Enwave Optronics

Low concentration natural methanol exists in most alcoholic beverages and usually causes no immediate health threat.  Nevertheless, it is possible to have natural occurring methanol in beverages with concentration as high as 18 g/L of ethanol; or equivalent to 0.72% methanol in 40% ethanol, in alcohol (1). Current EU regulation limits naturally occurring methanol to below 10 g/L of ethanol; or equivalent to 0.4% methanol in 40% ethanol. 

Raman spectroscopy has been shown to be an effective tool in compositions analysis as well as adulteration identifications in foods (2). In the alcoholic beverage industry, the standard composition analysis method were more expensive and time-consuming gas chromatography (3). Here, we present a Raman spectroscopy method for a quick and lower cost alternative to verify the existence of low concentration methanol in alcohol.  

40% ethanol/water solution was prepared using 200-proof ethanol and distilled water. HPLC grade methanol was added into the 40% ethanol solution to make samples with methanol concentration ranging from 50 ppm to 2.5%. An Enwave Optronics' ProRaman instrument with laser excitation at 785 nm was used for the measurements. The sample solutions were measured in quartz cuvette in a sample holder. Figure 1 depicts the results of the measured spectra in the fingerprint region of the Raman spectra. 

Figure 1: The fingerprint range spectra of the various solutions.			

Partial least square (PLS) regression method was used for calibration and prediction model for methanol. The spectral region from 950–1200 cm

  was chosen for developing the calibration model. The actual vs. predicted concentration value of methanol is shown in Figure 2. It is shown that the measured data and PLS prediction match very well with correlation coefficient 

Figure 2: Actual and predicted methanol concentrations using PLS regression model.			

An affordable, high sensitivity ProRaman instrument was used with PLS method to successfully analyze low concentration methanol in 40% alcohol. Based on our findings, the detection limit for methanol in 40% of ethanol is much better than 50 ppm and reliable quantitative determination using PLS prediction could reach 50 ppm of methanol in 40% alcohol. 

(1) F. Bindler, E. Voges, and P. Laugel, 

(3) L.M. Reid, C.P. O'Donnell, and G. Downey, 

				18200 McDurmott St., Suite B, Irvine, CA 92614

				tel. (949) 955-0258, fax (949) 955-0259 

Articles

Low concentration natural methanol exists in most alcoholic beverages and usually causes no immediate health threat.

Determination of Low Concentration Methanol in Alcohol by an Affordable High Sensitivity Raman Instrument

High sensitivity Raman analyzers could provide solutions for continuous and automated measurements of low concentration ions or molecules in aqueous solutions for applications in industries including biomedical, mining, agrichemical, pharmaceutical, cleaning, and environmental. As an example for similar applications, the quantitative analysis of low concentration sulfate ion in aqueous solution down to as low as 2.5 ppm by conventional (Non-SERS) Raman is demonstrated. 

Solutions of sulfate with concentration below 100 ppm were prepared by dissolving sodium sulfate in distilled water. One ppm is defined as one milligram of SO

 in one liter of water. An Enwave ProRaman instrument with a wavelength-stabilized 785 nm laser, a high throughout TEC-controlled CCD spectrograph, and a fiber optics probe was used for the measurements. All solutions were kept in quartz cuvette during the measurements.  

  for sulfate aqueous solutions with concentrations of 2.5, 5, 10, 25, 50, and 100 ppm. The spectra are backgrounds corrected. Notably the water O-H bending mode at around 1640 cm

 peak in these solutions. The close-up view of the sulfate 

 peak with increasing concentration from 2.5 to 100 ppm is shown in the inset. The second inset shows the areas of the sulfate 

 peak scaled linearly with the concentration. The fitting parameter R

 ~0.99986 indicates a very good correspondence in the instrument readings and the concentrations of the prepared solutions.    

Quantitative determinations of sulfate ion concentrations down to as low as 2.5 ppm in aqueous solutions were conducted by using a high-sensitivity Raman spectrometer. Aqueous solutions of ions like nitrate, phosphate, carbonate, sulfite, nitrite, silicate, bicarbonate, and ammonium, etc, could also be analyzed to similar or lower concentrations. Quantitative analyses of these ionic species along with other molecular species to similar concentration level are important in many industrial applications. 

The developments of SERS techniques in recent years gave hopes for measurements of low concentration aqueous samples with Raman spectrometers. Nevertheless, difficulties exist in the requirements of robust, continuous, and automated measurements in various industrial settings. This application note shows the possibility of analyzing low concentration ionic species in water with a state of the art conventional dispersive Raman instrument. For even lower concentration measurements, different configurations of Raman instrument, e.g. using shorter wavelengths excitation lasers, could also be devised to meet the sensitivity requirements.     

18200 McDurmott St., Suite B, Irvine, CA 92614

High sensitivity Raman analyzers could provide solutions for continuous and automated measurements of low concentration ions or molecules in aqueous solutions for applications in industries including biomedical, mining, agrichemical, pharmaceutical, cleaning, and environmental. As an example for similar applications, the quantitative analysis of low concentration sulfate ion in aqueous solution down to as low as 2.5 ppm by conventional (Non-SERS) Raman is demonstrated.

Enwave Optronics

Articles

Determination of Low Concentration Methanol in Alcohol by an Affordable High Sensitivity Raman Instrument

Raman Analysis of Low Concentration Ions in Water