TOF mass spectrometry

Question 1

What is mass spectrometry?

Answer 1

A method of analysis, which helps us to determine relative atomic masses of unknown substances. It also helps us to determine the abundances of specific isotopes.

Question 2

Outline the 4 steps in TOF mass spectrometry

Answer 2

1) Ionisation

2) Acceleration
3) Ion Drift
4) Detection

Question 3

what are the 2 types of ionisation that can happen in TOF mass spectrometry?

Answer 3

electrospray ionisation

electron impact ionisation

Question 4

Describe the steps involved in ElectroSpray Ionisation

Answer 4

Electrospray Ionisation:

Dissolve the sample in a polar solvent

Push it through a tiny nozzle at high pressure

Apply a high voltage

Each particle will gain a proton

The sample will be a gaseous sample of positive ions

Question 5

Describe Electron Impact Ionisation

Answer 5

Vaporise the sample

Sample is bombarded by high energy electrons

An electron will be removed from each particle

This will create +1 ions

Question 6

Describe the Acceleration phase of TOF Mass Spec

Answer 6

The positive ions move through a negative electric field, accelerating toward the negative plate. Lighter ions and ions with a higher charge achieve a higher speed here.

Question 7

Describe the Ion Drift phase of Mass Spectrometry

Answer 7

During ion drift, the different ions will move across the flight tube. Positive ions with smaller m/z values will have the same KE as those with a large m/z values and will move faster.

The ions are distinguished by different flight times.

Question 8

Describe Detection

Answer 8

Assuming they have the same charge, lighter ions will reach the detector first. When these positive ions hit the detection plate, they gain an electron,

producing a flow of charge. The greater the abundance, the greater the current produced.

Question 9

When do we use electrospray ionisation over electron impact?

Answer 9

Electron impact can cause larger, organic molecules to fragment. Hence, we use electrospray ionisation for larger, organic molecules - the ‘‘softer’’ conditions of this technique means fragmentation won’t occur

Question 10

What can happen to molecules during an electron impact ionisation stage? What will this show on the mass spectra?

Answer 10

Molecules will often break up and give a series of peaks caused by the fragments.

Question 11

What do we mean by molecular ion / parent ion?

Answer 11

The largest peak.This is when the complete molecule passes through the mass spectrometer - it has the largest m/z and will be equal to the relative molecular mass.

Question 12

What must we consider when working out Mr from a mass spectra, in which Electrospray Ionisation was used?

Answer 12

The peak will be equal to MH+.
To get M, the molecule, we must subtract 1 - the mass of a H+.

If the peak is 521.1, there Mr is 520.1

Question 13

Why does the TOF mass spectometer need to be under a vaccuum?

Answer 13

To prevent air particles ionising and registering on the detector

Question 14

What are the equations for TOF?

Answer 14

kenetic energy = 1/2m(v squared)

change in velocity = (2KE/m) square root
2xKinetic energy divided by mass and sqaure root answer

time = distance/ velocity

change in time = distance x (mass divided by 2xkinetic energy ) square rooted