When a sample contains a high matrix concentration, a signal suppression or enhancement due to a matrix effect might be seen. Clogging at the nebulizer or interfaces might also occur. In general, the maximum concentration for a matrix should be no more than 1000 ppm.
Drinking water can be directly analyzed after acidification with HNO3. Drinking water typically contains a few tenths ppm of Na, Ca, Mg, K and Cl. In general, ArNa at m/z 63 interferes with Cu, but m/z 65 can be used. ArMg interferes with Zn at m/z 64, but m/z 66 and 68 can be used. CaO and KO interferences at m/z 60 and 55 are negligible; ClO and ArCl interferences are also negligible in most cases, the signal from ArCl at m/z 75 for As is corrected by an interference correction equation and m/z 82 is used for Se.
For more information, see Analysis of Samples.
These samples are filtered by a 0.45 mm membrane filter right after collection to separate particulate and dissolved materials. Then the sample has to be acidified to prevent absorption on the inner surface of the container and precipitation of trace elements with major elements; HNO3 should be used for acidification to obtain a pH less than 2.
For more information, see Analysis of Samples.
Although higher salt concentrations might cause clogging at the tip of the interface cones, high matrix samples, even undiluted seawater which contains about 3% (w/v) salts can be directly introduced using the Ni interface cones. Matrix suppression, however, occurs in the case of seawater. The signal can be reduced to about 1/3 to 1/5 of normal sensitivity, and polyatomic ions such as ArNa, CaO, CaOH, ArMg, ArCl and ClO interfere with elements at m/z less than 80. Se can not be analyzed at m/z 82 due to BrH interference. However, heavier elements such as Cd and Pb can be analyzed using internal standards.
Matrix suppression is compensated for by internal standard correction. An element whose mass is close to that of the analyte is used as an internal standard. However, the behavior of higher ionization potential elements such as As and Cd are sometimes different from that of lower ionization potential elements. Therefore, Te would be recommended as an internal standard element for Cd.
For analysis of lighter mass elements in seawater, matrix separation is the only way to analyze them. Cation exchange chromatography separates matrix elements from the analytes.
Many different matrix elements may be found in waste water. Therefore it is difficult to give a general description of their analysis. Matrix interference and matrix suppression are important considerations. The best way to analyze such an unknown sample is to dilute it 100 times with pure water and do a semiquantitative analysis. That will give an approximate concentration for each element in the sample, then proceed to a more detailed analysis using that information.
To digest these samples completely, HF is commonly used with other acids such as HNO3 or HClO4. Open or closed vessel digestions are commonly used. However microwave digestion has several advantages and therefore is recommended. A combination of HNO3, HF and H2O2 with the microwave can digest these samples completely. For more details, refer to Analysis of Samples.
In the case of EPA Method 6020, a mixture of HNO3 and H2O2 is used for sample preparation. The purpose of these analyses is to monitor toxic elements leached out from landfills, so complete digestion is not required. A 1 g sample is acid digested, and the digested sample is diluted with water up to 100 mL and filtered. The matrix concentration in the final solution will be much less than 1%, because silicates cannot be digested by the acids.
In general, for long term analyses, the maximum matrix concentration in a sample solution should be less than 0.1%, or 1000 ppm.
The alkali fusion method is also used for digestion of these samples, but this method should be limited to the determination of Si.
Basically same as the digestion procedure as for soils and sediments.
Animal tissues are digested by HNO3 using the microwave digester.
The classic open vessel digestion with HNO3 and HClO4 is not recommended because As and Se, which are very important elements in biological study, may be lost during the digestion.
Urine contains some particulates as well as higher concentrations of salt and protein. Therefore, filtration by a 0.45 mm membrane filter, followed by 10 times dilution with dilute HNO3 is the commonly used technique.
Because of its high viscosity and matrix, the direct analysis of whole blood is not possible, and many researchers are working on best digest methods. The following are some preparation procedures:
Carbon enhances the signal for selenium and arsenic (high ionization potential) relative to an internal standard. To compensate for this effect, an alcohol can be added to standards, samples and blanks (about 1%v/v of butanol, or isopropanol).
Metals can be dissolved by the chemicals described above. The final concentration of matrices should be less than 0.1%. Polyatomic interferences and matrix suppression should be taken into account.
This sample type is one of the more difficult. Carbon may deposit at the tip of the sampling cone, higher vapor pressure solvents may put out the plasma, lower vapor pressure solvents may remain in the sample introduction devices and some organic solvents dissolve the plastic used for the sample introduction system.
Some organic solvents can be analyzed directly by ICP-MS.
The following is a brief summary of organic samples.
Methanol has a high vapor pressure, which restricts its introduction into the plasma. Too much sample loading may put out the plasma. Therefore, a special introduction device has to be used.
Using a small inner diameter torch injector reduces sample loading into the plasma, and makes possible the analysis of 100% methanol. The standard torch injector is 2.5 mm i.d., there are 1.5 mm i.d. torch injectors as an option. Tubing can be used, although the silicon tube is better.
The recommended preparation is dilution by water after pre-concentration by evaporation to dryness.
Acetone also has a high vapor pressure and the same technique as for methanol can be applied. However, the tube cannot be used. Therefore, self-aspiration must be used. The addition of oxygen is required to prevent carbon deposition by forming CO2. The recommended preparation is dilution by water after pre-concentration by evaporation to dryness.
Xylene is commonly used for solvent extraction. 100% xylene can be analyzed with the addition of oxygen. In general, 5% oxygen in the total flow of nebulizer (carrier) and blend gas should be added. Excessively high concentrations of oxygen might cause deterioration of the interfaces.
Crude oil is a difficult organic sample to analyze because of its higher boiling point and viscosity. There are two techniques for its analysis:
Photoresist is also extremely difficult because of its higher viscosity and boiling point.