Inside job: Fatty acid stability marker in skin creams
- Published: Sep 3, 2013
- Author: Steve Down
- Channels: Gas Chromatography
Skin cream stability
Many commercial skin creams contain unsaturated fatty acids which have been added to help moisturise the skin, protect it from exposure to UV light, and give it a more youthful appearance. Typical fatty acids include the omega-3 type essential fatty acids such as α-linolenic acid and eicosapentaenoic acid, and the omega-6 type, like linoleic acid.
These fatty acids are susceptible to oxidation but it is difficult to tell whether or not a cream has begun to degrade like this. One way is to look for the decomposition products which are formed by oxidation and azelaic acid is one candidate. Correctly known as 1,7-heptanedicarboxylic acid, it is produced from unsaturated fatty acids like oleic acid.
However, anyone checking for azelaic acid must be careful because it is also used as a legitimate constituent of creams that are used to treat skin conditions like acne and dermatitis due to its antimicrobial and anti-inflammatory effects. It has also been employed as a whitening agent due to its anti-hyperpigmentive effect.
There are some HPLC methods for measuring azelaic acid, all of which rely on the introduction of a chromophore for detection by UV-visible spectroscopy. One such method has been reported by Muhammed Alzweiri and colleagues from the University of Jordan in Amman.
Now, the same group has turned to gas chromatography to see how useful that method can be. Writing in the Journal of Separation Science, the research team explained how derivatisation was also necessary for GC, since fatty acids are too polar and involatile to pass through the column.
Azelaic acid derivatisations compared
Two derivatisation procedures were tested. The dimethyl ester was prepared using methanol in concentrated sulphuric acid and, alternatively, the trimethylsilyl ester of azelaic acid was prepared. Their structures were confirmed by GC/MS.
Both compounds were measured by GC with a flame ionisation detector using a 5% phenyl methylpolysiloxane column. This was a revealing exercise, as it demonstrated that the trimethylsilyl diester proved to be unstable. Just 4 hours after being prepared, 50% of the compound had degraded and this rose to 100% within one week.
So, the researchers decided to concentrate on the dimethyl ester as a means of measuring azelaic acid. It took twice as long to prepare the dimethyl ester than the trimethylsilyl ester, but this was overridden by worries about the stability of the silylated derivative.
After method validation, the detection and quantitation limits of the dimethyl ester were 10 and 100 ng/mL, respectively, and the calibration curve, prepared using nonane as an internal standard, was linear from 100 ng/mL to 100 mg/mL. Recoveries were also near quantitative.
The GC method was applied to the measurement of azelaic acid in three commercial creams using the method of standard additions and extracting with methanol. One portion of each cream was doped with a known amount of standard azelaic acid and another was undoped. The dimethyl ester was estimated from the area under the curve.
The label on the first cream declared that it contained 10% by weight of azelaic acid and this was confirmed by GC analysis with a measured value of 9.9 % by weight. So, it appears that the fatty acids in this cream had not undergone any oxidation or the level of azelaic acid, which is the breakdown product, would have been higher.
The labels on the two other creams declared no azelaic acid but they were found to contain 0.4 and 0.1 % by weight, implying some degree of decomposition, probably from the oleic acid present. The manufacturing date of the former was one year earlier than the latter, which is in agreement with increasing oxidation over time and the corresponding rise in azelaic acid content.
This result suggests that azelaic acid formed by the oxidation of fatty acids can be used as a stability marker in skin creams, although it would have to be confirmed against a much larger number of different creams. In addition, the GC procedure can be used to measure azelaic acid as an active ingredient in pharmaceutical formulations.
Journal of Separation Science 2013 (Article in Press): "Gas chromatography/trace analysis of derivatized azelaic acid as a stability marker"
Article by Steve Down
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.