Solvent Desorption

J. J. Vreuls, R. T. Gliijsen, G. J. de Jong and U. A. Th Brinkman, Drying step for introduction of water-free desorption solvent into a gas cliromatograph after on-line liquid clir omatograpliic Cace enrichment of aqueous samples , 7. Chromatogr. 625 237 - 245 (1992). 

To analyze charcoal tubes or vapor monitors for hydrocarbons by IR, the desorption solvent would have to be IR inactive in the region 3100 to 2750 cm-. Two such solvents Freon R 113 (1,1,2 trichloro-1,2,2-trifluoroethane) and perchloroethylene meet this requirement. 

Many of the charcoal tube methods are based on NIOSH Method P CAM 127 (4) for organic solvents. In this method, a known volume of air is drawn through a charcoal tube to trap organic vapors, the charcoal is transferred to a vial, and the sample is desorbed with carbon disulfide. The sample is analyzed by gas chromatography (GC) with flame ionization detection (FID). Most methods use CS2 as the desorption solvent because it yields good recoveries from charcoal and produces a very low flame response. 

Method No. Substance OSHA Std. (mg/m3) Sorbent Desorption Solvent Analytical Method Range (mg/m3) Coefficiei Variation... 

When selecting a desorption solvent, the effect of the solvent on recovery of sample matrix contaminants should be considered. If available, a control sample matrix should be screened against potential elution solvents to assess which solvents can be used to maximize recovery of the analyte of interest and minimize the elution of sample contaminants. 

Compared with thermal desorption, solvent desorption is plagued by a number of shortcomings. For example, VVOCs are lost when the liquid sample is reconcentrated prior to its analysis. Moreover, solvent peaks may overlap with the peaks of VVOCs. A recent comparison of thermal and solvent desorption efficiencies also showed that with few exceptions solvent desorption consistently underestimates various classes of VOCs found in typical indoor air [59]. According to Wolkoff [5], solvent desorption leads to considerable loss in analytical sensitivity. 

B. Thermal Desorption, Solvent Extraction, and Supercritical-Fluid Extraction. 627... 

Thermal desorption, solvent elution, and solvent extraction are used in VOC preparation schemes for samples collected on solid sorbents. Thermal desorption methods require determination of the sensitivity of the target analytes to the desorption temperature. It is also critical to remove all traces of O2 from the gas used to purge the sorbent and transfer the analytes to the preconcentration trap. Quantitative recovery of monoterpenes from Tenax is accomplished at thermal desorption temperatures of 250°C. ° Multibed sorbent tubes consisting of graphitic carbon solids and molecular... 

In offline techniques, the desorption solvent most often used for SPE with C18 sorbent is ethyl acetate, sometimes associated with isooctane.MTBE is also used. In the case of polymeric phases, methanol, acetonitrile, and acetone are used. ... 

Desorption solvent Ethyl acetate + Acetonitrile Etiiyl acetate + Acetone + MTBE Etiiyl acetate D... 

Desorption of phthalate esters from cartridges can be performed by extraction with organic solvents or by thermal desorption. Solvent extraction can be made using direct elution, Soxhlet extraction or extraction assisted by ultrasounds. 

Extraction of the spiked PUF was done simulating as much as possible interaction of WPA and PUF this means the WPA had to cross the complete PUF (the spiked side of PUF was aligned to the desorption solvent inlet). 

The recovery of volatile analytes by partial concurrent solvent evaporation was significantly improved by using a volatile desorption solvent (e.g. methyl acetate). This allowed a low transfer temperature to be used and the addition of a presolvent to ensure the early formation of a solvent film to enhance solvent trapping [77,117]. The concentration of analytes in the initial portion of the desorption solvent is high and falls to a low level or zero at the completion of the desorption process. The early escape of weakly trapped volatile analytes deposited at the front of the solvent film, where the analyte concentration is highest, is the main cause of their low recovery. Volatile analytes poorly trapped by the solvent film in the retention gap are transported through the gas phase faster than the front of the solvent film advances during solvent transfer. 

Once these analytes have moved ahead of the film boundary they are no longer retarded and are lost through the solvent vapor exit. The loss of partially trapped analytes is more critical in on-line SPE-GC than for conventional large volume injection because of the nonuniform distribution of analytes in the solvent film. The introduction of a small volume of organic solvent, presolvent, immediately in front of the sample plug to ensure that a solvent film is already present in the retention gap when sample transfer starts, can be used to minimize the loss of volatile analytes. Acceptable recovery of analytes as volatile as xylene and chlorobenzene was obtained using an on-column interface when 30-50 jl1 of methyl acetate was introduced immediately in front of 50 xl of the same desorption solvent. 

Humidity. High humidity during collection may produce low recoveries for some compounds such as those which are easily hydrolyzed. If a large amount of water is collected, it may prevent good contact with a nonpolar desorption solvent, or change the equilibrium. However, high humidity may also improve desorption for some chemicals. 

For many systems the desorption efficiency can be written in terms of an equilibrium constant it is dependent on the ratio of solvent to sorbent for the distribution of the compound between the two phases. An equation has been derived by Dommer and Melcher which relates the desorption efficiency to the volume of solvent and the amoimt of sorbent. The equation assumes the system is in equilibrium and can be approached from either direction. That is, the same desorption efficiency should be obtained when the compound is initially in the solvent or the solid phase. This has been shown to apply to most organic compounds in the concentration range of interest in industrial hygiene analyses. Desorption efficiency using the phase equilibrium method is similar to direct injection into the sorbent, only the test compund is prepared in the desorption solvent. 

Several workers use commercial SPME probes in conjunction with CE and related techniques. In all cases, probes were conditioned both before first use and between uses. Poly acrylate (PA) probes were conditioned in 50 50 methanol/water (30 min) or the desorption solvent, for example, acetonitrile. A carboxen/PDMS probe was cleverly conditioned by exposing it to GC inlet conditions (300°C for 2 h). The solutes in this case were halophenols. Obviously, this treatment would not be useful for ionic compounds, for example, cations in a PA phase (although if the cation were a low molecular weight protonated nitrogen base, it may work). Thirty minutes in methanol for a PDMS/divinyl benzene (DVB) probe proved suitable. Other workers used a 30 min initial conditioning for four different types of probes, and used a 20 min conditioning between analyses. The theme is clear. A 30 min exposure to a suitable solvent will work to condition SPME probes for use in CE both before and between analyses. 

Removal of analytes from a tube is an elution problem analogous to frontal chromatography and has been discussed in detail. In general, if the desorption temperature of a gas chromatogram is high and thin coatings are used, then all the analytes are in the gas phase as soon as the coating is placed in the injector. The desorption time then corresponds to the elution of two void volumes of the capillary. For liquid desorption, the desorption volume can be even smaller since the analytes can be focused at the front of the desorption solvent. 

SPE processes may be subjected to computer assisted optimization. As an example, an orthogonal array design was employed for the optimization of an SPE process applied to atrazine, diazinon, ame-tryn, and fenthion in surface water. Seven parameters (type of desorption solvent, type of sorbent, flow rate of the elution solvent, sample pH, sample volume, elution volume, organic modifier addition, and flow rate of the water sample) were studied and optimized. 

Provided the desorption solvent is sufficiently powerful and polar, it should recover between 50 and 100 % of the additive present in the silica gel fraction and provide sufficient material for examination by UV or IR spectroscopy or MS. 

The use of benzene as the desorption solvent in-aeases the hazards of analysis because of the potential carcinogenic effects of benzene. 

FIGURE 11.25 Flowthrough SPME extraction diagram the fiber is contained within a tube through which the analyte solution and later, a desorption solvent, flows [reprinted with permission from R. Eisert and J. Pawliszyn, J. Chromatogr. A 776, 293 (1997), Figure Id copyright 1997, Elsevier Science B.V.],... 

Syringe pump on/off 2, nitrogen valve on/off 3, start of SVE controller. Abbreviations i-PrOH, isopropanol MeOAc, methyl acetate (desorption solvent). 

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