Polycyclic aromatic hydrocarbons PAHs
Polycyclic aromatic hydrocarbons, abbreviated as PAHs, belong to a group of chemical compounds with unsaturated cyclic hydrocarbons. These compounds are found in cigarette smoke. Their presence in the environment can be detected by spectroscopic instruments used to analyze chemical substances. At different wavelengths, they exhibit specific energy transitions that can be represented in the form of the emission, absorption, or excitation spectrum.
Literature Review
This class of compounds has a negative impact on and human beings. Their presence in the environment poses a human health risk due to their carcinogenic properties (Agency for toxic substances and Disease Registry [ATSDR], 1995). Therefore, their detection is of importance to monitor and control their exposure to the environment.
The detection of PAHs relies on three primary spectroscopic techniques that utilize the behavior of PAHs particles. When these particles are exposed to energy, they get excited and yield absorption, fluorescence, or excitation spectrum (Douglas et al., 2006). Depending on their behavior, they can be detected through analysis of the spectrum obtained in spectroscopic instruments. Generally, PAHs absorb energy in the form of light at a wavelength of 200-400nm. When these molecules absorb light, they are excited and change their electronic energy state from ground state S0 to an excited state S1. When these changes occur in a sample of PAHs, an absorption spectrum, in the form of a wavelength, is recorded, which is determined by the decline in light intensity relative to the light passed in a blank sample.
Conversely, molecules of PAHs can return to the ground state by emitting light n the form of energy, by a process known as fluorescence (Shane et al., 2000). In this case, the fluorescence spectrum is obtained. Therefore, though similar to the absorption spectrum, the excitation spectrum involves a variety of excitation wavelength and measuring the intensity of fluorescence.
Method on Analysis
- Determination of the Absorption spectrum
Heptane was used to make 10 ml solutions of each PAHs with a concentration of 5ppm from stock solutions of 200 mg/L of benzo[a]anthracene, benzo[k]fluoranthene, chrysene, and phenanthrene. From each of these 5ppm concentrated solutions of benzo[a]anthracene , dilute solutions of 1ppm, 0.80ppm, 0.60ppm, 0.40ppm, and 0.20ppm were made. These diluted samples were introduced to a UV-VIS Spectrophotometer. A wavelength of 200-600nm was chosen. The largest peak, wavelength, and absorbance of each solution were recorded in the form of spectra readings. This procedure was repeated for the remaining three PAHs and their data recorded. An empty cuvette was placed into the farther holder. The cuvette containing heptane was placed into closer holder, and spectra of 20 readings were recorded. A plot of absorbance versus concentration was plotted for each compound using Beer-lamberts law.
- Determination of Fluorescence spectrum and detection limits of PAHs
1ppm of solutions of each PAH was made. Spectrofluorometer was used to measure the wavelength at which the maximum absorbance occurred for each compound. Heptane was used to dilute each 1ppm concentrated solutions of the four PAHs to 0.2 ppm, 0.15ppm, 0.1 ppm, and 0.05 ppm. The fluorescence spectrum of each solution was recorded. Also, the wavelength at which maximum fluorescence intensity occurred for each solution was recorded, and plotted against concentration (mol L-1). A selective PAH was chosen and its limits of detection determined through collecting 20 fluorescence spectra, using equation: SM=Sbl+3sbl where sbl is blank signal standard deviation, SM is minimum detectable analytical signal. Limits of detection (LD) was determined using equation; LD= (SM– sbl)/m=3sbl/m. A comparison of LDs for selective PAH using absorption in part A was made.
C: Stern-Volmer quenching of the fluorescence signal
- 0 ml was diluted from 1 mg/L solution of benzo[a]anthracene using heptane and 1-bromoheptane as shown in the table below; and fluorescence spectrum pf each solution using the peak absorption wavelength at the excitation wavelength. A plot of Stern-Volmer was drawn and ratios of KQ/Kf determined.
| Volume of I mg/L benzo[a]anthracene(mL) | Approximate volume of Heptane (Ml ) | Volume of 1-bromoheptane(Ml) |
| 2.0 | 3.0 | 0 |
| 2.0 | 2.8 | 2.0 |
| 2.0 | 2.6 | 0.4 |
| 2.0 | 2.4 | 0.6 |
| 2.0 | 2.2 | 0.8 |
| 2.0 | 2.0 | 1.0 |
- Determinatin of PAHs excitation spectrum
The excitation spectrum for each compound using 1mg/L was recorded. The excitation and absorption spectra was compared and similarity identified.
Results and Discussion
Part A:
Table 1: A table of absorbance of the PAHs.
| PAHs | Chrysene | Phena | Fluora | Anthra |
| Concentration | Abs | |||
| 0.2 | 0.217194 | 0.021561 | 0.018356 | 0.088501 |
| 0.4 | 0.352646 | 0.095993 | 0.063736 | 0.23558 |
| 0.6 | 0.563004 | 0.168228 | 0.106949 | 1.279114 |
| 0.8 | 0.718155 | 1.251801 | 0.158676 | 1.409775 |
| 1 | 0.8211 | 0.707886 | 0.35643 | 0.49176 |
Graph 1: A graph of molar absorbance against concentration of the PAHs
| PAHs | λmax absorption (nm) | molar absorbtivity,ε (M/cm) |
| Chrysene | 268 | 0.7867 |
| Phena | 251 | 1.2642 |
| Anthra | 288 | 0.9904 |
| Fluora | 307 | 0.3855 |
Sbl=0.02 and 3sbl=0.06
m=Gradient= (419.67-103.804)/(1-0.2).
From equation (7), LD=(3sbl)/ m=(0.06)/ (419.67-103.804)/(1-0.2)
= 1.5196 x 10-4
Obtained values were not precise because of the small differences exhibited in their deviation.
From analysis, the deviation was; 0.03-0.002=0.028
The percentage error was found to be 9.33%, which is a significant figure.
Table 2: Maximum Emmission data of the PAHs
| Concentration (ppm) | Fluoranthene | Chrysene | Anthracene | Phenanthrene |
| 1 | 419.627 | 6.76 | 35.851 | 1.843 |
| 0.2 | 103.804 | 5.131 | 10.28 | 1.224 |
| 0.15 | 80.771 | 3.821 | 7.845 | 0.762 |
| 0.1 | 59.018 | 2.661 | 5.721 | 0.607 |
| 0.05 | 28.541 | 1.469 | 2.814 | 0.469 |
Graph 2: A graph of Intensity against Concentration for PAHs
Wavelength 365nm was chosen as the biggest wavelength.
Sbl=0.03 and 3sbl=0.09
m=Gradient= (1.224-0.762)/(0.2-0.15).
From equation (7), LD=(3sbl)/ m=(0.09)/ (1.224-0.762)/(0.2-0.15).
=9.74 x 10-3
The limit of detection in part B was larger than the one in part A. This difference is attributed to the less energy of emitted spectrum.
Table 3. A table of quenched and unquenched fluorescence of signals of benzo (a) anthracene
| Conc(ppm) | Iº | I | Iº/I |
| 1 | 35.851 | 17.035 | 2.1045495 |
| 0.2 | 1.28 | 14.816 | 0.0863931 |
| 0.15 | 7.845 | 16.055 | 0.4886328 |
| 0.1 | 5.731 | 14.191 | 0.4038475 |
| 0.05 | 2.814 | 15.606 | 0.1803153 |
The data collected shows that unquenched PAH compound shows higher reading of florescence. This phenomenon is attributed to insufficient of energy that can excite the particles to higher energy, hence emitting the energy.
Graph 3. A graph of intensity ratio against concentration of a quencher on benzo(a)anthracine
Given density of 1-bromoheptane=1.114g/ml,
Concentration (ppm) in 0.2 ml =(1.14g/ml) x (0.2 ml/5ml) x 106=4.56 x 104 ppm
Concentration (ppm) in 0.4 ml =(1.14g/ml) x (0.4 ml/5ml) x 106=9.12 x 104 ppm
Concentration (ppm) in 0.6 ml =(1.14g/ml) x (0.6 ml/5ml) x 106=1.368 x 105 ppm
Concentration (ppm) in 0.8 ml =(1.14g/ml) x (0.8 ml/5ml) x 106=1.924 x 105 ppm
Concentration (ppm) in 1.0 ml =(1.14g/ml) x (0.4 ml/5ml) x 106=2.28x 105 ppm
Intensity observed for unquenched benzo(a) anthracene at 387nm =35.851
Intensity observed for quenched benzo(a) anthracene at 387nm = 10.28
The value of KQ/KF is given by the slope of the curve, hence =(2.10-0.08)/0.8
= 2.525
Conclusion
The absorbance is directly proportional to the concentration of the analyte. From the Beer-Lamberts equation, the molar absorptivity decreased with increase in the value of wavelength. Similarly, the intensity of fluorescence increased with increase in concentration. There were no much disparities in the measurement of limits of detection because the data range was big. It is evident that a quencher causes a decrease in fluorescence.
Works Cited
E.C. Shane, M. Pierce Everet, T. Hanson, Fluorescence Measurement of Pyrene wall absorption and pyrene association with humic acids, Journal of Chemical Education ,2000. Pp.77
Skoog, Douglas A., et al. Principles of Instrumental Analysis. Cengage Learning Asia Pte Ltd, 2018.
Toxicological Profile for Polycyclic Aromatic Hydrocarbons: Draft. The Agency, 1993.