ICP Expert Help
Click here to see this page in full context

 

More About Internal Standards

Internal standards can be used as part of your analysis. This page provides information about:

Why Should I Use Internal Standards?

Internal standardization is a method used to compensate for the effects of intensity changes and fluctuations (noise) due to the sample matrix. Therefore it can be used to improve both the accuracy and the precision of analysis.

How Are Internal Standards Applied?

Internal standardization is applied by taking the intensity of the internal standard measured with the first blank (or standard) and referencing that as 1.0. The intensity of the internal standard in subsequent standards and samples is referenced against the first internal standard measurement and calculated as a ratio. This ratio is then applied to the calculated value of the subsequent standards and samples to give a corrected value. That is, the analyte standards and samples are corrected for any changes which may affect both the internal standard and the analytes.

What Matrix Effects Do Internal Standards Overcome?

There are two kinds of matrix effects that internal standardization can be used to overcome:

  • The effect of differences in viscosity between standards and samples. Such differences cause changes in the rate of sample transport to the plasma.
  • The effect of the sample on excitation conditions within the plasma.

A sample matrix that causes a change in sample transport rate to the plasma (effect 1) will almost invariably cause a difference in excitation conditions in the plasma (effect 2), and vice versa. Therefore it is difficult to separate the effects of viscosity differences from the effects on the plasma discharge itself. As a result, internal standardization is usually carried out with the aim of compensating for both kinds of effect.

How Do I Choose an Internal Standard?

  • To compensate for transport differences, you can use any suitable reference element.
  • To compensate for the effects of differences in plasma conditions you must use a reference element and a line which have similar spectroscopic properties to the analyte.

Therefore when selecting an Internal Standard you should ensure that:

  • The internal standard and analyte properties match as closely as possible. This will ensure that any variations in plasma conditions will affect the analyte and the internal standard similarly. In contrast, these variations can have differing effects on atomic and ionic lines.
  • You match the state of ionization and the excitation energy of the internal standard line to the analyte line. To a first approximation, the energies of the lines’ excited states can be matched by choosing lines of similar wavelength. Thus, the line Mn II 257.610 nm is a good choice of internal standard for Fe II 259.940 nm, as it has the same state of ionization and a similar excitation energy (wavelength). Conversely, the line Cu I 324.754 nm would not be a good choice of internal standard for Fe II 259.94 as it has a different state of ionization and its energy is also quite different.

What Other Properties Should an Internal Standard Have?

Internal Standards should have the following additional properties.

  • The internal standard must not be present in detectable amounts in the samples.
  • The internal standard should not suffer spectral interference from the sample. Any spectral interference on the internal standard that does occur should be easily corrected.
    Additionally, the internal standard must not itself cause spectral interference on the analyte.
  • The internal standard solution must be chemically compatible with the analyte and sample solutions.
  • The internal standard should be obtainable as a pure material.

What Are Some Examples of Common Internal Standards?

Common elements used for internal standardization in ICP-OES include Sc, Y, La and Lu. These elements are seldom present in contaminants. The only exception is Y, which is found in certain environmental samples.

How Do I Add Internal Standards to the Standards and Samples?

The internal standard must be added to the standards and samples and the concentration of the internal standard must be the same throughout. This can be done either by manual addition of the internal standard to each of the solutions, or by using the third channel of a three-channel pump.

If dilutions are performed and you have spiked your solutions with an internal standard manually, then the diluent must contain the internal standard at the same concentration as in the solutions, or an incorrect result will be reported.

How Do I View Internal Standard Ratios?

Click here for details.

Does Internal Standardization Have Other Benefits?

Yes. A final benefit of internal standardization occurs when the analyte and internal standard are measured simultaneously.

This simultaneous measurement makes it possible to compensate for the noise known as flicker noise. This is because fluctuations in the analyte intensity caused, for example, by short-term changes in sample introduction rate to the plasma, will also be passed on to the internal standard intensity. Therefore the ratioing of intensities can be used to compensate for the short-term fluctuations, just as the ratioing compensates for the effect of the sample matrix on analyte intensities. This ratioing can result in an improvement in analytical precision (RSD) for lines whose noise is limited by flicker noise.