What is Purge and Trap...

Purge and Trap is a type of dynamic headspace extraction of VOC compounds where a purge gas, such as helium or nitrogen, is passed through a sample and the VOCs are carried to a trap to concentrate them. Used globally for environmental testing, Purge and Trap can easily perform part per trillion levels of detection using GCMS. Many EPA methods mandate the use of this technique such as EPA 8260, 524 for drinking water, 624, etc.

Typically, a 5ml aqueous sample is placed into a sparge vessel. This can be done manually but is more commonly done by an autosampler that can also add an internal standard. Once the sample is in the vessel, the system begins to purge the sample by passing helium or nitrogen over the sample for 8-11 minutes at 40ml/min. These parameters can change depending upon the method but usually fall within this range. After purging, a small amount of gas is passed through the trap without going through the sample. This is called dry purge and is done in effort to remove some water that was purged over to the trap during the purge process.

After Dry Purge, the sample is ready to be sent to the GC for analysis. A valve inside the instrument turns, allowing GC carrier flow to pass through the trap which is heated during the step. The GC carrier gas sweeps the trap and the volatiles to the GC. Once the desorb step is completed, the valve rotates back to home position.

Valve purge diagram
Purge and trap desorb flow.

After desorption, the trap must be prepared for the next sample. To reduce carryover, the trap is heated an additional 10 degrees C and gas sweeps the trap, carrying any residual VOCs out a vent. The trap is then cooled and is ready for the next sample.

The control of moisture is key to performing reliable purge and trap results. Systems such as the ARID moisture reduction system in the EST Evolution 2 remove water during the purge process. Other systems remove water during the process of sending the sample to the GC. Water removal in this way is independent of the flow rate from the GC, while the ARID system is independent of GC flow offering a more stable and reliable method.

Automated systems such as the Evolution 2 Purge and Trap concentrator and Centurion Series of autosamplers can provide highly reproducible, reliable results while reducing carryover from sample to sample.