VOLATILE ORGANIC COMPOUNDS IN WATER, SOI

2.1 Volatile organic compounds (VOCs) are purged or desorbed from liquid or solid samples, and conveyed directly into the mass spectrometer ion source by means of a stream of helium (or air and/or other gas for suitable mass spectrometers), where the compounds are ionized by electron impact (EI) and/or chemical ionization (CI). Full-scan mass spectra are acquired continuously and are used to identify the VOCs. Characteristic ions unique to the target analytes are monitored for a selected period of time to establish an accumulated (integrated or averaged) response and the response is compared to that from a comparably generated calibration factor for quantitation. Selective ionization, multiple stage/mass spectrometry (MS/MS), and spectral subtraction may be employed in cases where additional selectivity is required. 2.2 Discrete samples of water or soil may be analyzed for VOCs by purging the sample with a stream of helium (or air and/or other gas) in a manner comparable to that performed in standard purge-and-trap GC/MS methods. However, the VOCs are monitored continuously, rather than being collected for subsequent analysis. Discrete samples of VOCs collected from air on sorbent traps are thermally-desorbed in a stream of helium (or air and/or other gas) and monitored similarly during the desorption process. 2.3 In situ continuous real-time measurement of VOCs may be performed using sampling probes that transfer the VOCs from their matrix into the vapor phase. An air-monitoring module employs a pump to deliver the VOCs to the mass spectrometer. Sampling probes are available which allow the in situ measurement of VOCs in groundwater, surface water, and soil, and the continuous monitoring of air, gaseous process streams, and aqueous process streams. The probes may be deployed with site characterization tools such as cone penetrometry and hydropunch or may be used at existing sampling wells and sites. 2.4 This procedure permits data to be acquired in both El and Cl modes, if desired, alternating between one and the other under computer control. In the El mode, structural information is obtained from the fragmentation pattern produced, while complementary molecular ion information is obtained in the CI mode. The CI mode also provides enhanced sensitivity and improved selectivity for certain compounds, including alkyl-aromatics, ketones, and aldehydes.