Minimizing mass discrimination effects

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... 

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. 

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. 

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 ). 

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 ... 

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. - ... 

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