Syringe which sucks air

[Jean_Noel]

Hello,

I work with a Agilent 6890N GC equipped with a CTC Combi Pal which is adjusted to a syringe with a needle lenghth of 7 cm (2.5 inch). I tried different solvents and syringes but I am not able to get the "same" peak area three times (it varies a lot).

So I don't know if a have to re-adjust the whole CTC Combi Pal to shorter syringes or if there is the possibility to change the wash volume of the CTC - to get rid of the air bubbles.

The GC itself works fine since with headspace analysis I get very good results.

Thanky for any help

[Peter Apps]

The settings on the autosampler need to match the syringe you are using, otherwise the needle depth settings for sampling, washing and injection will not be accurate.

There are also a lot of other reasons why you might be getting air bubbles and poor repeatability.

Peter

[Jake]

First off - while CombiPALs may have many attributes, precision and accuracy aren't ones. That being said, provide more details: what PAL cycle are you using? What type of syringe and needle type? How many filling strokes? What's your volume of solvent pre-rinse? Draw speed? Etc. As Peter alludes to, there are many variables (and hence, reasons) that contribute to poorer-than-usual precision on a PAL.

[Jean_Noel]

Thanks for the answer

So I use a CTC Combi Pal (MXY02-01 B with an injection tower MZ02-00E).

I use a Hamilton syringe with a needle lenght of 7 cm, 23s, either conical or beveled. I wash the syringe 6 times with the solvent before injecting the sample. There is no difference when using different solvents ( I tried it). There is also no change when using other washing solvents.

I have an filling speed of 5 µl/s for 1 µl and I plunge 6 times ( to get rid of the most bubbles). The washing volume is constant and about 5µl but this parameter I can't change (as I see).

It seems that I either change the whole needle to a smaller syringe (which has to be avoided) or that I wash my syringe with a higher amount of washing sample but this isn't programmable (as I saw) with the CTC handheld.

I get now in SIM Mode and using an Internal Standard a R^2 of 0.98 for Limonen (in the best cases), which would look bad but I can't show anyone the calibration curve since every injection point of each standard varies due to a lack of reproducibility.

I don't know if it's the CTC Combi Pal since with Headspace I achieve a R^2 of 0.99 with external calibration.

[James_Ball]

Are you doing split or splitless injections?

I have seen times when the splitless solenoid valve in the 6890 can get dirty and not completely seal when in splitless mode and it will give bad reproducibility between injections but in split mode it will be normal.

[Peter Apps]

What volume syringe are you using ?, and why is it a particular problem to use another volume ? Does it have a fixed or removable needle, and do you have a spare that you can exchange it for to check that the needle is properly sealed to the barrel ?

To get rid of bubbles you need to draw up slowly and expel quickly. Reduce your uptake speed to 1 ul/s.

Peter

[Jake]

Peter and James make good points.

Additionally, have you tried a different syringe? Is it fixed- or removable-needle? We use removable-needle Hamilton syringes in our PALs, and on rare occasions, they do loosen, allowing air to be drawn in too during filling.

[Jean_Noel]

Are you doing split or splitless injections?

I have seen times when the splitless solenoid valve in the 6890 can get dirty and not completely seal when in splitless mode and it will give bad reproducibility between injections but in split mode it will be normal.

So I always inject splitless since I have low concentratred molecules. But I will try it split. What woul be a normal split ration to begin with ? 1:100 1:3 or 1:200 ?

[Jean_Noel]

What volume syringe are you using ?, and why is it a particular problem to use another volume ? Does it have a fixed or removable needle, and do you have a spare that you can exchange it for to check that the needle is properly sealed to the barrel ?

To get rid of bubbles you need to draw up slowly and expel quickly. Reduce your uptake speed to 1 ul/s.

Peter

Thanks Peter. I have a Hamilton 10 µl with a fixed needle. I will draw the sample with a much slower speed.

I meant with using another volume that I would have to change the washing volume from 5 µl to I don't know 8 µl but I didn't saw any changing option on the handheld to do that. I thought that with a higher washing volume I would be able to rinse the syringe in a better way so that I can get rid of the bubbels. I saw that when I wash the sytinge manually by pulling up 10 µl of washing solution for 6 or 7 times I get no bubbles in my volume.

[Jean_Noel]

Peter and James make good points.

Additionally, have you tried a different syringe? Is it fixed- or removable-needle? We use removable-needle Hamilton syringes in our PALs, and on rare occasions, they do loosen, allowing air to be drawn in too during filling.

I used 2 different needles from two different brands where I have to say that the Hamilton works better. They are all fixed.

[Jean_Noel]

To summarize the whole problem and to give advise what to do, if there is a lack of reproducibility - here is my solution:
-First of all use a split liner filled with glass wool and a split mode.
- Increase the washing volume of the syringe to 60% ( at the CTC handheld it is done in setup in the service menu which is available by pressing F3 and enter. There you can change under object the Wash Volume)
-Use a short needle (5 cm) with a gauge of 26S
- Use less volume to inject than 1 µl ( you can change it in the service menu by changing the smallest volume of the syringe to 0 µl) - With this you can inject 0.5 µl
- Use a backflash calculator to see, if the solvent is adequate:
http://www.restek.com/images/calcs/calc_backflash.htm


Good luck