Chapter 17 — Introduction to Mass Spectrometry

What does the molecular ion peak (M peak) represent?

Molecular mass of the compound after losing one electron.

What do the M+1 and M+2 peaks help identify?

M+1 → approximate number of carbons

M+2 → presence of Cl or Br

How do chlorine and bromine isotope patterns differ?

Cl → M:M+2 = 3:1

Br → M:M+2 = 1:1

What does the nitrogen rule say?

Odd molecular mass → odd number of nitrogens

Even molecular mass → even/zero nitrogens

What is fragmentation in mass spectrometry?

Molecules break into smaller ions/radicals after ionization.

What common fragment losses should you recognize?

−15 = CH₃ loss

−18 = H₂O loss (alcohol dehydration)

Which fragments are most stable in mass spectrometry?

More substituted, allylic, and benzylic carbocations/radicals.

How do you determine possible molecular formulas from a molecular ion (M) peak in mass spectrometry?

1. Use the M peak to get the molecular mass.

2. Apply the nitrogen rule:

• odd M peak → odd number of N atoms

• even M peak → even or zero N atoms

3. Look for M+2 peaks:

• 3:1 ratio → Cl present

• 1:1 ratio → Br present

4. Estimate carbon count from M+1 peak (~1.1% per carbon).

5. Build formulas using common atomic masses:

• C = 12

• H = 1

• N = 14

• O = 16

• Cl = 35

• Br = 79

6. Check if the formula gives a reasonable degree of unsaturation and hydrogen count.