Seeing Green

October 23, 2008

E-Separation Solutions

Hian Kee Lee from the National University of Singapore is at the cutting edge of developing an array of simple and ?environmentally friendly? sample preparation techniques for environmental analysis, writes Alasdair Matheson.

Hian Kee Lee from the National University of Singapore is at the cutting edge of developing anarray of simple and "environmentally friendly" sample preparation techniques forenvironmental analysis, writes Alasdair Matheson.

You are currently involved in developingenvironmentally friendly extractionprocedures for environmental applications?Why is this type of research important?

Lee: Environmental analytical chemists (and probablymost chemists!) are contributing in some way toworldwide chemical pollution and contamination. Ithink these scientists need to be more aware of thisand address the problem. It is ironic that inattempting to learn more about the environmentalimpact of pollutants and contaminants on theecosystem, we may be generating too much chemical andsolvent waste in the process.

Thus, the development and adoption of environmentallyfriendlier analytical techniques, particularly in samplepreparation, pretreatment and extractions, should be a primaryaim of those working in environmental analysis. It is importantbecause we must have the credibility to practise what we arepreaching to the public at large and the decision-makers. As acommunity, scientists should try to minimize — and, if possible,completely eliminate — the emission of potentially harmfulchemicals into the environment.

What “green” techniques are you developing forseparation scientists and what are the benefits?

Lee: Our primary interest is in the development and applications ofsample preparation procedures that require minimal amounts ofsolvents, reagents and materials. These are microscale orminiaturized techniques so they are environmentally benign andchemically sustainable (in the sense that there is very little, if any,wastage). A major focus is the application of these techniques toenvironmental samples, particularly aqueous matrices.

I think environmental analysis is perhaps the mostcommon application of analytical chemistry, so itseems a desirable area to develop techniques for.Another important philosophy that I have about thisis that the procedures should be simple enough tolearn and to use, so that those with access to basictools and apparatus can apply them to performuseful and meaningful analytical chemistry. Ofcourse, there are some esoteric techniques thatappear to be very exciting, but they are not globallyaccessible to the general analytical community. Ithink there are a lot of scientists in the less developed regionsof the world who would like to plug into the global grid butare unable to do so because the costs are prohibitive. Simpleand affordable procedures allow such accessibility..

Can you describe some of the techniques you havedeveloped, the main obstacles that you had toovercome and the benefits of these over existingtechniques?

Lee: I did not realize it previously, but it seems that our group firstcoined the term “liquid-phase microextraction” (LPME) that isnow widely used in the literature. In our original work, the ideaof using this term was to represent a procedure that used verysmall amounts (low microlitre amounts) of solvent forextraction from water samples. Solid-phase microextraction(SPME), a solventless extraction and preconcentrationtechnique developed by Pawliszyn’s group at the University ofWaterloo, Canada, was already well-known (and commerciallysuccessful) then, and it seemed to us it was natural to try andhave a complementary counterpart in which small volumes ofsolvent were involved.

This is basically liquid–liquid extraction, probably still themost common extraction procedure for water samples, but at aminiaturized scale. (Note that I don’t use “miniaturized” tomean dimensions as applied to the microfluidics field. In ourcontext, we do not need any special fabrication or handlingtools.) I must mention and emphasize that, in our work, wewere inspired by the early research on solvent-minimized ororganic drop-based extraction by Cantwell from the Universityof Edmonton, Canada and Dasgupta who worked at the TexasTechnical University, Lubbock, USA) at the time, as well asPawliszyn’s work on SPME.

Since then there have been various implementations ofLPME, including the use of polypropylene hollow fibremembranes (essentially just fine tubing with porous walls), tobetter stabilize the extracting solvent. Again the inspirationcame from several colleagues in Scandinavia includingPedersen-Bjergaard and Rasmussen (both at the University ofOslo, Norway) and Jönsson from Lund University in Stockholm,Sweden. Apart from this, we developed polymer-coated hollowfibre-LPME (similar to the SPME approach), solvent-barmicroextraction (in which the solvent is entrapped within thechannel of a hollow fibre, and during extraction, tumblesaround in the sample, picking up analytes efficiently), andcontinuous-flow microextraction (CFME) in which samplesolution is automatically delivered to impact on an organic dropinto which the analytes are extracted.

Lately, we have been using solid sorbents and placing smallquantities of them in sealed membrane envelopes to serve asmicroextraction devices. We refer to this procedure as microsolid-phase extraction (μ-SPE). Casually, I term it “tea bagextraction”, because the device actually looks like a teabagalthough it is only 1.8 mm 0.5 mm in size.

The main advantages of these techniques are theirenvironmental friendliness, simplicity, and convenientimplementation so that they are easy to set up for usestraightaway. Of course — as has also been demonstrated byother researchers — they provide good analytical results, whichis, obviously, of paramount importance (1–9).

The use of hollow fibres has another important advantage:Such techniques prevent the extracting medium from beingaffected by the matrix as well as by strong stirring, thus protectingand stabilizing it. With hollow fibres, even complex aqueoussamples of environmental matrices, such as slurries, can beconsidered, as well as biological fluids, such as urine and blood.

One of our other objectives in developing these microscaletechniques is to provide a simple platform to conduct field or on-site sample preparation. The ultimate aim is to bring theanalytical system to the environment rather than having to bringsamples back to the laboratory for processing and analysis.

Do you see this miniaturized approach to samplepreparation becoming more predominant? Willclassical liquid–liquid extraction ever become totallyobsolete?

Lee: Of course, I hope to see more people using these techniques.They work well, give good analytical results, do not generateany significant waste, and can be very quickly implementedwith basic apparatus and materials. Basically, for the analysis,all you need is a gas chromatograph or a liquidchromatograph, which are accessible to virtually all analyticalchemists nowadays.

Classical liquid–liquid extraction has served us well for along, long time, it works very well, and is generally very easy touse. It is obviously not environmentally benign, and the analystis exposed to potentially toxic organic solvents. However, Idoubt whether it will become obsolete soon. The hope is thatthe newer microextraction techniques (whether solid or liquidbased)will eventually supplant it. It will take some time but thetrend is increasingly clear to us. It is up to us to conscientiouslymake it happen.

What other areas of sample prep do you seebecoming popular in the future? Can you illustratethis with a practical example?

Lee: For those needing to perform large numbers of samples,automation is the key. I am in two minds about this. On theone hand, for the laboratories with routinely high samplethroughputs, automation would be a boon. On the other,adding automation to an existing instrument, no matter howaffordable, will involve a significant capital outlay, andcomplexity. The current microextraction techniques arefundamentally characterized by simplicity. Once the complexitycomes in, it is possible that the wide accessibility to a largesection of the community may be lost. I believe automation isan inexorable trend, but I still hope to see simplicity in futuredevelopments in sample preparation. More affordable andportable analytical instruments with the same capacity asbenchtop systems but suitable for field analysis immediatelyafter sample preparation would be useful.

There are already efforts, by others and ourselves, to developsuch on-site microextraction approaches. More versatile andhigher-performance on-site instrumentation has also received greater attention in recent years. The convergence of these twolines of development will allow us to have a dedicated on-siteanalytical system.

Is there any advice you would give to researchers whowant to develop environmentally friendly sampleprep methods?

Lee: I think the primary motivation should be creativity withsimplicity. I would strongly suggest giving free rein to studentsor junior researchers to devise innovative solutions. My owngraduate students have never complained about being giventoo much independence in the laboratory! Additionally, it is notunusual for undergraduate students in my laboratory to beassigned projects that are completely untested and are onlyrepresented by scribbles on the pages of a notebook. Evenshort-term visiting foreign students who spend a few monthswith me are not spared this type of project allocation! Notevery idea works, of course, but some nice preliminary workcan come out of this that is then more rigorously tested bythese students if they subsequently join our graduateprogramme or by other graduate students. I also enjoy workingwith other researchers involved in sample preparation andenvironmental analysis, as well as other disciplines, such asxenobiotics and materials science. This gives us usefulperspectives on what can be done to move the field forward.

References: 1. M.A. Jeannot and F.F. Cantwell, Anal. Chem., 68, 2236 (1996).
2. J. Liu and P.K. Dasgupta, Anal. Chem., 68, 1817 (1996).
3. Y. He and H.K. Lee, Anal. Chem., 69, 4634 (1997).
4. Y. Wang et al., Anal. Chem., 70, 4610 (1998).
5. J.Å. Jönsson and L. Mathiasson, Trends Anal. Chem., 11, 106 (1992).
6. S. Pedersen-Bjergaard and K.E. Rasmussen, Anal. Chem., 71, 2650(1999).
7. G. Shen and H.K. Lee, Anal. Chem., 74, 648 (2002).
8. L. Zhao and H.K. Lee, Anal. Chem., 74, 2486 (2002).
9. S. Pedersen-Bjergaard and K.E. Rasmussen, J. Chromatogr. A., 1184,132 (2008) (review).

Hian Kee Leeis professor and deputy head of the Departmentof Chemistry at the National University of Singapore. He holdsBSc (Honours) and PhD degrees from the University ofCanterbury, New Zealand. His research interests are focused onthe development of microscale sample preparation proceduresin combination with microseparation techniques forapplications to environmental and biological samples. ProfessorLee is an editor of Analytica Chimica Acta and also serves onthe editorial board of the Journal of Chromatography A and onthe international advisory board of The Analyst.