Instrumental background

Stable isotope mass spectrometer

The stable isotope mass spectrometer specially designed for measurement of light environmental stable isotopes D, 13C, 15N, 18O, 34S is a sensitive and selective instrument with application in hydrology, geology, environmental protection, radwaste management. The high precision stable isotope ratio mass spectrometer is provided with two ion sources (H, CNOS sources) and ten separate Faraday cups. Its analyser of 3.7 cm (H) and 13.6-15.1 cm (CNOS) radii and 180o deflection angle provide first order focusing for ion beams diverging in the radial plane.

The sensitivity of the instrument for nitrogen is better than 4x10-4 A/mbar using the ion source adopted for higher masses with an exit slit of 0.2 mm and collector entry slit of 1 mm width. The collector slits are of 1 mm width for nitrogen, carbon, oxygen, sulphur and 2.5 mm for hydrogen, resulting a working resolution of M/DM = 120 (10 % valley) and M/DM = 8 for hydrogen. The slits are width enough to produce flat topped trapezoidal peaks with the shape of about 10-3 top flatness. The H3+contribution of the hydrogen source depends strongly on the operational pressure. It is in the region of 12 ppm (equivalent to 4% contribution to minor current at natural level) for the standard operating major current of 8x10-9 A H3+ contribution is compensated by the software.

Trap regulation of a magnetically and electrostatically focused electron beam is used to achieve stable ion production in the ion sources. Ion currents are amplified with low bias-current solid state amplifiers and integrated with voltage to frequency converters and scalers. Mass setting is performed by varying the ion accelerating voltage.

A symmetrical dual viscous inlet system was constructed for measuring the sample and standard gases alternately. Pneumatically actuated bakeable gold sealed metal valves exclude cross mixing between sample and standard gases. The instrument, including the inlet system with variable reservoir volumes, is controlled by computer thus providing automatic and reproducible operation.

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