Chapter 3: Mass Spec and IR

Question 1

What is mass spectrometry used for?

Answer 1

Determining a molecule’s molecular weight, molecular formula, and structure.

Question 2

What type of ions are detected in mass spectrometry?

Answer 2

Only positively charged ions

Question 3

What is the mass-to-charge ratio (m/z)?

Answer 3

The mass of an ion divided by its charge (usually charge = +1, so m/z = mass).

Question 4

What is the molecular ion (M⁺)?

Answer 4

The unfragmented molecular ion (same molecule but missing one electron).

Question 5

What is the base peak?

Answer 5

The tallest peak in the mass spectrum (set to 100% intensity)

Question 6

How can you determine a molecule’s molecular weight from a mass spectrum?

Answer 6

Look for the M⁺ peak (molecular ion peak).

Question 7

How do fragment peaks help determine molecular structure?

Answer 7

They indicate possible substructures that were cleaved from the molecule.

Question 8

What is the Rule of 13 used for?

Answer 8

To estimate the molecular formula by:

1. Dividing the molar mass by 13 → gives the number of carbons
2. Adding the remainder to the quotient → gives the number of hydrogens

Question 9

What is the nitrogen rule?

Answer 9

• If M⁺ is even, the compound has 0 or an even number of N atoms
• If M⁺ is odd, the compound has an odd number of N atoms

Question 10

How can you tell if a compound contains chlorine in MS?

Answer 10

M⁺ and M⁺2 peaks appear in a 3:1 ratio, due to ³⁵Cl (75%) and ³⁷Cl (25%) isotopes.

Question 11

How can you tell if a compound contains bromine in MS?

Answer 11

M⁺ and M⁺2 peaks appear in a 1:1 ratio, due to ⁷⁹Br (50%) and ⁸¹Br (50%) isotopes.