Photometry involves measuring light absorption across the ultraviolet (UV) to visible (VIS) to infra-red (IR) spectrum. This technique determines the quantity of an analyte present in a liquid or solution. Photometers utilize a specific light source and detectors, such as photodiodes, photoresistors, or photomultipliers, to convert light passing through the sample into an electric signal. By applying Beer-Lambert's law and using coefficients from transmittance measurements, a calibration function establishes a correlation between absorbance and analyte concentration, ensuring precise results.

Photometry is a widely used quantitative analysis in research labs to quantify inorganic and organic compounds in solutions and liquids. It also finds extensive applications in various industries, including detecting contaminants in drinking and wastewater, analyzing nutrients in soil, food, and beverages, examining building material composition, and more.

Construction and Working of Photometer

A typical photometer comprises several components, including a light source, monochromator, sample, and detector. The light source can be a tungsten-halogen lamp, commonly used for visible light analyses, or LEDs. For measurements in the UV-Visible range, a xenon flash lamp may be employed. The monochromator filters the light emitted by the source, allowing only a narrow spectrum to pass through. This filtered light then passes through the cuvette or sample holding cell. Depending on the amount of analyte or its derived dye in the sample solution, some of the light is absorbed while the rest is transmitted. The transmitted light is directed towards the detectors, generating an electric current proportional to the light intensity.

Beer–Lambert Law

The Beer-Lambert law, also called Beer's law, establishes that the amount of light absorbed by a sample is directly linked to the concentration of the analyte and the path length of light passing through the sample. In this context, "sample" refers to either the analyte itself (for direct measurement) or a dye derived from the analyte (when employing reagents or kits). This relationship is mathematically expressed by the following formula:

A = elc

where

A = absorbance of the sample

l = optical path length (cm)

c = concentration of the analyte (M)

e = molar extinction coefficient (M-1cm-1)

The spectrophotometer measures the intensity of light before and after passing through a solution and relates it to the transmittance (T) by the following equation:

Transmittance(T)=It/Io

It and Io are the intensity of light after and before passing through the solution, respectively. The transmittance is related to the absorbance by the equation:

Absorbance(A)=−log(T)

REFLECTOMETRY

Reflectometry, also known as remission photometry, is a non-destructive analytical method that employs the reflection of light from surfaces and interfaces to assess properties such as color intensity, film thickness, and refractive index.

Like other photometers, reflectometers consist of essential components, including a light source, typically long-life LEDs with specific wavelengths, which are focused onto a sample surface using a lens system. The reflected light is then measured by detectors.

Reflectometers are commonly designed to measure physical characteristics of surfaces, such as color changes on a test strip. In this approach, a sample can be placed on a test strip, and its remission (REM) value can be compared against appropriate controls and standards. Similar to photometry, the difference in intensity between emitted and reflected light enables a quantitative determination of the concentration of specific analytes.

Reflectometry finds widespread use in industrial applications due to its rapid and sensitive nature, making it an effective method for quantifying a broad range of organic and inorganic parameters in water, food, beverages, and environmental samples. Additionally, it has diverse applications, such as surface analysis of building materials and quantification of skin color.