Page 192 - Tandem Techniques

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The ICP ion source is very similar to the volatalizing unit of the ICP atomic emission spectrometer, and
a diagram of the device is shown in Figure 5.8. The argon plasma is an electrodeless discharge, often
initiated by a Tesla coil spark, and maintained by rf energy, inductively coupled to the inside of the
torch by an external coil, wrapped round the torch stem. The plasma is maintained at atmospheric
pressure and at an average temperature of about 800 K.

The ICP torch consists of three concentric tubes made from fused silica. The center tube carries the
nebulizing gas, or the column eluent from the gas chromatograph. Argon is used as the carrier gas, and
the next tube carries an auxiliary supply of argon to help maintain the plasma, and also to prevent the
hot plasma from reaching the tip of the sample inlet tube. The outer tube also carries another supply of
argon at a very high flow rate that cools the two inner tubes, and prevents them from melting at the
plasma temperature. The coupling coil consists of 2-4 turns of water cooled copper tubing, situated a
few millimeters behind the mouth of the torch. The rf generator produces about 1300 watts of rf at 27 or
40 MHz which induces a fluctuating magnetic field along the axis of the torch. Temperature in the
induction region of the torch can reach 10,000°K but in the ionizing region, close to the mouth of the
sample tube, the temperature is 7000-9000 K.

The sample atoms account for less than 10-6 of the total number of atoms present in the plasma region,
and thus there is little or no quenching effect due to the presence of the sample. At the plasma
temperature, over 50% of most elements are ionized. The ions, once formed, pass through the apertures
in the apex of two cones. The first has an aperture about 1 mm I.D., and ions pass through it to the
second skimmer cone. The space in front of the first cone is evacuated by a high-vacuum pump. The
region between the first cone and the second skimmer cone is evacuated by a mechanical pump to about
2 mbar and, as the sample expands into this region, a supersonic jet is formed. This jet of gas and ions,
flows through a slightly smaller orifice into the apex of the second cone. The emerging ions are
extracted by negatively charged electrodes (-100 to -600 V) into the focusing region of the
spectrometer, and then into the mass analyzer.