Gas Chromatography - Tandem Techniques - Gas Chromatography Mass Spectroscopy (GC-MS) Systems > Ion Generation > The ICP Mass Spectrometer Ion Source > Page 71


The ICP ion source is almost identical to the volatalizing unit of the ICP atomic emission spectrometer. A diagram of the ICP ion source is shown in figure 58. The argon plasma is an electrodeless discharge, usually initiated by a Tesla coil spark, but maintained by rf energy, that is inductively coupled to the inside of the torch by an external coil, wrapped round the torch stem. The plasma is at atmospheric pressure and is maintained at an average temperature of about 8000 K.


Physically, the ICP torch takes the form of three concentric fused silica tubes. Argon is used as the carrier gas, which, on leaving the column enters the center tube and acts as the nebulizing gas. The next tube carries an auxiliary supply of argon to maintain the plasma, and acts as a coolent to prevent the hot plasma from reaching the tip of the sample inlet tube. The outer tube carries yet another supply of argon at a very high flow rate to cool the two inner tubes so they do not melt at the plasma temperature. The coupling coil is 2-4 turns of water cooled copper tubing, placed a few millimeters behind the mouth of the torch. About 1300 watts of rf are generated at 27 or 40 MHz which induces a fluctuating magnetic field along the axis of the torch. The temperature in the induction region of the torch can reach 10,000K but in the ionizing region the temperature is kept to 70009000 K.


The sample atoms amount to less than one in 106 of the total number of atoms present in the plasma region so little or no quenching results from the presence of the sample. At 70009000 K, over 50% of most elements are ionized. The ions then pass through the apertures in the apex of two cones. The first aperture is 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. In the region between the first cone and the second skimmer-cone, the pressure is maintained by a mechanical pump at 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.

The ICP ion source has a number of unique advantages; the samples are introduced at atmospheric pressure, the degree of ionization is relatively uniform for all elements, and singly charged ions are the principal ion product. In addition, sample dissociation is highly efficient and few, if any, molecular fragments of the original sample remain to pass into the mass spectrometer. As a result of the high ion concentration of trace components in the sample, the system is extremely sensitive.