Mass bias

This effect has a much stronger mass dependence than the gas dynamic effect and dominates the overall bias. Anything that affects this space charge field, affects the observed bias. Mass bias in ICP/MS thus depends on sample preparation (as more sample matrix is put into the ICP, the composition and intensity of the... 

Equations (8.1) and (8.2) are not corrected for instrumental mass bias (mass discrimination) and blank values. In the literature, one can therefore find much... 

A clear cut answer was provided by Irving Roberts and Harold C Urey of Colum bia University in 1938 They prepared methanol that had been enriched in the mass 18 isotope of oxygen When this sample of methanol was esterified with benzoic acid the... 

The result of the Back-to-Basics series is an accumulation of some 50 separate but interrelated expositions of mass spectrometric principles and apparatus. Some areas of mass spectrometry, such as ion cyclotron resonance and ion trap instruments, have not been covered except for passing references. This decision has not been due to any bias by the authors or Micromass but simply reflects the large amount of writing that had to be done and the needs of the greatest proportion of users. 

The ANN model had four neurones in the input layer one for each operating variable and one for the bias. The output was selected to be cumulative mass distribution thirteen neurones were used to represent it. A sigmoid functional... 

Consider now a situation in which the bias limits in the temperature measurements are uncorrelated and are estimated as 0.5 °C, and the bias limit on the specific heat value is 0.5%. The estimated bias error of the mass flow meter system is specified as 0.25% of reading from 10 to 90% of full scale. According to the manufacturer, this is a fixed error estimate (it cannot be reduced by taking the average of multiple readings and is, thus, a true bias error), and B is taken as 0.0025 times the value of m. For AT = 20 °C, Eq. (2.9) gives ... 

In contrast to thermal ionization methods, where the tracer added must be of the same element as the analyte, tracers of different elemental composition but similar ionization efficiency can be utilized for inductively coupled plasma mass spectrometry (ICPMS) analysis. Hence, for ICPMS work, uranium can be added to thorium or radium samples as a way of correcting for instrumental mass bias (e g., Luo et al. 1997 Stirling et al. 2001 Pietruszka et al. 2002). The only drawback of this approach is that small inter-element (e g., U vs. Th) biases may be present during ionization or detection that need to be considered and evaluated (e.g., Pietruszka et al. 2002). 

In another approach to improving the significance of mass matches, Demirev et al. have constructed a truncated database comprising proteins predicted to be abundant by estimating codon bias.72,73 The size of the entries for each organism can be selected, for example, as the 10 proteins predicted to be most abundant. 

Equation (1) suggests that tunnel junctions should be ohmic. This is true only for very small bias. A much better description of the tunneling current results when the effects of barrier shape, changes in barrier with applied potential, and effective mass of the electron are all included. An example of such an improved relationship is given by (2), where / is the current density, a is a unitless parameter used to account empirically for non-rectangular barrier shape and deviations in the effective electron mass, and barrier height given by B = (L + work function of the left-hand metal ... 

Anthropometric measurements are gross measurements of body cell mass used to evaluate LBM and fat stores. The most common measurements are weight, height, limb size (e.g., skinfold thickness and midarm muscle, wrist, and waist circumferences), and bioelectrical impedance analysis (BIA). 

O. Vorm and P. Roepstorff. Detector Bias Gating for Improved Detector Response and Calibration in Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry. J. Mass Spectrom., 31(1996) 351-356. 

Three methods were applied to detect bias measurements in this process network where only total mass balances are considered. These are the Generalized Ratio... 

Since the particles are randomly located in grid cells, interpolation and particle-field-estimation algorithms are required. Special care is needed to ensure local mass conservation (i.e., continuity) and to eliminate bias. 

In principle, the three isotope method may be widely applied to new isotope systems such as Mg, Ca, Cr, Fe, Zn, Se, and Mo. Unlike isotopic analysis of purified oxygen, however, isotopic analysis of metals that have been separated from complex matrices commonly involves measurement of several isotopic ratios to monitor potential isobars, evaluate the internal consistency of the data through comparison with mass-dependent fractionation relations (e.g., Eqn. 8 above), or use in double-spike corrections for instrumental mass bias (Chapter 4 Albarede and Beard 2004). For experimental data that reflect partial isotopic exchange, their isotopic compositions will not lie along a mass-dependent fractionation line, but will instead lie along a line at high angle to a mass-dependent relation (Fig. 10), which will limit the use of multiple isotopic ratios for isobar corrections, data quality checks, and double-spike corrections. 

Figure 1. Transmission of Zn isotopes as a function of the collector position along the focal plane. This position is parameterized hy mass M. Transmission is the ratio between the number of ions of mass At, arriving at the collector at mass M divided by the number of atoms of mass At, introduced in the mass spectrometer. The parameter u is the linear mass bias coefficient.

Mass bias, or the instrumental mass fractionation, is the variable transmission of the ion beam into the mass spectrometer. A variety of phenomena create conditions that lead to variable transmission of ion beams. For modem instmments, the transmission in the flight tube and the efficiency of ion conversion to electrons at the collector are almost quantitative. Most fractionation processes, therefore, take place within the source, namely in the area where the analyte is introduced into the mass spectrometer and ionized, or at the interface between the source and the mass analyzer. 

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