5–6
Transpector CPM Operating Manual
The mass of ions transmitted (M) is directly proportional to the RF amplitude
(provided f is constant). As the RF amplitude is increased, progressively higher
mass ions will be made to oscillate in phase with the RF field and thus gain
sufficient energy to strike the poles. The DC voltage must be increased to maintain
the high mass rejection properties of the filter. A mass spectrum is generated by
sweeping the RF amplitude along with the DC voltage.
The next section (Scanning Characteristics) discusses the variation in the
efficiency of transmission of ions through the filter with mass. Following that,
section 5.2.2.2, The Zero Blast, discusses the behavior of the filter at very low
masses where the applied voltages approach zero.
5.2.2.1 Scanning Characteristics
The quadrupole acts as a mass filter for a mixed beam of ions, rejecting those of
both high and low mass, while passing those of an intermediate mass. The
selectivity of the mass filter is expressed in terms of resolution, R, which is
numerically given by the ratio of the center mass, M, to the width, M (both in
AMU), of the pass band. Since the number of the ions passed by the filter falls off
gradually as the edge of the pass band is approached, the width is defined at the
point where the ion current falls to some specified fraction (usually 1/2 or 1/10) of
the maximum value. The width of the pass band is determined by the DC-to-RF
ratio.
The quadrupole can be designed so that R varies in any desired manner with M.
However, keeping M constant ensures adequate separation of masses that are
1 AMU apart. This mode of scanning is called Constant M. As a result, R is
proportional to M, and therefore the efficiency with which ions of mass M are
transmitted through the quadrupole decreases with M. Therefore, the sensitivity of
the sensor decreases as M increases.
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