Page 396 - Tandem Techniques

Page 380

Figure 9.38
The Electrospray Interface Fitted
with an Ultrasonic Nebulizer

In other respects, the system closely resembled the standard type of electrospray interface, typically
described by Yamashita and Fenn [33]. It was found essential that, for effective nebulization, the
frequency of the ultrasonic vibrator must be adjusted to the resonant frequency of the device, and this
critical frequency was identified by experiment. Furthermore, the optimum frequency had to be
carefully controlled, as a deviation of 0.1 kHz at 180 kHz was found to reduce the spray efficiency by
nearly two thirds. The sample used for optimizing the conditions of ion production was adenosine
dissolved in pure water (100 pmol/µl) which was considered a 'worst case' example. The interface was
employed in the separation of some nucleosides, using a microbore column and a Hewlett-Packard mass
spectrometer model HP88A. The nucleosides were derived from a tRNA digest, reduced to its
substituent nucleosides through the combined action of nuclease P1, and bacterial alkaline phosphatase.
The samples were separated on a C18 column, 1 m long and 0.25 mm I.D., using a gradient from 5% to
20% aqueous methanol solution. Good spectra were obtained from as little as 1.5 pmol of sample. The
interface operated well with liquids having high water contents, and in fact, operated well when
nebulizing samples in solution in pure water. It was