Mass Spec

Question 1

What is mass spec

Answer 1

Separating the different isotopes of the elements based on their mass to charge ratio (m/z)

1Da = 1amu

Question 2

What is nominal mass

Answer 2

The integer number MW of the molecules most common isotope
Ex. CH4 16Da

Question 3

Why do we use mass spec

Answer 3

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)

Question 4

What are the three step to get mass spec to work

Answer 4

Ionization

Mass separation

Detection

Question 5

What is ionization for mass spec and what are the types of ionization

Answer 5

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)

Question 6

What are the types of hard ionization

What are the soft ones

Answer 6

Hard: Electron ionization

Soft:

Chemical ionization

MALDI

Electro spray

Question 7

What is electron ionization

Answer 7

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

Question 8

In electron ionization what leads to the extra fragmentation

Answer 8

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

Question 9

What happens in chemical ionization

Answer 9

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

Question 10

What happens in Maldi

What type of analyte it is used for

Answer 10

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

Question 11

What happens in electro spray

Answer 11

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

Question 12

What is god about electron spray

Answer 12

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)

Question 13

What is mass separation for mass spec and what are the ways to do it

Answer 13

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

Question 14

What is a Mass sector analyzer

Answer 14

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)

Question 15

What is a quadrupole mass analyzer

Answer 15

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

Question 16

What is the time of flight mass analyzer

Answer 16

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

Question 17

What is detection in mass spec and what are the way to do it

Answer 17

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

Question 18

What is a discrete dynode

Answer 18

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

Question 19

What is a continuous dynode

Answer 19

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.

Question 20

What is a micro channel plate

Answer 20

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.

Question 21

Why is there a high vaccum in between the mass analyzer and the ion detector in mass spec

Answer 21

It’s helps avoid interference (chemical or physical) from air

It’s very very low pressure

Question 22

What are the quantitative and qualitative uses of mass spec data

Answer 22

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

Question 23

If something is poorly resolved what do the peaks look like in mass spec

Answer 23

They don’t have separate full width half maxes

It’s difficult to see both separate peaks or their height/areas

Question 24

If something has a bigger resolving power at FWHM what does this mean

Answer 24

It has better resolution of peaks

Question 25

What is ion mobility spectrometry

Answer 25

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