Page 380 - Tandem Techniques

Page 364

The electrospray interface has been widely used in LC/MS tandem instruments, and there are many
applications reported in the literature. Nevertheless, the device in some forms does have some
drawbacks. The electrospray system may not work well with mobile phases having high water content,
and as the majority of chromatographic separations are carried out employing reverse phase distribution
systems, this limitation can be a serious problem. However, certain interface designs work better than
others with aqueous mixtures, and some electrospray nebulizers are designed specifically to cope with
solvents of high water content. It follows that in purchasing an electrospray interface, its efficacy for the
particular analyses with which it is to be used, must be ascertained. Its areas of use have increased
rapidly over the past five years, and some particular applications that illustrate its versatile capabilities
will be described.

Applications of the Electrospray LC/MS Interface

Davis et al. [21], used an electrospray interface to couple a microbore column to a Finnigan MAT TSQ
700 triple sector quadrupole mass spectrometer. The basic layout of the tandem system is shown in
Figure 9.25. The microbore column was 15 cm long, 0.25 mm I.D. and packed with a C18 reversed
phase support. The flow rate was only 1-2 µl /min and thus a special electrospray assembly was
designed to accommodate these low flow rates. The gradient was performed and stored in the manner of
Snyder and Saunders [22] and Katz and Scott [23]. The eluent was monitored by an on-column UV
detector, and the eluent passed directly to the micro electro-jet assembly. The micro-spray was
constructed from a flame-drawn length of fused silica tubing, 5 cm long, 350 µm O.D. and 150 µm I.D.
The aperture diameter of the drawn jet ranged from 1-5 µm. Two lengths of 150 mm O.D. and 25 mm
I.D. tubing were placed inside the tube, and sandwiched between them was a hydrophilic PVDF frit.
These insert tubes reduced the dead volume of the spray jet, and provided a support for the filter, which
was necessary to avoid the jet becoming blocked.