In extractable and leachable studies there are a range of extraction techniques that can be used to either produce a solution for further analytical study or directly analyse the materials. A selection of the most common extraction techniques are listed in Table 1. Over the coming weeks each of the techniques will be discussed, detailing what is involved to set up the equipment, the advantages and disadvantages as well as some of the limitations of each technique.
The primary aim of any extractable study is the acceptance or rejection of a given material. The acceptance or rejection can only be achieved with knowledge. This can be achieved by a number of ways, including; understanding of the materials likely composition, manufacturers information and the definitive testing. In all aspects of extractable testing it is important to remember that the extraction is not so vigorous as to deform or degrade the material which would likely produce extractables that will not be observed as leachables. Extractables are potential leachables, it is the leachables that the patient is exposed to and are of toxicological concern. There is a sweet spot for extractable studies in that they should be aggressive enough to produce worst case leachables but not so harsh as to still allow for a correlation between extractables and leachables.
There is no single extraction technique that can provide all the information needed for an extraction study and so multiple techniques are typically used.
Table 1 Potential extraction techniques:
Sonication is one of the simplest extraction techniques in terms of equipment, solvent selection and sample preparation. A known weight of material is placed in a container with a known volume of solvent. The ratio of sample-to-solvent and the overall amounts of material required are dependent upon: method requirements; analytical limits of detection; and the sample volumes required for testing. For example, standard metal analyses by inductively couple plasma will require far more solvent volume (typically 10-20 mL) compared with gas chromatography (GC) or high-performance liquid chromatography (HPLC) analyses (≤1 mL).
The sample is placed in the sonic bath and sonicated for the prescribed time (or over a range of times) until asymptotic levels are reached. Potential issues can arise with sonication duration due to the relative efficiencies of sonic baths. One sonic bath can be more efficient than another in terms of energy supplied and temperature increase of the solution. The degree of temperature increase of the sample (which can have a significant influence on the degree of extraction) can vary significantly from machine to machine.
The choice of solvent and the analyte being extracted can have a large impact on the efficiency of this technique [1-3]. In general, the more volatile the solvent, the greater is the efficiency of extraction when compared with other extraction techniques such as reflux. Sonication results using a low-boiling-point solvent such as dichloromethane (DCM) (boiling point 40 °C) will more closely match those achieved with an extraction technique such as reflux than if a solvent such as isopropanol (boiling point 82 °C) is used. This could be due to the kinetics involved because the reactions will be occurring at similar temperatures with DCM compared with the wide differential with isopropanol.
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- R. Banjoo and P.K. Nelson, Journal of Chromatography A, 2005, 1066, 9.
- F. Guerin, Journal of Environmental Monitoring, 1999, 1, 1, 63.