Accurate mass definition

Notwithstanding the efforts of groups attempting to precisely define acceptable practices and requirements, [20] confusion still exists on the differences between definition of the terms high resolution and accurate mass. [21] High resolution, as in any context, implies the ability to make confident measurements of small differences. In mass spectrometry, high resolution implies the ability to measure the small differences deriving from different elemental compositions, which have the... 

It has been stated that measured accurate masses when used to assign molecular formulae should always be accompanied by their mass accuracies. [34] Ideally, this can be done by giving the mean mass value and the corresponding error in terms of standard deviation as obtained from several repeated measurements of the same ion. [35] This is definitely not identical to the error which is usually provided with the listing from mass spectrometer data systems, where an error is given as the difference of calculated and measured mass value. 

Example Cesium iodide is frequently used for mass calibration in fast atom bombardment (FAB) mass spectrometry (Chap. 9) because it yields cluster ions of the general formula [Cs(CsI)n] in positive-ion and [I(CsI)J in negative-ion mode. For the [Cs(CsI)io] cluster ion, m/z 2730.9 is calculated instead of the correct value m/z 2731.00405 by using only one decimal place instead of the exact values Mi33Cs = 132.905447 and M1271 = 126.904468. T e error of 0.104 u is acceptable for LR work, but definitely not acceptable if accurate mass measurements have to be performed. 

Systematic Errors in Accurate Mass Measurements. 1. Problem Definitions The value of high resolution mass spectrometry is diminished if the mass measurements do not give unambiguous elemental compositions. Accurate mass measurements in FTMS require a precise measurement of ion frequencies and an accurate calibration law for converting ions frequencies to mass. The ion frequencies can be measured to nine significant figures with modern electronics however, the relationship between ion frequencies in the cubic cell and mass still requires further development. 

If the peak shape is approximately Gaussian, the resolution can be obtained by a single peak. In fact, as shown by Fig. 2.1, the mass difference, AM, is equal to the peak width at 5% of its height and, accordingly to the gaussian definition, it is about two times the fwhm. Consequently, with this approach it is possible to estimate the resolution of a mass analyzer simply by looking at a single peak, without introduction of two isobaric species of different accurate mass. 

A more general approach that can be employed for all mass analyzers [as time-of-flight (TOF), ion cyclotron resonance (Fourier trans-form-mass spectroscopy, FT-MS), Q-TOF] is based upon measuring the full width half-maximum, as shown in Fig. 2.2. In the same figure, the definition of mass accuracy is also reported. This parameter reflects on the specificity of mass measurements. In fact, on one hand it allows to determine the accurate mass of a selected ion (and, consequently, its elemental composition). On the other hand it allows to operate in accurate mass mode in order to identify species of interest present in complex matrices on the basis of their elemental composition. 

Fundamental to the basis for the calculation of the roaster operational data is a full mineralogical analysis and a complete definition of the chemical and physical properties of each concentrate. Data available from such analyses form the basis for an accurate mass and energy balance. The model then calculates all process ii ut data required for any prescribed output product requirements. Consistent calcine quality can be achiev only if the concentrates are properly assessed and predictions and consequent adjustments are made to the roaster input parameters. 

Stable isotope dilution mass spectrometry (IDMS) IDMS is a technique (method) based on the addition to an analytical sample of a known amount of a stable isotopically labelled analogue of a desired analyte as an internal standard. The ratio of the amount of added isotopic analogue and the naturally occurring compound, as measured by mass spectrometry, provides highly accurate values. Definitive accuracy of the IDMS technique requires full equilibration between endogenous natural amounts of the compound... 

One must also accept the challenge and responsibility of using accurate mass measurements correctly. Kondrat and Gross illustrate some of the prevalent misconceptions about the utility and appropriateness of accurate measurements, and illustrate the confusion that still exists on the differences between definition of the terms high resolution and accurate mass. Busch illustrates the use of and limitations of accurate mass measurements. Biemann s discussion drives the point home in much more detail. Biemaim lists three typical uses for accurate mass measurements elucidation of fragmentation pathways, confirmation of structure of the synthetic products, and elucidation of completely unknown structures. The demands for necessary... 

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