Minimizing mass discrimination

All mass spectrometers consist of four common elements (1) the ion source, (2) the mass analyzer, (3) the detector, and (4) the vacuum system [56]. Quadrupole, time-of-flight, and magnetic sector instruments have all been used successfully to obtain thermodynamic data from metallic and alloy systems. It is important that the instrument introduce no mass discrimination effects or that corrections be applied for these effects. Thus, in general, magnetic sector instruments are preferred because they can be designed to minimize mass discrimination. 

One strategy that has been employed in our laboratories in an effort to minimize mass discrimination effects has been to couple GPC, either off-line or directly on-line to MS. In GPC, the separation mechanism involves an equilibrium between solutes in the mobile phase and those which can permeate the inner volume of a stationary phase that is porous. Molecules with hydrodynamic volumes (or size in solution) that are smaller than the pore sizes in the stationary... 

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. 

Mass discrimination Ions injected along the z-axis into a linear ion trap as shown in Figures 2 and 4 encounter minimal rf field in the z-direction. Thus, mass discrimination during ion injection and ion trajectory stabilization processes is much reduced in a linear ion trap a wide mass range from 150 to 2000 Th can be trapped simultaneously with high efficiency. 

Laser-ablation, inductively coupled-plasma mass spectrometry (LA-ICPMS) is an instrumental technique in which a laser-ablahon cell and ophcal microscope supplant the spray chamber/nebulizer apparatus of a standard ICP-MS instrument. Subsamples of questioned material are ablated from a solid sample via laser (often a pulsed Nd-YAG tuned to 266 or 213 nm). Ablated specimens are transported in a stream of Ar to a plasma torch for ionization and mass discrimination as per solution ICP-MS. Only minimal sample prep is required, and few restrictions are placed on the nature of questioned solid samples (Brundle et al. 1992 Vickerman 1998). While laser spot sizes can be reduced to several micrometers, sensitivity is degraded as a result, and usual spatial resolutions are on the order of 10-100 pm. Matrix-matched standards are also necessary for accurate trace-element and isotopic quantitative analyses in LA-ICPMS. Depending on the quality of such primary standards, LA-ICPMS accuracies are typically 1-10%, with limits-of-detection in the parts-per-billion (ppb) range (O Table 62.1). 

Although thermal emission sources provide equal ionization efficiency of isotopes of a given element, fractional vaporization may occur. This is one of the several possible sources of mass fractionation in the technique, which must be corrected in order to obtain high quality results. Mass discrimination (relatively constant) and thermal fractionation (variable, at the filament) effects may be minimized by one or more of the following ... 

Reactions involving a transfer of a proton or a hydrogen atom are an extremely common type of ion-molecule reaction and are particularly suited for study by the pulsed source technique. The secondary ion will usually occur at an m/e ratio where it is not obscured by abundant primary ions, and the product and reactant ions frequently will differ only slightly in mass, thus minimizing discrimination effects. 

Zoller and Johnston applied the above method to a copolymer containing units of acrylonitrile and butadiene. They used mass spectral data to discriminate between first-order Markoff and Bernoulli distributions. First they assumed Bernoulli, performed the best-fit, and formd an agreement of AF = 0.11. Thereafter they assumed first-order Markoff and the minimization yielded AF = 0.09. As a consequence they concluded that first-order Markoff gives better results. 

The chemical characterization of forensic evidence from a crime scene or the criminal has some different requirements from that of many other types of chemical analysis. High sensitivity is important because the quantity of material for examination is often limited to minute traces found at the scene. The material under scrutiny must be characterized as comprehensively as possible to ensure maximum discrimination from other material in the same class. Forensic laboratories are multiinstrument facilities required to deal with many types of evidence found at a crime scene therefore, the routine methods used should preferably employ relatively inexpensive instrumentation. In order to protect integrity, samples should be analyzed as received if possible and any workup minimized. The method should preferably not be labor intensive. Pyrolysis-gas chromatography (Py-GC) and pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) have proven to be an effective means of satisfying these requirements in many forensic science laboratories. - ... 

Based on the discussion in Section 10.5 of the various effects that can limit the mass resolution, there seans to be room for improvement in the measurement accuracy by a factor of 10-100 and, hence, mass measurement with an accuracy of m/Am = lO -lO should be within reach. In this respect, the main error sources to be minimized are charging effects during loading of ions (Section 10.5.4) and imperfections in the RF-field configuration (see Section 10.5.3.2). By a modest optimization of our current experimental arrangement, we expect to be able to achieve a relative mass measurement accuracy of < 10, and thus be able to discriminate between various mass doublets (for example, MgH and Mg ). 

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