Purging and Trapping

Thermal desorption (TD) is a technique in which solid or semisolid samples are heated under a flow of inert gas. Volatile and semivolatile organic compounds are extracted from the sample matrix into the gas stream and introduced into a gas chromatograph. Samples are typically weighed into a replaceable FIFE tube liner, which is inserted into a stainless steel tube for heating. 

The thermal desorption must take place at a temperature below the decomposition point of other materials in the sample matrix. Solid materials should have a high surface area (e.g., powders, granules, flbers). Bulk materials are ground with a coolant such as solid carbon dioxide prior to weighing. This technique simplifies sample preparation and avoids the necessity of dissolving samples or solvent extraction. Thermal desorption is well suited for dry or homogeneous samples such as polymers, waxes, powders, pharmaceutical preparations, solid foods, cosmetics, ointments, and creams. There is essentially no sample preparation required. 

An example of the use of TD is for the analysis of water-based paints for organic volatiles. The TD tube is used in combination with a second tube containing a sorbent that removes the water, which cannot be introduced into the capillary GC column. A small aliquot of paint (e.g., 5 /jIS) is placed on glass wool in the TD tube. Solids from the paint, which would harm a GC column, remain behind. 

In thermal desorption, the volatile analyte is desorbed from the sample by heating and introduced directly into the GC. 

See www.markes.com for literature on sorbent selection for thermal desorption. 

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Schematic diagram of a purge-and-trap system. Anaiyte is coiiected in the primary adsorption trap. The secondary adsorption trap is monitored for evidence of breakthrough.

Adjusting the Analyte s Concentration Analytes present at concentrations too small to give an adequate signal need to be concentrated before analyzing. A side benefit of many of the extraction methods outlined earlier is that they often concentrate the analytes. Volatile organic materials isolated from aqueous samples by a purge and trap, for example, can be concentrated by as much as 1000-fold. 

Environmental Analysis One of the most important environmental applications of gas chromatography is for the analysis of numerous organic pollutants in air, water, and wastewater. The analysis of volatile organics in drinking water, for example, is accomplished by a purge and trap, followed by their separation on a capillary column with a nonpolar stationary phase. A flame ionization, electron capture, or... 

To satisfy the Resource Conservation and Recovery Act (1977) and its amendment for hazardous and solid waste (1984), the 80(K) Series Methods have been designed to analyze solid waste, soUs, and groundwater. In particular, methods 8240/8260 require the use of a purge-and-trap device in conjunction with packed or capillary GC/MS, respectively, for the analysis of purgeable organic compounds. Methods 8250/8270 concern analyses for the less-volatile bases, neutrals, and acids by GC/MS after extraction from the matrix by an organic solvent. 

Target compounds are specified for each Series Method. Volatile compounds that need to be analyzed can be extracted from the matrix by a purge-and-trap device. 

Gas Chromatography. Gas chromatography is a technique utili2ed for separating volatile substances (or those that can be made volatile) between two phases, one of which is a gas. Purge-and-trap methods are frequently used for trace analysis. Various detectors have been employed in trace analysis, the most commonly used being flame ioni2ation and electron capture detectors. 

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."... 

Liu JM, Jiang GB, Zhou QF (2001) Comprehensive trace-ievei determination of methyitin compounds in aqueous sampies by cryogenic purge-and-trap gas chromatography with fiame photometric detection. Analytical Sciences, 7(11) 1279-1283. 

Purge-and-trap methods have also been used to analyze biological fluids for the presence of trichloroethylene. Breast milk and blood were analyzed for trichloroethylene by purging onto a Tenax gas chromatograph to concentrate the volatiles, followed by thermal desorption and analysis by GC/MS (Antoine et al. 1986 Pellizzari et al. 1982). However, the breast milk analysis was only qualitative, and recoveries appeared to be low for those chemicals analyzed (Pellizzari et al. 1982). Precision (Antoine et al. 1986) and sensitivity (Pellizzari et al. 1982) were comparable to headspace analysis. 

Analysis of environmental samples is similar to that of biological samples. The most common methods of analyses are GC coupled to MS, ECD, a Hall s electrolytic conductivity detector (HECD), or a flame-ionization detector (FID). Preconcentration of samples is usually done by sorption on a solid sorbent for air and by the purge-and-trap method for liquid and solid matrices. Alternatively, headspace above liquid and... 

Water Purged and trapped on Tenax - GC/HECD <0.1 ppb 98 (HECD) Otson and Williams... 

Water Purged and trapped on Tenax -GC thermally desorbed GC/HECD. 05 ppb 50-90 Wallace et al. 1986a... 

Grains, grain-based foods Purged and trapped on Tenax /XAD-4 resin desorb with hexane GC/ECD Low- to sub-ppb 86-100 Heikes and Hopper 1986... 

Analysis of soils and sediments is typically performed with aqueous extraction followed by headspace analysis or the purge-and-trap methods described above. Comparison of these two methods has found them equally suited for on-site analysis of soils (Hewitt et al. 1992). The major limitation of headspace analysis has been incomplete desorption of trichloroethylene from the soil matrix, although this was shown to be alleviated by methanol extraction (Pavlostathis and Mathavan 1992). 

The Environmental Health Laboratory Sciences Division of the National Center for Environmental Health, Centers for Disease Control and Prevention, is developing methods for the analysis of trichloroethylene and other volatile organic compounds in blood. These methods use purge and trap methodology, high resolution... 

Heikes DL, Hopper ML. 1986. Purge and trap method for determination of fumigants in whole grains, milled grain products, and intermediate grain-based foods. J Assoc Off Anal Chem 69 990-998. 

USEPA] US Environmental Protection Agency. 1996. Method 1631 mercury in water by oxidation, purge and trap, and cold vapor atomic fluorescence (C VAFS). Draft method EPA 821-R-96-012. 

Schumacher BA and Ward SE (1997) Quantitation reference compounds and VOC recoveries from soils by purge-and-trap GC/MS. Environ SciTechnol 31 2287-2291. 

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. 

Figure 8.27 A, apparatus for dynaaic headspace analysis of urine with sorbent trapping. B, gas phase stripping apparatus (purge-and-trap).

Solid-phase microextraction eliminates many of the drawbacks of other sample preparation techniques, such as headspace, purge and trap, LLE, SPE, or simultaneous distillation/extraction techniques, including excessive preparation time or extravagant use of high-purity organic solvents. SPME ranks amongst other solvent-free sample preparation methods, notably SBSE (Section 3.5.3) and PT (Section 4.2.2) which essentially operate at room temperature, and DHS (Section 4.2.2),... 

Figure 4.3 Schematic of an off-line purge-and-trap apparatus. After Cole and Woolfenden [208a]. Reprinted with permission from LC.GC, Vol. 10, Number 2, February 1992, pp. 76-82. LC.GC is a copyrighted publication of Advanstar Communications Inc. All rights reserved...

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