mass spec isotopes (1 Viewer)

[indeed]

These are some isotopes for mass spec, do you guys think its worth memorising the highlighted ones or all or ?



Thanks!

 

[synthesisFR]

These are some isotopes for mass spec, do you guys think its worth memorising the highlighted ones or all or ?

View attachment 40085

Thanks!

yes

 

[Luukas.2]

Definitely need to be aware that ³⁵Cl : ³⁷Cl is approximately 3 : 1 and that ⁷⁹Br : ⁸¹Br is approximately 1:1.

 

[indeed]

Definitely need to be aware that ³⁵Cl : ³⁷Cl is approximately 3 : 1 and that ⁷⁹Br : ⁸¹Br is approximately 1:1.

So assume most Cl is 35, but occasionally 37?
And Br can be either?

 

[synthesisFR]

So assume most Cl is 35, but occasionally 37?
And Br can be either?

No basically on a mass spec if a compound contains those halogens would have to two peaks if the halogens r included in that ratio. so like the molecular ion peak for would have two peaks in a 3:1 ratio telling u it’s chlorine

 

[indeed]

No basically on a mass spec if a compound contains those halogens would have to two peaks if the halogens r included in that ratio. so like the molecular ion peak for would have two peaks in a 3:1 ratio telling u it’s chlorine

Oh ok thanks

 

[wizzkids]

Regarding H, C and O. Look at those values for abundance. There is only one dominant isotope, the other isotope is totally insignificant for the mass spectrum, so no need to memorise those ones.
Regarding those other elements, as part of your general knowledge in Chemistry, you should be aware of the natural isotopic mixture present in boron, magnesium, potassium, chlorine, bromine and zinc. Also probably uranium too, because of its technological significance for atomic power.
How do you know which elements are isotopic mixtures?
Go look up the Periodic Table. Any element that has an atomic mass that is significantly different to a whole number is probably an isotopic mixture. For example, the A-number of potassium is 39.10. It is a mixture of isotopes 39 (93.3%) and 41 (6.7%) with a tiny bit of 40 thrown in.

 

[Luukas.2]

For Module 8 spectroscopy, there are only three or four isotopic distributions that matter:

• Cl and Br as 3:1 and 1:1, as noted above
• C being mostly ¹²C but some ¹³C matters for
  • mass spec - to explain the peaks at m / z = M + 1, where M is the molar mass of the parent ion
  • ¹³C NMR - to explain the absence of splitting that is observed in ¹H NMR
• Related to the last point: ¹H being nearly, but not actually, 100% abundant matters for
  • explaining why there is splitting in proton NMR spectra
  • explaining why we can get deuterated solvents that lack ¹H atoms to dissolve samples without drowning out the spectrum of the compound of interest... for example, using D₂O, CD₂Cl₂, CDCl₃, or C₆D₆ as solvents.

Questions relating to these are in decreasing frequency in the above list.