GPC - Negative Peaks

[chiralcenter1552]

I am trying to resolve a strange GPC issue regarding negative peaks that appear on the low-MW end of my analyte peaks.

Here is as much information as I can think to include about my current operating parameters and instrument status:

Shimadzu RID-10A detector, mobile phase is THF, samples are also in THF, flow rate ~1 mL/min, columns at 40C, cell at 40C, baseline is stable, pressure is stable, tubing is new, no back pressure distal to RID, detector lamp life 2100/20000 hours, voltage above spec at 4.0V, two columns, both are 10E3A Phenogels with a guard.

As an example, here is a screenshot of what I see when I run a 1.5k PS standard: http://postimg.org/image/jwh6iodsj/

Other MW PS standards as well as compatible polymer samples give similar results, repeating the same sample multiple times give reproducible (almost identical) retention times, so there is no peak drift over time. Finally, the retention times relatively correspond to what is expected for a given Mn but I have been uncomfortable working this up more exactly against a calibration curve until I can get these peaks to start looking better.

I tried injecting samples without the columns attached, and I observe the same exact peak pattern (just at a much lower retention time, of course). My first impression is that this may have to do with the detector cell. It is also possible that this may be as a result of a faulty injection system, although I am unsure exactly how either one of these things may affect the GPC tracing in this particular way.

I have not yet cleaned the detector, still waiting on reply from Shimadzu rep about proper cleaning procedure. If anyone has any other suggestions or perhaps has seen something like this before, I would greatly appreciate any input based on your experience. Also, if you need any additional information, I will be happy to provide.

Thanks!

[tom jupille]

It may simply be that those compounds have lower refractive index than THF.

[Gerhard Kratz]

Please rinse, flush reference and sample cell with fresh distilled THF. Keep THF in brown bottles and in addition protect against day light with aluminum folio. Use always same THF for reference cell, mobile phase and sample.

[kubowicz.tomasz]

Hello

It could be BHT (THF stabilizer). I know that some GPC methods use stabilizer peak as marker (to compensate retention time drift - which is critical for mass distribution).

Regards

Tomasz Kubowicz

[shouthurray]

chiralcenter1552- did you find a solution to this problem? I am experiencing the exact same kind of double positive/negative peaks with my PEG standards (aqueous solvent, Shimadzu RID-10A detector) as well. I would love to know if there is an easy fix!

[darthcolvin]

I think Tom and the other respondents are on the right track. Are you using stabilized THF? How fresh is the THF and under what conditions is it stored upon receipt in your lab?