Absorption for aldehydes: Polymeric monolith moulds for SPME

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  • Published: May 7, 2012
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: Absorption for aldehydes: Polymeric monolith moulds for SPME

Polymer monoliths as sorbents

A novel monolithic polymer has been prepared using the network of a porous polymer frit as a mould for use as the SPME medium in the extraction of aldehydes from human serum.

Scanning electron micrograph of the polymeric monolith frit at a magnification of 10,000

 

Porous polymer monoliths are becoming increasingly popular in the analytical world and have found countless applications as stationary phases for chromatographic separation and sorbents for extraction. They differ from many particulate separation phases in that they consist of a single entity, rather than many particles, so that there are no interparticular void spaces. As a result, all of the applied solution passes through the material in convective flow, rather than diffusive flow, increasing the separation speed.

This type of material can be prepared in bulk but it is often preferable to prepare it in situ, within a chromatographic column or a microfluidic device, so that it fits its surroundings exactly and no packing is required. Now, a team of Chinese scientists has demonstrated a novel way to produce monolithic polymers using a porous frit as the mould. Hui Xu, Zhihua Yan and Dandan Song from the Key Laboratory of Pesticide & Chemical Biology, Central China Normal University, Wuhan, explained their procedure in the Journal of Separation Science.

Their aim was to produce material suitable for offline SPME. There have been many reports of monoliths used for SPME where the polymer has been prepared within devices like capillaries, PEEK tubes, pipette tips and specially designed holders. However, there tends to be a tricky balancing act between generating a material with a low back-pressure during extraction and a high specific surface area, especially for column-shaped systems.

If the surface area-to-thickness ratio of the polymer becomes too small, then the back-pressure builds up and hinders efficient extraction. But by changing the geometry from a rod to a disk, this ratio is increased while retaining the high column capacity, to lower the back-pressure.

 

Porous polymer network

The researchers used a commercial porous polypropylene frit just 4.8 mm diameter and 1.6 mm thick with a pore size of 20 µm. The diameter is the same as the internal diameter of a 1-mL syringe barrel, allowing a perfect fit for subsequent extractions.

All of the ingredients required to prepare poly(methacrylic acid-ethylene glycol dimethacrylate) were mixed and added to the frit that was held in the syringe, then the whole assembly was irradiated for 90 seconds with UV light at 365 nm to initiate polymerisation. The polymerisation conditions were optimised, including the choice of solvent, the amount of reaction solution added to the frit, and the reaction time.

After washing, the frits were examined by scanning electron microscopy. The virgin frits displayed a highly interconnected channel network but this was filled with the monolith after polymerisation. The polymeric monolith contained many nanometre-sized pores formed by small spherical particles which surrounded the larger polypropylene particles.

The new material operated with low back pressure, typically 1 bar at 2.5 mL/min for an aqueous methanol solution. Under these conditions, there appeared to be no loss of monolithic material, so the high mechanical strength and low back pressure implied that it would be a promising substance to use for extractions.

SPME of stress-derived aldehydes

The performance of the new material as an SPME medium was assessed for the extraction of biological aldehydes in human serum. Compounds like hexanal and heptanal are markers of free radical damage in the body brought about by a variety of factors such as diet, stress and pollutants. The radicals are thought to be active agents in inducing many types of disease, including heart disease, Alzheimer’s disease, atherosclerosis and some types of cancer.

The ultimate aim was to analyse the aldehydes by HPLC with UV detection, so they needed to be derivatised first, using dinitrophenylhydrazine (DNPH), to form the respective hydrazones which have an inbuilt chromophore. So, the flexibility of the polymer monoliths was demonstrated by in situ derivatisation before the derivatives were desorbed and eluted for analysis.

Working with standard solutions first to optimise the conditions, a phosphate buffer at pH 2.2 was added to the monolith frit, before the DNPH solution was drawn through followed by a solution containing hexanal and heptanal. The resulting derivatives were washed out with acetonitrile for HPLC. The recoveries of the aldehyde hydrazones were greater than 90% and the HPLC detection limits were 1.86 and 1.38 nmol/L for hexanal and heptanal, respectively.

In the final step, serum from healthy subjects and lung cancer patients was centrifuged and diluted with phosphate buffer before being added to the monolith frit for derivatisation and extraction. The concentrations of hexanal and heptanal were found to be 2.41-10.41 and 1.41-13.12 µmol/L, respectively, in the cancer patients, compared with 0.044-0.27 and 0.22-0.90 µmol/L, respectively, in healthy people.

The new method for preparing the monolithic frits is simple, quick and inexpensive and it produces materials having good mechanical strength and biocompatibility. They should provide a promising alternative for the simultaneous extraction and derivatisation of biomarker aldehydes from biological liquids.

Related Links

Journal of Separation Science 2012, 35, 713-720: "Development of a novel monolith frit-based solid-phase microextraction method for determination of hexanal and heptanal in human serum samples"

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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