A Timesaving Approach to Sample Prep

Food safety is a growing global concern; in fact the food safety testing industry is estimated to reach $15 billion by 2019. While a large portion of this market involves pathogen testing, contaminant testing by techniques such as gas chromatography, high-performance liquid chromatography, and liquid chromatography coupled to tandem mass spectrometry is a growing sector because these techniques provide rapid results and allow analysts to detect and quantify contaminants at extremely low concentrations. While the actual chromatographic analysis is a rapid procedure, samples must be properly pretreated prior to analysis, which can add significant time requirements. As concerns over food safety grow, laboratories must process their samples as quickly as possible, making it extremely important to determine the quickest and most effective sample preparation technique.

(left) Image 1: Initial sample prior to cleanup by SLE. At left, coffee with 2% ammonium hydroxide. At right, control ground coffee.
(right) Image 2: Sample after cleanup by SLE. At left, clean sample after extraction. At right, Novum SLE 6 cc tube after extraction.
Image Credit: Phenomenex

There are many sample preparation techniques available to food safety analysts, each of which has its pros and cons. The most effective sample preparation technique is perhaps solid phase extraction (SPE) because it results in extremely clean, concentrated samples; however this technique requires a significant amount of method development time. A more popular technique in the industry is to perform a liquid-liquid extraction (LLE). LLE is fairly quick, does not require as much method development time as SPE, and produces a rather clean sample. To perform LLE, the food sample is first homogenized into a liquid form. Once liquid, the food sample is mixed with a water-immiscible organic solvent by shaking the two solvents in a flask or separatory funnel. During the shaking process, target analytes partition out of the aqueous sample and into the water-immiscible solvent, leaving behind interferences such as lipids, proteins, and salts. The water-immiscible solvent can then be collected by manually separating this layer from the aqueous layer. While this procedure is rather easy to perform, it does introduce challenges such as analyte loss due to emulsions, or bubbles, that can form at the interface of the two liquid phases as well as incomplete collection of the water-immiscible layer during the liquid separation process. These challenges also make the technique difficult to automate, which can eliminate the ability to perform high-throughput sample processing.

This technique ­eliminates the formation of ­emulsions and the need to manually ­separate liquid phases.

Supported liquid extraction (SLE) has become a popular means to avoid the challenges associated with LLE in bioanalytical laboratories; however this technique has not yet been rapidly adopted in the food safety industry. The technique traditionally relies on diatomaceous earth to provide a solid support on which a liquid separation can be performed. Aqueous-based sample is loaded onto the sorbent, which acts like a sponge to distribute the sample across the surface and inside the pores of the sorbent. Water-immiscible solvent is then added to the sorbent and the two liquid phases interact, allowing target analytes to partition into the water-immiscible solvent, which then drips out of the sorbent and into a collection vessel. This technique eliminates the formation of emulsions and the need to manually separate liquid phases. In addition to the ease of use over LLE, SLE can also be automated, which provides further timesavings, particularly for high-throughput laboratories. Traditional SLE products are packed with diatomaceous earth which is a natural product made up of fossilized diatoms. The material can be found in many different mines across the world, and variances in the product can occur if it is mined from multiple locations. To eliminate the potential challenges such as consistency and performance variances, a synthetic SLE sorbent has recently been engineered by