Mass Spectrometry Flashcards
What is Mass Spectrometry?
A powerful instrumental method of analysis
What is the purpose of Mass Spectrometry?
- To find the abundance and mass of each isotope in a sample
- From this you can work out the relative atomic mass/ relative molecular mass of atoms and molecules
In the real world, what are 2 uses of mass spectrometry?
- Forensic scientists can use this to identify substances like illegal drugs
- When probes arrive in other planets mass spectrometry can identify the elements there
What is actually meant by the term
a) relative atomic mass
b) relative molecular mass
a) the average mass of 1 atom divided by 1/12 of the mass of an atom of Carbon (12)
b) the average mass of 1 molecule divided by 1/12 of the mass of an atom of Carbon (12)
Relative atomic masses are measured on which scale?
On a scale of which an atom of Carbon (12) has a mass of exactly 12
What other isotopes have a mass of an exact whole number? Explain your answer
None because neither protons nor neutrons have a mass that is an exact whole number
Why is Carbon 12 used as a standard?
- It is stable
- It is a solid at room temperature
- It’s abundance in all organic compounds
- It is consistent to Avogadro principle
What had previously been used as the standard, before Carbon 12 and why were they scrapped?
- Hydrogen, there was inaccuracy because of its extremely low mass
- Oxygen, there was an issue due to the fact naturally occurring oxygen is a mixture of 3 isotopes; O16, O17 and O18
What is the general principle of a mass spectrometer, no matter which method is used?
The basic principle is to form ions from a sample, to then separate them according to their mass to charge ratio
What are the six steps of Time of Flight mass spectrometry?
V- Vacuum I- Ionisation A- Acceleration I- Ion drift D- Detection D- Data Analysis
The whole apparatus must be kept under a certain condition
a) what is this condition
b) why is this important
a) A vacuum
b) It is vital because otherwise the ions would collide with molecules from the air and this would slow them down
What are the two different ways in which a sample can be ionised?
By electron impact or by electrospray ionisation
Describe the three steps of Electron Impact
- The sample is vaporised
- High energy electrons are fired at the sample from an electron gun
- This knocks off an electron from each particle so positive ions are formed
What is an electron gun?
A hot wire filament with a current running through it that emits electrons
What is the equation for electron impact
X(g) -> X+(g) + e-
When an electron is knocked out of a particle to form a 1+ ion, what do we call this ion?
The molecular ion
What type of elements and substances do we use electron impact to ionise?
Elements and substances with low formula mass
What can happen to the molecular ion when electron impact was used to ionise?
It can break down into smaller fragments. This can be detected by the detector and included in the mass spectrum
What are the 3 main steps of electrospray ionisation?
- Dissolve the sample in a volatile solvent
- Inject the sample through a fine hypodermic needle (with a positive charge)to give a fine mist into the ionisation chamber
- The spray emerges and can be ionised by gaining a proton or by losing an electron
How do we get the needle to have a positive charge?
By attaching the tip to the positive terminal of a high voltage power supply.
Give two examples of a volatile solvent that could be used in electrospray ionisation?
Water or methanol
Electrospray ionisation can ionise particles by gaining a proton. Explain how this happens
- the particle leaves the needle and gains a proton (i.e a single H+) from the solvent
- this produces XH+ ions
- the solvent evaporates
What is produced when the particles in the sample gain a proton? What is the mass of this and why?
XH+ ions are produced. They have a single positive charge and a mass of Mr+1 because it has gained a proton (which has a RAM of 1)
Give the equation for when a particle is ionised by electrospray ionisation and gains a proton
X(g) + H+ —> XH+(g)
Alternatively to gaining a proton, particles in the sample can lose an electron. Explain how this happens
- the spray emerges and the particles lose an electron to the positively charged needle
- this produces positively charged droplets
- the solvent evaporates from the droplets and they get smaller and smaller until they may contain no more than a single positively charged ion
When is electrospray ionisation used? Give an example.
For substances with higher molecular masses like biological molecules such as protein
Which is the harshest technique to ionise by?
Electron impact
What is the next step after ionisation?
Acceleration
How are the ions accelerated and why?
using an electric field so that they all have the same amount of kinetic energy
What is used to attract the ions?
A negatively charged plate
Explain how an ions velocity depends on its mass
All the ions have the same kinetic energy, and kinetic energy = 0.5 mass x velocity ^2. meaning, if an ions mass is heavier than another ion it will have to have a slower velocity so the kinetic energy still matches the other ions
How do the ions get to the flight tube?
Positive ions travel through a hole in the negatively charged plate, forming a beam, to get into the flight tube
What happens in the flight tube?
The ions drift down to the detector
What is the equation for the ToF?
distance / velocity
How are the ions detected?
They hit a negatively charged electric plate
Which ions arrive at the detector first and why?
The lightest ones because they have higher velocities
How are the positive ions discharged?
By gaining an electron from the negatively charged plate
Why is a current produced at the detector?
Because there is a movement of electrons
What does the size of the current give a measure of and why?
The abundance of ions because the more ions that hit it the greater the movement of electrons and the greater the current
What is used to determine the mass of ions?
The time of flight
