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LVI-SV with MMI Inlet Using a Restek Premium 2.0 mm Single Taper Liner w/ Wool

23 Apr 2015

Agilent’s Multimode (MMI) Inlet can be used as a PTV (programmable temperature vaporization) type inlet for large volume injections, with the advantage of using standard sized split/splitless liners.  This technique requires careful optimization of a number of parameters, which starts by choosing a proper inlet liner.  I recommend trying a Restek Premium 2.0 mm Single Taper Liner with wool.

When performing large volume injections with solvent venting (LVI-SV), you are essentially trying to inject your sample at a low temperature, evaporate most of the solvent out, and then rapidly elevate the inlet temperature (after closing the vent) to transfer analytes to the column.  In order to perform solvent venting, there should be a “platform” on which to catch and hold your sample, and serve as a “staging area” for the solvent venting to occur.  Wool makes a great platform for this, as it has a large, relatively even surface area and can contain a great amount of heat energy with which to evaporate the solvent from.  The wool will essentially catch your sample as it’s injected (preventing it from going straight out the vent) and simultaneously allow for evaporation of solvent, while retaining analytes of interest.  Solvent venting allows for the introduction of large volumes of liquid, which can potentially lower detection limits, reduce solvent use by starting with lower sample volume, or require less prep time by not needing to concentrate the final extract as much.

As an example, I performed some injections of Restek’s QuEChERS performance mixes using an MMI inlet in solvent venting mode with a Restek Premium 2.0 mm single taper liner with wool.  The mixes were diluted in acetonitrile, which can be a tricky solvent due to its polarity and high expansion coefficient.  With solvent venting, however, you can eliminate most of the solvent prior to transferring your analytes to the column, allowing for better focusing and no issues with overloading the liner with solvent.


MMI_Cgram1

MMI_Cgram2

MMI_Cgram3


MMI_Parameters

Choose an inlet starting temperature that is below the boiling point of the solvent.  I chose 80⁰ C, which though very close to the boiling point of acetonitrile (~82⁰C), worked fine due to the inlet actually being significantly cooler at the bottom, where the wool resides.  Once you choose a starting temperature, injection speed and venting time must be optimized around the elimination rate of your solvent.  I recommend using a solvent elimination calculator, such as the one included in Agilent’s software, to get a basic idea of where to start.  From here you can adjust parameters until you get desired results.  A venting time that is too long can lead to loss of analytes, whereas a short venting time will lead to excess solvent in your column, causing focusing issues and/or a large tailing solvent peak.

Vent pressure is a variable that will affect the amount of solvent that gets vented vs. loaded onto your column.  Higher vent pressures will lead to more solvent loading onto the column.  Vent flow affects elimination rate; with higher flows leading to quicker venting.  For inlet ramp rate, you generally want to choose a relatively high rate to help reduce time for negative interactions between the analytes and active sites in the inlet, once heating begins.

I kept the oven starting temperature well below the boiling point of acetonitrile to ensure good focusing at the head of the column.

As a final warning, while wool makes an efficient and consistent platform for solvent venting and retaining analytes during this process, there is always the risk of activity towards certain sensitive analytes.  From the QuEChERS mixes, I had issues with dicofol breaking down into DCBP, which may have been due to interactions with the wool.  I also saw decreasing responses of dichlofluanid, captan, and folpet (three related pesticides) with consecutive injections.  I did find this latter issue to be the case in liners without wool, as well though, and believe that the acetonitrile may have been negatively impacting these base sensitive pesticides and their interaction with the deactivation.  I’d be interested to know if anyone else has run into this and found ways around it.  Fortifying with acetic or formic acid may make a difference, though I did not get a chance to try this.

I’d love to hear about your experiences with solvent venting and what works/doesn’t work for you.