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Transpector MPS 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. Of course, the DC voltage must also be
increased to maintain the high-mass rejection properties of the filter. A mass
spectrum can therefore, be obtained by sweeping the RF amplitude, along with the
DC voltage.
The next Section (
) discusses the variation in the
efficiency of transmission of ions through the filter with mass. Following that,
section 3.4.2, The Zero Blast, on page 3-9
discusses the behavior of the filter at
very low masses where the applied voltages approach zero.
3.4.1 Scanning Characteristics
As described above, 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.
While the quadrupole drive circuits can be designed so that R varies in any desired
manner with M, it is usually most convenient to keep
M constant at a value, which
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. Thus, the sensitivity of the sensor decreases as M increases.
