Mass Spectrometry

Question 1

What is mass spectrometry?

Answer 1

It is the methodology we use to identify the weight of a molecule.

Question 2

What are the three processes of mass spec?

Answer 2

1) Ionise the molecules (Ion Source)
2) Separate the charged molecules and fragments according to their m/z (mass : charge) ratio (mass analyzer)
3) Once separated the charged ions are detected (Detector)

Question 3

What phase must the molecules be tested in?

Answer 3

Gas phase - to lessen the likelihood or reactions and to improve results. this is achieved by vaporisation

Question 4

How are the molecules separated by mass:charge ratio

Answer 4

By using a magnet on the ionised particles to break things up by mass and ratio.

Question 5

What type of injection methods are there

Answer 5

1) Static - Injection

2) Dynamic - Separation

Question 6

What is the drawback of static sample injection?

Answer 6

It can only be done using pure samples.

Question 7

With ionisation, what causes fragmentation?

Answer 7

Using hard (high) energy in the ionisation process, the higher the ionisation the more likely you will get fragmentation.

Question 8

What is an example of a hard ionisation?

Answer 8

Electron impact ionisation, this uses a rhenium or tungsten filament as a electron source. As these electrons pass the source region they break the molecular bonds of the analyte causing significant fragmentation and cation production. The electrons figuratively tear the electrons straight off the molecule. The cations are then repelled away using a repeller electrode.

Question 9

What is the most common sample introduction used for Electron impact ionisation

Answer 9

Gas-Chromatography.

Question 10

what is an example of soft ionisation?

Answer 10

Electrospray ionisation. This involves a solution of analyte and solvent which is then sprayed in a fine mist. This mist is then electrically charged, when the aerosol dries out (desolvation) the droplets undergo fission due to there being to great a density of electrical energy. the process of desolvation and fission occurs until there is nothing but analyte left.

Question 11

What does ESI do better then EI?

Answer 11

ESI allows for higher molecular masses to be analysed as you can increase the charge and reduce the mass according to the mass:charge ratio.

Question 12

What is a matrix effect?

Answer 12

A matrix effect is the effect that a sample matrix has on the interpretation of the data, the composition of the matrix may cause shadowing or enhancements of particular molecules. This means we need to do a study on our matrices as well.

Question 13

How do you do a matrix analysis?

Answer 13

By running a pure sample of the analyte you can determine a reference point. By spiking a known ‘clean matrix’ that will be used in spectrometry we can determine the effect that the matrix has on the analyte.

Question 14

When do we need to do a matrix analysis?

Answer 14

Whenever there is a new analyte-matrix combination.

Question 15

What does the mass analyser do?

Answer 15

The analyser is responsible for separating the molecule out according to its mass charge ratio.

Question 16

What quality of the results is the analyser responsible for?

Answer 16

The analyser is responsible for the range, accuracy, resolution and sensitivity of the results.

Question 17

What range of mass do small molecules exist in?

Answer 17

A range of 0 to 1000 molecular mass.

Question 18

What is a quadrupole mass analyser?

Answer 18

This analyser uses four rods to exert an oscillating field between them, this oscillating field uses specified frequencies to causes target molecules to travel in a sine wave into the detector. The other molecules become unstable and crash into the rods causing them to be removed from the sample. This is great for removing atmospheric contamination such as water, nitrogen and oxygen.

Question 19

What is an ion trap analyser?

Answer 19

An ion trap analyser uses the same idea of a quadrupole analyser but instead of using the magnetic field to fire the analyte into the detector it uses it to trap the ion for later detection. This brings has a higher sensitivity and resolution.

Question 20

What is a TOF analyser?

Answer 20

It is a Time of Flight Analyser. This uses a same sourced kinetic force to push the ions into the detector over a length. The resulting velocity gives the molecular mass. This has no upper limit on molecular size which makes it useful for large bio-polymers such a proteins.

Question 21

What is a Fourier transform mass spec?

Answer 21

This creates a magnetic field and measures the disturbances (movements) of the ions in the field. A computer then deciphers this data. This is super expensive to run.

Question 22

What is the importance of isotopes is mass spectrometry?

Answer 22

This allows for further structural and compositional data to be interpreted. Isotopes appear as different peaks and the recorded spectra.

Question 23

What importance does Atomic weight play in the interpretation of mass spectrometry?

Answer 23

It allows us to interpret the molecular mass recorded in the mass spectrometry data.

Question 24

How do we determine the difference between fragments and molecular ions in the graph?

Answer 24

The molecular ion should theoretically be the largest peak, however if you use a hard ionisation technique you may lose the molecular ion peak due to fragmentation. If this is the case we use the Nitrogen rule.

Question 25

What is the Nitrogen rule?

Answer 25

This rules suggests that if the molecular mass is an even number then the presence of Nitrogen is either 0 or in an even quantity. If a molecular mass is odd then the presence of nitrogen occurs in odd numbers. This rule states that if the nitrogen count (therefor molecular mass) is uneven then the peak is a fragment.

Question 26

What compounds does the nitrogen rule work for?

Answer 26

Heaps, particularly those containing Hydrogen, Carbon, Oxygen and halogens.

Question 27

How do we calculate atomic mass?

Answer 27

Using the common abundance of a certain element we average out the presence of all isotopes. For example 12 Carbon exists is a abundance of 98.93% at 12g/mol whereas 13 Carbon exists in a abundance of 1.07%.
at 13.0035 g/mol Therefor we average these values to get a atomic mass of 12.011g/mol

Question 28

How do isotopes help us identify molecules?

Answer 28

by comparing intensities of isotopes we can predict the element and its quantity. for example a molecule containing 6 carbons would have a minor isotope intensity multiplied by 6.