Chromatography Forum

dear all
we are not getting consistant areas of n-propnaol by headpace , what could be the reason, column zb-624, 30mx0.53mm, 3.0µm, please suggest

thanks

Hi,

Please give us more info:
Which incubation temperature and time are you using?At what temperature is the inlet and detector?Are you working isothermal?Which liner type are you using? How much %RSD are you getting?

Regards,
Atom

Hi,

Please give us more info:
Which incubation temperature and time are you using?At what temperature is the inlet and detector?Are you working isothermal?Which liner type are you using? How much %RSD are you getting?

Regards,
Atom

Chromatographic Parameters:

Column : ZB 624 column
(30 m x 0.53mm ID), 3.0 µm film thickness or Equivalent.
Injector Temperature : 250°C
Detector : FID
Detector Temperature : 300°C
Flow : 5.0 ml/min, constant flow
Carrier gas : Helium
Oven Temperature : 40°C for 5 min and then increased to 80°C at the rate of 5°C/min and then increase to 200°C at the rate of 10 °C per minute and hold at 200°C for 2 min.
Injection type : Split
Split ratio : 10:1

Headspace Parameters:

Vial Oven Temperature : 100°C
Transfer line Temperature: 120°C
Loop Temperature : 120°C
GC Cycle Time : 37 minutes
Vial equilibration Time : 10 minutes
Pressurization time : 0.15 minutes
Loop fill time : 0.5 minutes
Loop Equilibration Time : 0.1 minutes
Injection Time : 0.5 minute
Vial Shake : High
Loop Size : 1.0 ml
Headspace Vial : 20 mL

Preparation of Standard Solution:

Weigh accurately about 250 mg of n-propanol into a 100 mL volumetric flask containing about 20 mL of N,N Dimethyl formamide. Dissolve and dilute to volume with N,N Dimethyl formamide and mix well. 5.0 mL of the above solution transferred into a 50 mL volumetric flask and then dilute to volume with N,N Dimethyl formamide.

Transfer 2.0 mL of the solution into a headspace vial and seal the vial immediately.

Preparation of Sample Solution:

Weigh accurately about 100 mg of sample into a headspace vial. Add 2.0 mL of N,N Dimethyl formamide and seal the vial immediately and mix gently.

Rewriting due to late post.......

My first guess:

a. 100°C in HS oven when using DMF (bp ~155°C) gives you a load of DMF entering the valve and load. Personally avoid using more than 70°C if possible. This in combination with b.....

b. Temperatures in valve/loop and transferline is below DMF bp. Increase to above bp of DMF.
Polar compounds like propanol can attach to residual/condensed DMF in valve/loop => inconsitant areas

c. loop fill time might be a bit long but likely not, never had to go outside the 0,2-0,4min range with similar applications.

d. Even though DMF retains solvents better than water, the high HS temp of 100°C compared to bp of propanol may contribute to the high RSD%, if your sample solvent had been water it most likely would have.
You should not need that high oven temp 60-70 °C would surely be enough.

Rewriting due to late post.......

My first guess:

a. 100°C in HS oven when using DMF (bp ~155°C) gives you a load of DMF entering the valve and load. Personally avoid using more than 70°C if possible. This in combination with b.....

b. Temperatures in valve/loop and transferline is below DMF bp. Increase to above bp of DMF.
Polar compounds like propanol can attach to residual/condensed DMF in valve/loop => inconsitant areas

c. loop fill time might be a bit long but likely not, never had to go outside the 0,2-0,4min range with similar applications.

d. Even though DMF retains solvents better than water, the high HS temp of 100°C compared to bp of propanol may contribute to the high RSD%, if your sample solvent had been water it most likely would have.
You should not need that high oven temp 60-70 °C would surely be enough.

i tried with Oven temperature 80°C and noticed inconsistant areas and then moved to 100°C

i tried with Oven temperature 80°C and noticed inconsistant areas and then moved to 100°C

Well that increase both amount of DMF and propanol entering the loop. It does not prevent DMF from condensating in the valve/loop, rather increasing the chance of contaminating surfaces in valve/loop with DMF.

As I said try elevating temp in valve/loop and transfer line above boiling point for DMF.

Is this the 1st time you're doing this method or in the past you could get good results with this same parameters?

Column max temp isn't it 260°C? Why do you have 300°C for FID. I would lower the inlet to 200°C and FID to 250°C.
Did you try to increase the vial equilibration time to 20minutes? Isn't 2ml in 20ml HS vial too less?

Regards,
Atom