Page 390 - Tandem Techniques

Page 374

pointed out, that in general, the lower the flow rate through the electrospray, the smaller the droplets
that are produced. Small droplets have high surface to volume ratios, and thus make a large proportion
of the analyte molecules available for desorption.

The authors fabricated capillaries with spraying orifices having only 1-2 µm I.D. The capillary tips
allowed reduced flow rates of 20 nl/min to be realized. The droplet diameter was claimed to be about
200 nm, in comparison with 1-2 µm, the diameter of droplets generated from conventional electrospray
sources. As a consequence, the droplet volume was also 100 to 1000 times smaller. In addition, the
miniaturized electrospray inlet could be operated without a sheath flow or pneumatic assistance, which
made the design of the interface much simpler. Spectra were obtained from a 1 µl sample of an aqueous
solution of ovalbumin containing 5 pmol/µl, and so it is clear that the interface functioned well with
aqueous solvent mixtures. Assuming a molecular weight of about 44,000 for ovalbumin, this sample
volume contains a mass of about 0.2 µg of protein at a sample concentration of about 0.02%.
An interesting extension of the utilization of this type of interface is its use with a MS/MS tandem
instrument. The results published by Wilm and Mann [28], are shown in Figure 9.33. The top curve
shows an MS analysis of the peptide over the m/z range of 600 to 850. Two ion adducts from the
preliminary ionization at about m/z values 670 and 810 were chosen for subsequent examination, and
the results obtained are shown by the two spectra in the lower part of the Figure 9.33. It is seen that
whereas the sample adduct ions had m/z values of 670 and 810, the m/z range of the MS/MS spectra
extend to m/z values of 1100 and 1500. One explanation for this difference might be that the original
adduct ions having m/z values of 670 and 810 carried multiple charges.

From the results discussed so far, it would appear that the sensitivity limits of the electrospray interface
can vary widely from one design to another, and also, perhaps, between one type of sample and another.
In any event, the ultimate practical sensitivity obtainable from the electrospray principle may well be
much greater than that which has been so far achieved.