Development of a new multipoint calibration method and uncertainty of a high-performance spectrophotometer for ensuring accurate water turbidity measurements

Turbidity is a key indicator of water quality influencing the proliferation of protozoa. Accurate turbidity measurements are essential for optimal water resource management and river basin planning particularly in contexts where water quality is a primary concern. Turbidity of unknown samples is usually measured by turbidity meters or spectrophotometers under one point calibration at 860 nm in the UV region. This creates a measurement limitation since the turbidity of an unknown sample should be the same or quite near turbidity to the CRM. To overcome this limitation, this work describes a new multi-point calibration method for a high-performance Lambda Spectrophotometer equipped with Gallium-Indium-Arsenic (GaInAs) detector. The calibration was carried out using three turbidity standards of 100, 500 and 1000 Nephelometric Turbidity Unit NTU which were measured in the NIR region at 1070, 1372 and 1569 nm respectively. The absorbance of each standard was measured 10 times and the CRM turbidly values were plotted against the measured absorbance values. A very good linear relationship was obtained with R2 of 1 and the linear calibration function can be used to obtain the turbidity of an unknown sample measured at each of the three used wavelengths. The type A and type B calibration uncertainty sources were quantified according to ISO GUM and the calibration results were found to be 100.14± 0.47, 498.03±1.60 and 997.81±3.54 NTU. The method has been applied for measuring turbidity of a CRM sample with a small error of approximately 1%. The developed calibration method will be useful for analytical laboratories serving the water quality in the environmental and industrial sectors.