Thermal desorber

Caffeine is extracted from beverages by a solid-phase microextraction using an uncoated fused silica fiber. The fiber is suspended in the sample for 5 min and the sample stirred to assist the mass transfer of analyte to the fiber. Immediately after removing the fiber from the sample it is transferred to the gas chromatograph s injection port where the analyte is thermally desorbed. Quantitation is accomplished by using a C3 caffeine solution as an internal standard. 

Gc/ftir has both industrial and environmental appHcations. The flavor and aroma components in fragrances, flavorings, and foodstuffs can be identified and quantified via gc/ftir (see Food additives). Volatile contaminants in air, water, and soil can be analy2ed. Those in air are usually trapped in a sorption tube then injected into the chromatograph. Those in water or soil are sparged, extracted, or thermally desorbed, then trapped and injected (63,64). 

A common approach for personal dosimetry is collection of pollutant on, e.g., silica gel, organic resins or activated charcoal in small tubes worn on the operator s lapel (Table 9.2). Silica gel is useful for polar chemicals charcoal finds wide use for non-polar substances. The pollutant is then solvent-extracted or thermally desorbed for subsequent analysis by, e.g., chromatography. 

Exhaled air Collected on Tenax -GC, thermally desorbed injected into GC HRGC/MS 0.3 ppb 95-99 Wallace et al. 1986a... 

Human milk Purged warm trapped in Tenax(I)-GC thermally desorbed HRGC/MS Qualitative identification NR Pellizzari et al. 1982... 

Collected in stainless steel canister preconcentrated in cooled adsorbent thermally desorbed... 

Adsorbed on Tenax -GC thermally desorbed to on-column cold trap heat-released... 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

To overcome the problems of relatively low sample capacity associated with SPME, a technique known as stir-bar sorptive extraction has been reported by Baltussen etal. A glass-coated magnetic stir bar was coated with 50-100 iL of PDMS. Sample extraction was performed by placing the stir bar in the sample with subsequent stirring for 30-120 min. After extraction, the stir bar was removed and analytes were thermally desorbed at 150-300 °C for 5 min for GC, or liquid desorbed for LC. Qualitative analysis of organochlorine residues in wine has been reported using a commercially available product known as Twister. ... 

As well as the methods discussed above headspace samplers, pyrolyzers, thermal desorbers, and column switching devices could be considered as specialized sample inlet systems. These are treated separately in Chapter 8. 

DP-MS suffers from system saturation sample loads of a few ig are to be used. DP-ToFMS equipped with El and FI sources is a thermal separation technique for solids which allows exact mass determination (Section 6.3.3). In order to detect and characterise polymer fragments of higher molecular weight, techniques such as DCI, in which the sample is thermally desorbed by the filament on which it is directly deposited, and laser desorption... 

Relatively few descriptions of direct mass spectral analysis of plastics compounds have appeared in the literature [22,37,63,240,243], Additives in PP were thermally desorbed into a heated reservoir inlet for 80 eV EI-MS analysis [240], Analysis of additives in PP compounds via direct thermal desorption ammonia CI-MS has been described [269] and direct mass spectrometric oligomer analysis has been reported [21],... 

Figure 9.15. Comparison of the total ammonia adsorption of coated and extruded V2O5/WO3—Ti02 catalysts. Catalyst volume = 7 cm3. Model gas for loading 10% 02, 5% H20, NH3 = 1000ppm, and balance N2. GHSV = 52000h 1. Model gas for temperature-programmed desorption (TPD) experiment 10% 02, 5% H20, NO = 1000 ppm, NH3 = 1000 ppm, and balance N2. NH3 desorbed is calculated as the sum of thermally desorbed NH3, directly measured at the catalyst outlet, and chemically desorbed NH3, measured by the reduction of the NO concentration due to the SCR reaction.

Air (particulate and organolead) Collection of particulate matter collected onto nucleopore filters filtered gases cryogenically trapped and thermally desorbed XRF (particulate) GC/GFAAS (gaseous) 0.3 pg/m3 0.2 ng/m3 46->90 90-100 De Jonghe et al. 19 ... 

The SPME process, adapted for solid or viscous matrix, is shown in Figure 10.1. A fused silica fibre, coated with a polymer, is installed inside a stainless steel hollow needle. In the first step, the needle is introduced in the sample vial through the septum. The fibre is then exposed to the headspace above the sample and the organic analytes adsorb to the coating of the fibre. After a variable sampling time, the fibre is drawn into the needle and the needle is withdrawn from the sample vial. Finally, in the same way, the fibre is introduced into the chromatograph injector where the analytes are thermally desorbed. 

The amount of hydrocarbons present in the fire effluents have been measured in two different ways 1) amount of non-burnt hydrocarbons 2) soot was separated from gas with a glassfilter and latei extracted with cyclohexane. After the porous filter an absorbent glass-tube was connected with either charcoal or Tenax GC as the absorbent. Charcoal tubes were later extracted with carbon disulfide for analysis and Tenax tubes directly thermally desorbed t( a gas chromatograph and a mass spectrometer. 

Results from thermally desorbed samples taken from the fire effluents of the different materials in the full scale fire experiments are presented in Table VI. The different species of hydrocarbons have been grouped together and presented in three different cathegories T< M and R representing Trace. Medium and Rich concentration. In this way it is possible to get an idea of the amount of contribution of different species of hydrocarbons to the fire effluents of each material. These results agree well in principle with results obtained by other researchers (6. 7). 

Thermal desorption is a technique that involves the pre-concentration of substances prior to chromatography. For example, it enables the build-up of volatile materials such as toxic solvent vapours in an industrial or laboratory environment to be monitored. The vapours are allowed to pass through a small tube containing an absorbent such as Tenax or Poropak by atmospheric diffusion for a prescribed period of time. The tube is subsequently connected to the injection port of the chromatograph and purged with carrier gas whilst being rapidly heated. This causes any previously adsorbed substance to be thermally desorbed and swept onto the column in a narrow band to be separated in the normal way. 

The GAGI samplers, installed along traverses at both Honerat and Unknown, were left in place for 5 months before being retrieved. Retrieved samplers from both locations were sent to Gore Laboratories where they were heated, and compounds were thermally desorbed and analyzed with a gas chromatograph/mass spectrometer (GC/MS). 

Hydrocarbon Anomalies in Soils Soil samples were collected at 200 to 500 meter intervals over the Lisbon and Lightning Draw fields and analyzed for thermally desorbed C1 to C12 alkanes by GC-FID and solvent-extractable C6 to C36 aromatics by fluorescence... 

Automatic Continuous Air Monitoring System (ACAMS) This system can detect G agents, VX, or mustard agents at very low levels. It is an automatic gas chromatograph that first collects agent on a solid sorbent and then thermally desorbs the agents into a separation column for analysis. 

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