Gas Dosimeters As Detector for Gas Chromatography

Abstract

Introduction Gas chromatography (GC) is an important tool for the analysis of trace gases. In a chromatography column, the components of the sample are completely separated from a mixture of substances. Individual gas pulses occur at the retention times of the respective gas species [1]. These are converted afterwards by a GC detector into an electrical sensor signal and a chromatogram originates It shows the detector signal, means the analyte-depending concentration peaks, versus analysis time. To achieve the total amount of each substance, i.e., for quantitative analysis, the detector signal must be timely integrated. For small peaks that are difficult to distinguish from background noise and that are in the range of the detection limit, peak integration is often subject to errors [2]. A concept for a novel kind of detector is presented in this study, where the timely integral of the concentration can be determined directly, without mathematical integration. The concept of an impedimetric gas dosimeter, that measures directly the amount of a certain gas species is evaluated as a GC detector [3]. The concept is validated with cancerogenic epichlorohydrin serving as a model substance. Dosimeter concept The gas dosimeter concept consists of two phases, the sorption and the regeneration phase (Fig. 1). During sorption, the analyte is sorbed on the functional material and simultaneously the electrical properties change linearly. If the concentration of the model substance is zero, the signal remains constant and no desorption should occur. The sensor signal increases as soon as the substance is available. Then, the change of the sensor signal corresponds directly to the amount of the analyte and the time derivative is determined by the concentration of the substance. After a certain sorption state is reached, a regeneration, e.g. thermally or by UV light, is necessary to reset the signal [3, 4]. The dosimeter concept itself is already proven for detection of NOx and NO2 at high working temperatures and even for devices operated at room temperature [3, 4]. Materials and Methods For the purpose of detection of epichlorohydrin and to proof the concept of gas dosimeters as GC detectors, we focused on room temperature operation. It has been reported in [5] that copper-containing zeolites change their electrical properties during epichlorohydrin sorption. We investigated different MFI-type zeolites, e.g. one with M = 200 and 0.27 wt-% Cu. They were applied by screen-printing on alumina substrates equipped with interdigitated Au electrodes and fired at 650 °C. The sensors were mounted in a gas purgeable tube and were exposed to dry synthetic air. Epichlorohydrin was added in the ppm-range by special experimental setups. The electrical properties are measured by impedance spectroscopy (f = 1 MHz, U eff = 1 V). Since zeolites are poorly conducting in dry atmospheres, the capacitance C was determined. As sensor signal, the relative capacitance change (C-C 0)/C 0 was calculated, with C 0 being the capacitance in base gas and C the capacitance during epichlorohydrin sorption. Results and Discussion Initial results of zeolite MFI 200 with 0.27 wt-% Cu are shown in Fig. 2. The relative capacitance change (C-C 0)/C 0 (given in ppm) during exposure to 16.5 ppm epichlorohydrin is plotted versus time. The sensor signal shows dosimeter-type behavior, meaning that the signal increases linearly during epichlorohydrin exposure and remains constant after the dosing pulse. No desorption effects are visible. In real application behind a GC column, a typical concentration peak is plotted schematically in Fig. 3. The expected sensor signal of a gas dosimeter shows, that the sensor signal increases directly depending on the current concentration (= slope of the curve) and remains at a constant sensor signal after the analyte peak, which corresponds directly to the total amount of the analyte. The dosimeter signal of MFI 200 with 0.27 wt-% Cu was determined in this operation mode, too. Epichlorohydrin was sorbed at an activated carbon filter for a certain loading time (40 min) and was pulse-like thermally desorbed (P = 250 W) afterwards. The dosimeter results, the sensor signal (C-C 0)/C 0, and the corresponding time derivative, are shown in Fig. 4. The filter was unloaded twice (100 and 400 s) and the dosimeter shows a clear signal change which depends on the desorbed amount of epichlorohydrin. The shape of the time derivative corresponds approximately to a typical concentration peak after a GC column. Conclusions This study shows, that a sensitive material for detection of epichlorohydrin with dosimeter-type characteristics at room temperature was found. Therefore, the potential exists to develop a detector for gas chromatography, whereby the sensor detects directly the total amount of an analyte peak downstream of a GC column. References [1] C.A. Cramers et al., High-speed gas chromatography: an overview of various concepts, J. Chromatography. A 856 (1999) 315–329. [2] K. Dettmer-Wilde et al., Quantitative Analysis, in: K. Dettmer-Wilde, W. Engewald (Eds.), Practical Gas Chromatography, Springer Berlin Heidelberg, Berlin, Heidelberg, 2014, pp. 271–302. [3] I. Marr et al., Resistive NOx dosimeter to detect very low NOx concentrations - Proof-of-principle and comparison with classical sensing devices, Sens. Act. B: Chem., 248, 848–855 (2017) [4] R. Wagner et al., Novel Operation Strategy to Obtain a Fast Gas Sensor for Continuous ppb-Level NO2 Detection at Room Temperature Using ZnO-A Concept Study with Experimental Proof, Sensors 19 (2019) 4104. [5] D. Kalló, Applications of Natural Zeolites in Water and Wastewater Treatment, in: D.L. Bish (Ed.), Natural zeolites: Occurence, properties, applications, Mineralogical Society of America, Washington, DC, 2001, pp. 519–550. Figure 1