Extractables and Leachables Extraction Techniques
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expert analytical testing Extractables and Leachables Extraction Techniques Second Edition A collection of articles designed to help improve your knowledge and skills. TABLE OF CONTENTS P.03 INTRODUCTION P.04 SONICATION P.05 REFLUX P.07 SOXHLET P.10 SEALED VESSEL P.11 PRESSURISED SOLVENT EXTRACTION P.14 MICROWAVE ASSISTED EXTRACTION P.16 SUPERCRITICAL FLUID EXTRACTION (SFE) P.19 OVEN EXTRACTION P.22 LIQUID EXTRACTION SURFACE ANALYSIS (LESA) P.24 DIRECT ANALYSIS IN REAL-TIME (DART) & DESORPTION ELECTROSPRAY IONISATION (DESI) P.27 HEADSPACE P.29 THERMAL DESORPTION P.30 SUMMARY P.31 CONCLUSION INTRODUCTION 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. Each of the techniques will then 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: Technique Solution based Complexity Sonication Yes Low Reux Yes Low Soxhlet Yes Low Sealed vessel e.g. Autoclave Yes Medium Pressurised Solvent Extraction e.g. ASE™ Yes High Microwave Assisted Extraction Yes High Supercritical uid extraction Yes High Oven Yes Low Direct analysis e.g. DART, DESI No High Liquid Extraction Surface Analysis E.g. LESA No High Headspace Either Low Thermal desorption Either High Dynamic headspace Either High Extractables and Leachables Extraction Techniques: Volume I Page 3 SONICATIONSONICATIONSONICATIONSONICATION SONICATION 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. 1. Saim, J.R. Dean, P. Abdullah and Z. Zakaria, Journal of Chromatography A, 1997, 791, 361. 2. R. Banjoo and P.K. Nelson, Journal of Chromatography A, 2005, 1066, 9. 3. F. Guerin, Journal of Environmental Monitoring, 1999, 1, 1, 63. Extractables and Leachables Extraction Techniques: Volume I Page 4 reflux REFLUX Reflux is another simple technique in which the sample is placed in a flask containing the solvent with a condenser fitted on top of the flask. The flask is heated for a set time, allowed to cool and then the solution can be analysed. The fundamentals of the extraction are around the boiling point of the solvent which makes the technique universal. With any setup using a condenser, the efficiency of the condenser can be important. Each condenser has a potential cooling capacity and efficiency. Different types of condenser exist, from the basic ‘Liebig condenser’. To the more complex ‘Graham condenser’ which has a spiral coil running the length of the condenser. a) b) c) Vapour Trail Water Trail Graham condenser, with three possible configurations: a) cooling with jacket, b) cooling with spiral and c) combined jacket and spiral cooling. [1] Extractables and Leachables Extraction Techniques: Volume I Page 5 There are two basic configurations for a Graham condenser. In the first, the spiral contains the coolant, and the condensation takes place on the outside of the spiral. This configuration maximises flow capacity because vapors can flow over and around the spiral. In the second configuration, the jacket tube contains the coolant, and condensation takes place inside the spiral. This configuration maximises collected condensate because all the vapors must flow through the entire length of the spiral, thereby having prolonged contact with the coolant. Other condensers such as the Allihn condenser and ‘Friedrich condenser’ are available. For reflux extraction and also for any other type of extraction, the correct sample-to-solvent ratio should be used. This is to make sure that a solution is obtained that spans a suitable concentration range e.g. too concentrated or dilute, as well as consideration of the amount of solution required for analyses and the required limit of detection based on the Analytical Evaluation Threshold (AET). Heating is commenced and the material extracted at the boiling point of the solvent. The boiling point of the solvent is the key controlling factor with this technique, along with the duration of extraction. Extraction should continue until asymptotic levels or exhaustive conditions are reached, depending on the guidelines that are being followed. Reflux may not be suitable if the material to be extracted is particularly thermally labile because it is exposed to complete heating of the solvent. This potential thermal impact can be exacerbated by the material sinking and being in contacted with the bottom of the flask and potentially be exposed to higher temperatures. Reflux extraction does have the advantage that any solvents can be used including mixtures. Other related techniques that also heat the sample in a solvent such as microwave-assisted extraction (MAE) or pressurised solvent extraction (PSE) will be discussed in more detail in following weeks. 1. Update on Undertaking Extractable and Leachable testing A Feilden ISBN 978-1-84735-455-6 Extractables and Leachables Extraction Techniques: Volume I Page 6 S XHLET SOXHLET Soxhlet extraction is probably the most common technique used for the extraction of materials from solid samples, and has been used for more than a century. It was originally used for the determination of fat in milk [1]. As such, all other extraction techniques are generally compared with Soxhlet extraction. The sample is typically placed in a thimble (usually made from cellulose) but doesn’t need to be and the solvent is placed in a flask below the thimble. The solvent is heated and recondenses into the thimble. Once the liquid reaches a defined level, it is siphoned back to the flask, transporting the extracted species back into the bulk liquid. The solvent is then distilled again, thereby delivering pure solvent to the material being extracted. The number of cycles (‘turnovers’) can affect the extraction. Hence, a minimum number is normally expected and this varies from material to material and species to species being extracted. The advantage of Soxhlet is the repeated delivery of fresh pure distilled solvent to the sample, allowing continual shifting of the transfer equilibrium. It is also a very simple system requiring a heater, condenser and inexpensive glassware. Soxhlet has one advantage over reflux in that fine-particulate matrices can be used because these are contained in the extraction thimble, resulting in only extracted analytes in solution and is more amenable to potentially thermally challenging samples. A standard Soxhlet apparatus can use large solvent volumes, a potential disadvantage to the technique. However, automated Soxhlet systems are available that offer additional functionality,