Heat fiber-sample adsorption

Heat Fiber-Sample Adsorption (AHfs) Enthalpy of the process related to the transfer of analyte(s) from the sample toward the hber. A negative value means that the process is exothermic, and thus, an increase in temperature can enhance the amount of compound extracted. On the other hand, a positive AH value indicates that the extraction process is endothermic thus, increasing the temperature lessens extraction efficiency. 

The fiber-sample heat of adsorption (AHk) values were negative for aU the analytes (-31.6,-14.8, and -10.8 kJ/mol for DCM, TCE, and PCE, respectively), thus confirming the exothermic nature of the adsorption process. 

Pawliszyn, 1995). In SPME, a phase-coated fiber housed within a syringe is exposed to the sample medium (water or air), allowing the analytes to be adsorbed on the fiber coating. For organic compounds having high partition coefficients, adsorption equilibrium can be attained within minutes. After sample adsorption, the fiber is withdrawn into the needle, which is then inserted into the heated injection port of the GC. The adsorbed analytes are thermally desorbed and analysis proceeds in the same manner as in normal GC/MS analysis. 

Solid-phase microextraction (SPME) is a technique that was first reported by Louch et al. in 1991 (35). This is a sample preparation technique that has been applied to trace analysis methods such as the analysis of flavor components, residual solvents, pesticides, leaching packaging components, or any other volatile organic compounds. It is limited to gas chromatography methods because the sample must be desorbed by thermal means. A fused silica fiber that was previously coated with a liquid polymer film is exposed to an aqueous sample. After adsorption of the analyte onto the coated fiber is allowed to come to equilibrium, the fiber is withdrawn from the sample and placed directly into the heated injection port of a gas chromatograph. The heat causes desorption of the analyte and other components from the fiber and the mixture is quantitatively or qualitatively analyzed by GC. This preparation technique allows for selective and solventless GC injections. Selectivity and time to equilibration can be altered by changing the characteristics of the film coat. 

Thermal desorption from a solid phase microextraction (SPME) fiber has shown considerable potential for selectively introducing semivolatile chemicals into an IMS. ° The SPME approach is a simple design patterned after the early platinum wire introduction thermal desorption system described. With SPME, semivolatile compounds are extracted by either absorption or adsorption onto a nonvolatile polymeric coating or solid sorbent phase that has been coated onto a small fiber. Normally, the adsorption liber is housed in the needle of a syringe to permit puncture of a sample bottle septum and to protect the fiber from contamination during transfer of the fiber from the sample to the IMS instrument. After the analytes are adsorbed onto the SPME fiber, the fiber is retracted into the needle and then injected in a normal syringe technique such that the fiber is extended into the heated region of the IMS and the analytes are desorbed from the fiber into the clean carrier gas of the IMS. 

Aliquots (500 nl) of urine were sampled by manual SPME prior to GC/MS. The vials used for SPME were steam-cleaned by hot distilled water, rinsed three times with triple-distilled water, and air-dried prior to the addition of urine. Either a reverse-direction insert top, conditioned for several days in a GC oven at 250 °C, or an aluminum foil cover through which the fiber and its holder were inserted was used. Based on the results from Asian female urine (Rasmussen, 2001), 100-pm polydimethylsiloxane (PDMS) SPME fibers were employed. No fibers were immersed rather, they were exposed in the headspace above the liquid sample, which was gently stirred by a tiny magnetic stirring bar. For each sample, the adsorption of volatile compounds on the fiber was conducted first at native pH and at ambient temperature (25 °C), and then heated to 37 °C. For selected duplicate samples, 1 mg/ml of non-specific bacterial protease (Sigma cat. no. P-5147) was added (Poon et al., 1999 Yamazaki et al., 1999). For some of these samples, the pH was adjusted to 4.0, the pH demonstrated to result in release of ligands from urinary albumin (Lazar et al., 2002). Selected samples were also reduced further to pH 1.0. 

Solid-phase microextraction (SPME) SPME consists in the adsorption of the target compounds on a thin polymeric film deposed on the surface of a capillary fiber. The mass transfer can be achieved from liquid media in direct contact with the extracting coated fiber as well as from gaseous environments. Volatile or semivolatile herbicides existing in solid samples can be easily transferred in the gas phase on heating in closed vials, followed by trapping of the resulting vapors in the coated fiber (procedure is known as Headspace HD/SPME). 

Precision of the method is controlled by several factors, first of all, the status of the SPME fiber. The mean fiber lifetime is 100 runs when desorbing into an injector heated to 220°C or immersing into water saturated with salt and at pH 2. Direct sampling from dirty matrices may cause faster degradation of the fiber, due to the adsorption of high-molecular-weight species such as proteins, salt crystals, or humic materials. If the process is reversible, the fiber can be soaked with a proper solution [8j. When a fiber used for quantitative analysis starts to degrade. 

In the SPME extraction method, the fiber was exposed to the headspace above the truffle sample for 30 min at either 80°C or at room temperature. In the HS Tenax adsorption procedure, thin strips of sample material were placed between plugs of glass wool in an empty stainless steel tube, which was then mounted in a desorber oven. The oven was heated to 60°C, and approximately 20 mL of He gas was used to flush the tube to the Tenax-filled cold (-40°C) trap over a 4-min period. The trapped volatiles were desorbed by rapid electrical heating to 250°C for 45 seconds and transferred to a GC column. 

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