Page 500 - Tandem Techniques

Page 486

tetramethylammonium bromide with a molecular weight of 159, the mass contained in the peak was,

Now assuming the signal to noise ratio for the peak was about 3, and the signal to noise ratio required
to unambiguously identify solute is 2,
Then the minimum detectable mass =

Sensitivity Enhancement Techniques

Wahl et al., [3] claimed to have increased the sensitivity of the system described by Smith et al. [1] by
reducing the diameter of the electrophoresis tube to 5 µm. Unfortunately, the advantages or
disadvantages that result from the use of the smaller diameter tubes, are confused by the method
employed to report sensitivity. As already discussed, the use of molecular units to report sensitivity can
be deceptive. The mass sensitivity of a measuring device, which is the basic specification of all
analytical measurements, will be different for solutes that have different molecular weights but the same
molar sensitivity. The results of this muddle are well illustrated by these two publications. The
sensitivity was quoted for a number of different proteins. Taking myoglobin with an m/z value of about
770 which, in the electropherograms published, carried 22 charges, the actual molecular weight would
be about,

770 x 22 = 16940

Consequently, the sensitivity published as 600 amol of the protein would be equivalent to

It is shown that the sensitivity, in terms of absolute mass, is nearly an order less than that described for
the smaller molecular weight compounds discussed in a previous report, albeit the molar sensitivity is
much greater. The problems associated with defining sensitivity have been discussed extensively
elsewhere [4,5], and as instrumental sensitivity continues to