MS: Resolution and Accuracy
 

The beauty of FT-MS is the elegant combination of both high resolution and mass accuracy. In its simplest form, resolution in mass spectrometry is the ability to separate components with the same nominal or unit mass using a mass spectrometer.  This is probably best described with an example.  CO, N2, and C2H2 all have the same nominal mass of 28 but can be separated by mass spectrometry at a medium resolution.  The actual masses (in amu) are CO = 27.9949, N2 = 28.0161, and C2H2 = 28.0313.  The difference in mass between N2 and C2H2 is 0.0152.  Divide the common mass number of 28 by 0.0152 to get 1842.  This implies that a resolution of about 1900 is required to separate these compounds.  Conventionally, low and medium resolution are less than 3,000, while high resolution is greater than or equal to 10,000. With a high field FT-MS, resolution is typically greater than 10,000. A low resolution spectra will produce the nominal mass of a compound with a plus or minus 0.5 amu degree of accuracy, reporting data only to the first decimal place.  For a more accurate mass measurement high resolution is required.  Restated another way, high resolution is required (a value of 10,000) to separate the peaks of isotopic of isotopic distribution .  A highly accurate mass value, used in determining an elemental formula, can be obtained by high resolution mass spectrometry in combination with an internal calibration standard. The formula used to determine resolution is: Resolution = M / DM where M is the integer (or common mass) and DM is the difference between the two actual masses. The FT-MS with the 4.7 Tesla magnet will be sufficient for Exact Mass up to ~2000 by EI / CI, ~5,000 by MALDI, and ~ 10,000 by ESI.  For elemental composition analysis, Exact Mass should be used, otherwise Unit Mass would be a better choice.