2.2.1 Gaseous Samples
Gaseous samples are the easiest samples to analyze. For on-site analysis, a gaseous sample can simply be drawn into a syringe and the sample injected into a sampling valve/loop. Sampling loops are necessary in GC analysis in order to inject a consistent volume of a compressible sample. When a gas sample is taken at atmospheric pressure and injected into a GC inlet, the pressure in the GC will compress the gas in the syringe and not allow all of the sample volume to be injected. A sampling valve and loop consist of a four- or six-port valve that allows the sample to be injected into a fixed-volume (loop of tubing that is at atmospheric pressure. A valve is then turned that transfers all of the gas contained in the sample loop into the GC injection port. For field gaseous samples that need to be transported to the laboratory for analysis, a variety of sampling containers are available including Teflon bags and metal cylinders (referred to as bombs) that can be filled with the sample gas. It should be noted that when these containers are analyzed that they be adjusted back to their field temperature in order to avoid condensation of some gaseous analytes to liquids; this is especially true when industry smoke stack or process gases are being sampled and analyzed. Another possibility for sampling gaseous analytes is a resin tube. To collect a sample a known volume of gas is passed through a glass or metal tube containing a resin that has a strong affinity for the analytes. The analytes adsorb to the resin and after a sufficient volume of gas has passed through the system, each end of the resin tube is capped and transported back to the lab. In the lab, the resin is extracted with a solvent specific to the analysis and the solvent/analyte solution is injected into the GC. A relatively simple calculation yields the concentration of each analyte in the original gas volume. The obvious advantage of this method is concentration of the gaseous analytes and the improvement of detection limits, as opposed to analyzing the gas by direct injection. The resin tube method is commonly used in the monitoring of solvents in the work place where an industry worker will wear a portable personal pump that takes in atmospheric gases at the same rate as a human would breath under working conditions. At the end of the day, the tube is extracted and analyzed to determine if the worker was exposed to chemicals in excess of workplace limits according to Occupational Safety and Health Association standards.
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