Tuesday, December 21, 2010

Uncertainty in Polymer Molecular Weights to Arrive in 2011

Not content to let 2011 arrive in peace and serenity, IUPAC is at it again, in this case changing the atomic weights of atoms from single values to ranges. The current single values are averages of the known natural distributions (Cl being probably the most famous, with the current single value of 35.453 being an average of 35Cl and 37Cl, the former isotope being about 3x more common than the latter.)
"According to Michael Berglund, a member of Iupac's Commission on Isotopic Abundances and Atomic Weights, which proposed the change, it is purely because more reliable and precise measurement data are now available. He points out that in its previous form, the standard atomic weight for an element like lithium was just an average value. 'It would actually be very difficult to find a material with this exact atomic weight,' he says"
Certainly a valid point.

I see potentially two issues down the road: 1) this is going to be extremely difficult for students (how can they calculate the MW of a molecule when the atomic values have a range? [*]), and 2) for our little world of polymers, the changes could be quite large simply because our molecules don't have a single digit quantities of any one atoms, but have immense amounts of them. As a somewhat extreme example, consider ultrahigh molecular weight PE. If I recall correctly, the molecular weight is about 4 million, meaning that there are about 290,000 carbon atoms and over half a million hydrogen atoms per molecule (4,000,000/14, the molecular weight of the methylene repeat unit). Changing the atomic mass value of either C or H by even as little as 1% will still result in a noticeable change in the molecular weight. End group analysis is going to be a lot more challenging.

And to really delight us, the changes are coming quickly:
The first ten elements for which atomic weights will be stated as intervals in the Table of Standard Atomic Weights are: hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium.
i.e., Let's try this out on the organikers first!

[*] And of course, we should always rearrange chemistry to please the students, right? But seriously, I'm just not sure how high school teachers will handle this.

1 comment:

Notmedchem said...

Wait...Are they trying to destroy the field of exact mass spectrometry. I've got a $175k instrument right here that gives me an exact mass differences.

In practice, we'd still use the Nominal average masses for calculations and structure confirmation.