Mass discrimination

Taylor PDF, De Bievre P, Walder AJ, Entwistle A (1995) Vahdation of the analytical linearity and mass discrimination correction model exhibited by a Multiple Collector Inductively Coupled Plasma Mass Spectrometer by means of a set of synthetic uranium isotope mixtures. J Anal At Spectrom 10 395-398... 

Variable mass-discrimination (generally a problem only for TIMS analyses of Th or of U when not double-spiked and not normalized via... 

The internal standard and analyte should be resolved chromatograph-ically to baseline (except for isotoplcally labelled samples when mass discrimination or radioactive counting are used for detection), elute close together, respond to the detection system in a similar way, and be present in nearly equal concentrations. 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

Table 7.23 Combinations of separation techniques and mass discrimination"...

The largest change in K for a given AM2 occurs when 0=180°, and so large scattering angles maximise the mass discrimination of the technique. 

On the other hand, for broad-MMD polymer, MALDI underestimates both Mn and Mw [4-6]. This problem is usually called mass discrimination [4-6]. Many... 

Mass discrimination with distillation effects. Let us assume that the isotope composition of an element is being measured by thermal ionization. This method consists in ionizing the sample atoms by evaporation on a metal filament. Statistical thermodynamics (e.g., Denbigh, 1968) tells us that, while vapor pressure is a function... 

Standard bracketing consists in interpolating the mass bias of an unknown sample between the biases inferred from two standard runs, one preceding and one following the sample analysis (less stringent orders of interpolation are also used). Let us divide Equation (20) for the sample by the same equation for a standard (1) run just before the sample. The mass discrimination law used is the exponential law but the principle is easily adapted to the power law. We get ... 

Figure 9. Sketch of the double spike Zn- Zn method. The surface is constructed by drawing an infinite number of straight-lines through the point representing the spike composition (supposed to be known with no error) and each point of the mass fractionation line going through the point representing the measured mixture. One of these straightlines, which is to be determined from the calculations, is the sample-spike mixing line (stippled line). Each determination of the Zn isotope composition of a sample involves only one run for the mixture of the sample with the spike. Since all natural samples plot on the same mass fractionation line, any reference composition will adequately determine isotope composition of the sample, note that, since the instrumental bias is not linear with mass, the mass discrimination lines are curved.

Because of its isotopic variability, background must be reduced at any cost. The matrix of samples and standards must be reduced by appropriate chemistry to trace amounts, typically to a total concentration far smaller than the element to be analyzed. This requirement is most critical when the mass bias is inferred not internally from the sample itself, but externally from bracketing standards or from a different element used for isotopic normalization. Even the most dilute heavy species may drastically affect mass discrimination. 

Vance D, Thirlwall ME (2002) An assessment of mass discrimination in MC-ICP-MS using Nd isotopes. Chem Geol 185 227-240... 

However, there are other considerations that make these ratios unattractive for this purpose. The accuracy of the mass discrimination correction depends on the precision with which the isotope ratio can be measured, which is poorer for lower abundance isotopes. Hence Ca is a poor choice, and the combination of Ca and H2a is particularly unattractive. Also, because the mass discrimination is mass dependent as discussed below, it is advantageous to use an isotope ratio that is similar in average mass to that of the target isotope ratio being measured. 

The result of the various considerations is that it is generally best to use either the " Ca/ Ca or " Ca/ Ca ratios to monitor the radiogenic enrichments of Ca, and to use KlaJ Ca to measure mass discrimination. Marshall and DePaolo (1982) chose to use CaJ Kla as the target ratio and to use Ca/ Ca for the mass discrimination correction. They found that when using the value of Ca/ Ca = 0.31221 reported by Russell et al. (1978b) for the mass discrimination correction, the initial solar system value of " Ca/ Ca is 151.016 + 0.008. 

Figure 1. Schematic representation of the calcium mass spectrum in (a) natural materials, (b) a Ca- Ca tracer solution used for separating natural mass dependent isotopic fractionation from mass discrimination caused by thermal ionization, and (c) a typical mixture of natiwal calcium and tocer calcium used for analysis. The tracer solution has roughly equal amounts of Ca and Ca. In (c) the relative isotopic abundances are shown with an expanded scale. Note that in the mixed sample, masses 42 and 48 are predominantly from the tracer solution, and masses 40 and 44 are almost entirely from natural calcium. This situation enables the instrumental fractionation to be gauged from the Ca/ Ca ratio, and the natural fractionation to be gauged from the sample Ca/ Ca ratio.

Using the three measured ratios, Ca/ Ca, Ca/ " Ca and Ca/ " Ca, three unknowns can be solved for the tracer/sample ratio, the mass discrimination, and the sample Ca/ Ca ratio (see also Johnson and Beard 1999 Heuser et al. 2002). Solution of the equations is done iteratively. It is assumed that the isotopic composition of the Ca- Ca tracer is known perfectly, based on a separate measurement of the pure spike solution. Initially it is also assumed that the sample calcium has a normal Ca isotopic composition (equivalent to the isotope ratios listed in Table 1). The Ca/ Ca ratio of the tracer is determined based on the results of the mass spectrometry on the tracer-sample mixture, by calculating the effect of removing the sample Ca. This yields a Ca/ Ca ratio for the tracer, which is in general different from that previously determined for the tracer. This difference is attributed to mass discrimination in the spectrometer ion source and is used to calculate a first approximation to the parameter p which describes the instrumental mass discrimination (see below). The first-approximation p is used to correct the measured isotope ratios for mass discrimination, and then a first-approximation tracer/sample ratio and a first-approximation sample CeJ Ca... 

The essential nature of the exponential parameterization of instrumental mass discrimination given by Russell et al. (1978b) is represented by the following two equations ... 

The tracer-subtraction procedure adds negligible uncertainty to the measured CaJ Ca ratios. However, it is in fact essentially impossible to entirely eliminate the effects of instrumental mass discrimination for the measurements of either the Ca- Ca mixed tracer or for the standard Ca isotope ratios. Hence, it is necessary to have a standard material with an agreed-upon value of 5 Ca. At the time of writing of this article there is no such standard. 

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