How the maldi-ms and esi can determine the accurate mass of the polypeptide?mechanism

how the accurate mass is determined??

Accurate mass—the experimentally determined mass of an ion measured to an appropriate degree of accuracy and precision used to determine, or limit the possibilities for, the elemental formula of the ion. A mass spectrum can be annotated with its nominal masses or accurate masses, to an appropriate number of significant figures. The IUPAC unit of mass is the unified mass unit (u) and is also referred to as the Dalton (Da) although this is not an SI unit. The term atomic mass unit (amu) is a redundant unit although it is still in wide use. In this article, discussions are limited to singly charged ions and the terms Da and mDa will be used (rather than the SI units u and mu (i.e., milli u), respectively) to refer to ion mass as these are widely recognized by mass spectrometrists. However, a mass spectrometer measures mass-to-charge, and m/z should normally be used when referring to the mass scale. There are a number of methods for accurate mass measurement, which involve different approaches and instrumentation, but all involve calibration of the mass scale using ions of known exact mass. It is necessary to emphasize the difference between the terms accurate mass and exact mass. Accurate mass is the experimental quantity that is measured and exact mass the calculated quantity. Exact mass—is the calculated mass of an ion whose elemental formula, isotopic composition and charge state are known, i.e., it is the theoretical mass. The IUPAC definition constricts the definition to using one isotope of each atom involved, usually the lightest isotope, but generalizes the definition to cover an ion or neutral molecule. The charge state is relevant as the mass of the electron (0.00055 Da), or multiple charges, may not be negligible in the context of mass measurement. There is a need to statistically treat accurate mass measurement data and apply terminology that describes these procedures in a consistent manner. The aim of this article is to clarify and define terms in common usage and to advise which are preferred. Whilst the terms “accurate mass” and “exact mass” are commonly used for the measured and calculated masses, has suggested using “measured accurate mass” and “calculated exact mass”, that leaves no doubt which is the experimental and the calculated mass. In this article the briefer terms accurate mass and exact mass will be used as they are in common usage. It is not our intention to make strong statements or recommendations on which terminology constitutes best practice, this should be the work of IUPAC. The difference between the measured value (accurate mass) and the true value (exact mass) is the “accuracy” of the “accurate mass measurement” (an unfortunate double use of the word) and it is suggested that the term “mass measurement accuracy” should be used to denote this difference. We have introduced the concept of the accuracy of measurements, which reflects the presence of systematic errors, and it is important to emphasize the difference between accuracy and precision. Accuracy—the proximity of the experimental measurement to the true value (exact mass). When a measurement is close to the true value we say it is accurate and when it is not we say it is inaccurate. Normally, mass measurement error would be used to describe the accuracy of a single reading. Precision—the repeatability of the measurement reflecting random errors. Random errors cause measurements to fall on either side of the average experimental measurement and affect the precision of the set of measurements. When a set of mass measurements of one ion species lie close together we say the measurements are precise, and when not we say the measurements are imprecise.