Shift ret.time ONLY SECOND HALF CHROMATOGRAM

Discussions about GC and other "gas phase" separation techniques.

19 posts Page 1 of 2
It is very strange and I have never seen before in many years of work with GC.

I have a TD-GCMS and some times, a standard show a great shift in retention time only for the second half of chromatogram. Benzene, Toluene-D8 and Toluene show the same ret.time of previous injection, while ethylbenzene, m,p-xylene and o-xilene shift of about 0.15min. Peak area and peak shape are always good even in the shifted chromatograms.

What I inject is a control standard solution of BTEX in methanol added to a sorbent tube by 1uL with syringe under inert gas flow.. I used this method and this tecqunique since 2016 so the method is good and validated. This thing happens for example just one time in a 5 replicated analysis and then happens againg after few days.

The method of thermal desorber is a primary desorption of 9minute form tube to trap at 45mL/min and then10minutes of trap heat at 280°C at 1mL/min so and the GCMS run starts exatcly when trap heating starts too. My method is in accordance with ISO 16000-6 and ISO 16017-1.

In my opinion the problem is note related to classic troubleshooting like leaks in carrier of bad column installation because the problem is related only to second part of chromatogram (full chromatogram is 30min, the problema start with ethylbenzene at about 14minutes), so do not give me simple and old explanation, but I need some useful tip.
I think that the problem is related to Thermal desorber and not to GCMS, beacuse trap heating stop after 10minutes and in that moment it changes some flows and pressure to go in stand by condition.

I cannot understand what happen. Can you help me? Thank you very much
CrisTheChemist wrote:
... so do not give me simple and old explanation, but I need some useful tip...

Simple, old and very useful - you may have this kind of leak which is present when ferrules get hot during chromatography runs (that's why it affects late eluters - I guess you have temperature programmed method). Such leaks, connected with thermal expansion of materials may not be present every run.
CristheChemist,

Does your TD have a 6-port valve that puts the trap in line and off line to the GCMS?

If so, how long are you in the desorb mode at temperature? Because... at temperature the viscosity of the carrier is much different than it would be at
a cooling or ambient trap viscosity.

Food for thought.

Best regards,

AICMM
dblux_ wrote:
CrisTheChemist wrote:
... so do not give me simple and old explanation, but I need some useful tip...

Simple, old and very useful - you may have this kind of leak which is present when ferrules get hot during chromatography runs (that's why it affects late eluters - I guess you have temperature programmed method). Such leaks, connected with thermal expansion of materials may not be present every run.


thanks for the reply, but I cannot understand: which ferrules do you mean? what is the extact expalantion of late leuters intead of the first eluters?
AICMM wrote:
CristheChemist,

Does your TD have a 6-port valve that puts the trap in line and off line to the GCMS?

If so, how long are you in the desorb mode at temperature? Because... at temperature the viscosity of the carrier is much different than it would be at
a cooling or ambient trap viscosity.

Food for thought.

Best regards,

AICMM


My valve is a 4 port valve. Its a Perkin Elmer TMX350. The valve is a VICI valve.
The TD method is:
-9min from tube to trap at 280°C valve and tube, and trap -10°C;
-10min from trap to GCMS at 280°C.
At this point TD stops with a sort of flow oscillation very short and the shift in the chromatogram is for sure at about 12-13min, beacuse toluene peak at 11.8min has no shift and ethylbenzene at 14min sometimes has shift.

In generale, when shift occurs, the difference in ret.time is about 0.2-0.3minutes.
Thank you very much for your help!
I don't know about a Perkin Elmer system but I have had such a shift on an Agilent system that was caused by the oven flaps not closing completely. It did not always happen but if it does not seal completely sometimes, then the oven will have difficulty reaching the target temperature in the ramp at the expected time and cause the later eluting compounds to be delayed.

To check for this, if it has a display of oven setpoint and oven actual, watch them for the whole run and see if they are not equal. If the actual lags behind the set point by more than a second or two then it could be what is happening. Some ovens also have an insulating strip on the flaps, if that tears or comes off it can cause leaks even when the flaps close.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
I don't know about a Perkin Elmer system but I have had such a shift on an Agilent system that was caused by the oven flaps not closing completely. It did not always happen but if it does not seal completely sometimes, then the oven will have difficulty reaching the target temperature in the ramp at the expected time and cause the later eluting compounds to be delayed.

To check for this, if it has a display of oven setpoint and oven actual, watch them for the whole run and see if they are not equal. If the actual lags behind the set point by more than a second or two then it could be what is happening. Some ovens also have an insulating strip on the flaps, if that tears or comes off it can cause leaks even when the flaps close.


Thank you for your help. Unfortunately I cannot check this temperature in real time beacuse the method has a temperature oven ramp and the setpoint is not visualized in the screen beacuse it changes continuosly. Anyway I have an other 2 of these GCMS and this is recent (almost 2year) so its difficult to think to flap troubles at this point. The GCMS is Shimdazu. I have an hybrid TD-GCMS.
CrisTheChemist wrote:
thanks for the reply, but I cannot understand: which ferrules do you mean? what is the extact expalantion of late leuters intead of the first eluters?

Leak I mentioned may affect all connections which are heated and cooled periodically. When the connection is heated metal parts expand causing a leak. When connection is cool it is tight. Thats why rather late eluters are affected.
Thanks for explanation, but what are you saying has no sense beacuse the peak at 12.0min is ALWAYS not shiftet, but the next peak at 13.55min is shifted at 13.65 (0.15min shift), so this is not a problem of late eluter.... this is a problema of half chromatogram, so half compunds, starting from ethylbenzene.
If the problem was a leak due to cooling-heating ferrule I would have a gradual shift and not a clean cut shift at 13min.
CrisTheChemist wrote:
Thanks for explanation, but what are you saying has no sense beacuse the peak at 12.0min is ALWAYS not shiftet, but the next peak at 13.55min is shifted at 13.65 (0.15min shift),...

How could anybody know about exact RT shifts. Show chromatograms in the future
so this is not a problem of late eluter.... this is a problema of half chromatogram, so half compunds, starting from ethylbenzene.
If the problem was a leak due to cooling-heating ferrule I would have a gradual shift and not a clean cut shift at 13min.

Definitely you're wrong.
Why are you asking for help when you know better ?
Let us know after finding the reason of dividing your chromatograms in half.
BTW - how it is halved - in terms of RTs or in terms of the number of compounds ?
leave me you mail address and I will send ypu chromatograms. Here I cannot do (I cannot find any way to load chromatograms here).

Anyway, I asked for help beacuse I do not know the answer but If you say something wrong and I thing so, I have to tell you, without malice. If I ask for help in physics and someone tell me gravity does not exist, what I have to reply? :)

Let's focus on this: the ret.times situation is

- benzene 8.75min (no shift, never!)
- toluene D8 11.7min (no shift never!)
- toluene 12min (no shift, never!)
- ethylbenzene 14.2min (sometimes shifts to 14.35min)
- m,p-xylene 14.42min (sometimes shifts to 14.58min)
- o-xylene 15.05min (sometimes shifts to 15.28min)
Hi CrisTheChemist ,

By the way , did you try to tighten the ferrules of the column connections , especially the ferrule on the inlet ?
CrisTheChemist wrote:
leave me you mail address and I will send ypu chromatograms. Here I cannot do (I cannot find any way to load chromatograms here).

Anyway, I asked for help beacuse I do not know the answer but If you say something wrong and I thing so, I have to tell you, without malice. If I ask for help in physics and someone tell me gravity does not exist, what I have to reply? :)

Let's focus on this: the ret.times situation is

- benzene 8.75min (no shift, never!)
- toluene D8 11.7min (no shift never!)
- toluene 12min (no shift, never!)
- ethylbenzene 14.2min (sometimes shifts to 14.35min)
- m,p-xylene 14.42min (sometimes shifts to 14.58min)
- o-xylene 15.05min (sometimes shifts to 15.28min)


I would tend to agree with dblux on this one. You could have a leak forming due to expansion. Expanding metal of a fitting is just like expanding metal used in a dial thermometer, once it reaches a specific temperature it expands enough to open up the connection and the leak begins. It would be very reproducible but can have some slight variations.

If you have a digital leak detector you could heat the oven to maximum temperature then insert the probe and see if it detects the leaking carrier gas.

I have also seen just the opposite of this at the mass spec interface, where you see m/z 44 from CO2 leaking in when the run begins and at a set temperature it goes away because the ferrule at the interface expands enough to seal the leak. And you will see it reproduce very well each run.
The past is there to guide us into the future, not to dwell in.
CrisTheChemist wrote:
peak at 12.0min is ALWAYS not shiftet, but the next peak at 13.55min is shifted at 13.65 (0.15min shift)


Is the GC getting enough carrier gas pressure to maintain the set flowrate when the oven heats up?

I'd figure out what the max injection port pressure is that the method needs. Then set the inj port pressure a bit higher than that, and make sure it can reach that setpoint.
uzman wrote:
Hi CrisTheChemist ,

By the way , did you try to tighten the ferrules of the column connections , especially the ferrule on the inlet ?


Yes. There is no air in the mass tuning. m/z 18 and 28 are good, so it demosntrate that there is no leak in the column
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