Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: upforair on September 24, 2012, 11:49:19 AM
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I isolated a product by column chromatography and I have some impure fractions that contain the product but also other junk. Rather than repurifying these small fractions, is there a way to determine how much yield (mass) of the desired product is in the impure fractions? I'm trying to get the best possible isolated yield possible to report. Thanks.
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Yes there is a way. But if you want to report isolated yields then you will have to re-purify them. Otherwise any yield you quote including these fractions is not correct and not-reproducible.
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Thanks, I will repurify then but I'd like to also know how to quantify it, say in general. Can you tell me how it's typically done?
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You can only calculate the yield of a mixture if you know the molecular weight of each component and their ratio. However, even if you do know what the impurities are, you cannot report an isolated yield from this calculation. For an isolated yield, you must isolate the compound.
You should purify the remaining mixture if you want that mass to contribute to the isolated yield.
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Let me rephrase this...I understand I cannot use this calculated yield as an isolated yield (I appreciate to all the comments on clarifying that) but I'm trying to learn how you can estimate the amount in impure fractions (NOT for isolated yields, just in general). That's what I'm trying to figure out here. Can someone enlighten me on this, instead of telling me it can be done but not mentioning how it's done?
Do you add a known amount of a standard or something? How do you go about the calcuation, if I know the MW of the desired compound in the mixture but not the impurities?
Thanks again. I hope someone can help me out here.
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Can someone enlighten me on this, instead of telling me it can be done but not mentioning how it's done?
If you have a mixture of compound A (MW 100) and compound B (MW 150), and you know from NMR integrals that they are in a molar ratio A:B of 1:1, and you know their combined mass is 250 mg, then it is quite straightforward to calculate the mass fractions. Try it.
Do you add a known amount of a standard or something? How do you go about the calcuation, if I know the MW of the desired compound in the mixture but not the impurities?
Yes, I guess you could add a known amount of a standard and get the molar ratio of the standard and you compound from the NMR integrals to calculate the yield.
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Can someone enlighten me on this, instead of telling me it can be done but not mentioning how it's done?
If you have a mixture of compound A (MW 100) and compound B (MW 150), and you know from NMR integrals that they are in a molar ratio A:B of 1:1, and you know their combined mass is 250 mg, then it is quite straightforward to calculate the mass fractions. Try it.
Do you add a known amount of a standard or something? How do you go about the calcuation, if I know the MW of the desired compound in the mixture but not the impurities?
I always used bromoform when a reference standard was necessary. Gives a nice singlet and is easy to measure (also high MW).
Yes, I guess you could add a known amount of a standard and get the molar ratio of the standard and you compound from the NMR integrals to calculate the yield.
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Typically, I get the numbers like Dan does, from the NMR integration. Especially if the chromatography is separating two compounds, I know the identity of both compounds, and I have mixed fractions. No, the numbers don't make for good publications, but when you're just trying to figure out how well the reaction worked and if there is enough product in your mixed fractions to make it worth chasing, it's worth the effort.
If you truly want good accurate numbers, you can do it by running standard curves on an HPLC or GC for your pure compound, then testing your impure compound. The same way you would be quantifying a compound in a solution of unknown concentration. Make sure the concentrations of your standard samples are on both sides of the concentration of your mixture so you can interpolate instead of extrapolate from the curve.
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Thanks for the replies. I don't know the identity or MW of the impurities though.
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If you are trying to calculate the yield, you only need the product. As noted by fledarmus, if you add a weighed standard to a weighed product, you can determine the ratio, hence the amount of product in a sample. In the case of NMR, you may not need to adjust for differential responses of the samples, but the integration may not be as precise. With GC, equal moles or weights can give quite different responses. However, if the samples had been weighed, then you can calculate the ratio of the responses and can calculate the amount of product from the amount of standard you added.
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To clarify the use of GC or HPLC - run samples of your pure product at, for example, 10, 20, 30, 40, and 50 μg/ml. Plot the integration of your product peak against the concentration. Then make a solution of 50 μg/ml of your mixed fraction product and run that. The integration of your peak compared to the standard plot you just made will tell you the amount (by weight) of desired product in your mixed fraction. For this method, it doesn't matter what the impurities are, as long as they don't elute at the same time as your desired product.