The construction of a small-size, magnetic sector, single focusing mass spectrometer (He-MS) for the continuous, on-site monitoring of He isotope ratios (3He/4He) is described. Nier type, electron impact ion source is usually installed. The ion source is the same as those used in modified-VG5400 mass spectrometers in our laboratory, which simplifies the evaluation and/or investigation of the performance as well as trouble shooting. A split-type flight tube was designed for the simultaneous detection of both 3He and 4He. Because of the low 3He/4He ratio (10?5) of naturally occurring He, the beam intensity of 3He is much weaker than the 4He ion beam, 3He is measured with a secondary electron multiplier operated in the ion-counting mode, while measuring 4He involves the use of a Faraday cup (High Faraday in Fig. 4). In the VX-770 flight tube, the 3He ion beam passes a narrow collector slit (300?m in width) so as to separate 3He ions from interfering HD and H3 ions with a resolving power of 500, while 4He is introduced into the outer flight tube, which passes through the wider collector slit (900?m in width). In order to simultaneously obtain 3He and 4He signals, the position of the collector slit for 4He can be adjusted by means of a manipulator and the focal point of the 4He ion beam can be moved slightly by applying positive voltages to a pair of deflecting plates that are located in front of the collector slit. Another Faraday cup (Axial Faraday) can be inserted into the 3He beam line to measure a strong ion beam such as 20Ne+ and 40Ar+ (Fig. 4). Fig.?4.?Design drawing of the He-MS instrument, consisting of the ion source, Q-lens, flight-tube, and collectors. High and axial Faraday mean the Faraday cup for 4He (high GP-IB, and then converted to imaginal voltage for convenience by the following formula, Fig.?5.?Schematic diagram of ion detectors and signal processing. Output signals of high and axial Faraday and ion counting are acquired by a computer through a GP-IB interface. where is an elementary charge (1.610?19 VX-770 C), counting rate (cps), and imaginal electric register Rabbit Polyclonal to RAB38 (11015 ). The intense 4He beam is usually collected in the Faraday cup and measured as a current mode by converting to voltage through an operational amplifier (OPA104CM: Burr-Brown Co.) with a 1.01010 feedback register (RHA2B: Hydragin Co.). The output voltage is measured using a digital multimeter (HP 34401: Agilent Technologies) and read by the computer GP-IB. Data acquisition is VX-770 usually carried out using the HP-Basic program reported by Nagao et al.15) and Sumino et al.16) A pair of electrodes (deflectors in Figs. 4 and 5) is usually installed in front of the collector slit for 4He, which functions as a zoom lens as well as a deflector of the 4He ion beam to focus the ion beam at the collector slit and to produce a flat top mass spectrum. Results and Evaluation of the He-MS Mass resolution, sensitivity, and detection limit VX-770 The basic specification of He-MS are summarized in Table 1, and the current instrument is compared with the miniature and conventional mass spectrometers for noble gas measurements developed by Sano et al.12) and the Modified-VG5400 installed at the University of Tokyo. Vacuum conditions, 5% valleys of the mass resolutions, sensitivities for 4He, background peak heights of HD, detection limits for 3He, and dynamic ranges of He detection, respectively, are listed. The mass resolution power is usually 430 for a 5% peak height (upper panel in Fig. 6), which is lower than the simulated value of 700 (Fig..