Isotopes: HRMS and recognition of isotope patterns Mass Spectrometry II Isotopes: HRMS and recognition of isotope patterns

High Resolution - HR MS Determination of exact mass using HR MS has mostly supplanted combustion analysis as a way of proving molecular formula for new compounds. Selenium 79.916250

Example of HRMS for formula proof HRMS FAB = 542.1203 Reviewer Assignment : Does the provided FAB MS data provide acceptable proof of molecular formula? Justify your reasoning quantitatively (show calculation).

Example 2 Assignment : Decide whether the ESI-TOF mass spectrum at left provides acceptable proof of molecular formula. Justify your reasoning quantitatively (show calculation).

Appendix 1A

Isotope Patterns Notice that in the MS of methane there is a small peak at M+1.

Effect of # of C’s on M+1 peak C6H14 C10H18

Organic Bromides

Dibromides

Tribromides

Organic Chlorides

Dichlorides

Isotope Patterns in General N, O, F, I, Si, and P are all essentially monoisotopic (so no isotope patterns) C is somewhat monoisotopic but with large numbers M+1 peaks become significant. S is 95% 32S, 5% 34S Se gives cool patterns For the presence of >1 of a diisotopic element use polynomial expansion to predict pattern: (a + b)n Software for isotope pattern and HRMS calculations exist.