Enzymatic single drop microextraction

Single drop microextraction (SDME), which was described in the middle 90´s, is an effective extraction technique which comprises isolation and preconcentration in one step, allowing the direct injection of the extracts in different analytical instruments. In SDME, a small volume of extractant is aspirated in a microsyringe which is introduced in the extraction vessel where the sample is located. A small drop of extractant, in the range of 1-5 µL, is finally exposed to the sample (direct immersion mode) or to its headspace (headspace mode) in order to extract the target analytes. After the extraction, the drop is retracted into the microsyringe which will be finally transferred to the appropriate analytical instrument.

In the usual situation, the extractant is an organic solvent with suitable properties. In this way, the solvent should present a high affinity towards the analytes in order to isolate them from the sample matrix. Moreover, the organic solvent should present a low volatility and low water solubility, especially when headspace and direct immersion modes are used, respectively. Aqueous drops can also be used, but this situation is only applicable in the headspace mode in order to avoid the dissolution of the extractant into the sample. A secondary reaction, acid/base reaction in the simplest case, is usually required to achieve the preconcentration of the analytes.

In a recent article accepted for publication in Analytica Chimica Acta, enzymatic SDME is firstly presented for the extraction of ethanol from cosmetic samples (1). In this case, the aqueous drop consists of a buffer containing alcohol dehydrogenase (ADH) which is the enzyme that transforms ethanol into acetaldehyde. To perform its biological activity, ADH requires the presence of the cofactor nicotidamide adenine dinucleotide (NADH). NADH is a cofactor that participates in biological redox reactions and it may coexist in two different forms: NAD⁺ (oxidizing agent) and NADH (reducing agent). Although both forms present native fluorescence, NADH can be selectively monitored using 365 nm and 446 nm as excitation and emission wavelengths, respectively.

In this SDME, ethanol is previously transferred to the headspace being finally recovered in the aqueous drop where the enzymatic reaction takes place. This enzymatic reaction plays two different roles. On the one hand, it is a secondary reaction which makes easier the extraction of ethanol. On the other hand, it acts as indicator reaction since it produces a change in the organic drop, a fluorescence signal, proportional to the ethanol concentration.

The article is interesting not only for theoretical reasons, the use of enzymes in SDME, but also for application purposes since the development methodology allows the detection of frauds in ethanol-free cosmetics commercialization

Reference:

(1) Enzymatic single-drop microextraction for the assay of ethanol in alcohol-free cosmetics using microvolume fluorospectrometry detection. Link