UV v/s Absorption Spectroscopy

This spectrophotometer records light attenuation in the wavelength region between 200 and 750 nm. The measurement is performed in-situ, without sampling or sample pre-treatment, thus preventing errors due to sampling, sample transport and storage etc. A measurement cycle takes between 20 and 60 seconds, making possible a high measuring frequency and detection of rapid changes. In order to combine this high measurement frequency with high accuracy and sensitivity, the instrument is equipped with a Xenon flash lamp as a light source and a concave holographic grating matched with a 256 pixel diode array as detector. For long-term stability of the signal produced, a split light-beam design is used; one beam passes through the sample while the other travels along a parallel pathway inside the instrument and thus acts as an internal reference beam. This second beam is used to cancel out fluctuations and interferences. These instruments are available for wide range of applications, from ultra pure water (dissolved organics in the μg/lt range) up to concentrated wastewaters (organics
and non-dissolved materials in tens of grams /lt range).

The spectra, referred to as fingerprints obtained with such on-line spectrometers are used for the characterization of the sampled water. The fingerprints are used to monitor changes in the water composition through analysis of the general shape of the spectrum or absorption at a specific wavelength. Furthermore, they are also used to derive more specific parameters, such as turbidity, TSS, Color, nitrate, concentration and sum parameters such as COD (chemical oxygen demand), BOD
(biochemical oxygen demand) & TOC (total organic carbon) and (dissolved organic carbon) DOC in water analysis.

Turbidity, due to suspended substances, causes light scattering and shading, thus influencing the absorption over the entire fingerprint. This is an important factor that influences in situ measurements and requires compensation in order to obtain reliable and reproducible readings. Furthermore, it is also used to estimate the concentration of single (groups of) substances, such as benzene, phenol, xylene and toluene. The wavelengths used for determining all these parameters have been selected
using various analysis. The use of such multi-wavelength algorithms allows higher specificity than can be achieved with conventional single, dual wavelength, Eight wavelengths etc. in UV range photometry.

The methods usually employ patterns recognition techniques (PRT) to quantify the absorption profile and convert it to an equivalent concentration for the particular water quality parameter of interest. Usually these methods do not require sample pre treatment or daily calibration at the monitoring site of interest. They are employed mainly because samples can be analysed with very little or no sample preparation, no chemicals are required, and they have low operational costs. Such methods have been developed for monitoring Chemical Oxygen Demand (COD), Total Organic Carbos (TOC), Biochemical Oxygen Demand (BOD) and Nitrate.