Page 147 - Tandem Techniques

Page 128

It is clearly seen that the dispersion resulting from the serpentine tube is drastically reduced in
comparison with the straight tube. According to the graph, the numerical value of the peak variance per
unit length for the serpentine tube (0.010 in. I.D.) is 0.05 mL /cm and consequently, a tube 10 cm long
2
would contribute a variance of 0.5 ml . In contrast, the dispersion of a straight tube of the same internal
2
diameter and only one centimeter in linear length would be 5.5 ml , which is an order of magnitude
2
larger.

It follows that in any conduit carrying eluent from a chromatographic system, great care must be taken
to maintain the integrity of the peak dispersion and thus conserve the resolution. If straight open tubes
must be employed, then they should be as short as possible and have the minimum radius that is
practical. Unfortunately, it is often difficult in practice to have the column or detector exit close to the
sample inlet of the mass spectrometer, and connecting tubes 30 cm or more long are often necessary. If
long lengths of conduit are needed, then low dispersion tubing should be utilized, or at least the tubing
should be tightly coiled to introduce radial mixing and reduce dispersion as much as possible.

It is also important to remember that all types of metal tubes can exhibit catalytic activity and cause
pyrolysis or the molecular rearrangement of any labile compounds that might be eluted from the
column. Consequently, fused silica or glass conduits should be used wherever possible. Unfortunately,
it would appear that, to date, no low dispersion conduits have been fabricated from glass or fused
quartz.

Synopsis

The integrity of the separation that is achieved in the column must be maintained during the passage of
the eluent to the tandem instrument. The conduits used to connect the two instruments in tandem
systems are usually simple tubes. The velocity profile in open tubes is parabolic in shape, the velocity at
the center being a maximum, and that at the wall almost zero. As a result of the differential velocity
across the tube, serious band dispersion can occur. The dispersion increases linearly with the tube