Using either the absorption of light during a concentration dependant colour reaction or the optical characteristics of the contents themselves, one can use the difference in the intensity measured by the photometer to calculate the desired measuring values.
Transmission and Absorption
The transmission T (%) is the proportion of intensity of the light (I) after the light beam (I0) has passed through the sample: T% = 1/10 x 100%.
The absorption or extinction is the reduction in light intensity: Absλ = - log (T).
The concentration of a substance that is to be measured is determined with the aid of a calibration curve. This is set up by measuring a dilution series using a standard of the substance which is to be measured where the concentration is already known. Modern photometers calculate the calibration curve from the individual measurements of a dilution series and identify the concentration of the sample automatically. Some devices have pre-programmed calibration curves. In days gone by, the absorption values of a dilution series were painstakingly plotted on millimetre graph paper and a line was drawn through the points to find the mean. An unknown sample can be measured and the relevant extinction value inserted on the line. The concentration is then easily found.
Many substances absorb light. Using a photometer we can determine the range in wavelengths where they absorb light particularly well. A scan is carried out using the whole spectral range that the device can offer, and one or more maxima can be identified (fig. 2). These maxima can then be used for choosing the optimal measuring wavelength for determining the concentration. A comparison of such a spectrum with the reference databank stored in the device can be used for analysis during quality control measures at incoming and outgoing goods points.
Multi Wavelengths and Multiple Measurements
Being able to set several measurements against each other makes it possible to carry out more complicated analytical tasks rather than just one single test.
One can ascertain changes in optical characteristics over time or using different wavelengths. In this way, taking a series of concentration measurements, one can determine the conversion rates of a reaction depending on temperature or added substances. Testing the same sample using several wavelengths is a process used in quality assurance (fig.3).
The Colour of Wine
The colour of a wine is an important characteristic of its quality. It is made up of several components. Besides chlorophyll, tannin and carotenoids, grapes carry a colour pigment called anthocyanidin. Usually anthocyanidins are present in glycosidic linked anthocyans. The amount of anthocyan depends on the grape variety and the wine production process, in particular the fermentation and storage conditions. The colour components determine the final colour of the wine during the fermentation and storage periods.
One differentiates between the intensity of the colour (its strength) and the hue (shade). The intensity of the colour is the sum of the red, brown and blue components (I = E 420 nm + E 520 nm + E 620 nm (I is absolute to 3 decimal places)), while the hue is made up of the ratio of the colour components (N = E 420 / E 520 (N is absolute to 2 decimal places)).