Plasma isotopic patterns

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. 

TABLE 21.2 Commonly Detected Isotopic Patterns from Rat Plasma Samples... 

Both the reliable determination of isotope ratios and the quantitation of trace elements generally accompanied by complete destmction of all molecular entities. Opposed to what is normally desired in organic and bio-organic mass spectrometry, ionization methods in inorganic mass spectrometry therefore use conditions which erase any molecular stmcture, i.e., inorganic mass spectrometry particularly employs high temperature plasmas to achieve atomization prior to ionization and mass analysis [27]. Furthermore, ion-neutral reactions need to be suppressed or their products destroyed in order to ensure isotopic patterns free from interferences with isobars [28,29]. 

In general, ICP-MS is a sensitive and selective detector for speciation analysis however, spectroscopic interferences can cause problems for quantitative determination of an analyte, as well as for species identification based on the analyte s isotopic pattern. Spectroscopic interferences by another element (isobaric interferences) can often be corrected for using equations involving the known natural isotopic distribution of the interferent and the analyte. Polyatomic interferences, on the other hand, are less easily evaluated and compensated for. The source of these interferences is often the plasma (e.g. At2+) or a combination of the plasma and the sample or separation medium (e.g. ArCl+). Both the collision cell (CC) and the dynamic reaction cell (DRC) are designed to reduce the effect of spectroscopic interferences on the determination of the elements of interest. 

From a series of Zn isotope measurements, Marechal et al. (1999) deduced in this way that the patterns of the Zn mass bias on the Lyon Plasma 54 are best accounted for by the exponential law. 

Nance WB, Taylor SR (1976) Rare earth element patterns and crustal evolution—I. Australian post-Archean sedimentary rocks. Geochim Cosmochim Acta 40 1539-1551 Nishio Y, Nakai S (2002) Accurate and precise lithium isotopic determinations of igneous rock samples using multi-collector inductively coupled plasma mass spectrometry. Anal Chim Acta 456 271-281 Nishio Y, Nakai S, Hirose K, Ishii T, Sano Y (2002) Li isotopic systematics of volcanic rocks in marginal basins. Geochim Cosmochim Acta 66 A556... 

These same six samples were also analyszed by inductively coupled plasma-mass spectrometry (ICP-MS) to determine if the same element patterns detected in the EMPA analysis could be observed with a less labor intensive method. ICP-MS works in a similar manner as TIMS, expect the sample is dissolved and injected into a plasma flame in order to atomize and ionize the sample. Isotopes ratios can be determined in this manner, but generally the precision is less than with TIMS. 

The advances in instrumentation and particularly the arrival of the multicollector inductively coupled plasma mass spectrometers (MC-ICPMS) opened a window for a number of new tracers that were difficult to tackle with the old instrumentation. A pattern for seawater evolution is thus emerging for isotopes of boron and calcium. 

Lead is an impurity in silver. The isotopic composition of the lead measured by inductively coupled plasma-mass spectrometry provides evidence for the origin of the silver because silver mined in each region retains the pattern of lead isotopes from that region. 

Figure 16 Distribution pattern of cobalamins and cobalamin analogs (insert) in normal human plasma, measured in the HPLC-eluate by radio-isotope dilution assay using R-binder ( ) or R-binder-blocked Intrinsic Factor ( ) OH/SOjCbl (fractions 7-12). CNCbl (fractions 15-17). AdoCbl (fractions 22-26), MeCbl (fractions 33-36), fraction volume 0.5 ml. (From Ref. 74.)...

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