GPC - Late eluting peaks

[Delker]

have a question related to a late eluting peak when doing GPC analyses.

Using either a pair of Oligopore columns or 4 PLgel columns and toluene mobile phase at 1 mL/min, I have a peak that elutes about 15 to 20 minutes after the solvent peak. For the Oligopore columns, the solvent peak elutes at about 20 minutes and this extra peak is between 35 and 40 minutes. For the PLgel columns, the solvent peak is at 40 minutes and this extra peak, which is about twice as broad, elutes at about 60 to 70 minutes. I am using a Water's Alliance system with RI detection. The columns are maintained at 40°C. The sparge and seal wash lines are in the same bottle of Toluene as is used for the mobile phase. The same mobile phase is used to prepare the samples. This peak occurs in both the Toluene blank, as well as in my samples.

[mbicking]

This extra peak is being retained by non-exclusion effects. That is, it is interacting with the statrionary phase, rather than just diffusing through the pores. Non-exclusion effects are known in the toluene systems. Identifying the peak is more difficult, since you say it appears in the blank. One possibility is water, which is likely to interact with the polymer support. It would appear due to slight differences in water content in the toluene, caused by atmospheric changes.

Perhaps one of the Waters people may have some instrument-specific suggestions.