Volatile organic compounds sample handling

The most variable aspect of carbon tetrachloride analysis is the procedure used to separate carbon tetrachloride from the medium and prepare a sample suitable for GC analysis. As a volatile organic compound of relatively low water solubility, carbon tetrachloride is easily lost from biological and environmental samples, so appropriate care must be exercised in handling and storing such samples for chemical analysis. Brief summaries of the methods available for extraction and detection of carbon tetrachloride in biological and environmental samples are provided below. 

Knowing the problems associated with the analysis of volatile organic compounds, you inquire about the handling of the samples. Here we go The samples (100 mL) were put into 1 L flasks, which were then sealed and stored at 5°C for several days. Then, in the cooling room, an aliquot of the water was withdrawn and analyzed for benzene. What was the original concentration of benzene in the water sample Assume that equilibrium is established between the gas phase and the water and neglect adsorption of benzene to the glass walls of the bottle. The data required to answer this question can be found in Table 3.4. 

Headspace-GC-MS analysis is useful for the determination of volatile compounds in samples that are difficult to analyze by conventional chromatographic means, e.g., when the matrix is too complex or contains substances that seriously interfere with the analysis or even damage the column. Peak area for equilibrium headspace gas chromatography depends on, e.g., sample volume and the partition coefficient of the compound of interest between the gas phase and matrix. The need to include the partition coefficient and thus the sample matrix into the calibration procedure causes serious problems with certain sample types, for which no calibration sample can be prepared. These problems can, however, be handled with multiple headspace extraction (MHE) [118]. Headspace-GC-MS has been used for studying the volatile organic compounds in polymers [119]. The degradation products of starch/polyethylene blends [120] and PHB [121] have also been identified. 

GC provides separations that are faster and better in terms of resolution than the older chromatographic methods (see Chapter 14). It can be used to analyze a variety of samples. However, many samples simply cannot be handled by GC without derivatization, because they are insufficiently volatile and cannot pass through the column or they are thermally unstable and decompose under conditions used for GC separations. It has been estimated that only 20% of known organic compounds can be satisfactorily separated by GC without prior chemical modification of the sample. 

Previous reports 13] emphasized the importance of sample handling, and indeed because of the very volatile nature of the compounds measured in this type of analysis, sample collection deserves special consideration. In general, narrow mouth glass vials with a total volume in excess of 50 ml are acceptable. The bottles need not be rinsed or cleaned with organic solvents, but simply cleaned with detergent and water, rinsed with distilled water, air dried, and dried in a 105°C oven for one hour. The vials are carefully filled with sample to overflowing (zero head space) and a Teflon faced silicone rubber septum is placed Teflon face down on the water sample surface. The septa may be cleaned in the same manner as the vials, but should not be heated more than one hour because the silicone layer slowly degrades at 105°C. 

The detection of low level concentrations of volatile petroleum hydrocarbons in either soil or water can be performed by static headspace analysis. In this technique, the gas phase in thermodynamic equilibrium with the matrix is analysed. The soil is placed in a headspace vial to which water and soluble salts such as sodium chloride are added to aid the transfer of hydrocarbons into the headspace. Internal standards and surrogate spikes can also be introduced. The vial is heated and an aliquot of the static headspace vapour is directly injected onto the column of the gas chromatograph. The advantages of this technique for volatiles such as gasoline range organics are less sample handling which minimises losses, no introduction of solvents which can interfere with the compounds of interest (MTBE), and the technique can be easily automated. 

This exercise affords the student an opportunity to further utilize gas chromatography in combination with two different sample preparation methods. Whenever a sample preparation method is used, there is most always loss of analyte due to sample handling, and if the method involves phase distribution equilibria, some analyte is lost between phases. This is particularly important when conducting VOCs analyses due to volatility losses of the analytes. It thus becomes imperative that, prior to actual sample analysis, a recovery study be undertaken to evaluate these losses. An analyst cannot assume that every method yields a 100% recovery of every organic compound Recovery studies thus become a major part of a good quality assurance program for the trace environmental analysis laboratory. 

The purpose of trip blanks is to assess the collected sample representativeness by determining whether contaminants have been introduced into the samples while they were handled in the field and in transit, i.e. in coolers with ice transported from the site to the analytical laboratory. A possible mechanism of such contamination is the ability of some volatile compounds, such as methylene chloride or chlorofluor-ocarbons (Freons), to penetrate the PTFE-lined septum and dissolve in water. Potential sources of this type of contamination are either ambient volatile contaminants or the VOCs that could be emanating from the samples themselves, causing sample cross-contamination. To eliminate ambient contamination, samples must not be exposed to atmospheres containing organic vapors. Cross-contamination is best controlled by such QA measures as sample segregation and proper packaging. 

Low Temperature Sublimation in Vacuum - Prepare volatile compound, place in low temperature oven in vacuum, collect vapor on suitable cold plate. The volatile compound is rendered non-volatile and the organic material destroyed by ignition. Preparation of extremely thin sources for highest resolution alpha energy analysis. Excellent sources -any kind of backing may be used. Only a small fraction of sample collected. Volatile compounds are difficult to handle and constitute a health hazard. Not a routine method. 

---