Mass Spectrometry II

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Mass Spectrometry II

• Isotopes HRMS and recognition of isotope patterns

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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
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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).
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Example 2

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

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Appendix 1A
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Isotope Patterns

• Notice that in the MS of methane there is a small
  peak at M1.

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Effect of of Cs on M1 peak

• C6H14
  C10H18

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Organic Bromides
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Dibromides
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Tribromides
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Organic Chlorides
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Dichlorides
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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
  M1 peaks become significant.
• S is 95 32S, 5 34S
• Se gives cool patterns
• For the presence of gt1 of a diisotopic element
  use polynomial expansion to predict pattern (a
  b)n
• Software for isotope pattern and HRMS
  calculations exist.