An improved quadrupole mass analyzer (QMA) has been proposed for use in determining the compositions of gas mixtures. The proposed QMA would incorporate two major innovations over traditional QMAs: (1) It would feature a simplified radio-frequency-excited quadrupole electrode structure that would be smaller, weigh less, and consume less power, relative to the quadrupole electrode structure of the corresponding traditional QMA; and (2) It would include dc end electrodes that would enable it to function as either a traditional transmission-mode mass spectrometer or an ion-trap mass spectrometer (ITMS).
The figure illustrates the proposed quadrupole electrode configuration and the corresponding traditional configuration, both designed for an inner radiusr0. In the traditional configuration, the outer cylindrical electrode is grounded, while a radio-frequency signal of amplitude V0 is applied in opposite polarities to successive rod electrodes at 90° angular intervals. The radius of the quadrupole electrodes (1.1468r0) and the radius of the outer shielding electrode (3.54r0) are chosen to eliminate a sixth-order departure from the ideal quadrupolar electric potential and thereby obtain a close approximation to the ideal quadrupolar electric field.
The proposed quadrupole electrodes would be made from a single, precisely machined hollow cylinder cut lengthwise into eight sectors with alternating angular widths of 30° and 60°. The 30° sectors would be electrically grounded, while the radio-frequency signal of amplitude V0would be applied in opposite polarities to successive 60° sectors. This configuration would also eliminate the sixth-order departure from the ideal quadrupolar potential, yielding a similar close approximation to the ideal quadrupolar electric field. A simple comparison of overall radii shows that the cross-sectional area of the proposed QMA would be less than 1/10 that of the traditional QMA. If the proposed and traditional QMAs were of the same length, then the proposed QMA could be made to weigh about 1/10 as much.
Further analysis of the electric fields reveals that the proposed QMA could achieve the same stability parameters as does the traditional QMA at an excitation voltage about 10 percent smaller, and that the capacitance of the proposed QMA would be about 1/4 that of the traditional QMA. As a result, the energy stored in the electric field of the proposed QMA would be only about 1/5 that of the traditional QMA. As a further result, if the resonance quality factors of the excitation circuits were the same in both cases, the proposed QMA would consume only about 1/5 the power of the traditional QMA.
The ITMS version of the proposed QMA would be based on a linear ion trap, in which the trapping volume would be roughly a cylinder of length L and small characteristic radius R about the axis of symmetry. In contrast, a traditional ITMS is based on a point-node ion trap, in which the trapping volume is roughly a sphere of radius R. Thus, the trapping volume of the proposed ITMS version would be about 3L/4R times that of a traditional ITMS. One could choose the dimensions of the electrodes to obtain L >> R, such that the trapping volume of the proposed ITMS version would be 100 to 1,000 times the volume of the corresponding traditional ITMS; the number of ions generated and trapped would be increased accordingly.
This work was done by John D. Prestage of Caltech for NASA's Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-20011