Page 413 - Tandem Techniques

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the device described previously, and changed the actual nebulizer to accommodate lower flow rates.
The basic design is shown in Figure 9.55, and is a modification of a nebulizer described previously by
Cappiello and Bruner [49].

Figure 9.55
The Micro-flow Nebulizer
In this modification the end portion of the coaxial tubing, carrying the helium, has been widened to
keep any slow-growing liquid droplets away from the internal gas conduit wall. This enlargement is
sharply reduced at the capillary tip, thus keeping the contact surface between the capillary and the wall
of the gas conduit to a minimum.
The unique character of this nebulizer is its capacity to nebulize very small liquid flows. In the absence
of a helium flow, the liquid is not forced out by the following liquid but remains as an expanding
droplet. When the helium flow is commenced, as soon as the droplet size exceeds the diameter of the
tip, the energy of the gas breaks the surface tension forces and droplets are formed. The performance of
the interface was demonstrated in separating and identification of low levels of caffeine and
testosterone.

Clear and unambiguous peaks were produced for 40 pg of caffeine (m/z of monitoring ion of 194) and
600 pg of testosterone (m/z of monitoring ion 124). The flow rate was 2 µl/min. In general, the authors
claimed very little nebulizer contamination over long periods of time, better overall performance over a
wide range of mobile phase composition, efficient nebulization over a wide range of flow rates and
more simple operational procedures.