Chromatography Forum

Hi all.

I'm running an HP 6890 GC/FID, looking at diesel. One day this week, I fired up my instrument and am getting about 50-60% recovery on my CCV and spikes. It hadn't been run in a few weeks, but the instrument is kept "on" at all times. What could cause this??

Conditions:

HP 6890 GC/FID (dual)
splitless 3uL injection
HP-5MS, 30mX0.25mmX0.5um
0.5min equil
Oven: 40°C for 1min
55°C/min to 140°C hold 0
45°C/min to 210°C hold 0
30°C/min to 300°C hold 4 min
Column flow: 1.5mL/min hold 4
7mL/min hold 999

Loss of signal can have many causes. The first questions that come to mind are:
1) when did you last to inlet maintanance - particularly change the septurm. (A corred septum will give a significant, and often irreproducable loss of signal.)

2) have you checked for leaks wiht a leak detector around the nut holding the column at the bottom of the inlet? Ferrules can work loose over time. And it is worth checking around the top of the inlet and around the gold seal to be sure that nothing is loose anwhere else around the inlet.

3) Are there any changes in retention time or peak shape? These changes, or lack thereof, are important clues.

crosemeyer,

I would work with Don's 3rd comment first. If retention times are the same, I would not suspect a leak but rather look at just the things that affect sensitivity. Then, using Don's comment about inlet maintenance, I would focus on the column position in the liner, in particular if you use a gooseneck liner since column position in or out of the gooseneck can have a significant impact on overall sensitivity. Then I would look at the detector flow conditions to make sure they are where they should be (think 10:1 air/hydrogen more or less.) Then I would look at the column position in the detector to make sure it is not sticking up above the jet. Finally, I would pull the jet to make sure it is not clogged.

Retention time is the key to splitting this problem in half. If retention times are fine, focus only on that which affects sensitivity.

Best regards,

AICMM

When you say that recoveries have decreased, how are you measuring them ?, and what are you comparing them against ?. In other words, have all the peaks on the chromatogram got smaller by about the same amount, or are only some affected ? If only some of them are affected we can immediately rule out detector problems.

Have you made up fresh standards, spikes etc ?

Peter

Well I did some inlet maintenence, changed liner, septum, gold seal, and cut a bit off of the column, and the problem seems to have cleared up. It appeared some glass wool from my liner made it's way down and clogged up the gold seal.

Thanks for the advice, I'm barely a year into GC, and am not a real chemist, just a biologist who fell into the field and has been doing IC and HPLC for ~5 years.

Thanks for the feedback.

"just a biologist who fell into the field" me too

Peter

Glad you got it fixed. FYI - if someone turned off the makeup gas to the FID, this can drastically change the sensitivity also. Not so much of a problem with the newer instruments, but it was with the old pneumatic instruments where you manually turned off the makeup to light the flame.

I run an old 5890II for GROs that has a manual turn on, and I have certainly forgotten to turn on the flame before!