Headspace solvent microextraction.

A hanging microliter drop of 1-octanol is shown to be an excellent preconcentration medium for headspace analysis of volatile compounds in an aqueous matrix by gas chromatography (GC) or gas chromatography/mass spectrometry (GC/MS). Model compounds benzene, toluene, ethylbenzene, and o-xylene (BTEX) are conveniently and rapidly preconcentrated in the microdrop. An internal standard, decane, is present in the organic extracting solvent, and linear calibration curves of relative peak area versus aqueous concentration are obtained for the four model compounds. Detailed kinetic studies reveal that the overall rate of mass transfer is limited by both the aqueous-phase stirring rate and the degree of convection within the organic phase. The very low vapor pressure of 1-octanol results in minimal evaporation of the microdrop during the extraction time. This system represents an inexpensive, convenient, and precise sample cleanup and preconcentration method for the determination of volatile organic compounds at trace levels.

Observation of gel-induced protein modifications in sodium dodecylsulfate [corrected] polyacrylamide gel electrophoresis and its implications for accurate molecular weight determination of gel-separated proteins by matrix-assisted laser desorption ionization time-of-flight mass spectrometry.

Matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry (TOFMS) can potentially provide accurate molecular weight information of proteins separated by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE). Several issues related to resolution and accuracy of molecular weight measurement are investigated by using a time-lag focusing MALDI-TOF mass spectrometer. The effects of the gel components SDS, glycerol, and tris buffer on the mass spectral signals are studied systematically. Glycerol and tris buffer are shown to have little or no effect on resolution and mass accuracy, whereas SDS degrades sensitivity, resolution, and mass accuracy even at low concentrations. A simple and fast gel extraction technique is presented which is capable of detecting proteins loaded at the low-picomole level on the gel. The sample preparation procedure used in this work appears to remove most of SDS from the gel, thereby reducing the peak broadening effect caused by SDS and resulting in high resolution and accurate measurement of proteins. However, for proteins containing cysteines, the molecular ions are composed of a distribution of acrylamide-protein adducts likely formed by reaction with unpolymerized acrylamide in the gel during the gel separation process. The implications of gel-induced protein modifications on the accurate molecular weight measurement of gel-separated proteins are discussed.

Solvent microextraction as a speciation tool: determination of free progesterone in a protein solution.

A 1-microL drop of n-octane suspended from the tip of a microsyringe needle in 500 mL of stirred aqueous solution is used to extract unbound progesterone in the presence of 1% (w/v) bovine serum albumin (BSA) for analysis by gas chromatography (GC). The observed extraction kinetics are in good agreement with a proposed convective-diffusive mass transfer model which accounts for diffusion of both free and bound species in the aqueous Nernst diffusion film adjacent to the interface. The equilibrium binding constant, which can be measured both at equilibrium and at nonequilibrium extraction times, is in good agreement with literature values. The very small phase ratio employed (i.e., 2 x 10(-6) mL of organic/mL of water) avoids perturbation of the aqueous solution equilibria.

Solvent microextraction into a single drop.

An analytical technique is described which combines solvent extraction with gas chromatographic (GC) analysis in a simple and inexpensive apparatus involving very little solvent consumption. A small drop (8 µL) of a water-immiscible organic solvent, containing an internal standard, is located at the end of a Teflon rod which is immersed in a stirred aqueous sample solution. After the solution has been stirred for a prescribed period of time, the probe is withdrawn from the aqueous solution, and the organic phase is sampled with a microsyringe and injected into the GC for quantification. The observed rate of solvent extraction is in good agreement with a convective-diffusive kinetic model. Analytically, the relative standard deviation of the method is 1.7% for a 5.00-min extraction of the analyte 4-methylacetophenone into n-octane.