Mass Spectrometry

Question 1

Mass Spectrometry Def

Answer 1

Measurement of charge to mass ratio of gaseous molecules. Measures molecular mass as charge due to iniosation is assumed to be +1

Question 2

Molecular Mass Def

Answer 2

Sum of all atomic weights in a molecule

Question 3

Fragments Def

Answer 3

Ionised molecules are unstable. They break apart into smaller molecules (radical cations, cations and radicals). Detected as cations

Question 4

Radical Cation (M .+)

Answer 4

Particles with properties of radicals (1 or more unpaired electrons) and cations (positive charge). Break down into radicals and catons. Only cations are seperated in magnetic field. Radicals removed by vacuum pump

Question 5

Steps of Mass Spectrometry

Answer 5

Vapourisation, Ionisation, acceleration, separation and detection

Question 6

Vapourisation Outline

Answer 6

Vacumn pump converts substance to gaseous form by decreasing pressure

Question 7

Ionisation Outline

Answer 7

Electrons are knocked off atoms by radiation

Question 8

Acceleration Outline

Answer 8

Ions are given the same kinetic energy

Question 9

Seperation Outline

Answer 9

Passing of an ion through magnetic field. Sperated due to masses when passing through bend. Ions that are too light/too heavy are deflected at angles too big/small to make it through bend. Varying magnetic field strength = varying mass of ions collected. Record of mass and no. of particles of that mass

Question 10

Detection Outline

Answer 10

Charged particles are detected by a chart recorder and an amplifier. Detector signal is proportional to number of molecules hitting it

Question 11

Mass Spectrum Plot Def

Answer 11

Graph showing atomic mass of Fragments (x-axis) and their relative abundance (y-axis). Base peak = the most abundant fragment (defined at 100% relative abundance)

Question 12

Characteristics of most abundant fragment

Answer 12

Most stable carbocation (if unstable it will keep fragmenting). Eg tertiary carbocations (positive C attached to 3 other Cs) > secondary carbocations > primary

Question 13

Acylium Ions

Answer 13

Ketones with a positive charge. Relatively stable and thus form easily

Question 14

What do you do when asked to give difference in frequency of certain molecules

Answer 14

Estimate ratio based on visual inspection of line

Question 15

Relationship between halogen presence, molecular mass and frequency

Answer 15

The higher the halogen presence = more weight given to fragment = lower frequency

Question 16

Indication of 1 halogen atom

Answer 16

2 peaks 2 molecular mass units apart. Peaks being in a ratio 3:1 (in favour of molecule of less weight)

Question 17

Indication of 2 halogen atoms

Answer 17

3 peaks, 2 molecular mass units apart. Peak frequencies in ratio 9:6:3 (in favour of molecules of least weight)