Mass Bias Correction

Using the mean of the results for R , calculate the mass bias correction factor, C, using Eqn. B. 1. 

Calculate the observed isotope ratio, R , for the sample, and multiply by the mass bias correction factor to obtain the true isotope ratio, R Hence, calculate C for the sample using Eqn. B.4. 

High precision determination of Rb/ Rb isotope ratios by MC-ICP-MS (Isoprobe, Micromass) is advantageous in comparison to MC-TIMS (VG Sector 54, Micromass) due to an unproved precision, and fraction capability problems can be easily considered using admixed Zr for mass bias correction via the Zr/ °Zr isotope ratio. Rb isotopic composition in geological materials (basalt, granite and greywacke) and synthetic samples (in house mica standard, NBS 607 K-fsp standard) have been examined with a reproducibility of Rb/ Rb measurements of 0.02-0.05 % (2 O ). No anomalous isotope fractionation was observed for the samples investigated. 

Whereas the abundance of Sr in rubidium rich rocks changes over time due to the radioactive 3 decay of Rb as a function of the primordial rubidium concentration and the age of the mineral, the abundance of the stable Sr isotope and consequently the Sr/ Sr is constant in nature. The constant Sr/ Sr isotope ratio is often used for internal standardization (mass bias correction) during strontium isotope ratio measurements of Sr/ Sr. In the rubidium-strontium age dating method, the isotope ratios Sr/ Sr and Rb/ Sr are measured mass spectrometrically (mainly by TIMS or nowadays by ICP-MS) and the primordial strontium ratio ( Sr/ Sr)o at t = 0 and the age t of the rock can be derived from the isochrone (graph of measured Sr/ Sr isotope ratios (represented on the ordinate) as a function of the Rb/ Sr ratio (on the abscissa) in several minerals with different primordial Rb concentrations). The age of the minerals will be determined from the slope of the isochrone (e — 1), and the primordial isotope ratio ( Sr/ Sr)o from the point of intersection with the ordinate (see Figure 8.9). Rb-Sr age dating is today an... 

MeHg using isotopically enriched Me Hg and Bus Sn wet and dry plasma conditions used to facilitate mass bias correction and signal optimization CRM 710 analyzed to validate method... 

Using appropriate ions of the natural analyte and the spike, the isotope amount ratios for the spiked sample and the spiked calibration standard are determined. It is suggested that alternating measurements of the isotope amount ratio are made on these two solutions (repeated measurement of the calibration blend allows mass bias correction to be performed for inorganic IDMS (see Section 3 10), repeating each five times. The mean value of the five measurements will minimise the effects of any instrument drift. An improved estimate of the natural analyte concentration in the sample can then be calculated from the data. 

After mass bias correction see Section 4.2.2 for an example of calculating the mass bias correction. 

The mass bias correction factor is calculated using the equation ... 

Table 3 Mass bias correction factor before and after analysis...

Hence the mean mass bias correction factor to be applied to the sample is... 

The isotopic abundance (isotope amount fraction) is usually obtained from the lUPAC standard tables. However, if the abundance of the element varies in nature (e.g. Li, B, Pb, U, Sr, Os) it must be determined experimentally on the actual material in use. In this case, the unknown sample and a pure natural isotopically certified standard e.g. SRM981, NIST for Pb), should be run alternately, so that the unknown sample is bracketed by the analyses of the standard. (The run sequence is the same as that used for checking for interferences, as shown in Figure 5.) This approach allows mass bias correction to be facilitated. The abundances for the unknown sample can then be determined, after allowance has been made for any mass bias effect. 

Figure 6 Run sequence to characterise the spike, using reverse IDMS of the second blend with the first blend for mass bias correction for key see Figure 4...

Interpolation In inorganic ID-MS, it is also possible to use an alternative element, with an isotope pair of similar mass to the isotope under study, and which has a certified isotope amount ratio. The mass bias correction can be performed by interpolating (with increasing accuracy), using either linear, power, or exponential equations ... 

The advantage of this approach is that the mass bias correction can be performed by spiking the sample with a mass bias correction standard (e.g., Tl) and measuring this isotope amount ratio at the same time as the sample. 

It should be taken into account, however, that the isotope ratio precision is typically worse than that with pneumatic nebulization as a means of sample introduction. Recently, Aramendia et al. provided a description of strategies to improve the isotope ratio precision attainable with the combination of LA and single-collector ICP-MS [62]. As the use of LA also affects the degree of mass discrimination, matrix-matched standards are required for adequate mass bias correction [63],... 

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