The Column - June 2009 - 9

Tips & Tricks: GPC/SEC How Does the Flow-rate Influence GPC/SEC Results? Thorsten Hofe and Daniela Held, PSS Polymer Standards Service GmbH, Mainz, Germany. Molar mass determination in GPC/SEC using a calibration routine with reference materials is based on the precise measurement of the elution volume of the sample. Neither the peak height, nor the peak area itself are evaluated, but the absolute peak position and the peak shape. This is one of the major differences between HPLC and GPC/SEC. GPC/SEC experiments are run under isocratic conditions. Solvent composition and ﬂow-rate are kept constant for the whole experiment. Method development for GPC/SEC requires the selection of the proper column and solvent,1 as well as the optimum ﬂow-rate. The ﬂow-rate inﬂuences the diffusion behaviour and, therefore, the column performance, resolution and accuracy. One of the aims in GPC/SEC is to minimize the peak spreading described by the Tung axial dispersion equation.2 If a GPC/SEC experiment is performed with the wrong ﬂow-rate, additional peak broadening and sample shearing may take place. Three terms (A, B, C) contribute to the van Deemter equation: the eddy diffusion (or eddy dispersion), the longitudinal diffusion of the solute while passing through the column and the (interphase) mass transfer. While, eddy diffusion does not depend on , longitudinal diffusion decreases with increasing and mass transfer increases linearly with (compare Figure 1). As a result of all three terms, Figure 1 also shows the overall dependency of the plate height as a function of the ﬂow. However, the ﬂow-rate set at the pump itself, is not universal enough for a general description. To compare different column dimensions, the ﬂow-rate has to be normalized by the cross sectional area of the column. Then the linear ﬂow can be discussed. Figure 2 shows van Deemter curves for columns with three different dimensions. It clearly shows that the minimum for the plate How Do I Find the Optimum Flow-rate? It is a well-known fact that there is an optimum ﬂow-rate for the mobile phase, at which the best resolution is achieved. The well-known van Deemter equation helps to select the proper ﬂow-rate in GPC/SEC.3 It is used to explain band broadening and relates the height of a theoretical plate (which can also be seen as 1/plate number, N) to the ﬂow. At the lowest point of the van Deemter curve (lowest plate height, H), the diffusion behaviour of the sample through the column is at its optimum and the band broadening of the peak is minimized. The height H of the theoretical plate is given by H A B C [1] where is the velocity. 9 1 9 Contents Tips & Tricks 2 13 Vosloo Rainville and Mather 6 17 News Meeting Review 8 20 Market Trends & Analysis Events & Contacts Ilovephoto Design/Getty Images

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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