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Last Update: 01/31/00

Methodologies For The Quantification Of Purge And Trap Thermal Desorption and Direct Thermal Desorption Analyses

INTRODUCTION

This manuscript describes methodologies which are useful for quantitative analyses involving purge and trap thermal desorption (P&T-TD) or direct thermal desorption (DTD) using the SIS Short Path Thermal Desorber and sample isolation accessories (1). Quantification techniques used for the determination of volatile organic compounds in water or wastewater involving P&T-TD have been extensively reviewed. These procedures form the basis of the US-EPA water and wastewater testing program and are outlined in great detail in methods #524 and 624 (2). Since this methodology has been extensively validated, it will not be reviewed in this manuscript. Rather, this discussion will address quantification problems which arise in the analysis of non-routine matrices such as foods, polymers, soils, sludges and other samples.

Analyte recoveries during purge and trap experiments can vary widely due to matrix effects, purging efficiency (volatility), purge cell design, sparging rate, choice of adsorbent, isolation temperature and many other factors. For these reasons, quantification of P&T-TD and DTD experiments are best carried out using various spiked internal standard methodologies which may offer some degree of correction for variable analyte extraction efficiency or recovery. Each individual sample matrix and analyte present unique problems  must be overcome in order to achieve accurate and precise quantification. Analytical strategies must be formulated on an individual basis and are thus not always universally applicable. Intensive methods development and validation studies are required for each individual sample matrix and analyte combination to guarantee accuracy and precision in the measurements. In our laboratory, we routinely employ several different internal standard methods to achieve quantification of P& T-TD and DTD analyses. These include the standard addition, internal standard, surrogate internal standard and stable isotope labeled internal standard methods. These methods are described in this manuscript and are illustrated with examples from current research projects.

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