Photometers: Types, Filters, and Applications
Photometers are simple, relatively inexpensive tools for absorption measurements. It is a convenient and easy-to-maintain alternative when compared to spectrophotometers.
They boast high radiant energy throughput, resulting in good signal-to-noise ratios. Filter photometers are particularly useful in:
- Portable instruments for field use
- Measuring absorbances in flowing streams
- Clinical laboratories
Following is the table of content on topic photometers:
Type 1: Visible Photometers: Colorimeters
Visible photometers, or colorimeters, come in two types: single-beam and double-beam.
1. Single-beam Photometers
Single-beam instruments consist of:
- A light source (tungsten filament lamp or LED)
- A lens
- A filter
- A photodiode transducer
The photodiode’s current is processed to provide a direct readout in absorbance or transmittance. Modern colorimeters store the reference signal, calculating absorbance as the logarithm of the sample and reference signals ratio.
2. LED-based Photometers
LED-based photometers have two variations:
- Automatic wavelength changes
- Fixed wavelength operation
Calibration is achieved using two or more standards.
3. Double-beam Photometers
Double-beam photometers are designed to measure absorbance in flowing streams. These instruments split the light beam using a bifurcated fiber optic, which directs the light through the sample and reference cells. Key features of this design include:
- Photodiodes with nearly identical response
- Electrical outputs are processed to deliver a readout proportional to the absorbance
Overall, photometers offer an accessible and cost-effective solution for various absorption measurement applications, making them an attractive choice for professionals and researchers alike.
Type 2: Probe-type Photometers
In addition to the previously mentioned photometers, probe-type photometers offer an innovative approach to absorption measurements.
A commercially available dipping-type photometer that uses an optical fiber to transmit light from a source to a solution layer between the fiber’s glass seal and a mirror.
The reflected radiation passes to a photodiode detector through a second glass fiber.
Some key features of probe-type photometers include:
- Amplifiers with electronic choppers synchronized with the light source, eliminate extraneous radiation response
- Filter options such as “drop-in” filters, a six-interference filter wheel for specific applications, and custom filters
- Probe tips made from stainless steel, Swagelok® Stainless Steel, Pyrex®, or Acid-resistant Lexan® Plastic
- Light path lengths ranging from 1 mm to 10 cm
To measure absorbance, the probe is first dipped into the solvent and then into the solution. This device is particularly useful for photometric titrations.
How to Choose a Filter for Analysis?
Filter selection is an important aspect of using general-purpose photometers, as the measurement sensitivity directly depends on the chosen filter.
To find the most suitable filter, the absorption spectrum of the solution is analyzed using a scanning spectrophotometer. The filter that closely matches the wavelength of maximum absorption is selected.
In some cases, measurements are made away from the absorption maximum to minimize interferences. Whenever possible, measurements are made near the absorption maximum to reduce Beer’s law deviations due to polychromatic radiation.
If a spectrophotometer is unavailable, filter selection can be guided by remembering that the color of the absorbed light is the complement of the solution’s color.
Solutions appear in specific colors because they transmit certain parts of the spectrum while absorbing others. For example, a red solution transmits the red portion of the spectrum but absorbs the green.
The intensity of green radiation varies with concentration, so a green filter should be used. Generally, the most suitable filter will be the color complement of the solution being analyzed.
Ultraviolet (UV) Absorption Photometers
Ultraviolet (UV) Absorption Photometers find applications in various fields:
1. High-performance liquid chromatography (HPLC) detectors
UV photometers often serve as detectors in HPLC, where a mercury vapor lamp is used as a source and the 254 nm emission line is isolated by filters.
2. Continuous monitoring of gas or liquid streams in industrial plants
UV photometers are employed to monitor the concentration of one or more constituents in industrial settings. These instruments are double-beam in space and frequently use mercury emission lines isolated by filter systems.
Common applications include:
- Determining low concentrations of phenol in wastewater
- Monitoring concentrations of chlorine, mercury, or aromatics in gases
- Calculating ratios of sulfur products in a gaseous sample.
UV photometers provide valuable information for various industries by enabling continuous monitoring and analysis of different components in gas and liquid streams. Their innovative designs and wide range of applications make them a go-to solution for absorption measurement challenges.