2 mass spectrometry - indiana university...
TRANSCRIPT
Molecular Ion Peak
• Determination of molecular mass• Rule of Thirteen (Possible molecular formulae)• Odd vs. Even molecular mass• Isotopes
– M + 1 peaks– M + 2 peaks
• Nominal Mass vs. Exact Mass
Empirical Formula and MS
• What is the MF of a compound with the following data: M+= 110; %H = 12.72; %C = 87.27?
• Answer: C8H14
Rule of 13
• What if you are only given M+ data?• Determine a range of possibilities using rule of 13
• Divide M+ by 13– Quotient = #C– Quotient + remainder = #H– Replace CH4 with O, etc.
• Example: Give three MF for M+ = 110
M+ + 1 Peak
• Arises from sum of all possible isotopes• C‐13 is most prevelant• Therefore, we can use M+1 to determine the likely number of carbon atoms in a molecule
Apply to CH4• In a group of CH4:
– Most are 12C and 1H M+ = 16 amu– About 1% are 13C and 1H M+1 = 17 amu– Small % are 12C and 2H M+1 = 17 amu– VERY SMALL % are 13C and 2H M+2 = 18 amu
• Application of M+ + 1 peak: determine #C= (M++1 intensity/M+ intensity)/(1.1/98.9)= (M+1 intesity/M intensity)/0.011
HRMS
• Nominal Mass• “Exact” Mass• Advantage of HRMS
– Unique mass– Compare C8H14 with C7H10O
– Nominal: 110 amu– Exact: 110.10962 vs110.07320
Fragmentation
• Unstable molecular ion: radical cation
• Only cationsobserved– f+ (directly)– f . (indirectly)
Mechanisms
• Draw Lewis structure with electron knocked out of least stable place– Lone pair– Pi bond– Sigma bond
• Fishhook arrow
Example: Ethane
Inductive Cleavage
Example: Propane
Typical Aliphatic Fragments: 15, 29, 43, 57, 71…
Functional Groups
• Benzylic group• Alkenes• Alcohols, amines, ethers, etc• Carbonyls: alpha fragemntation• Carbonyls: McLafferty fragmentation
Identifying Fragments• What are the most likely identities of the ions that causes peaks at 41, 43, 69, and 84?
• What causes the little peak above 84?
• What is the molecular ion peak? Why is it not visible?• Why is the peak at 59 bigger than the peak at 73?• Draw a mechanism for the loss of neutral hydroxide radical. What is the ion peak visible from this fragmentation?
Case Study• M+ = 114 and C=O known to be present– Nitrogen?– Cl/Br?– 72 suggests…
• Strategy– Rule of 13– Table of mass peaks and fragments