Accurate mass error, causes

The question remains whether the mass of the electron m (0.548 mmu) has really to been taken into account in accurate mass work as demanded by the lUPAC convention (Chap. 3.1.4). This issue was almost only of academic interest as long as mass spectrometry never yielded mass accuracies better than several mmu. Nowadays, extremely accurate FT-ICR instruments become more widespread in use, and thus, the answer depends on the intended application The electron mass has to be included in calculations if the result is expected to report the accurate mass of the ion with highest accuracy. Here, neglecting the electron mass would cause an systematic error of the size of m. This cannot be tolerated when mass measurement accuracies in the order of 1 mmu or better are to be achieved. 

There are several causes of errors in the determination of an accurate mass, illustrated in Figure 6.5, from the centroid of the peak ... 

The error in accurate mass measuronents is composed of two components the error attributable to the measurement itselfi such as the distortion caused by an isobaric interference, and the error arising from instrument design and manufacturing tolerances. Examples include how effectively ions can be focused and the quality of power supplies. 

When multiple measurements are made of a parameter, such as mass, the precision of the result should be determined. Precision is a measure of how closely clustered are the values of a set of repeat measurements, but it does not provide information as to whether the data are accurate. Eor example, if the calibration of the mass scale is incorrect, then replicate mass measurements may be very close to each other, i.e., they are very precise, but the results will not be accurate because of the error caused by the... 

Calibration of peak position for accurate mass determination can be performed internally or externally to minimize systematic errors. Internal calibration can be conducted when compounds with known molecular weight (called calibration compounds or calibrants) are mixed with the sample prior to the introduction into the ion source. This calibration can be performed, for example, by adding the calibrant to the liquid-phase sample while diluting it prior to analysis. The best result is achieved when multiple calibration signals are used to interpolate the m/z of ions within the range of interest. In proteomics, a tryptic digest of albumin from horse heart is typically used as the calibrant because it covers a wide m/z range (e.g., m/z 800-3000) that is ideal for mass calibration of low- to medium-sized peptides. In external calibration, the calibrants are analyzed before the analysis of real samples. The peaks of the calibrants are used to create and set the calibration equation in the data acquisition software. This method provides less mass accuracy because the instrument condition may still vary between the calibration and analyses of real samples. However, external calibrations save time and calibration compounds, and such methods also make analyses of analytes free from interferences caused by calibrants. 

Now we are ready to make one step forward and discuss some aspects of interpretation for real conditions when the gravitational field is measured with some error that is, the numbers that describe the field are accurate to only some decimal places. This is a fundamental difference from the previous case where we assumed that the field generated by masses of some body is known exactly. The presence of error is caused by the two following factors, and these are... 

The masses of isotopes can be measured with accuracies better than parts per billion (ppb), e.g., m40Ar = 39.9623831235 0.000000005 u. Unfortunately, determinations of abundance ratios are less accurate, causing errors of several parts per million (ppm) in relative atomic mass. The real limiting factor, however, comes from the variation of isotopic abundances from natural samples, e.g., in case of lead which is the final product of radioactive decay of uranium, the atomic weight varies by 500 ppm depending on the Pb/U ratios in the lead ore. [8]... 

The basic idea behind the VOF method is to discretize the equations for conservation of volume in either conservative flux or equivalent form resulting in near-perfect volume conservation except for small overshoot and undershoot. The main disadvantage of the VOF method, however, is that it suffers from the numerical errors typical of Eulerian schemes such as the level set method. The imposition of a volume preservation constraint does not eliminate these errors, but instead changes their symptoms replacing mass loss with inaccurate mass motion leading to small pieces of fluid non-physically being ejected as flotsam or jetsam, artificial surface tension forces that cause parasitic currents, and an inability to calculate accurately geometric information such as normal vector and curvature. Due to this deficiency, most VOF methods are not well suited for surface tension-driven flows unless some improvements are made [19]. 

In the best of instruments, isotope ratio errors may occur as a result of inefficient counting, from mass bias, and other causes. Correction standards at approximately the same concentrations as quality control (QC) samples and unknown samples are used to correct for these sources of error. A natural uranium spike is used because low-concentration endogenous uranium in the base urine and any small amounts of contamination from environmental sources are presumed to be natural, thus will not affect the true ratio in the event of environmental contamination. Correction with an unadulterated isotope ratio is assured. Considering the nature of the urine sample and the problems that must be overcome for uranium accurate isotope ratio analysis with acceptable sample throughput, two sample preparation... 

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