Interpreting Mass Spectrometry Flashcards
What is the point of using a mass spectrometer?
To find the Mr of molecules in a sample
The Mr of this hydrocarbon is 56.
Why is there another peak at 57?
There are isotopes in the compound.
E.G.
²H instead of H
¹³C instead of ¹²C
What does the y-axis and x-axis of a mass spectrometer represent?
Y-axis = relative abundance
X-axis = m/z
(m/z = mass / charge)
What is m/z?
The mass of the ion divided by the charge on the ion
How would you find the Mr or Ar of a molecule or element through interpreting a mass spectrum?
If the sample is an ELEMENT…
…it contains isotopes…
> look for the most abundant Ar
If the sample is a MOLECULE…
…it contains fragments…
> look for the HEAVIEST m/z or mass
(because the heaviest is the maximum the mass could possibly be for there to be fragments)
A sample of silicon contains 3 isotopes. Use the table below to work out the relative atomic mass of silicon.
Isotope Relative abundance
28 Si 9.2
29 Si 0.5
30 Si 0.3
(28 x 9.2) + (29 x 0.5) + (30 x 0.3) / (9.2+0.5+0.3) x 100 = 28.1
Σ relative abundance = 9.2 + 0.5 + 0.3 = 10
A sample of pure Kr was put through a mass spectrometer.
(a) Why are there multiple peaks?
(b) Sometimes the mass spectrum of Kr has a very small peak with a m/z value of 42.
Explain the occurrence of this peak.
(a) Kr has different isotopes
(b) One of the Kr ions may have lost another electron and gain a 2+ charge, so dividing the mass by 2 produces a smaller m/z value.
m / 2 < m / 1
Why is a molecular ion referred to as a free radical?
Molecular ion is the molecule with 1 electron removed
So there is 1 unpaired electron.
Then the molecule fragments.
We show this with a * (pretend that’s a floating dot)
E.G.
M ━━> [M]⁺* + e⁻
(Not technically on the spec, but comes up)
Chlorine exists as 2 isotopes, ³⁵Cl and ³⁷Cl in the ratio 3:1
What is the probability for the Cl₂ molecule to be ³⁵Cl ━ ³⁵Cl
Probability of 1st atom is ³⁵Cl = 3/4
Probability of 2nd atom is ³⁵Cl = 3/4
Probability for the molecule to be ³⁵Cl ━ ³⁵Cl = 3/4 x 3/4
Probability for the molecule to be ³⁵Cl ━ ³⁵Cl = 9/16 (ANS)
Chlorine exists as 2 isotopes, ³⁵Cl and ³⁷Cl in the ratio 3:1
What is the probability for the Cl₂ molecule to be ³⁵Cl ━ ³⁷Cl
Probability of ³⁵Cl = 3/4
Probability of ³⁷Cl = 1/4
(3/4 x 1/4) + (1/4 x 3/4) = 3/8
Why is high resolution mass spectrometry used?
To determine the molecular formula of a compound from the accurate mass of the molecular ion.
Measures the mass to 4 or 5dp
This helps differentiate between compounds that have similar (or the same) Mr to 1 dp.
A compound is thought to be either CH₃CH₂OH (ethanol) or H₂NCH₂NH₂ (diaminomethane), both have an Mr of 46.0.
The sample is found to have an accurate relative formula mass of 46.0417. Calculate the Mr of each compound to 4 decimal places and therefore the identity of the compound.
H = 1.0078
C = 12.0000
O = 15.9949
N = 14.0031
Mr CH₃CH₂OH = (12.0000 x 2) + (15.9949 x 1) + (1.0078 x 6) = 46.0417
Mr H₂NCH₂NH₂ = (12.0000 x 1) + (14.0031 x 2) + (1.0078 x 6) = 46.0530
Identity of sample is ethanol (CH₃CH₂OH)
State why the precise relative atomic mass quoted in the table for the ¹²C isotope is exactly 12.0000 [1 mark]
MARK SCHEME:
By definition [1]
The standard / reference value or isotope [1]
