低频率射频辉光放电的研究

辉光放电电离源可用来检测固态、液态及气态样品，目前主要用于无机固体样品的元素分析和定量检测。本研究将低频率射频辉光放电（Lf-RFGD）与飞行时间质谱仪（TOF MS）联用用于分析液体和气体样品。研究了电离腔真空度、进样管尺寸、辅助气压力、放电频率及电极板压差对电离效率的影响。最优分析条件为：电离腔真空度约30 Pa，进样管长150 mm、内径0.25 mm，辅助气压强约1.0 MPa，放电频率2 MHz，电极板压差约100 V。在此条件下，放电功率约20 W，对液体样品的检出限约0.1 pg、对挥发性样品的检出限约20 mg/m³，检测半峰分辨率约3 000（FWHM）。将Lf-RFGD TOF MS应用于实际药品溶液和挥发性有机物（VOCs）的检测。本仪器对药品溶液的检测灵敏度与常用检测仪器相比无优势，但对挥发性有机物的检测效果较好，为开发低成本的VOCs检测仪器提供了有吸引力的选择。

Study on Low Frequency Radio Frequency Glow Discharge

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 about 1.0 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.