Ask me anything

[kaberett]

I understand I'm not always great at explaining what I'm up to here, so -- if you have questions about how things are going for me or what I'm doing or how exactly my life fits together, please feel encouraged to ask. <3

(Today on my way home from work I saw: (1) someone busking with a tuba that belched flames every time they played a note; (2) a cavalcade of police motorcycles, blues flashing, who as far as I can tell were learning how to halt a busy junction in the rush hour. It was kind of endearing.)

Comments

[kaberett]

a;lkdjfasl;kfjsadfj MASSIVE COMPLIMENT IS MASSIVE :D :D :D

And yes, connecting things up is awesome - you're spot-on with radioactive/unstable!

1. There are several reasons we've gone for thallium! One is that it's only in the past few years that it's really become possible to detect variations in isotope ratio for such heavy elements, and nobody has yet (at all seriously) applied this to the problem that we're looking at. Another is that we know it's basically nonexistent in the ambient mantle, whereas there's a relatively enormous amount in marine sediments, so we're pretty sure *any* thallium we see in an erupted magma has to have originated as sediment, and therefore lets us work out some stuff about (1) what the magma source was and (2) mantle convection dynamics.

(2) Lots of things cause the numbers to shift! Unfortunately some of it involves quantum maths in a way that makes me pull faces. I ended up explaining this to That One Gentleman the other night, but I got to wave my hands around for that, so I will see if I can manage without the physical handwaving... ;)

What I care about is mass-dependent stable isotope fractionation. Every chemical species has a range of energy levels available to it - it can be in its ground state or lowest energy level, or it can be many different kinds of "excited" (for a molecule, think "bonds wiggling about faster" - it takes energy to wave your arms about). Where exactly the ground state is - what a molecule's minimum possible energy is - is dependent on mass (it's not zero!). The heavier isotope has a lower minimum energy, Because Of Quantum.

There are two main processes I'm interested in: equilibrium fractionation and kinetic fractionation. The next bit is easier to understand with diagrams -- here's an example of a potential energy map, which shows how you have your starting materials (reagents) at one energy level; they pass through a transition state (energy maximum); and end up as product (lower energy state).

Let's take equilibrium first: for this, you want to end up with the lowest overall energy of the system. This tends to favour the heavier isotope ending up in the lower-energy product (where the lighter isotope is more likely to end up in unreacted starting material). Equilibrium processes tend to be slow - it takes them a while to work everything out - and to dominate at lower temperatures.

At higher temperatures or in unidirectional reactions (i.e. the products are removed once made, so they can't react to form the starting material again), kinetic fractionation is the dominant process. (This is the case when you're e.g. evaporating water from the ocean - the evaporated water becomes part of atmospheric humidity and gets more-or-less taken out of contact with the surface of the ocean). In this situation, what you care about is the energy gap between starting material (reagents) and transition state - a smaller gap makes it easier to get over the hump. And atoms of and species containing the lighter isotope have a slightly higher ground-state energy, so more of them will make it over the transition-state energy-maximum to make it into product.

With thallium in particular, there's an additional weird effect going on - the Nuclear Field Shift effect - due to the fact that it's quite heavy (~205 atomic mass units), to the point where the nucleus can no longer be treated as a point charge. Instead, the atomic nucleus is big enough that it starts to interact with the orbits of the innermost electrons, which has knock-on effects for the rest of the electrons, which in turn alters how different isotopes (which show differing degrees of this effect) undergo chemical reactions - which leads to much bigger fractionations than would be expected for an element this big if this effect didn't exist. (Because most isotopes have only 1 amu difference in mass - and while this is a Big Deal proportionally when you're comparing hydrogen-1 with hydrogen-2, when you're looking at thallium-205 versus thallium-206 the relative difference in mass is much smaller - a fraction of a percent, rather than 200%, so you don't expect to see such big mass-dependent effects.)

Erm. I think I have run out of steam again, but feed me another question and I'll give you more answer... ;)