mass spectrometry

how does a mass spectrometer work?

what is measured? what form does the sample need to be?

what is output?

molecular weight measured

ionise sample to create positive and negative ions from neutral sample

output is measurement of mass/charge ratio of ions

how are ions generated and in what phase

the source generates ions in gas phase

time of flight mass spectrometer diagram

what is accelerating energy for time of flight?

E=zV

depends on charge and applied voltage

what are the 3 main ion sources

electrospray (ESI)

matrix assisted laser desorption/ionisation (MALDI)

electron impact (EI)

what are soft ionisation methods? 2 examples

what types of molecules are detected

ESI and MALDI

low energy to molecular ion is observed without breaking up

detection of large molecules eg proteins

what are hard ionisation methods? example

EI

imparts energy to molecule = extensive molecular fragmentation h

how does electrospray ESI work

what other analytical technique is it coupled with (think state)

generates ions from molecules in solution

• coupled with liquid chromatography

how does matrix assisted laser desorption/ionisation MALDI work

generates ions from sample dried together with matrix on solid plate

electrospray ionisation ESI diagram

• what state ?

what ions are seen in ESI (think what type of ionisation is it?) 3

what isn’t seen

pseudo molecular ions - gained or lost H⁺

multiply charged ions

other adducts (e.g. Na⁺ or K⁺)

• don’t see fragmentation

MALDI diagram

(think what states is it in, what is analyte held on)

what ions are seen in MALDI (think what type of ionisation is it?) 2

what isn’t seen

molecular ion peak seen

multiple charged ions

• don’t see fragments

how is matrix chosen in MALDI? how does this affect what ions are seen

don’t see fragments as matrix (e.g. c acid) absorbs laser energy

electron impact EI diagram

where do electrons come from, how do analyte molecule become charged

high energy electrons created by filament and accelerated towards analyte

electrons knock valence electron off to make analyte molecules charged

is fragmentation seen in electron impact? why or why not?

electrons accelerated to high energy, much more than needed to ionise

so excess energy is rumbling around leading to significant bond breaking = fragmentation

what is mono isotopic mass?

exact mass value for most abundant isotope

what is nominal mass?

integer mass value for most abundant isotope

what is average/chemical mass?

average atomic mass value taking into account isotope abundance

how to work out ppm error?

what value do you want for it to be considered a match?

within 5ppm

what are the ratios for Cl isotopes

ratios of peaks?

75% ³⁵Cl

25% ³⁷Cl

1:3 ratio

why is ¹³C containing ion peak smaller

much less abundant

about 1%

what are the ratios for Br isotopes

ratios of peaks?

50% ⁷⁹Br

50% ⁸¹Br

1:1 ratio

how do peaks from molecules with different isotope compositions differ?

differ in mass by 1 unit (neutron)

what is m/z gap for doubly charged species

what if mass is +4 gap?

0.5 gap

0.25 gap

which fragmentation pathways are favoured for EI?

what does this mean about peaks

generation of stable cations and radicals are favoured

means molecular ion peak may not be most intense

show benzylic group fragmentation to give 91 m/z peak

show carbonyl fragmentation to give m/z 43

show carbonyl fragmentation to give m/z 57

O positively charged

show carbonyl fragmentation to give m/z 15

show carbonyl fragmentation to give m/z 29

what is alpha cleavage?

when bond next to O broken

what is the mclafferty rearrangement?

what does a mass analyser do?

filter or sorter - only allows ions of specific m/z through

what are double bond equivalents equation? what does high DBE suggest?

suggests there may be aromatic ring