Chromatography Forum

Hi all,
I am new in the GC, I am using the GC 6980 and the software Chemistation from Agilent.
In my setting from Chemistation software i set the injection amount is 1 ul (using 10 ul syringe), howewer I always observed that the syringe in the front injector takes "different amount" (i am not really sure if it is different because I saw the level from the barrel)
i am wondering if something wrong or it is the way machine setting up.
For instance:

if I set 1 ul > syringe pump 3 times and i see 2ul level in barrel
if i set 2ul > the syringe pumps 4 times and I see 3 ul level in barrel
Thank you

Welcome to the forum

The reason that you see the tip of the syringe plunger at a volume 1 ul more than you set is that after the syringe has picked up the volume you set, and the needle has been lifted out of the sample, the plunger moves up an extra 1 ul to pull the sample up out of the needle. There is 1 ul of air in the needle below the sample.

Peter

Hi Peter,
Thank you for the quick response.
I would like to ask that if there is any way to change the setting to without taking air in the needles in Agilent GC6890. I am trying to set from electric blook but it does not help.
Thank you.

I don't work with Agilents, so I can't help with the technical side.

Why do you not want air in the needle ?

Peter

...
if I set 1 ul > syringe pump 3 times and i see 2ul level in barrel
if i set 2ul > the syringe pumps 4 times and I see 3 ul level in barrel
Thank you

If you mean "pumps" = syringe wash than number of pumps has nothing to do with injection volume and should not change when you change injection volume.

I don't work with Agilents, so I can't help with the technical side.

Why do you not want air in the needle ?

Peter

Thank you, i want to make sure that the the amount injection is correct.

...
if I set 1 ul > syringe pump 3 times and i see 2ul level in barrel
if i set 2ul > the syringe pumps 4 times and I see 3 ul level in barrel
Thank you

If you mean "pumps" = syringe wash than number of pumps has nothing to do with injection volume and should not change when you change injection volume.

I am sorry for my poor English, I mean that If i set to inject 1ul in the chemistation - for the front injector (from desktop computer) then I check the GC system, by press to the Front Injector (in the method storage and automation) I see in the small screen in GC system appear:
Front injector
Injection Vol 2
#Sample pums 3
...
Here I mean that the GC system automatic convert to 2 ul, even I set the front injector as 1ul.
Similarly, when I set to inject 0.2ul, the GC will move to 1 il, 1 to 2, 2 to 3, 3 to 4 and 4 to 5.
If i changed the syringe volumes to 5 ul, the similar thing was also happened,
0.1 to 1,
0.5 to 2
1 to 3,
1.5 to 4
and 2 to 5

So I am wondering if how the volumne in GC system can be understand: it is the volumne that actual amount of sample to be injected, or the is number to indicated the level to be injected from chemistation setting.

Moreover, I also saw that the niddle inside synringe moved according to the volumne setting from GC system. That what I mean in the previous post, if I set inject 1 ul from chemistation > GCsystem read as inject 2ul > The neddle move up two times when taking the sample (so I observed that the head of needle is at 2 ul in barrell).
I hope it is clear to you. Thank you

The "needle inside the syringe" is the plunger whose movement brings sample into the syringe and then expels it into the injector.

If you want to know the actual volume injected then look at the liquid in the syringe before injection (it will probably be close to 2 ul) and then after injection (close to 1 ul) and subtract after from before.

The volume injected is always larger than the volume displaced by the movement of the plunger (which is the volume you set on the Chemstation) unless the inlet is cold, or the needle moves so quickly that it is not heated at all by the hot inlet. If the needle warms up the sample in the needle expands and more goes out of the tip of the needle, or if the needle gets above the boiling point of the solvent the sample boil sin the needle and blows liquid and vapour out of the needle, leaving the needle partly empty.

As long as samples and standards are injected under the same conditions the actual volume injected makes no difference.

Peter