Lactic acid standards?

[Karen01]

I have never worked with lactic acid before and I need to make calibration curve for it in aqueous solution for LC (H2SO4 mobile phase) over a broad range going to maybe 50 g/L...

I know it has tendency to from dimers and trimers... Would it be better to to buy a 1 N aqueous solution (~90 g/L), Pure lactic acid, or some salt?

Obviously diluting a 1 N solution would be easiest but is it stable enough to buy it that way (if I store it refrigerated)?

- Karen

[KM-USA]

We use sodium lactate as our standard, then dilute and neutralize with H2SO4. The we saponify our samples, dilute with water neutralize with H2SO4.

We also use dilute H2SO4 as the mobile phase; we developed and GLP-validated that method here so we can use either conductivity detector or low wavelength UV detector.

[Karen01]

We use sodium lactate as our standard, then dilute and neutralize with H2SO4. The we saponify our samples, dilute with water neutralize with H2SO4.

We also use dilute H2SO4 as the mobile phase; we developed and GLP-validated that method here so we can use either conductivity detector or low wavelength UV detector.

Thanks... a few questions. Why neutralize the sodium lactate solution? With a (I assume 5 mM) H2SO4 mobile phase, should not just injecting it be enough to to take care of that?

Why do you saponify the samples? Do you need to base hydrolyze something else to get the lactate or is it to breaking down dimers and trimers of lactic acid already in solution?

Thanks
- Karen

[KM-USA]

Why do you saponify the samples? Do you need to base hydrolyze something else to get the lactate or is it to breaking down dimers and trimers of lactic acid already in solution?

Yes, we saponify the samples to get all the dimers, etc., back to the lactate salt form, so all is in the same form for the analysis step.

Thanks... a few questions. Why neutralize the sodium lactate solution? With a (I assume 5 mM) H2SO4 mobile phase, should not just injecting it be enough to to take care of that?

We neutralize so that the NaLactate standard and the saponified sample will be at near-neutral pH. If we just injected the saponified sample, then we'd need to deal with whether there was enough mobile phase acidity was present to acidify the entire sample. If we keep raising the concentration of the H2SO4 in the mobile phase, our background with conductivity detector is higher.

[magda roth]

We use sodium lactate as our standard, then dilute and neutralize with H2SO4. The we saponify our samples, dilute with water neutralize with H2SO4.

We also use dilute H2SO4 as the mobile phase; we developed and GLP-validated that method here so we can use either conductivity detector or low wavelength UV detector.

KM: what are the condiitons for the saponification? I need to do an in-process analysis and cannot have the plant waiting for too long.
Also, did you ever try to use other acids but sulfuric for teh testing? My mobile phase has CF3-COOH, do toy think that wouyld work well?
I tried injecting sodium lactate in solution (made sure that the pH is less than 2!) and got three nice peaks, and was expecting only one. Any suggestions?
Thnaks

Magda

[KM-USA]

KM: what are the conditions for the saponification? I need to do an in-process analysis and cannot have the plant waiting for too long. Magda

We currently add 0.5 N sodium hydroxide and heat inside a steam bath or on robust steam bath 30 minutes, we have no "need for speed" at R&D. We have not explored less time or more aggressive conditions. The USP Lactic Acid monograph boils for 20 minutes. I will mention that we have successfully done other saponifications in 10 minutes on the steam bath.

Also, did you ever try to use other acids but sulfuric for the testing?

No. We had used dilute sulfuric acid eluent on the same column for other organic acids, so it was first tried that way.

I tried injecting sodium lactate in solution (made sure that the pH is less than 2!) and got three nice peaks, and was expecting only one. Any suggestions?

We used Sigma-Aldrich/Fluka #71720-5G. We have only observed one peak, and we did validate the test procedure for our product.

[magda roth]

KM: what are the conditions for the saponification? I need to do an in-process analysis and cannot have the plant waiting for too long. Magda

We currently add 0.5 N sodium hydroxide and heat inside a steam bath or on robust steam bath 30 minutes, we have no "need for speed" at R&D. We have not explored less time or more aggressive conditions. The USP Lactic Acid monograph boils for 20 minutes. I will mention that we have successfully done other saponifications in 10 minutes on the steam bath.

Also, did you ever try to use other acids but sulfuric for the testing?

No. We had used dilute sulfuric acid eluent on the same column for other organic acids, so it was first tried that way.

I tried injecting sodium lactate in solution (made sure that the pH is less than 2!) and got three nice peaks, and was expecting only one. Any suggestions?

We used Sigma-Aldrich/Fluka #71720-5G. We have only observed one peak, and we did validate the test procedure for our product.

Oh thank you!

How are you preparing your standard? Simple dissolving it in the mobile phase? Do you have an internal standard?

[magda roth]

KM: what are the conditions for the saponification? I need to do an in-process analysis and cannot have the plant waiting for too long. Magda

We currently add 0.5 N sodium hydroxide and heat inside a steam bath or on robust steam bath 30 minutes, we have no "need for speed" at R&D. We have not explored less time or more aggressive conditions. The USP Lactic Acid monograph boils for 20 minutes. I will mention that we have successfully done other saponifications in 10 minutes on the steam bath.

Also, did you ever try to use other acids but sulfuric for the testing?

No. We had used dilute sulfuric acid eluent on the same column for other organic acids, so it was first tried that way.

I tried injecting sodium lactate in solution (made sure that the pH is less than 2!) and got three nice peaks, and was expecting only one. Any suggestions?

We used Sigma-Aldrich/Fluka #71720-5G. We have only observed one peak, and we did validate the test procedure for our product.

Oh thank you!

How are you preparing your standard? Simple dissolving it in the mobile phase? Do you have an internal standard?

I changed to mobile phase water + H2SO4 - pH 2.0. Dissolved the sodium lactate (0.1 grs/10 mls) in the mobile phase: pH of the salt solution is 4.5, not a good idea. How did you prepare your lithium salt standard? It should not be such a difference with the sodium salt?

Thanks

Magda

[KM-USA]

How are you preparing your standard? Simple dissolving it in the mobile phase? Do you have an internal standard?

No internal standard.

Sodium lactate standard, no lithium used anywhere here by us.

We dissolve the sodium lactate in water, add a little 0.500 N sulfuric acid solution to acidify, then mix and dilute to volume with water.