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Things you can do with a MultiFlex (No. 3) – Twister or Stir-Bar Sorptive Extraction and Desorption

Martin Perkins

19th November 2013


Twister Stir-Bar is removed from a sample bottle

Twister or Stir-Bar Sorptive Extraction (SBSE) was the final great sample preparation invention of the 20th Century.

Analytical chemists throughout the 90’s had appreciated the usefulness of poly-dimethylsiloxane (PDMS) as a trapping sorbent because it:

  • Eliminates the need for costly or hazardous extraction solvents,
  • Easy to predict recovery using the widely available Octanol/Water coefficient KO/W
  • Stable at high temperatures and in strong solvents.

However at the time the existing methods had drawbacks: Tubular Coated PDMS Traps suffered from practical limitations; while SPME, which had been developed at the beginning of the 90’s had great success for many applications, but was limited for trace level work by its low phase ratio.

Then in 1999 Pat Sandra and his colleagues at the Research Institute of Chromatography (RIC) developed Stir-Bar Sorptive Extraction:

An inert, glass-encased magnetic stir-bar with a thick coating of PDMS could be spun inside a vial or flask on a magnetic stirrer and analytes would partition into the polymer.  Then the Twister was popped into a Thermal Desorption Unit that sat above a cold focusing GC inlet, and the full volatile and semi-volatile profile could be chromatographed.

The results were impressive, and within months of invention, the technique had been applied to a diverse range of problems from flavour profiles in dairy foods to monitoring waste water.

Twister has the big advantage of a thick phase coating, with typical PDMS volumes of 100 microlitres, so for certain hydrophobic analytes enrichment of over 1000 fold can be achieved.

The concentration power of the large phase ratio on Twisters makes SBSE ideal for testing drinking water for mal-odour compounds, since the chemicals that cause certain bad smells can still be noticeable to the consumer’s nose even at tiny levels that still challenge modern analytical systems.

This year, nearly 300 articles have been published on SBSE and the second conference dedicated to Twister was held at Paris in February.

The range of phases available now includes PolyAcrylate (PA) and Ethylene Glycol (EG) chemistries which enable extraction of higher polarity species.

The US EPA routinely use Twisters to monitor water pollutants, since spinning twisters is easy to do near sampling sites, and sending tiny stir-bars back to the lab is easier than transporting bulk bottles of water.  SBSE was ideal for monitoring Gulf-of-Mexico seawater after the Horizon oil rig disaster.

Cosmetics manufacturers use Twisters for monitoring and matching the fragrances of their products, without fouling their GC inlets.

Food and beverage companies use SBSE to objectively monitor aroma and flavour.

The Anatune MultiFlex is an MPS MultiSampler configured for automated thermal extraction with a GERSTEL Thermal Desorption Unit (TDU) and Cooled Injection System (CIS4).  It can desorb from Stir-Bars just like other desorption tubes, using the same standard configuration.  This means that you don’t need to spend any time adjusting your hardware, just rack up your Twisters and go.

If you have an analytical challenge that you’d like to try cracking with Twister Stir-Bar Sorptive Extraction, then call us on 01223 297210, or email enquiries@anatune.co.uk.