Isotope ratio measurements applications

Accurate, precise isotope ratio measurements are important in a wide variety of applications, including dating, examination of environmental samples, and studies on drug metabolism. The degree of accuracy and precision required necessitates the use of special isotope mass spectrometers, which mostly use thermal ionization or inductively coupled plasma ionization, often together with multiple ion collectors. 

Accurate, precise isotope ratio measurements are used in a variety of applications including dating of artifacts or rocks, studies on drug metabolism, and investigations of environmental issues. Special mass spectrometers are needed for such accuracy and precision. 

Applications Sector instruments are applied for niche applications such as high-resolution measurements and fundamental ion chemistry studies. Magnetic sector mass spectrometers remain the instrument of choice in areas of target compound trace analysis, accurate mass measurement and isotope ratio measurement. 

A major attraction is the ability to perform isotope ratio measurements, e.g. in many geological applications to determine the age of rocks, and isotope dilution analysis. The latter in particular is gaining-popularity as a highly accurate, precise and hence traceable, method of analysis, so it is worthwhile describing these techniques in more detail. 

Yip Y-Ch, Chung-wah Lam J, Tong WE (2008) Applications of lead isotope ratio measurements. Trends Anal Chem 27 460-480 and references therein. 

Instrumental developments (e.g., of sector field instruments with multiple ion collection, introduced in 1992, or the insertion of collision and reaction cells in order to reduce disturbing isobaric interferences), the progress in applications for ultratrace analysis, also in combination with on line hyphenated separation techniques (HPLC, CE), especially routine capability as well as decreasing price and user friendly maintenance mean that sales are increasing by 10 % every year. To improve the analytical performance of ICP mass spectrometers for precise isotope ratio measurements (e.g., for geochronology or for the study of fine isotope variation in nature) powerful instrumentation with high mass dispersion and multiple ion collector systems instead of single ion collection are commercially available on the analytical market. 

As discussed before, quadrupole based ICP-MS allows multi-element determination at the trace and ultratrace level and/or isotope ratios in aqueous solutions in a few minutes as a routine method with detection limits of elements in the sub pgml-1 range and a precision for determined trace element concentration in the low % range (RSD - relative standard deviation). The precision for isotope ratio measurements varies between 0.1% and 0.5% RSD. This isotope ratio precision is sufficient for a multitude of applications, e.g., for evidence of contamination of sample with depleted or enriched uranium in urine (this technique is used in the author s laboratory in a routine mode14) or the isotope dilution technique for the quantitative determination of trace element and species concentration after doping the sample with enriched isotope spikes. 

Further applications of ICP-MS with a collision/reaction cell in trace, ultratrace analysis and isotope ratio measurements are described in Chapter 8.16 17 20 22 27 28... 

A sensitive high resolution ion microprobe (SHRIMP, with a maximum mass resolution of m/Am of 30 000) was designed on the basis of the double-focusing sector field Matsuda type instrument190 as discussed by Compston in 1996.191 The first SHRIMP and a modified version for precise isotope ratio measurements were constructed in the 1980s and later produced commercially.192 The main application of this instrument so far has been in situ U-Pb dating of zircon.192,193... 

An excellent possibility for quantifying analytical data in inorganic mass spectrometry is isotope dilution analysis (IDA) based on more precise isotope ratio measurements. IDA uses highly enriched isotope spikes of analytes of known concentration for calibration and is the method of choice if a high accuracy for element concentrations is required. The principles and applications of this method will be discussed below. 

The fundamentals and several applications of isotope dilution mass spectrometry requiring accurate isotope ratio measurements are reviewed by Heumann.50,51 Today isotope dilution mass spectrometry (IDMS) is recognized as a primary measurement method, by means of which accurate results with sufficiently small uncertainties can be achieved and therefore it has been used in certifying the composition of reference materials. A requirement of isotope dilution analysis in mass spectrometry is to achieve equilibration of spike and sample so that very careful sample preparation steps, especially in solid mass spectrometry, are necessary when a homogeneous sample spike mixture is to be prepared. 

This chapter focuses on selected developments and applications of mass spectrometry for the analysis of gases and volatile compounds relevant in inorganic mass spectrometry. A brief introduction to isotope ratio measurements by gas source mass spectrometry is presented in Section 8.3. 

Precise and accurate isotope analyses by mass spectrometry have attained growing importance in the last few years due to instrumental improvements with respect to sensitivity, detection limits, precision and accuracy.1 As mentioned before, because the isotope abundances of several elements are not constant and vary as a result of nuclear, biological, chemical, geochemical and physical processes, isotope ratio measurements are required for different research and application fields. Isotope ratio measurements are therefore necessary for elements with two or more isotopes for inves-... 

Figure 8.1 Fields of application for isotope ratio measurements. (Modified from J. S. Becker, J. Anal. At. Spectrom., 17, H72 (2002). Reproduced by permission of The Royal Society of Chemistry.)...

Of all the different mass spectrometric techniques for isotope analysis (such as ICP-MS, LA-ICP-MS, TIMS, GDMS, AMS, SIMS, RIMS and isotope ratio mass spectrometry of gases), the greatest proportion of pubhshed papers today concern ICP-MS with single and multiple ion collection.19 Due to its benefits, ICP-MS has now become a widely accepted method for isotope analysis and allows isotope ratios to be measured in a short time with good accuracy and precision.9,19,75 78 As discussed above, as a powerful and universal tool, ICP-MS has opened up new applications for isotope ratio measurements of elements with a high first ionization potential, which are difficult to analyze with TIMS (such as Mo, Hf, Fe). Of all the heavy metals studied, uranium was favoured by ICP-MS and LA-ICP-MS. 

Numerous applications of quadrupole ICP-MS (without a collision cell) for isotope ratio measurements can be found in quite different fields. For example, magnesium isotope ratios on... 

Selected applications in isotope ratio measurements by LA-ICP-MS using different equipment are summarized in Table 8.4.50,126 136... 

Table 8.4 Selected applications in isotope ratio measurements by LA-ICP-MS. 

Sample ICP-MS Laser system/ Wavelength Measured isotope ratio RSD (%) Application Reference... 

Further applications of LA-MC-ICP-MS using the Plasma 54 by Zr isotope ratio measurements in zircon and baddeleyite samples are described by Hirata.147 The isotope ratios 92Zr/90Zr = 0.333 94 0.000 02 and 96Zr/90Zr = 0.054 63 0.000 01 have been determined on a Merck reagent with a precision of 0.01-0.02% and 0.03-0.04%, respectively. Neither isotopic variation in the... 

Van den Boom et al.150 have reported on the determination of silicon isotope ratio measurements in silicate materials by MC-ICP-MS (at a mass resolution of 2500 to resolve isobaric interferences) after sodium hydroxide sample digestion and purification of silicon. 829Si and 830Si have been determined for several silicon isotope standard reference materials. A precision for 830Si of 0.18-0.41 %o was achieved. Precise and accurate measurements of isotope ratios on transient signals by HPLC-MC-ICP-MS for nuclear application was performed by Giinther-Leopold et al.151... 

Table 8.5 Application of stable isotope ratio measurements.

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