GPC internal standard

[ilycip]

Hi,
Could I ask you what in internal standard in GPC?I use an GPC WATERS BREEZE SYSTEM with RDI.

[gstaepels]

I use toluene as internal reference, meaning as flow rate marker. Is that what you mean by internal standard?

[ilycip]

Yes, it's about toluene like internal reference.In my analysis till now, I didn't used , because I don't know if it's necessary.Can you explained me :Why should I used use toluene like internal reference?
Thank you
Chem. eng Ileana Curca

[gstaepels]

In GPC, the retention time is very important. To calculate the molecular weight avarages, the retentiontime of each slice is converted to a molecular weight, which was determined by the calibration using e.g. narrow distributed polystyrene. The relationship between the slice molecular weight and the retention time is logaritmic. This means, that a slight change in retention time can result is a large variation of the result of the molecualr weight. The retention time is onfluenced by the flow rate of the mobile phase through your columns. That is why toluene as a flow rate marker is added. I assume that uyou work with an Empower CDS as you mention "internal reference". With every sample (calibrant of unknown) about 0.05% toluene is added. The retention time of toluene is inputted in the calibration parameters. During the calculation the retention time of the toluene in the unknown sample is comapred to that of the calibration and the retention times are adjusted. This compensates for any change in the flow rate.
Note that you can put in a time window for the retention time of the internal reference material. I put that number to be 1%. If the retention time of the toluene inmy unknown differs more than 1% of that of the calibration, then I know something is wrong. Most of the time it is due to very small leaks on the seals of the pump.
Hope this clarifies somewhat your quenstion.

[ilycip]

Hi,
I must make an analysis of a polyol with molecular weight M aprox. 470-500 Da.What kind of calibration must use:relative or universal with k and alpha?
I have a Waters Breeye system with RDI , 3 Styragel columns:HR1 and 2XHR0.5 (7,8x300 mm), eluent THF 1ml/min.
I used polystyrene standards 400, 800, 1000,2000,3000.

[Uwe Neue]

As mentioned above, you should include toluene as the unretained peak marker.

For the analysis itself, you have two options:

The simplest one is to just report your data for the polyol as "polystyrene-equivalent molecular weight". This is good enough for QC, but may not satisfy the needs of the people who request this analysis.

The better approach is to report the data in the true molecualr weight for the polyol. This requires knowledge of the relationship between the molecular weight of the polyol and the polystyrene. With other words, you will need to know the Mark-Houwink constants for polystyrene and for the polyol at your operating temperature to convert one molecular weight axis into another.

There is a chapter in my book on GPC, unfortunately without Mark-Houwink constants. Also, your GPC software may have the information to make this conversion.

[ilycip]

Hi,

I must make an analysis of a graft polyol with high
molecular weight >50000Da and the base polyol with 3500 Da.
I have a Waters Breeze system with RDI ,and the following columns:Styragel columns:HR0.5;HR1;HR2;HR3;HR4;HR4E;HR5 (7,8x300 mm), eluent THF 1ml/min.
I have polystyrene standards: 2000,3000,10000,30000,70000,100000.
So, I want to ask you :wich combination columns it's better for a good results?
Thank you, chem. eng Ileana Curca