A) Comparison of NO2 - and NO3 - actinometer light dosages (μEinstein/cm2) using the Tecan and the Aqualog
Chemistry
Comparing Ultraviolet Light Dosage using Chemical Light Meters
Chemistry
Comparing Ultraviolet Light Dosage using Chemical Light Meters
Maya Weathers '27 counducted this research as a part of CHE 490: Chemistry Research I.
Overview
Photolysis plays a significant role in atmospheric and aquatic chemistry, assisting in many natural processes. Photolysis allows for the formation of ozone in our atmosphere, protecting life on Earth from harmful ultraviolet (UV) rays, as well as the degradation of trace organic molecules including pesticides and pharmaceuticals. During photolysis, molecules absorb light and decompose into new, excited molecules. When studying photochemical reactions, it is essential to accurately measure the amount of light the sample is exposed to, or the light dosage. One way to quantify light dosage is using chemical light meters, or actinometers.
Researcher
Maya Weathers ’27
Chemistry, Environmental Science
College of Arts & Sciences
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
Figure 1. Standard addition curves of the fluorescence of SA from exposed and control NO3 - actinometers using the Tecan
Results
B) Comparison of NO2 - and NO3 - actinometer light dosages (μEinstein/cm2) when exposed to different weather conditions using the Tecan.
Findings
- The comparison of the Tecan Infinite MPlex Microplate Reader and the Aqualog Spectrometer showed a 9.94% difference for the light dosages produced from the NO2 - actinometers and an 85.78% difference for light dosages produced from the NO3 - actinometers.
- As expected, more photolysis occurred in the exposed samples on the sunny day with snow than on the cloudy day with precipitation, with a 64.42% increase in light dosage for the NO2 - actinometer and 236.3% increase for the NO3 - actinometer.
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.
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