Principles of Isotope Dilution Mass Spectrometry

The fundamental principles of IDMS have been extensively described in several textbooks [1, 8, 9], from which detailed information can be obtained. Here, those characteristics of ICP-IDMS that are specific in comparison with other analytical techniques are reviewed and attention is also paid to the fundamental topics to be considered carefully for successful analysis via ICP-IDMS. 

In isotope dilution analysis, a sample with an unknown concentration of the target element, usually in its natural isotopic composition, is mixed and chemically equilibrated with a known amount of an isotope-enriched spike compound of the corresponding element. This approach self-evidently requires the element to be determined to have at least two isotopes measurable by ICP-MS. For poly-isotopic elements, such as lead with four stable isotopes, only two isotopes need to 

One of the exceptional advantages of the isotope dilution technique compared with other methods is that a ratio (isotope ratio, R) is measured, whereas in all other analytical methods the amount of analyte needs to be measured, such that non-quantitative recovery during the sample pretreatment needs to be corrected for. For IDMS analysis, on the other hand, once the isotope dilution step has taken place, the recovery does not need to be known. This is because, for example, subsequent chemical separation procedures will usually not change the isotope ratio of the isotope-diluted sample to a significant extent. From this, it follows that sample loss does not affect the analytical result in IDMS analysis. An important precondition for successful element determination via isotope dilution is an equilibration of the sample and spike as early as possible in the analytical procedure. This prerequisite is best fulfilled in the case of solutions. Solid samples therefore need to be completely digested and thereby equilibrated with the spike, usually added in the form of an aqueous solution. If the analyte and the spike in the isotope-diluted solution are not equilibrated and exist in different chemical forms, sample preparation steps such as chromatographic separation will fractionate the analyte from the spike and the corresponding isotope ratio measurements will produce incorrect results. 

Another advantage of IDMS is its independence of matrix effects, which present an important drawback for other calibration approaches. Even if matrix effects affect the signal intensity in ICP-MS, the reference and spike isotope are affected in the same way and to the same extent, so that the measured isotope ratio R is not 

Equations (8.1) and (8.2) are not corrected for instrumental mass bias (mass discrimination) and blank values. In the literature, one can therefore find much 

---

Chapter 8 provides the reader with the basic principles of isotope dilution mass spectrometry used for elemental analysis and also discusses more advanced features of this calibration approach, such as its use in direct solid sample analysis and in elemental speciation work, wherein not the total amount but that of various chemical species of a target element need to be determined. 

---