Getting into hot water: Sunscreen agents separated by green HPLC method at high temperature

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  • Published: Apr 2, 2012
  • Author: Steve Down
  • Channels: HPLC
thumbnail image: Getting into hot water: Sunscreen agents separated by green HPLC method at high temperature

High temperatures get the green light

High temperature liquid chromatography with commercial columns and pure water as the mobile phase has been explored for the analysis of sunscreens in skincare products. Although the resolution was not ideal, the potential for this green technology was demonstrated.

Green chemistry is making steady inroads into industry, although there is a long way to go. A report published by a major market research organisation in June 2010 estimated the green chemistry industry at $2.8 billion but forecast that it would grow to about $100 billion over the next decade. The major growth areas were predicted to be waste minimisation from chemical production processes, the development of renewable feedstocks, and the use of less toxic materials for existing products.

Routine operations in industrial labs are also being targeted to become part of this sea change and there has been some progress. For instance, ionic liquids have been introduced as mobile phases in HPLC, solvent use has been minimised by microextraction techniques and chip-based analyses, and organic solvents are being replaced by benign solvents.

Another recent green innovation is high-temperature liquid chromatography (HTLC). In this emerging technique, the mobile phase is preheated and the raised temperatures lead to improved peak shapes and faster run times. In addition, the mobile phase becomes less polar, so that organic solvents can be replaced by pure water to elute non-polar compounds from reversed-phase stationary phases. Consequently, solvent gradients can be substituted by temperature gradients.

The rising interest in HTLC has been matched by the development of columns that are stable at temperatures of 100-200°C and an increasing number of HTLC studies are being published. Since many of these are academic studies, there remains a need to transfer them to real applications in an industrial setting.

One research team in the US achieved this goal by developing an HTLC method for the separation and analysis of niacinamide and niacin in skincare creams using pure water as the mobile phase. The work was published by lead researcher Yu Yang from East Carolina University, Greenville, SC with scientists from Procter & Gamble Company, Cincinnati, OH.

Now, the same two groups have collaborated in another project for the HTLC analysis of sunscreens in skincare products, which has been reported in the International Journal of Cosmetic Science.

Screening columns

Conventional HPLC methods for sunscreen agents employ relatively expensive and/or toxic solvents for the mobile phase with methanol one of the most common, Yang wanted to explore the possibility of transferring the method to an HTLC system using heated water as the mobile phase in a technique referred to as subcritical water chromatography (SBWC).

The six cosmetic agents targeted were avobenzone, ensulizole, homosalate, octisalate, octocrylene and octinoxate, the most common sunscreen agent in cosmetics. These compounds covered several different classes including dibenzoylmethanes, benzimidazole sulphonic acids, salicylates and cinnamates.

A standard mixture of sunscreen agents and extracts of commercial skincare creams purchased locally were both made up in methanol for analysis on a lab-built HTLC system. It was fitted with a pre-heating coil for the mobile phase that was positioned in front of the column.

The performances of three commercial columns were compared for separating the agents at high temperature. The first was a C18-clad zirconia column designed especially for operation over the full pH range and at high temperatures. The second and third were both C18 columns that were produced by a hybrid particle technology and were also stable at high pH and high temperature.

HTLC with hot water almost there for sunscreens

On the zirconia column with mobile phase at 90 or 150°C, there was reasonable separation of the sunscreens but only when the mobile phase contained a large proportion of methanol in water. This was improved slightly at 190°C but the overall reduction in methanol use remained small.

There was improvement for the hybrid C18 columns. On one operated with a mobile phase at 200°C, only 2% methanol in water was required but all of the peaks were not well separated. Both cis- and trans-homosalate appeared as small shoulders on the peaks of octisalate and octocrylene, respectively. Raising the temperature to 250°C allowed the use of pure water but the resolving power on this column was weak and significant peak broadening occurred.

The performance of the second hybrid C18 column was similar to the first, so the researchers introduced a novel mobile phase program which they used at 200 and 250°C. Pure subcritical water was used for the first 10 minutes before a gentle gradient of methanol was begun. This meant that the aqueous methanol could be separated from the pure water for correct disposal. Reasonable separation was achieved with this mobile phase but recovery of two of the sunscreens was low due to their poor separation.

When the temperature was raised to 230°C and pure water was used, the results were similar to those from the first hybrid C18 column, with reasonable separation that was not sufficiently good for quantitation of the sunscreens.

So, while the current experiments did allow the use of heated pure water as a mobile phase, the performances were not as good as conventional HPLC. However, they do illustrate the potential of subcritical water chromatography and the elimination of organic solvents in the pursuit of more green methods.

Related Links

International Journal of Cosmetic Science 2012, 34, 169-175: "Separation of sunscreens in skincare creams using greener high-temperature liquid chromatography and subcritical water chromatography"

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.

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