Calibration

The first step in any GPC analysis is to calibrate the system. Below, you see a polystyrene narrow standard calibration curve that was obtained on Alliance® using THF as the eluent. The molecular range covered is ~250 to 3M. The column set consisted of 2 HR 5E's (mixed bed) and a single HR2 (500A). The columns were heated to 40 °C in the column heater, and the flow rate was 1.0 mlL/min. The calibration curve is a 5^th order fit. The curve looks excellent, but there is also something very interesting to note. There are three injections of each standard shown on the curve, (3 injections each from three different vials). So the total number of points on the curve is 39! (If you look hard enough you may be able to see some evidence of a very small amount of scatter for a couple of the standards). The retention time reproducibility of the narrow standards is less than 0.04%, a result of the superior flow delivery of the Alliance system. We sometimes need to do our GPC analysis in solvents other than THF.

Below is a narrow standard calibration curve using poly(methyl methacrylate) standards, with dimethylformamide being used as the eluent.

We prefer to use PMMA's rather than polystyrene when working with DMF, as low molecular weight polystyrene standards tend to have inconsistent retention times, eluting later than expected. The polystyrene oligomer standards, (molecular weight under ~700, for example), may show retention beyond the total volume, V_T. The PMMA narrow standards do not exhibit this tendency, and are preferred for work in DMF. The same column set that was used for the polystyrene calibration was used here, (2 HR 5E's plus a single HR2). The only difference is that these columns were packed in DMF. Lithium bromide, at a concentration of 0.05M, was added to the DMF. This is to prevent any polar interaction between sample and eluent, as most samples run in DMF tend to be very polar. As for the polystyrene curve, there are three injections for each standard, so, in this case, there are 36 calibration points on the curve. There seems to be even less scatter on this curve than the polystyrene curve. The columns were heated to 80° C to reduce the viscosity of the DMF.