Study on Low Frequency Radio Frequency Glow Discharge

Abstract

Glow discharge ionization source can be used to analyze solid sample, liquid sample and gas sample. At present, glow discharge optical emission spectrometer (GD-OES) and glow discharge mass spectrometer (GD-MS) are mainly used to the elemental analysis of inorganic solid samples and quantitative depth profile analysis, including direct current (DC) power supply and radio frequency (RF) power supply. In this paper, the ionization of liquid sample and gas sample by low frequency radio frequency glow discharge (Lf-RFGD) was researched, and the glow discharge structure constructed was connected to timeof flight mass spectrometer(TOF MS). The influences of the vacuum degree of ionization chamber, the size of sample inlet capillary, the auxiliary gas pressure, the discharge radio frequency and the voltage difference between electrode plates on the ionization efficiency were studied. The optimal analysis conditions are the vacuum degree of ionization chamber is about 30 Pa, the length of sample inlet capillary is 150 mm and the inner diameter is 0.25 mm, the auxiliary gas pressure is about1.0 MPa, the low discharge frequency is 2 MHz, the voltage difference between electrode plates is about 100 V. Under these conditions, the discharge power is about 20 W,the detection limit of liquid sample is 0.1 pg and volatile sample is 20 mg/m3, and the detection resolution (full width at half maximum, FWHM) is about 3 000. Combined with samples testing, the Lf-RFGD TOF MS was preliminarily applied to the drug solution and volatile organic compounds (VOCs). However, the sensitivity of this instrument for detecting non volatile liquid sample was not good compared with the conventional detection instrument. The reason is that high proton affinity and low ionization potential are needed to obtain good detection sensitivity in the positive ion mode of mass spectrometer, and non volatile liquid sample without pretreatment is not easy to meet the condition of high proton affinity under low power glow discharge condition. Successfully, a lowcost, portable approach based on the application of atmospheric sampling Lf-RFGD TOF MS has been developed for near real time measurement of trace VOCs. In this context, the glow discharge excitation source offers attractive alternative for development of low cost VOCs and elemental analysis instrument. Lf-RFGD as an excitation source has unique advantages with respect to development of hand held instrument such as low cost, low power consumption, and analytical versatility.

Keywords

• low frequency
• radio frequency; glow discharge (gd)
• time of flight mass spectrometer (tof ms)