered before extraction, showed the effectiveness of Tenax, and the extracts obtained for organochlorine and organophosphorus pesticide analysis prove to be much less contaminated by the liquid-liquid technique. Leoni et al. [156] showed that for the extraction of organic micropollutants such as pesticides and aromatic polycyclic hydrocarbons from waters, the recoveries of these substances from unpolluted waters when added at the level of 1ppb averaged 90%.

Leoni et al. [155, 156] conclude that the extraction of insecticides from waters by adsorption on Tenax yields results equivalent to those by the liquid-liquid procedure when applied to mineral, drinking and surface waters that completely or almost completely lack solid matter in suspension. For waters that contain suspended solids that can adsorb some insecticides in considerable amounts, the results of the two methods are equivalent only if the water has previously been filtered. In these instances, therefore, the analysis will involve filtered water as well as the residue of filtration. Compared with liquid-liquid extraction, the main advantages of Tenax are the considerable amount of time saved, the possible automation of the process and the fact that gas chromatographic analysis shows the 'extracts' obtained with Tenax to be less contaminated by interfering substances. Another advantage of Tenax is that the product can be used 'as received', without preliminary treatment.

Gosink [157] and Aue [158] have also studied the determination of organochlorine insecticides and PCBs in water samples.

Sackmauereva et al. [14] used columns filled with silicic acid-Celite to separate organochlorine insecticides from PCBs. The PCBs were eluted with petroleum ether. To elute insecticides from the column they used a mixture of acetonitrile and hexane and methylene chloride.

C -silica-bonded phase columns [92, 159] have been used to prec1o8ncentrate chlorinated insecticides prior to gas chromatographic [159] and thin-layer chromatographic analysis [92]. Noy et al. [160] have discussed an on-line liquid chromatography-gas chromatography combination for the preconcentration of chlorinated insecticides, Noroozian et al. [94] have described a sample on-line technique involving sorption on a liquid chromatography micro precolumn packed with reversed-phase adsorbent followed by direct elution gas chromatography with hexane to determine a series of chlorinated insecticides and polychlorinated biphenyls in natural waters.

Amberlite XAD-2 resin [89, 161-163], cellulose triacetate membrane filters [164] and Tenax polymer [155] have all been used to obtain concentrated extracts of chlorinated insecticides and PCBs from water samples.

9.2.2 Sea water

9.2.2.1 Gas chromatography

Girenko et al. [165] noted that it was difficult to analyse samples of sea water because they are severely polluted by various coextractive substances, chiefly chlorinated biphenyls. To determine organochlorine insecticide residues by gas chromatography with an electron-capture detector, the chlorinated biphenyls were eluted from the column together with the insecticides. They produce inseparable peaks with equal retention times, thus interfering with the identification and quantitative determination of the organochlorine insecticides. The presence of chlorinated biphenyls is indicated by additional peaks on the chromatographs of the water samples and aquatic organic organisms. Some of the peaks coincide with the peaks of the o,p' and p,p' isomers of DDE, DDD and DDT and some of the constituents are eluted after p,p'-DDT.

9.2.2.2 Preconcentration

Organochlorine insecticides and PCBs have been preconcentrated from sea water on to a column of Tenax-Celite as discussed in Section 9.2.1.5 [155, 156].

9.2.3 Potable waters 9.2.3.1 Gas chromatography

The gas chromatographic method described by the Department of the Environment [130] discussed in Section 9.1.4.1 has also been applied to the analysis of chlorinated insecticides and PCBs in potable water.

9.2.4 Waste waters

9.2.4.1 Gas chromatography

A method has been described [166] for the determination of the following PCBs (Arochlors) at the nanogram level in 15:85 methylene chloride-hexane extracts of water and waste water: PCB-1016, PCB-1221, PCB-1232, PCB-1242, PCB-1248, PCB-1254 and PCB-1260. This method is an extension of the method for chlorinated hydrocarbons in water and waste water (described by Goerlitz and Law [167]). It is designed so that determination of both PCBs and the following organochlorine insecticides can be made on the same sample: Aldrin, DDT, Mirex, BHC, Heptachlor, pentachlorinitrobenzene, Chlordane, Heptachlor epoxide, Strobane, DDD, Lindane, Toxaphene, DDE, Methoxychlor and Trifluralin.

The PCBs and the organochlorine insecticides are coextracted by liquidliquid extraction and insofar as possible, the two classes of compounds are separated from one another prior to gas chromatographic determination. A combination of the standard Florasil column clean-up procedure and a silica gel microcolumn separation procedure are employed. Identification is made from gas chromatographic patterns obtained through the use of two or more unlike columns including SE-30 or OV-1 (3%), OV-17 (1.5%) or OV-210 (1.95%) on Gas Chrom. Detection is accomplished using an electron-capture, microcoulometric or electrolytic conductivity detector. Techniques for confirming qualitative identification are suggested by these workers. The interference in industrial effluents is high and varied and poses great difficulty in obtaining accurate and precise measurement of PCBs and organochlorine insecticides. Separation and clean-up procedures are generally required and may result in the loss of certain organochlorine compounds. Therefore, great care should be exercised in the selection and use of methods for eliminating or minimizing interferences. Phthalate esters, certain organophosphorus insecticides and elemental sulphur will interfere when the microcoulometric or electrolytic conductivity detectors are used in the halogen mode. Organochlorine insecticides and other halogenated compounds constitute interferences in the determination of PCBs. Most of these are separated by the method described. However, certain compounds, if present in the sample, will occur with the PCBs. Included are: sulphur, Heptachlor, Aldrin, DDE, Chlordane and Mirex.

Workers at the US Environmental Protection Agency [168] have evaluated protocols for chlorinated insecticides and PCBs in raw waste water. They concluded that the gas chromatographic method performed satisfactorily at the parts per billion level.

Millar et al. [17] have described a gas chromatographic method for the determination of 25 organochlorine insecticides and PCBs in waste water. Data are presented on extractability, recovery from clean-up columns, and preservation of samples for analysis.

Mclntyre et al. [169] determined organochlorine pesticides and PCBs by electron-capture gas chromatography. The waste water sample is extracted with hexane and the extract cleaned up on alumina-silver nitrate and silica gel columns prior to electron-capture gas chromatography.

Dunn et al. [170] used gas chromatography data to carry out pattern recognition for classification and determination of PCBs in waste waters and waste dump extracts.

Ericksen et al. [171] used high-resolution gas chromatography-electron-impact mass spectrometry to determine PCBs in halocarbon solvent process and product wastes.

9.2.5 Sewage effluents

9.2.5.1 Gas chromatography

Mattson and Nygren [172] devised a solvent extraction method for extracting PCBs from sewage sludge containing lipids. They point out that lipids and some other impurities in the crude extracts of sewage sludge can be destroyed by treatment with fuming sulphuric acid, either by shaking the acid [173] or by eluting on a fuming sulphuric acid-Celite column [174, 175]. Dieldrin is decomposed by this treatment but DDT and its metabolites, DDD and DDE, are not. Extracts of sewage sludges often contain large amounts of elemental sulphur, particularly after treatment with sulphuric acid. These interfere with early eluting compounds in the gas chromatographic step.

Sulphur was removed by the Bartlett and Skoog [176] method in which the sulphur is reacted with cyanide in acetone solution to produce thiocyanate. BHCs are decomposed to some extent, probably to pentachlorocyclohexane. An alternative procedure for the removal of sulphur utilizing barium hydroxide is also described. Alkali hydroxides should not be used as they cause dehydrochlorination of BHCs [173]. Lindane and its isomers are dehydrochlorinated to trichlorobenzenes

[177] and are eluted together with the solvent. Cochrane and Maybury

[178] have used the reaction with sodium hydroxide in methanol for the identification of BHCs. Dieldrin is not decomposed in the potassium hydroxide treatment and can thus be detected in the chromatogram of that aliquot. Some common chlorinated hydrocarbon pollutants and the internal standard hexabromobenzene were treated, according to the general procedure described above with sulphuric acid, potassium cyanide and potassium hydroxide. The results of the recovery experiments are shown in Table 9.14. When using packed columns, a precolumn of sodium and potassium hydroxides will give the same effect as the potassium hydroxide treatment described above [179]. Mattson and Nygren [172] have also tested a column with a packed alkaline postcolumn to remove the sulphur peak from the chromatogram. In the postcolumn, DDT and DDD are dehydrochlorinated but this does not affect their retention times. This method has good reproducibility and has a detection limit for the total amount of PCBs in the dried sample of at least 0.1mg kg-1 and for DDT, DDD and DDE limits of 0.01, 0.005, and 0.005mg kg-1, respectively.

Jensen et al. [180] have described a procedure for the determination of organochlorine compounds including PCBs and DDT in sediments and sewage sludge in the presence of elemental sulphur. The method can also be used for a search for both volatile and/or polar pollutants. The sulphur interfering in the gas chromatographic determination is removed in a nondestructive treatment of the extract with tetraburylammonium sulphite.

Table 9.14 Effect of treatment of a solution of chlorinated hydrocarbons and the internal standard hexabromobenzene with fuming sulphuric acid (I), fuming sulphuric acid plus potassium cyanide (II), and potassium hydroxide (III) expressed as percentages of the compounds in an untreated solution

Compound name (no.) Concentration Method of treatment

I II Itl

Table 9.14 Effect of treatment of a solution of chlorinated hydrocarbons and the internal standard hexabromobenzene with fuming sulphuric acid (I), fuming sulphuric acid plus potassium cyanide (II), and potassium hydroxide (III) expressed as percentages of the compounds in an untreated solution

Compound name (no.) Concentration Method of treatment

I II Itl

a-BHC(l)

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