liquid chromatography and mass spec Flashcards
difference of isocratic and gradient mode in LC
isocratic = mobile phase composition is the same every run
gradient = mobile phase changes throughout the run
what causes separation of anaylte bt mobile and stationary phase
differential equilibrium aka partition coefficient
what is another name for partition chromatography
liquid liquid chromatography bc both mobile and stationary are liquid
purpose of a column in LC
prevent dissolution of stationary phase by covalent binding solica
explain normal and reverse phase LC column
normal phase - stationary phase is more polar than mobile phase
reverse phase - mobile phase more polar than stationary
two most common solvents use in LC
methanol
acetonitrile
what is considered when choosing a solvent
polarity and UV cut-off
purpose of buffers
control pH (selectivity)
reduce peak tailing
give well shaped peaks
What should the buffers pH be
+/-1 of buffer’s pKa
are volatile or non volatile buffers preferred in LC-MSMS
Volatile because non volatiles contaminate ion source
Thoery of UV detector
UV and photometers with intense light sources are quantified using beers law
theory of electrochemical/amperometric detector
- effluent passes voltage electrode
- large voltage = movement of electrons = current
- current is proportional to concentration of analyte
what is voidtime (to)
time to elute unretained substance
what is retention time
time elasped from injection of sample to recording of peak
what value is good resolution
1.25 or greater
what factors affect peak retention and peak width
retention
- capacity factor (k)
- selectivity (a)
width
- efficiency (N)
what does capacity factor (k) measure and how to adjust it
retention time - change organic solvent
what does selectivity factor (a) measure
retention time of two peaks - most important factor
what does efficiency factor (N) measure and how
peak width - calculate number of theoretical plates
why is selectivity the most imporant factor for resolution
selectivity is a function of column packing, mobile phase, and solute chemistry
what does a large number of theoretical plates mean
narrow peaks = efficient column
what factors increases efficiency (narrower peaks)
well packed and lengthy column
increasing flow rate
less volume injected
why are samples extracted before injection
-prolong life of column
- peaks of interest
-improve sensitivity
what does mass spec detect
fragmentation patterns
theory of ionization -electron impact- in mass spec
- electron beam of 70eV from a heated filament onto ion source
- sample loses e- (unstable) = fragmentation
- voltage on repeller plate causes ions to be accelerated to analyzer
what is a quadrupole
- mass analyzer
- four rods (dc and rf voltages)
what is the path of mass spec
- inlet
- ionization (electron impact)
- mass analyzer (quadrupole)
- detector and data
what is the molecular ion and base peak
molecular ion = largest m/z peak
base = most abundant peak
problems of gc mass spec
heat labile compounds; methadone
acetaminophen has poor chromatographs
full scan vs SIM (select ion monitoring)
full scan = every ion
SIM = derivitization step for selected ions (increase sensitivity)
what is deuterated internal standard
3 hydrogen replaced with deuterum for controls
pros and cons of LC MS
pro
- LC is good for volatile and non volatile (GC only uses volatiles)
- LC works with polar analytes (GC is poor extraction with polar)
con
- Ion suppression with high salt
steps in electrospray ionization
- form charged droplets
- evaporation
- ionic repulsion
- ion enter mass analyzer
what happens in triple quadrupole (tandem) MS
Q1 = single m/z selection
Q2 = fragmentation in collision cell
Q3 = single m/z selection to the detector
principle of QTOF MS
separation of m/z by TOF
-smaller = faster
- heavier = slower
Q1= filter
Q2= fragmentation
TOF = separation of ions
