[DontStealMyBacon]

What types of spectroscopy could be used to detect the difference between two oxidation states of the same element? I know that I could use a ligand that binds to the element and turns a color that is dependent on the oxidation state and use spectrophotometry to measure the colors, but using a ligand does not perfectly fit what I want to do. What other methods of spectroscopy could I choose from?

[Corribus]

More information on what you're doing, and especially what metals you are using, would definitely be helpful. For instance, Mossbauer spectroscopy is very useful for this kind of thing, but is only applicable to a few metals and certain sample formulations. There are also a number of non-spectrometric methods that have utility, such as magnetic susceptibility measurements or voltammetry, that could provide the information you need.

Without more detail, the question is impossible to answer in anything but the most general terms.

[DontStealMyBacon]

I am dealing with the copper stress response system inside E. coli. I plan on running many trials with different genetically engineered strains and measuring the extracellular concentration of copper 1+ and 2+ ions. I am able to kill off cells in a container at the end point of a trial, but if I want to take concentration measurements at multiple time points, I will need a method of spectroscopy that does not kill my cells, and I can't drain all of the fluid away from my cells in the middle of a trial. Is that enough information? I could give more if needed.

[Corribus]

Real-time, nondestructive measurement of metal ion oxidation state, presumably at low concentrations, and especially in a complex solution, is going to be a difficult challenge.

Some kind of X-ray photoemission spectroscopy may be an opinion. In principle you should be able to distinguish between metal oxidation states of copper.  (E.g.: http://pubs.rsc.org/en/Content/ArticleLanding/2013/TC/C3TC00639E#!divAbstract). The technique is used mostly for solids but discrimination of metal ions in solution has been reported.

http://www.sciencedirect.com/science/article/pii/S0301010410004052#

I have little practical experience in the technique so I'm not sure if it can be used in your system. Also, X-ray spectroscopy instrumentation is not widely available, so that may be an issue.

Maybe there is a better way. If I think of anything, I'll post it here.

[Borek]

Why do you want it to be spectroscopy? Perhaps some potentiometric method would work?

[DontStealMyBacon]

Why do you want it to be spectroscopy? Perhaps some potentiometric method would work?

Spectroscopy is the word that comes to mind, but more broadly I am interested in measuring the concentration of aqueous copper in terms of individual oxidation states. They will likely either be in the form of copper sulfates or copper chlorides.

[Corribus]

Potentiometric methods could certainly work, but they're not necessarily nondestructive.

[kriggy]

Any particular reason you cant take like 0,5 mL of your sample and do the experiment? (Even that mossbauer is best for what you want to measure, I dont think it works in solution so its IMO no go for you).

What about EPR? Cu²⁺ has one unpaired electron

[Borek]

Potentiometric methods could certainly work, but they're not necessarily nondestructive.

Same can be said about spectroscopic methods - it is a matter of selecting a correct one.

[Corribus]

I think running a sample through a range of voltages is far more likely to be irreversibly destructive to other components in the sample than most spectroscopic methods. Of course, irreversible photochemistry can happen, but it can happen sitting out on a sample bench as well. No measurement method is truly sample-neutral. But my opinion based on many years doing both types of measurements is that electrochemistry samples generally cannot be reused for other experiments but (many) spectroscopy samples can. If nothing else, voltametric methods usually require addition of a considerable quantity of additional electrolytes directly into the sample to increase conductivity of the sample medium, which may kill off his cells or otherwise interfere with other measurements or processes he desires to observe.

[Borek]

I think running a sample through a range of voltages

Are you sure you don't mistake potentiometry for voltammetry?

If nothing else, voltametric methods usually require addition of a considerable quantity of additional electrolytes directly into the sample to increase conductivity of the sample medium, which may kill off his cells or otherwise interfere with other measurements or processes he desires to observe.

In typical approach, yes, but if you use microelectrodes (nm diameter range) currents are so low supporting electrolyte can be not necessary. But I think this is off topic, as in potentiometry we don't need additional electrolytes at all.

[Corribus]

Yes you are right, I misread. My apologies.