As previously mentioned, vodkas are produced from various raw materials of agricultural origin such as grains and potatoes. Due to diversity of raw materials, the final products are also highly diversified. At present, numerous brands of vodka are offered on the market, including pure and flavoured vodkas produced from one or more raw materials. The types of vodka production also differ, which influences the final composition of the product. Due to the ever increasing number of vodka products and the client’s interest in new products, it is necessary to precisely determine their composition.
Low concentrations of the compounds present in vodkas pose a big challenge for chemical analysts. The majority of studies are conducted by means of one-dimensional gas chromatography because this technique has many advantages such as high resolution and high sensitivity. This allows the identification of a large number of analytes. Moreover, the possibility of coupling GC with different detectors makes this technique applicable to a wide spectrum of alcohol-based products. Flame ionization detector (FID) is most commonly used because of its relatively low price and universal application. A GC-FID system was used, among others, to determine methanol content in commercial and illegally produced vodkas. The obtained results differed depending on the vodka type, and ranged from 17 to 376 mg/l (Chłobowska et al. 2000). The admissible concentration of methanol in pure vodka is 100 mg/l of vodka; while in case of flavoured vodkas, the admissible concentration of methanol is 2 g/l of vodka. All the investigated samples were within these limits. A GC-FID system was also applied to analyse the volatile fraction of vodkas originating from Brazil (Pereira et al. 2013) and Vietnam (Lachenmeier et al. 2009). In the case of Brazilian vodkas, 32 brands were analysed with regard to the content of higher alcohols, acetaldehyde, ethyl acetate and methanol. Both methanol and acetaldehyde were present in these vodkas at the concentrations below the limit of quantification. For most samples, the content of higher alcohols and ethyl acetate did not meet the EU standards although the total content of contaminants was definitely lower than the values prescribed by the Brazilian regulations (Pereira et al. 2013). Lachenmeier et al. (2009) analysed 11 samples of alcoholic beverages available in local stores in Hanoi, which included three vodkas, one whiskey, one brandy, one rum and others. The collected samples were analysed with regard to the content of ethanol, methanol, acetaldehyde, 1-propanol, 1-butanol, 2-butanol, isobutanol, amyl alcohols, 1-hexanol, 2-phenylethanol, ethyl acetate, ethyl lactate and ethyl octanoate. None of the analysed vodkas contained 1-butanol, 2-butanol, 1-hexanol, 2-phenylethanol, ethyl acetate, ethyl lactate and ethyl octanoate (Lachenmeier et al. 2009). The GC-FID technique was also used to determine diethyl phthalate in vodka, ethanol and illegal alcoholic products (Savchuk et al. 2006) as well as for assessing changes in the composition of vodka before and after filtration through activated charcoal (Siříšťová et al. 2012). In both cases, besides GC-FID analysis, the analysis by means of gas chromatography coupled with mass spectrometry (GC-MS) was performed as it gives better results compared to GC-FID analysis. A gas chromatograph coupled with a mass spectrometer is a configuration often used in the analysis of alcoholic beverages. In comparison to FID, the MS detector is more sensitive and allows easier identification of the analysed compound. As previously mentioned, the GC-MS system was used to determine selected compounds in vodkas, ethanol and illegal alcoholic products (Savchuk et al. 2006). A total of 13 samples were analysed, which included three samples purchased at the grocery stores in Stavropol, one reference sample purchased legally in the store in the city, and nine samples bought from individual home owners by the agents from the Kryzyl distillery. All samples were analysed with regard to the content of ethanol, ethyl acetate, methanol, 2-propanol, n-propanol, n-butanol, ethylene glycol and diethyl phthalate. The composition of all analysed samples differed from the composition of reference sample (Savchuk et al. 2006). The content of diethyl phthalate was also the object of investigation in the article on the risk of consuming this compound via intake of various alcoholic beverages, among others, vodkas. Phthalates are highly durable esters of phthalic acid commonly utilized in the chemical industry. They are used as plasticizers in many products such as furniture, car air fresheners, medical devices, toys for children or food packages. Phthalates are not chemically bound to plastic materials, which means they can migrate into the environment. Thus, people are exposed to phthalates via swallowing, inhalation or skin contact. Diethyl phthalate is applied as ethyl alcohol denaturing agent. Acute toxicity of phthalates is LD50 1–30 g/kg. Moreover, chronic toxicity is observed, too. Aforementioned diethyl phthalate is also considered a potential carcinogenic and teratogenic agent. Due to this fact, it is of utmost importance to test spirit-based beverages for diethyl phthalate presence, especially in case of the products sold in Eastern Europe where alcohol is frequently denatured with phthalate in many production processes. Leitz et al. (2009) conducted the study by means of GC-MS, while the sample preparation involved the use of liquid-liquid extraction (LLE). Vodka was used as a blind sample because it did not contain diethyl phthalate (Leitz et al. 2009). Due to the presence of sulphur compounds (including dimethyl sulphide, DMS) in some spirit-based beverages, Cardoso et al. (2004) analysed selected products for the presence of DMS; alcohols popular in Brazil such as cachaça, whiskey, rum, brandy, grappa, tiquira, tequila and vodka were among the investigated samples. The application of GC-MS was described however this technique did not detect DMS in the samples of vodka, tequila and rum (Cardoso et al. 2004). GC-MS was used to determine fatty acids and esters in some alcoholic beverages and tobacco. Vodka was also among the analysed alcohols. Samples were pretreated by solid-phase microextraction (SPME). This allowed for detecting the compounds present at low concentrations such as, ethyl dodecanoate, ethyl tetradecanoate, ethyl hexadecanoate, ethyl hexadecenoate, ethyl oleate, ethyl stearate and ethyl linoleate (Ng 2002).
Siříšťová et al. (2012) described changes in the vodka composition after filtering through activated charcoal; the GC-MS system was used to create a list of volatile organic compounds that had been detected in the analysed vodka samples. As in the previous case, the head space (HS)-SPME technique was applied to preconcentrate the analytes. A total of 29 compounds were detected, including acetaldehyde, limonene, dodecane, hexyl acetate and 2-methylfuran, which were identified based on their retention times and mass spectra (Siříšťová et al. 2012). Studies on the presence of ethyl carbamate (EC) in spirit-based beverages are frequently conducted. Ethyl carbamate occurs naturally in fermented foods and alcoholic beverages such as, bread, yoghurt, soy sauce, wine, beer and particularly in spirits made from stone fruits and the stone fruit pomace obtained from cherries, prunes, mirabelle plums and apricots. The conducted animal studies proved that ethyl carbamate is a carcinogen. The International Agency for Research on Cancer has classified this compound as probably carcinogenic to humans (Balcerek and Szopa 2006; Commission recommendation 133/2010).
The content of EC in Brazilian vodkas (Pereira et al. 2013) and various spirit-based beverages, including vodkas purchased in Ontario (Clegg et al. 1988), was determined by using GC-MS. The gas chromatography coupled with tandem mass spectrometry (GC-MS/MS) was used to detect EC in Vietnamese vodkas (Nordon et al. 2005). In all these studies, ethyl carbamate has not been detected.
Besides the above-mentioned detectors, the electron capture detector (ECD) and flame photometric detector (FPD) have been used for analysing vodkas. The ECD is a nondestructive detector which allows the determination of the concentrations of halogenated compounds at ppb-ppt level. The articles describing the development and application of the method for determining carbonyl compounds in alcoholic beverages can serve as an example here. In both studies, the samples were subjected to derivatization with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA) in order to separate the investigated compounds (Wardencki et al. 2003; Sowiński et al. 2005). Wardencki et al. (2003) employed the HS-SPME technique for this purpose, while Sowiński et al. (2005) compared the results obtained by headspace injection with those obtained by SPME. In both studies, the analysis included methanal, ethanal, propanal, propenal, butanal, isopentanol, 2-butenal, pentanal and hexanal. Additionally, dimethyl ketone was determined by Wardencki et al. (2003) and isobutanal by Sowiński et al. (2005). Most carbonyl compounds have a negative impact on the aroma and taste of spirit-based beverages. Some of them, for instance propenal (acrylaldehyde), are highly carcinogenic substances and irritating to the eyes and respiratory tract. That is why it is important to conduct research aimed at their control. Both techniques described in the aforementioned papers proved to be effective in the analysis of carbonyl compounds. HS-GC-ECD analysis revealed higher concentration of some investigated compounds compared to SPME-GC-ECD. This technique allows faster analysis than in the case of using SPME, thanks to the exclusion of preliminary preparation of the samples via solid-phase microextraction technique. With this method, the HS-GC-ECD technique occurred to be better compared to SPME-GC-ECD for most of the investigated carbonyl compounds.
The FPD registers the intensity of light emitted by analyte particles returning to the ground state after excitation in the hydrogen flame. This detector is mainly used to determine the concentrations of compounds that contain sulphur (spectral line at 393 nm) and phosphorus (spectral line at 526 nm). The FPD was used by Leppänen et al. (1979) to determine volatile sulphur compounds present in alcoholic beverages at low concentrations. Even small amounts of sulphur compounds can have a negative effect on the quality of consumed alcoholic beverages. The samples of wine, beer, cognac, brandy, whiskey, rum and vodka were analysed. The vodka brands originating from Finland, Russia and Poland were among the analysed samples. The analysed substances included dimethyl sulphide, diethyl sulphide, dimethyl disulphide and dimethyl trisulfide. The application of FPD allowed the detection of only dimethyl disulphide in vodkas originating from Poland and Russia (Leppänen et al. 1979). Dimethyl sulphide was present in vodkas at very low concentration so its influence on the aroma and taste of the vodkas was insignificant.
Besides one-dimensional gas chromatography, it is also possible to employ two-dimensional chromatography (GC × GC) (Fig. 1) for analysing spirit-based beverages. Despite its many advantages (e.g. improved resolution, better sensitivity and structured chromatograms), two-dimensional chromatography is not used often. This is due to the fact that these techniques require qualified personnel and expensive equipment, the latter definitely more expensive than a one-dimensional chromatograph. In the case of GC × GC, time-of-flight mass spectrometer (TOFMS) is the most frequently used detector. This technique was employed for analysing the volatile organic compounds in selected spirit-based beverages such as, cachaça, rum, vodka, whiskey, tequila, gin and some liqueurs (melon, banana, strawberry and Tia Maria) (Cardeal and Marriott 2009). The lowest number of compounds was detected in vodkas which demonstrate their poor aroma profile compared to other analysed alcoholic beverages. Among the detected groups of compounds were alcohols, aldehydes, ketones, esters, terpenes and aromatic compounds.
Although the vodka composition is mainly analysed by means of gas chromatography, there are studies in which spectrophotometry, atomic absorption and high-performance liquid chromatography (HPLC) have been applied. The aforementioned techniques are used to determine specific compounds which cannot be determined or are difficult to determine by GC.
In the case of spirit-based beverages, HPLC is used rather rarely. This is due to the composition of such beverages which contain many volatile organic compounds therefore their analysis is easier by using gas chromatography. Coumarin is one of the vodka components which is analysed by means of HPLC. It belongs to lactones and can be found in some plants. Coumarin is present, among others, in Polish vodka Żubrówka which is made from rye and flavoured with the grass species Hierochloe odorata growing in Białowieża Forest in Poland. The HPLC analysis of Żubrówka showed that coumarin concentration was at the level admissible by norms, i.e., below 10 mg/kg (Sproll et al. 2008).
In the case of flavoured vodkas, studies aimed at detection of calcium and citrate was also conducted by means of UV–VIS spectrophotometry and artificial neural networks (ANN). The aim of this research was the evaluation of aforementioned techniques in comparison to NMR technique (McCleskey et al. 2003).
Near infrared (NIR) spectroscopy and Raman spectroscopy were used to determine the ethanol content in vodkas (Nordon et al. 2005). NIR spectroscopy is a nondestructive technique characterized by fast and precise measurements, low costs and the possibility of concurrent determination of multiple components. The technique uses radiation in the range of 750–2500 nm (Chodak 2005). In Raman spectroscopy, the mechanism of operation is based on the scattering of radiation by a sample. Both these techniques allowed the ethanol content determination with only a slight deviation from the true value (Nordon et al. 2005). The aforementioned techniques possess some important advantages as compared to the standard techniques utilized for determination of alcohol content. These are non-invasive techniques that can be applied to the already bottled alcohols without the need to open them. Analysis takes a short time, which makes it suitable for online techniques. NIR and Raman spectroscopies can be employed to determine falsification without destroying the sample. Unfortunately, it is extremely important to verify additional parameters such as bottle glass thickness or the bottle movement on the production belt. These elements limit the applicability of the above techniques on the production lines. Mid infrared (MIR) spectroscopy in attenuated total reflectance (ATR) mode was utilized to analyse ethanol, sugar and tartaric acid content in selected alcohol-based beverages including vodka. This technique was employed as an alternative to chemical analyses. The results were comparable with the ones obtained using the classical chemical analyses. ATR method is fast, precise and easy to operate, which can contribute to its broader implementation for analysis of alcohol-based beverages in future (Nagarajan et al. 2006). Spirit-based beverages, e.g. vodkas were analysed by means of atomic emission spectroscopy and atomic absorption spectroscopy. Atomic absorption spectroscopy (AAS) is characterized by high selectivity, the detection limit at ppb level, and a possibility to analyse ca. 70 elements. Because of that, AAS was used to determine the content of lead and copper in Brazilian vodkas for which the measurements were below the detection limit (Pereira et al. 2013). Atomic emission spectroscopy allows for the concurrent detection or determination of many elements even when they are present in infinitesimal amounts. Both techniques were employed to determine selected metal ions, e.g. sodium, magnesium, aluminium, iron and calcium in spirit-based beverages including vodkas (Nascimento et al. 1999). Unfortunately, the detailed results have been published for cachaça only.
Spectrofluorometry was used to determine formaldehyde in vodka samples (De Andrade et al. 1996; Tsuchiya et al. 1994). This technique is characterized by high sensitivity and good selectivity. Formaldehyde is an irritating and carcinogenic substance so investigation of its content in spirit-based beverages is very important. Spectrofluorometry is the technique suitable for determination of substances, which upon light absorption, emits the radiation of different wavelengths. In the case of aldehydes, it is necessary to conduct derivatization into the radiation-emitting products. Due to this fact, spectrofluorometry is not a common approach to determine other aldehydes. The measured concentrations of formaldehyde in Russian (De Andrade et al. 1996) and Japanese (Tsuchiya et al. 1994) vodkas were 0.33–0.65 mg/l and 18.4 nmol/ml, respectively. Formaldehyde was determined in alcohol-based beverages including two vodka samples using flow injection analysis. A method based on the reaction between Fluoral-P and formaldehyde was used, which yields DDl compound that reveals fluorescence at λex = 410 nm and λem = 510 nm. Formaldehyde was not detected in one sample, while in the other one, it was at the lowest level with respect to the other investigated alcohols (De Oliveira et al. 2007).
Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine metals in vodkas (Lachenmeier et al. 2009). ICP-MS is a very sensitive technique with high precision, which can be employed to make concurrent determinations of multiple elements and selective determinations of specific isotopes of the same element in complex matrices. It also has low detection limit (at the level of pg/L in solutions) due to highly efficient plasma ionization, and a wide linear range of calibration curves, which allows for determining trace and macro elements by a single measurement (Szpunar and Łobiński 1999; Vanhaecke and Moens 1999). ICP-MS enabled detection of alkaline earth metals, e.g. sodium, potassium, calcium and magnesium at the level of milligrams per liter in vodkas originating from Vietnam (Lachenmeier et al. 2009). This technique supplemented with photochemical vapour generation (PVG) was also utilized for determination of cobalt, nickel and tellurium in three cachaça samples, one vodka sample and one sweet vermouth sample. It occurred to be superior to traditional ICP-MS due to lower limit of detection. The highest content of tellurium was detected in vodka, whereas nickel and cobalt content values are higher than the case of two out of three cachaça samples and lower than the case of vermouth and the third cachaça sample analysis (De Quadros and Borges 2014).
Moreover, studies aimed at assessing the influence of water hardness on the transparency of vodka were also conducted. The samples of tap water, artesian well water and commercial bottled water were analysed. The hardness of water was determined by titration with Na2H2EDTA. Based on the study results, it can be concluded that the transparency of vodka depends, to a large degree, on the type of water used in vodka production (Krosnijs and Kuka 2003).