Gas Chromatograph Base line Problems

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tjaya
tjaya's picture
Gas Chromatograph Base line Problems

We have Shimadzu GC-14A
I did not zero the base line before my sample analysis.
I have run lot of samples like that
Can I take that data into consideration

Thank you very much for your valauble information

qiang wang
qiang wang's picture
would recommend not to use

would recommend not to use

but if all your samples are in the same buffer, the results maybe used after subtract buffer (blank exp)

Greg Pronger
Greg Pronger's picture
wrote:

wrote:

could you give a bit on the application (solvent, etc) and detector.

Did you run a calibration or any other type of known QC under the same conditions as you ran the samples?

Greg

Dr. Analytical
Dr. Analytical's picture
You can run your GC without

You can run your GC without adjusting the zero, but be careful.

1. Your GC detector has a maximum output. If, for example, the maximum is 1000 mV, and your baseline rises to 900 mV (a very bad situation), any peak that is larger than 100 mV will cause the detector to reach its maximum value, and your results will not be accurate. Smaller peaks will not be affected, but you may find that your baseline has more noise. If you baseline drifts up a small amount (100 mV), this should not cause any problems.
2. The results will only be accurate if your integration program corrects for the baseline change. Most modern systems will do this.

OnurM
OnurM's picture
I am running petroleum

I am running petroleum samples in GC and have similar problem. My baseline quite close to zero at the beginning of the analysis (find the analysis conditions below). However once the oven temperature reaches 320oC  (max. allowance of column is 350oC) I have a waiting time of 60 min in order to elute heavy part of petroleum. At that point I am getting an increasing upward drift.
(analysis conditions; beginning 35oC - wait 8 min - 3oC/min ramp upto 320oC - wait at 320oC for 60 min. and the column is conditioned before analyses)
Is there any suggestion which may solve this problem.

Greg Pronger
Greg Pronger's picture
First, from my experience

First, from my experience some level of baseline rise is typical for the type of work you're performing. Whether the rise is due to some heavy boilers that did not quite elute from previous injections or is simply column bleed may be very difficult to discern (at least remotely).
If it's column bleed, there are a variety of columns out which offer low bleed (mostly targetting GCMS systems to stop from putting the column bleed into the mass-spec) which would improve this. This may be an easier solution, but at the same time a more expensive one.
Be certain you're carrier gas is clean. If you have some oxygen going in, it will damage your column, which will then increase the bleed. Heavier hydrocarbons, though an uncommon problem, in the carrier can do this (I saw this once about a dozen years ago and it took the lab a great deal of effort to replace the tubing from the tanks to the instruments and then clean-up the plumbing on the effected instruments). Both of these problems are solved through a quality trap on your carrier gas, and ensuring that you're using a good quality product.
If it's carry-over, about all you can do is increase you're final hold, or (if the instrument is capable) add a post run bake out at the column max for 5 minutes or so. When we're running petroleum products, there's almost always some material that's heavier than what we're looking for, and this will invariably end up building up on the column causing some level of baseline ris (if we're not baking out above the temperature of the GC temperature program inbetween).
If the instrument is a HP 5890, you can get build-up in the split vent line causing back-pressure, and material depositing up into the top of the inlet. If you don't routinely clean the top of the inlet, and once in a while clean the split vent line, these may help your situation. We tend to be thinking that everything that goes into the liner either goes down the column or split line, but experience tells me at the instant of injection, and you have the sample injected into the (very hot) inlet, when the solvent goes from liquid to gas, this needs to be considered closer to a "little explosion" with stuff going in nearly every direction, than (what we tend to consider it) a highly controlled process.
As was mentioned earlier in this thread, your integrator should be able to handle the rise, or worst case is that you need to add an integrator event to "pull" the baseline up.
Finally, if after all this (and what's been previously suggested) and your issue is that you need an optimized chromatogram (i.e., picture) and you're not working with a data system (which would allow you to change display parameters and then re-print)  you could simply increase the injection volume (say 1uL to 2 uL) and then increase your attenuation. This is somewhat stating the obvious but if the issue is simply the "picture", it may be the easiest solution.

Greg Pronger
Greg Pronger's picture
Just came across an option

Just came across an option (typically used on GCMS) that decreases carry-over, which is backflushing the column between runs. The Agilent PDF is located at: http://www.chem.agilent.com/Library/applications/5989-8588EN.pdf
Though this application is for GCMS, the device would work to clean runs between any type of GC application where heavy material from one injection may be causing problems on subsequent injections.
 
Greg
 
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rajaniprabha
rajaniprabha's picture
my problem is also same. i am

my problem is also same. i am using solvent toluene. my baseline is also not straight i.e. on zero.  its going in upward direction as per time. my oven temp is 220 degree C. kindly give me suggestion
 

Dr. Analytical
Dr. Analytical's picture
Try reducing the column

Try reducing the column temperature in 20 C steps and see if the baseline changes.
If the baseline decreases and becomes stable, then you probably have a column bleed problem, which may mean that you column should be replaced.
And please give us more information about your problem: GC, column, analyte, GC settings.
 

Sudheer K.Singh
Sudheer K.Singh's picture
yes you can use that

yes you can use that chromatographs if thease are lower concentration .but if the sample concentration is hihger then it affect the response and the value is not come accurate .so first you check your concentration if your analysis is ppm level and baseline is below 400mv then you can use otherwise not .are you have don calibration on same base line.