The mass spectrum for zirconium
The number of isotopes
The 5 peaks in the mass spectrum shows that there are 5 isotopes of zirconium - with relative isotopic masses of 90, 91, 92, 94 and
96 on the 12C scale.
The abundance of the isotopes
This time, the relative abundances are given as percentages. Again you can find these relative abundances by measuring the lines
on the stick diagram.
In this case, the 5 isotopes (with their relative percentage abundances) are:
zirconium-90 51.5
zirconium-91 11.2
zirconium-92 17.1
zirconium-94 17.4
zirconium-96 2.8
Working out the relative atomic mass
Suppose you had 100 typical atoms of zirconium. 51.5 of these would be 90Zr, 11.2 would be 91Zr and so on.
The total mass of these 100 typical atoms would be
(51.5 x 90) + (11.2 x 91) + (17.1 x 92) + (17.4 x 94) + (2.8 x 96) = 9131.8
The average mass of these 100 atoms would be 9131.8 / 100 = 91.3 (to 3 significant figures).
91.3 is the relative atomic mass of zirconium.
http://www.chemguide.co.uk/analysis/masspec/elements.html
The mass spectrum of chlorine
Chlorine is taken as typical of elements with more than one atom per molecule. We'll look at its mass spectrum to show the sort of
problems involved.
Chlorine has two isotopes, 35Cl and 37Cl, in the approximate ratio of 3 atoms of
35
Cl to 1 atom of 37Cl. You might suppose that the mass spectrum would look like
this:
You would be wrong!
The problem is that chlorine consists of molecules, not individual atoms. When
chlorine is passed into the ionisation chamber, an electron is knocked off the
+
molecule to give a molecular ion, Cl . These ions won't be particularly stable,
2
and some will fall apart to give a chlorine atom and a Cl+ ion. The term for this is fragmentation.
If the Cl atom formed isn't then ionised in the ionisation chamber, it simply gets lost in the machine - neither accelerated nor
deflected.
The Cl+ ions will pass through the machine and will give lines at 35 and 37, depending on the isotope and you would get exactly the
+
pattern in the last diagram. The problem is that you will also record lines for the unfragmented Cl ions.
2
+
Think about the possible combinations of chlorine-35 and chlorine-37 atoms in a Cl ion.
2
Both atoms could be 35Cl, both atoms could be 37Cl, or you could have one of each sort. That would give you total masses of the
+
Cl ion of:
2
35 + 35 = 70 35 + 37 = 72 37 + 37 = 74
That means that you would get a set of lines in the m/z = 70 region looking like this:
These lines would be in addition to the lines at 35 and 37.
The relative heights of the 70, 72 and 74 lines are in the ratio 9:6:1. If you know the
right bit of maths, it's very easy to show this. If not, don't worry. Just remember that
the ratio is 9:6:1.
What you can't do is make any predictions about the relative heights of the lines at
35/37 compared with those at 70/72/74. That depends on what proportion of the
molecular ions break up into fragments. That's why you've got the chlorine mass spectrum in two separate bits so far. You must
realise that the vertical scale in the diagrams of the two parts of the spectrum isn't the same.
The overall mass spectrum looks like this:
http://www.chemguide.co.uk/analysis/masspec/elements.html