1.4 mass spectroscopy

Question 1

underline 2 principles of a TOF mass spectrometer

Answer 1

1. find the mass and abundance of each isotope in an element, allowing the Ar to be determined
2. identify molecules by determining their relative molecular mass

Question 2

underline the 4 different regions of the mass spectrometer

Answer 2

1. ionisation region
2. acceleration region
3. drift region
4. detection

Question 3

what happens in the ionisation region?

Answer 3

the neutral atoms are ionised (lose an electron/ gain a proton) to form +1 ions

Question 4

what are the 2 methods of ionisation

Answer 4

1. electron impact
2. electrospray

Question 5

outline electron impact ionisation

Answer 5

1. sample is vaporised
2. **a beam of high energy electrons is fired from an electron gun **
3. the electron gun has a hot wire filament with a current running through it; heated wire emits electrons
4. electrons are **accelerated by attraction to a positively charged electrode **
5. high energy electrons** knock off only 1 electron **
6. forming a +1 ion
   ** X(g) + e- —–> X+(g) + 2e-**

Question 6

explain what happens in electrospray ionisation

Answer 6

sample is dissolved in a volatile solvent
injected through a hypodermic needle to give a fine mist (aerosol)
the needle tip is attached to the positive terminal of a high voltage power supply
particles are ionised by gaining a proton (H+) from solvent
as they leave the needle producing XH+ ions (ions with a single +ve charge and a Mr of +1)
solvent evaporates and XH+ ions attracted to negative plate

Question 7

explain what happens in the acceleration region

Answer 7

+1 ions accelerated using an electric field to give them all the same kinetic energy

Question 8

what does the speed of each particle depend on (given that kinetic energy is equal)

Answer 8

mass

Question 9

what happens in drift region?

Answer 9

no accelerating field
heavy ions drift more slowly and take longer to reach the detector
lighter ions drift faster and reach the detector first

Question 10

explain what happens in the detection zone

Answer 10

positive ions strike a negatively charged plate
positive ions neutralised as they gain electrons
process generates a flow of electons (electric current)
current is amplified to produce a signal on computer

Question 11

what is m/z

Answer 11

mass to charge ratio

Question 12

the relative intensity of the peak in the resulting mass spectrum produced by __ i__ with a particular __/__ value is proportional to the n_______ of i_____ hitting the p______

Answer 12

the relative intensity of the peak in the resulting mass spectrum produced by** an ion **with a particular m/z value is proportional to the number of ions hitting the plate

Question 13

what is the maximum value of m/z in electron impact

Answer 13

m/z = mr

Question 15

what is the maximum value of m/z in electrospray

Answer 15

m/z = Mr + 1