Purge and trap techniques

Figure 15-12 is a schematic illustration of a technique known as acid volatile sulfides/ simultaneously extracted metals analysis (AVS/SEM). Briefly, a strong acid is added to a sediment sample to release the sediment-associated sulfides, acid volatile sulfides, which are analyzed by a cold-acid purge-and-trap technique (e.g., Allen et ai, 1993). The assumption shown in Fig. 15-12 is that the sulfides are present in the sediments in the form of either FeS or MeS (a metal sulfide). In a parallel analysis, metals simultaneously released with the sulfides (the simultaneously extracted metals) are also quantified, for example, by graphite furnace atomic absorption spectrometry. Metals released during the acid attack are considered to be associated with the phases operationally defined as "exchangeable," "carbonate," "Fe and Mn oxides," "FeS," and "MeS."... 

Gas phase stripping (purge-and-trap) techniques can iaq>rove the yield of organic volatiles from water or biological fluids by facilitating the transfer of volatiles from the liquid to the gas phase it is also more suitable than dynamic headspace sampling when the sample volume is restricted (320 23,347-351). Tbe technique is used routinely in many laboratorl B for the analysis... 

In general terss, the purge-and-trap technique is the nethod of choice Cor detemining organic volatiles in water because of its ease of operation. If greater sensitivity is required, the closed loop stripping apparatus should be used. 

Askari et al. [15] have compared purge and trap, methanol immersion and hot solvent extraction methods for the determination of volatile organic compound in aged soil. These workers found that hot solvent extraction is much more effective than the US Environmental Protection Agency approved purge and trap technique [7, 8]. 

The mass spectrometer GC detector has a high degree of compatibility with the purge and trap technique, and GC/MS has been employed widely with this isolation and concentration procedure for the analysis of volatile organic compounds. The mass spectrometer is strongly recommended for samples where there is a possibility of unexpected compounds, and for broad spectrum analyses f 6J of poorly defined samples. 

APHA. 1992b. Methos 6040C. Purge and trap technique. In Standard methods for the examination of water and wastewater. 18th ed. Washington, DC. American Public Health Association, 17-36. 

Dynamic mode instead of working in a closed environment, a carrier gas such as helium is either passed over the surface of the sample or bubbled through it in order to carry the volatile components into a trap where they are adsorbed and concentrated (Fig. 20.5). The sample is then introduced into the chromatograph by thermal desorption. This purge-and-trap technique is semi-quantitative and delivers a sample without residue. 

Results of interlaboratory studies conducted by the American Society for Testing and Materials (ASTM) Committee D-19 for the purge and trap technique (26) and liquid-liquid extraction (27) have... 

Extensive review article of purge-and-trap techniques geared towards environmental applications. Good discussion of choice of extraction conditions including purge rate and time. 

In general, the purge and trap technique is apphed to analyze substances that have boiling points below 200°C and are insoluble or slightly soluble in water. 

Table 2.9.1 lists some commonly used volatile halogenated hydrocarbons. Most of these are U.S. EPA listed pollutants. The term volatile indicates that these substances may be extracted by purge and trap technique. This also includes a few compounds of relatively moderate boiling range, responding adequately to the purge and trap method. The characteristic masses for GC/MS determination are also presented in the table. 

Obviously, some procedures take less time than others. For example, sample concentration by the purge and trap technique that precedes VOC analyses takes only about 20 minutes. It is performed immediately prior to analysis on a multisample automated concentrator combined with an analytical instrument. The shortest of all sample preparation procedures is the waste dilution procedure, commonly known as dilute and shoot, which takes minutes. It consists of diluting a known volume of a concentrated waste sample in a known volume of a compatible solvent, followed by an injection into a gas chromatograph. 

Purge-and-trap techniques in which volatile analytes are evolved from blood or urine in a gas stream and collected on a trap for subsequent chromatographic analysis have been developed. Such a technique employing gas chromatographic separation and Fourier transform infrared detection has been described for a number of volatile organic compounds in blood.6... 

Miscellaneous organics Hall electrolytic and photoionisation Purge and trap technique 0.1- 0.9ppb [475]... 

A number of techniques can be used to isolate analytes from water. The technique used will depend on the volatility of the analyte. Volatile compounds (i.e., more volatile than n-C12) can be analyzed using Purge and Trap techniques or by Headspace analysis. Semivolatile compounds are extracted using liquid—liquid or solid phase extraction techniques. 

Wylie PL. 1987. A comparison of headspace purge and trap techniques for the analysis of volatile priority pollutants. Proceedings of the Water Quality Technology Conference 14 185-202. 

It was not until the advent of the electron capture detector (BCD) and the development an appropriate BCD cahbration routine when precise and rehable N2O measurements were made possible (Cohen, 1977 BUdns, 1980 Rasmussen et al, 1976 Weiss, 1981). Up to now the use of an BCD in connection with equilibration or purge-and-trap techniques followed by gas chromatographic separation is state of the art for the determination of dissolved NoO (Butler and BUdns, 1991). 

Previous studies have established the presence of NHCs in several retort water samples. Leenheer et al. [2] fractionated compounds into hydrophobic/hydrophilic basic and acidic fractions and demonstrated the presence of a number of alky 1-pyri dines and -quinolines by bigh-performance liquid chromatography. Hawthorne and Sieveis [ 3 ] used gas chromatography/mass spectrometry to identify, in 3 retort water/gas condensate sample pairs, a limited number of NHCs in tbe ambient headspace, and many more sucb compounds by an exhaustive purge and trap technique. Also reported in retort water have been a series of Cg to alkyIpyridines t 7 ]. 

The analysis of natural compounds in foods is also assisted by the use of the purge and trap technique in methods for distinguishing strawberry varieties [100] the aroma of unprocessed foods including gherkin [101], durian [102], garlic [103] and meat [104-107] cheese [108,109] and other dairy products [110] tobacco, tea and coffee [111,112] and peanuts [112]. Food additives including sulphur dioxide [113,114] and food contaminants such as VOCs [115-126], have been recovered by PT, particularly from table-ready foods. Animal [127,128] and plant tissues [129,130] have also been subjected to PT for separation of volatile compounds. 

If the analytes of interest are volatile or semivolatile, solvent extraction is not always necessary, and head-space techniques (HS) can be applied for the analysis, typically utilizing GC as the final analytical step. HS analysis can be defined as a vapor-phase extraction, involving ftrst the partitioning of analytes between a non-volatile liquid or solid phase and the vapor phase above the liquid or solid. The vapor phase is then transferred further and either analysed as vapor or (ad)sorbed to an (ad)sorbent. The head-space techniques have been widely utilized in the analysis of volatiles, such as fi agrances and aroma compounds, in various food and agricultural samples (81-84). The dynamic head-space (DHS), or purge-and-trap technique, is easily coupled on-line with GC. In an on-line system, desorption of trapped analytes for subsequent analysis is usually performed using on-line automated thermal desorption (ATD) devices. 

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