The Purge and Trap Technique

FIGURE 2.8 Schematic of a single-drop extraction apparatus. (Reprinted from Buszewski, B. and Ligor, T., LC-GC Europe, 15, 92-97, 2002. With permission from Advanstar Communications.) 

It is quite time-consuming and labor intensive, particularly when many samples are involved. In addition it requires complex instrumentation. 

Another sample preparation possibility is the method based on trapping the analytes. Using this methodology the analyte is trapped in an adsorbent material, and then washed off with a minimal volume of solvent. The most popular trapping method is SPE. 

The history of SPE dates from more than fifty years ago, with granulated active carbons previously used in water treatment technologies. The pioneer work was conducted by the U.S. Public Health Service (Cincinnati, OH). After that, other approaches were investigated with petroleum pollutants, insecticides and VOCs. Disadvantages such as irreversible adsorption, analyte reactions on the activated carbon surface and low recoveries started research in new sorbent materials. See the historical review of SPE by I. Liska.  

SPE began to be extensively used in sample preparation processes in the 1970s but the great breakthrough in SPE occurred over the past seven or eight years with many improvements in formats, automation and introduction of new phases. In fact, SPE is currently accepted as an alternative extraction method to LLE for 22 of the official methods for the U.S. EPA. 

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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. 

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. 

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... 

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. 

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. 

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. 

Ramstad, T., Nestrick, T. J., Peters, T. L. Applications of the Purge-and-Trap Technique, American Laboratory, July, 65 (1981)... 

In the dynamic headspace method, the sample is put in a thermal desorption unit in order to desorb the RS a continuous flow of a carrier gas pushes the RS into a trapping system which is refrigerated and where they are accumulated prior to analysis. Then the RS are rapidly desorbed by rapid heating and carried onto the column via the carrier gas. There are different ways to apply this technique. The arrangement when purge gas passes through the sample is often called the purge and trap technique (some other equipment uses the acronym DCI (desorption, concentration, injection)). This method is particularly useful for very low concentrations of RS as the total amount of a substance is extracted and can be applied directly to powders without need to dissolve them. The main drawback is that the dynamic headspace methods are not readily automated. ... 

Mehran, M.T. Nickelsen, M.G. Gokar, N. and Cooper, W.T. Improvement in the purge and trap technique for the rapid analysis of volatile organic pollutants in water. Journal cf High Resolution Chromatography 1990, 13, 429 33. 

Dynamic headspace sampling (DHS) is a nonequilibrium process in which air or an inert gas such as nitrogen is passed over the sample (in the case of a solid) or through the sample (in case of a liquid). In the case of a liquid sample, this is more commonly referred to as the purge-and-trap technique, which is used widely in the environmental field. 

Figure 2.23 Glass apparatus for the purge and trap technique (Tekmar). (a) U-tube with/without frit (5 and 25 mL sizes), (b) Needle sparger left single use vessels, middle glass needle with frits, right vessels with foam retention, 5, 20 and 25 mL volumes.

Careri M, Dalcanale E, Mangia A, Ruffini M (1997) Cavitand sorbents for the selective adsorption of organic compoundsby the purge-and-trap technique. Anal Commun 34(1) 13-15. doi 10. 1039/A607363H... 

Although it is sometimes regarded as a universal sorbent, Tenax is not suitable for every application, and many analysts choose to augment or replace it with other sorbent materials (9). In an effort to extend the purge-and-trap technique, the U.S. Environmental Protection Agency (EPA) has devised additional traps, which use Tenax as the primary sorbent backed by other, more retentive sorbents. To concentrate on a wide range of volatiles, such as in EPA method 502.2, which... 

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