Page 359 - Tandem Techniques

Page 343

The main body of the interface is constructed of stainless steel and is fitted to the side flanges of a
Finnigan quadrupole mass spectrometer, such that the interfaces are re-entrant to the ion source, and
terminate a few millimeters from the electron beam. The two chambers are separated and terminated by
sapphire jewels 0.1 in. O.D. and 0.018 in. thick. The jewels in the left-hand interface, where the sample
is introduced have apertures 0.010 in. I.D. The jewels in the right-hand interface, where the wire
transport leaves the mass spectrometer to the winding spool, have apertures 0.007 in. I.D. The larger
diameter apertures on the feed side are employed to reduce the surface of the wire being 'scuffed', which
might result in possible solute loss. Sapphire jewels are necessary to prevent frictional erosion of the
apertures by the stainless steel wire. The first chamber of each interface is connected to a 150 1/min.
rotary pump which reduces the pressure in the first chamber to about 0.1 mm of mercury. The second
chamber of each interface is connected to an oil diffusion pump backed by a 150 1/min. rotary pump.
The pressure in the second chambers is reduced to about 5-10 µm of mercury. The entrance and the exit
of each interface is fitted with a helium purge that passes over the aperture through which the wire is
entering or leaving and ensures that only helium is drawn into the interfaces. In this way background
signals from air contaminants are greatly reduced. The purge also allows the use of methane as a
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chemical ionization agent if so required. The pressure in the source was maintained at about 1 x 10
mm of mercury.
The sensitivity of the device to diazepam, evaluated by monitoring the total ion current, was found to be
about 4 x 10 g/ml. Bearing in mind that only a small proportion of the eluent is taken on the wire, this
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was calculated to be equivalent to about 7 x 10-10 g/spectrum (0.7 ng). A total ion current
chromatogram, of a sample of the mother liquor from some vitamin A acetate crystallization, is shown
in Figure 9.11. The total ion current is taken as proportional to the sum of all the mass peaks in each
spectrum. The separation in Figure 9.11 was carried out employing incremental gradient elution using
12 different solvents. It is seen that a good separation is obtained and the integrity of the separation is
maintained after passing through the interface. It also shows the complete