Exposition

Quadrupole & Curved Pre-Filter

Exhibit no. 774

The quadrupole mass analyzer separates ions based on the stability of their trajectories in the oscillating electric fields. It consists of four parallel rods with a hyperbolic or circular cross-section. The opposing quadrupole rods are connected together electrically, and a radio frequency (RF) voltage and a DC voltage is applied between one pair of rods and the other. The quadrupole mass analyzer was developed in parallel with the quadrupole ion trap by the Nobel prize-winning mass spectrometry pioneer, Wolfgang Paul.

The exhibit is a 3 MHz quadrupole analyzer with an S-shaped prefilter and an entrance aperture. It served in the Varian 820-MS inductively coupled plasma mass spectrometer (ICP-MS) manufactured by Varian Inc. in 2002. In this instrument, an ion mirror reflected the ion beam through 90 degrees while photons and neutrals pass to the vacuum system. The mirror created a parabolic electrostatic field to focus the analyte ions with optimum efficiency at the quadrupole entrance aperture. Precision-machined, stainless steel, round rods manufactured to micrometer tolerances were locked into ceramic mounts. The use of the S-shaped prefilter (fringe electrodes) permitted a further increase of sensitivity for low mass isotopes and a decrease in the background count rate. The mass range of the quadrupole was 3 – 256 Da.

This particular quadrupole has been developed in the eighties by a team of Varian engineers led by Petr Zdařil. Although the quadrupole including the curved prefilter was developed for an in-axis geometry, it was finally used with the ion mirror reflecting the analyte ion beam by 90 degrees. The ICP-MS division of Varian was purchased by Bruker Corporation and later acquired by Analytik Jena, but since 2000, the quadrupole remained unchanged.

Wikipedia: Quadrupole mass analyzer
Wikipedia: Wolfgang Paul

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