Time of Flight Mass Spectrometer

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This instrument is a method of analysis.  It is used:

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• to find the masses and abundance of isotopes of an element in order to determine the relative atomic mass

• determine the relative moleculer mass of a molecule

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There are 4 important stages to this process:

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• ​ionisation

• acceleration

• flight tube

• detection

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Ionisation can be achieved in 2 ways:

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Electrospray ionisation

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This technique is useful for large organic molecules as there is very little chance of fragmentation.  The sample is first dissolved in a polar volatile liquid that provides protons and allows ionisation.  The mixture is passed through a hyperdermic where a high voltage is applied to the needle causing the particles to gain a proton.  The mass to charge ratio for a molecule that has become protonated is:

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m/z = Mr + 1    the process can be represented as      \(X(g) + H^ +\)            \(XH^+(g)\) 

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Electron impact (Electron ionisation)

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The sample is first vapourised then passed into the mass spectrometer where electrons are fired at the particles using an electron gun (filament that current is passed through releasing electrons accelerated by a p.d.).  This normally knocks one electron from each particle to form a 1+ ion as shown below:

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\(X(g)\)            \(X^+(g) + e^-\)

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This technique is used for low formula mass molecules.

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Acceleration

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The positive ions produced are then accelerated using an electric field so that they all have the same kinetic energy.

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\(KE = \frac{1}{2}mv^2\)  so  \(v = \sqrt{\frac{2KE}{m}}\)

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Flight tube

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The positive ions pass into the flight tube through a small hole in the negatively charged plate.  As they are given the same energy the time taken for particles to travel through the tube will be dependent on their mass. The lower the mass the greater will be their velocity so the lower the time taken to pass across the flight tube.

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Detection

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When the positive ions reach the end of the flight tube they will hit a detector and be discharged by gaining electrons from the plate which causes a small current to flow.  The size of current is a measure of the number of ions hitting the plate per unit time so is proportional to the abundance of the ion.

                                                                           Time of flight mass spectrometer summary

                                      sample: chlorine \(Cl_2\)                                                                            mass spectrum

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1. Ionisation using electron impact method where an electron gun made up of a filament releases electrons when a current is passed through it.  The electrons are accelerated by a postive plate and collide with the sample particles causing ionisation.

2. Acceleration using a negatively charged plate makes the positive ions enter the flight tube,

3. Lighter particles will move faster and reach the end of the flight tube quicker.  In this case chlorine 35 ions being lighter than chlorine 37 reach the detector first.  

4. The greater number of chlorine 35 ions being discharged at the detector per unit time produces a greater current and so a higher % abundance on the graph and the lower time of flight produces a smaller m/z given as 35 on the graph.

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Calculating relative atomic mass given the weighted means of isotopes

 

The following formula can be used to calculate relative atomic mass given the abundance and relative isotopic masses of each isotope:

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                       \(relative \ atomic \ mass \ Ar =\) \(\frac{combined \ mass \ of \ all \ isotopes}{combined \ abundance \ of \ isotopes}\)

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Exercise 1

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1. State the meaning of the word isotope.

2. There are two methods of ionisation used in the mass spectrometer, briefly describe these two methods and the type of molecule suitable for:                                                                                                                                                                                          a) electron impact                                                                                                                                                                              b) electrospray ionisation

3. When particles are accelerated in the mass spectrometer why do different isotopes have different speeds in the flight tube?

4. Exlain how a small current is produced when particles reach the detector in a mass spectrometer.

5. For the mass spectrum of chlorine shown above, what produces the peaks at                                                                                         a) m/z 72                                                                                                                                                                                           b) m/z 74

6. Write down the formula for calculating relative atomic mass.

7. Calculate the relative atomic mass of the Boron isotopes B-10 and B-11 if their abundances are 19.8% and 80.2% respectively.

8. Calculate the relative atomic mass of Silicon having isotopes Si-28, Si-29 and Si-30 with relative abundances of 92.21%, 4.70% amd 3.09% respectively.

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