Organic MS Flashcards
Organic MS Pros
Qualitative Quantitative Blind analysis
Molecular ion Peak
Highest m/z equal to molecular weight
Base Peak
most abundant all other peaks intensity relative
Instrument Parts
sample inlet ionisation source mass spectrometer detector computer
Why vacuum
Collisions deflect ion trajectory may react with ions
Sample Introduction methods
- direct infusion - liquid/gas through capillary 2. direct insertion - solid vaporised on probe
Electron Ionisation
analyte impacted with electron beam to ionise by ejecting electron
high energy thus high fragmentation often no molecular ion lower energy reduces fragmentation but reduces ionisation/sensitivity
Chemical Ionisation
Analyte ionised by collision with reagent gas soft ionisation - proton transfer - adduct formation - anion abstraction - charge exchange
Reagent Gases
methane - no selectivity isobutane - some hydrocarbons ammonia - no hydrocarbons only polar/amines
Negative ion formation
electron capture hydride abstraction (less sensitive)
Desorption Chemical Ionisation
Rapidly heated and ionised pros - less decomposition - molecular ion cons - spectrum depends on temp analytes - sugars, nucleic acids
Field Ionisation
Ionised by electric field very soft, only molecular ion pros - soft cons - low sensitivity analytes - fossil fuels
MALDI Process
- analyte dissolved in matrix - dried to crystals - laser ablation vaporises crystals - proton transfer causes ionisation - desolvation - analysis
MALDI pros
- high sensitivity - wide range of analytes - easy sample prep - tolerant to contamination - simple spectra - only need one laser/matrix
MALDI cons
little fragmentation matrix determines spectrum quality
MALDI analytes
biopolymers peptides organics/inorganics carbohydrates lipids
Atmospheric pressure ionisation pros
On line detection easier introduction
Electrospray Ionisation
Strong electric field pulls charged droplets from liquid sample
ESI pros
can detect high weight proteins due to multicharge high mass sensitivity couples with LC, CE
ESI cons
can’t analyse non-polar
ESI Analytes
proteins polymers polar
Atmospheric pressure chemical ionisation (APCI)
analogous to CI but primary ions produced by corona discharge pros couples with LC
Atmospheric Pressure PhotoIonisation (APPI)
vaporised analytes ionised by UV exposure direct or indirect with dopant pros: analyse non ionisable Analytes: - non-polar - non-ionisable
Desorption Electrospray Ionisation (DESI)
sample directly exposed to ion beam desorbed species drawn into MS Analytes polar compounds pros no sample prep ambient conditions direct analysis cons can’t do non-polar
