4.2.2 Sample Introduction
The most common sample introduction system for an ICP-MS is made up of a nebulizer and spray chamber like an ICP-OES system (Section 3.3.2). While the majority of applications use this setup, there are some specialized applications that allow solid samples to be analyzed. Solid samples can be placed directly into the ICP with a graphite rod that contains a small quantity of sample (Section 3.3.2). Other sample introduction procedures cause solid samples to sublimate before they enter the plasma. One common form of sample introduction not presented here is the glow discharge system that is used heavily by the semiconductor and metallurgy industries.
Another solid introduction technique is laser ablation which is becoming more common especially for geological and materials science applications. In laser ablation, the automatic sampler and peristaltic pump for liquid samples are replaced with the working components of the laser ablation system. This consists of a small chamber to hold the solid sample on a movable stage, a laser to ablate (heat and vaporize the solid), a viewing window to align the laser to a specific spot on the sample, and an argon gas stream to purge the ablation chamber and rapidly transport the vaporized sample to the inlet of the plasma. The laser is focused on a 10- to 25-μm section of the sample and a pulse of energy from the laser vaporizes the sample. The sample is transported to the plasma as a short pulse of vapor that is atomized and ionized in the plasma, and the generated cations are analyzed by the MS unit. Given the relatively small sampling area of the laser, numerous analyses can be conducted for a given sample and an average of analyte concentrations are determined. Common laser types include Nd-YAG, ruby, CO2, and N2. The only requirement of the laser is that it has sufficient power to ablate and vaporize refractory (high bond energy) sample matrixes. Obviously one of the quantitative limitations of the laser ablation technique is obtaining reference standards. While solid reference standards can be relatively easily made and obtained, it is almost impossible to match the matrix of all samples. Detection limits for this technique are in the range of 0.1 to 10 ppm which is much higher (poorer) than aqueous sample detection limits in an ICP-MS. As a result of the difficulties encountered with instrument calibration, qualitative analysis is commonly performed.
| Frank's Homepage |
©Dunnivant & Ginsbach, 2008