What Every GC-MS User Should Know About the GERSTEL TDU

Martin Perkins

21st July 2011

GERSTEL Multi Purpose Sampler Loads a Twister Stir Bar into the TDU

GERSTEL MultiPurpose Sampler Loads a Twister Stir Bar into the TDU

Many analysts find that their workload consists of a mixture of the routine and the unpredictable. A GC-MS is a great tool for both kinds of work, but sample preparation can be a problem. Most labs are well equipped for the regular work that they do, however, there is no telling what form non-routine samples may take or the kind of answers that the business may be looking for.

In this context, the GERSTEL TDU is becoming an increasingly significant product. It is at the heart of an outstandingly useful and  flexible sample preparation and automation system that brings great power to GC-MS users, by automating routine tasks and providing great flexibility to cope with issues that pop-up without warning..

What is it then that makes this combination of instrumentation so powerful?

The best place to start is with the story of how the TDU was born.

Roll back 10 years or so to the invention of Stir Bar Sorptive Extraction (SBSE). The original concept was that once analytes had been sorbed onto the Twister stir bar, they would be desorbed in a thermal desorber to both focus the analytes as they emerged from the PDMS coating and to transfer the analytes to the column in a tightly focused band. As the Twister became popular, it also became clear that there was room for a specialised thermal desorber that was smaller, less expensive and could be automated using the GERSTEL MultiPurpose Sampler (MPS). The Twister Desorption Unit (TDU) was born.

The new TDU fitted onto the top of a GERSTEL Cooled Injection System (CIS) in place of the septumless head assembly and was very closely coupled to the injection port liner. No transfer line, no valves and no places for analytes to be lost during transfer to the column. The vertical alignment, also meant that tubes containing Twister stir bars could be loaded and unloaded using the magnet on the bottom of the MPS syringe carriage. A nice, neat, simple arrangement.

The desorption of analytes from within the PDMS on the stir bar is relatively slow and so to ensure decent peak shapes, secondary focusing of the analytes is essential. When using the TDU, the CIS is operated at a sub-ambient temperature (using liquid nitrogen or in some cases liquid CO2) to trap the analytes as they emerge from the TDU. The CIS then heats rapidly to transfer the trapped analytes to the column in a nice tight band. All of this can be done either split or splitless, so everything desorbed from the stir bar can be transferred to the column, if trace analysis is the order of the day.

In next to no time, it became clear that the TDU could be used for the thermal desorption of packed tubes and the Twister Desorption Unit was increasingly referred to as the Thermal Desorption Unit.

Thermal Extraction of solids was the next technique to emerge, followed by the large volume injection of dirty extracts (so no clean-up needed). All of these techniques taking advantage of the systems ability to focus analytes in circumstances where the analytes emerge from the extractant or matrix slowly.

The combination of a TDU, CIS injector and an MPS sampler (AKA “The TDU Bundle”) is a formidable sample prep and automation tool. In addition, the introduction and continuous development of the Maestro control software has made it possible to “invent your own” sample preparation technique, since pretty much everything can be programmed and you can assemble any sequence of events that makes sense to you.

We have a system like this set-up on a GC-MSD in our application laboratory and it is in pretty much constant use – it is such a useful problem solver.

To see the TDU Bundle working in our Cambridge laboratory, or to learn more, call 01954 212909 or email enquiries@anatune.co.uk.