Thursday, October 18, 2012

Column chemistry

It's been a productive week in the lab.  On Monday, I observed the Nu Plasma 54 mass spectrometer in action, on Wednesday I ran the second round of Column "0", and today I ran Column "1".  Running Column "0" twice is necessary for samples >300 mg (most of my samples are around 600 mg), due to the limited capacity of the column.  Columns "1" and "2" don't have to be run twice, though, which is nice.  So here's the cartoon again, updated:

After running Column "0" twice, we have collected all the Hf separate (which goes on to Columns "1" and "2" to purify Hf) and the REE separate (which goes on to extraction chromatography to separate individual REE, to be done later, after Hf chemistry).  Of course, the "Hf separate" obviously doesn't just contain Hf (or else we would be done!), but contains Ti (which we need to eradicate via Column "2"), Zr (which we fortuitously don't need to worry about because there isn't a mass interference), and some other elements, such as W (which we do need to worry about, as W 180 interferes on Hf 180, even more important because our spike is Hf 180), and Ta (and probably Nb).  After running Column "1", all we will be left with is Hf, Zr, and Ti.  Column "2" will then take care of Ti, leaving us with just Hf and Zr (and I just told you Zr is OK to have together with Hf).

In contrast to Column "0", Column "1" is an anion-exchange column.  This means that anions (negatively charged ions) will adhere ("stick") onto the resin.  We first take up the sample in 1 ml of 0.5 M HF:0.5 M HCl, do the necessary heating, ultrasonicating, and centrifuging, then load the sample onto the resin.  Next, we elute with this same acid combination (0.5 M HF:0.5 M HCl).  Note this is a pretty dilute acid.  In such a dilute acid, Hf will complex with the F- ion in HF, forming an anion complex that will stick to our resin.  However, all the other elements will fortunately not complex with F-, and will thus be eluted.  So how do we get that Hf (and Zr and Ti) out?  We next elute with a stronger acid, 6 M HCl, which is strong enough to strip away the complexed Hf on the resin.  This is what we collect at the final step, and which will go on to the final column (Column "2") to remove Ti leaving behind just Hf and Zr.

Here's a picture of Column "1".  It's a smaller column that "0", but with a wider resin bed diameter, meaning that care must be taken while loading sample and acids in order to not disturb the resin.

Unfortunately no cool colored band representing an element within the resin in this one.

After we've collected our Hf, Zr, and Ti in clean teflon beakers, the next step is to add ~10 drops (1 drop ~ 30 ul) of perchloric acid, and then we evaporate on a hot plate, but NOT to complete dryness.  If everything gets evaporated, an insoluble precipitate will form and you'll have to re-dissolve it all over again using HF.  We don't want HF on the next column (Column "2"), because if we had HF on this column, it will just strip all the Hf away.  So we are faced with the delicate task of keeping Hf in solution, which means not forming a precipitate, before we can run the next column.  This is done by evaporating with perchloric and leaving a small drop behind in the beaker.  If a precipitate does form (or even if it doesn't and you want to pre-empt one from forming), add 2 drops of HF to keep Hf in solution.  Then when ready to run Column "2", evaporate a little to remove as much of that HF as possible.

Next week, I'll run Column "3".  Till then. 

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