Guidelines for Achieving High Accuracy in Isotope Dilution Mass Spectronometry (IDMS)

1 Introduction

The technique of isotope dilution mass spectrometry (IDMS) was initially developed during the 1950s for elemental analysis. With readily available and user-friendly instrumentation, the number of applications for which elemental IDMS was used developed rapidly. IDMS was extended into the field of organic compounds in the 1970s and the range of applications for which isotopically labelled organic compounds are available continues to grow. IDMS applications range from routine survey work, such as residue analysis of dioxins, to use as a reference technique.

In mass spectrometry, unlike spectrophotometric techniques, there is no fixed relationship between the amount, and concentration, of a particular substance and the instrument response. Sensitivity for a particular compound or ion varies with, for example, time and instrumental tune parameters. These variations are in addition to those caused by, for example, sample introduction and chromatography. To achieve even moderately accurate quantification requires the use of an internal standard. An advantage of mass spectrometry lies in its ability to use isotopically enriched analogues (inorganic mass spectrometry) or isotopically labelled analogues (organic mass spectrometry) as internal standards. Indeed, in some instances IDMS is referred to as 'stable isotope internal standardisation'. Provided the isotopic analogue is added to the sample at the very beginning of the analytical method and it comes into equilibrium with the analyte without losses or isotopic fractionation, it enables exact compensation to be made for errors at all stages of the analysis, from sample digestion/preparation through to the final instrumental measurement.

Initially, IDMS of inorganic analytes was most frequently performed using thermal ionisation mass spectrometry (TIMS). More recently, inductively coupled plasma mass spectrometry (ICP-MS) based IDMS has become more prevalent, because it requires much less sample preparation prior to analysis, and can provide results of the required accuracy and precision. The inorganic sections of this guide are concerned only with the use of ICP-MS for high accuracy IDMS measurements. IDMS of organic analytes can equally well be carried out using the full range of sample inlet systems available, such as GC-MS, LC-MS and the newer technique of capillary zone electrophoresis-mass spectrometry (CZE-MS). Tandem mass spectrometers (MS-MS) can also be used. In some instances, however, the inlet system/mass spectrometer type can have an influence on IDMS analysis. The use of a GC split injector, for example, can cause isotopic fractionation.

The advent of compact and economic instrumentation, such as quadrupole and ion trap mass spectrometers, has led to the increasing use of mass spectrometry in the field of analytical chemistry. As a result, IDMS is playing an increasingly important role in trace analysis. This is also due to its greater accuracy than other calibration methods and its ability to compensate for matrix effects. Notwithstanding this, there are several reasons why, in spite of the unrivalled accuracy and precision possible with the IDMS technique, it is not more widely used. In principle, the method is simple and allows for knowledge or control of all the variables that can lead to error. In practice, achieving accurate results requires careful design of the experiment and considerable attention to detail and is hence quite time consuming. Such factors have led to a slow acceptance of the technique. It has become popular, however, in some analytical fields where the sample matrix makes sufficiently accurate quantification difficult, e.g. clinical analysis. A theoretical discussion on the influence of some instrumental parameters on the precision of IDMS measurements has been published.

1.1 Advantages and Disadvantages of IDMS

The use of IDMS has a number of advantages and disadvantages, which the prospective user should consider.

1.1.1 Advantages

Providing the sample is homogeneous and isotopic equilibrium has been reached, the advantages of IDMS include:

1. It is a definitive method because of its precision, accuracy and provision of definable uncertainty values.

2. Once equilibration of the spike and analyte isotopes has been achieved, the total recovery of the analyte is not required, because the determined value is based on measuring the ratio between the analyte and the isotopic analogue (spike).

3. The accuracy of the method is determined by the precision of this ratio measurement.

4. Analyte transformation (e.g. breakdown) during sample preparation (most applicable to organic analysis) is compensated for by use of the isotopic analogue as internal standard.

5. The methodology is less time consuming and can provide greater accuracy than standard additions.

This guide aims to provide a structured approach to the use of IDMS to achieve high accuracy analytical measurements in both organic and inorganic IDMS. The approach outlined in Sections 4 and 5 has the advantage over normal IDMS of:

1. Negating the need to accurately characterise the isotopic analogue in terms of isotopic abundance and concentration (most applicable to inorganic analysis).

2. Providing a logical framework of understanding, which allows the analyst to identify specific problems, related to their application.

3. Providing a lower uncertainty value.

4. Simplifying the equations used for organic analytes.

1.1.2 Disadvantages of IDMS

1. The cost and availability of suitable isotopic materials.

2. The cost of the mass spectrometry instrumentation required.

3. Training of the analyst is of the utmost importance; otherwise less accurate results are often achieved.

4. Isotopic equilibration needs to be shown to have been achieved.

5. Differences in the physical (e.g. solvation) and chemical properties (e.g. pKa value) between the analyte and the...