Page 367 - Tandem Techniques

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from electrospray where the ions assume a charge determined by the direction of the ionizing field. The
consensus of opinion appears to be that as these charged droplets evaporate, the diameter of the droplets
are reduced and the charge density increases. There comes a point when the electrical forces become
equal to the surface tension forces which contain the drop, and the droplet explodes. This is called
Raleigh's limit and the maximum permissible charge (qr) that can be carried by a drop radius (r) can be
mathematically expressed by the following equation.

where (g) is the surface tension of the liquid

and (e ) is the permittivity of free space.
0

The thermospray produces larger drops (ca 10 µm) than electrospray (1-2 µm). The small size of the
droplets produced by electrospray probably accounts for the production of multiple ions, which as will
be seen later, allows very high masses to be measured.

An example of the thermospray Ionizer is shown in Figure 8.17. The device consists of a stainless steel
tube, containing in one end a metal cap made from a high-conductivity metal such as copper. Through
the center of the stainless steel tube and copper cap passes, either the column itself if a capillary
column, or a conduit of small internal diameter carrying the column eluent. The conduit or column
projects slightly out from the end of the heater cap. In the copper cap is placed a cartridge heater and a
thermocouple, which measures the temperature of the probe tip, and also provides the controlling signal
to maintain the cap at a selected temperature. As the pumping rate of the mass spectrometer vacuum
system is limited, either a microbore column can be used, or if a normal packed column is employed,
the eluent is split. The flow rate range that can be accepted by the thermospray interface is about 100-
1000 µl/min. In contrast, the electrospray, which will be described later, can only accept a flow rate that
lies between about 1 to 10 µl/min. The flow capacity of the