Chromatography Forum

i am doing residual solvents GC method validation. encounter puzzle。
first prepare 0.1ppm benzene solution。then transfer into vial like this：
5ml water to 20ml vial，then transfer 1ml benzene solutioninto the vial using pipette，last cap. parallel six vials. then run sequence to analysis using agilent 7890-7697 GC system. the peak area data as follows:
13.9/13.7/6.4/6/5.7/5.6
bad reproductivity, what's the reason? not once, the phenomenon often emerges. usually the first vial peak area maximum，the other gradually decrease in area。

You should cap each vial immediately after the benzene solution is added, not after you have added it to the six vials.

You should have your benzene solution in a non-polar solvent, instead of water, if that is the case.

You should have exellent RSD values, around 2% unless your technique is poor.

best wishes,

Rod

You should cap each vial immediately after the benzene solution is added, not after you have added it to the six vials.

You should have your benzene solution in a non-polar solvent, instead of water, if that is the case.

You should have exellent RSD values, around 2% unless your technique is poor.

best wishes,

Rod

i'm still confused, i have operated just as you say, long time ago. even two people coorperate, one transfer liquid, the other cap.
i'm do as USP467 say, it's forbided not do as usp. by the way,the other components such as methanol,acetone,isopropanol,furan, i can reach less than 1% for rsd of area occationally. just the benzene hardly to pass the criterion of 15% rsd。
let me say the solution process:100ml volumetric, add 10ml DMSO,then weigh 40mg benzene into it, dilute with water to volume, and mix,400ppm.Transfer 1.0 mL of this solution to a 200-mL volumetric flask, previously filled with about 90 mL of water, dilute with water to volume, and mix,2ppm. Transfer 5 mL of this solution to a 100-mL volumetric flask, previously filled with about 50 mL of water, dilute with water to volume, and mix,o.1ppm.

i have started this validation from july，so far，not amazing development，drive me crazy. could u give me sone excellent suggestions to resolve the problem?thanks for ur attention.
best wishes,
dogcatlike

If the pattern of results that you show i.e. the early vials higher than the first, is consistent then I suspect that something is happening while the vials are waiting for analysis. Try a series of six vials, but with each one filled and capped just before it is analysed, so that they are all queued for the same short time.

What kind of septa are you using on the headspace vials ?

Peter

Peter's point is a good one. If you are using plain butyl rubber septa and are holding the vials for hours before they are tested then you CAN lose benzene into the septa.

Your results indicate that your HS analyzer is working properly and it is not a transfer or evaporation issue.

To check, make preparations in sealed vials of containing 100 ng of benzene.

Put 100 ng of benzene into 0.1mL of DMAc or DMF or MeOH. (Make dilutions of higher concentrations with the non-aqueous solvent you choose, do not use water to make this std addition solution.) Add 5mL of water to each vial and add 0.1mL of non-aqueous solution containing 100 ng of benzene, cap immediately and process as you normally would.

If your results are still poor then it is caused by your septa. Teflon lined silicone septa are generally the best for HS analysis of solvents.

best wishes,

Rod

in past two month，i have been using agilent septa 9301-0976，a grey one，have gap @ edge of septa。aluminium cap not agilent，we buy ourselves to save money。

GC circle time：66min。

incubation temp：105 ℃，45min

i found the standard solution not steady，i mean，the concentration vary，along with time。it's impossible to transfer solution，not six vial almost same time。

You have found your problem.

Congratulations.

Rod

You have found your problem.

Congratulations.

Rod

thanks for ur concern。
if really the 0.1ppm benzene solution is easily influenced by environment，so that lead to bad rsd。in that way，what should i do to resolve the problem？

You should use HS qualified septa. The best in my experience are those of silicone rubber, teflon or aluminum coated, (I prefer teflon). Bare rubber septa are unsatisfactory.

Look at the literature and you will see benzene as an analyte is not a problem. Note how they prepared their samples in a way different from your procedure. Change your procedure to match the literature.

best wishes,

Rod

Aglinet have a guide for HS septas on their website.

thanks for rod's suggestions, exactly i need immediate help，best from someone do same work：usp467，benzene analysis。i have not enough time to practice repeatedly，feeling boss‘s pressure。

i often use a splitless liner id 2mm，while split ratio 5:1，maybe it lead to bad rsd，also low concentration solution not steady than high one。this two factor maybe primary cause。

usually the first vial data，distinctly big，abnormal。
yesterday data：27.5 11.3 8.2 7.1 6.9 8.4
what cause this？can't gloomy any more。

Your vials are leaking, or your septa are absorbing benzene from the headspace.

best wishes,

Rod

Your vials are leaking, or your septa are absorbing benzene from the headspace.

best wishes,

Rod

give me more help，please。
all septa i used，agilent product，have PTFE film。
cap operation，i exert all my strenth，even one cap，rotate different angle to repeat 6 times ，make sure tight enough。

what kind of rubber is used in the septa?

You can overtighten vials. Are they screw-on caps? Or do you use a crimper?

I still suspect that since other solvents give you excellent reproducibility (am I wrong?) that it is the septa that is the problem.

The other issue is the exact procedure used in making the preparations.

You should prepare spiking solutions in a non=polar solvent such as DMF, including all dilutions, then add a reproducible amount of solvent (DMF) containing the benzene to the vials immediately before crimping the vials shut.

If you are not getting enough sample created by the pressuring the vial and heating the vial that you flush the sample loop completely at least three times its volume then you could also have variability. But since your FIRST vial is higher in response, it appears to be a leak OR loss by absorption by your septa.

Rod