Ascorbic acid assay from Vinpocetine etc solution

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

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We've recently tried several chromatography methods for ascorbic acid assay:
- SAX, 0.1M AcONH4 / AcN 1/1: ideal peak and specificity
- NH2, 0.1M AcONH4 / AcN: tailing peak, no specificity
- C18, TBA HSO4 pH 5 / MeOH 95/5: tailing peak, poor specificity
Finally SAX wins :lol:
However method validation shows
+ quantification limit ~ 1 % from nominal concentration
+ recovery 100 ± 7 %
+ linearity with r2 > 0.999
- residual member ~ (minus 10 %) from nominal area
- unstable (day-to-day) slope coefficient (probably because of pH and UV spectra instability, however adsorption maximum seems to be constant)
- Adcorbic acid - 0.5 mg/ml
- Vinpocetine - 5 mg/ml
- Sorbitol
- Benzyl alcohol
- Metabisulfite sodium
- Tartric acid - up to pH from 3 to 4
Test solution: test sample as is.
Reference solution: ascorbic acid, sorbitol, tartric acid, water.
Any ideas are welcomed!
Best regards,
Dmitriy A. Perlow
Any tips please?
Best regards,
Dmitriy A. Perlow
What is your question?

One thing I can tell you is that vitamin C is a highly unstable molecule in solution. For a good analysis, you want to prepare your calibration standards daily from scratch (solid vitamin C).

It is sensitive to light, oxygen and metals, especially at higher pH. Most analytical procedures extract in an acidic environment, together with the use of reducing agents and chelating agents.

I would advice to not reinvent the wheel and use an application note or article as a fundament of your analysis.
I used ~1% meta-phosphoric acid (not ortho) in our RP mobile phase in order to prevent formation of de-hydro-AA.
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