The Column - February 2008 - (Page 20)

Tips & Tricks The Column www.thecolumn.eu.com February 2008 Tips & Tricks: GPC/SEC Friedhelm Gores and Peter Kilz, PSS Polymer Standards Service GmbH, Mainz, Germany. Examining accuracy and precision in GPC/SEC. Any analytical method has its intrinsic inaccuracy. It is important to know these limits and the precision and accuracy of the analytical method to interpret the results in the right way. Furthermore, it necessary to know how these entities can be influenced and improved with simple tools or proper experimental set-up. Before discussing the tools, the general definitions of the factors should be clear: • The accuracy of an analytical procedure expresses the closeness of the agreement between the value that is accepted, either as a conventional (true) value or a generally accepted reference value, and the value found. • The precision of an analytical procedure expresses the closeness of the agreement (degree of scatter) between a series of measurements obtained from multiple sampling of the same homogeneous sample under the same conditions. Figure 1 shows the difference between accuracy and precision. In more detail, precision can be discussed in terms of short-and longterm precision: • The repeatability describes the intralaboratories empirical variance of the results of multiple measurements of a sample during a short period of time. • The intermediate precision expresses intra-laboratories variations over a long time period. • Another important term is the reproducibility, that is assessed by means of inter-laboratory deviations. How Accurate is GPC/SEC? GPC/SEC is a relative method and the molar masses obtained can only be accurate if the calibration standards and the samples are matching with regard to their chemical composition. Standards such as the ISO 13885 for GPC/SEC do not give any references for accuracy but many analytical laboratories report an accuracy of 5% for Mw and 10–15% for Mn depending on the complexity of the samples. In instances where no matching calibration standards are available deviations of several 100% are possible. Fortunately many GPC/SEC users only focus on repeatable and precise measurements (e.g., when only quality Figure 1: Difference for accuracy and precision. Accuracy and precision Error Random Systematic “True value” Analytical quality Accurate Precise 20 Authors: Friedhelm Gores and Peter Kilz Contact: PKilz@polymer.de http://www.thecolumn.eu.com

Table of Contents for the Digital Edition of The Column - February 2008

The Column - February 2008
Contents
News
Zosimus
The Winning Hand
Market Trends and Analysis
Comprehensive GC with Quadrupole Mass Spectrometric Detection
Tips and Tricks: GPC/SEC

The Column - February 2008

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