Mass Spectrometry (Lecture 25)

Question 1

What is mass spectrometry is a technique for?

Answer 1

studying the masses of atoms or molecules or fragments of molecules

Question 2

What does a mass spectrum display?

Answer 2

the number of ions detected at each value of mass-to-charge ratio (m/z)

Question 3

What types of analysis does mass spec do?

Answer 3

Qualitative and quantitative analysis.

Question 4

What is the sensitivity level of mass spec?

Answer 4

High sensitivity.

Question 5

What is mass spec coupled with?

Answer 5

chromatography

Question 6

What is mass spec used for?

Answer 6

Determining the amino acid sequence in protein, structure of carbohydrates, types of lipids in an organism, mass of cells and viruses etc.

Question 7

What do all mass spectrometers have?

Answer 7

A source of ions.
A mass separator.
A detector.

Question 8

What is atomic mass?

Answer 8

weighted average of the masses of the isotopes of an element

Question 9

What is molecular mass?

Answer 9

sum of the atomic masses listed in the periodic table for a molecule or ion

Question 10

What is nominal mass?

Answer 10

integer mass of the species with the most abundant isotope of each of the constituent atoms

Question 11

What is mono isotopic mass?

Answer 11

exact mass of the species with the most abundant isotope of each of the constituent atoms

Question 12

What does the molecular ion tell us?

Answer 12

the molecular mass

Question 13

Why do some compounds not exhibit a molecular ion in electron ionisation?

Answer 13

the molecular ion breaks apart too efficiently

Question 14

What clues do available fragments provide?

Answer 14

clues about the structure of the analyte

Question 15

How do you find the molecular mass?

Answer 15

gather a chemical ionisation mass spectrum because it usually has a strong peak for MH⁺

Question 16

What does M+1 refer to?

Answer 16

an ion with a mass one unit greater than that of the molecular ion (we don’t write the positive charge)

Question 17

What does base peak refer to?

Answer 17

The most intense peak in a mass spectrum. The intensities of other peaks are expressed as a percentage of the base peak intensity.

Question 18

What can the M+1 peak be used for?

Answer 18

estimation of the number of carbons in an organic compound

intensity of M+1 (relative to molecular ion for CnHm) = (n x 1.08%) + (m x 0.012%)

Question 19

What does the nitrogen rule help us to propose?

Answer 19

compositions for molecular ions

Question 20

What is the nitrogen rule?

Answer 20

If a compound has an odd number of nitrogen atoms (in addition to any number of C, H, halogens, O, S, Si and P) then the molecular ion has an odd nominal mass.
If a compound with an even number of nitrogen atoms, the molecular ion has an even nominal mass.
E.g.: A molecular ion at m/z 128 can have 0 or 2 N atoms, but it cannot have 1 N atom.