An ordinary (low resolution) mass spectrometer gives m/z values to the nearest whole number. A high resolution mass spectrometer measures m/z values very precisely, to four or more decimal places.This matters because relative isotopic masses are not exact whole numbers. By definition, carbon-12 has a relative isotopic mass of exactly 12 (12.0000). Every other isotope differs slightly from a whole number, because of the way mass changes when nucleons bind together:1H = 1.0078 12C = 12.0000 14N = 14.0031 16O = 15.9949 So molecules that have the same whole-number mass usually have slightly different precise masses. A low resolution instrument cannot tell them apart, but a high resolution instrument can.Worked example: carbon monoxide, nitrogen and ethene all have a nominal Mr of 28.
CO: 12.0000 + 15.9949 = 27.9949
N2: 2 × 14.0031 = 28.0062
C2H4: (2 × 12.0000) + (4 × 1.0078) = 28.0312These values differ in the second decimal place, which a high resolution instrument easily distinguishes. A molecular ion peak at m/z = 28.0312 must therefore be ethene, not carbon monoxide or nitrogen.High resolution mass spectrometry is widely used to confirm the molecular formula of a newly made compound, and it is often combined with chromatography to identify each component of a mixture.
CO: 12.0000 + 15.9949 = 27.9949
N2: 2 × 14.0031 = 28.0062
C2H4: (2 × 12.0000) + (4 × 1.0078) = 28.0312These values differ in the second decimal place, which a high resolution instrument easily distinguishes. A molecular ion peak at m/z = 28.0312 must therefore be ethene, not carbon monoxide or nitrogen.High resolution mass spectrometry is widely used to confirm the molecular formula of a newly made compound, and it is often combined with chromatography to identify each component of a mixture.