Chromatography Forum

I'm testing residual solvents with agilent 7890 using ctc/pal.
Unfortunately the method is in splitless mode.
How do I determine the Purge time and Purge flow? and will they have any influence on peak shape?

Thank you.

Hi Chemist83,

Short answer is "yes" the effects will be on the solvent and early-eluting sample peaks' shape(s). Please follow this link:

http://www.chromacademy.com/Essential_G ... ection.pdf

Chromacademy is an excellent source for learning about a variety of things related to chromatography. Page 34 of 44 is directly relevant to your question...about setting purge time. You want the purge flow to be fairly high to sweep out the sample/solvent from the inlet liner that was not transferred to the GC column.

Let's see how others weigh in, and my thanks!

Thank you.
The method is for residual methanol,acetone,IPA,Hexanes (5 isomers) and trichloroethylene using propanol as internal standard.
Column is DB-624, 75m x 0.53mm x 3um
Flow (nitrogen)- 13ml/min
inlet-140c - splitless 4mm liner. splitless purge time 0.5min
Detector - 220c
injection - 800ul
incubation temp- 120c incubation time 15min
injection speed -500ul/sec
I have a problem with high RSD of hexane peaks, all others are O.K.
The area of all hexane peaks is erratic.
What could be the reason?

An incubation temperature of 120C implies that you have a high boiling solvent or a solid as matrix. What is the matrix ?

13 ml.min should flush the inlet fast enough for there to be no serious effects on peak shape, and any effects that there are would affect all the peaks, not just the hexanes. You might be getting better stationary phase focussing of the polar compounds, but this will affect peak shape, not area. Are the hexane peaks distorted ? Are the hexane peaks much bigger or smaller than the peaks for the other compounds ?

Peter

Solvent is dimethylacetamide. Hexane peaks are not distorted and n-hexane peak is indeed much bigger than all the others.
I should mention that all the peaks are erratic but propanol (internal STD) behaves the same. The area of all hexane isomers increase when all the others decrees.

From earlier posts;

"I have a problem with high RSD of hexane peaks, all others are O.K."

vs

"all the peaks are erratic"

Which is it ?

Peter

Because I'm using internal standard there is no problem if there is a variation in peak area from on vial to the next as long as all of them act the same.
My intention was that hexane acts differently from the rest of the peaks.
Sorry for not clearing that up.

How are determining repeatability; multiple injections from one vial ?, single injections from different vials filled with the same standard ?, single injections from multiple vials each spiked separately ?

Have you checked the vials for leaks ? - a lot of pressure builds up at 120C.

What is the temperature of the syringe ?

Peter

Single injections from different vials filled with the same standard.
I'm using CTC/PAL with 20ml headspace vials adding 2ml of a solution containing all the standard solvents in dimethylacetamide.
Syringe temp is 125c
what is the procedure for checking leaks in headspace vials

Single injections from different vials filled with the same standard. Good
I'm using CTC/PAL with 20ml headspace vials adding 2ml of a solution containing all the standard solvents in dimethylacetamide. Should be OK
Syringe temp is 125c OK
what is the procedure for checking leaks in headspace vials

put about 1 ml volatile solvent (hexane, pentane, CH2Cl2 etc) in a vial and crimp as normal. Immerse in water above the BP of the solvent. A stready stream of bubbles from around the cap shows a leak

Peter

Thank you.
If there is a leak, what would cause hexane to behave differently than all the other solvents?

Hi

if you do find that you have a leak after following Peter's suggestion then the short answer is yes for hexane and similarly for pentane

Regards

Ralph

I've followed the procedure using hexane and there is no leak from the vial.

Have you run an equlib. test for your head space parameters? My gut feeling is your incubation temp is quite high.

How do I perform equilibrium test?