Development of ion current measurement and recording system for MI1201IG mass spectrometer

T14N4

V.V. Kolobov, V.N. Selivanov, M.B. Barannik

We performed complex modernization of the MI1201IG mass spectrometer. Ion current measuring and recording system was designed. The new system consists of electrometric amplifier and recording system with high-precision analog-to-digital converter. The electrometric amplifier is based on the specialized integral microcircuit and high-resistance (1 TOhm) measuring feedback resistor. The electrometric amplifier is placed in electrometer head. The recording system is located in a new separating magnetic field control and ion current measuring unit. The new ion current measuring and recording system was developed not only to increase sensitivity and expand the dynamic range but also to decrease the time constant of the analog part of the measuring system. Reducing the time constant is necessary for an undistorted ion current signal transmission to ADC input in full-scan mode. Special shielding system to distribute the electric field gradient on surface of the high-resistance feedback resistor was designed. This allowed the compensation of a distributed capacitance of the feedback measuring resistor and the reduction of the time constant of the electrometer to 0.2 sec. The time constant value is determined by the value of in-to-out stray capacitance of the electrometer’s amplifier chip. Therefore, a parasitic capacitance compensation circuit was developed. As a result, the end-to-end frequency response of ion current measuring and recording system is determined only by the upper cutoff frequency 10 Hz of low-pass filters. The usage of the new measuring system allowed us to increase the mass spectrometric resolution, mass accuracy and full-spectrum scan speed. The measuring range of ion currents is 1 × 10–15 – 3 × 10–12 А. The measured equivalent input root-sum-square of noise is 0.86 fA at sampling frequency of 10 Hz and integration time of 1 sec. The measured equivalent input root-sum-square of noise is 0.14 fA at sampling frequency of 10 Hz and integration time of 10 sec.

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Комплекс средств для модернизации системы управления и регистрации масс-спектрометра МИ-1201АГ

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