1.1 Atomic Structure - Mass Spectrometry

Question 1

When is electron impact used?

Answer 1

For elements and low Mr compounds

Question 2

Electron impact ionisation:

Answer 2

The sample is dissolved and vaporised; high energy electrons are fired at the sample from an electron gun; this knocks off one electron from each atom/molecule to form a 1+ ion - X(g) -> X+(g) + e-

Question 3

Electrospray ionisation:

Answer 3

The sample is dissolved in a volatile solvent (e.g methanol, water) and injected through a fine hypodermic needle as a fine spray into a vacuum in ionisation chamber; very high voltage applied to end of needle where spray emerges (needle is positive); particles gain a proton and become ions as a fine mist; solvent evaporates leaving 1+ ions
X(g) + H+(g) -> XH+(g)

Question 4

When is electrospray used?

Answer 4

For high Mr compounds (e.g. proteins)

Question 5

Ionisation stage:

Answer 5

Electron impact or electrospray

Question 6

Acceleration stage:

Answer 6

Positively charged ions are accelerated using an electric field so that all ions have the same kinetic energy - this means the lighter ions move faster than the heavier ones
KE=1/2 x m x v^2

Question 7

Separation if charged ions (ion drift) stage:

Answer 7

The ions enter the flight tube without an electric field so they just drift through it; lighter ions have a quicker time of flight

Question 8

Detection stage:

Answer 8

Detector is a negatively charged plate - a current is produced when the ions hit the plate - the more ions that hit the plate the greater the current; the charged ions can be detected and mass can be calculated using the time of flight

Question 9

y-axis of mass spectrum:

Answer 9

The relative atomic abundance of ions (often as a percentage); also referred to as intensity

Question 10

x-axis of mass spectrum:

Answer 10

‘Mass/charge’ ratio - m/z

Question 11

Electron impact ions on a mass spectrum:

Answer 11

One electron has been knocked off each particle to turn them into 1+ ions so the mass/charge ratio remains the same as that of the relative isotope

Question 12

Electrospray ions on a mass spectrum:

Answer 12

An H+ ion has been added to each particle to form 1+ ions so the mass/charge ratio of each peak would be one unit greater than the relative mass of each isotope

Question 13

Calculating relative atomic mass from mass spectrum:

Answer 13

sum (abundance x relative isotopic mass) / 100

Question 14

Identifying elements/molecules from mass spectrum:

Answer 14

Elements with isotopes will produce multiple peaks due to their varying masses which will produce characteristic patterns specific to each element or molecule.

Question 15

2+ ions formed during ionisation:

Answer 15

2+ charged ion may be produced as second electron knocked off in process
This means that it will be more affected by the magnetic field producing a curved path of smaller radius
This means its mass to charge ratio is halved so expected m/z value on graph is halved

Question 16

Ar calculation:

Answer 16

m/z x abundance / total abundance