Mass Spectrometry (Lecture 25) Flashcards
What is mass spectrometry is a technique for?
studying the masses of atoms or molecules or fragments of molecules
What does a mass spectrum display?
the number of ions detected at each value of mass-to-charge ratio (m/z)
What types of analysis does mass spec do?
Qualitative and quantitative analysis.
What is the sensitivity level of mass spec?
High sensitivity.
What is mass spec coupled with?
chromatography
What is mass spec used for?
Determining the amino acid sequence in protein, structure of carbohydrates, types of lipids in an organism, mass of cells and viruses etc.
What do all mass spectrometers have?
A source of ions.
A mass separator.
A detector.
What is atomic mass?
weighted average of the masses of the isotopes of an element
What is molecular mass?
sum of the atomic masses listed in the periodic table for a molecule or ion
What is nominal mass?
integer mass of the species with the most abundant isotope of each of the constituent atoms
What is mono isotopic mass?
exact mass of the species with the most abundant isotope of each of the constituent atoms
What does the molecular ion tell us?
the molecular mass
Why do some compounds not exhibit a molecular ion in electron ionisation?
the molecular ion breaks apart too efficiently
What clues do available fragments provide?
clues about the structure of the analyte
How do you find the molecular mass?
gather a chemical ionisation mass spectrum because it usually has a strong peak for MH⁺
What does M+1 refer to?
an ion with a mass one unit greater than that of the molecular ion (we don’t write the positive charge)
What does base peak refer to?
The most intense peak in a mass spectrum. The intensities of other peaks are expressed as a percentage of the base peak intensity.
What can the M+1 peak be used for?
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%)
What does the nitrogen rule help us to propose?
compositions for molecular ions
What is the nitrogen rule?
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.
