Structure & Reactivity


Introductory Mass Spectrometry

MS9. High vs Low Resolution

Two common categories of mass spectrometry are high resolution mass spectrometry (HRMS) and low resolution mass spectrometry (LRMS). Not all mass spectrometers simply measure molecular weights as whole numbers. High resolution mass spectrometers can measure mass so accurately that they can detect the minute differences in mass between two compounds that, on a regular low-resolution instrument, would appear to be identical.

The reason is because atomic masses are not exact multiples of the mass of a proton, as we might usually think.

As a result, on a high resolution mass spectrometer, 2-octanone, C8H16O, has a molecular weight of 128.12018 instead of 128. Naphthalene, C10H8, has a molecular weight of 128.06264. Thus a high resolution mass spectrometer can supply an exact molecular formula for a compound because of the unique combination of masses that result.

HRMS relies on the fact that the mass of an individual atom does not correspond to an integral number of atomic mass units.

Problem MS9.1.  Calculate the high-resolution molecular weights for the following formulae.

a)  C12H20O  and C11H16O2

b)   C6H13N  and C5H11N2

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These materials are protected by copyright 2008, Chris P. Schaller


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