usp467,o.1ppm benzene solution,rsd

[dogcatlike]

Are you headspace oven is at 105? If water is in the sysytem it should be maximum 85

yeah，105 degree exactly。

[chemist10]

Your headspace oven temperature should be 20'C lower than boiling point of your diluent. For water is 80'C.
Also you mention that you have problem mostly with benzene for RSD, but not with other 5 solvents. In the sysytem DMSO/Water you likely to have a problem with the solvents that have limited solubility in water: Benzene, Heptane, Hexane, Toluene, etc. Methanol, Acetone, IPA should give you much less problems. Stop using mixture DMSO/Water. Stick with water for Benzene (to increase response). Use DMSO for Heptane or Hexane (for solubility), use whatever you want for Toluene, Methanol, Acetone, IPA. Do not use mixture.

[chromatographer1]

I have never had ANY problems using mixtures for headspace, but then I always had the mixer going during the heating process. I also prefer DMAc over DMSO due to reactions and reaction impurities which are bothersome.

As always, doing your own method using reduced amounts of dissolution solvent is SO MUCH BETTER no matter what solvent or mixture you use.

best wishes,

Rod

[dogcatlike]

in all， usp467 method validation work，for me，not easy work，never easy-going。 i have to deal with deviations of validation work，upset。

why 105 degree not suitable for water？how to persuade the other？water in vial can't boil in 105 degree，although reach boiling point。

[chromatographer1]

I would think you are supersaturating the headspace and exceeding the linearity limits of Henry's Law.

I would recommend a different solvent if temperatures greater than 85 degrees are utilized.

best wishes,

Rod

[Peter Apps]

in all， usp467 method validation work，for me，not easy work，never easy-going。 i have to deal with deviations of validation work，upset。

why 105 degree not suitable for water？how to persuade the other？water in vial can't boil in 105 degree，although reach boiling point。

105 C is above the boiling point of water at 1 atmosphere pressure. Presuming that your vials are leak tight the water will not boil, until the needle pierces the septum and the headspacer valve switches to fill the loop by venting to atmosphere. Then the water will suddenly boil - spraying droplets into the gas stream that will end up in the loop and other parts of the sampler plumbing. This cannot be good.

Peter

[dogcatlike]

in all， usp467 method validation work，for me，not easy work，never easy-going。 i have to deal with deviations of validation work，upset。

why 105 degree not suitable for water？how to persuade the other？water in vial can't boil in 105 degree，although reach boiling point。

105 C is above the boiling point of water at 1 atmosphere pressure. Presuming that your vials are leak tight the water will not boil, until the needle pierces the septum and the headspacer valve switches to fill the loop by venting to atmosphere. Then the water will suddenly boil - spraying droplets into the gas stream that will end up in the loop and other parts of the sampler plumbing. This cannot be good.

Peter

exactly as u say? when fill loop, water will suddenly boil.
why in usp467 there are one set of parameters:105 degree and 45min? when to use it? the method developer doesn't realise the problem u say?

[dogcatlike]

i think your opinion very reasonable. but i never saw by my eye. still suspect if or not the penomenon happen in the vial as you say.

[chemist10]

You can use 105'C if DMSO is your solvent. That what is USP <467> for water insolule substances states. But even USP with their poor methods will not wright 105'C for water. It's a nonsense.
Any way, in my opinion, increasing headspace oven temperature helps very little with sensitivity. 80C is perfect for 99% of the test for any analysis. If you have to analyze DMSO, DMF, Propylene Glycol or other highboilers by headspace, you'll have problem with sensitivity at any temperature.

[chromatographer1]

Peter's comment is solid.

Think for a moment:

You have a headspace at 105C containing nearly 100% of all the benzene you are going to partition from the water. Other solvents may still have some of their content remaining in the water.

You pressurize this headspace (or not) for release through your sample loop to vent.

At ambient pressure at 105C the water will 'boil' or release more vapor which then flushes the air containing benzene through the lines to vent, effectively removing most of the benzene from the sample loop, filling it with mostly water vapor.

Golly, your benzene peak is mostly GONE, flushed from the system, while a portion of the other solvents 'boil' out of the water with the vapor through your sample loop.

You inject the sample loop which contains some of the solvents originally in the water, but almost NONE of the benzene is to be seen.

Does this sound familiar?

best wishes,

Rod

[dogcatlike]

thanks peter and rod. u both are reasonable.

the incubation temp of 105 degree must be a problem. i will asked research department to change parameters and validation roughly. then i will run the method again. once i gain enough data, i will feedback my progress.

[tinsang]

I was using:
Septa of pharma Fix (Butyl/PTFE 50° shore A, 3,0 mm) and Electronic crimper Thermo fisher Modell - Nr.: ECR-20C. I've never problem to seal the HS-vials (to tight or to loose). They are the best in my experience.The reproducibility of six injections of Ethanol, Aceton, MEK, 1-propanol (1ppm) usually were within less than 2%.

[shekhar30]

Check your headspace conditions.

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。

[vdaliessio]

Welcome to headspace analysis. Conditions that are optimum for one analyte in one solvent are completely wrong for another analyte or solvent. I am a noob but this much I can say with some authority. Don't be afraid to vary conditions one at a time until optimized. Then take the data and go do battle with the boss.

[dogcatlike]

thanks for guys。 in past several month i have been busy in other task all along。no time read posts and reply。i‘m sorry。thanks for all reasonable suggestions。