|Application of an Atmospheric Micro Hollow Cathode Discharge Set-up in a Novel GC-detector
|Interfacing and Ionisation
|Mr Kris Wolfs
|Dr Niels van Boxtel
Mr Juan Aspromonte
Prof Ann Van Schepdael
Prof Erwin Adams
|Pharmaceutical Analysis - KU Leuven
Abstract Information :
Since the development of gas chromatography (GC) as analytical tool for volatile analytes, many different detectors were developed to analyse a broad range of different analytes. The most well-known detector for GC is the flame ionisation detector (FID), due to its robustness and excellent linearity, making it ideally suitable for quantitative GC-analysis. The FID offers picogram range sensitivity for hydrocarbons, but it has poor sensitivity towards halogenated solvents such as flame retardants, polychlorinated biphenyls (PCBs), etc. Although the electron capture detector (ECD) is often used for halogenated analytes, it uses a radio-active element as an electron emitter, which gives rise to legislation issues. The use of a conventional glow discharge (GD) in combination with mass spectrometry (MS) has proven to provide adequate sensitivity for halogenated analytes after GC separation, thereby circumventing the aforementioned issues with the FID and ECD. Nevertheless, this approach presents a main disadvantage since the system needs a rather expensive vacuum system to sustain the GD and to operate the MS. In order to obtain a stable plasma at atmospheric pressure, the cell dimensions have to fulfil the Paschen minimum criteria. The created plasmas are referred to as microplasmas or micro cavity plasmas. One particular interesting microplasma device is the DC operated micro hollow cathode discharge (MHCD). The MHCD shows stable operation at atmospheric pressure in both noble and molecular gases and a special mode of operation (called the hollow cathode mode). This phenomenon is caused by electrons that are moving as pendulum within the hollow cathode as they are constantly repelled by the opposing cathode surface. During their movement, they can ionise other neutral species and generate other electrons which in turn can undergo the same pendulum motion. The net result is that the MHCD geometry provides increased ionisation efficiency compared to a GD.
In this work, a GC detector using a MHCD as ionisation source was constructed and coupled to a headspace (HS)-GC system as an alternative to the hydrogen flame used with the FID. After GC separation and ionisation, ions were extracted and collected by means of a biased collector electrode after which the resulting current was recorded. This new detector was able to quantitatively detect different analyte species, including chlorinated ones, providing adequate sensitivity (pg-range) thereby competing with the FID. Furthermore, depending on the collecting distance, peaks can change polarity and might open up identification possibilities or provide additional selectivity.