plastic digestion for Cr6 analysis?

[Fat Jonny]

I have some plastic powder samples that I need to analyse for Cr6 by HPLC with ICPMS detection. Usually we only run soil samples (alkaline digestion) or water samples (dilute and shoot).

I tried the alkaline digestion method but this seemed to do nothing to the samples.

How do I get this stuff into solution? Is there another standard method that applies? What about solvent extraction?

Any suggestions would be appreciated!

[Mark Tracy]

I think you are posting in the wrong forum, but I'll take a hack at it anyway. I used to run an ICP for heavy metals and selenium analysis... First, Cr is not volatile so you don't need to be gentle. I would first try a dry-ashing with Mg(NO3)2 in a muffle furnace; then redissolve in HNO3. This should vaporize the plastic and oxidize everything else. You might also try molten NaOH/Na2O2. We used to do total destruction of the organic matrix by HNO3/HClO4/H2SO4 digestions, but that is pretty extreme, and requires a special fume hood.

[Fat Jonny]

I know this isn't quite the right forum, but I'm hoping somebody might know

The only issue I am worried about is conversion of species. We've already run these samples for a full-scan analysis by conventional ICPMS. This was just a standard microwave digestion with nitric acid, but won't this result in oxidation/reduction of Cr3 and Cr6?

I guess we could spike with both then microwave digest. If recoveries look OK that would show there was no interconversion during digestion...

[Mark Tracy]

I missed the point about speciating the Cr. This will be genuinely hard. Most of the means of dissolving or destroying the polymer will cause some redox chemistry with the Cr(VI) because they use extremes of acid, base or temperature. Do you know what the polymer is? Some can be volatilized easily, some not at all. Some dissolve in organic solvents, some don't.

[Fat Jonny]

It's a black polyester resin at very small particle size. I tried adding solvent (hexane, acteonitrile, acetone) and then blowing it off. All solvents seemed to dissolve the powder easily (at least it formed a consistent solution), but after reconstituting with water or mobile phase, it forms small black clumps again!

I'm stumped.... don't you hate it when management accepts "routine" samples without consultation!!!

[Mark Tracy]

I used to work in a veterinary toxicology lab. We got anything and everything, no matter how long it had been dead.

Polyester will withstand dilute aqueous acids or alkalis, but you should be able to depolymerize it in molten 85% KOH in a Teflon capsule. Keep the exposure time short; a couple minutes ought to be enough. Chromate is not an aggressive oxidizer under basic conditions, but the hydrolyzed ester will have oxidizable alcohol groups. Your sample probably has carbon black in it, so be sure to filter it with a 0.2µ membrane filter.

Unfortunately, without reference materials, you will be flying blind when it comes to %recovery. Make sure your managers understand that, and insist that they tell the customer.

[hookeslaw]

years ago i used to do extractions on plastics ( mainly poly carbonates) using DCM or hybrid mixes of hexane / methanol / acetonitrile etc . Accelerated solvent extraction was also used for particulaly hard plastics. the main thing we used to isolate were organic extractables ( antioxidants etc)

i wonder if extracting in a mix of DCM / methanol followed by a liquid / liquid extraxtion (via seperating funnel) would work? i am guessing that the chromium is complexed by organic ligands?

extraction efficiencies can be determined on standard solutions etc so it should be possible to calculate fairly accurately the concentration.

just a thought