Difference Between UV and Visible Spectrophotometer

 Key Difference – UV vs Visible Spectrophotometer
 

There is no difference between UV and visible spectrophotometer because both these names are used for the same analytical instrument.

This instrument is commonly known as the UV-visible spectrophotometer or Ultraviolet-visible spectrophotometer. This instrument uses the absorption spectroscopy technique in Ultraviolet and visible spectral region.

CONTENTS

1. Overview and Key Difference
2. What is UV Spectrophotometer or Visible Spectrophotometer
3. Summary – UV vs Visible Spectrophotometer

What is UV Spectrophotometer (or Visible Spectrophotometer)?

UV spectrophotometer, also known as visible spectrophotometer, is an analytical instrument that analyzes liquid samples by measuring its capability to absorb radiation in ultraviolet and visible spectral regions. This means this absorption spectroscopic technique uses the light waves in visible and adjacent regions in the electromagnetic spectrum. The absorption spectroscopy deals with the excitation of electrons (movement of an electron from the ground state to excited state) when the atoms in a sample absorb light energy.

Figure 01: A UV-Visible Spectrophotometer

The electronic excitations take place in molecules containing pi electrons or non-bonding electrons. If the electrons of molecules in the sample can easily be excited, the sample can absorb longer wavelengths.  As a result, the electrons in pi bonds or non-bonding orbitals can absorb energy from light waves in UV or visible range.

The major advantages of UV-Visible spectrophotometer include simple operation, high reproducibility, cost-effective analysis etc. In addition, it can use a wide range of wavelengths to measure analytes.

Beer-Lambert’s Law

The Beer-Lambert’s law gives the absorption of certain wavelength by a sample. It states that the absorption of wavelengths by a sample is directly proportional to the concentration of the analyte in the sample and the path length (the distance travelled by the light wave through the sample).

A = εbC

Where A is the absorbance, ε is the absorptivity coefficient, b is the path length, and C is the concentration of the analyte. However, there are some practical considerations regarding the analysis. The absorptivity coefficient depends only on the chemical makeup of the analyte. The spectrophotometer should have a monochromatic light source.

Basic Parts of the UV-Visible Spectrophotometer

1. A light source
2. A sample holder
3. Diffraction gratings in a monochromator (to separate different wavelengths)
4. Detector

A UV-visible spectrophotometer may use a single light beam or double beam. In single beam spectrophotometers, all the light passes through the sample. But in double beam spectrophotometer, the light beam splits into two fractions, and one beam passes through the sample while the other beam becomes the reference beam. This is more advanced than using a single light beam.

Uses of UV-Visible Spectrophotometer

The UV-visible spectrophotometer can be used to quantify the solutes in a solution. To quantify analytes such as transition metals and conjugated organic compounds (molecules containing alternating pi bonds), one can use this instrument. We can use this instrument to study solutions, but sometimes scientists use this technique to analyze solids and gases as well.

Summary – UV vs Visible Spectrophotometer

The UV-visible spectrophotometer is an instrument that uses the absorption spectroscopic techniques to quantify the analytes in a sample. There is no difference between UV and visible spectrophotometer because both names refer to the same analytical instrument.

Reference:

1. “Ultraviolet–Visible Spectroscopy.” Wikipedia, Wikimedia Foundation, 10 Apr. 2018. Available here 
2. “Spectrophotometry and Visible Spectrophotometers.” Aurora Biomed. Available here

Image Courtesy:

1.’Spectrophotometer Model 1’By Viv Rolfe – Own work, (CC BY-SA 4.0) via Commons Wikimedia 

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