Page 395 - Tandem Techniques

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ions, when using an LC/MS tandem system with an electrospray interface. The addition of
submillimolar levels of sodium acetate to the mobile phase, encourages the formation of sodium
replacement ions in addition to the normally observed protonated species. The m/z spacing of the
sodium adducts allows unambiguous identification of the charged state of the ions and hence their
actual mass. It was also found that at the necessary sodium concentration (ca. 250 µM) the sodium salt
did neither interfere with the chromatographic process, nor cause undue fouling in the mass
spectrometer ion source. In general, as the quantity of sodium salt in the mobile phase is increased, the
number of sodium replacement ions also increases. The spectra exhibit a number of replacement ions.
The m/z values for the sodium replacement ions can be plotted against the number of sodium atoms in
the respective replacement ion, which results in linear graph. the slope of the line indicates the number
of charges on the sodium adduct ion, e.g. a slope of 11.5 would indicate that the ions carried two
charges. Consequently from the slope and intercept of the graph the molecular weight of the substance
can be calculated

In an attempt to eliminate the restrictions imposed by the nature of the mobile phase on the function of
the electrospray interface, Banks et al. [32] developed an ultrasonically assisted nebulizer, to allow
aqueous mixtures of nucleosides to be separated and monitored by a LC/MS tandem instrument, using
the electrospray interface. The major interest of the authors was to employ the electrospray interface in
the analysis of nucleosides, and the modified electrospray interface they developed is depicted
diagramatically in Figure 9.38. Except for the ultrasonic nebulizer, the interface was similar to those
already described. It consisted of a 0.005 in. I.D., 1/16 in. O.D., stainless steel tube, which had been
ground to a sharp point at one end fitted into a stainless steel body which was in two parts. A pair of
piezoelectric crystals were fitted between the two stainless steel parts, and were driven by a function
generator and a power amplifier. The device was designed so that there was independent control of the
needle potential, the cylindrical electrode potential, the nosepiece potential, and the capillary entrance
potential. In the configuration shown, the needle was always maintained at ground potential and the
temperature of the drying gas was held at 82°C.