1.4 Mass spectrometer Flashcards
What does a mass spectrometer do
. It finds the relative atomic masses by determining the mass of separate atoms.
No atom other than carbon (12) has a relative atomic mass of an exact whole number, because neither the proton or neutron has a mass of exactly 1
Why is the mass spectrometer apparatus kept in a vacuum
The whole apparatus is kept under a high vacuum to prevent the ions that are produced colliding with the air
Describe how
Electrospray ionisation occurs
2) ionisation:
. The sample is dissolved in a volatile solvent
. It’s forced through a fine hollow needle that is connected to the positive terminal of a high voltage supply.
. This produces tiny positively charged droplets that gained a proton in the solvent
. The solvent evaporates from the droplets into the vacuum until they contain no more than a single positively charged ion
What are mass spectrometers used by
Forensic scientists can identify illegal drugs
How does ionisation
electron impact occur
ionisation:
. The sample is vaporised, and high energy electrons are fired at it from an electron gun, which is a hot wire filament with a current running through it, that emits beam of high energy electrons.
. This usually knocks off one electron from each particle forming a 1+ ion
Once there are positive ions formed, how does accelleration occur
3) Acceleration: The positive ions are attracted towards the negatively charged plate and accelerate towards it.
. Lighter ions and more highly charged ions achieve a higher speed
What are the final stages of mass spectrometry after accelleration
4) Ion drift: The ions pass through a hole in the negatively charged plate, forming a beam and they travel along a tube, called the flight tube to a detector
5) Detection:
. When ions with the same charge arrive at the detector, the lighter ones are first as they have higher velocities.
. The flight times are recorded.
. The positive ions pick up an electron from the detector which causes a Current to flow
6) Data analysis: The signal from the detector is passed to a computer which generates a mass spectrum
How can mass spectra be used
- It can identify different isotopes that make up an element.
- It detects individual ions, so different isotopes are detected separately because they have different masses.
The peak height gives the relative abundance of each isotope, and the horizontal scale
What is the difference between high and low resolution mass spectrometry
High: Mass spectrometers can measure RAM to 5 decimal places of an atomic mass unit
Low: However most work is done to 1 decimal point
Equation to find relative atomic mass of neon
It has abundances 90% of mass number 20
and 10% of mass number 22
(90 X 20)+ (10 X 22)
—————————- = 20.2
100
What are the two isotopes of chlorine
find the relative atomic mass
Cl 35 and Cl 37
They occur in the ratio 3:1
SO for every 3 Cl 35, there is 1 Cl 37
This means there is 75% of Cl 35, and there is 25% Cl 37
so relative atomic mass is 35.5
How can you identify elements using mass spectrometry
.All elements have a characteristic pattern that shows the relative abundances of their isotopes
. This can be used to help identify a particular element
Eg chlorine has two peaks, at 35 and 37.
The peak at 35 is 3x the height of the peak ar 37 because there are 3x as many 35s as there are 37s.
The spectrum will also show peaks caused by ionised Cl2 molecules, which are called molecular ions
How are mass spectrometers used in space
Space probes such as Mars Rover Curiosity carry mass spectrometers.
These identify elements in rock samples.
. Also an aircraft lands on titan and carried a mass spectrometer to identify and measure amounts of gas in titans atmosphere
What is mini mass spectrometry
A unit small enough to carry as a backpack, which includes batteries, weighs 10kg so can be carried by crime scene officers to detect drugs etc
They can be used by people with little or no knowledge as it has software to identify samples
what are the two equations required to calculate time of flight mass spectrometry
Ke =1/2 mv^2
distance= time X velocity
