Chromatography Forum

Hi,

As part of an analytical method I am testing, I will be extracting analytes (at ultra-trace levels which have been deposited on a solid matrix) using various extraction solvents. These will be recovered, evaporated to dryness and will then be reconstituted in my analytical solvent ready for GC-MS.

Because my analytes are at very low levels, I would like to minimise reconstitution volume and wonder if anyone had any experience of small volume samples. I would like to go down to around 10µL (if that is practicable) and have bought a bunch of vial inserts (the ones with plastic feet) with a retention volume of 1µL which I guess gives me the best chance.

With this in mind, I would be really grateful if you might consider the following questions:

1. Is 10µL a feasible volume? Has anyone gone lower successfully?
2. Will the needle depth require adjusting given that the plastic springs push the inserts up fairly high?
3. Obviously auto-sampler 'sample wash' (where the sample is drawn up to remove residual solvent from the syringe) is not feasible... Should I also get rid of the auto-sampler bubble removal step (where the sample is drawn up and pumped) too?

I recognise that issue 3 is far from ideal and that my RSD will suffer, but on the plus side, the method is qualitative...

Many thanks for your opinions on this.

Kind Regards

TD2

1. Is 10µL a feasible volume? Has anyone gone lower successfully?
2. Will the needle depth require adjusting given that the plastic springs push the inserts up fairly high?
3. Obviously auto-sampler 'sample wash' (where the sample is drawn up to remove residual solvent from the syringe) is not feasible... Should I also get rid of the auto-sampler bubble removal step (where the sample is drawn up and pumped) too?

10µL is feasible if you use good quality tapered vials or inserts and have an autosampler needle that is not the type with a side port way up the side.
I'm not a fan of the drawn vials that rely on feet or other attachments to maintain a vertical position in an autosampler. This type frequently gets mis-aligned on insertion and either jams the autosampler or gets flung across the room. When that is not happening, they're trying to Z your needles because of inconsistent depths from vial to vial or insert to insert. Good small volume vials are not cheap, but they're very much worth their cost.

Altering your needle depth, assuming the needle draws from the bottom or close enough to the bottom, will probably be necessary. If you use any type of insert, you need to make sure your needle is hitting the centers of your vials too.

Do all of your needle washing after each injection. If possible, go for a little wash solvent, an air gap, then your sample... It may not be 100% accurate, but you should be using an internal standard for this type of method anyway, so you're more interested in precision than accuracy. Keep the bubble (or at least try it that way). This can be done, but it all requires a lot more close watching than usual, and you'll probably go through a needle or two at some point.

I used these same inserts, with the plastic spring feet and found they work well, but you need to make sure the needle doesn't push down too hard into them or it can cause a seal around the tip of the needle and the sides of the smallest portion at the bottom and will prevent the needle from drawing sample because of vacuum. I just adjusted the sample depth up 1mm at a time until I got consistent injections.

For such a small volume and to reduce bubbles, you may want to experiment with plunder draw speed and set it low and see if that improves consistency with results.

Dear Both,

Great advise and thank you both for taking the time to provide some brilliant (and comprehensive) responses.

Have a great Christmas and New Year

TD2

We never used little spring inserts at my employer, we bought the small-volume vials. Yes, they were more expensive, but we concentrated on getting results the first time and not having to repeat sample preparations. My 2¢.

You might consider increasing the injection volume and the sample volume in proportion to one another - how practical a large volume injection will be depends on your application.