[Babcock_Hall]

A graduate student I know would like to determine the molar absorptivity for a new molecule.  He is planning to use a high grade of methanol as his solvent.  It is a liquid and he was planning to weigh some into a volumetric flask and to work with this solution.  I am not sure how much material he can commit to this experiment, and I am worried that weighing a small amount of liquid into a volumetric flask will not be terribly accurate.  Does anyone have any ideas for alternative methods of how to quantitate how much material is present in the solution? 

[Irlanur]

if you want to know the MOLAR absorptivity(is that the word for it?) you need to know the mass. so weigh it...? Dilutions can then be made with pipettes.

[TheUnassuming]

Seems like mass would be the most straight forward and should be quantitative as long as you are using a good analytical scale and the compound being tested is pure by NMR.  Double check the tolerances for the scale you are using and just make sure to keep well above them and you should be fine with a smaller amount.  Can he not recover the material from the MeOH afterward?
If you want to be even more accurate, have him do the mixing/measuring enough times to get the RSD to a respectable number.  If he has good hands it should only take three or four runs.

[Babcock_Hall]

Maybe I can use one or two hypotheticals to clarify what I am trying to get at.  Suppose I have just made a new, hydrophilic phosphorus-containing compound and want to know its molar absorptivity.  Perhaps its degree of hydration is unknown or variable (sodium thiophosphate would be an example).  As an alternative to mass, I could measure the amount of inorganic phosphate produced by ashing, followed by a colorimetric assay for inorganic phosphate.  I would probably be better off doing the latter than attempting to weigh it under the circumstances I suggested.

Or perhaps I have so little of a new compound that a mass is not going to be very accurate (an example from my own research would be the isolation of micrograms of a possibly unknown carotenoid).  I am wondering whether or not there are other practical ways to measure small amounts of a compound containing carbon, hydrogen, and oxygen.

RSD?

[TheUnassuming]

If its a salt than hydrates are of course a concern, but couldn't you determine degree of hydration by NMR as well (never had to deal with this problem yet)? Either way in the first example aren't you are still measuring the initial mass of unknown so you can back calculate the % of your desired compound in the unknown?  Also, once you know the composition of your unknown, won't you will still need to accurately measure the mass you put into solution for the absorptivity experiment?
If you want to measure composition (like in your ashing example) you could always send out for elemental analysis, but that won't help you I think. 
As to how you can measure the amount of a new compound you will be putting into the solution without using a scale, you sir have me stumped.   Everything I can come up with relies on already knowing constants that you won't know with a new compound.   
Ah, sorry, RSD=relative standard deviation.  The analytical chemist I TA'ed for, preferred to look at the relative standard deviation to determine how significant your average actually is.   For the quantitative analysis class as well as instrumental analysis he wouldn't even look at something above 5% .

[Irlanur]

so how much compound do you have? do you know nothing about it? I don't think general discussions make any sense in this context. Can you do Kryoskopie in water? Hydration wouldn't really affect the result and you could determine the molarity and then the molar absorptivity. You just wouldn't know the molar mass then. But then again you could just use NMR/MS

[Babcock_Hall]

If its a salt than hydrates are of course a concern, but couldn't you determine degree of hydration by NMR as well (never had to deal with this problem yet)? Either way in the first example aren't you are still measuring the initial mass of unknown so you can back calculate the % of your desired compound in the unknown?  Also, once you know the composition of your unknown, won't you will still need to accurately measure the mass you put into solution for the absorptivity experiment?
If you want to measure composition (like in your ashing example) you could always send out for elemental analysis, but that won't help you I think.   

In the hypothetical example I mentioned, one would put an amount into solution, measure the absorbance spectrum, and perform a phosphate assay.  Strictly speaking, one would not have to measure the mass, but one might do so anyway in order to get a ballpark figure.  With respect to thiophosphate (which I mentioned in a previous comment), it is offered commercially as the dodecahydrate and also as the "x" hydrate, both from Sigma-Aldrich.  I recrystallized it once from water/ethanol, but IIRC I just assumed what crystallized was also the dodecahydrate, but it was a long time ago.  [off-topic tangent] related questions pop up not that infrequently in my work, which is typically done with hydrophilic compounds.  We buy 4-nitrophenylphosphate as the hexahydrate of the disodium salt, but one preparation we purchased is impure.  One possible purification route is to recrystallize from acetone.  I doubt that what will come out is the hexahydrate. [/off-topic tangent]

[Babcock_Hall]

so how much compound do you have? do you know nothing about it? I don't think general discussions make any sense in this context. Can you do Kryoskopie in water? Hydration wouldn't really affect the result and you could determine the molarity and then the molar absorptivity. You just wouldn't know the molar mass then. But then again you could just use NMR/MS

I asked on behalf of someone in a different lab.  It is a divinely ketone containing only C, H, and O, and it lacks H-bond donors.

[TheUnassuming]

In the hypothetical example I mentioned, one would put an amount into solution, measure the absorbance spectrum, and perform a phosphate assay.

Ah! That makes sense and would avoid ever having to measure using mass.  Thank you for explaining .

[Enthalpy]

If only the investigated molecule contains phosphorus, you can measure its amount through the X-ray absorption.

X spectra show a sharp opacity increase when the photon energy suffices to eject electrons from the 1s shell, and this energy ~A²*13.6eV characterizes the element. There (thanks to NIST again)
http://physics.nist.gov/PhysRefData/XrayMassCoef/ElemTab/z15.html
it's around 20keV for phosphorus.

People use this to discriminate the amount of each element, like nitrogen and oxygen , or iron, in luggage. The source emits a range or X-rays, screens of the sought element and the previous atomic number are swapped on the path to achieve different spectra, and the detector tells how the unknown item attenuates both.

The selectivity isn't brilliant, but after calibration by known materials, software tells the proportion of the element - possibly many elements at once. This is also done on complete pictures, where software can raise alarm on suspicious elemental proportions.

Hum, possibly overkill for your need...

[Babcock_Hall]

Enthalpy,

That might be very useful for a project of mine, which does involve phosphorus.  Thank you.

[Corribus]

I'm not sure I understand what the problem is. If you know the molecular weight, you weigh out the sample, dissolve in a known volume of solvent, take an absorption spectrum. You'll have to finesse the concentration range, but that's straightforward. If your available mass is small, your problem is in getting an accurate value for the mass, and sensitivity of your spectroscopic equipment. The former problem can be solved by using a very accurate balance.  At some point the amount of mass becomes so small that an accurate mass isn't so much a problem as does the sensitivity of your spectrometer. You can purchase cuvettes that hold a very small volume of solution, though (as little as 10-20 microliter), which will allow you to amp up the concentration considerably. Keep in mind that for good molar absorptivity determinations you need to acquire spectra at several different concentrations and do a linear regression.

If you don't know the molecular weight, your problem has nothing to do with the molar absorptivity measurement itself. Your problem is finding a molecular weight of your unknown. There are a lot of ways to go about this, depending on the nature of the compound and how much you know about it. If you know the basic molecular weight of your unknown but it is contaminated or contains hydrate or some such, TGA may be an easy way to determine the % by mass that your sample is the actual active substance you are trying to measure. Hydrates are often very easy to process using this technique, particularly if it's an inorganic hydrate.

[Babcock_Hall]

The problem is the relatively small mass of the liquid sample being weighed.  The student made what I consider a reasonable compromise.  He weighed out 30 mg into a small vial, then quantitatively transferred into a 100 mL volumetric flask.  The vial weighed much less than the flask, which presumably reduced the error in weighing.

With respect to another type of problem that I mentioned, which is having sub milligram quantities of a carotenoid, the molar absorptivities of carotenoids tend to be greater than 100,000; therefore, they are relatively easy to detect.  However, I don't see an obvious solution to the problem of needing to weigh out microgram quantities accurately.

[Corribus]

Sub-mg masses is certainly a challenge. Using a standard balance is going to be tricky. You might have to be creative. QCM can accurately weigh out samples like that I believe (never used one). I could conjure up a way you might be able to do it with TGA, which also uses a very sensitive balance. Are your molecules fluorescent? If so, an indirect method of determining molar concentration may be by using quenching experiments - a kind of Stern-Volmer experiment. Usually this is done by varying the amount of quencher, but it may be possible to vary the amount of fluorophore instead and back-determine the concentration. I'd have to think about that; maybe it isn't possible. There may also be some kind of reaction chemistry, monitored through UV-Vis, that can help you determine concentration... but nothing is jumping out at me, I admit.

[Enthalpy]

A salt containing the phosphorus, dissolved in deionized water, would drop the resistivity even in tiny amount, and makes an easy measure.

What I ignore but someone here may know: how to convert a compound of phosphorus into a known phosphor-containing salt and keep all the phosphorus in the operation but introduce no other conductive ion in unknown amount.