Mass Spectrometry Flashcards
Advantages of choosing measurands with a high mass:charge ratio?
Greater signal to noise/ lower background
Less likely to have isobars/isomers
With what m/z is chemical interference most common?
200-500
Steps that may be used in sample preparation
- Protein precipitation followed by centrifugation and/or filtration
- Solid phase extraction
- Liquid-liquid extraction
- Affinity enrichment
- Derivatisation
What is derivatisation?
Addition of a functional group to the target compound of interest so it is easier to separate chromatographically or detect by mass analysis.
Possible goals of derivatisation?
a) make a compound more volatile (GCMS)
b) give a compound more thermal stability (GCMS)
c) give it greater ionisation efficiency (LCMS)
d) modified chromatographic ppties (GC or LCMS)
e) favourable fragmentation ppties (GC or LCMS)
What is selective ion monitoring?
The mass spectrometer is set to detct only ions with a specific mass spectra. This mass spectra must be known in order for the system to be programmed to detect it.
Advantages and disadvantages of selective ion monitoring?
Advantages include: Greater specificity, sensitivity, higher signal to noise, more accurate quantification. One drawback is the potential for isobaric interference.
What is selected reaction monitoring?
Similar to selective ion monitoring, however, SRM is used with a tandem mass spectrometer, and data from both precursor and fragment ions are used to improve selectivity and reduce isobaric interference
Basic components of a mass spectrometer
- Chromatograph
- Ion source
- Vacuum system
- Mass analyser
- Detector
- Computer
Different types of ion source/ionisation for MS?
- GC-MS
Electron ionisation
Chemical ionisation - LC-MS (these techniques operate at atmospheric pressure)
Electrospray ionisation
Atmospheric pressure chemical ionisation - Other
Inductively coupled plasma
MALDI
How does electron ionisation work?
Gaseous sample flows through ionisation chamber towards vacuum in mass analyser.
Electrons generated by a heated filament are accelerated across the ionisation chamber perpendicular to the sample from cathode to anode using a potential of 70eV
As electrons collide with sample molecules, the molecules are ionised and sucked into mass analyser
Advantages and disadvantages of electron ionisation?
Ions must be volatile and thermally stable
A “hard” ionisation technique - sample molecules fragment in the ion source
This can be of benefit as fragmentation at 70eV is reproducible so libraries of fragment data can be created and searched
How does chemical ionisation work?
Reagent gas (methane, ammonia, isobutane) injected into high pressure compartment
Electrons fired to fragment and ionise reagent gas
Ionised reagent gas then reacts with sample to produce ions which are sucked into high vacuum mass analyser
Advantages/disadvantages of chemical ionisation?
A softer ionisation technique, preserving some molecular ions.
Requires volatile and thermally stable analyte
Better for seeing negative ions
Can alter reagent gas to produce different fragmentation patterns
How does electrospray ionisation work?
Sample injected through a heated metallic capillary
A high voltage is applied - this both ionises the sample and accelerates it towards the mass analyser.
As it accelerates towards mass analyser, ions pass through curtain gas and a series of staged pressure compartments separated by skimmers or cone orifices
As the pressure in each chamber decreases, solvent evaporates (heated curtain gas, usually N2, assists with solvent evaporation) until eventually only the charged particle enters the mass analyser vacuum
