Determination of Concentration by Means of a Calibration Curve – Exemplified by Surfactant Measurements According to PD CLC/TS 50677:2019 Application Note · SPECORD 210 PLUS

Introduction

A new technical specification for determination of the rinsing effectiveness of clothes washing machines and washer-dryer combos for household use was released in 2020 by the European Committee for Electronical Standardization (PD CLC/TS 50677:2019). It applies to all washing machine manufacturers which are producing for the European market. Herein, the amount of residual linear alkylbenzene sulfonate surfactant (LAS), which was extracted from unstained test swatches after the washing performance test, is determined. LAS is used as a reference material for the total amount of base powder detergent IEC-A*, assuming a fixed linear relation between the quantity of surfactant in total and LAS. This simplification is made due to the easy detection of LAS in UV/Vis measurements at 223 nm and 330 nm with 1 nm spectral bandwidth. The characteristic absorbance of LAS at 223 nm is based on a π-π* transition of the aromatic group of LAS and enables the determination of the concentration of the detergent.

Initially, a calibration curve is created and the absorbance values are plotted against the concentration of known standards. This calibration curve is then used to determine the concentration of the unknown samples, based on their absorbance values. The challenge is the very low concentration of LAS which is remaining in the samples after the washing procedure – therefore, a very low limit of detection is required for the measurement. The SPECORD 210 PLUS instruments have a very low deviation of the baseline and high reproducibility, which is vital for the accurate creation of a calibration curve with low concentration values.