ChemistryUnit 2: Assignment

1Module 2: Analytical methods

and separation techniques.

Mass Spectrometry!

Objective 7.1:

Explain the basic principles of mass spectrometry; include block diagram. (simple schematic diagram of the process)

Mass spectrometry

Mass spectrometry is a physical method available for identifying the structure of a compound (the mass # and % relative abundance of each isotope present in a sample of an element). From that data the relative atomic mass of the element could also be determined.

The basic principle!

Imagine yourself in this scenario:You and your friend heard about a new game called ‘Deflect the ball’ and wanted to try it out. This is how the game is played: 1 person throws a ball while the other tries to ‘Deflect the ball’ by spraying the hose with water towards the ball.The aim of the game is to divert the ball from its original course with the water from the hose!

Now consider these materials:• 2 balls- wooden ball with an iron core

and a tennis ball (both of different sizes and mass)

• A hose with water

You and your friend gathered these materials and decided to play the game.

Now that you have your “materials” and you have an idea of how to play the game:• What do you think would happen if your

friend threw the wooden ball with the iron core towards you?

would the ball be deflected by water?

• What do you think would happen if your friend threw the tennis ball towards you?

would the ball be deflected by water?

• This would most likely be the case with the wooden ball:

It would barely deflect the ball with the action of the hose because of the heavy weight of the ball.

• However with the tennis ball the action of the water will deflect the ball.

The ball is deflected as a result of its light weight and ends up on the ground.

So basically the principle of mass spectrometry is to separate atoms according to their mass and determine the relative numbers of different atoms present.The instrument used in mass spectrometry is the MASS SPECTROMETER!

The mass spectrometer

The mass spectrometer works on a principle that when charged particles passes through a magnetic field the particles are deflected by the magnetic field, and the amount of deflection depends upon the mass/charge ratio of the charged particles.

Once the sample of an element has been placed in the mass spectrometer, it undergoes 5 stages:

• Stage 1: Vaporisation• Stage 2: Ionisation• Stage 3: Acceleration• Stage 4: Deflection• Stage 5: Detection

A full diagram of a mass spectrometer!

A sample is injected into the vaporisation chamber

• Vaporization!The sample has to be in its gaseous form. If the sample is a solid or liquid then a heater is used to vaporise some of the sample.

• Ionisation! The sample is bombarded by a steam of high energy electrons from an electron ‘gun’. The high energy electrons can ‘knock’ an electron from an atom producing a positive ion:

X(g) + X+(g) + e-

• Acceleration!The positive ions are attracted towards an electric field which is used to accelerate the ions towards a magnetic field. The accelerated ions are focused and passed through a slit which produces a narrow beam of ions.

• Deflection!In the magnetic field the accelerated ions are deflected. Ions which are too light or too heavy are deflected. The lighter ions are deflected too much while the heavier ions are not deflected enough. The deflected ions are removed from the mass spectrometer via the vacuum pump.

• Detection!The ion detector is connected to an amplifier and a recorder. Ions with different masses are detected - these are recorded on the mass spectrum.

Sample is vaporised

Sample undergoes ionisation

Sample is accelerate

d

Sample is deflected

Sample is detected!!

!

Objective 7.2:

Explain the significance of the (M+1) peak in mass spectra

Terms:• M/Z mass to charge ratio.• M+ molecular ion peak for the molecule.• M+1 peak this is one mass unit greater

than the molecular ion peak.• M+2 peak this is two mass units greater

than the molecular ion peak.• Base peak this peak is the peak with the

relative intensity of 100%. It is the tallest peak and is caused by the most stable cation.

What is the mass spectrum?

A mass spectrum is usually represented by a graph in which each line in the graph represents an ion having a specific mass-to-charge ratio (m/z) and the length of the line indicates the relative abundance of the ion.

Significance of the (M+1) peak in the mass spectra!

• In a mass spectrum, each isotope produces a separate peak with its own mass to charge value.

• The height of the peak is proportional to the relative abundance of that isotope.

The diagram below shows the M+1 peak and the base peak in a mass spectrum.

• The M+1 peak is due to the presence of isotopes of the same molecule. For example carbon13 and hydrogen.

• The carbon-13 isotope

The M+1 peak is caused by the presence of the C isotope in the molecule. C is a stable isotope of carbon (don't confuse it with the C isotope which is radioactive. Carbon-13 makes up 1% of all carbon atoms.

• The hydrogen2 isotope

Hydrogen2 makes up 0.01% of all hydrogen atoms.

• Take for example the compound methane, CH4, if the M+1 peak was caused by the carbon atom, the mass 1 unit heavier than the molecular ion would be 1 x 1=1% of carbon 13.

• If however it was caused by the hydrogen atom, it would have been 4x0.01=0.04% of H.

• The number of carbon atoms present in a molecule could be determined by the presence of the M+1 peak.

• The main significance of the M+1 peak in the mass spectra is to distinguish between unknown molecules which have similar relative molecular masses!

Objective 7.3:

(i) Determine relative isotopic masses; and relative isotopic abundance;

(ii) Distinguish between molecules of similar relative molecular mass;

(iii)Predict possible identities of simple organic molecules based on their fragmentation pattern.

• Isotope - is an atom which contains a different number of neutrons in its nucleus than some other atom of the same element. This means that different isotopes of an element will have different masses, since both the protons and the neutrons contribute about equally to the mass of an atom.

• Relative Isotopic Mass - The mass of a particular isotope of an element, on the scale carbon-12 = 12 exactly.

• Relative isotopic abundance - The relative number of atoms of a particular isotope in a mixture of the isotopes of an element, expressed as a fraction of all the atoms of the element.

• A relative molecular mass - can be calculated easily by adding together the relative atomic masses of the constituent atoms. There are no units.

The mass spectrometer is also used to measure relative molecular masses. The molecular ions formed in the instrument can often fragment, and it is from the relative masses and abundances of these fragments that information about molecular structure can be deduced.

Example using the mass spectrum!

Chlorine consist of isotopes, naturally occurring chlorine consists of atoms of relative isotopic masses 35 (75%) and 37 (25%). Its relative atomic mass is 35.5. The relative masses of atoms are measured using an instrument called a mass spectrometer, which produces information in the form of a mass spectrum.

The diagrams below represent the mass spectrum of naturally occurring chlorine.

The above right spectrum has been represented so that the most abundant isotope has a relative abundance of 100%, with the other mass peaks scaled in relation to this. The relative atomic mass of chlorine is now calculated as shown below:

Ar = (75/100x 35) + (25/100x 37) = 35.5

Pastpaper time!

Here are 2 Module 2 pastpapers:

• 2005/Module 2/Question7

• 2008/Module 2/Question 5

Enjoy

2005/Module 2/Question7

• 2008/Module 2/Question 5

References!

• Chemistry in context• Chemistry for Cape• AS Chemistry. Carol Hibbert• www.chemguide.co.uk

For further information read Chemistry for Cape by Susan Maraj.