Mass Spec Flashcards
What is mass spec
Separating the different isotopes of the elements based on their mass to charge ratio (m/z)
1Da = 1amu
What is nominal mass
The integer number MW of the molecules most common isotope
Ex. CH4 16Da
Why do we use mass spec
For identifying molecular structure (from the functional groups fragments)
Identifying atomic isotopes (the geographical origins can be found from the isotopes ratios
Sensitive identification of molecules (single molecule/atoms LODs)
What are the three step to get mass spec to work
Ionization
Mass separation
Detection
What is ionization for mass spec and what are the types of ionization
First the sample is in the system, the charge (z) comes from making cations
Hard: giving lots of energy to the ionizing molecule so that it splits a lot
Soft: the ionizing molecule will be ionized but will split apart less (adding things to the molecule so that sometimes M>the molecules actual mass)
What are the types of hard ionization
What are the soft ones
Hard: Electron ionization
Soft:
Chemical ionization
MALDI
Electro spray
What is electron ionization
First a hot filament made of tungsten at 1000 degrees generates electrons
These electrons accelerate (to 70ev) and collide with the molecule and kicks of its valence electrons, this makes M into M+ e- e-
The postive charged molecule then move toward the - voltage side of the machine which increases in negative voltage to make an accelerated ion beam
Ions go to the mass analyzer through this beam
In electron ionization what leads to the extra fragmentation
Some of the 70v that turned to 70ev to accelerate the electrons gets transferred over to the M+
This makes the M+ excited and leads to more fragmentation
What happens in chemical ionization
CH4 is added in lower pressure than the M sample in the machine
The ch4 is more likely to be hit by the electrons than the sample.
So the ch4 gains positive charge and applies a postive hydrogen to the sample and the sample gets sent through the accelerated ion beam
The interaction with the ch4+ + M is lower in energy which leads to less fragmentation
What happens in Maldi
What type of analyte it is used for
This is for large heavy or insoluble analyte
The matrix that the sample is in gets a uv laser pulse
The matrix is ionized by the laser, which causes the sample within it to also gain charge and be ionized
Once the sample is ionized, heat and Columbic repulsion’s between the sample and charges in the matrix cause expansion of the matrix
The charged sample then blasts through the accelerated ion beam
What happens in electro spray
In a small capillary tube made of metal, the high voltage power supply is applied from the capillary to the mass spec insert + to -
This voltage pulls the liquid out of the tube and applies a + charge to it
The liquid is full of positive charges and when it’s sprayed out, it forms large droplets of multiple + charges
Once the large droplets evaporate, they are very unstable so they go under columbic explosion. They turn into single postive charges instead of a bunch.
Then they go into mass spec
What is god about electron spray
No fragmentation happens but
Multiple positive charges are applied to the sample so the m/z ratio is smaller for large molecules
This smaller m/z is good for mass analyzer because the molecule is below its upper m/z limit for the machine (because the m is not higher)
What is mass separation for mass spec and what are the ways to do it
Using a mass analyzer to separate each of the molecules ions based on their m/z, usually by using magnetic and electric fields to change the velocity and direction of the moving ions
Mass sector analyzer
Quadrupole mass analyzer
Time of flight mass analyzer
What is a Mass sector analyzer
Once the sample is ionized and goes through the accelerator ion path, a magnets magnetic field (B) causes the ions to curve.
These ions go through a slit that has a specific m/z range that it’s measuring
The lighter ions curve more and the heavier ions curve less (less acceleration)
What is a quadrupole mass analyzer
There are 4 poles (2 vertical two horizontal)
A electric field is applied across each pair to change the ions path (because of attraction and repulsion to the poles)
The electric field occilates between the poles so the ions do too. You select a frequency of the two pole pairs to make it so that only one m/z resonates and make it through
The poles are basically mass filters and can scan thrown many alternating current frequencies to find the entire range of m/z in the sample
What is the time of flight mass analyzer
This is better for finding mass of larger molecules
The ionized particles go through a 2m drift region with no electric field. They’re initially all accelerated to begin moving
Because F=ma, the smaller particles (lower m/z) go faster than large
We monitor the signal of the micro channel plate ion detector (things ions are travelling to) over time. The time taken for one thing to get there translates to speed and finally m/z
What is detection in mass spec and what are the way to do it
The use of detectors to turn the chemical info into a usable electronic signal
Need very sensitive detector so we are able to put in the least amount of sample (single ion) and still get info
Ways to do it:
Most are based of the use of dynodes
Discrete dynode
Continuous dynode
Micro channel plate
What is a discrete dynode
The ions go into the detector, hit a cathode go through to the dynode, get amplified and multiplied at 90V then ends at anode
Just like a pmt
What is a continuous dynode
One whole cone shaped curved dynode
Surrounded in lead-doped glass (resistor)
The electrons go in at a continuous voltage and bounce and multiplies through the cone (electron cascade)
The voltage increases closer-to the tip where the multiplied ions are and we measure the current at this point.
What is a micro channel plate
There are holes in the plate that are v shaped
The ion from the mass separator goes into these holes and gets amilified, each hole acts as a dynode horn
The amplified ion then gets sent to a long detector anode that can detect the entire mass spectrum.
Why is there a high vaccum in between the mass analyzer and the ion detector in mass spec
It’s helps avoid interference (chemical or physical) from air
It’s very very low pressure
What are the quantitative and qualitative uses of mass spec data
Qualitative: you can find the molecular structure from a pattern of m/z peaks
Quantitative:
you can use “counts” of the peaks to tell you the number of molecules (ions at that m/z)
Can find the concentrations of a component in the solution by montiering a specific m/z
Can find the relative ratios of atoms or isotopes in a molecule
Used as a detector in chromatography
If something is poorly resolved what do the peaks look like in mass spec
They don’t have separate full width half maxes
It’s difficult to see both separate peaks or their height/areas
If something has a bigger resolving power at FWHM what does this mean
It has better resolution of peaks
What is ion mobility spectrometry
Like mass spec, but separation is based on size
Add a constant voltage (acceleration) along the the drift region of the machine. The voltage is against the flow of the carrier gas
If the ion is larger it has more interactions with the gas and moves slower
If the ion is smaller it has less collisions so it’s faster
