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Ethanol As Internal Standard for Determination of Volatile Compounds in Spirit Drinks by Gas Chromatography

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Ethanol As Internal Standard for Determination of Volatile Compounds in Spirit Drinks by Gas Chromatography Ethanol as Internal Standard for Quantitative Determination of Volatile Compounds in Spirit Drinks by Gas Chromatography Siarhei V. Charapitsa, Anton N. Kavalenka, Nikita V. Kulevich, Nicolai M. Makoed, Arkadzi L. Mazanik, Svetlana N. Sytova Research Institute for Nuclear Problems of Belarusian State University, POB 220089, Gruschevskaya Str., 124, Minsk, Belarus, Tel/Fax: 375-172-26-2517, [email protected] Abstract The new methodical approach of using ethanol as internal standard in gas chromatographic analysis of volatile compounds in spirit drinks in daily practice of testing laboratories is proposed. This method provides determination of volatile compounds concentrations in spirit drinks directly expressed in milligrams per liter (mg/L) of absolute alcohol according to official methods without measuring of alcohol strength of analyzed sample. The experimental demonstration of this method for determination of volatile compounds in spirit drinks by gas chromatography is described. Its validation was carried out by comparison with experimental results obtained by internal standard method and external standard method. Keywords: spirit drinks; ethanol; internal standard; volatile compounds; gas chromatography Introduction According to the official methods (1–8) the analytical laboratories should determine the following volatile compounds in spirit drinks: acetaldehyde, methyl acetate, ethyl acetate, methanol, 2-propanol, 1- propanol, isobutyl alcohol, n-butanol, isoamyl alcohol. Concentrations of these compounds are expressed in milligrams per liter (mg/L) of absolute alcohol (AA). For calculation of concentrations the internal standard (IS) method is used (1–3, 9–11). Usually pental-3-ol is used as IS. Some researchers (4–8, 12) make calculations by means of the external standard (ES) method to avoid the introduction of another source of error, such as the addition of an internal standard. To get quantitative values of impurities concentration per liter of absolute alcohol it is also required to measure alcohol strength of analyzed sample (1–8). Early (13, 14) an idea to use the main component (solvent) for determination of impurities concentration was proposed. It is possible at the present time to introduce this approach for routine practice of analytical laboratories due to modern GC with wide range of signal registration from flame ionization detector (FID). The linear range of modern FID is generally more than 107. Signal registration from impurities compounds and from main component ethanol takes place without any distortions. In (15, 1 16) demonstration of possibility of using ethanol as internal standard method (“ethanol as ISTD”) for quantitative analysis of spirit drinks by gas chromatography was realized on results obtained in the Laboratory of Analytical Research of the Institute for Nuclear Problems (INP) of Belarusian State University. In this paper our aim is the further verification and validation of the method “ethanol as ISTD” by comparison of experimental results of all methods cited above. Results presented were obtained in INP and in the Control Laboratory of Bobruisk Hydrolysis Plant from Belarus (BHP). Theoretical background The main difference of the proposed method “ethanol as ISTD” from classical method of IS in this case is the following. In the classical case calibration of chromatograph includes the measuring of relative detector response factors for every analyzed compound relative to IS. Numeric values of these factors RFi are calculated from chromatographic data for standard solutions with known concentrations of analyzed compounds and may be expressed by the following equation: st st st st AIS CIS (/) mg L AIS ⋅ Ci (/) mg L RFi = st / st = st st , [1] Ai Ci (/) mg L Ai ⋅ CIS (/) mg L st st st st where Ai and AIS are peak areas of i-th compounds and IS respectively; Ci (/) mg L and CIS (/) mg L are concentrations of i-th compounds and IS respectively expressed in mg per 1 liter of solution. Concentration of i-th sample compound relative to absolute alcohol Ci [mg/L] is expressed by the following formula (1–3): Ai 100 Ci= RF i × × CIS (/) mg L × , [2] AIS Strength where Ai and AIS are the peak areas for i-th compound and IS respectively, CIS (/) mg L is concentration of IS, Strength is concentration of alcohol in solution expressed in % volume. In the case of “ethanol as ISTD” the formulas [1] and [2] looks as follows: st st st st AIS CEt ( mg / L ( AA )) AIS ⋅ Ci ( mg / L ( AA )) RFi ( Et__)/ as IS = st st = st , [3] Ai Ci ( mg / L ( AA )) Ai ⋅ ρEt st where Ci ( mg / L ( AA )) is concentration of i-th compounds expressed in mg per 1 liter of absolute alcohol, ρEt =789300 mg/L is the density of ethanol. Concentration of i-th sample compound relative to absolute alcohol Ci [mg/L(AA)] is expressed by the following formula: 2 Ai Ci= RF i (__) Et as IS × × ρEt . [4] AEt According to [4] we obtain value of i-th sample compound concentration directly expressed in mg per 1 liter of absolute alcohol directly without any additional measurement of strength and without of any procedure of IS adding in an analyzed sample. Standard and sample preparations All individual standard compounds were purchased from Sigma-Fluka-Aldrich (Berlin, Germany). The standard solutions for graduation and sample solutions were prepared by adding of individual standard compounds in ethanol-water mixture (96:4) by weight. Ethanol of high grade quality was purchased from Minsk-Kristall Winery and Distillery Plant (Minsk, Belarus). The state standard ethanol- water (96:4) solutions GSO-8404 (“ethanol solution”) and state standard ethanol-water (40:60) GSO-8405 (“vodka solution”) solutions were purchased from VNIIPBT (Moscow, Russia). Fig.1. Typical chromatogram of the standard ethanol-water (40:60) solution. 1 – acetaldehyde, 2 – methyl acetate, 3 – ethyl acetate, 4 – methanol, 5 – 2-propanol, 6 – ethanol, 7 – 1-propanol, 8 – isobutyl alcohol, 9 – n-butanol, 10 – isoamyl alcohol, 11 – 1-pentanol (internal standard). Gas Chromatographic conditions Analyses were carried out in laboratories of INP and BHP on the gas chromatographs Crystal5000 (JSC SDB Chromatec, Yoshkar-Ola, Russia) equipped with FID, a split/splitless injector, liquid autosampler, Unichrom software (New Analytical Systems Ltd., Minsk, Belarus), capillary column Rt- Wax, 60 m x 0.53 mm, phase thickness 1 μm (Restek, Bellefonte, PA, USA). The oven temperature was: initial isotherm at 75 °C (9 min), raised to 155 °C at rate 7 °C/min with final isotherm of 155 °C (2.6 min). 3 Carrier gas was nitrogen. Gas flow was 2.44 mL/min; injector temperature 160 °C; detector temperature 200 °C; injector volume 0.5 μL and split ratio 1:20. This high split ratio was chosen to achieve good separation between peaks of 2-propanol and ethanol. Once the gas chromatographic conditions had been optimized the satisfactory separation under these conditions has been achieved. Typical chromatogram of the used standard solutions is presented in Fig. 1–2. In Fig. 1 in order to show the dominant compound ethanol and another minor compounds synchronously the logarithm scale of response signal is chosen. Fig.2. The same chromatogram as in Fig. 1, but linear scale of response signal is chosen. Results and discussion To compare proposed methodical approach “ethanol as ISTD” with classical commonly used method of internal standard experimental researches were planned and executed in laboratories of INP and BHP. Seven standard ethanol-water (96:4) solutions VC and VCC were prepared by weight. Concentrations of analyzed volatile compounds in the prepared solutions are presented in Table 1. The standard solutions of volatile compounds VC-1, VC-2 and VC-3 were used to generate calibration curves. The standard solutions VCC-0, VCC-1, VCC-2, VCC-3 and certified reference materials GSO-8404 and GSO-8405 were used to control trueness (17) of the proposed methodical approach. Every standard solution was injected two times. 4 Table 1. Concentrations of analyzed volatile compounds are expressed in mg/L (AA), 1-pentanol was introduces as internal standard Concentration, mg/L (AA) Relative Compound error, % VC-1 VC-2 VC-3 VCC-0 VCC-1 VCC-2 VCC-3 (Р=0,95) acetaldehyde 111 11,2 1,13 4275 1096 56,2 2,22 ± 3 % methyl acetate 114 11,5 1,17 4397 1128 57,8 2,29 ± 3 % ethyl acetate 108 10,9 1,11 4173 1070 54,9 2,17 ± 3 % methanol 1092 113,3 14,3 41995 10774 555,5 24,96 ± 3 % 2-propanol 105 12,1 2,70 3991 1025 54,1 3,69 ± 3 % 1-propanol 104 10,5 1,06 4012 1029 52,8 2,08 ± 3 % isobutyl alcohol 103 10,4 1,05 3975 1020 52,3 2,06 ± 3 % n-butanol 106 10,7 1,08 4071 1044 53,5 2,11 ± 3 % isoamyl alcohol 106 10,7 1,08 4071 1044 53,5 2,11 ± 3 % 1-pentanol (IS) 27,13 27,13 27,13 27,13 27,13 27,13 27,13 ± 3 % In all cases calibration coefficients were been generated by the considered three methods. At the first case 1-pentanol was used as IS in accordance with (1–3). At the second case the ES method was used in accordance with (4–8). And in the third case the ethanol as IS was used. Analytical characteristics of the obtained calibration coefficients are presented in Table 2 and Table 3. Table 2. Analytical characteristics of the calibration coefficients from INP 1-pentanol as IS ES Ethanol as IS Slope Correlation Slope Correlation LOD* Compound Correlation coefficient, (mg/L)/ coefficient, Slope (mg/L) coefficient R2 R2 (pA*min) R2 acetaldehyde 2,396 0,9997 266,1 0,9997 1,710 0,9997 0,344 methyl acetate 2,491 0,9997 276,7 0,9996 1,779 0,9999 0,683 ethyl acetate 1,757 0,9997 195,1 0,9997 1,254 0,9999 0,322 methanol 2,133 0,9998 236,9 0,9997 1,523 0,9999 0,046 2-propanol 1,400 0,9998 155,5 0,9997 0,999 0,9999 0,119 ethanol 1,413 N/A 155,5 N/A 1 N/A N/A 1-propanol 1,179 0,9997 130,9 0,9996 0,841 0,9999 0,222 isobutyl 1,018 0,9998 113,0 0,9997 0,727 0,9999 alcohol 0,178 n-butanol 1,117 0,9999 124,1 0,9998 0,798 0,9999 0,189 isoamyl alcohol 1,030 0,9999 114,4 0,9998 0,735 0,9999 0,179 1-pentanol 1 N/A 110,1 N/A 0,708 N/A 0,271 * limit of detection (LOD) 5 Table 3.
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