intro to mass spec

Question 1

Mass spectrum

Answer 1

a histogram of signals representing the distribution of separate masses in a sample
abundance/intensity on the y-axis and mass-to-charge ratio (m/z) on the x-axis

Question 2

MS basic principles

Answer 2

technique performing “mass selective separation” i.e. the ability to separate something by their molecular weight. First must ionise

Question 3

Ionisation

Answer 3

process of producing an ion from your analyse (M). Either a molecular ion M+. or an adduct ion [M+X]+ (where X=cation)
M–> M+’ + e-
M + X+ –> [M+X]+

Question 4

five stages of MS

Answer 4

sample inlet
ion source  - atmospheric/vacuum pressure
mass analyser - high vacuum
ion detector - high vacuum
data collection

Question 5

analyte

Answer 5

the thing that is being analysed

Question 6

cation

Answer 6

positively charge ion

Question 7

anion

Answer 7

negatively charged ion

Question 8

gas-phase chemistry

Answer 8

MS analyses gas-phase ions in a high vacuum
means that many species are stable for long enough to contribute to mass spectral studies
e.g CH5+ is stable but highly reactive

Question 9

unit of mass.

Answer 9

Dalton

1 D = 1.66E-27 kg

Question 10

isotopic mass

Answer 10

mass calculated from the sum of the most abundant isotopes making ip the molecule

Question 11

average mass

Answer 11

mass calculated from th average atomic masses of the elements making up th molecule
often used when working out amounts of reactants to use in a reaction or when working out yields

Question 12

nominal mass

Answer 12

the mass calculated from the sum of the most abundant isotopes making up a molecule rounded to the nearest whole number
convenient for disussions

Question 13

isobars

Answer 13

are molecules with the same nominal mass but different formulae and therefore isotopic mass

Question 14

accurate mass

Answer 14

experimentally determined mass performed in a way to determine or confirm the molecular formula
error (ppm) = ((difference in accurate mass and theoretical isotopic mass) / (theoretical mass) ) * 1E6

Question 15

accuracy

Answer 15

for an experimental measurement is the ‘degree of closeness’ to the actual/reference value

Question 16

precision

Answer 16

degree to which repeated measurements remain unchanged under the same conditions

Question 17

high accuracy, low precision

Answer 17

improve precision by averaging data

Question 18

low accuracy, high precision

Answer 18

systematic error - recalibrate

Question 19

isotopes

Answer 19

atoms of the same element with differing numbers of neutrons

Question 20

calculation of no. carols

Answer 20

maximum no. carbons = Nc
Nc = (Y/X)*(100/1.1) or Y/1.1 (assumes scaled to 100% axis)
Y = intensity of [M+1]+
X = intensity of M+

Question 21

Nitrogen rule

Answer 21

organic compounds containing exclusively H, C, N, O, S, Is, P and the halogens can have either:
- an odd nominal mass that indicates an odd number of N atoms or
- an even nominal mass that indicates an even number of N atoms (or none)
General principle when attempting to identify unknown

Question 22

Resolution

Answer 22

ability of a mass spectrometer to separate ions that are close in mass .
two overlapping peaks considered fully resolved if the point of overlap is less than 10% of the intensity
R10% = (left peak m/z )/ (difference in m/z between top of peaks)

Question 23

High resolution

Answer 23

allows the separation of isobaric ions - ions with the same nominal mass, bu different isotope composition and therefore accurate mass
Required to determine the accurate mass of an ion - i.e. to prevent other isotopic compositions from skewing the measured mass

Question 24

Sensitivity

Answer 24

a measure of signal strength or ion intensity when analysing a fixed amount of sample