While Purge and Trap systems have not changed much over the past few years, capillary column technology and mass selective detector technology has improved dramatically. These enhancements have provided the opportunity to drastically reduce analytical run times by using shorter and smaller internal diameter capillary columns without a loss in resolution or sensitivity. Analyses that once were performed in 30 minutes could now be accomplished in less than 10 minutes!
Since EPA Purge and Trap methodology stipulates Purge, Desorb and Bake parameters generally in excess of 20 minutes, this reduction in analysis time could not be efficiently utilised in the VOC environmental laboratory. The Purge and Trap cycle time is now the limiting factor in a laboratory's ability to increase sample throughput.
By adding a valve-switching device, known as the PT2, to add a second Purge and Trap Concentrator to a GC/MS system, sample throughput can be improved. As the first concentrator is purging a sample, the second concentrator is preparing for the next sample, thus alternating sample injections into the GC/MS. This overlap of Purge and Trap cycle time approximately doubles the system's sample throughput.
Experimental
The PT2 is used to interface the two EST Encon Purge and Trap Concentrators and one EST 8100 vial autosampler to one GC/MS system. The PT2 directs the sample purge flow from the autosampler to one of the concentrators through the use of a heated 6 port valve. The PT2 also directs the sample desorption flow from the concentrator to the GC/MS through the use of a second heated 6 port valve.
The PT2 communicates electronically with the autosampler, the two concentrators and the GC with a remote interface cable to assure system synchronisation. To take full advantage of the PT2 benefits, it is necessary to optimise the GC operating conditions to reduce the GC oven cycle time to less than 15 minutes.
Results
To demonstrate the system's linearity, six levels with concentrations ranging from 1 to 100 µg/L were run on each concentrator. An initial calibration curve was then constructed for each concentrator with its respective standards. Next, a single initial calibration curve was constructed using alternating points from each concentrator (Standard 1 on PT#1, Standard 2 on PT#2 etc).
Discussion
With PT2, the sample flow paths of the system are separate. As a result of these separate sample pathways, if one purge and trap concentrator becomes contaminated, the integrity of the other concentrator remains protected. The separate pathways are also equal in length, which allows the user to perform either a single calibration using both concentrators, while still satisfying all the quality assurance criteria of EPA Method 8260B, or running two separate calibration curves, one on each concentrator.
There are several benefits to running a single calibration curve besides the added throughput it will provide. The single calibration option also provides a quick and easy visualisation diagnostic tool to ensure the performance of both purge and trap concentrators in the system. By using the dual calibration option, problems may be masked in one purge and trap concentrator which could easily escalate to additional problems. Running two calibration curves also requires two methods, each with a different set of response factors, MDLs etc. When using the single calibration option, it is imperative to run continuing check verifications (CCVs), blanks, laboratory control standards (LCSs) and matrix spike/matrix spike duplicates on each purge and trap concentrator. If the dual calibration is used it is suggested that a 'marker' standard be run consistent with only one purge and trap concentrator. For this work fluorobenzene was used.
Conclusion
Until now, the purge and trap cycle time has been the deterrent to improving productivity in the environmental volatile organic laboratory. Simply by adding one purge and trap concentrator in conjunction with the PT2 interface to an existing system, productivity can be improved by increasing the number of analyses one GC/MS system can achieve during each 12 hour 'Tune' period.
With the conditions outlined in this paper sample throughput was increased to where 48 analyses could be performed in a given 12 hour 'Tune' period. This increase in productivity allows environmental laboratories to be more profitable and competitive.
