Thought Leaders

Solid Phase Micro Extraction

Dr. Pawliszyn, University of Waterloo, talks to AZoM about the Solid Phase Micro Extraction and his upcoming talk at Pittcon 2017.

SM: Can you tell us a bit about how you became interested in sample preparation techniques as an area of study?

JP: When I started my scientific career in the mid-1980s, there was beginning to be a lot of discussion about pollution. Agreements like the Montreal Protocol stated that we should try to address the impact on the air and on the environment – and today this is only becoming more and more important.

Sample preparation is a very important part of the practice of analytical chemistry, environmental chemistry, and many other fields – despite the fact that it is not always seen as the most crucial or the most highly “scientific” area. The ability to isolate components of interest into some form which can be introduced into sophisticated instrumentation, such as chromatography or mass spectrometry or direct spectroscopic measurement, is highly important for this type of work.

The traditional method of preparing a sample was to use tens of milliliters, or even larger volumes of organic solvents per sample to extract the chemicals of interest. But then what do you do with the solvent afterwards? Usually it is evaporated, and goes straight into the atmosphere. So by using analytical chemistry to do environmental monitoring, you are contaminating the environment!

The reason that this had been the situation for so long was simply that there were few scientific groups working on better sample preparation or sample extraction methods.

I spotted that there was therefore an opportunity to make a really big impact on analytical chemistry, not by working on new, exciting analytical techniques, but by streamlining the analytical process, and improving the science of sample preparation.

So looking at this problem for analytical chemistry and environmental monitoring specifically, we realized that the most powerful sample preparation process would be one which can integrate sampling on-site with preparing the sample for convenient introduction into analytical instrument. That’s actually the main area of my research – trying to develop devices which can facilitate that approach, both in the laboratory and in the field.

Such strategy involves sampling on-site, but increasingly in the future, the analysis is going to be carried out there as well, with portable instrumentation. I’m not saying there will be no labs, of course, but most of the chemical analysis will be done on-site. So these devices will help take us towards that future of on-site analysis.

SM: Tell us about Solid Phase Micro Extraction, how does it work, and how does it differ from conventional extraction methods?

JP: When I introduced Solid Phase Micro Extraction in 1990, it was referred to by many people as a paradigm shift in extractions. The most popular format for an SPME device is basically a fiber about the diameter of a human hair – around 100 microns – coated with another 100 microns of an extraction phase, involving appropriate sorbent. About 1 cm of the coated fiber is placed inside the needle of a syringe using piece of metal tubing – we chose the syringe format because its highly portable, and everybody understands how to handle and use it.

SPME faced considerable skepticism in its early days – it was hard for scientists experienced in conventional separations to understand how we could get the same sensitivity in measurements using microliters of sorbent volume as they could by using many milliliters of solvent! It sounds impossible, but of course this is because the principles of microextraction are somewhat different – rather than flooding the sample with an excess of solvent to be ensure you have extracted majority the target compounds, you limit the volume of the extraction phase to a very small amount, but take full advantage of the entire capacity of that extraction phase leading to high enrichment.

Then, rather than concentrating those mililiters of solvent down to a 100 microliter volume and injecting a microliter into the instrument, you simply inject the entire extraction phase into analytical instrument. So this was certainly something of a paradigm shift in the thinking around extractions, and in the early days it was challenging to attract funding!

SM: How does SPME deal with complex, mixed-phase samples, such as soil samples, biological samples etc.?

JP: Yes, this is another issue where it seems like there might be a shortcoming of the microextraction approach – if you put this small fiber directly inside a complex sample, with thousands or millions of components, it seems like there might be all sorts of things like macromolecules that could interfere with the analysis. I’s not as difficult as you might think to avoid this, however.

One approach that is traditionally used in these situations is headspace analysis – and in the early years of SPME we did use this approach, since it does give a very clean extraction and analysis. All the compounds which go into the headspace will desorb from the fiber, and of course will also go through the GC column.

The problem with that, however, is that it doesn’t always give you an accurate representation of what is in the original sample. Headspace extraction is typically done from an aqueous solution, which means hydrophobic compounds are going to be expelled into the headspace more readily, so they will be overrepresented. There are more polar compounds that you might not see at all.

For some food samples, like mint leaves, so much of the aroma compound goes to the headspace that you can smell it all over the lab – in this case the fiber actually becomes saturated or swollen, which makes it impossible to obtain properly quantify your target analytes. The solution in such cases is to use shorter extraction times, but this results in lower sensitivity.

To address this challenge and facilitate the direct extraction we developed an approach that adds a third component to the device – you have the fiber support, the extraction phase coating, and then a very thin layer of a biocompatible, “restricted access medium” – essentially a selective membrane which will allow small molecules to pass through, but will block macromolecules and ions which might interfere with the analysis.  This “protection layer” is designed to eliminate adsorption of macromolecules as well.

If we use an extraction phase like HLB, we can extract a wide range of compounds varying in polarities in one sampling, and you get a beautiful cleanup in the process.            

SM: What are the main applications that SPME has been used in?

JP: Well originally, as we have discussed, it was used in environmental research and food chemistry. I did not really design the technology for a specific area of application, however – really it is applicable in many different disciplines.

Food analysis is probably the area where the most devices are used. It has been commercialized by a number of companies that we have been working with for a long time, primarily for use in that area.

Devices have also been commercialized for the military, for use in identifying explosives etc. in the field. SPME is ideal for this application, since it doesn’t need qualified chemists to operate the device.

Right now the most exciting area is using the fibers to sample living systems, for animal and medical research or clinical applications. We’ve done lots of work on plants, animals, and fish. We are working with surgeons to try the method for monitoring organs prior to, and during, transplantation procedures. This is not an FDA-approved method yet, but Toronto General, our local hospital and the largest hospital in Canada, is working with us on these trials.

These devices work really well for these biological applications, since we don’t actually extract any tissue, only the small molecules – we call it a chemical biopsy tool. We have a project on sampling brain tissue, where before they were using microdialysis which heavily favors polar compounds and ions. It works well as an extraction method, but of course the samples are not compatible with certain instruments, like a mass spectrometer. Using our fibers, the sample we get is so clean, it is not only compatible, but we can inject it directly into the mass spectrometer! We don’t even need to do LC beforehand.

So what you have is a rapid diagnostic tool, where you can sample directly investigated living object and put that straight into an instrument with no further preparation – medical applications are a big area of interest for this naturally, but you can see how it is great for environmental applications, food screening, forensics, and so on.

SM: How does the approach to quantification differ when using SPME rather than conventional solvent extraction?

JP: Typically, the traditional analytical chemist will work with a fixed volume of sample, and exhaustively extract the components into the extraction phase. This is a very limiting approach, because you need to have a very well-defined sampling process, to collect a precise volume of the sample, which in many cases can be very restrictive. With simple samples, this is quite easy, but for more complex samples, this quickly becomes more difficult. It is also difficult to ensure that you perform a truly exhaustive extraction.

So with SPME, I proposed to not bother with that at all, and simply place the extraction phases directly within the sample, and quantify it based on the amount of the components which migrated and enriched into the extraction site over a fixed length of time. We called that kinetic extraction, and proposed calibration procedures based on timed extractions, and the diffusion coefficient and the mass transfer conditions for the sample.

SM: You have also done some work on extracting proteins from complex biological samples – SPME typically excludes macromolecules from the extraction, so how do you modify the technique to select for just proteins rather than small molecules?

JP: When you want to extract the proteins, that is a completely different challenge. You need to introduce some molecular recognition, in the same way some biosensors and affinity chromatography work – we have worked on this and made some progress, but it is very difficult to perform well in practice.

First of all, you need to design a surface containing high-density molecular recognition. The surface area available on the fiber is not very high, so a high concentration of ligands is required to make sure there is sufficient sensitivity.

Then, you need sorbent which will only selectively interact with proteins and eliminates no-specific adsorption. We only leave the device in contact with the sample for a few minutes to an hour, which is an advantage over some sensor designs which work this way, which might have to keep working for days, weeks, or years. But still, the biggest challenge is to design a strategy to eliminate non-specific adsorption.

SM: How do you see direct extraction techniques like SPME developing in the future?

JP: I think there is a lot of future promise in using nanoparticles as the collection medium. That gives a very fast extraction – however, it’s not very convenient to operate, compared to the fiber-based devices.

The other technology which I’ve been working on, which has historically been used on an industrial scale, is membrane extraction. In my opinion, membrane extraction is under-developed, primarily because the membrane will work best if you interface it directly with portable instrumentation. That way, the membrane acts as a barrier between the complex sample and the instrument, which is sensitive to contamination – only the volatile components penetrate the membrane, making it a simple, effective cleanup method. The main challenge in such approach is the calibration method able to account for varying convection conditions, the task we have accomplished by using calibrant in the stripping media.

SM: What are you looking forward to at Pittcon 2017?

JP: As every year, I am looking forward to see the exhibition, discuss the new developments in analytical instrumentation with my academic and industrial colleagues and interact with the participants interested in my research area.  Pittcon has been the most influential analytical meeting over many decades. The conference with its comprehensive approach to analytical chemistry both during scientific sessions and on the exhibit floor helped to deepen my understanding of the area I studied as graduate student. This is the meeting where I gave my first conference talk.  During the following decades Pittcon was integral part of education strategy for my students.   I have always been admiring and impacted by the contributions made by recipients of Pittcon Awards and now I am the one who is going to address the Pittcon audience. This is a real honor.

About Janusz Pawliszyn

Janusz Pawliszyn, native of Gdansk, Poland obtained his engineering BS degree and M.S. in bio-organic chemistry at the Technical University of Gdansk.  In 1979 he moved to the United States, where he completed his Ph.D. degree in Analytical Chemistry at Southern Illinois University as a first Ph.D. student of Professor John Phillips. There he introduced the concept of concentration modulation including thermal modulation, which is the basis of the comprehensive two-dimensional gas chromatography.  During 1982-1984 he worked at the University of Toronto as a Postdoctoral Fellow with Professor Michael Dignam designing photothermal deflection FT-IR instruments to study interfaces.

Professor Pawliszyn began his independent scientific carrier as Assistant Professor in 1984 at Utah State University, developing new concepts of universal and absorption concentration gradient detection based on probe laser beam deflection.  He applied this scheme to investigate transport through membranes and microfluids detection in high efficiency separation techniques including capillary electrophoresis.  He has also proposed the use of laser desorption to facilitate rapid gas chromatographic separation of poorly volatile high MW polymeric compounds. In his research, he explored the application of new electrooptical devices such optical fibres, light emitting and laser diodes to design microfluidic detectors.

Continuation of this work, a few years’ latter, led to the invention and development of a Solid Phase Microextraction. In 1988, Professor Pawliszyn moved to University of Waterloo, where he progressed through the ranks and in 1997 he was promoted to a Full Professor.  He developed a strong analytical program focusing in the area of analytical separation and sample preparation. He is internationally recognized, particularly in the latter area, for his fundamental contributions to solvent-free techniques including supercritical fluid, solid phase and membrane extraction techniques.

The breakthrough came in 1989, with the publication of the first paper on the application of fused silica fibers for the extraction of organic chemicals from water and their rapid transfer to capillary gas chromatographic columns. The paper heralded the invention of Solid Phase Microextraction, or SPME as it is generally known around the world. The term “SPME” was used for the first time in a paper published in 1990. It took just three years for the technology to be commercialized by Supelco, Inc. (now Millipore-Sigma).  SPME has been one of the most important sample preparation techniques in gas chromatography ever since.  Several other sample preparation techniques, such as hot water extraction and accelerated solvent extraction were proposed after his and co-workers' fundamental work on high temperature supercritical fluid extraction. He also developed needle trap technology and membrane extraction with a sorbent interface, sample preparation techniques for monitoring of organic compounds in on-site environments. In the area of analytical separation, he introduced the concept of the whole column detection by combining capillary separation with CCD imaging technology. This technology developed by his group formed the foundation of Convergent Bioscience (now “Protein Simple”), a Toronto-based company whose technology is now considered the Platinum Standard for characterizing proteins and peptides and it is widely accepted in Biotech Industry.

Dr. Pawliszyn has developed strong collaboration with manufactures of analytical instrumentation, which allowed him to commercialize his new concepts.  In 1995, Dr. Pawliszyn became the Industrial Research Chair in Analytical Chemistry and in 2003 Canada Research Chair, positions he holds to date. He has published over 550 publications to date and the Hirsch Index (H index) of Janusz is 85.

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