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torch, and not by a cooled coil. In addition, with the ICP, the isotopes of argon, oxygen, nitrogen and
hydrogen can combine with themselves, or with other elements, to produce isobaric interferences. The
use of helium, which is essentially mono-isotopic, significantly reduces the number of interferences
compared with the argon plasmas.

Laser Desorption Mass Spectrometry

The laser can be used for two purposes, first to desorb the sample as vapor into the ionizing system, and
second to actually produce the sample ions. However, ions that are created by pulsed lasers produce
bursts of ions and are obviously unsuitable for use with the scanning spectrometer. Intermittent ion
production, however, is compatible with the time of flight mass spectrometer, which can record all the
ions produced by each laser pulse. Another advantage of this combination is the nearly unlimited mass
range of this particular type of mass spectrometer. A diagram of the laser desorption/time of flight mass
spectrometer system is shown in Figure 9.4.

Figure 9
A The Laser Desorption Mass Spectrometer Combination

The sample is placed on a probe that receives high-energy laser pulses from a carbon dioxide laser. The
bursts of vaporized sample are propelled