Modern Analytical Techniques I

Question 1

How does a mass spectrometer work?

Answer 1

1. Ionisation - electrons are fired at vapourised sample particles (mostly) 1+ ions
2. Acceleration - the ions are accelerated by an electric field
3. Deflection - the ions are deflected by a magnetic field
4. Detection - the ions hit the charged plate linked to a computer which processes the information into a mass spectrum

Question 2

How do you use mass spectrometry to find the Mr of the compounds?

Answer 2

• The first species formed is called the molecular ion, [M+] (parent ion)
• The signal for [M+] gives the Mr of the compound (i.e. the peak with the greatest m/z value)
• Note: common isotope (bromine 79Br:81Br = 1:1;, chlorine 35Cl:37Cl = 3:1)

Question 3

How do you use mass spectrometry to find the Mr of the compounds?

Answer 3

• The first species formed is called the molecular ion, [M+] (parent ion)
• The signal for [M+] gives the Mr of the compound (i.e. the peak with the greatest m/z value)
• Note: common isotope (bromine 79Br:81Br = 1:1;, chlorine 35Cl:37Cl = 3:1)

Question 4

Why can’t low resolution mass spectrometer find out the molecular formula of compounds?

Answer 4

We cannot be sure of the molecular formula of the compound
Often more than one compound with the same molecular formula. This means that while high resolution mass spectrometry gives the molecular formula, it may not identify the compound

Question 5

What is fragmentation:

Answer 5

The molecular ion can fragment (split up) into another positive ion, which is detected and a neutral radical which is not

Question 6

What is fragmentation:

Answer 6

The molecular ion can fragment (split up) into another positive ion, which is detected and a neutral radical which is not

Question 7

Why does fragmentation happen?

Answer 7

Because the covalent bonds have been weakened by the loss of the electron from the molecule

Question 8

Why does fragmentation happen?

Answer 8

Because the covalent bonds have been weakened by the loss of the electron from the molecule

Question 9

What are the relatively stable ions?

Answer 9

Carbocations (eg/ CH3+, CH3CH2+) and acylium ions (eg/ CH3CO+) are more common fragments in the spectrum
The more stable the ion, the greater the intensity of the peak

Question 10

What are the relatively stable ions?

Answer 10

Carbocations (eg/ CH3+, CH3CH2+) and acylium ions (eg/ CH3CO+) are more common fragments in the spectrum
The more stable the ion, the greater the intensity of the peak

Question 11

How are isotopes useful in distinguishing ions of the same mass?

Answer 11

As the mass of individual ions is measured, ions which are otherwise the same but contain different isotopes of the same elements appear at different m/z values

Question 12

What are the two main uses of IR spectroscopy:

Answer 12

1. Using the fingerprint region to identify a compound by comparing it to reference spectra in data books
2. Identify characteristic bonds (functional groups) within molecules