Combining large volume injectors and solid phase extraction

The most common protocol adopted by environmental laboratories when determining trace organics in a range of matrices is: liquid-liquid extract from large volume of sample, perform some sort of extract clean-up, vent off the large quantity of solvents, reconstitute the sample in a small volume of solvent, and finally inject 1-2 µL into a GC.

The combination of Solid Phase Extraction (SPE) and Large Volume Injectors (LVI) will simplify the above protocol as well as deliver lower detection limits and the opportunity of working with less quantities of sample with considerable savings of time, solvent and efforts. As an example, let's look at how to perform the same work under the two distinct protocols. Let's assume that the traditional protocol involves extracting 1 L of sample with up to 100 mL of solvent, cleaning-up over an florosil bed, bringing the extract to dryness, making up to 2 mL and inject 2 µL into a GC. This could be altered to involve passing 100 mL of sample over an SPE column, extract into 1 mL of solvent and inject 100 µL into a GC. This is a 10-fold increase in sensitivity although using only one tenth of the sample. The time and solvent savings are self-evident.

SPE

SPE is a sample preparation technique that through some specific retention mechanism (ie, normal phase, reversed phased and ion exchange) concentrates the compounds of interest, purifies them and they are eluted into a solvent appropriate for the final analysis.

The vast array of SPE sorbent material available through specialist suppliers like IST means that the chemist can tailor the protocol fairly precisely to eliminate interferences and improve all the aspects of chromatography. The introduction of multimode and layered SPE columns by IST has allowed entire classes of compounds of more than one class of compounds to be extracted concurrently from matrices as difficult as body fluid with chromatograms showing baseline resolution for all compounds.

LVI

LVI are devices that can be retrofitted to any GC and replace conventional split/splitless injectors. In traditional GC, the injector is held at high temperature (normally 30-50°C above the final temperature of the GC) so that any sample entering the injector is flashed vaporised and the resulting vapours transported quickly into the analytical column. The limitations of this type of injector are well known. Large sample sizes can't be injected since these would cause cooling of the injector, poor vaporisation and too large a volume of solvent vapours that flood the analytical column. LVI is set at a temperature below the boiling point of the solvent but yet high enough to allow the solvent to vaporise at a steady rate. A volume of 10-500 µL of sample is injected into the cold injector, and the solvent is vented during this period. When the amount of solvent vented falls below an operator-set value, the vent line closes, the pressure in the LVI is increased and the injector is heated very rapidly (eg, at a rate up to 16°C/sec) to its final temperature. During this period, all the vapours enter the analytical column. Once the final temperature is reached, the injector, opened to vent, is held at its final temperature for a period up to the end of the analysis while the septum is purged. When the analysis is completed, the injector is cooled to its initial temperature ready for the next injection.

System Performance

A mixture containing more than 50 compounds was analysed on an HP6890 fitted with a DB5 column (30 x 0.32 mm ID x 0.25 µm) and FID. The front split/splitless injector was replaced with the ATAS Optic 2. The analysis time was about 30 minutes with a final column temperature of 320°C.

The concentrations of the mixture tested were in the range 0.5 to 10.0 ng/mL and 50 µL was the injection volume.

Late eluting peaks in such a complex mixture are often troublesome both in retention time reproducibility, resolution and linearity. For these reasons a pair of peaks eluting at 23.6 and 23.8 min was used for assessing the Optic 2. This pair was baseline resolved throughout the concentration range.

The results indicated that large volume injectors combined with solid phase extraction is a worthwhile investment in the environmental laboratory as it presents the opportunity to improve detection limits while reducing costs.

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