A Novel Approach for Profiling VOCs in Water Samples Using a Purge-and-Trap Concentrator

U.S. EPA drinking water Methods 524 and 8260 implement purge-and-trap (P&T) technology and are widely used by laboratories for the analysis of volatile organic compounds (VOCs). In order to effectively run water samples via these methodologies, a P&T concentrator must be able to effectively control the large amounts of water being transferred to the adsorbent trap prior to analysis by GC-MS. Minimizing the amount of water being transferred significantly improves data quality. An additional hurdle for P&T analysis is carryover between samples. Since most drinking water samples are typically clean (containing no sludge/sediment), it is imperative that the P&T concentrator be capable of minimizing any possibility of false positives for subsequent samples after a high-concentration sample is run. The Stratum PTC (purge-and-trap concentrator) (Teledyne Tekmar, Mason, OH) addresses both water management and carryover issues associated with P&T techniques.

Instrumentation

The instrument used was a Hewlett Packard 5973I mass selective detector (MSD) with an Agilent 6890 gas chromatograph (Agilent Technologies, Palo Alto, CA). A Stratum PTC concentrator and Aquatek70 autosampler (Teledyne Tekmar) were used for automated sampling.

P&T analysis

Sample concentration was carried out using an Aquatek70 autosampler for transferring a 5-mL sample to the Stratum PTC. Prior to sample transfer, internal standards (IS) and surrogates (surr) were added.

A Vocarb® 3000 analytical trap (Teledyne Tekmar) was chosen because of its desorption characteristics. Higher desorption temperatures allow for quick release of target analytes in the aforementioned U.S. EPA methods. Use of a higher desorption flow (20 mL/min) also helps to release the analytes into a tight band before they are passed onto the GC-MS. Both the desorption temperature and flow rate contribute to excellent peak shape of all target analytes, especially the lighter compounds.

Table 1 - Optimum Stratum/Aquatek70 performance parameters

The GC used was equipped with an electronic pressure control (EPC). This feature can be operated in constant pressure, constant flow, and pressure programmed modes. In the constant flow mode, the pressure at the head of the column is maintained and the flow within the column is kept constant throughout the entire run. By maintaining a constant flow and pressure, the linear velocity of the column is optimized, resulting in better resolution and decreased run times. The optimum parameters used for data collection for this paper are listed in Table 1.


Figure 1 - Seventy replicate water blanks to show precision, accuracy, and prevention of water accumulation.

With the combination of a mass flow controller, condensate trap configuration, redesigned heaters, and high efficiency fans, the Stratum PTC opens a new dimension for controlling moisture. A condensate trap temperature of 20 °C with a flow of 100 mL/min for 1 min achieved optimal water removal and no water buildup when analyzing large sample batches. Removal of water vapor interference when using the concentrator resulted in increased response for the early eluters, especially the gases, when using the concentrator. More than 50 water blanks were run consecutively to observe precision and accuracy (see Figure 1).

Table 2 - Percent carryover results: three consecutive blanks following a 200-ppb standard*

System cleanliness is enhanced by the Stratum PTC’s sample flow path. The flow path design was improved with highly efficient Siltek® coated inert tubing (Restek, Bellefonte, PA). Furthermore, great strides have been taken to ensure heating consistency throughout the entire sample pathway, minimizing any potential cold spots, which may trap or slow down certain compounds. Background contamination is dramatically reduced with the Stratum PTC, offering better analytical performance throughout analyses by reducing noise.

A high standard (200 ppb) followed by three blanks were analyzed using the concentrator connected to an Aquatek70 autosampler. Carryover results were then tabulated after each blank; the data associated for these runs are presented in Table 2.

Conclusion

More than 50 continuous blank samples were evaluated for water management and buildup using the Stratum PTC. The highly efficient condenser allowed for effective water management and improved target response. Seventy-five target compounds found in U.S. EPA Methods 524 and 8260 were also analyzed in a 200-ppb standard followed by subsequent blanks to fully evaluate the concentrator’s inert sample pathway. This pathway provides suitable conditions for enhanced system cleanliness, resulting in less carryover from high-level samples, therefore reducing the possibility of false positives on subsequent analysis. Additional studies are currently under way with the Stratum PTC to further improve sample run time, analyte response, and overall data quality.

Ms. Dattilio is VOC Product Specialist/Applications Chemist, Teledyne Tekmar, 4736 Socialville Foster Rd., Mason, OH 45040, U.S.A.; tel.: 513-229-7000; fax: 513-229-7050; e-mail: [email protected].

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