Internal Standardization and Isotope Dilution

Isotope Dilution Mass Spectrometry

As discussed in part 10, ICP-MS suffers form matrix related effects upon the nebulizer and the signal intensity (quenching). In addition, even slight deposition on the sampler cone will cause drifting. Due in part to drifting, analysts have chosen to use the calibration curve technique with internal standardization over the technique of standard additions. Although the standard additions technique should work well in theory, the drifting associated with ICP-MS is too pronounced. The use of a ratio technique such as internal standardization is a reasonable compromise with the understanding that the internal standard is not influenced to exactly the same degree as the analyte signal. This is due to mass dependence. The internal standards commonly used are only used over relatively narrow mass ranges making the use of multiple internal standard elements required for broad mass range applications. The most common internal standard elements listed from low to high mass are ⁶Li (isotope 6 enriched), Sc, Y, In, Tb and Bi.

ICP-MS has the unique capability of using an enriched isotope of the element of interest as the internal standard. This technique, which is known as isotope dilution mass spectrometry (IDMS), has been known for nearly 50 years¹. IDMS is made possible through the availability of enriched stable isotopes of most of the elements from the electromagnetic separators in Oak Ridge, Tennessee (U.S.A). IDMS is therefore not applicable to monoisotopic elements.

The IDMS technique involves the addition of a known amount of an enriched isotope of the element of interest to the sample. This addition is made prior to sample preparation during which the spiked addition of the enhanced isotope is 'equilibrated' with the sample. By measuring the isotope ratio of the sample and sample + spike isotope addition and knowing the isotopic ratio of the enhanced addition, the sample concentration can be calculated. The entire measurement is based upon ratio measurements of one isotope of the element to another. Drift, quenching and other related matrix effects do not present an interference with IDMS. This technique is considered a definitive₂ method and is well suited and established for the certification of certified reference materials.

IDMS is free from matrix effects (physical interference) but it is not interference-free in that mass interference must still be dealt with (isobaric, MO⁺, M⁺⁺, etc.) in addition to correction of the signal intensity for detector dead time and mass bias interference.

To view an example of an IDMS method, reference EPA Method 6800.

1. Hintenberger, H, Electromagnetically Enriched Isotopes and Mass Spectrometry, Proceedings Conference, Harwell, (1955): pg 177; Butterworths Scientific Publications, London.

2. Definitive is defined as, "A method of exceptional scientific status, which is sufficiently accurate to stand alone in the determination of a given property for the Certification of a Reference Material. Such a method must have a firm theoretical foundation so that systematic error is negligible relative to the intended use. Analyte masses (amounts) or concentrations must be measured directly in terms of the base units of measurements, or indirectly related through sound theoretical equations. Definitive methods, together with Certified Reference Materials, are primary means for transferring accuracy -- i.e., establishing traceability.

Traceability is defined as, "The property of a result or measurement whereby it can be related to appropriate standards, generally international or national standards, through an unbroken chain of comparisons."

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