Applicability of High-Resolution ICP−Mass Spectrometry for Isotope Ratio Measurements

Effect of the detector dead-time uncertainty on the analytical result of minor elements in low-alloy steel by isotope dilution/ICP sector field mass spectrometry

In the present study the effects of the detector dead-time and its uncertainties on the accuracy and uncertainty of isotope dilution mass spectrometry (IDMS) were considered through an interlaboratory study on the analysis of low-alloy steel by using an ICP-sector field mass spectrometer. Also, an optimized mixing ratio of the sample and the spike to obtain highly precise results was theoretically and experimentally investigated. The detector dead-time used in the interlaboratory study showed a negative value. However, it less affected the trueness of the analytical result if the dead-time correction for the measured isotope ratio was done properly. As many researchers have pointed out, the detector dead-time showed a clear mass dependence. Therefore, it is desirable to check the dead-time in every target element by using assay standards or isotopic standards, which would lead to an accurate result even if the detector dead-time is a negative value. On the other hand, the effect of the uncertainty of the detector dead-time can be minimized when both isotope ratios and ICP-MS signals of the [sample + spike] blend in IDMS are equal to those of [spike + assay standard] in reverse IDMS. From standpoints of error magnification theory and the precision of the isotope ratio measurement, an optimized isotope ratio of the sample-spike blend would be 1.0 for an element with a large difference in ten times and more between the atomic fractions of two isotopes used for IDMS. In the case of an element with no significant difference between the atomic fractions of two isotopes, an optimized isotope ratio can be calculated by a formula expressed as a function of the atomic fractions of the sample and the spike as well as the signal of ICP-MS.

Evaluation of the Isotope Ratio Performance of an Axial Time-of-Flight ICP Mass Spectrometer

The isotope ratio performance of an axial time-of-flight ICP mass spectrometer (Renaissance TOF-ICPMS, LECO Corp.) was evaluated. The isotope ratio precision, expressed as the relative standard deviation (RSD) for 10 successive measurements, was evaluated using multielement standard solutions with analyte concentrations of 50-500 μg/L. The influence of the acquisition time per replicate measurement was studied by varying it between 0.5 and 300 s. For an acquisition time of 30 s per replicate and an elemental concentration of 500 μg/L, typical isotope ratio precisions of ≤0.05% RSD were obtained. The fact that this isotope ratio precision can be obtained for many ratios simultaneously is an especially attractive feature of TOF-ICPMS. In contrast to what was expected, increasing the acquisition time per replicate to values of >30 s resulted in a slightly deteriorated isotope ratio precision. At short acquisition times (<10 s), isotope ratio precisions similar to, or better than, the best values ever reported for quadrupole-based instruments were obtained. The latter observation remained valid when working with transient signals of corresponding duration. Mass discrimination was observed to be analogous to that observed with other types of ICPMS instrumentation (∼1% per mass unit at midmass). The accuracy attainable was evaluated by comparing Pb isotopic results for a "natural" Pb standard solution obtained by TOF-ICPMS with those obtained by thermal ionization mass spectrometry.

Assessment of lead availability in contaminated soil using isotope dilution techniques

Isotope dilution methods using a stable isotope tracer ((207)Pb) were developed for the determination of Pb availability in contaminated soils. The methods included determination of E values (isotopically exchangeable pool), L values (plant labile pool) and isotopic exchange kinetics (IEK). Isotopically exchangeable Pb was monitored at different exchange times based on measurement of the (207)Pb/(208)Pb ratio in soil solution following addition of the tracer. The rate of decrease in the (207)Pb/(208)Pb ratio in solution could be described by using the same IEK equation as used previously with radioisotope tracers. The amounts of isotopically exchangeable Pb in Pb-contaminated soils estimated from long-term IEK parameters were in good agreement with directly determined E values up to 15 days. However, values of some of the fitted IEK parameters cast doubts on the validity of using the IEK approach with (207)Pb, most probably as a result of irreversible fixation of some of the spike by reactive surfaces in the soils. Estimation of isotopically exchangeable Pb using short-term kinetics data was unsuccessful, substantially underestimating E values. Results for the control (uncontaminated) soil were highly variable, most probably as a result of fixation of tracer by the soil and poor analytical precision due to low solution Pb concentrations. A compartmental analysis of the variation in E values with time indicates a good potential for estimating bioavailable Pb in contaminated soils. The amounts of available Pb obtained from summation of the E(1)(min) and E(1 min-24 h) pools (E((available))), accounting for an average of 57.62% of total soil Pb, were significantly correlated with both the L values and with Pb extracted from soil with EDTA.

Measurement of labile Cu in soil using stable isotope dilution and isotope ratio analysis by ICP-MS

Isotope dilution is a useful technique to measure the labile metal pool, which is the amount of metal in soil in rapid equilibrium (<7 days) with the soil solution. This is normally performed by equilibrating soil with a metal isotope, and sampling the labile metal pool by using an extraction (E value), or by growing plants (L value). For Cu, this procedure is problematic for E values, and impossible for L values, due to the short half-life of the 64Cu radioisotope (12.4 h), which makes access and handling very difficult. We therefore developed a technique using enriched 65Cu stable isotope and measurement of 63Cu/65Cu ratios by quadrupole inductively coupled plasma mass spectrometry (ICP-MS) to measure labile pools of Cu in soils using E value techniques. Mass spectral interferences in detection of 63Cu/65Cu ratios in soil extracts were found to be minimal. Isotope ratios determined by quadrupole ICP-MS compared well to those determined by high-resolution (magnetic sector) ICP-MS. E values determined using the stable isotope technique compared well to those determined using the radioisotope for both uncontaminated and Cu-contaminated soils.

Cadmium reactivity in metal-contaminated soils using a coupled stable isotope dilution-sequential extraction procedure

To better understand the intrinsic reactivity of Cd, four soils having diverse sources of Cd contamination (total Cd: 22-34 mg kg(-1)) were investigated using a stable isotope dilution-sequential extraction procedure (SID-SEP) during a 59-week incubation. Samples were spiked with carrier-free 111Cd and periodically extracted into five operationally defined fractions; the 111Cd:10Cd ratios were measured by inductively coupled plasma-mass spectrometry. The total labile pool of Cd (E value) was calculated as well as the labile Cd within each extracted fraction. Results for three of the soils (Cd sources: natural, sewage sludge, smelter emissions) were quite similar. The overall %E after 2-week equilibration was 35-49% of total soil Cd. Within fraction 2 (sorbed/carbonate), 70-75% of the Cd was isotopically labile, while within fraction 3 (oxidizable) only 35-41% of the Cd was labile within 2 weeks. The fourth (reducible) and fifth (residual) fractions were dominated by nonlabile Cd. Although all E values increased somewhat from 2 to 59 weeks, none of the extracted fractions reached isotopic equilibrium with the soluble/exchangeable Cd extracted during step 1. Because fractions 2 and 3 dominated the native Cd in all three soils, the total labile pool was contributed primarily (85-98%) by these two fractions. A fourth soil (mine spoil-contaminated) was demonstrably different: after 2 weeks, the overall %E was just 13 and, although 82% of the total Cd was present in the oxidizable fraction, just 2% of that was isotopically labile. The nonlability of Cd in this soil could be ascribed to the predominance of inorganic forms, most likely occluded Cd in sphalerite. No single Cd fraction from the SEP nor any combination of fractions showed a good correspondence with the size of the isotopically labile pool. Our results suggest that conventional SEPs may be of limited utility for predicting bioavailability, for example, during ecological risk assessment.

Radiochemical neutron activation analysis in the life sciences. A look back and ahead

The contribution of radiochemical neutron activation analysis (RCNAA) to a better understanding of trace element analytics and physiology in the life sciences is outlined. Now, various non-nuclear powerful techniques for trace element analysis have become available, competing with RCNAA. This necessitates re-evaluation of the position of RCNAA, in particular versus inductively coupled plasma--mass spectrometry (ICP-MS). On basis of the characteristic features of RCNAA and the capabilities of various competing non-nuclear analytical techniques, future niches for RCNAA in the analytical market are indicated.

Plasma source mass spectrometry in experimental nutrition

The development and commercial availability of plasma ion source, specifically inductively coupled plasma, mass spectrometers (ICP-MS) have significantly extended the potential application of stable isotopes for nutritional modeling. The status of research and commercial ICP-MS instruments, and their applications and limitations for stable isotopic studies are reviewed. The consequences of mass spectroscopic resolution and measurement sensitivity obtainable with quadrupole, sector, time-of-flight, and trap instruments on stable isotope analysis are examined. Requirements for reliable isotope measurements with practical biological samples including tissues and fluids are considered. The possibility for stable isotope analysis in chemically separated compounds (speciation) also is explored. On-line compound separations by chromatography or electrophoresis, for example, have been combined instrumentally with ICP-MS. Som possibilities and requirements are described for stable isotope speciation analysis.