BS ISO 21543:2020 Milk and milk products — Guidelines for the application of near infrared spectrometry.
6.1 Near infrared (NIR) instrument.
NIR instruments are based on diffuse reflectance, Lransn LLan u Ii nsIlcLancw me.iuttiienL principle by NIR spectroscopy in the whole NIR wavelength region of 780 nm to 2 500 nm (12 800 cm1 to 4 000 cm’) or segments of this or at selected wavelengths.
The optical operation principle may be dispersive (e.g. grating monochromators), interferometry or non-thermal (e.g. light-emitting diodes, laser diodes, lasers). The instrument should be provided with a diagnostic test system, e.g. for testing photometric instrument noise, wavelength/wavenumber accuracy and wavelength/wavenumber precision/repeatability (for scanning spectrophotometers) according to the manufacturers recommendations.
The instrument should be equipped with sample containers (cups, glass or polystyrene Petri dishes, flow cells, etc.), which allow repeatable filling of the sample. It is important that the window, Petri dish or flow cell material that allows the NIR light to interact with the sample is manufactured to a consistent standard by the manufacturer or supplier to avoid biased results or even re-calibration work. A sufficient sample volume or surface is required to eliminate any influence from inhomogeneity derived from the chemical composition or physical properties of the test sample. The sample path length (sample thickness) in transmittance measurcnwnts should be optimized according to the manufacturer’s recommendations with respect to signal intensity for obtaining linearity and maximum signal/noise ratio. In reflectance measurements in a top-down instrument set-up, a quartz window or other appropriate material to eliminate drying effects should preferably cover the interacting sample surface layer. In a bottom-up instrument setup, the container and the sample are covered and protected against drying by the measurement window.
The sample containers may he re-usable or made of disposable material.
6.2 GrindIng or grating device.
An appropriate device for preparing the sample (e.g. a food processor for semi-hard cheese) can be required. A procedure should be established to ensure consistency over time and with different operators.
Changes in grinding or grating conditions may influence the NIR measurements and results.
7 Calibration and initial validation
7.1 SelectIon of calibration samples
The instrument should be calibrated before being used or installed pre-calibrated. Because of the complex nature of NIR spectral data, which consists mainly of overtones and combination bands of fundamental vibrations in the mid-infrared region, the instrument should be calibrated using a series of natural samples (often at least 120 samples). No specific procedure for the calibration of a specific application can be given due to the wide variations over all aspects in sample types, constituents, concentration ranges, instrument setups and possible calibration algorithms. Examples and results are published in an IDF bulletin.BS ISO 21543 pdf download.