Absolute error, mass spectrometry

After introducing the reference procedure values for cholesterol, based on IDMS measurements, the different peer group target values have now been replaced by one reference procedure value, which in our case is represented as the exact middle of the screen. The corresponding limits of acceptance are shown as the solid square. As a consequence, methods with inherent systematic errors like the Liebermann-Burchard method and the cholesterol oxidase/iodide method disappeared off the market and today only procedures which are within the limits of acceptance with the reference procedure values established by isotope dilution mass spectrometry exist. In fact, until 1988 there was an unacceptably wide scatter of procedure-dependent target values for many clinical chemical parameters. In order to improve accuracy in clinical chemistry it was absolutely essential to replace these method-dependent target values with reference procedure values. 

Yanson et al. [41] using field-ionization mass spectrometry studied the formation of gas-phase GC, CC, AT and TT pairs. From measurements of temperature dependence of equilibrium constants, an interaction enthalpy for the base pair formation was derived. This technique was sometimes questioned because the determination of enthalpy from the slopes of appropriate van t Hoff curves might not be unambiguous. From Table 6 it is evident that the agreement with the present theoretical values is good, and concerns not only the relative interaction enthalpies but even the absolute values the average absolute error is less than 1.5 kcal/mol. 

The isolation of a chemical fraction is only the first part of a radiochemical analysis. The relative concentrations of the analytes must be measured, a chemical yield applied, and the absolute concentrations of the analytes in the original sample must then be derived. Chrono-metric determinations can be based directly on relative concentrations, often with smaller propagated errors. Concentration data arise from mass spectrometry or by radiation counting. Samples submitted for mass spectrometry typically require no special handling beyond the requirements of cleanliness these samples are submitted in small volumes of dilute nitric acid or as a dry deposit, preferably in a Teflon container. For radiation counting, the geometric extent of the radionuclide sample, and whether liquid or solid, affects the efficiency of the detector and the accuracy with which the concentrations of the radionuclides can be determined. 

As mentioned previously, FT/MS is capable of providing more detailed structural information compared to most any other analytical technique. For example, it is possible to measure the absolute mass of an oligomer or a polymer fragment ion, and from that mass measurement the most probable elements composition can be computed by the mass spectrometer data station. A more accurate mass measurement (i.e. a lower error on the measurement) limits the chemical formulas that can be computed. Generally this elemental composition information is all that is needed to confirm a suspected molecular structure or substructure. However, if this is not sufficient, mass spectrometry/mass spectrometry (MS/MS) techniques (24) can be applied. 

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