Thank you Martin for raising a good question. You mentioned about switching from a temp programme to isothermal  in the said analysis without a change in the setting for isothermal. I will advise you to check the following  parameters before the start of the analysis:

1. Since, a proper solvent peak is being detected by you. This means that the FID flame is in ignition state. Perhaps, the carrier flow may not be proper for this analysis in present conditions. Hence, please check the column flow (if a constant flow mode enabled) or pressure. If not constant or proper, then try to increase the same if cylinders are being installed. 

2. The second most important point is type of the samples being analyzed at present. Are these samples contained similar compounds if compared to your previous samples ? If yes, certainly, you should see these sample peaks. Otherwise, system may have issues.

3. If sample type is different with respect to previous one. Then you should increase the oven temperature to get an idea about the peak detection.

4. Inject a freshly prepared sample again.

5. Check the solvent drawing capacity of the injection syringe in use. Sometime, a blocked syringe gives solvent signal because of the solvent volatility.

With regards

Dr CS Chanotiya, CSIR-CIMAP, India

Thank you for your reply.

The analytes of interest are the same in all samples I am testing.  The solvent is likewise the same.

Here is an update on what is happening.  I came in this morning and setup my temperature program method, and some compounds that did not elute came out before my solvent peak in this method.  This does not make sense to me because my start temperature on my programmed method is lower (85) than on my isothermal (175).  Also my flow rate is higher on my isothermal method (18 cm/s) than my programmed method (15.75 cm/s).  So I do not understand why it would have lingered in the column.

The inlet and detector temperature are lower on my isothermal method (220 both) than my programmed (250 both) but I tried the isothermal with the higher temperature for both and got similar dysfunction.

I am going to leak check my system but my elution times are fairly consistent with my programmed method.

The final temperature on my program is 10 less than the limit of my column but I have only had this column for ~8 weeks.  Is it possible that column damage could have occurred that would cause dysfunction in my isothermal method but not my programmed method?

dear martin the temperature of the isothermal program maybe is to low and maybe the compounds that you are testing are very similar , you could try a higher oven temp or a lower flow of mobile phase

best regards

Dear Benjamin, 

From your explanations, I think you might have a small leak somewhere in your system. In order to make sure that's the case you can monitor the detector signal with the change in the oven temperature. If you set your system at constant flow and see a substantial change in the detector signal as a function of temperature, your system has for sure a small leak, which makes you lose the analytes.

You also can have a leak in the injection port. You might want to change the liner and also check for the cleanness of the golden plate on the bottom of the injection port. 

Cheers,

Saer 

Dear Benjamin,

If you are switching from isothermal analysis (@175C) to programmed one (starting @85C) you can expect the compounds to elute much later (I do not know what is the ramp in you programed run or the Rt in the isothermal analysis). If you prolong the final time (at highest temp) you should see the peaks. The presence of the peaks before solvent might be related to the compounds that stayed in the column since last injection (too short time or too low temperature to elute them). Injection temp should not influence your analysis time (assuming it is sufficient to vaporate analytes).

Regards,

Henryk Jelen

Dear Banjiman

The only reason to your problem is that the analytes did not elute completrly and u r experiencing a carry over  its better to increase the flow rate of gas, appox double to ur method, and maintain a maximum temperature for column, appox 10 less than limit. Run this for at leadtv1 hour  

Niw run ur analytes and problem will b rwdolved.

Good luck.

Thank you again for elaborating your problem to this forum. Though, most the experts (from Drs Boris to Chaudhary) have already talked the probable causes extensively.  However, I wish add one more dimension to this issue in the following lines:

You wrote that, "Here is an update on what is happening. I came in this morning and setup my temperature program method, and some compounds that did not elute came out before my solvent peak in this method. This does not make sense to me because my start temperature on my programmed method is lower (85) than on my isothermal (175)....."

The most probable answer may be that the system is working properly. Except for the column oven temp programme, which seems to be slightly low. Therefore, either increase your oven temp or carrier gas flow/ pressure.

Why no detection?

This means that your compounds of interest have a very strong chemical affinity with the column stationary phase. Due to this affinity, these compounds retained inside the column or with stationary phase for longer duration.

Further, what you have observed (early elution of compounds) today (in your blank run, I suppose) was the outcome of your previous days analysis. If your previous days sample (referring to your isothermal analysis) was completed without any interference. Then these early eluting peaks were definitely the same to whom you were waiting for.

During the said isothermal analysis, your compounds of interest traveled with very slow speed because of their strong chemical affinity but could not reach upto the detector in set time frame/ limit (if total run time is of 60 min). E.g., in a 30m column, let say these compounds covered 20 m distance. Once, you change your isothermal programme to temp programme, it was quite obvious that they have covered the remaining distance in the this run.

Moreover, its not a surprising fact because temp programme is based on an increase of oven temp in degree cel. per min (ramp rate). An increase of 3 degree cel. per min. oven temp shall be sufficient to distract the bonding in between analytes and stationary phase. Consequently, one shall observe such detection.

Some other important aspects may be the leaked Injector septum, damaged ferrule if graphite is used and low injector temp. Further, detector temp if kept above 200 degree, has no issue here.

If you are working with a electronically controlled GC system (for carrier and detector gases), leakage may not be a major cause. However, you may check injector and detector ferrules.

Secondly, you wrote about column damage. I do not see a reason if detection is fair in one particular analysis ? Because damaged column shall have equal response for all type of analysis. I will suggest you to carry out a blank analysis everyday as a starting sample.

Please workout on these points.

Good luck

Dr CS Chanotiya, CSIR-CIMAP, India

Dear Martin, there are meny factors influencing GC separaton. Check the septum integrity.My suggestion is, after a reasonable while of seeing nothing in an isothermal run,, to rise te temperature and see what happens. If some peaks appears, then adjust the isothermal run some 15degrees below that temperature,Then, do the fine tunning to get the best chromatogram. The peaks eluting before your solvent, are compounds from earlier injections, also notice that very high retention times in isothermal runs leads to broad peaks and if the concentration is low your comound appears masked  with the noise..