headspace oven temperature and resolution？

[dogcatlike]

different headspace oven temperature will lead to different resolution of Methylene chloride and Acetonitrile？i'm using Rtx-624,0.32mm ID column as residual solvent analysis。i found 105 ℃ incubation better resolution than 80 ℃ incubation ？why？seems not big difference in peak area。
i'm using water as solvent in headspace vial。

[Peter Apps]

Probably some strange solvent effect as super-heated steam condenses onto the column. How are you measuring resolution and how big is the change ? Are the peaks narrower, or further apart ? How repeatable are the peak areas at 105 C ?

Peter

[chromatographer1]

I agree with Peter. The use of a bare retention gap instead of the column alone may also make a difference. A dirty transfer line also makes a difference.

Ain't science grand?

Rod

[dogcatlike]

thanks for ur concern。

i found peak width increase 80 ℃ than 105 ℃，compound retention time smaller，more distinct。although 105 ℃ not

ideal area repeatable，but resolution between acetonitrile and dichloromethane same importance for me。

[dogcatlike]

i have to use smallest volume inlet liner to try，if still not enough resolution，i can't change incubation temp。unless i anew find a proper column for 467.

[chromatographer1]

If the water doesn't condense in the column head at 105C, but it does at 80C then I can understand the narrower peak width for the solvents at the higher temperature.

The water layer on the surface affects the focusing of the vapor plug of the solvents onto the column at the lower temperature. I suspect that whatever temperature your injector is held is not enough to compensate for the cooler temperature of the column oven in keeping the water as a vapor as it enters the column.

Increasing the split SHOULD help the resolution of the solvents but the water may be a larger factor than the split. As Peter mentioned, this is called a 'solvent effect'.

A 3 meter piece of Wax column attached to your 624 column will also change the separation of AcN and DCM.

good luck in your work,

ROd

[dogcatlike]

If the water doesn't condense in the column head at 105C, but it does at 80C then I can understand the narrower peak width for the solvents at the higher temperature.

The water layer on the surface affects the focusing of the vapor plug of the solvents onto the column at the lower temperature. I suspect that whatever temperature your injector is held is not enough to compensate for the cooler temperature of the column oven in keeping the water as a vapor as it enters the column.

Increasing the split SHOULD help the resolution of the solvents but the water may be a larger factor than the split. As Peter mentioned, this is called a 'solvent effect'.

A 3 meter piece of Wax column attached to your 624 column will also change the separation of AcN and DCM.

good luck in your work,

ROd

i mean headspace incubation temp， not column oven。 maybe i didn't express clearly。 the column temp initial stage all 40 ℃ for 40min。

[dogcatlike]

Probably some strange solvent effect as super-heated steam condenses onto the column. How are you measuring resolution and how big is the change ? Are the peaks narrower, or further apart ? How repeatable are the peak areas at 105 C ?

Peter

not ideal data，sometimes。validate this method，puzzle me，not a easy work。
1 2 3 4 5 6 mean st devi RSD（%）
71.3 72.5 67.7 65.5 66.2 67.8 68.5 2.8 4.1
666.4 666.6 591.3 555.6 586.6 548.7 602.5 52.3 8.7
479.9 481 431.3 406.5 426.2 430.5 442.6 30.7 6.9
228.9 228.6 201 190.4 201.4 199.2 208.3 16.4 7.9
14 10 8.3 8.4 8.6 8.1 9.6 2.3 23.8
1541.1 1541.9 1340.8 1309.5 1279.9 1271.7 1380.8 126.8 9.2


105℃ R=1.07， 85℃ R=0.90

[chromatographer1]

Yes, I understood you.

The higher temperature of the sample plug (105 versus 80C) affects how the water vapor condenses on the cool column. The temperature of the injector you have not provided. But the higher temperature sample plug should condense over a longer distance on the column, giving a thinner layer of water, allowing the surface of the column phase more interaction initially with the other solvents.

Your data is not exceptionally precise. I would not be happy with it.

best wishes,

Rod

[dogcatlike]

exactly i doesn't know how to upload data well。try picture format， but fail。 in all， peak area not ideally repeatable 105℃ equilibrium。

i'm using agilent 7697A headspace sampler--7890A GC：

resolution criterion：more than 1.0

oven temp for equilibrium 105℃ last 45min，loop 115℃，transfer line 125℃，good R=1.07，but peak area RSD not good enough。

oven temp for equilibrium 80℃ last 60min，loop 105℃，transfer line 120℃，bad R=0.90，peak width increase。

I thought to change into 80℃ equilibrium，but resolution restrict me。I’m in dilemma。I worked in a small pharmaceutical company，have no power to construct our own residual solvent assay method，have to do completely following USP467。I hope expert can show me a brilliant road，leading me out of difficulties。

[chromatographer1]

You could use the smallest ID injection liner, or increase your split ratio to reduce sample plug size on column, or use direct injection with a much smaller sample loop in your sampling valve.

I think these changes may be done in USP 467.

best wishes,

Rod

[dogcatlike]

You could use the smallest ID injection liner, or increase your split ratio to reduce sample plug size on column, or use direct injection with a much smaller sample loop in your sampling valve.

I think these changes may be done in USP 467.

best wishes,

Rod

i forget what size liner i have used when try the equilibrium temp 80 ℃，but i exactly have used id2mm direct splitless liner to try，which is my smallest liner at that time。afterwards i gain id 1.5mm liner，maybe not big difference than id2mm one。i have ordered a kind of liner id 1mm，split liner，but now i have't received it。

present split ratio is 5:1，467 value，it's permission to modified，but increase it，mean less sample into column，maybe decrease S/N test pass rate。
change the GC system for direct injection not very convenience，for we just have one 7697-7890 system。when change a assay item，it's not convenient。

[Bigbear]

I think you are incubating at too high a temp. I am under the impression you should never exceed the boiling point of your solvent. For ACN and MeCl2 incubation somewhere about 50 should be fine.

[dogcatlike]

I think you are incubating at too high a temp. I am under the impression you should never exceed the boiling point of your solvent. For ACN and MeCl2 incubation somewhere about 50 should be fine.

in usp 467 there are only three options：80 ℃，45min；80℃，60min；105℃，45min。i doesn't want to construct and

validate my own method。