PerkinElmer, Inc.

Determination of VOCs in water using static headspace GC/MS with simultaneous full scan and selective ion recording

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Courtesy of PerkinElmer, Inc.

The combination of static headspace sample introduction and gas chromatography/mass spectrometry (HS/GC/MS) provides the analyst with a powerful, fully automated technique for the determination of trace volatile organic compounds (VOCs) in water. Samples may be hermetically sealed in the headspace vial at the point of sampling. This approach avoids errors due to glassware cross-contamination and loss of volatile components during liquid sample transfers.

Precision is highly dependent on the reproducibility of the thermostatting and sampling procedure. On the PerkinElmer® HS 40XL automated headspace sampler used here, the samples are intelligently queued in an oven that is machined from a solid metal block to provide unparalleled temperature stability. Samples are transferred to the head of the capillary column via an inert fusedsilica transfer line using a pressurebalanced sampling system. This unique approach to headspace sampling, which has been a feature of PerkinElmer systems for some 30 years, avoids contact of the sample components with materials used in valve rotors and sample loops.

There is also a link between precision and detection limits. By definition, it is difficult to get excellent reproducibility for analytes present at or near the limits of detection. Anything that improves the detection limits (signal-to-noise ratio) of the analytical method is also likely to prove beneficial in terms of precision.

The PerkinElmer TurboMass™ mass spectrometer utilizes MS methods that are composed of “scan functions.” No restrictions exist to prevent mixing full-scan functions and the monitoring of specific ions using Selected Ion Recording (SIR), also called Selected Ion Monitoring (SIM). Under normal full-scan conditions, it is likely that any particular mass (for example, m/z 78) is only being monitored for 1 to 2 milliseconds in each scan. Adding a SIR function lasting for 0.02 to 0.05 seconds to the same run increases the scan duration by only a few percent. However, this same SIR function means that the specific mass is being monitored for a dedicated 20 to 50 milliseconds, yielding a dramatic increase in the signal-to-noise ratio and, thus, sensitivity.

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