The lowest concentration detected of naphthalene in ethanol solution was indicated through MIR absorption spectroscopy. Spectra of naphthalene were obtained by using different ATR probes. Comparisons were made of the sensitivity of three separate FlexiSpec® ATR probes; diamond, zinc selenide and silicon, from Art Photonics.
Analysis of MIR & NIR absorption is a largely applied spectroscopic method offering chemical identification of the samples with high levels of accuracy and sensitivity . Incorporating the spectrometer with optical fiber ATR probes in the MIR & NIR range forms an effective tool of in situ spectroscopic analysis.
Simple implementation of distant measurements through probe immersion in the sample, has led to probe areas of application including: food quality analysis, chemical composition , biotechnology, monitoring of chemical reactions in real time , completely automated fabrication control, etc.
An essential application iscontaminant detection, like naphthalene, in the environment. Naphthalene emission sources include industry, gasoline and oil combustion, the employment of mothballs, deodorant, etc . For these reasons, naphthalene exposure is inevitable for a large number of individuals and impacts environmental and health risk.
Areas where it can be found are; air (5.19 µg/m3), soil (between 66 µg/kg – 16.7 mg/kg) and food (2.85 µg – 16.6 µg of daily intake) . Predictions of the average intake rate per day for human inhalation is 19 μg, and 0.002 - 4.0 μg for ingestion of water .
Additionally, naphthalene is a toxic substance soluble in ethanol (77 g/l or 0.6 M) and insoluble in water. Its solubility in water is 0.2 mM or 30 mg/l. Contact with naphthalene, through ingestion or inhalation, has been associated with adverse impacts on an individual’s health like changes in respiratory and olfactory epithelium and incidences of cancer [6,7].
Due to the fact that naphthalene is soluble in organic solvents, the practice of using ethanol for the extraction of naphthalene or other polycyclic aromatic hydrocarbon contaminants from soil and water samples is employed .
Following extraction, naphthalene concentration is figured out on the basis of chromatographic measures. Spectroscopic techniques, however, are much simpler and faster in application . The presence and concentration of naphthalene can be determined based on MIR absorption spectral analysis down to sensitivity limit. This study presents the sensitivity limits for acquiring the MIR absorption spectrum of naphthalene in ethanol when a variety of fiber ATR probes that are utilized.
During this study, different FlexiSpec® ATR probes were employed from art photonics GmbH: diamond (12 mm diameter), zinc selenide – ZnSe (12 mm diameter) and silicon (6.3 mm diameter). Probes were combined with the MATRIX-MF FTIR spectrometer from Bruker. ATR background spectra were gathered without the presence of a sample. Every presented spectrum was averaged of three measured spectra, utilizing 128 scans and 4 cm-1 resolutions.
The tests were performed for naphthalene in 80% ethanol solutions in a variety of concentrations; 80, 60, 40, 20, 10, 5, 3 and 1 mM. This was done by diluting 90 mM naphthalene solution with respective level of ethanol. In relation to the attained spectra, calibration curve was produced and sensitivity value was determined.
Figure 1. Different Fiber Optic ATR probes for immersion measurements connected to the spectrometer. Image Credit: Art Photonics
To be able to acquire naphthalene spectrum, ethanol spectrum was taken away from each spectrum of naphthalene in ethanol solution acquired with different probes, as seen in Figure 2. Naphthalene is able to be discerned by spectral bands at: 787 cm-1, 962 cm-1, 1011 cm-1, 1270 cm-1, 1391 cm-1, 1511 cm-1 and 1597 cm-1 which are shown by areas shaded in the figure.
Figure 2. Differential spectra of 90 mM naphthalene in ethanol solution obtained via diamond (D), zinc selenide (ZnSe) and silicon (Si) ATR probes. Shaded areas indicate spectral bands of naphthalene. Image Credit: Art Photonics
To determine each probe’s sensitivity limit, the highest intensity concentration dependent spectral band of naphthalene at 787 cm-1 was selected. Figure 3 shows the differential spectra of decreasing naphthalene concentration in ethanol solution when diamond (a), ZnSe (b) and silicon (c) ATR probes were employed accordingly.
Figure 3. Differential MIR absorption spectra (region 860 cm-1 – 715 cm-1) of naphthalene solutions with decreasing concentration. Spectra obtained: a) via diamond; b) via ZnSe; c) via silicon ATR probe. Image Credit: Art Photonics
The intensity of naphthalene spectral band was reduced from 90 mM to 1 mM in addition to S/N ratio for spectral band at 787 cm-1 (30 to 1.5). Figure 4 demonstrates that it gets too low for observable identification of naphthalene.
Figure 4. Signal to noise ratio (S/N) of MIR absorption spectra of different concentration naphthalene solutions obtained via diamond (D), zinc selenide (ZnSe) and silicon (Si) ATR probes. Image Credit: Art Photonics
Based on the outcomes, the highest sensitivity is shown using the diamond probe. With the diamond probe the detection limit of 1 mM concentration of naphthalene was reached. On the other hand, ZnSe and Si probes are appropriate to reliably find the naphthalene with concentration reaching reduced to roughly 5 mM.
Lastly, the integral area of spectral band at 787 cm-1 was determined from the ethanol subtracted spectra. The dependency result of the calculated intensity to the concentration was recorded and analyzed in more detail, as shown in Figure 5. The intensity depends linearly on the concentration with somewhat different inclination for diamond; for ZnSe and for silicon.
Figure 5. Dependency of the integral area intensity of spectral band at 787 cm-1 on naphthalene concentration in ethanol solutions. Image Credit: Art Photonics
Spectra of naphthalene were obtained by using different ATR probes. Comparisons were made of the sensitivity of three different FlexiSpec® ATR probes, diamond, zinc selenide and silicon, manufactured by art photonics GmbH.
The diamond probe showed reliable sensitivity for naphthalene detection and was determined to be 1 mM. On the other hand, sensitivity of ZnSe and silicon probes were almost able to find naphthalene of 5 mM concentration. When concentrations were further reduced, S/N ratio was becoming to low for reliable detection.
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About art photonics GmbH
In the global market, art photonics GmbH is the leader in manufacturing and supplying InfraRed chalcogenide and polycrystalline specialty optical fibers, spectroscopy fiber probes and fiber bundles, high power fiber cables for industrial and medical applications.
With over 30 years of experience, art photonics' aims to engineer, design, development and produce specialty optical fibers and various fiber systems for the broad spectral range from 200 nm to 18 µm with their unique technology.
Clubbed with innovation, art photonics aims to provides optimized solution to suit specific customer requirement. The products FlexiSpec® and FlexiRay® stands testimony to their success.
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