| Popis: |
As direct real-time analysis techniques, Selective Ion Flow Tube Mass Spectrometry (SIFT-MS) and Proton-Transfer Reaction Mass Spectrometry (PTR-MS) provide on-line measurement of volatile organic compounds (VOCs). Both techniques are widely-used across several disciplines, e.g. atmospheric chemistry, food science and medicine. However, the humidity of the sampled air greatly influences the quantified mixing ratio, and must be accounted for in the calibration of both platforms. Here we present both an evaluation of calibration functions taking account for humidity and a side-by-side comparison of the performance of the two instruments based upon a calibration of 15 different VOCs over a relative humidity range from 0–90 % at 25 °C. While we made several improvements to a Voice 200 ultra SIFT-MS instrument to reduce background levels, overall detection limits for a PTR-QMS 500 are an order of magnitude lower. Sensitivity (here defined as the slope of the calibration curve) was higher for the SIFT-MS, and its calibration was more robust against humidity compared to PTR-MS. Thus, PTR-MS is the method of choice for simple, low-concentration, low-humidity analyses with a limited number of compounds whereas SIFT-MS is better-suited for more complex systems with varying humidity like our experiments investigating the change in VOC emissions of soils during dryout. |
