Page 25 - Inorganic Mass Spectrometry : Fundamentals and Applications

Thermal Ionization Mass Spectrometry 15

the two magnetic fields track together with enough precision to preserve the flat-
topped peaks required for isotope ratio measurements. The only other time high-
voltage scanning is desirable is when the smallest possible samples must be
analyzed; because the voltage can be scanned many times faster than the magnetic
field, more informiction can be obtained from a rapidly diminishing signal when it

is near its maximum intensity. I(
In all cases, one goalpof sweeping the mass spettrum is to combine it with
slit settings to achieve flat-topped peaks. The sharp, triangular peaks commonly
seen in organic applications are undesirable when the primary goal is to dethne
the areas of the peaks (which are more reliable measures
peak heights) rather than their locations on the mass scale.
ratio mass spectrometers is usually about 500; therefore, it
rate molecular isobars from the isotopes of interest. This places stringent demands
on sample preparation, a topic addressed in Sec. 1.6.

1.
The goal of any sector mass spectrometer is to transfer an ion beam from the image
point of the ion source to the detector without significant degradation of beam
shape. One of the earliest extensive treatments of ion optics in mass spectrometry
was presented by Hintenberger and Konig in 1959 [46]. As computers have
become more powerful, programs to assist in designing mass spectrometers have
become more cornmon. There are now programs of great sophistication available
to help design instruments of whatever characteristics of almost any configuration.
A critical measure of performance in isotope ratio measurements is abun-
which
dance ~en~itivi~, is defined as the intensity of a large peak divided by the
intensity of the background 1 mass unit lower; it is sometimes defined as the
reciprocal of this ratio. This means that, for uranium, for example, if 106 counts are
collected for 238U and there is one count at mlz = 237, the abundance sensitivity is
106 (or 10-6). The reason the low-mass side of the intense peak is specified is that
gas-phase collisions cause scatter in the ion beam that reduces abundance sensi-
tivity; these collisions result in loss of energy far more often than gain, which
means scatter is greater on the low-mass side of the peak. Most mass spectrome-
ters used with thermal ionization sources are designed to have high abundance
sensitivity. These most cornmonly feature a single magnetic sector. A drawing of
one of these instruments is given in Fig. 1.9. First developed at what was then the
National Bureau of Standards by Shields [47], it is in use in several laboratories
around the world. Its magnet has a 30.5-cm radius of curvature with a 90-degree
angle of deflection. An extended flight path combined with nonnormal exit from
the magnetic +field (82") are used to correct for fringe fields [48].
Very high abundance sensitivity (>5 x lo5) can be achieved by adding a
second magnet after the first, as first suggested by White and Collins 1491. The