Thermal desorption modulation

E. S. Erancis, M. Wu, R B. Eamswoith and M. L. Lee, Supercritical fluid extraaion/gas cliromatography with thermal desorption modulator interface and niti O-specific detection for the analysis of explosives , 7. Microcolumn Sep. 7 23-28 (1995). 

Z. Fiu and J. B. Phillips, Sample introduction into a 5 p.m i.d. capillary gas chr omatog-raphy column using an on-column thermal desorption modulator , /. Microcolumn Sep. 1 159-162(1989). 

A method for the determination of diazinon in human serum has recently been published by researchers at the Centers for Disease Control and Prevention (Liu et al. 1994) in which 2-dimensional chromatography was used to determine 15 pesticides in 4 minutes. Supercritical fluid extraction (SFE) was used to recover pesticides into methylene chloride and this extract was analyzed using two 2-meter columns connected by an on-column thermal desorption modulator. Sensitivity for diazinon was reported to be 1.8 pg on-column no details about overall recoveries were provided. 

Z. Liu and J. B. Phillips, Large-volume sample introduction into narrow-bore gas chromatography columns using thermal desorption modulation and signal averaging , J. Microcolumn Sep. 2 33-40 (1990). 

The effectiveness of using SEE coupled to GC with a thermal desorption modulator (TDM) interface and TEA detection to analyse explosives has been evaluated by Francis et al. [33] in soil samples and standards. 

BTT and BTN, butane-1,2,4-triol-trinitrate DNT, dinitrotoluene EGDN, ethylene glycol dinitrate HPLC, high-performance liquid chromatography LOD, limits of detection MDQ, minimum detectable quantities NB, nitrobenzene NG, nitroglycerine NN, nitronaphthalene NT, nitrotoluene PETN, pentaerythritol tetranitrate RDX, cyclotrimethylene trinitramine SFE, supercritical fluid extraction SGC, solvating gas chromatography TDM, thermal desorption modulator TNB, trinitrobenzene and TNT, trinitrotoluene. 

The first report on the concept of thermal modulation was published in 1985 by Phillips s group [26]. By then, the modulation principle was not yet involved in GC xGC but rather in the so-called multiplex gas chromatography concept that was used in the field of headspace sampling without preconcentration for low-level analytes. On-column thermal desorption modulation was then applied in the field of sample introduction as a device capable of narrow injection pulses for high-speed GC [27]. 

The concept of thermal desorption modulation (TDM) finally found its own specific area of application as the essential interface device between the two columns of a comprehensive two-dimensional separation system [28]. The TDM device consisted of a short section of a column that could be rapidly and reproducibly heated by an electrical current passing through an electrically conductive film. The authors used a segment of a capillary column with a narrow film thickness, covered with a layer of electrically conductive paint as the modulating interface between the two columns (Figure 8). Substances retained on the stationary phase of this modulator section were pulsed into the second column by resistive heating. 

LMCS, longitudinally modulating cryogenic system. TDM, thermal desorption modulator. 

Thermal desorption is the method of choice to monitor air pollution. A known amount of air is drawn through an adsorbent tube filled with activated carbon, Tenax, silicagel, or mixtures of these. Analytes of interest are trapped and concentrated on the adsorbent. The ad.sorbenl tube is sealed and transported to the laboratory where it is installed in the thermal desorption unit. An example of a thermal desorption unit is shown in Figure 33. After the adsorbent tube is placed in the desorption module, the carrier gas is directed over the adsorbent, which is heated from room temperature to 250-300 "C. Since thermal desorption is a slow process, the solutes are focused by cold trapping on a fused silica trap. The cryogenic trap is then heated rapidly to transfer the sample to the column. The method works well for apolar and medium-polarity components in air. Highly polar solutes are very hard to desorb, and other sampling methods must be selected,... 

A semiportable or stationary thermal desorption/gas chromatographic type of instrument, this device Is operable in field environments in a mobile laboratory unit. It Is designed for Identification of a long list of TICs and nerve and blister agents down to or below AEL levels through the use of different modules equipped with various signal detectors. 

The adaptation of external devices such as headspace, P T or thermal desorption is straightforward, with the pressure sensor installed close to the injector head which is generally the case with commercial forward-regulated GC instruments. If the pressure sensor is installed in the split line of the injector, it has to be moved close to the inlet to provide a short carrier gas regulation loop. Carrier gas flow and pressure in the external device is controlled by the EPC module of the GC. The position of the pressure sensor in the flow path of an inlet is important for accurate measurement of the inlet pressure to get rapid feedback control. 

---