A Capacitance Sensor for Water: Trace Moisture Measurement in Gases and Organic Solvents

Abstract

The determination of water in various matrices is one of the most important analytical measurements. We report on a high-resolution capacitance-based moisture sensor utilizing a thin film of a perfluorosulfonate ionomer (PFSI)-H(3)PO(4) composite in a flow-through configuration, for both gas and liquid samples. Incorporation of H(3)PO(4) into a PFSI sensing film improved the limit of detection (LOD) (signal-to-noise ratio, S/N = 3) by a factor of 16 in the gas phase to 0.075% relative humidity (RH) (dew point = -56 °C). The response time was dependent on the sensing film thickness and composition and was as low as ∼60 ms. The temperature dependence of the sensor response, and its relative selectivity over alcohol and various other solvents, are reported. Measurement of water in organic solvents was carried out in two different ways. In one procedure, the sample was vaporized and swept into the detector (e.g., in a gas chromatograph (GC) without a column); it permitted a throughput of 80 samples/h. This is well-suited for higher (%) levels of water. In the other method, a flow injection analysis system integrated to a tubular dialysis membrane pervaporizer (PV-FIA) was used; the LOD for water in ethanol was 0.019% (w/w). We demonstrated the temporal course of drying of ethanol by Drierite; the PV-FIA results showed excellent correspondence (r(2) > 0.99) with results from GC-thermal conductivity detection. The system can measure trace water in many types of organic solvents; no reagent consumption is involved.

Pulsed Discharge Helium Ionization Detector for Highly Sensitive Aquametry

“…25 Since the system uses a smaller flow/ volume than commercial flow-through systems, this makes it relevant to our small-analysis volume measurements described in this paper. Overall, the PDHID-D2 data were similar to the RH measurements with the perfluorosulfonate ionomer or KFT, 26 but without the disadvantages of these latter techniques.…”

“…25 The PDHID-D2 detection limit was comparable to the RH values of 0.075 to 1.20% reported using a perfluorosulfonate ionomer composite. 26 Early detection and prevention of moisture-induced damage is critical to the oil and natural gas industry, where an LOD of 10 to 50 ppmv is desirable with the chosen aquametry technology. The aquametry sensitivity using the PDHID-D2 is near to this range (using only 1.0 μL of gas) for gaseous aquametry and 100 to 500-fold below this range for liquid aquametry, as applicable to the oil and natural gas industry.…”

Sensitivity Enhancement of a PPM Level Capacitive Moisture Sensor

“…This causes volume expansion of the oxide, resulting in a gradual decrease in surface area and porosity [2]. The sensor does not show any drift due to the change in working temperature [9,18,19,23].…”

A fiber optic sensor with a metal organic framework as a sensing material for trace levels of water in industrial gases

“…Calibration gases containing high ppmv levels of water were generated by bubbling ultra-pure water or saturated LiCl solution at 25 °C [29]. Calibration gases containing low ppmv levels of water were prepared with permeation tubes.…”