New operation principle for ultra-stable photo-ionization detectors

Abstract

Photo-ionization detectors (PIDs) are suited ideally to be combined with gas chromatographs (GCs). Without a GC, a PID can be used as a cheap unspecific sensor for noxious substances in the air. In this case, the unpredictable slow offset drift of the device leads to a significantly worsened detection limit. The dependence of the sensitivity on ambient humidity further deteriorates the accuracy, so that the usability of a classical PID without GC in practice is highly limited. We present a new PID design that eliminates the offset and thus offset drift problems and reduces the cross-sensitivity to humidity. It is based on an additional grid that allows modulation of the ion flow in a part of the ionization chamber. This modified PID exhibits a 20 times improved detection limit and a reduction of the cross-sensitivity to humidity from 58% of the signal to 10% of the signal. Further reduction is possible.

Flow-through microfluidic photoionization detectors for rapid and highly sensitive vapor detection

“…In addition, in order to increase the ion collection efficiency, a relatively high voltage (a few hundreds of volts) is needed to reduce ion recombination and quenching that adversely impact the detection signal. 27,32,40 In contrast, the microfluidic PID uses a serpentine channel that reduces the chamber volume and eliminates the dead volume while maintaining a large VUV illumination area. Furthermore, the ion collection efficiency is improved due to the significantly reduced distance between the two electrodes and increased electrode area.…”

“…In this work, we developed a rapid, flow-through, and highly sensitive microfluidic PID that is micro-fabricated directly on a conductive silicon wafer with an Archimedean spiral channel commonly used in μGC columns 38,39 and can be operated with low voltage (<10 VDC, over 10 times lower than that used in a regular PID 27,32,40 ). The microfluidic PID has a significantly reduced ionization chamber volume of only 1.3 μL, nearly 10 times smaller than that of state-of-theart PIDs and over 100 times smaller than commercial PIDs.…”

A robust calibration technique for concentration measurements using ionisation detectors

“…Calibration errors may arise due to changes in the ambient conditions of the flow such as temperature and humidity, and also due to sensor degradation, dust accumulation, etc. For instance, Liess and Leonhardt (2003) observed a significant reduction (approximately 50%) in the sensor sensitivity, when the gas sample had 67% humidity compared to a dry sample. Likewise, temperature variations are shown to affect the operation of TIP, FID and PID; for example, temperature change causes condensation of water vapour or deposition of gaseous compounds that can reduce the response of an ionisation detector over the duration of an experiment (Lovelock 1961).…”