Ex) Article Title, Author, Keywords
Ex) Article Title, Author, Keywords
New Phys.: Sae Mulli 2023; 73: 549-557
Published online June 30, 2023 https://doi.org/10.3938/NPSM.73.549
Copyright © New Physics: Sae Mulli.
Syed Bilal Junaid, Furqanul Hassan Naqvi, Jae Hyeon Ko*
School of Nano Convergence Technology, Nano Convergence Technology Center, Hallym University, Chuncheon 24252, Korea
Correspondence to:*E-mail: hwangko@hallym.ac.kr
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
The use of hand sanitizers is one of the important measures recommended by the Centers for Disease Control and Prevention to prevent the spread of diseases. The main component of hand sanitizers is ethanol, and its concentration should be greater than 60% as regulated. Several spectroscopic techniques have been developed to determine the actual ethanol concentration in sanitizers. One such method involves measuring the frequency of a specific vibrational mode of ethanol, which is proportional to its concentration. In this study, Raman and Brillouin spectroscopies were used to investigate seven hand sanitizers, and the results were compared with those of pure ethanol–distilled water (subsequently referred to as ethanol–water) solutions. The results showed that the mode frequency of the CCO stretching vibration of several sanitizers deviated from the fitted line, indicating that the actual ethanol content in these commercial products differed from the values stated on the product labels. Additionally, the concentration deviation was correlated with an anomalous increase in the sound velocity. This suggests that acoustic methods may be used as an additional tool to investigate the actual ethanol concentration in commercial hand sanitizers.
Keywords: Hand sanitizers, Ethanol, Raman spectroscopy, Brillouin spectroscopy
After the outbreak of COVID-19 since the beginning of 2020, various precautionary measures have been taken to stop the spread of the pandemic. According to the World Health Organization (WHO) guidelines, regular washing of hands and the use of masks and hand sanitizers were the foremost effective ways to reduce the spread of the disease. Although the Centers for Disease Control and Prevention (CDC) advised frequent washing of hands with soap and water, it also suggested using alcohol-based hand sanitizers containing at least 60% alcohol as an alternative[1]. This caused the use of hand sanitizers to increase rapidly worldwide as customer demand increased. However, the proliferation of hand sanitizer production facilities across the world sometimes resulted in some products not meeting their stated specifications, and thus, not being effective in the elimination of the virus.
Therefore, it is important to maintain the product quality; ethanol concentration must be high enough to meet the specific requirement for effectiveness. Moreover, to avoid risk to human health, contamination in hand sanitizers must be prevented; e.g., methanol contamination may cause methanol to be absorbed through the skin, resulting in chronic toxicity[2]. The ethanol concentration is written on the product label. However, due to lax regulations and the absence of proper inspection protocols, the actual ethanol percentage might vary from that claimed in the label.
In this research, we aim to investigate the ethanol concentration in seven commercial hand sanitizers indirectly by comparing their spectroscopic properties with those of pure ethanol–water solutions. Previous studies have shown that techniques such as Raman spectroscopy[3], Infrared spectroscopy[4,5], Fourier transform infrared spectroscopy[6], and UV–visible absorption spectroscopy[6] can be used for the measurement of ethanol content in hand sanitizers. In particular, the Raman mode frequency at 880 cm-1, which corresponds to the CCO symmetric stretching mode, was found to be linearly proportional to the ethanol content[7]. Various experimental techniques such as dielectric constant measurement[8,9], through transmission ultrasonic method[10,11], and laser spectroscopy[12-14] have been applied to explore the dynamic behavior of pure ethanol–water solutions. In this study, we employed Raman and Brillouin light scattering spectroscopies to explore the vibrational and acoustic properties of both gel- and liquid-type hand sanitizer products and compared them with those of pure ethanol–water solutions. This will help determine their actual ethanol content and help in making evaluation standards for them.
Seven commercial hand sanitizer products were purchased from the local market with different ethanol concentrations (volume percentage): 62%, 63%, 66%, 70%, and 83%. There were two products each for 62% and 70%. Thus a total of five concentrations and seven different products were investigated. The products were numbered as P1 to P7 with increasing ethanol content (P1 and P2 for 62% and P5 and P6 for 70%). The label did not specify any other ingredients in the products. Ethanol (anhydrous 99.5%) was purchased from Sigma-Aldrich. Pure ethanol–water solutions were prepared with varying ethanol concentrations: 0%, 20%, 40%, 60%, 80%, and 100%.
The Raman measurements were carried out by using a standard Raman spectrometer (LabRam HR800, Horiba Co., Japan) in the frequency range of 400 to 4000 cm-1. A diode-pumped, solid-state green laser with 532-nm wavelength was used to probe the samples. For all Raman measurements, an optical microscope (BX41, Olympus, Japan) with a 50X magnification objective lens was used at backscattering geometry. Prior to the measurement, the Raman spectrometer was calibrated with a silicon substrate as the reference. The samples were poured into quartz cuvettes for analysis. All measurements were performed at room temperature (20 °C ± 1 °C). The intensities of the measured Raman spectra were reduced using the Bose-Einstein correction factor.
Brillouin spectra were obtained using a standard tandem multipass Fabry-Perot interferometer (TFP-2, JRS Co., Zürich, Switzerland) and a 532-nm excitation source. A modified microscope with backscattering geometry was used (BH-2, Olympus, Tokyo, Japan) for the measurement. A conventional photon-counting instrument was linked to a multichannel analyzer (1024 channels) to measure and average the signal. The free spectral range was 14 GHz for identifying the longitudinal acoustic (LA) mode in the Brillouin spectrum. The sound velocity and the absorption coefficients were calculated from the Brillouin scattering spectrum by measuring the refractive index of each sample using a homemade, prism-based refraction measurement tool.
Seven hand sanitizer products were investigated by Raman and Brillouin light scattering spectroscopy. Raman spectroscopy has been used earlier to quantitatively analyze the relative amount of ethanol in commercial hand sanitizer products[3]. The Raman results of pure ethanol and pure ethanol–water solutions were compared to verify the percentages mentioned on the labels of commercial hand sanitizers. The Brillouin results were used to compare the acoustic behavior, i.e., sound velocity and the absorption coefficient, of pure ethanol and pure ethanol–water solutions with those of hand sanitizers.
Figure 1 shows the Raman spectra of pure ethanol and hand sanitizer P7 with the highest ethanol concentration of 83%. A wide frequency range from 400 cm-1 to 4000 cm-1 was chosen for measurement as the Raman modes of ethanol are expected to be observed in the high-frequency range because of the presence of light elements such as carbon and hydrogen. The Raman spectra of P7 exhibit nearly the same spectral shape as pure ethanol, and several distinct peaks corresponding to the optical modes of pure ethanol can be observed. Mode assignment of each Raman band is listed in Table 1. Raman spectra of the other hand sanitizers were also very similar to that of pure ethanol as shown in Fig. 2. Thus, we can conclude that the Raman vibrational behavior of hand sanitizers is dominated by the ethanol molecules. However, the peak positions and full width at half maximum (FWHM) might differ depending on the ethanol content. The changes in Raman shifts are caused by the increased ethanol concentration. The inset of Fig. 2 shows the CCO symmetric stretching mode, the frequency of which was confirmed to be linearly proportional to the ethanol concentration[7].
Table 1 Mode Assignments of Raman peaks of pure ethanol and seven hand sanitizers.
Pure Ethanol | P1 | P2 | P3 | P4 | P5 | P6 | P7 | Mode Assignment*[15-20] |
---|---|---|---|---|---|---|---|---|
435 | 436 | 436 | 436 | 435 484 541 782 815 849 | 435 | 436 | 435 | CCO bending |
883 | 880 | 881 | 881 | 881 | 881 | 881 | 882 | CCO symmetric stretching |
1051 | 1048 | 1049 | 1049 | 1049 | 1049 | 1049 | 1050 | CCO asymmetric |
1095 | 1089 | 1090 | 1089 | 1091 | 1090 | 1091 | 1092 | CH3 rocking COH deformation |
1275 | 1277 | 1277 | 1277 | 1276 | 1276 | 1277 | 1276 | CH2 wagging |
1385 | 1382 | 1384 | 1381 | 1382 | 1382 | 1383 | CH2 wagging CH3 symmetric deformation | |
1454 | 1454 | 1454 | 1454 | 1454 | 1454 | 1454 | 1454 | CH3 asymmetric deformation |
1484 | 1485 | 1485 | 1485 | 1483 | 1485 | 1485 | 1485 | CH2 deformation |
2715 | 2722 | 2719 | 2720 | 2719 | 2719 | 2719 | 2718 | Combinational frequencies*[23] |
2750 | 2753 | 2753 | 2753 | 2752 | 2753 | 2752 | 2752 | Combinational frequencies*[23] |
2837 | 2832 | 2831 | 2832 | 2831 | 2831 | 2831 | 2831 | Combinational frequencies*[23] |
2889 | 2884 | 2883 | 2884 | 2883 | 2883 | 2883 | 2882 | CH3 symmetric stretching |
2928 | 2932 | 2931 | 2931 | 2930 | 2931 | 2931 | 2930 | CH3 asymmetric stretching |
2973 | 2977 | 2976 | 2976 | 2975 | 2976 | 2976 | 2975 | CH3 asymmetric stretching |
3252 | 3227 | 3238 | 3239 | 3243 | 3241 | 3242 | 3244 | O-H stretching |
3374 | 3431 | 3431 | 3431 | 3428 | 3431 | 3428 | 3426 | O-H stretching |
To compare the spectral changes in hand sanitizers with those of the pure ethanol–water system, we measured the Raman spectra of pure ethanol–water solutions with different volume concentrations. Figure 3 shows the Raman spectra of ethanol (
To obtain quantitative Raman shifts, the Raman spectra were fitted using a superposition of Lorentzian functions. Table 1 shows the Raman shifts of pure ethanol and all sanitizer samples with respective mode assignments. Additional peaks in the frequency range of 450–850 cm-1 were observed only for P4 and not for any other products. This might be due to the presence of additional and distinct ingredients added during manufacturing.
Figure 4 shows a correlation between the ethanol concentration and the peak at 882 cm-1 of pure ethanol–water solutions. This clearly indicates that the mode frequency changes linearly with the ethanol concentration, which was also confirmed in hand sanitizers[7]. This linear correlation can be used to evaluate the actual ethanol concentration in commercial hand sanitizers as confirmed by previous studies[3,4,7]. For instance, if the Raman shifts of this peak in hand sanitizers deviate from the fitted line for the pure ethanol–water solutions, it indicates that the ethanol content in the product is different from that written on the label. The Raman shifts of this mode for all hand sanitizers are shown in Fig. 4 for comparison.
The Raman shifts of P2, P3, and P7 were close to the fitted line, indicating that their measured ethanol concentrations were close to the values on the product label. However, the mode frequencies of other products, especially P1, substantially deviated from the fitted line even though the label values of P1 and P2 were the same. As estimated from the Raman frequency of P1, the ethanol content is only 40%. This result indicates that, for many hand sanitizers, the actual ethanol content may be different from that specified on the label. This difference may have been due to the volatile nature of ethanol, which caused it to evaporate during the manufacturing process or usage period, resulting in a gradual deviation from the original value.
Brillouin light spectroscopy was used to investigate the acoustic properties of the hand sanitizers and pure ethanol–water solutions. Figures 5(a) and (b) illustrate the Brillouin spectra of pure ethanol–water solutions and hand sanitizers, respectively, which clearly reveal spectral changes in the LA mode with increasing ethanol concentration. Figure 5(a) shows a red shift in the LA mode when the ethanol concentration is increased up to 40%, followed by a blue shift as the ethanol concentration is increased further up to 100%. However, as seen in Fig. 5(b), the LA mode of the hand sanitizers displays minor differences from those of Fig. 5(a) because of the small changes in the ethanol content in these products.
The Brillouin shift and the FWHM of the LA mode of the pure ethanol–water solutions and hand sanitizers as a function of ethanol content are shown in Fig. 6(a) and (b), respectively. The results show that the Brillouin shift attains a maximum at the ethanol ratio of 20–40% and then decreases rapidly as the ethanol content rises above 40%. The mode frequency of P1 deviates most from the fitted line of the pure ethanol–water solution while others deviate slightly; this is corroborated by the Raman data shown in Fig. 4. The FWHM of LA mode of the pure ethanol–water solution increases monotonically till 40%, and then rapidly decreases upon further increase in the ethanol content. As per Fig. 6(b), hand sanitizers have much higher acoustic damping than pure ethanol–water solutions.
For quantitatively determining acoustic properties such as the sound velocity and the absorption coefficient, the refractive index was obtained as described in Fig. S1 in the Supplementary Materials, and the results are shown in Fig. S2. The measured refractive index of the ethanol–water solution in this work is slightly different from those published previously[24-26]. This can be ascribed to differences in temperature and probe wavelength values. The sound velocity
Both Raman and Brillouin data suggest that some hand sanitizers exhibit vibrational and acoustic behaviors substantially different from those of pure ethanol–water solutions. This indicates that the difference in the ethanol concentration between the actual content in the product and that specified on the label is not only associated with a Raman mode near 882 cm-1 but also with the acoustic properties probed by Brillouin scattering. In this context, it would be interesting to investigate the correlation between Raman and Brillouin data.
The equation provided below was employed to determine the degree of deviation (
In the equation,
One interesting observation is that the absorption coefficients of all hand sanitizers were significantly larger than those of pure ethanol–water solutions irrespective of the ethanol content. This could be due to other additives present in the sanitizers. The additives, usually organic compounds, contain polar and nonpolar groups in general, which affect the hydrogen bonding and the size of the hydrogen-bonded dynamic clusters in the ethanol–water solution[24]. Another possibility is the change in temporal fluctuations of local ethanol concentration. These spatial and temporal changes in the dynamic clusters caused by the additives may dissipate the acoustic waves, resulting in an increase in the absorption coefficient.
Finally, this technique can also be applied to methanol–added solutions. The addition of methanol in the formulation of sanitizers is strictly prohibited by the Food and Drug Administration (FDA) and the World Health Organization (WHO) due to its toxicological implications. Thus, it would be interesting to study the effect of a small amount of methanol on the vibrational and acoustic properties of sanitizers, and the study would also help in detecting this toxicological material. The detection of the specific vibrational modes of methanol molecules and the change in the sound velocity of ethanol–methanol–water solutions depending on the methanol content may be useful offshoots of this study.
The vibrational and acoustic properties of seven commercial hand sanitizers were investigated by using Raman and Brillouin spectroscopies. The obtained results were compared with those of the pure ethanol–water solutions. For some hand sanitizers, the Raman shift of the CCO stretching mode, known to be linearly proportional to the ethanol content, showed deviations from the fitted line indicating that the actual ethanol content was different from that on the label. The Brillouin frequency shift and the FWHM of the LA mode of hand sanitizers exhibited substantial deviations from those of the pure ethanol–water solutions. In particular, the deviations between the Raman and Brillouin frequencies (and, thus, between the Raman frequency and the sound velocity) displayed an approximately linear behavior when the degree of deviation was large. The findings of this study indicate that Brillouin spectroscopy is another noncontact and nondestructive method for evaluating the ethanol content of hand sanitizers. It should be noted that these results are valid for dry-type sanitizers. To confirm that this testing can be more widely applied, a thorough investigation of various hand sanitizers, including wet-type sanitizers, by Brillouin spectroscopy is required.
The online version of this article contains a supplementary material.