Sample application thermal desorption

Applications The potential of a variety of direct solid sampling methods for in-polymer additive analysis by GC has been reviewed and critically evaluated, in particular, static and dynamic headspace, solid-phase microextraction and thermal desorption [33]. It has been reported that many more products were identified after SPME-GC-MS than after DHS-GC-MS [35], Off-line use of an amino SPE cartridge for sample cleanup and enrichment, followed by TLC, has allowed detection of 11 synthetic colours in beverage products at sub-ppm level [36], SFE-TLC was also used for the analysis of a vitamin oil mixture [16]. 

These liner exchange systems make feasible yet another analysis mode direct thermal desorption (DTD). Here the liner or an insert is packed with the solid sample. The liner exchange system can then be used in place of a conventional autosampler. The liner is automatically inserted into the PTV and the volatiles thermally desorbed onto the column. Some analysts may feel uneasy about such desorption from the solid phase how does one know that all of the volatile analytes have been released from the sample crystal lattice However, where applicable, this approach may not be as difficult to validate as one might imagine. For instance, the PTV can be cooled after the analyte transfer, and then, at the end of the chromatographic temperature programme, reheated to repeat the process. Ideally all of the analyte should transfer in the first cycle and none in the second, demonstrating that complete desorption occurs in the method. 

Notes The most commonly used porous polymer sorbent is Tenax-GC, although the Porapakand Chromosorb Century series have also been used Tenax-GC has been used with thermal desorption methods, but can release toluene, benzene, and trichloroethylene residues at higher temperatures in addition to Tenax-GC, XAD 2-8, Porapak-N, and Chromosorbs 101, 102, 103, and 106 have found applications, sometimes in stacked sampling devices (for example, a sorbent column of Tenax-GC — Chromosorb 106 in tandem) Chromosorb 106, a very low polarity polymer, has the lowest retention of water with respect to organic materials and is well suited for use as a backup sorbent... 

R.M. Black, R.J. Clarke, D.B. Cooper, R.W. Read and D. Utley, Application of headspace analysis, solvent extraction, thermal desorption and gas chromatography-mass spectrometry to the analysis of chemical warfare samples containing sulfur mustard and related compounds, J. Chromatogr., 637, 71-80 (1993). 

Thermal Desorption Thermal desorption is an alternative GC inlet system particularly used for VOC analysis. However, the analytes subjected to thermal desorption must be thermally stable to achieve successful analysis. Otherwise, decomposition occurs. This technique is mainly used for determination of volatiles in the air. Such a methodology requires sample collection onto sohd sorbents, then desorption of analytes and GC analysis. Traditionally, activated charcoal was used as a sorbent followed by extraction with carbon disulfide. However, solvent desorption involves re-dilution of the VOCs, thus partially negating the enrichment effect. Therefore, the sampling method is to pump a sample of gas (air) through the sorbent tube containing certain sorbents in order to concentrate the VOC. Afterwards, the sample tube is placed in thermal desorber oven and the analytes are released from the sorbent by application of high temperature and a flow of carrier gas. Additionally, desorbed compounds are refocused in a cold trap and then released into the GC column. Such a two-step thermal desorption process provides a narrow chromatographic band at the head of the column. 

Typical applications of thermal desorption are related to determination of volatiles and semivolatiles in air samples [55-58]. This technique has been applied for investigation of insect pheromones [59] and drugs in urine [60]. The same principles can be applied for solids or semisolids (soil, sediment, pharmaceutical raw materials, cream, ointments, polymers, etc.)... 

Schnelle-Kreis, J., Orasche, J., Abbaszade, G., Schafer, K., Flarlos, D.P., Hansen, A.D.A., Zimmermann, R. Application of direct thermal desorption gas chromatography time-of-flight mass spectrometry for determination of non-polar organics in low-volume samples from ambient particulate matter and personal samplers. Anal. Bioanal. Chem. 401, 3083-3094 (2011)... 

Another configuration of MAP gas-phase extraction relates to dynamic headspace sampling, often referred to as purge and trap sampling. The container can be fitted with an aperture enclosing a trap, or a sorbent, cooled by some common means. This allows the application of a prolonged, low-power irradiation, or of a multi-pulse irradiation of the sample, thus providing a means to extract all of the volatile analytes from the matrix. The contents of the trap can then be transferred (by elution for a chemical or sorbent trap, or by thermal desorption for a cold trap) to an analytical instrument, such as a... 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

Baltussen, E., David, F., Sandra, P., Janssen, H.-G., and Cramers, C. A., Retention model for sorptive extraction-thermal desorption of aqueous samples application to the automated analysis of pesticides and polyaromatic hydrocarbons in water samples, J. Chromatogr. A, 805, 237-247, 1998. 

Differences between different formulations were detected using a mass spectral based chemical sensor with static headspace introduction. Results were validated using traditional GC/MS with SBSE-thermal desorption introduction. Advantages of using the chemical sensor include fast sample throughput and easy to interpret results. For this particular application, flavor formula 1 was found to be very similar to flavor formula 4. This result was also confirmed by the flavor supplier. 

Thermal desorption from the SPE cartridge is a further possibility [77,122]. In this approach, the sample is introduced at a controlled speed into the packed liner of a PTV injector set to a low temperature with the water eliminated via the split vent. Salts and involatile polar material are rinsed from the sorbent with water and the sorbent dried by purging with a high carrier gas flow rate. The trapped analytes are subsequently desorbed in the splitless mode by rapidly heating the PTV to the injection temperature. The most commonly used sorbents are Tenax and Carbofrit. The method is restricted to a narrow range of applications by the low breakthrough volume of polar analytes on the... 

Headspace and thermal desorption are thermal extraction methods which can be directly connected to gas chromatography and do not need additional sample preparation. Usually both methods are applied for the determination of volatile compounds in air and water. Only few applications are known for the direct treatment of soil samples. The investigations for analysis of phenylarsenic compounds were carried out with an Headspace Sampler HS40 (Perkin-Elmer Inc.) and a Thermal desorption system TDS 2 (Gerstel GmbH, Germany). 

The experimental technique for the trace analysis of metals simply involves the production of an emitter of acceptable quality. In general, 10 /im tungsten wires are activated at high temperature with benzonitrile in a multiple activation device. As the result of such an activation process, the tungsten wire is covered with dendrites of partially ordered pyrocarbon. Due to the small radii of curvature of the tips of the microneedles, the field strength is enhanced to a. level suitable for FDMS. These emitters are mechanically stable, which is important for repeated use they can also be chemically and thermally strained. This property is a prerequisite for the pyrolysis of the organic matrix and desorption of the metal cations, and last not least, the surface area of the emitter is sufficient for sample application. 

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