Ion Chromatography - Inductively Coupled Plasma Interfaces (LC/ICP-AAS) 2
The sampling cone has an orifice of about 1 mm I.D. where the gas expands and is pumped away. Part of the jet stream from the torch then passes through a second orifice called the skimmer cone. The ICP produces singly charged positive ions from most elements and is, thus, a very efficient ion source for coupling to a mass spectrometer. A series of electrostatic lenses direct and focus the ions into a quadrapole mass spectrometer analyzer. After mass dispersion, the ions are detected in the normal way with an electron multiplier tube. A diagram of an ICP interface associated with a quadrapole mass spectrometer is shown in figure 48.
The quadrapole mass spectrometer separates ions on a mass-to-charge ratio (m/z) basis. The ICP is not a very efficient excitation/ionization source for non-metals such as the halogens and for elements such as arsenic and selenium and so it is not a good system for examining anions from an ion exchange column. Although argon is normally employed as plasma gas, helium plasma has an ionization potential of 24.5eV compared with that of argon, 5.75eV, and is consequently a more efficient excitation/ionization source. The plasma is induced by a resonant microwave cavity surrounding the body of the torch. 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.
Dorn and Frame (6) employed a SEC/ICP-AAS combined instrument to determine traces of silicon compounds in water. Organosilicone compounds have been used extensively in foods, pharmaceuticals, health and beauty products, adhesives, sealants and many other products. Silicones appear in the environment in a number of forms, as polar silanols, as nonpolar poly(dimethylsiloxane) (PDMS), polymers, or volatile cyclic compounds such as octamethyl-cyclotetrasiloxane. Dorn and Frame assembled an apparatus specifically for this purpose the details of which are shown in figure 49A.
As the silicon compounds had relatively high molecular weights, size exclusion chromatography was employed as the separating technique. The chromatograph was the Hewlett-Packard Model 1090M fitted with an auto sampler.
The spectrometer was a Jobin-Yvon sequential JY Model 24S equipped with dual monochromators. A standard Scott double-pass glass spray chamber was used with a low dead volume pneumatic nebulizer as an interface between the chromatograph and the ICP-AAS.
The SEC separations were carried out on a Phenogel 5 Linear size exclusion column 30 cm long, 2.2 mm I.D. packed with 5 mm particles. Xylene was use as the mobile phase in which the silicones were readily soluble and it was possible to maintain a stable plasma in the presence of this solvent. An example of the type of separation that was obtained is shown in figure 49B.
(Karas and F. Hillenkamp, Anal. Chem., (60(20))(1988)2299
Figure 49A. The LC/ICP-AAS Tandem Combination Used for The Determination of Silicones in Water (ref. 23)