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Water Sample Preparation Techniques for Capillary Gas Chromatographic Analysis
759
WATER SAMPLE PREPARATION TECHNIQUES FOR CAPILLARY GAS CHROMATOGRAPHIC ANALYSIS
M. TERMONIA State Secretariat for Agriculture, Institute for Chemical Research Museumlaan 5, B - 1980 Tervuren, BELGIUM
ABSTRACT Recent improvements of well-known sample preparation techniques, together with new methodologies a~ presented. Their possibilities for different kinds of samples are reviewed. Problems associated with the use of standard on-line techniques are discussed in relation to vaporizing and on-column sample introduction methods. On-line selective sampling techniques such as headspace analysis and multidimensional gas chromatography are shown to be best suited in the case of samples with complex matrices. Finally, off-line selective sampling methodologies are reviewed. Micromethcds, especially designed for capillary gas chromatography are described including sample purification, fractionation of preconcentration by short liquidchromatography columns, steam-distillation extraction, liquid-liquid extraction, closed loop stripping, and pyrolysis g s chromatography.
1. INTRODUCTION
The introduction of wallcoated open tubular columns for gas chromatography by Golay in 1957 has led to a considerable increase in separation performances of the chromatographic techniques used in water analyses [ 1--3 1. Column technologies have been developed to such an extent that sample preparation (iacluding sampling and injection techniques) now most often appears as the limiting step which determines the quality of the global analytical method.
760 Accordingly, assuming that the capillary gas chromatographic (c.g.c.) columns were optimized in terms of separation efficiency, thermal stability, stationary phase selectivity, and adsorptivity towards the sample components, and that a proper detection system was selected, there is only one way left to improve the gas chromatographic analysis, i.e., through sample preparation. This is particularly true for samples from the aquatic environment because of the high complexity of the sample matrices involved, which often impart the use of the most selective detectors (even mass spectrometry operating in the single ion monitoring mode) for the determination of single components, especially at the trace level (ppb and beyond).
As many recent.reviews of standard water sample preparation techniques are available [4--7 1 , this contribution will only present recently improved well-known methodologies or new developments, in relation with the following sample characteristics: --
vapour pressure of the sample components;
-- concentration range of the analytes;
--their polarity and overall chemical stability; --the nature of the matrix; --
the nature of the interactions of the components with the matrix.
This presentation will be organized according to the following sequence: -- on-line standard techniques; --
on-line selective sampling techniques; and
-- off-line techniques.
2. ON-LINE STANDARD TECHNIQUES 2.1. VAPORIZING INJECTORS
In the analytes have high vapour pressure (already in the gaseous phase at room temperatures) or amenable to the gas phase by vaporizing the sample at convenient temperatures and without thermal degradation into a heated injection chamber, then standard sampling techniques such as split of splitless injection can successfully be applied, provided a few precautions are taken. With non-specific detection methods such as flame ionization, they apply only to components in concentrations above the ppm level. Although these techniques are the most popular, some problems can be encountered in quantitative analysis due to solvent effects and recondensation phenomena occuring i n the column inlet. These were systematically described by Grob and co-workers [ 8-13] -
761 In particular, it has been shown that both the nature of the solvent and the column inlet temperature during the injection is of primary importance in split injection. By influencing the true splitting ratio, they give rise to erroneous results when working with external standards which are not dissolved in the same solvent as the sample or when the column temperature is not precisely reproduced [ l l ] . Temperature of the column inlet again plays an important role in splitless injection where solvent effects can seriously affect the results. During the injection, the oven temperature has to be properly adjusted in function of the volatibility of the solvent, especially for the volatile components eluting just after the solvent peak [12, 131 .
For low volatility components such as polychlorobiphenyls or tetrachloro-paradibenzodioxins (TCDD), it is possible to make use of a solvent effect in order to perform oncolumn enrichments. Thus, Poy [ 141 injected 4 times 2 pl of a sample containing TCDD isomers without destroying the separation performance. The low boiling components were strongly retained in the thick solvent layer formed when the solvent recondensed so that all 4 injections refocused in the column inlet before the chromatographic process took place. An interesting new development in injector design is the PTV (Programmed Temperature Vaporizer) [15, 161 which allows for solvent backflush in the injector. With this system, the sample can be injected at low temperature onto an adsorbing cartridge and the non-adsorbed sample components are backflushed from the injector to vent. The temperature is then raised, vaporization of the analytes occurs and injection into the analytical column is performed. In another working mode, the same injector can be used for retention of low or nonvolatile cornpounds on the Same adsorbing cartridge, avoiding the accumulation of high voiling cornpounds or inorganic material in the column inlet. 2 3 . DIRECT (ONCOLUMN) INJECTORS
It has been shown that direct injectors, such as the cold oncolumn where the liquid sample enters the inlet of a capillary column, are able to produce quantitative results with improved reproducibility and accuracy [17, 18 1 . Moreover, nonderivatized compounds with low physicochemical stability can efficiently be analyzed using this type of injector.
It is particular concern in the analysis of aqueous samples, because the solvent effects involved during the direct aqueous injection (DAI)allow the 'direct analysis of levels (pg/l) of trihalomethanes and related compounds in water, using well deactivated thick film columns and ECD detection [19,20 1 . Other applications of DAI describe the analysis of lindane in water at the 5mg/L level El] and the determination of free carboxylic acids at the mg/Llevel [22].
762
3. ON-LINE SELECTIVE SAMPLING TECHNIQUES 3 -1. HEADSPACE ANALYSIS
When the sample matrices are so complex that t..ey interfere with the analysis, sample preparation of the clean-up become necessary in order t o pre-separate the components. The different fractions obtained can then be separately prepared and analyzed, according t o their chemical properties. However, if the vapour pressure of the analytes is important and if they are not too tightly bound to the matrix (no chemisorption), headspace analysis is possible [23-25 1’ . Static equilibrium headspace can successfully apply t o the determination of high volatility compounds present in complex matrices such as sludges from water treatment stations [ 24, 261 . This can be operated automatically, with a high sample throughput using commercial headspace samplers. The samples are either introduced by means of a syringe (Carlo Erba), of a gas sampling valve (Dani, Siemens), or by a contact of programmable duration between the headspace sample and the carrier gas (Perkin Elmer). The uses of headspace analysis in connection with capillary columns is well documented [24--27 1 and are the subject of important developments. With this technique coupled to a g.c. equipped with thick film columns and ECD detection, it is possible to analyze trihalomethanes in aquatic media with a detection limit of about 1 ppb, expressed for CHCI3, when L ml of headspace gas is injected in the split mode (1 : 4) [ 22 ] . In principle, it is possible to enhance sensitivity by a factor of 100 or more if larger volumes are injected, using a cold trapping effect at the g.c. column inlet. This can be done by cryofocusing with liquid nitrogen[ 281 . However, it has been shown that for highly volatile compounds (with b.p. 7OoC), very long capillary cold-traps or packed capillary traps are needed to avoid sample loss caused by breakthrough of volatiles [29,30 1 . Determination of volatiles at the trace level is also possible by preconcentrating the headspace volatiles on a suitable adsorbent. The trapped compounds are subsequently recovered by thermal desorption in front of a cooled trap connected t o the capillary column or by solvent elution followed by splitless or on-column injection. These methods, called “dynamic headspace enrichment” or “ purge-and-trap”, have been applied to trace level analysis of volatiles, using conventional electrically heated systems [31, 32 1 , a Curie-point Pyrolyser [33 1, or a microwave cavity [34] as thermal desorbers. 3 2 . PRESEPARATION BY G.C.
For compounds of intermediate volatility (typically preseparation of the samples is more conveniently performed by means of packed pre-columns.
763
As chromatography is the most efficient separation method which can be applied to these samples, it has been shown that the on-line coupling of such columns can successfully apply to water analysis 35 1 . This method is derived from the multi-dimensional gas chromatography (MDGC) technique. Double oven instruments were recently developed which are specifically' dedicated t o this type of MDGC analysis &I. In such a system, the pre-column acts as a real g.c. column for which the temperature can be independently programmed. A fraction of the g.c. effluent from this column can be cold trapped in the inlet of a second column, of the analytical type, most often a capillary one with high separation power. In certain difficult cases, it is worthwhile to replace the packed pre-column by a capillary column of high capacity.
4. OFF-LINE SELECTIVE SAMPLING TECHNIQUES 4.1 - SAMPLE PREPARATION ON SHORT
L.C. COLUMNS
The samples may also be prepared off-line using cartridges which can be filled with any standard packing material used in liquid chromatography. The sample components are recovered by solvent elution and subsequently analyzed according to a standard technique such as those described in section 2. Carbopack F, C, or B traps have been used for the enrichment of polyaromatic hydrocarbons from water samples [ 3 7 1. The water to be analyzed is forced through the trap which is finally extracted with toluene. Recoveries higher than 86% were reported for compounds ranging from acenaphtylene to benzo g, h, i perylene. Micro adsorption cartridges, capable of fractionation, purification or enrichment can also be introduced directly into the body of a syringe, allowing sample preparation to occur during the injection [38 1. Milligram amounts of a material in which partition can take place (A1203, charcoal, Florisil, Lipidex 5000, --) are sufficient to handle the ng amounts needed for high performance c.g.c. 4 2 . STEAM-DISTILLATION-EXTRACTION
Samples from aquatic environments (sediments, fish), containing appreciably high amounts of fatty material can efficiently be prepared by Steam-Distillation-Extraction (SDE). In particular, pesticides and herbicides which have appreciably greater vapour pressure than those of water soluble chemicals can be extracted with high recovery yields using different SDE units [39-431 . The produced extracts are generally suitable for direct g.c. analysis, without concentration and cleanup procedures. A closed loop version of this technique has
7 64
successfully been applied to nonylphenols and nonylphenolethoxylates in secondary sewage effluents [44 I .
A recently introduced microversion of the Likens and Nickerson SDE apparatus has been shown to be able to analyze polychlorinated biphenyls [ 411 and fatty acids [45 ]in aqueous samples. It has further been improved and studied for use with different kinds of solvents [42,1. 4 3 . LIQUID-LIQUID EXTRACTION
Well known liquid-liquid extraction procedures have been adapted to c.g.c. analysis by developing micro-techniques, as only a few microliters are usually sufficient. Low solvent volumes in a ratio of 200 pl solvent to 900 d o f water have been reported
f46-48 1 .
A syringe extraction procedure using 1-50 ml of water and 20 pLof solvent has been described [49 ]which allows for the determination of trihalomethanes and chlorinated pesticides in water at the &/L level with a sample throughput of 50 samples a day. 4.4. CLOSED LOOP STRIPPING
Trace amounts of organic micropollutants in the C1420 range can be analyzed by c.g.c. after stripping and trapping in a closed circuit [ 501 . The headspace gas of the system being studied is recirculated by means of a pump via the liquid of the system and a trap connected in closed circuit. The components to be determined are gradually removed by the gas from the liquid and accumulated in the trap. Finally, the concentrate is either recovered from the trap by solvent elution [ 50-53 1 or by thermal desorption [54, 55 1 . This method appears to be a very powerful means of headspace gas trace and ultratrace analysis. It permits high concentration effects to be attained while covering a wide range of compounds. As the liquid being analyzed is stripped by the headspace gas itself, the danger of introducing artifacts into the system is significantly reduced. 4 5 . PYROLYSIS
Non-volatile organic compounds can he characterized by c.g.c. if pyrolyzed directly into the injector chamber of a g.c. [56, 57 1 . It has been shown that size exlusion chromatography and gel permeation chromatography are adequate techniques for fractionation of non-volatile components from water samples. The fractions are then to pyrolysis-g.c.- mass spectrometry for the characterization of humic acid and fulvic acids, sugars, and proteins [57 1 .
765
5 . CONCLUSIONS Instrumentation and column technology have undergone improvements to such a degree that new demands must be made on sampling methodology. As some difficult samples components that were not previously amenable to gas chromatography can now be analyzed, preparation procedures that ensure the recovery of these components are under development. In this respect, selective sampling techniques coupling different kinds of chromatographic processes such as multi-dimensional g.c. or separation on microcolumns seem to be promising. It is hoped that in the near future, automated g.c. analysis systems will be able to integrate all sample preparation techniques.
REFERENCES 1. M. L. Lee, F. J. Yang, and K. D. Bartle, Open Tubular Column G a s Chromatography, Theory and Practice, John Wiley and Sons, 1984. 2. F. I. Onuska, and F. W. Karasek, Open Tubular Column Gas Chromatography inEnviDnnmrtal Sciences, Plenum Press, 1984. 3. W. G. Jennings, ed., Applications of Glass Capillary Gas Chromatography, Marcel Dekker, 1981. 4. W. G. Jennings, A. Rapp, Sample Preparation for Gas Chromatographic Analysis, Hiithig Verlag, 1983. 5. F. J. J. Brinkmann, Advance in concentration techniques for organic micropollutants, Proc. First Europ. Symp. Analysis of Organic Micropollutants in Water, Berlin 1979, Ed. by C.E.C.
(OMP/29/82XII/ENV/l7/82). 6. B. Josefsson, Recent concepts in sampling methodology, Proc-Sec. Europ. Symp. Analysis of Oganic Micropollutants in Water, Killamey, 1981, D. Reidel Publ. Co., 1982,7-15. 7. Proc. Third Eurp. Symp. Analysis of Organic Micropollutants in Water, Oslo 1983, D. Dreidel Publ.CO. 1984, 3-36. 8. K.Grob Jr., See ref.6,pp. 57-60. 9. K.Grob Jr., J.Chromatogr., 2 7 9 , 1983, 225. 10. K. Gmb Jr, Sympsium Pfactice of Capillary Column Gas Chmmatography, Pittsbulg, March 5-9, 1984. 11. K.Grob Jr., and H. P. Neukom, a) J. HRC and CC, 2, 1979, 563, b) J,, Chromatogr., 236 1 9 8 2 , 291. 12. K.Gmb Jr., J.Chromatogr., 213, 1981, 3. 13. K.Grob Jr., and B.Schilling, J.Chromatogr., 264, 1983, 7 . 14. F. Poy, in: Applications of Mass Spectrometry to Trace Analysis, S. Dachetti (ed.), Elsevier Publ.Co, 1982, pp. 135.
766 15. F.Poy, Chromatographie, 16, 1982, 345. 16. G . Schombulg, H. Husnann, H.Behlau, and F.Schulz, J.Chromatogr, 279, 1983, 251. 17. N-Galli, S.Trestianu. and K. Grob Jr., H-HRC and CC, 2, 1979, 366. 18. K-Grob, andK-Grob Jr., J-Chromatogr., 151, 1978. 311. 19. E. Fogelc@st, and M.Larsson, J.Chmmatogr., 279, 1983, 297. 20. K. Crob, J-Chroktogr., 2 9 9 , 1984 , 1. 21. A.Zlatkis, F.S.Wang, and H-Schonfield, Anal.Chem., 55, 1983, 1848. 2 2 . M.Termonia, unpubl. results. 23. L. S.Ettre, J. E..Purcell, J-Widomski, B.Kolb, J-Chmmatogr. Sci., 18, 1980,116. 24. B. Kolb, M.Aver, and P.Pospisi1, J. Chromatogr., 279, 1983, 341. 25. P-Gagliardi, and G . R. Verga, J.Chromatogr,
279. 1983, 323.
26. Applied Headspace Analysis, Ed. by B. Kolb. Publ. by Heyden and Sons, 1980. 27. Analysis of Volatiles, Methods and Applications, Ed. by P. Schreier, Publ. by Walter de Gruyter, 1984. 28. M. Kuck, ref. 25, p. 12. 29. J.A.Rijks, J. Dmzd, and J.Novak, J.Chmmatogr. 186, 1979,167. 30. J.W. Graydon, and K. Gmb, J.Chromatogr., 254, 1983 , 265. 31. A. R. Trussel, F. Y. Lieu, and J. G . Moncur in Advances in the Identification and Analysis of Organic Pollutants in Water (L. H. Keith ed.). Ann Arbor Science Publ., Inc. Ann Arbor, Mich. 1981, 171. 32. E. D. Pellizzari, and L. Little, Collection and Analysis of Purgeable Olganics Emitted Wastewater Treatment Plants EPAd00/280-017, 1980.
from
33. B. A.Colenutt, and S.Thiorburn, Chmmatografii, 12 , 1979 , 519. 34. H. J.Neu, W. Merz, and J. Panzel, J. HRC and CC, 5,1982,
382.
35. G. Schombulg, E. Bastians, H. Behlau, H. Husmann, F. Weeke, HRC and CC, I , 1984 ,4.
M.Oreans, and F. Miiller, J.
36. M.Oreans. F. M a e r , and D. Leonhard, J.Chromatogr., 279 1983,357. 37. F. Bruner, G . Furlani, and F. Mangani, J.Chmmatogr., 302, 1984 , 167. 38. P.Sandra, J.HRC and CC, 6, 1983, 690. 39. G. D. Veith, and L, M. Kiwus, Bulletin of Environm., Contem. and Toxicology, 17, 1977, 631. 40. T.L. Peters, Anal. Chem., 52, 1980 , 211. 41. M. Godefroot, M-Stechele, P. Sandra, and M. Verzele, J. HRC and CC, 5, 1982,75. 42. J. Rijks, J.Cuners, Th.Noy, and C. Cramers, J.Chromatogr,
279
. 1983 ,395.
43. X.Monseur, J. Walrlvens, P. Dourte, and M. Termonia, J. HRC and CC, 4, 1981 , 49. 44. C-Schaffner. E. Stephanou, and W. Giger, ref. 6, p. 330. 45. W-Janda, F. Pehal, and J. Hwinak, J. HRC and CC, 7 , 1984, 540 46. K.Cmb, K-Gmb Jr. and G-Grob, J.Chmmatogr,
106, 1975, 299.
767 47. D.A.J.Murray,
J.Chmmatogr., 177, 1979. 135.
48. M.M.Thomason, and W.Bertsch, J-Chmmatogr., 279, 1983 ,383. 49. J.F.J.Van Rensburg, and A.J. Hassett, J.HRC and CC.5, 1982,574. 50. K.Grob, and F. Zurcher, J.Chmmatogr., 117, 1976 ,285.
5 1. M. Termonia, Int. J. Mass Spectmm. and Ion Phys., 48,1983 ,299.
52. J.Novak, I . Gouas, and J-Diozd, J.Chmmatogr., 204, 1981 ,421. 53. A. Habich, and K-Grob, J. HRC and CC, 7, 1984 ,492. 54. J.W. Gmydon, K-Gmb, F. Zuercher, and W. Giger, J,Chmmatogr, 285, 1984 , 307,
55. M.Termonia, J.Walravens, P.Doude, and X. Monseur, ref 7 , p. 162.
56. J. Mallevialle, A. Bruchet, and Y.Tsutsuni, Proceed of theThird International Congress Analytic. Techn. in Environm. Chem., November 21 -23 1984, Barcelona. 57. L-Stieglitz, W.Roth, 1ef.7, p141.
on
WATER SAMPLE PREPARATION TECHNIQUES FOR CAPILLARY GAS CHROMATOGRAPHIC ANALYSIS
M. TERMONIA State Secretariat for Agriculture, Institute for Chemical Research Museumlaan 5, B - 1980 Tervuren, BELGIUM
ABSTRACT Recent improvements of well-known sample preparation techniques, together with new methodologies a~ presented. Their possibilities for different kinds of samples are reviewed. Problems associated with the use of standard on-line techniques are discussed in relation to vaporizing and on-column sample introduction methods. On-line selective sampling techniques such as headspace analysis and multidimensional gas chromatography are shown to be best suited in the case of samples with complex matrices. Finally, off-line selective sampling methodologies are reviewed. Micromethcds, especially designed for capillary gas chromatography are described including sample purification, fractionation of preconcentration by short liquidchromatography columns, steam-distillation extraction, liquid-liquid extraction, closed loop stripping, and pyrolysis g s chromatography.
1. INTRODUCTION
The introduction of wallcoated open tubular columns for gas chromatography by Golay in 1957 has led to a considerable increase in separation performances of the chromatographic techniques used in water analyses [ 1--3 1. Column technologies have been developed to such an extent that sample preparation (iacluding sampling and injection techniques) now most often appears as the limiting step which determines the quality of the global analytical method.
760 Accordingly, assuming that the capillary gas chromatographic (c.g.c.) columns were optimized in terms of separation efficiency, thermal stability, stationary phase selectivity, and adsorptivity towards the sample components, and that a proper detection system was selected, there is only one way left to improve the gas chromatographic analysis, i.e., through sample preparation. This is particularly true for samples from the aquatic environment because of the high complexity of the sample matrices involved, which often impart the use of the most selective detectors (even mass spectrometry operating in the single ion monitoring mode) for the determination of single components, especially at the trace level (ppb and beyond).
As many recent.reviews of standard water sample preparation techniques are available [4--7 1 , this contribution will only present recently improved well-known methodologies or new developments, in relation with the following sample characteristics: --
vapour pressure of the sample components;
-- concentration range of the analytes;
--their polarity and overall chemical stability; --the nature of the matrix; --
the nature of the interactions of the components with the matrix.
This presentation will be organized according to the following sequence: -- on-line standard techniques; --
on-line selective sampling techniques; and
-- off-line techniques.
2. ON-LINE STANDARD TECHNIQUES 2.1. VAPORIZING INJECTORS
In the analytes have high vapour pressure (already in the gaseous phase at room temperatures) or amenable to the gas phase by vaporizing the sample at convenient temperatures and without thermal degradation into a heated injection chamber, then standard sampling techniques such as split of splitless injection can successfully be applied, provided a few precautions are taken. With non-specific detection methods such as flame ionization, they apply only to components in concentrations above the ppm level. Although these techniques are the most popular, some problems can be encountered in quantitative analysis due to solvent effects and recondensation phenomena occuring i n the column inlet. These were systematically described by Grob and co-workers [ 8-13] -
761 In particular, it has been shown that both the nature of the solvent and the column inlet temperature during the injection is of primary importance in split injection. By influencing the true splitting ratio, they give rise to erroneous results when working with external standards which are not dissolved in the same solvent as the sample or when the column temperature is not precisely reproduced [ l l ] . Temperature of the column inlet again plays an important role in splitless injection where solvent effects can seriously affect the results. During the injection, the oven temperature has to be properly adjusted in function of the volatibility of the solvent, especially for the volatile components eluting just after the solvent peak [12, 131 .
For low volatility components such as polychlorobiphenyls or tetrachloro-paradibenzodioxins (TCDD), it is possible to make use of a solvent effect in order to perform oncolumn enrichments. Thus, Poy [ 141 injected 4 times 2 pl of a sample containing TCDD isomers without destroying the separation performance. The low boiling components were strongly retained in the thick solvent layer formed when the solvent recondensed so that all 4 injections refocused in the column inlet before the chromatographic process took place. An interesting new development in injector design is the PTV (Programmed Temperature Vaporizer) [15, 161 which allows for solvent backflush in the injector. With this system, the sample can be injected at low temperature onto an adsorbing cartridge and the non-adsorbed sample components are backflushed from the injector to vent. The temperature is then raised, vaporization of the analytes occurs and injection into the analytical column is performed. In another working mode, the same injector can be used for retention of low or nonvolatile cornpounds on the Same adsorbing cartridge, avoiding the accumulation of high voiling cornpounds or inorganic material in the column inlet. 2 3 . DIRECT (ONCOLUMN) INJECTORS
It has been shown that direct injectors, such as the cold oncolumn where the liquid sample enters the inlet of a capillary column, are able to produce quantitative results with improved reproducibility and accuracy [17, 18 1 . Moreover, nonderivatized compounds with low physicochemical stability can efficiently be analyzed using this type of injector.
It is particular concern in the analysis of aqueous samples, because the solvent effects involved during the direct aqueous injection (DAI)allow the 'direct analysis of levels (pg/l) of trihalomethanes and related compounds in water, using well deactivated thick film columns and ECD detection [19,20 1 . Other applications of DAI describe the analysis of lindane in water at the 5mg/L level El] and the determination of free carboxylic acids at the mg/Llevel [22].
762
3. ON-LINE SELECTIVE SAMPLING TECHNIQUES 3 -1. HEADSPACE ANALYSIS
When the sample matrices are so complex that t..ey interfere with the analysis, sample preparation of the clean-up become necessary in order t o pre-separate the components. The different fractions obtained can then be separately prepared and analyzed, according t o their chemical properties. However, if the vapour pressure of the analytes is important and if they are not too tightly bound to the matrix (no chemisorption), headspace analysis is possible [23-25 1’ . Static equilibrium headspace can successfully apply t o the determination of high volatility compounds present in complex matrices such as sludges from water treatment stations [ 24, 261 . This can be operated automatically, with a high sample throughput using commercial headspace samplers. The samples are either introduced by means of a syringe (Carlo Erba), of a gas sampling valve (Dani, Siemens), or by a contact of programmable duration between the headspace sample and the carrier gas (Perkin Elmer). The uses of headspace analysis in connection with capillary columns is well documented [24--27 1 and are the subject of important developments. With this technique coupled to a g.c. equipped with thick film columns and ECD detection, it is possible to analyze trihalomethanes in aquatic media with a detection limit of about 1 ppb, expressed for CHCI3, when L ml of headspace gas is injected in the split mode (1 : 4) [ 22 ] . In principle, it is possible to enhance sensitivity by a factor of 100 or more if larger volumes are injected, using a cold trapping effect at the g.c. column inlet. This can be done by cryofocusing with liquid nitrogen[ 281 . However, it has been shown that for highly volatile compounds (with b.p. 7OoC), very long capillary cold-traps or packed capillary traps are needed to avoid sample loss caused by breakthrough of volatiles [29,30 1 . Determination of volatiles at the trace level is also possible by preconcentrating the headspace volatiles on a suitable adsorbent. The trapped compounds are subsequently recovered by thermal desorption in front of a cooled trap connected t o the capillary column or by solvent elution followed by splitless or on-column injection. These methods, called “dynamic headspace enrichment” or “ purge-and-trap”, have been applied to trace level analysis of volatiles, using conventional electrically heated systems [31, 32 1 , a Curie-point Pyrolyser [33 1, or a microwave cavity [34] as thermal desorbers. 3 2 . PRESEPARATION BY G.C.
For compounds of intermediate volatility (typically preseparation of the samples is more conveniently performed by means of packed pre-columns.
763
As chromatography is the most efficient separation method which can be applied to these samples, it has been shown that the on-line coupling of such columns can successfully apply to water analysis 35 1 . This method is derived from the multi-dimensional gas chromatography (MDGC) technique. Double oven instruments were recently developed which are specifically' dedicated t o this type of MDGC analysis &I. In such a system, the pre-column acts as a real g.c. column for which the temperature can be independently programmed. A fraction of the g.c. effluent from this column can be cold trapped in the inlet of a second column, of the analytical type, most often a capillary one with high separation power. In certain difficult cases, it is worthwhile to replace the packed pre-column by a capillary column of high capacity.
4. OFF-LINE SELECTIVE SAMPLING TECHNIQUES 4.1 - SAMPLE PREPARATION ON SHORT
L.C. COLUMNS
The samples may also be prepared off-line using cartridges which can be filled with any standard packing material used in liquid chromatography. The sample components are recovered by solvent elution and subsequently analyzed according to a standard technique such as those described in section 2. Carbopack F, C, or B traps have been used for the enrichment of polyaromatic hydrocarbons from water samples [ 3 7 1. The water to be analyzed is forced through the trap which is finally extracted with toluene. Recoveries higher than 86% were reported for compounds ranging from acenaphtylene to benzo g, h, i perylene. Micro adsorption cartridges, capable of fractionation, purification or enrichment can also be introduced directly into the body of a syringe, allowing sample preparation to occur during the injection [38 1. Milligram amounts of a material in which partition can take place (A1203, charcoal, Florisil, Lipidex 5000, --) are sufficient to handle the ng amounts needed for high performance c.g.c. 4 2 . STEAM-DISTILLATION-EXTRACTION
Samples from aquatic environments (sediments, fish), containing appreciably high amounts of fatty material can efficiently be prepared by Steam-Distillation-Extraction (SDE). In particular, pesticides and herbicides which have appreciably greater vapour pressure than those of water soluble chemicals can be extracted with high recovery yields using different SDE units [39-431 . The produced extracts are generally suitable for direct g.c. analysis, without concentration and cleanup procedures. A closed loop version of this technique has
7 64
successfully been applied to nonylphenols and nonylphenolethoxylates in secondary sewage effluents [44 I .
A recently introduced microversion of the Likens and Nickerson SDE apparatus has been shown to be able to analyze polychlorinated biphenyls [ 411 and fatty acids [45 ]in aqueous samples. It has further been improved and studied for use with different kinds of solvents [42,1. 4 3 . LIQUID-LIQUID EXTRACTION
Well known liquid-liquid extraction procedures have been adapted to c.g.c. analysis by developing micro-techniques, as only a few microliters are usually sufficient. Low solvent volumes in a ratio of 200 pl solvent to 900 d o f water have been reported
f46-48 1 .
A syringe extraction procedure using 1-50 ml of water and 20 pLof solvent has been described [49 ]which allows for the determination of trihalomethanes and chlorinated pesticides in water at the &/L level with a sample throughput of 50 samples a day. 4.4. CLOSED LOOP STRIPPING
Trace amounts of organic micropollutants in the C1420 range can be analyzed by c.g.c. after stripping and trapping in a closed circuit [ 501 . The headspace gas of the system being studied is recirculated by means of a pump via the liquid of the system and a trap connected in closed circuit. The components to be determined are gradually removed by the gas from the liquid and accumulated in the trap. Finally, the concentrate is either recovered from the trap by solvent elution [ 50-53 1 or by thermal desorption [54, 55 1 . This method appears to be a very powerful means of headspace gas trace and ultratrace analysis. It permits high concentration effects to be attained while covering a wide range of compounds. As the liquid being analyzed is stripped by the headspace gas itself, the danger of introducing artifacts into the system is significantly reduced. 4 5 . PYROLYSIS
Non-volatile organic compounds can he characterized by c.g.c. if pyrolyzed directly into the injector chamber of a g.c. [56, 57 1 . It has been shown that size exlusion chromatography and gel permeation chromatography are adequate techniques for fractionation of non-volatile components from water samples. The fractions are then to pyrolysis-g.c.- mass spectrometry for the characterization of humic acid and fulvic acids, sugars, and proteins [57 1 .
765
5 . CONCLUSIONS Instrumentation and column technology have undergone improvements to such a degree that new demands must be made on sampling methodology. As some difficult samples components that were not previously amenable to gas chromatography can now be analyzed, preparation procedures that ensure the recovery of these components are under development. In this respect, selective sampling techniques coupling different kinds of chromatographic processes such as multi-dimensional g.c. or separation on microcolumns seem to be promising. It is hoped that in the near future, automated g.c. analysis systems will be able to integrate all sample preparation techniques.
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