Sample Insertion DSI

In practice, direct insertion of samples requires a somewhat more elaborate arrangement than might be supposed. The sample must be placed on an electrode before insertion into the plasma flame. However, this sample support material is not an electrode in the usual meaning of the term since no electrical current flows through it. Heating of the electrode is done by the plasma flame. The electrode or probe should have small thermal mass so it heats rapidly, and it must be stable at the high temperatures reached in the plasma flame. For these reasons, the sort of materials used 

Sample Inlets for Plasma Torches, Part C Solid Inlets 

These direct-insertion devices are often incorporated within an autosampling device that not only loads sample consecutively but also places the sample carefully into the flame. Usually, the sample on its electrode is first placed just below the load coil of the plasma torch, where it remains for a short time to allow conditions in the plasma to restabilize. The sample is then moved into the base of the flame. Either this last movement can be made quickly so sample evaporation occurs rapidly, or it can be made slowly to allow differential evaporation of components of a sample over a longer period of time. The positioning of the sample in the flame, its rate of introduction, and the length of time in the flame are all important criteria for obtaining reproducible results. 

Solid samples can be analyzed using a plasma torch by first ablating the solid to form an aerosol, which is swept into the plasma flame. The major ablation devices are lasers, arcs and sparks, electrothermal heating, and direct insertion into the flame. 

Before discussing light sources generally, it may be useful to consider some basic characteristics of light. 

The smallest unit (packet) of electromagnetic energy (a photon) is related to frequency by the formula, E = hv, in which E is the energy and h is Planck s constant. Alternatively, the relation can be written, E = hc/A,. Frequency (v) is a number with units of cycles per second (cps, the number of times a wavefront passes a given point in unit time, sec ) and is given the name Hertz (Hz). Planck s constant is a fundamental number, measured in J sec or erg-sec. 

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Direct Sample Insertion. In direct sample insertion (DSI) [82], the sample is placed on a rod, metal loop, or cup on a rod. After desolvation (by inductive heating of the rod or use of a heat gun), the sample is inserted into the plasma. The advantages of the DSI system include nearly 100% sample transport efficiency into the ICP and use of a single power source. The most exciting capability of DSI is preconcentration using aerosol deposition that can provide two orders of magnitude of improvement in ICP-MS detection limits [83]. Detection limits as low as 0.06 parts per trillion were obtained. 

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