| Popis: |
Proton Transfer Reaction-Mass Spectrometry (PTR-MS) and thermal desorption Gas Chromatography-Mass Spectrometry (GC-MS) allow for absolute quantification of a wide range of atmospheric volatile organic compounds (VOCs) with concentrations in the ppbv to pptv range. Although often neglected, routine calibration is necessary for accurate quantification of VOCs by PTR-MS and GC-MS. Several gas calibration methods currently exist, including compressed gas cylinders, permeation tubes, diffusion tubes, and liquid injection. While each method has its advantages and limitations, no single technique has emerged that is capable of dynamically generating accurate concentrations of complex mixtures of VOCs over a large concentration range (ppbv to pptv), is technically simple and field portable, and affordable. We present the development of a new VOC calibration technique based on liquid injection with these features termed Dynamic Solution Injection (DSI). This method consists of injecting VOCs (0.1–0.5 mM) dissolved in cyclohexane (PTR-MS) or methanol (GC-MS) into a 1.0 slpm flow of purified dilution gas in an unheated 25 mL glass vial. Upon changes in the injection flow rate (0.5–4.0 μL min−1), new VOC concentrations are reached within seconds to minutes, depending on the compound, with a liquid injection flow rate accuracy and precision of better than 7% and 4%, respectively. We demonstrate the utility of the DSI technique by calibrating a PTR-MS to seven different cyclohexane solutions containing a total of 34 different biogenic compounds including volatile isoprenoids, oxygenated VOCs, fatty acid oxidation products, aromatics, and dimethyl sulfide. In order to validate the new DSI method, a GC-MS and PTR-MS calibration intercomparison with VOC standards generated by dynamic dilution of NIST traceable permeation tubes (α-pinene, acetone, and ethanol) and a compressed gas cylinder (acetaldehyde) was made. The results revealed that while calibration of acetone is comparable between the methods, calibration curve slopes for other VOCs obtained by using permeation tubes and the compressed gas cylinder are lower than those obtained by the DSI technique by up to a factor of 2. This implies that concentration measurements of some VOCs may be overestimated using permeation tubes and/or compressed gas cylinders for calibration. Because of its high accuracy and precision, small size, low cost, and simplicity, we conclude that the Dynamic Solution Injection method will be of great use to both laboratory and field VOC studies. |
