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Re: Shouldered Ethanol Peak?
Posted: Tue Jun 10, 2014 7:36 pm
by JaceAssured
We are using 99.5% Ethanol for our standards. Do you really think that something in the 99.5% Ethanol could be causing that noticeable of a peak?
Re: Shouldered Ethanol Peak?
Posted: Tue Jun 10, 2014 9:11 pm
by Peter Apps
What are the settings on the autosampler - pre-injection dwell, eject rate, post injection dwell ? You might be getting two pulses of sample into the inlet due to the needle heating up before or after the plunger drops. Is the autosampler drawing the sample back up into the syringe ? Is the plunger properly zeroed ?
This could affect the ethanol and not the butanol because the butanol is a little bit stationary phase focussed.
Also what kind of liner do you have, and is there any packing in it ?. When did you last do inlet maintenance ?
What solvent are you using to wash the syringe between injections ?
Peter
Re: Shouldered Ethanol Peak?
Posted: Wed Jun 11, 2014 12:16 am
by MSCHemist
I did ethanol last week with SPME on a innowax column. It had a significant tail/shoulder I just went with it I chalked it up to should have used a thicker phase column for such a volatile polar compound. They sell Alc columns that are designed for ethanol as well.
Re: Shouldered Ethanol Peak?
Posted: Wed Jun 11, 2014 1:11 pm
by JaceAssured
Peter,
The settings for our autosampler are:
Pre and post-injection dwell times are 0 min.
Injection rate is at 6000 uL/min
I don't believe the syringe is pulling sample back up into the syringe and I believe the plunger is properly zeroed.
We have an Ultra Inert Liner with wool packing (Agilent P/N 5190-2295). As far as inlet maintenance, we changed the septum on Monday (June 9th) and we checked the inlet liner on Tuesday (June 10th). The liner looked clean with only a tiny amount of blackening at the top of the wool.
We are using Acetone as the solvent to wash the syringe between injections.
Re: Shouldered Ethanol Peak?
Posted: Wed Jun 11, 2014 1:40 pm
by Peter Apps
Peter,
The settings for our autosampler are:
Pre and post-injection dwell times are 0 min.
Injection rate is at 6000 uL/min
I don't believe the syringe is pulling sample back up into the syringe and I believe the plunger is properly zeroed.
We have an Ultra Inert Liner with wool packing (Agilent P/N 5190-2295). As far as inlet maintenance, we changed the septum on Monday (June 9th) and we checked the inlet liner on Tuesday (June 10th). The liner looked clean with only a tiny amount of blackening at the top of the wool.
We are using Acetone as the solvent to wash the syringe between injections. Try spiking the sample with a bit of acetone and see if the "shoulder" gets bigger - I think that it is acetone left in the syringe after washing. Also try washing with water after the acetone and see if the shoulder goes away. Peter
Re: Shouldered Ethanol Peak?
Posted: Wed Jun 11, 2014 2:43 pm
by Consumer Products Guy
Recently a contract manufactuer (during a test method transfer) used acetone to try to "dry" volumetric flasks, and we saw acetone residual in their chromatograms. They had to re-do.
I'd rinse syringe with water. Second choice would be methanol if it elutes/resolves before the ethanol peak.
Re: Shouldered Ethanol Peak?
Posted: Wed Jun 11, 2014 2:54 pm
by JaceAssured
Peter,
The acetone was the problem! Switched to ethyl acetate as the wash solvent and the problem disappeared.
Thank you so much.
Re: Shouldered Ethanol Peak?
Posted: Wed Jun 11, 2014 3:03 pm
by Peter Apps
Peter,
The acetone was the problem! Switched to ethyl acetate as the wash solvent and the problem disappeared.
Thank you so much.
Pleasure. Glad it is sorted.
As an additional precaution, inject some ethyl acetate to make sure that it does not co-elute with the ethanol. If it does you will get inflated estimates for ethanol content. I'm pretty sure that Agilent autosamplers can wash with two solvents, so I would in any case follow the organic wash with some water.
Peter
Re: Shouldered Ethanol Peak?
Posted: Thu Jun 12, 2014 3:05 am
by MSCHemist
I find acetone tends to hang arround. I'm not sure why. Whenever i clean the inlet with it I see traces of it on the GC/MS for several hours.
Re: Shouldered Ethanol Peak?
Posted: Thu Jun 12, 2014 2:45 pm
by KM-USA
I'm pretty sure that Agilent autosamplers can wash with two solvents, so I would in any case follow the organic wash with some water. Peter
Yes, 6890 and newer. "Maybe" for 5890, would need to check.
Re: Shouldered Ethanol Peak?
Posted: Thu Jun 12, 2014 5:38 pm
by Klaus I.
Peter,
The acetone was the problem! Switched to ethyl acetate as the wash solvent and the problem disappeared.
Thank you for this feedback. Nervertheless I'm assuming that you still think the chromatographic seperation is neglectable...
Re: Shouldered Ethanol Peak?
Posted: Fri Jun 20, 2014 5:38 am
by kka626
Hi, I am also running ethanol with water solvent . but my problem is my precision > % 3 %. my Area count of ethanol and n-propanol are up and down. Do you good % RSD for each std ?
instrument : Shimadzu GC-2014 with AutoInjector
head pressure : 15 Psi initially hold for 0.5 min (I am using pressure control mode)
the pressure program is 15 psi hold for 0.5 min
then pressure down to 10 psi hold the rest of the run
inj. Vol: 0.5 uL
Split : 1:50
split Liner : Restek split liner
Column: ZB-WaxPlus (phenomenex, zebron)
30 m: 0.25 mm ID, 0.25 film thickness
temp: 50 C hold for 5 min
then ramp to 200 C at 25C/min
INlet temp: 220C
detector: 240C
Re: Shouldered Ethanol Peak?
Posted: Fri Jun 20, 2014 6:43 am
by Peter Apps
Flash vaporizing injections in water are always going to give worse precision than those with organic solvents, because the water vaporizes to such large volumes that the resulting pressure pulse disrupts flows through the inlet. Use the smallest injection volume that you can and the lowest inlet temperature to reduce the problem. A different inlet liner shape might also help.
Peter
Re: Shouldered Ethanol Peak?
Posted: Fri Jun 20, 2014 2:00 pm
by kka626
Thanks Peter. But for the %rsd, do we calculate base on peak-area Ration (STD/Istd) or base on Area of each component. Because %rSd of peak-area Ratio give me < 2 % , but if I calculate each component Area , % rsd fail the > 3%
Re: Shouldered Ethanol Peak?
Posted: Fri Jun 20, 2014 2:22 pm
by Peter Apps
What matters is the variability of the results - so as long as you are ratioing areas to calculate results you should ratio them to calculate instrument repeatability.
Note that some official methods also have official ways of calculating repeatability.
Peter