Titanium samples can be selectively dissolved in aqua regia, leaving graphite and other metals in the selection unaltered. This is a standard method used for ICP-MS preparation of titanium samples before analysis. The graphitic nature of carbon in most refractory alloys also makes it particularly susceptible to dissolution during this procedure. The problem becomes more pronounced when using low percentage reagent grade acids commonly available from chemical suppliers. This article outlines a technique that avoids these problems by preparing a relatively inert interface for ICP-MS analysis.
There are five different models of skimmer cones available for the ICP-MS. These include Gilson, Agilent Technologies, Perkin Elmer/Cheshire, Dionex, and UVP/Nu Instruments. The two types of cones that are used in conjunction with multi-collector ICP MS Cones instruments are the “gas sampling” cone (the first four manufacturers listed above) and the “liquid sampling” cone (UVP/Nu Instruments). The main difference between these cones is whether they are intended to be sampled by placing them into a liquid or solid sample, respectively.
Gas Sampling Cone
This type of cone is designed to be inserted directly into the plasma of the ICP torch. The sample gas is ionized by the plasma and transported with any reagent gases (such as methane for mercury analysis) through the double-aperture skimmer. This type of sampling cone can only be used with a “gas standard,” i.e., it cannot be used on liquid samples or tissue homogenates. With this type of cone, there are no openings in the top surface to allow liquid from a fluid sample to pass through since the whole interface assembly is submerged in a liquid bath anyway!
Liquid Sampling Cone
This type of cone has holes at its tip that allow an amount of liquid from a solid or liquid sample to pass through and into the plasma torch, thus creating a “liquid standard.” The holes in the cone are not big enough for large pieces of tissue or solid samples. However, small amounts (up to about 5ml) can be quickly introduced by placing a few drops on the cone tip before inserting it into the plasma.
In order to use either type of skimmer cone, it is necessary to fasten them securely onto one end of a gas-tight connector that attaches to a sampling valve on your ICP-MS instrument. There are two types commonly used: needles and tubular fittings. Cone/Connector combinations come in polarity configurations for positive and negative ions as an integral pre-concentrator for preconcentration.
Skimmer cones are made from either titanium or stainless steel. The choice of material does not affect the quality of the results, but it does affect the price. Titanium costs more than stainless steel since there is less demand for this metal in the industry.