Large Volume Injection Revisited
For every microliter of sample injected into a hot GC injector, the explosive vaporisation of the solvent would create a volume of around 0.7ml of vapour and so injection volumes were limited by the capacity of the injection port liner. Your injection volume simply had to be in the range 0.1-2 microliters.
As a consequence, when designing sample preparation procedures, the allowable injection volume represented a small target to aim for. If your sample was too concentrated, it was easy enough to dilute it, if the concentration was low, then you would have to use a concentration step – which is always more complex, time-consuming and problematical.
When the programmed temperature vaporisation (PTV) injector became available, it enabled samples to be injected at temperatures below the boiling point of the solvent and so avoid the sudden generation of a large solvent cloud as the sample was introduced into the instrument.
This meant that the PTV could be held at a low initial temperature, with the split line wide open, until the solvent evaporated to vent. With most of the solvent gone, and the split line closed, the injector could then be gently programmed to a high temperature to transfer the analytes onto the column. The volume limitation then shifted to how much liquid phase sample the injector could accommodate – something that was much easier to manage.
In the mid-1990s, large volume injection became something that lots of labs tried on real samples and the pros & cons became apparent…
The advantages of large volume injection…
- Used in addition to sample pre-concentration, much lower detection limits are possible than ever before. This is especially true if you only have a small amount of sample to work with in the first place (saliva samples for example).
- It can eliminate the need for the pre-concentration of samples – reducing work and human error.
- LVI can permit a big reduction in the volume of individual samples that a laboratory needs – 100ml is a whole lot easier to transport and process than a 1 litre sample.
The limitations of large volume injection…
- Methods are more complex to develop and trickier to troubleshoot when things go wrong. Some analysts have great success with LVI, others – none at all.
- Of the PTV/autosampler combinations that are commercially available, not all are well suited to large volume injection.
- Solvent purity can be a real issue, since you are concentrating artefacts in the solvent along with your analytes.
- If you are working with high matrix samples and you can’t see your analytes for background components – LVI will do nothing for you. You end up just putting more of everything on the column.
Nowadays, while large volume injection is used by a small number of devoted advocates, it has never become a mainstream technique.
Of late, we have experienced a spike in interest in the topic of LVI. The reasons in favour of using it haven’t really changed. The difference has come from the increasing use of highly selective GC-MS systems such as the GC triple quad and GC high resolution accurate mass time of flight instruments.
If you consider the limitations I have listed above, the use of a very selective MS as a detector goes a long way to solving problems of solvent purity and matrix background. Plus modern electronic gas controls are very precise which makes LVI methods more robust and predictable in everyday use. That is why LVI is back in favour again.
What you need to be successful with large volume injection
This comes down to having a PTV/autosampler combination with all of the characteristics needed and knowing how to use them. Let’s look at the PTV first.
- Generally, when using a PTV to inject small injection volumes, you only need to get it to a start temperature that is below the pressure corrected boiling point of your solvent. With LVI, with the need to eliminate the solvent in a controlled fashion, the initial temperature is an important parameter to optimise and needs to be lower – typically, around room temperature. An air cooled injector is unsuitable as it will struggle to cool sufficiently. You must have an injector that uses Peltier cooling or if not, at least cryogenically cooled.
- Your autosampler must be able to accommodate a syringe of sufficient size. Some autosamplers can only inject a maximum of 50% of the syringe volume – so best to check.
- Your use of syringe wash solvents will increase in proportion to your increased injection volume. If you aim to inject 50 microliters, you may well need 0.5ml of wash sample for each sample you run. Take care that your maximum sample batch isn’t limited by the volume of wash solvent you have available.
- Slow injection is the norm. You need to be able to match your speed of injection to the speed of solvent evaporation. Check that you can handle slow injection speeds and these can set finely.
- Software that calculates the solvent elimination rates for you will save you a lot of time and heartache.
- Access to someone with prior experience of LVI who can provide help and advice when needed.
A GERSTEL CIS 4 PTV with a GERSTEL MPS sampler will do all of this for you and it is what we use for this work in our lab.
To summarise, large volume injection may no longer be as fashionable as it once was, but it is probably more useful than it has ever been. We have more LVI lab work scheduled on our MultiFlex GC/qTOF and we will write about it here before too long.
Here is an application note showing some successful work that we carried out recently on the LVI of explosives (the ultimate thermally labile analytes).
If you want to know more, feel free to call on 01223 279210 or email email@example.com.