Improving Headspace analysis with capillaries

Discussions about sample preparation: extraction, cleanup, derivatization, etc.

20 posts Page 1 of 2
Hey people,

I'm trying to improve the repetitiveness and cleanliness of my VOC Headspace analysis through usage of glass Glass micro capillaries

I got the idea when the local technician used those to test repetitiveness of our GC/MS and GC/FID in the yearly operational qualification test.

If they're good enough for this, they must be constant enough to carry standards, right?

I want to try using them to carry internal standards and calibration standards into headspace vials both for solvent assay tests and for EPA 8260.

There's a bit of added cost involved, but I'm thinking that the advantages overweigh the costs by far, those include:

1) More consistent standard volumes
2) Less, or even, no cross contamination at all from using syringes.
3) Less solvent and standard usage, no need to pre-rinse syringes with standards, no need to post-rinse syringes with clean solvent.
4) Less syringe wear-and-tear by simply using them less.

And I'm sure that there are even more advantages.

What do you think, is this a good solution or am I missing something important?
Hi Mark

There have been some discussions before on the use of glass micro capillaries - it may be worth looking at them.

Personally I have had great success in their use either

a) to use a pre-tared capillary tube to take up a volume of a low volatility liquid (2-10uL) - reweigh then drop the tube into a volumetric flask partially filled with solvent - then make up to the mark

b) Alternatively, use a prepared w/v solution. Dip the tube in to fill to the mark by capillary action ( if overfilled touch the tip onto some tissue to bring it down to the mark) - then drop into the solvent as above for dilution

Your query may be more complicated than it first appears

If using b) and dropping into a 25ml headspace ( you may have had to cut a portion off the top of the tube before use for it to fit in) you only need to ensure that all of the contents get out of the tube and into the headspace.

Without further information then yes to your queries 1 - 4 :-)

Kind regards

Ralph
1)The glass capillaries I've linked to in the first post are 32mm in length, short enough to fit below the 10mL MMS solution in a 8260 headspace vial, so I don't think there should be a problem with the solvent dispensing, although that's precisely one of the reasons I've asked here in the first place.

2) I don't even have to worry if 100% of the standard exits from the capillary into the headspace IF the recoveries are consistent since I'm prepared to calibrate using the capillaries so for example, if, consistently, 90% of the standard is recovered I still calibrate and thus compensate for it.
Hi Mark


I don't even have to worry if 100% of the standard exits from the capillary into the headspace IF the recoveries are consistent since I'm prepared to calibrate using the capillaries so for example, if, consistently, 90% of the standard is recovered I still calibrate and thus compensate for it.


If added to an empty vial I would still prefer to see that capillary empty. It really wouldn't take long for that to happen under normal conditions in your sealed headspace vial and for you to get everything into the headspace and equilibrate and your recovery would be consistent



The glass capillaries I've linked to in the first post are 32mm in length, short enough to fit below the 10mL MMS solution in a 8260 headspace vial,


do you mean that you already have 10ml solvent in the headspace vial and you are adding your capillary tube to it?

If added to a headspace vial containing solvent just give it a good shake

I may well be misinterpreting your post and method details. I am not familiar with this method and for that I apologise

Regards

Ralph

By standard I am assuming a standard analyte in solvent. If so what is your solvent and analyte and the size of your headspace vial?
In EPA8260/5021, we put solids in 10mL MMS solution (DDW saturated with NaCl) or 5mL liquid sample + 5mL MMS solution, and add 10uL Internal Standard to it.

I'm planning on using capillaries to add these 10uL internal standard, and for CCV tests, which use 50uL calibration standard, so I'm hoping the capillaries will help.
Microcapillaries work as accurately as syringes as long as you use them properly. They are not good for volatile solvents (but do work for solutions of non-volatiles in volatile solvents) and they are calibrated to contain not to deliver, which means that they have to be rinsed to transfer their contents. When filling, dip only the tip of the capillary in the liquid, otherwise you will get liquid stuck to the outside, you can wipe it off with a clean tissue but that risks drawing liquid out of the tube.

As an exercise to show whether capillary contents transfer into liquid; fill one with a coloured liquid (iodine dissolves in most organic solvents to give an intense colour), and see what it takes to get all the colour out of the capillary and into the solution.

Peter
Peter Apps
Sorry to be a bit dense but I am really uncertain as to why you are using syringes for dispensing in lieu of pipettors? We've been using Drummond pipettors for years to pipet in internal standards and calibration standards for headspace. There is no carryover; you change tips, and they are TD, not TC like the microcapillaries.

Dispense below the surface of the MMS solution, cap and then give it a good shake.
Mark Krause
Laboratory Director
Krause Analytical
Austin, TX USA
1) I don't want to use plastic pipette tips with solvents.
2) I know that smaller volume pipettes are available but the smallest we have are 50-200uL. I'm using 10uL InSt in each sample (yes I could dilute but I'm trying to simplify the process, not add more steps.)
Hi Mark



Just to take a step back

You said that your solvent is DDW = deionised distilled water?

1) I don't want to use plastic pipette tips with solvents.


Why not? I can understand it if using an aggressive solvent that will damage the seals of the pipetter

2) I know that smaller volume pipettes are available but the smallest we have are 50-200uL. I'm using 10uL InSt in each sample (yes I could dilute but I'm trying to simplify the process, not add more steps.)


Bearing in mind the previous helpful replies. Why not purchase a 10-20ul pipetter? Used correctly it should be fine

Am I missing something? :-)

Regards

Ralph
Plastic pipette tips are probably cleaner than you think - I did some tests and was pleasantly surprised. YMMV.

McKrause's comment about carryover does not apply if you are adding IS because you will be using the same solution over and over - you do not need to drop the tip of an autopipettor (but beware that the first dispense will deliver a bit less than the subsequent ones - dump it to waste) but also you do not need to rinse a syringe, so between a syringe with a plunger stop and an autopipette I doubt that there is much to choose in speed and convenience.

Peter
Peter Apps
Drummond micropipettors use glass capillaries. I, too, have used both plastic and glass and haven't found a lot of difference, but using the Drummonds addresses all of the doubts.

My carryover comment was in regard to calibration standards. Peter is obviously correct; for internal standards there is no carryover.
Mark Krause
Laboratory Director
Krause Analytical
Austin, TX USA
Just received the capillaries, kinda bummed out with it, they don't seem to draw the standard end-to-end like advertised, any liquid kinda stops filling up ~1.5 mm from the top...I have the pipetting aid, and it too does not seem to help much.

I tried searching youtube for a guide on usage, but did not find anything.

Wondering if it's a problem with my technique or something else entirely.
That's odd. What volume are the capillaries, and what liquid are you working with ?. Organic solvents do not draw up as well as water, and larger volume capillaries do not draw up as well as thin ones. What happens if you lean them to one side ?

Peter
Peter Apps
I have been thinking about that. I had problem with Headspace. I always had good calibration curve r2>95. However after calibration, repetetion test fail. For 1 ppm I got, 1, 1.2, 1.5, 2... and go up. Technican solved my problem (methanol pressure in vial was bigger than the vial filling pressure). But I was going to use cappiler. I hope it works fine for you.
Peter Apps wrote:
That's odd. What volume are the capillaries, and what liquid are you working with ?. Organic solvents do not draw up as well as water, and larger volume capillaries do not draw up as well as thin ones. What happens if you lean them to one side ?

Peter


I'm trying with 10uL and 50uL capillaries from Brand
http://www.brand.de/en/products/clinica ... pillaries/

So far I've failed filling them end-to-end both with our methanol-based standard, and with DDW, that's why I said that I'm not sure that my technique is right. The tube seems to fill to the end when lowered further into the liquid, then the liquid "recedes" 1-1.5 mm down the tube. I've tried lowering it straight, and at an angle.
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