The analysis of organic water pollutants by gas chromatography and gas chromatography-mass spectrometry.
Four methods for the analysis of trace levels or organic compounds in water have been discussed: direct aqueous injection, vapor stripping, solvent extraction, and lipophilic adsorption. Table 6 presents a comparison of these techniques. Direct injection is applicable to compounds having a wide range of polarities and volatilities, but it is not a very sensitive technique. For applications in which low sensitivity is acceptable, direct injection offers a very rapid and potentially accurate means of measuring organics in water. Vapor stripping is a factor of 10(6) more sensitive than direct injection, but this is achieved at the expense of simplicity. This method is applicable to compounds of volatilities less than that of eicosane. A subtle operational problem with most vapor stripping techniques is that the contents of the trap are consumed with one analysis; if anything goes awry, the analysis of that trapped sample cannot be repeated. Solvent extraction has a respectable sensitivity (about 0.5 ppb) and operationally it is very simple. Furthermore, aliquots of the resulting extract can be analyzed many times with different techniques if necessary. Because of the evaporation step, very volatile compounds (greater than that of decane) cannot be measured effectively with solvent extraction. The introduction of a solvent into the water carries with it the potential for contamination. Thus, ultrapure solvents must be used and all glassware should be carefully cleaned [96]. Lipophilic adsorption has the unique property of sampling many hundreds of liters of water. Thus, the method is quite sensitive (potentially in the ppt range) and, in addition, it can provide several milligrams of an isolated component. This latter feature is of considerable advantage if one wants to complement GC-MS analyses with infrared or NMR for difficult structural identifications. As a solvent extraction, the evaporation of the final solvent limits the applicability of adsorption methods to compounds with volatilities greater than that of decane. Finally, the importance of avoiding contamination should be emphasized. Many of the compounds encountered in environmental samples are also common laboratory artifacts, for example plasticizers and antioxidants. When one finds such compounds in a water sample, it is absolutely essential to demonstrate that they are indeed present in the water and are not the result of laboratory contamination. The best such demonstration is procedural blank analyses that are indeed blank.
The analysis of organic water pollutants by gas chromatography and gas chromatography-mass spectrometry. Four methods for the analysis of trace levels or organic compounds in water have been discussed: direct aqueous injection, vapor stripping, solvent extraction, and lipophilic adsorption. Table 6 presents a comparison of these techniques. Direct injection is applicable to compounds having a wide range of polarities and volatilities, but it is not a very sensitive technique. For applications in which low sensitivity is acceptable, direct injection offers a very rapid and potentially accurate means of measuring organics in water. Vapor stripping is a factor of 10(6) more sensitive than direct injection, but this is achieved at the expense of simplicity. This method is applicable to compounds of volatilities less than that of eicosane. A subtle operational problem with most vapor stripping techniques is that the contents of the trap are consumed with one analysis; if anything goes awry, the analysis of that trapped sample cannot be repeated. Solvent extraction has a respectable sensitivity (about 0.5 ppb) and operationally it is very simple. Furthermore, aliquots of the resulting extract can be analyzed many times with different techniques if necessary. Because of the evaporation step, very volatile compounds (greater than that of decane) cannot be measured effectively with solvent extraction. The introduction of a solvent into the water carries with it the potential for contamination. Thus, ultrapure solvents must be used and all glassware should be carefully cleaned [96]. Lipophilic adsorption has the unique property of sampling many hundreds of liters of water. Thus, the method is quite sensitive (potentially in the ppt range) and, in addition, it can provide several milligrams of an isolated component. This latter feature is of considerable advantage if one wants to complement GC-MS analyses with infrared or NMR for difficult structural identifications. As a solvent extraction, the evaporation of the final solvent limits the applicability of adsorption methods to compounds with volatilities greater than that of decane. Finally, the importance of avoiding contamination should be emphasized. Many of the compounds encountered in environmental samples are also common laboratory artifacts, for example plasticizers and antioxidants. When one finds such compounds in a water sample, it is absolutely essential to demonstrate that they are indeed present in the water and are not the result of laboratory contamination. The best such demonstration is procedural blank analyses that are indeed blank.