Extraction and Preconcentration of Residues and Contaminants in Food Samples Using Membrane Techniques -

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Extraction and Preconcentration of Residues and Contaminants in Food Samples Using Membrane Techniques

Feb 1, 2009
By: Titus A.M. Msagati, Luke Chimuka, Ewa Cukrowska, Hlanganani Tutu
LCGC North America

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Ronald E. Majors

Over the last few decades there has been tremendous advancement in the field of sample preparation methods, going hand in hand with the developments of state-of-the-art analytical instruments (1–3). The driving force behind the rapid development in the methods for sample preparation has been the need to replace traditional sample preparation methods, which involved many handling steps and, hence, was laborious, time-consuming, and uneconomical in terms of their high demands for consumables, for example, large volumes of solvents (4,5). More recently, an interest in the invention of new, nontraditional sample preparation methods that negate these disadvantages have come about (6,7). Many nontraditional sample preparation methods have been shown to be very selective and efficient in terms of the selective extraction, clean up, and enrichment processes (8).

Figure 1

The modern sample preparation methods can be classified based upon whether the extraction is exhaustive or nonexhaustive (9,10). A category of nontraditional extraction methods that take place at either exhaustive steady state or nonexhaustive state involving the use of membranes include supported liquid membrane extraction (SLM); microporous membrane liquid–liquid extraction (MMLLE); polymeric membrane extraction (PME); and membrane extraction with sorbent interface (MESI). In this review, we will discuss the various membrane techniques that have been applied in the selective extraction and preconcentration of contaminants in foods. The main attractive feature of membrane techniques in extraction of food samples is that macromolecules such as long chain proteins and carbohydrates are excluded as they cannot pass through the pores of the membrane (Figure 1). Large molecules also have slow mass transfer across the membrane. Selectivity can be finetuned further by optimization of the various conditions such as the donor and acceptor pH (or composition). For liquid membranes, a selective carrier can be incorporated in the organic liquid impregnated in the pores of the membrane increasing the selectivity.

Principles of Membrane Extraction

Both porous and nonporous liquid membranes have been used for extraction and separation of a variety of contaminants in different types of food samples. The difference between the porous and nonporous liquid membranes in terms of their ability to separate mixtures lies from the fact that the porous membrane functions as a boundary separating two solutions, one the receiving phase. The nonporous membranes however, play a role as a selective barrier that governs the selective passage of one analyte species contained in one phase to the other (11). Further, in porous membrane systems, the transport mechanisms, the driving force for mass transfer and the mode of selectivity, are solely dependent upon the partitioning property of the analyte between the contacting phase, which can be liquid–liquid or gas–liquid (12,13). Another important feature with porous membrane is that, the membrane does not have any significant influence as far as selectivity is concerned, though the pore size can exert influence on the selectivity of some specific solutes in some instances (12–15). On the other hand, nonporous membranes normally show the resistance problem for the mass transfer across the membrane, which can affect the mass transfer kinetics negatively (14). In recent reviews of the advances in membrane techniques, various membrane configurations have been detailed (16).

Dialysis: Dialysis and its variant techniques such as microdialysis and electrodialysis are porous membrane techniques widely used in analytical processes (17–21). In typical operations, both the aqueous phases separated by the porous membrane are flowing. Target analytes diffuse from the donor phase to the acceptor phase by a concentration gradient. Dialysis, unlike the nonporous membrane techniques, is not characterized by analyte enrichment; instead there is only partial clean up due to lack of a discrimination mechanism between other nonanalyte molecules that might have the same size as the analyte molecules (22,23). For this reason, dialysis is not regarded as an extraction technique per se, and often an additional sample clean-up step is included in the process. However, it has been applied widely to food samples because macromolecules are excluded from passing through the pores of the membrane (24–26).

Liquid membrane extraction techniques: In liquid membrane techniques, the permeability of any molecule through the membrane is governed by the extent of its solubility to diffuse through the liquid held in the porous membrane and the concentrations of analyte species in the feed and permeate (27). The theory and mathematics of membrane extraction techniques has been well worked out (27,28). The extraction efficiency (E) is defined as the fraction of analyte extracted from the donor phase into the acceptor phase. This is an important parameter commonly measured in membrane technique applications. It is a measure of the rate of mass transfer through the membrane, which is constant at specified extraction time, flow rate, phase composition, temperature, and ionic strength (29,30). The concentration enrichment factor E_n is a ratio of the concentration found in the acceptor phase to that in the original sample. It is used to estimate the detection limit of the membrane-extraction technique. The volume ratio of the acceptor phase to that of the sample influences the enrichment factor. Usually, acceptor volumes are kept small in the range of a few milliliters to microliters (29).

Supported liquid membrane (SLM): SLM extraction involves a three-phase system (31). An aqueous sample phase (feed–donor) is separated from an aqueous receiver (acceptor) phase by a layer of organic solvent impregnated in the porous membrane. To enhance extractability and selectivity of solutes in SLM, the conditions in the aqueous sample phase normally are adjusted so that the analyte of interest is forward-extracted out of the sample phase into the organic phase and back-extracted out of the organic phase into the receiver phase, in a concerted fashion (29,32,33). In typical operations, the donor is flowing while the acceptor phase is stagnant, thus resulting in high enrichment factors because large sample volume can be extracted. The SLMs can sometimes incorporate a chemical extractant or a carrier to enhance the process of selective transport of analyte components across the membrane interface (29,33). SLM generally is the most selective membrane technique. Ideally, only compounds belonging to the same family should be extracted at a time.

Microporous membrane liquid–liquid extraction (MMLLE): MMLLE is a two-phase technique that involves an aqueous phase and an organic phase. The organic phase is impregnated in the porous membrane and is also part of the acceptor phase.

Selectivity in a two-phase system is based upon the partitioning of the target analytes into the liquid membrane and on the membrane pore size that excludes bigger molecules. A two-phase system is therefore still suitable to extract target analytes from food samples. With a stagnant acceptor phase, the amount of analyte extracted into the organic acceptor phase is limited by its partitioning coefficient into the organic liquid. This system is more applicable for the extraction of nonpolar organic compounds (34). Initially, the flat sheet module was used commonly. However, recently, more applications are using the hollow fiber as the module of choice because phase breakthrough is almost eliminated because no pumping is required as the sample is stirred often. Hollow-fiber modules are also simple and cheap, and high enrichment factors are obtained. Memory effects are also not a problem because of the small thickness of hollow fibers. One limitation is that unlike in the flat sheet module, automation or online extraction is not easy except in the form of the extraction syringe (35), hollow-fiber liquid-phase microextraction (36), and fiber-in-tube liquid-phase microextraction (37) devices.

Hollow-fiber supported liquid membrane (HFSLM) microextraction technique: In hollow-fiber microextraction, a porous polypropylene hollow-fiber strand is used and a very small volume of acceptor solution (in the microliter range) is added to it. The filled hollow fiber is then exposed to an organic liquid to impregnate the pores, and it is then placed in an aqueous sample where extraction will proceed. Hollow-fiber microextraction can be carried out in either a two-phase (MMLLE) or three-phase (SLM) process depending upon the analyte being extracted. The principles of the extraction process that determines the selectivity are therefore similar to those described earlier. The application of the hollow-fiber technique in the extraction and separation of food contaminants has not been reported widely (38–40). However, it is now seen as the module of choice in liquid membrane extractions.

Polymeric membrane (silicone rubber) extraction techniques: A polymeric solid membrane (usually silicone rubber) is used to partition and separate the feed (donor) and stripping (acceptor) phases (41). Selectivity is based upon the differences in the partition coefficients of the analyte and interfering matrices into the silicone rubber, as in MMLLE (41). Conditions of the donor and acceptor phases also can be adjusted, as in the SLM extraction technique (41). Various phase combinations can, thus, be realized, such as aqueous (42) and organic (41). Phase breakthrough is eliminated. Silicone hollow fibers also are available, which can be used as hollow-fiber liquid-phase microextraction (43). The main drawback of PME is that it does not allow additions of chemical extractants (41).

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