Using a Purge and Trap Autosampler: An Overview
An autosampler inserts laboratory samples into the inlets of a chromotograph. In the past, the function of an autosampler was performed by hand. In addition to becoming time consuming, this technique placed samples at risk for contamination, and the operator at risk for coming in contact with harmful substances such as Volatile Organic Compounds (VOCs). Today, autosamplers have a number of mechanisms for inserting samples automatically. For VOC analysis, the most helpful of these mechanisms is the purge and trap type, which purges compounds from the sample and traps them in the concentrator, exactly where they are heated. Other types of autosampler insertion mechanisms consist of:
- S/SL injection mechanisms, which insert the sample by way of a syringe via a septum. - On-column inlet mechanisms, which insert the sample into the column without heating it. - PTV injector mechanisms, which insert a low heat sample at a controlled price. - Gas source inlet mechanisms, which insert a gaseous sample into the carrier gas stream by way of a switching valve that connects to a sample loop.
In addition to being mechanically distinguishable from the insertion mechanisms above, this type of mechanism is also distinguishable for its capability to isolate low levels of VOCs and other substances. A purge and trap autosampler is used for measuring substances whose presence can only be detected by a concentrator. This type excels at sequestering trace amounts of substances that other mechanisms would not capture. Water analysis, soil analysis, and food and beverage analysis all use the this technique to detect VOCs in infinitesimal amounts.
According to the Environmental Protection Agency's (EPA) "Test Techniques for Evaluating Strong Waste, Physical/Chemical Methods" (SW-486), there are two techniques for testing a sample for VOC analysis: Purge and Trap Method 5035B, in which a strong sample should undergo methanol extraction, with an aliquot from the extraction becoming added to the mechanism and Closed System Purge and Trap Method 5035, in which the sample is sealed in an airtight vial and then analyzed with out breaking the seal.
How the Purge and Trap Technique Functions
This method consists of 3 steps that happen prior to the compounds becoming delivered to the column via carrier gas:
- Carrier gas is bubbled via the sample to release its compounds - The compounds are released and trapped in the concentrator - The concentrator is heated until the compounds vaporize
From the column, the vaporized compounds travel to the detector exactly where their traits are analyzed. When the detection process is total, the outcomes are displayed on the data interface of the chromotograph, and can be recorded by a chart recorder when one is attached.
varian autosampler details and misconceptions.
