Introduction
Congeners are biologically active chemicals and are often contained in alcoholic beverages, in addition to ethanol, the key biologically active ingredient of alcohol. They are produced in the process of fermentation or ageing, when organic chemicals in the beverage break down. They may also be added during the production process to contribute to the taste, smell and appearance of the beverage.
Methanol, also known as wood alcohol, is a congener contained in alcoholic beverages which is thought to contribute to hangover symptoms and is toxic. It is a constituent of many commercially available industrial solvents and of poorly adulterated alcoholic beverages. Methanol toxicity remains a common problem in many parts of the developing world, especially among members of lower socioeconomic classes. Hence it is essential to prevent / monitor adulteration.
Determination of methanol is carried out using FTIR spectroscopy, HPLC / RI and Spectrophotometry. In this study, an attempt has been made to quantify methanol in synthetic samples using Attenuated total reflectance (ATR) technique and then extending the focus to work on real alcoholic beverages. ATR to technique is used to overcome the problem of strong absorbance in mid IR region.
Materials & Methods
Highly pure methanol (99.8%) and ethanol (99.8%), obtained from the Merck, were used for preparing samples in the following concentration: methanol, 0.2, 0.5, 1.0, 1.5, 2.0% and ethanol, 40%. Samples were prepared in HPLC grade water.
- Perkin Elmer Spectrum 100 series spectrophotometer with resolution 4 cm-1 was used for experiments. Horizontal type ATR accessory with Zinc Selenide (ZnSe) crystal was used for spectral measurements. Spectra were collected in the wave length region 650 – 4000 cm-1 with air as background.
- Perkin Elmer HPLC 200 series HPLC with Refractive Index Detector was used for analysis. Reverse phase C18 column was used for measurements with water as mobile phase.
- Spectrometry analysis was done using Perkin Elmer Lambda 25 UV/Vis spectrophotometer following IS 3752 method. Working solutions were 0.2%, 0.4%, 0.6, 0.8, 1.0% of methanol in HPLC grade water. In acid medium methanol reacts with chromotropic acid followed by KmnO4 solution at 80oC to develop violet – red colour. Absorbance of the colored complex was measured at 575 nm.
Results & Discussion
- ATR spectrum of synthetic sample (Fig.1) shows peaks at 1640 cm-1 is associated to OH deformation. C – OH deformation causes a peak at 1450 cm-1. Two closely resolved peaks, present at 1086 & 1045 cm-1, are attributed to the C – O stretch of ethanol and one peak at 1018 cm-1 is due to the C – O stretch of methanol respectively. Concentration variation of the constituents was observed easily from the overlaid spectra of synthetic samples (Fig. 2). Calibration curve was drawn in the range 0.2 – 1.5% of methanol content in 40% ethanol with Correlation coefficient (R2) of 0.9981.
Actual and predicted values of methanol% in prediction set
| Actual values | Predicted values |
| 0.3 | 0.34 |
| 0.8 | 0.72 |
| 1.2 | 1.31 |
| 1.8 | 1.79 |
- Chromatograms of synthetic sample (Fig.3) using HPLC show two distinct peak at 2.03 and 2.54 minute for methanol and ethanol respectively. Standard calibration curves were drawn for methanol and ethanol with 0.2, 0.5 & 1.0% of methanol and 0.2, 0.5 & 1.0% of ethanol with 10 times dilution. Correlation coefficients of methanol and ethanol calibration curves were 0.9983 and 0.9970 respectively. Synthetic samples were prepared by spiking known concentrations of methanol in vodka purchased from market and quantified against the drawn calibration curve. This shows a good agreement with the actual concentration and those obtained using other methods.
Actual and predicted values of methanol% in prediction set
| Actual values | Predicted values |
| 0.3 | 0.28 |
| 0.6 | 0.57 |
| 0.8 | 0.84 |
- Spectrometric determination has been done using chromotropic acid. A calibration curve was drawn in the range of 0.2 – 1.0% of methanol in ethanol with a correlation coefficient of 0.9990. The same vodka sample was spiked with 0.5% of methanol and quantified against the drawn calibration curve shows 96% recovery.
Conclusion
With the developed calibration curve a number of samples are used for prediction of methanol concentration. ATR predicted results are in good agreement with the actual concentration and those obtained using other analytical methods. Hence this could be used in routine analysis for determination of methanol as an adulterant in alcoholic beverages.