Molecular interferences

An example is shown in figure 1 of the molecular interferences which must be dealt with around mass 87 if one wishes to use a mass spectrometer for rubidium/strontium measurements in a geological sample [22]. The major elements in this lunar sample all have mass numbers less than 48. Thus, the mass 87 region should be completely free of atomic peaks except for the minor components such as rubidium and strontium. This is clearly not the case and at most mass numbers in the rubidium region there are major interferences from molecules. 

Until recently the only satisfactory way to separate these molecular interferences has been on the basis of nuclear mass defects, i.e., the mass of molecules having the same mass number differs from that of the atoms of the same mass number. Figure 2 shows the resolution that is needed to resolve the molecular impurities present in the previous example. Clearly, an unambiguous identification can be made, and all molecular fragments can only be eliminated for an instrument with resolution M/AM approximately 20,000. Once again, the need for high resolution will cause the transmission efficiency to be low. 

Molecular interferences can be completely eliminated by exploiting the fact that multiply charged molecules fragment with 100 percent probability, because of the internal coulomb forces, when several electrons are removed [1]. 

Why then, is such a complicated and expensive set up necessary AMS combines mass spectrometric features with efficient discrimination of isobaric and molecular interferences. Therefore, it can detect and quantify atomic species of very low abundance. In the case of 14C dating, before AMS was utilized, about 1 g of carbon was needed to date an archaeological item. One gram of fresh carbon contains about 6 x 1010 14C atoms, of which 14 decay per minute. To get 0.5% statistical precision using decay counting, a 48 h acquisition time is necessary. The same result can be obtained with AMS in about 10 min and with only 1 mg of carbon. 

Matrix effects are typically divided into spectral (isobaric) and non-spectral types. The spectral or isobaric effects include 1) elemental isobaric interferences such as Cr at " Fe, 2) molecular interferences such as Ca O at Fe and Ar N at Fe, 3) double charge interferences such as Ca at Mg. Non-spectral matrix effects are largely associated with changes in the sensitivity of an analyte due to the presence of other elements (Olivares and Houk 1986). Changes in sensitivity correspond to a change in instrumental mass bias, and therefore non-spectral matrix effects can have a significant impact on the accuracy of isotope measurements. 

However, at that time it was not possible to carry out a quantitative evaluation of the electron scattering intensity distribution. The positions of maxima and minima of the molecular interference pattern could be determined surprisingly well against the steeply falling background of atomic scattering intensity, due to the exaggerating... 

Another type of background correction system that has found some use is that developed by Smith and Hieftje. The Smith-Hieftje background correction technique is of especial use when there is strong molecular interference, such as that observed by phosphate on selenium or arsenic determinations. If the hollow-cathode lamp is run at its normal operating... 

The first factor in square brackets represents the Thomson cross-section for scattering from a free electron. The second square bracket describes the atomic arrangement of electrons through the atomic form factor, F, and incoherent scatter function, S. Finally, the last square bracket contains the factor s(x), the molecular interference function that describes the modification to the atomic scattering cross-section induced by the spatial arrangement of atoms in their molecules. 

Once the effective atomic number of the scattering species is known, for example using the HETRA method described in Section 2.3.1., it is possible to account for the IAM component of scattering in the diffraction profile, allowing the molecular interference function to be extracted. For this purpose, a universal free atom scattering plot, which can be extrapolated to non-integral values of the effective atomic number, is needed. The IAM total scattering curves for the elements with 6 < Z < 9 normalized to unit l/e... 

Knowledge of the effective atomic number allows the true width and height of the IAM curve to be determined and hence permits the molecular interference function, s(x), to be uniquely extracted on dividing the measured diffraction profile by the IAM function (cf. Eq. 7). 

The radial distribution function, g(f), generally forms the starting point for analysis of the liquid structure once the molecular interference function, j(x), is known. As discussed... 

The plots of alcohol and water mixtures presented here serve to illustrate the usefulness of XRD for liquid identification from the molecular interference function s(x). The curves shown here differ from the s (x) discussed in Section 2.3.1. in that they portray the square of the ratio of s (x) of the sample to s(x) of a white scatterer , a calibration object of proprietary composition whose scattering characteristics are fairly constant over the x range of interest. Notwithstanding these manipulations, the plots have absolute ordinate scale. 

In Section 2.3.1., the diffraction profile was fitted to IAM atomic scatter cross-sections in the region where the molecular interference function is practically unity. This procedure yields the effective atomic number and, particularly for liquids, several further parameters derived from peaks in the molecular interference function. With... 

Inspection of Eq. 7 reveals that the molecular interference function, s(x), can be derived from the ratio of the total cross-section to the fitted IAM function, when the first square bracketed factor has been accounted for. A widely used model of the liquid state assumes that the molecules in liquids and amorphous materials may be described by a hard-sphere (HS) radial distribution function (RDF). This correctly predicts the exclusion property of the intermolecular force at intermolecular separations below some critical dimension, identified with the sphere diameter in the HS model. The packing fraction, 17, is proportional for a monatomic species to the bulk density, p. The variation of r(x) on 17 is reproduced in Fig. 14, taken from the work of Pavlyukhin [29],... 

Density Organic explosives tend to have higher density than equivalent harmless plastic materials Diffraction profile yields density descriptor based on analysis of molecular interference function... 

Micromass has developed a potentially powerful new technique that eliminates many of these molecular interferences and also removes ions with an energy that differs from that of the analyte, such as components of the Ar support gas. This has a dramatic effect on the performance of the instrument. The technique deploys a hexapole ion lens (Szabo, 1986) located behind the skimmer cone and surrounded by a gas cell (Fig. 8.5). The hexapole uses a hexagonal array of rods between which a 400-V rf field is applied, confining the ions of interest to stable trajectories be-... 

Molecular Interferences High abundance of ions is likely to lead to the formation of polyatomic or molecular ions with the same mass as the elements of interest [51, 52]. Polyatomic interferences arise mainly from various combinations of Ar, C, Ca, Na, Cl, N, O, and S matrix, elements, which are present in sample matrices, plasma gas, and reagents used in the digestion of samples [53]. Arsenic has serious problems from polyatomic interferences arising from Ar plasma gas and matrix constituents (C and Cl). The use of correction equations, the addition of N2 or the use of a DRC or CC can minimize and sometimes eliminate these interferences [36-40, 54]. The use of correction equations should be avoided, especially if the As mass counts are small compared with the interference present. Correction equations suffer from inherent problems in that the masses used to correct for interferences may themselves suffer from interferences. 

Accelerator mass spectrometry (AMS) extends the capabilities of atom-counting using conventional mass spectrometry, by removing whole-mass molecular interferences without the need for a mass resolution very much better than the mass difference between the atom and its molecular isobar. This technique has been used with great success for the routine measurement of C, Be, " Al, C1 and, recently, (see Table 5.15). Analysis of " C by AMS can, for example, generate dates with a precision that is at least equal to the best conventional beta-particle-counting facility. In many cases, where small sample analysis is required, the AMS method has proved superior (Benkens, 1990). A complete description of AMS can be found in review articles (Litherland et al., 1987 Elmore and Philips, 1978) or recent conference publications. The application of AMS to measurement has been discussed in detail in Kilins et al. (1992). 

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