The Column - June 2009 - 11

The Column www.thecolumn.eu.com Tips & Tricks: GPC/SEC reducing the ﬂow-rate for analytical GPC/SEC columns from 1 mL/min down to 0.5 or 0.25 mL/min, sometimes even 0.1 mL/min. Table 2 summarizes the ﬂow-rate recommendations for samples with different molar masses. A positive side effect of the reduced ﬂow-rate is that there are less shear effects. High molar mass samples tend to degrade if the applied shear forces are too strong. The shear force itself is a function of the ﬂow-rate, the solvent viscosity and the interstitial volume of the column. So to avoid shear degradation, the ﬂow-rate must be reduced. A negative effect of the reduced ﬂow-rate is the increased analysis time. However, if a proper GPC/SEC software is used the change in ﬂow-rate is not affecting the time consumption too much. In contrast to HPLC software, GPC/SEC software plots the elution volume and not the elution time. Plate height (mm) Table 1: Comparison of columns with different dimensions: optimum linear ﬂow and ﬂow-rate set at the pump. Dimension (i.d length) 8 mm 300 mm (analytical) 20 mm 50 mm (HighSpeed) 4.6 mm (micro) Linear flow (cm/min) 2.0 Flow-rate (mL/min) 1.0 Application Traditional analytical GPC/SEC Approx. 12.5 mL eluent consumption/column time required: approx. 12.5 min/column Save time: HighSpeed GPC/SEC Approx. 12.5 mL eluent consumption/column time required: approx. 2 min/column Save eluent: (semi-)micro GPC/SEC Approx. 3.5 mL eluent consumption/column time required: approx. 10 min/column 2.0 6.25 250 mm 2.0 0.33 Because the elution volume is not affected by the ﬂow-rate, it is easily possible to construct a calibration curve of standards measured with different ﬂow-rates (e.g., the low molar masses with 1 mL/min, the high molar masses with 0.5 mL/min).4 (b) High viscous solvents For analytical columns with 5 µm particles and low or medium viscous solvents, a ﬂow-rate of 1 mL/min can be recommended as long as the column pressure is below the recommended column pressure values on the supplier column certiﬁcate. If the solvent viscosity is increasing (e.g., when working with DMF or DMAc) the back pressure also increases. Therefore, it might be required to reduce the ﬂow-rate and/or to switch to a larger particle size (e.g., from 5 µm to 10/20 µm). Besides this, it is also recommended to work at higher temperatures (e.g., 40–80 °C), to reduce the viscosity of the solvent and thus the back pressure. If the back pressure is reduced, the life time of the column is increased. How Does a Variation in the Flow-rate Inﬂuence the Molar Mass Determination? Because the absolute peak position is evaluated, ﬂow precision and accuracy is one of Figure 3: Plate height as a function of the linear ﬂow for various molar masses. 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.0 0.5 1.0 1.5 Molar mass dependency Mp 128 000 g/mol Mp M 2.0 9130 g/mol 374 g/mol 2.5 3.0 Linear ﬂow (cm/min) Table 2: Optimum ﬂow-rate depending on the molar mass of the sample. Molar mass Up to 1 million Da 1 million M 5 million 5 million M 10 million 10 millionen Flow-rate in mL/min 1 0.5 0.25 0.10 Optimum particle size (µm) 5 10 10 10–20 Table 3: Flow-rate deviation between calibration and sample evaluation: example data for a poly(styrene) with a molar mass of 220 000 Da. Please note that the actual values also depend on the resolution of the column. Flow-rate variation 5% (compared with calibration) Molar mass deviation 5–10% 7–12% 20–50% 60–160% 1. Statistical pump fluctuations 2. Suddenly increasing or decreasing flow-rate 3. Constantly increasing or decreasing flow-rate 4. Constant but permanent to high or to low flow-rate 11 1 9 Contents Tips & Tricks 2 13 Vosloo Rainville and Mather 6 17 News Meeting Review 8 20 Market Trends & Analysis Events & Contacts

The Column - June 2009

Table of Contents for the Digital Edition of The Column - June 2009