Variation in dtector sensitivity

[mojo]

Hi All,

We have 2 Agilent 6890 GC's. When we run our performance verification (6 injections of an FID test mix supplied by Agilent) on one GC, the peak areas are at about 400 pA. However, when we run the same sample on our other GC, the peak area is about 2000 pA. The method parameters are identical. This pattern is repeated across a number of our methods (methods include headspace and autosampler injecting, different columns, different liners, etc)with one GC response consistently about 20-25% of the other GC. The liners and grade of gases used on each GC are identical. Can anyone think of a reason? The only difference between the systems that I am aware of is that one system is capillary optimised, whereas the other has the option of using capillary or packed columns. On the system which has the loewr responses, the length of the jet for our FID is smaller than for the other system - the head of the jets and diameters are identical, it's just that one is longer by about 15 mm along it's shaft.

[chromatographer1]

But is the orifice of the jet the same?

A packed column jet will not give the sensitivity of a capillary column jet.

And your results sound about right if that were the case.

best wishes,

Rod

[mojo]

Hi Rod,

It is the GC which can handle packed columns that is giving us the greater response!

[chromatographer1]

But do you have the proper jet in the proper GC?

You need to measure the orifice or at least compare the jets.

Do you have a blockage or a mar at the jet tip?

Have you checked the flow rates for the detectors.

A factor of four is not unusual for detectors if settings are not optimized for the detector. You don't have to have noice for there to be a problem.

If it is important to you, then you should investigate further and confirm all the gas settings and the cleanliness of the detectors and jet. Electrical contacts can also make a difference.

It sounds like your capillary GC detector has a problem.

best wishes,

Rod

[GaryR]

Hi

Why don't you swap the jets over and see what response you get?

Gary

[AICMM]

Mojo,

As I recall, the reason the jets are different lengths is because they are different orifice sizes, one for capillary one for packed. Gary is right, you could swap jets but if you have the same signal to noise ratio, do you care? That is, do you have one fourth the noise on the one fourth response GC?

Best regards.

[gcguy]

I would put new jets of the same type into both detectors. The recommended jet for capilliary columns on the 6890 is the shorter jet. I would also check that the epc units are delivering the correct flows for the detector using a seperate flow meter. I have been caught out before by this. The instrument setting can be wrong and may require calibration, or in extreme cases the epc unit can be malfutioning and give a flow no where near the one the instrument says it it.
Do the responses vary the same amount for all the peaks in your chromatogram? If they do then it may be down to the injection, split flows etc.

GCguy