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

The Column - June 2009
Laboratory Equipment Qualification: Back to Basics
News
Market Trends and Analysis
Tips & Tricks: GPC/SEC
Confirming Peak Identification in Bioanalytical Studies Using Dual-Scanning MS Technology to Achieve ProductIon Confirmation
Meeting Review
Events
The Column - June 2009 - The Column - June 2009
The Column - June 2009 - Laboratory Equipment Qualification: Back to Basics
The Column - June 2009 - 3
The Column - June 2009 - 4
The Column - June 2009 - 5
The Column - June 2009 - News
The Column - June 2009 - 7
The Column - June 2009 - Market Trends and Analysis
The Column - June 2009 - Tips & Tricks: GPC/SEC
The Column - June 2009 - 10
The Column - June 2009 - 11
The Column - June 2009 - 12
The Column - June 2009 - Confirming Peak Identification in Bioanalytical Studies Using Dual-Scanning MS Technology to Achieve ProductIon Confirmation
The Column - June 2009 - 14
The Column - June 2009 - 15
The Column - June 2009 - 16
The Column - June 2009 - Meeting Review
The Column - June 2009 - 18
The Column - June 2009 - 19
The Column - June 2009 - Events
The Column - June 2009 - 21
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