Solvent desorption for

The tubes for solvent desorption are typically constructed of glass tubing, with both ends flame-sealed and containing two sections of suitable sorbent. The first section contains the adsorbent for the sample and the second section a back-up to test for breakthrough. Packed tubes are commercially available, alternatively empty tubes can be packed in the laboratory. The commercial sorbent tubes for solvent desorption, for example XAD glass tubes, are assumed to be clean but their cleanliness must, regardless of the assurances of the manufacturers, be checked by analyzing a minimum of five blank tubes by the same desorption and analytical procedures as used for the real samples. 

As for workplace monitoring, there have been two main types of diffusive sampler used for monitoring of indoor air (i) badge type samplers containing a strong adsorbent such as charcoal that requires solvent desorption for GC analysis and (ii) tube type samplers with weaker adsorbents such as the porous polymer Tenax that can be thermally desorbed. The samplers most widely used have been developed for monitoring of workplace atmospheres and applied to indoor air through modification of the exposure period and the analytical method. 

Steam-solvent distillation using diethyl ether has been used to remove and analyse for odour and taint from additives in food packaging films. Another technique that has been used is vacuum/thermal extraction. This procedure has been applied to polyamides and fluorocarbon polymers. The procedure is used for the direct isolation or release of volatile components from a polymeric matrix and may involve the combined use of vacuum and heat, as for example in the mass spectrometer direct insertion probe or during dry vacuum distillation. Alternatively, the volatiles may be swept from the heated sample by a flow of inert gas for concentration by freeze trapping and/or collection on to a solid adsorbent prior to thermal or solvent desorption for GC or mass spectrometric (MS) examination. 

Charcoal Usually for solvent desorption especially of non-polar compounds... 

Porous polymer Various types usually for solvent desorption suitable for a range of organic compounds including highly polar substances... 

MDHS 14 General method for the gravimetric determination of respirable and total dust MDHS 15 Carbon disulphide MDHS 16 Mercury vapour in air Laboratory method using hopcalite adsorbent tubes, and acid dissolution with cold vapour atomic absorption spectrometric analysis MDHS 17 Benzene in air Laboratory method using charcoal adsorbent tubes, solvent desorption and gas chromatography MDHS 18 Tetra alkyl lead compounds in air Continuous on-site monitoring method using PAC Check atomic absorption spirometry... 

MDHS 26 Ethylene oxide in air Laboratory method using charcoal adsorbent tubes, solvent desorption and gas chromatography MDHS 27 Protocol for assessing the performance of a diffusive sampler... 

For the charcoal, XAD, and PUF adsorbents discussed above, solvent extraction techniques have been developed for the removal and concentration of trapped analytes. Although thermal desorption has been used with Tenax-GC in some specialized air sampling situations [primarily with sampling volatile organic compounds (EPA, Method TO-17 )], this approach is not a viable alternative to solvent extraction for the charcoal, XAD, and PUF adsorbents. The polystyrene and PUF adsorbents are thermally unstable and the charcoal chemisorption bonding is more easily broken by... 

Toluene in air (charcoal diffusive samplers, solvent desorption and gas chromatography). General methods for sampling airborne gases and vapours. 

Melcher RG, Borders RA, Coyne LB. 1986. Development and validation of personal monitoring methods for low levels of acrylonitrile in workplace atmosphere I. Test atmosphere generation and solvent desorption methods. Am Ind Hyg Assoc J 47 152-157. 

To avoid this, we have employed hydrophobic resins for concentration and isolation of the products from aqueous media [49]. Organics are retained on the resin and subsequently can be desorbed with solvents such as ethanol, which is useful for green chemistry as it is readily recyclable, renewable and biodegradable. Nonextractive processes offer convenience, can be conducted with high throughput and afford low waste owing to ready disposal of the spent water, recyclability of the resin and the solvent used for desorption. 

Adsorption on XAD-2 and XAD-4 resins followed by solvent desorption and head space GS has been employed for the preconcentration and determination of volatile organosulfur compounds in estuary and seawater [330]. 

Extensive literature has developed related to the preferential interaction of different solvents with proteins or peptides in bulk solution.156-5X1 Similar concepts can be incorporated into descriptions of the RPC behavior of peptides and employed as part of the selection criteria for optimizing the separation of a particular peptide mixture. As noted previously, the dependency of the equilibrium association constant, /CassoCji, of a peptide and the concentration of the solvent required for desorption in RPC can be empirically described1441 in terms of nonmechanistic, stoichiometric solvent displacement or preferential hydration models, whereby the mass distribution of a peptide P, with n nonpolar ligands, each of which is solvated with solvent molecules Da is given by the following ... 

The analyte must be efficiently recovered. The usual mechanism for solvent desorption is selective displacement of the analyte. Selective displacement occurs as a more polar solvent displaces a less polar one on charcoal, just as a more active ion displaces a less active one on ion exchange resins. CS2 is frequently used to recover substances from charcoal, but simple alcohols cannot be displaced from charcoal by CS2, and it is necessary to add l%-5% of another alcohol to the CS2 to facilitate desorption. Frequently, low recoveries can be increased by increasing the quantity of solvent, if analytical sensitivity permits. Prospective solvents may be chosen based on polarity or solubility of the analyte. 

A mixture of benzene and methanol (19 to 1) was used for spreading the alkyl phosphonates. To minimize the influence of benzene on the film properties, the concentrations of the spreading solutions were > 1.5 X 10 3 gram per ml., and the experiments were performed at tt > 4 dynes per cm. (25). Moreover, higher proportions of methanol in the spreading solution did not alter the film properties under study for selected monolayers. For the sulfates, a mixed solvent containing water-benzene-2-propanol (1 10 10) was used because with the benzene-methanol solutions the properties of the films depended on the age of solution from which the films were prepared. Stearic and palmitic acids were spread from either hexane or the benzene-methanol solvent used for the phosphonates. Identical desorption results were obtained with the two solvents. 

While the exact recovery depends on the volatility of the flavor compounds, most compounds can be detected with this method when present at ppb (mg/liter) concentrations. Reproducibility (CV) is between 5% and 10%. To quantify the amount trapped, an internal standard curve can be made by adding the standards in solvent directly to the trap just before thermal desorption on the side of gas entry during thermal desorption. For liquid homogeneous samples, quantification of the amount in the matrix can be done by a standard addition methodology. 

The attitude implied in most current publications restricts (or extends) the term solvent-free to the stoichiometric application of solid or liquid reagents, with less than a 10% excess of a liquid or soluble reagent and/or less than 10% of a liquid or soluble catalyst. It seems widely accepted in the field that solvents used for pre-adsorption of reagents to a support or for desorption, purification, and isolation of the products are not counted in solvent-free syntheses. On the other hand, photolysis of insoluble solids in (usually aqueous) suspensions undoubtedly qualifies for inclusion as a solvent-free technique, but not the taking up of reagents from a liquid for reaction with a suspended solid. 

An example of an evaluation undertaken according to this protocol is for the measurement of benzene in ambient air by tube type samplers subject to thermal desorption (EN, 2005b) and samplers subject to solvent desorption (EN, 2005a). These standards were developed specifically to provide measurement methods meeting requirements for checking compliance with the air quality standard for benzene set by the European Ambient Air Quality Directive (Directive 96/62/EC, 1996). Evaluation of published data concerning the samplers under the test conditions enabled an evaluation of the expanded relative uncertainty of measurement at a limit value (5 pgnT3) for benzene in air and for solvent and thermally desorbed samplers a value of 13.4% was obtained. 

The OVM 3500 monitor was applied by Cohen et al. (1989) to determine VOC concentrations in a study of 35 homes in West Virginia, USA. Samplers were placed in the main bedroom at a height of approximately 1.5 m, and outdoors in a specially fabricated aluminum shelter that kept the sampler dry. They were exposed for one 3 week period before analysis by solvent desorption using 1.5 ml carbon disulphide and analysis with GC-FID. Results for indoor concentrations of some of the 17 compounds reported are shown in Table 3.2. 

EN (2005a) 14662-5. Ambient Air Quality-Standard Method for Measurement of Benzene Concentrations Diffusive Sampling Followed by Solvent Desorption and Gas Chromatography, The British Standards Institution, London, UK. 

The mobile phase for HPLC analysis, as well as the solvent media for desorption-aqueous 1-heptane sulfonic acid and acetonitrite mixture pH 3.5 flow rate 1 mL/min at ambient temperature. 

Another important mode of operation in SPME is in-tube SPME.65 In this system, usually coupled on-line to HPLC, a finite portion of sample is drawn through an internally coated capillary tube and then ejected into the sample vial. This technique requires more complex instrumentation than that used for standard SPME, but a greater sensitivity is obtainable with a longer tube (and consequently more sorbent). Two solvent desorption modes—are usually applied for introducing species into HPLC off-line desorption and on-line desorption. In the latter, the HPLC mobile phase is used for desorbing the analytes. 

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