Development and fundamental investigation of He plasma ionization detector (HPID) for gas chromatography using DC glow discharge

In analytical chemistry, gas chromatography (GC) has been widely used because of the short measurement time and the low running costs. High-sensitivity detection is required for further improvement of the analytical performance. To date, several types of detectors have been developed. Thermal conductivity detectors (TCDs) are the most common GC detectors, and they are applicable for almost all samples. However, element-specific detection is impossible, and the limit of detection (LOD), which is on the order of 100 ppm, is not good. In this study, a new type of GC detector using atmospheric-pressure He plasma was developed. He has the highest ionization (24.58 eV) and metastable (19.82, 20.62 eV) energies among the elements. This means that He plasma can effectively ionize and excite all elements. In the helium plasma ionization detector (HPID), DC-powered He plasma and rod-like electrodes were utilized for ionization of the samples. For an ionization detector, the generation of very stable plasma is important. Therefore, we used DC He plasma. The sample was ionized when it was mixed into the He afterglow plasma. The concentration of each sample was then measured by the detection of the ion or electron current. When a 2 mL mixture of 20 ppm H-2, N-2, O-2, CH4 and CO was introduced for GC, a LOD of 21-67 ppb was achieved. Thus, the detection ability of HPID was more than 1000 times better than that of TCD.