mass anaysis Flashcards
mass analysis definition
process of selectively separating ions by their individual mass-to-charge ratios (m/z)
factors to consider for mass analysis method
mass of analyte
what mass accuracy, resolution and sensitivity do you need?
do you need to see fragmentation to obtain structural information
must be compatible with the ionisation method best suited to your analyte
time-of flight mass analysis
Consists of a long evacuate tube with no electric or magnetic fields
Ions enter at the start with the same kinetic energy and thus traverse the tube at different speeds depending on their mass-to-charge ratios
small ions traverse the TOF faster than larger ions and will achieve a measure of mass selective separation
TOF potential energy
2 ions are extracted by the potential difference Va into the TOF region. potential energy, Ep = zeta
z = charge on ion
e = charge on electron
Va = accelerating voltage
TOF kinetic energy
assuming 100% conversion
Ep=Ek=mv^2/2
time of flight equation =
t= (m/z)^1/2*(d/(2eVa)^1/2
t= K (m/z)^1/2 where K is proportionality constant
TOF directly proportional to the square root of m/z
broadening in TOF
all the ions don’t start with the same potential energy as they will not all be at the same start point. Results in ions of the same m/z arriving at slightly different times which causes broadening
Corrected using reflection: ions are bounced back down the TOF and are refocused by energy to arrive at the reflection detector. This energy refocusing improves the resolving power of TOF
Cons of TOF
pulsed technique - needs an ion source that generates ions in pulses or needs an ion butcher
Can suffer from ion speed and thus low resolution doe to potential energy variations
Pros of TOF
very high m/z range
high sensitivity - all ions generated are measured
resolution problems can be corrected for by use of a reflection
2-Sector mass analysis
use of double focussing instrument using a combo of electro-magnetic and electrostatic sector to bring about mass selective separation
mass filtering is achieved by the magnetic sector.
scanning the magnetic field focuses ions of different m/z at the monitor slit
energy correction is achieved by the electrostatic sector
to achieve maximum sensitivity the ions are focussed by energy and m/z at the double focussed point on the detector slit
2-sector mass analysis - magnetic sector
ions accelerate out of the source by high potential difference, V
ions focussed by the lenses and enter the magnetic sector through the source slit
narrowing the source slit will decrease the beam spread bu physically constricting the ion beam
the electro-magnet field strength is scanned over predetermined range
the ions are spirited and focussed sequentially at the monitor slit.
A specific m/z is in focussed for each specific field strength according to
m/z= (B^2r^2)/2V
B= magnetic field strength
r = radius of magnet
V = accelerating voltage
m/z = mass-to-charge ratio
By scanning the magnetic field, and measuring the intensity of each m/z, the mass spectrum is recorded
2-sector mass analysis - electrostatic sector
ions from the magnet enter the electrostatic sector through the monitor slit ( where ion current can be measured)
the electrostatic sector corrects for kinetic energy variations of a given m/z
this reduces the beam width due to energy variations and increases sensitivity and resolution
ions with higher kinetic energies will traverse a wider arc and vice versa. therefore ions accelerated towards the detector slit by slightly different amounts depending on their kinetic energy. this focuses the ions at the ‘double focussing’ point at the detector slit
2-sector mass analysis spectrum
measured by scanning the magnet across a predetermined and calibrated field range. the electric sector has fixed voltages applied which are pre-tuned to maximise the sensitivity for the m/z range being scanned
2-sector mass analysis cons
magnet is scanned to produce a single scan, have to sum many scans to obtain a spectrum - many mins
mass precision can be a problem due to magnet drift
low sensitivity as is a scanning method
restricted m/z range
2-sector mass analysis pros
high resolution is achievable
high mass accuracy is achievable ( better than 5 ppm is possible)
