Dilution of sample has positive and negative effects on measurement results. The following points have to be considered. While the effect and the purpose of dilution are easy to define, it is not the case with the limits of dilution. We did a study about the limits of dilution by our standard analyser setup (one pump for sample and one pump for dilution) and by a two step dilution.

Graph 1 details:
horizontal = wavelength, vertical = absorption.
dark blue … sample diluted x10.
light blue … sample diluted x100.
green … sample diluted x250.
turquoise … sample diluted x500.
in red … measurement window. Window of interest.

Process of Sample Investigation:

  • investigate the sample background spectrum and solubility.
  • interference study, proved by sample spiking.
  • repeatability and precision study.

Complex versus Background.
Signal versus Noise.

Graph 2 details:
horizontal = wavelength, vertical = absorption.
dark blue … sample diluted x150.
light blue … 1ppm CrVI.
turquoise … 0.5ppm CrVI.
violet … sample diluted x150 and reagents.
Light blue and turquoise lines show the Chromium spectra with reagents (buffer and dye) added. These absorption values (and 4 more Chromium concentrations) are used for calibration table. The undiluted sample – dark blue line – shows different peaks and also in the range of focus between 500 nm and 600 nm. By dilution factor 150 this influence can be reduced to a minimum, show by violet line.

Relative standard deviation. Sample diluted x150.

zero measure wait extinction
3457 3047 2731 1024
3428 3024 2732 1000
3428 3006 2699 1028
%RSD 1.90

When dilution makes sense:

  • reduce intensity of colour, background, noise and interferences.
  • dilute sample to get optimum concentration range, defined by cuevette light path and reagent (dye) concentrations.
  • if sample background is easy to soluble to get a homogeneous sample.
  • great idea with ISE, because of logarithmic behavior of ISE. mV signal versus concentration.

In modern online analyser you’ll find the dilution mode as an integral part of the measurement tactics. The automated dilution is turned on whenever a specific threshold value is passed and if the measured concentration is below, dilution will turn off automatically. SEIBOLD Wasser analysers series COMPOSER always have automated dilution embedded. Avoid dilution in bucket systems (non flow-through-systems) because of greater problems with fouling and mixing to get a homogeneous solution.

See more information about our installation in Argentina at the Riachuelo.

Dilution: Limits and Purpose.

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