Abstract:

An online derivatization device for the analysis of polar organic compounds by gas chromatography-mass spectrometry (GC-MS) is designed. The derivatization reaction occurs in the hot GC injection port, and this is also known as injection port derivatization (IPD). IPD is usually performed in two ways: 1) direct IPD and 2) ion-pair extraction, followed by IPD. In both cases, the derivatization reagent reacts in liquid form. However, a method for online derivatization using gaseous derivatization reagents is provided. A special needle is designed and placed on the carrier gas transfer line to the injection port. The carrier gas is introduced into a glass bottle containing the derivative reagent (N-methyl-N-(trimethylsilyl)trifluoroacetamide, MSTFA), and then, the gaseous derivative reagent in the headspace is pressed out and introduced into the injection port of the GC instrument at a constant speed. The filter to be analyzed is placed directly in the liner, and the polar organic compounds on the filter react with gaseous MSTFA at 310 ℃ for 10 min in the injection port. During derivatization, the column oven is maintained at room temperature, and all the derivatives stay on the column head. When the reaction is complete, the MSTFA supply is ceased. The oven temperature is programmed, and the solvent delay is set until the excessive MSTFA is removed. The derivatives are allowed to pass through the column and analyzed by the MS detector. To prevent a large number of derivative reagents from entering the column, the injection port is set in split mode with a split ratio of 5∶1. Variables such as the injection-port temperature and derivatization time are investigated. The GC-MS responses of the ten silylated derivatives increase with increasing injection-port temperature (290-310 ℃), indicating that high temperatures can enhance the silylation efficiency. The derivatization times were also investigated. The GC-MS responses increased with an increase in the reaction time from 0 to 10 min, while higher temperatures or longer reaction times lead to the loss of some derivatives. The reproducibility of the derivatization reaction was 0.27% to 7.28%, and the linear correlation coefficient was 0.976-0.996. This device can be used for the online silylation of most polar organic compounds such as organic acids, alcohols, and phenols. The advantage of this device over offline derivatization is that the derivatization reagent and derivatives are protected by helium, which eliminates the risk of decomposition caused by moisture in the air, and the high temperature assists the reaction. The analytes were directly desorbed on the filter and derived online, and the sample quantity required was only 1/200 of that in the traditional solvent extraction method. Meanwhile, only the gaseous part of the headspace in the derivative reagent bottle was used, and the amount of derivative reagent was greatly reduced. Additionally, the operation is simple and solvent free, the entire analytical procedure was executed in a “green” manner. A PM2.5 filter was analyzed, and 26 different polar compounds were successfully derived, including monoacids, binary acids, aromatic acids, and alcohols, covering most of the common target polar compounds in atmospheric chemical analysis. Polyols such as glucose and sorbose that bear 5-6 hydroxyl groups and have large steric hindrance were also successfully derived. This device is expected to be an efficient and convenient analytical tool for tracing the sources of organic matter in atmospheric particles such as soil dust, biomass combustion, cooking oil smoke, and automobile exhaust, or for investigating atmospheric photochemical reactions. This gas-phase derivatization provides new insights for the development of chromatographic analysis methods for polar compounds. This device is simple and modular, and it has a wide range of applications; it is suitable for different brands of gas chromatographs and has great prospects for commercialization.

Key words: gas chromatography (GC), mass spectrometry (MS), silylation reaction, online derivation, polar organic compounds, atmospheric particulate matter, device