Sample Preparation
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Solid phase microextraction
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This extracts compounds from liquids, air, or even slude without using any solvent. The key component is a fused silica fiber coated with a 10 to 100-μm-thick film of stationary phase similar to those used in gas chromatographry.
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Stir-bar sorptive extraction
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This method is closely related to solid-phase microextraction, but is approximately 100 times more sensitive for trace analysis. A magnetic stirring bar enclosed in a thin glass jacket as the stationary phase in nonpolar gas chromatography columns. The stirring bar is placed in an aqueous liquid sample such as fruit juice, wine, urine, or blood plasma and stirred for 0.5-4 Hours to absorbed hyrdophobic analytes. 50 to 250 times more analyte is extracted from the stir bar. It's touched with a tissue to remove water droplets and placed in a thermal desorption tube. Desportion is typically conducted by heaing the tube to 250°C for 5 min in the flowing carrier gas. The volatile analytes are collected by cold trapping and sperated by gas chromatography.
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Purge and Trap
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This method is for removing volatile analytes from liquids or solids (such as groundwater or soil), concntrating the analytes, and introducing them into a gas chromatograph. In contrast with solid microextraction, which removes only a protion of analyte from the sample, the goal in purge and trap is to remove 100% of analyte from the sample. Quantitative remoal of polar analytes from polar matrices can be difficult. During the purge and trap process, gas flows through the adsorbent tube from end A to end B. After purging all analyte from the sample into the adsorption tube, reverse the gas flow to go from B to A and purge the trap at 25°C to remove as much water or other solvent as possible from the adsorbents. Desorbed analytes flows into the chromatography column where they are concentrated by cold trapping.
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INJECTION
Split Injection
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Analytes of interest constitue >0.1% of the sample, split injection is usually prefered. For high-resolution work, best results are obtained with the smallest amount of sample (≤ μL) that can be adequately detected -- preferable containing ≤1 ng of each compenent. A complte injection contains too much material for a 0.32-mm-diameter or smaller column. A split injection delivers only 0.2-2% of the sample in the comlumn.
Splitless Injection
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Force trace analysis of analytes that are less than 0.01% of the sample, spliteless injection is appropriate. This will use the same injection port as the Split. The glass liner is a straight empty tube with no mixing chamber. A large volume of dilute solution in a low-boiling solvent is injected slowly into the liner, with the split vent closed. Slow flow through the septum purge is mainteined during the injection and chromatography to remove any vamport that escape from the injectiion liner.
Cold Trapping
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An alternative means of condensing solutes in a narrow band at the beginning of the column is called cold trapping. The initial column temperature is 150°C lower than the boiling points of the solutes of interest. Solvent and low-boiling components are eluted rapidly, but hihg-boiling solutes remain in a narrow band aat the beginning of the column. The column is then rapidly warmned to initiate chromatography of the high boiling solutes.