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Quadrupole mass filters operated in higher stability regions Chen, Wei


This thesis describes experimental and theoretical studies of the properties of a quadrupole mass filter operated in the fourth stability region (Mathieu parameters a≈0 and q≈21.3) and the sixth stability region (Mathieu parameters a≈8, q≈6). The experimental work is done with a thermal ionization quadrupole mass spectrometer. The objective is to improve the resolution of inductively coupled plasma mass spectrometers for elemental analysis. With a quadrupole operated in the fourth region, a resolution measured as full peak width at half height (R[sub ½]) of up to 13,900 with 40 eV ³⁹K⁺ ions is obtained and a resolution of several thousand at m/z 39 with ions of several keV energy is achieved. The transmission is somewhat lower than that of the second stability region for a given resolution between 1500 and 5000, but much higher than that expected from acceptance calculations. Operation in the sixth stability region gives a maximum resolution of R[sub ½]=2000 for ³⁹K⁺ ions with an axial ion energy of 20 eV, and resolution of several hundred with high energy ions (>1000 eV). Operation at the upper (M) tip usually gives relatively higher resolution and transmission than operation at the lower (S) tip for the same ion energy. Thus for ions of intermediate energy, a quadrupole operated in the sixth stability region can also be used as an independent mass filter. This region could also be used as an additional mass filter to eliminate alias ions when a quadrupole is operated in regions II or IV. This thesis also describes a theoretical calculation of the ion optical properties of a quadrupole operated near the fourth and the sixth regions. For these regions, ions experience fewer RF cycles inside the quadrupole field because relatively higher energies are required to penetrate the fringing field at the entrance of the quadrupole rods. Ions outside the stability regions can thus pass through the quadrupole. Thus the stability boundaries are considered diffuse. For the fourth stability region, a narrower peak could not be obtained with a>0 for ion energies larger than 500 eV, and split peaks were observed with a scan line operated above the tip of the region. For the sixth stability region, broader peaks are obtained with higher energy ions, and the y-stability band dominates the ion transmission. Peaks were observed when using a quadrupole operated beyond the two tips of this region.

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