Mass spectrometry - WM

Question 1

Using the picture, how many isotopes are in this sample. And calculate the relative atomic mass.

(https://www.chemguide.co.uk/analysis/masspec/zrmasspec.GIF)

Answer 1

5 isotopes

(9051.5)+(9111.2)+(9217.1)+(9417.4)+(96*2.8)/100 = 91.3

Question 2

What does a mass spectrum of an element show

Answer 2

the different isotopes and their abundances in which you can calculate the Ar

Question 3

What happens during mass spectrometry

Answer 3

the sample particles are ionised (an electron is lost making positive ions). If those particles were molecules then the molecular ion is given. Once produced, some of these molecular ions travel through the rest of the machine intact and give a peak. But some break into fragments and the positively charged fragments also travel through the machine and give several peaks.

Question 4

What are the peaks called if the positively charge fragments travel through

Answer 4

fragmentation pattern

Question 5

What does the mass spectrometry graph look like

Answer 5

several peaks spread across a large range of m/z values

Question 6

m/z values definition

Answer 6

mass to charge ratio. But almost always the charge of each particle recorded will be +1 as just one electron is lost in the ionisation, so m/z usually gives us the mr for each one

Question 7

What are the features of a mass spectrum of a molecule (M⁺)

Answer 7

1. the molecular ion peak, M⁺. Biggest m/z value. M/z of this peak = mr of the compound

Question 8

What are the features of a mass spectrum of a molecule (fragmentation pattern)

Answer 8

• anything giving a peak is positive
• each fragment is given by 1 bond in the molecule breaking
• taller peaks are given by more stable fragments
• formula of species giving a fragment peak needs a + charge
• sometimes easier to work out the formula of species lost from the molecule (will be neutral)

Question 9

Common fragment peaks

Answer 9

15 = CH₃⁺
29 = C₂H₅⁺ or HC=O⁺
77 = C₆H₅⁺ (benzene)

Question 10

Why will there sometimes be a very small peak to the right of the M⁺ peak

Answer 10

most carbon atoms are ¹²C but about 1% are ¹³C. If one of those is present in the molecule in place of one ¹²C it will give a molecular ion peak one place further right (M+1)