The Column - October 2007 - (Page 15)

October 2007 www.thecolumn.eu.com The Column Tips & Tricks Tips & Tricks: GPC/SEC Daniela Held and Günter Reinhold, PSS Polymer Standards Service GmbH, Mainz, Germany. A look at the importance of molar mass average values. Q: Why does a macromolecule have several molar masses and what do these indicate? A: Macromolecules (with the exception of natural proteins and DNA) have a molar mass distribution (MMD). This means that even the simplest homopolymers consist of homologue chains with a different number of repeat units. Therefore, unlike low-molecular-weight components, macromolecules do not exhibit one definite molar mass. They are normally described with statistically based molar mass average values such as the molar mass averages of the number (Mn) or weight (Mw) and the polydispersity index (PDI). because they have different MMDs. Molar mass averages can be determined from the MMD, but it is not possible to reconstruct the MMD from the averages alone. Q: How broad is broad? A: Figure 1 shows an overlay of the MMD for two different samples with nearly the same Mw. One of the samples has a MMD that is normally called “narrow” or “fairly narrow” because the PDI is below 1.1. The other sample has a higher PDI of around 2 and is normally addressed as a sample with a broad MMD. Figure 1 illustrates the range of the molar masses present in the samples. Even for the sample with the low PDI the molar mass range is from 58000–230000 Da. The sample with the broad MMD covers the molar mass range from 4000–800000 Da showing that samples with a relative low Mw of 124000 Da can Figure 1: Comparison of a sample with a broad (blue) and a narrow (green) molar mass distribution with comparable Mw values. 4.5 Molar mass averages Mn 130 000 57 000 Mw 136 000 124 000 PDI 1.04 2.17 Molar mass peak maximum Mp 142 000 107 000 Molar mass range 58 000 230 000 4000 800 000 Q: Why are these averages important? A: Mn and Mw are calculated by averaging the number or the weight of polymer chains with a defined molar mass. Mn Mw and PDI Mi .ni ni Mi .wi wi Mw/Mn M2i .ni Mi .ni A simple example with a number of stones of different weight can help to understand the molar mass averages (Table 1).1 Mn and Mw can be related to macroscopic properties of the macromolecules and are important parameters in, for example, the investigation of polymer kinetics. Additional information is available when the PDI is taken into account. Macromolecules can have similar Mw values but quote different Mn values. These differences in the average distribution values are indicated by the PDI value. However, the molar mass averages do not fully describe the macromolecule. This can only be done by the MMD.2 Two samples can have the same molar mass averages Mn and Mw, but their properties can be different Authors: Daniela Held and Günter Reinhold E-mail: DHeld@polymer.de 4.0 3.5 W(log M) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 1 103 1 104 1 105 1 106 Molar mass (Da) 15 http://www.thecolumn.eu.com

Table of Contents for the Digital Edition of The Column - October 2007

The Column - October 2007
Contents
News
Time to Blow the Budget on Chromatography Research?
Market Trends and Analysis
Tips & Tricks: GPC/SEC
Improving Injection Consistency Using N-Methyl-2-pyrrolidone as a GC Syringe Wash
Meeting Preview: Extech-10 and HTC-10
Boosting Biological Analysis

The Column - October 2007