Comparing Ultraviolet Light Dosage using Chemical Light Meters

Introduction

Photolysis and Fluorescence

• Photolysis is the process by which molecules absorb light and decompose into new, excited molecules. When studying photochemical reactions, it is essential to accurately measure the light dosage of the sample. Light dosage can be quantified by measuring the fluorescence of salicylic acid (SA) produced from chemical light meters, or actinometers.¹

Nitrite and Nitrate Actinometers and the Production of Salicylic Acid

• Nitrite (NO₂^-) and nitrate (NO₃^-) are photosensitizers. When they absorb ultraviolet (UV) radiation, they produce reactive species (such as ·OH) that can undergo secondary reactions.¹

Photolysis of Nitrate:

NO₃^- + hν → NO₂* + O^-*

Photolysis of Nitrite:

NO₂^- + hν → NO* + O^-*

Production of Hydroxyl Radical (·OH):

O^-* + H₂O → ·OH + OH^-

Production of salicylic acid (SA) and phydroxybenzoic acid (p-HBA):

·OH (radical) + benzoic acid → SA + p-HBA

Aims

• Analyze UV light dosage (μEinstein/cm²) by quantifying the concentration of SA (nM) produced through the photolysis of NO₂^- and NO₃^- actinometers and compare results from two
  instruments: an Aqualog Spectrometer and a Tecan Infinite MPlex Microplate Reader.
• Analyze UV light dosage (μEinstein/cm²) by comparing the concentration of SA produced in samples given varying weather conditions: a sunny day with snow vs. a cloudy day with precipitation.

Materials and Methods

Sampling

• 10 nM NO₃^- or 1.0 nM NO₂^- in 1 mM benzoic acid solution buffered with 2.5 mM sodium bicarbonate were used as chemical actinometers.²
• Paired 20 mL borosilicate vials of each solution were placed outside for ≈ 1 hour, one vial was exposed and the other was wrapped in aluminum foil (control).
• Quantification of SA was determined via standard addition of SA with a final concentration range of 50-2,000 nM.²

Analysis

Tecan Infinite MPlex Microplate Reader

• Excitation: 305 ± 7.5 nm, Emission: 410 ± 7.5 nm.²
• 3/4 of 200 μL well plates filled with NO₂^- and NO₃^- actinometer solution. 1/4 of the well plates filled with a ratio of increasing SA and decreasing high purity water (HPW) to create a standard addition curve.

Aqualog Spectrometer

• Excitation: 305 nm, Emission: 410 nm.²
• 3.5 mL of a 4.5 mL glass cuvette filled with NO₂^- or NO₃^- actinometer solution.
• Increasing volumes of SA stock solution were added to create a standard addition curve.
• SA concentrations were converted into light dosages accounting for SA quantum yields, absorption coefficient, irradiation temperature, and nitrite or nitrate concentrations.³

Future Work

• This research was used to create a protocol for an Instrumental Analysis lab that was implemented into coursework this semester.
• This research will be continued next year, and further experimentation will be done regarding the comparison of instruments, chemical actinometers, and UV light dosage with varying weather conditions. The difference in results for NO₂^- vs NO₃^- actinometers will be investigated. 

References

¹Jankowski, J. J., et al. 1999. J. Photochem. Photobiol. doi: 10.1562/0031-8655(1999)070

²Jankowski, J. J., et al. 2000. J. Photochem. Photobiol. doi: 10.1562/0031-8655(2000)071

³Kieber, D. J., et al. 2007. Aquat. Sci. doi: 10.1007/s00027-007-0895-0

Professional Application

"This project has prepared me for my future career in many ways. Creating a procedure based on academic journals and research is a challenge, but seeing results is a great reward. From this project, I was able to create a complete protocol for an Instrumental Analysis lab, which was added to the coursework this semester. This experienced allowed me to practice my scientific writing abilities and challenged me to be as precise as possible. I was also able to work more in depth with new instruments, which allowed me to gain a greater understanding of how they operate. As a chemistry and environmental science major, I am very passionate about human health and sustainability, and I hope to start a career within the pharmaceutical industry. Understanding how pharmaceuticals decompose in nature is essential for the health of not only our environment, but also our people. Studying photolysis has been a great experience and I look forward to continuing this research in the future!" - Maya Weathers '27

For Further Discussion

This serves as an overview of the project and does not include the complete work. To further discuss this project, please email Maya Weathers. 

Course Overview

In CHE 490: Chemistry Research I, students work closely with a faculty mentor on a chemistry research project.

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