Accurate Mass measurements are routine experiments performed by modern mass spectrometers. The accuracy of a measurement refers to the degree of conformity of a measured quantity to its actual true value. Well, the more accurate a mass spectrometer is, the more expensive it is. Usually we refer to this quality parameter via the statistical error in ppm or ppb. The error of an mass measurement should be reported with a standard deviation, for real world experiments the 3-sigma rule (3σ) should be sufficient.

Why 3-sigma? Because under real world conditions, spray conditions can fluctuate, ion statistics can be bad, low analyte concentration and not enough abundance, or the signal-to-noise ratio is low. So even a high mass accuracy FT-ICR MS which can achieve 1 ppm or even 0.5 ppm mass accuracy over a wide mass range will follow the eternal rule of statistics, hence report values which are outside the value of the advertised prospect flyer. Other specifications like dynamic range, scan speed and sensitivity are also very important. For very accurate FT-MS measurements it is also important to include the mass of the electron (0.00054858026 u) into calculation of neutral masses.

High mass accuracy is not sufficient if mixtures are analyzed or if the isotopic fine structure of compounds needs to be evaluated. In such a case high resolving power is needed. High resolving power can be achieved by chromatography or comprehensive chromatography (GCxGC, LCxLC) or multidimensional chromatography (LCxGC). For chromatography we usually refer to a high efficiency or high number of theoretical plates. High resolving power can be obtained by mass spectrometry itself (Orbitrap or better FT-MS).

We have already proven (BMC Bioinformatics 2006, 7:234) that high mass accuracy (less than 1 ppm or even 0.1 ppm) is not enough for obtaining correct molecular formula results. Instead, additional rules and orthogonal filters need to be applied.

Additional Literature:
Intercomparison study on accurate mass measurement of small molecules in mass spectrometry
Debating Resolution and Mass Accuracy in Mass Spectrometry
High-Resolution Mass Spectrometers (Annual Reviews)

Additional Material:
The impact of the electron mass on accurate mass determinations and mass calibrations [XLS]

Accurate mass reference tables:
ATOMIC WEIGHTS OF THE ELEMENTS: REVIEW 2000 - (IUPAC Technical Report)
Pure Appl. Chem., Vol. 75, No. 6, pp. 683–800, 2003.
J. R. DE LAETER, J. K. BÖHLKE2, P. DE BIÈVRE, H. HIDAKA, H. S. PEISER, K. J. R. ROSMAN, AND P. D. P. TAYLOR

ASCII Tables for direct import/export
Atomic Weights and Isotopic Compositions
J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset
Office of Electronic Commerce in Scientific and Engineering Data
http://www.nist.gov/physlab/data/comp.cfm