Mass Spectrometry Flashcards
What is MS?
Separation of ions by mass to charge ratio
Also the production of ions that are subsequently separated of filtered according to their mass to charge (m/z) ration and detected
Where does the molecular ion peak occur?
Molecular ion mass + 1
Advantage of MS
- Provides MW info and structure info
- Very low limits of detection
- Minute amount of sample required (1 nanogram is all you need)
- Fast
- Can be used for a wide variety of possible sample
_______ requires much lower amount of sample than NMR
Mass Spectrometry
Difference between mass spectrometry and mass spectroscopy
Mass Spectrometry: based on particle
Mass Spectroscopy: based on frequency
Applications of Mass Spectrometry
- Structural determination of organics, biomolecules, polymers, catalysts etc
- Mixture Analysis
- Elemental Analysis
- Quality Control
- Environmental Monitoring
What is the most common type of mass spectrometry?
Electron Ionisation
What is the process of electron ionisation?
- Volatile sample is vaporised
- The sample is blasted with electrons and excess energy is absorbed (M+)
- Uni-molecular decomposition of M+
- Produce ions
- a- mass analysis of all ions
b- data recording
c- plot of bar graph - mass spectrum
What is M+?
Molecular ion
What is a mass spectrum?
It’s a relative abundance of ions of different m/z produced in an ion souce
What is the purpose of operating mass spectrometers at low pressures
- Minimise ion/molecule collisions
2. Prevent from high voltage discharges
Disadvantages of rough (vacuum) pump
- The gas to be exhausted outside the lab
- Time consuming: as it requires to heat/cool oil for hours
Disadvantages of diffusion pump
- Expensive
- The machine requires a constant flow of water
- Doesn’t do well with power outages (Causes oil to become carbon)
Advantages of Diffusion Pump
- Cheaper as they’re an older machines
-
- Method of ionisation
- Electron ionisation
- Introduce sample as a gass/vapourise to a gass
- Ionise through interaction with 70eV electron beam
- Can accept sample from a GC
- Method of ionisation
- Chemical ionisation
- Chemical ionisation: a progression from EI
- Accept samples from GC
- Method of ionisation
Atmospheric Pressure Ionisation (API)
- Spray liquid into the ion source and use hot gas to remove the solvent molecules
- A voltage gradient will cause ions to be pulled into the mass analyser
- Can accept sample from an LC
- Method ionisation
Matrix-assisted laser desorption/ionisation (MALDI)
- put sample on a metal plate and cover with a matrix compound
- place inside ion source and focus a laser beam onto the surface
- ions will created and pulled into the mass analyser
- Mostly performed on built time of flight instruments
Whats the typical speed of diffusion pump?
Diffusion pump has typical speeds between 500-800 L/sec
What’s the pressure of diffusion pump?
It has pressure of 10^-9 to 10^-10 Torr but with pump oil background
What’s turbomolecular pump
A clean vacuum pump that’s suitable as the main pump of a mass spectrometer.
What is a turbomolecular pump used for?
Used in a wide range of high and ultra-high vacuum applications, covering both clean application.
Disadvantages of turbomolecular pump
- Expensive to repair
- Random solids can damage the blades
Advantages of turbomolecular pump
- Cheap to operate
- Dependable
- Require no re-generation
- Operate at high pumping speeds in the high and ultra high vacuum range
Cyropump
- Gases are frozen (condensed) on surfaces cooled to 10-20K with liquid nitrogen
- Works well with gasses that freeze
- Gasses that don’t freeze are absorbed w/activated charcoal traps in the condenser
- Used in conjunction with turbopumps
Disadvantages of Cryopumps
- It has to stay cold
- Nitrogen pump can not be emptied
Define volatile
A sample can go into the gas phase and remain stable
Define direct introduction
Sample that is place directly into the ion source region of the mass spectrometer
Direct introduction for gases and volatile liquids
- Syringe, leak valve/pulse valve introduction of sample into a reservoir
- Volatile liquids have to be vaporised - through pumping them alone/heating the inlet system
Define Hyphenated-MS technique
Sample goes from a chromatograph, IR / ICP through an interface and into the MS source
Electrostatic focusing
Ions are moved and focus by placing voltages on metal components (plates, tubes, etc)
Positive voltages repel positive ions - attract negative ions n vice versa
The higher the voltage, the faster ions are accelerated away from or towards the voltage
Can repel ions by using the same charge
Define “Steering beams” of electron ionisation
Beams of varying voltages that can be applied to move the ions up, down, left or right
Uses of electrostatic
- Photocopies
- Laser printer
- Ink jet printer
- Electrostatic air filters
What does electrostatic focusing can do
- Extract ions out the ionisation source
- Form a focused ion beam that goes into the mass analyser
- Accelerate ion into the mass analyser
Define electron volt
The energy acquired by a singly charged particle in moving across a potential difference of 1 volt
What is the first ionisation potential
Measures of the energy that is required to remove one valence electron from a neutral atom and is an atomic property which reflects the outermost electronic configuration
What happen if the electron energy is lower than the ionisation potential?
The ions wont be made
If the electron energy is too high, it will cause what problem with ions?
The number of ions made and the current plateau off because there is so much energy - can break any bond.
Why would large electron energies such as 70eV be used in routine analysis?
- At high energies, the ionisation process is saturated (all molecules come into contact with the electron beam and ionise)
- Small changes in energy have negligible effects on the spectra
Advantages of EI
- Good sensitivity
- Can do + and - ions but negative ion sensitivity is low for most compounds
- Can withstand pressures to about 10^-5 - too high could result to burning the filament
- Works with high resolution and MS/MS experiments
