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C. Chambers-Bédard and B.M. Ross, Eur. J. Mass Spectrom. 22, 159–164 (2016) Received: 20 July 2016 n Revised: 9 September 2016 n Accepted: 10 September 2016 n Publication: 14 October 2016 159 EUROPEAN JOURNAL OF MASS SPECTROMETRY Quantification of methanol in the presence of ethanol by selected ion flow tube mass spectrometry Catherine Chambers-Bédarda and Brian M. Rossa,b,* a Department of Biology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B5E1 b Northern Ontario School of Medicine, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B5E1 *Professor Brian M. Ross, Division of Medical Sciences, Northern Ontario School of Medicine, 955 Oliver Road, Thunder Bay, ON P7B5E1 The quantification of trace compounds in alcoholic beverages is a useful means to both investigate the chemical basis of beverage flavor and to facilitate quality control during the production process. One compound of interest is methanol which, due to it being toxic, must not exceed regulatory limits. The analysis of headspace gases is a desirable means to do this since it does not require direct sampling of the liquid material. One established means to conduct headspace analysis is selected ion flow tube mass spectrometry (SIFT-MS). The high concentration of ethanol present in the headspace of alcoholic drinks complicates the analysis, however, via reacting with the precursor ions central to this technique. We therefore investigated whether methanol could be quantified in the presence of a large excess of ethanol using SIFT-MS. We found that methanol reacted with ionized ethanol to generate product ions that could be used to quantify methanol concentrations and used this technique to quantify methanol in beverages containing different quantities of ethanol. We conclude that SIFT-MS can be used to quantify trace compounds in alcoholic beverages by determining the relevant reaction chemistry. Keywords: methanol, ethanol, SIFT-MS, flavorant analysis, headspace Introduction Rapid and inexpensive techniques for the analysis of trace compounds in foodstuffs are of importance for the production, safety, and quality control of food and drink. For example, in alcoholic beverages, significant trace compounds can range from toxic contaminants such as ochratoxin A1 and other mycotoxins 2 to indicators of bacterial contamination such as acetic acid,3 and flavorant compounds such as acetaldehyde, ethyl caproate and 2-phenylethyl acetate.4,5 Furthermore, the identification of particular malting barley varieties,6 detection of biogenic amines7 and of volatile sulfur compounds,8 can play an important role in the quality control of beer and wine during the fermentation process. In addition, the alcohol methanol is naturally produced in small quantities during the production of alcoholic beverages, and ISSN: 1469-0667 doi: 10.1255/ejms.1438 is toxic for humans. 9 Ingestion of methanol can result in nausea, vomiting, blurred vision, blindness and death, with these effects being produced by the buildup of toxic methanol metabolites, particularly formic acid.10 Due to its toxicity, the methanol content of beverages is strictly regulated, with a general European Union limit of 5 g methanol per litre ethanol of agricultural origin.11 In addition to the naturally occurring methanol in alcoholic beverages, methanol has also been found to be added to imitation spirits and wines due to its low cost in comparison to ethanol, with this practice linked to the death or blindness of consumers. For example, a recent case of counterfeit beverages containing methanol in the Czech Republic resulted the deaths of 50 persons and the permanent injury of many more.12 © IM Publications LLP 2016 All rights reserved 160 One possible method for the detection of trace compounds such as methanol is gas chromatography-mass spectrometry (GS-MS). Unfortunately, there are some difficulties associated with analysis using this technique. These include that real time analysis cannot be performed,13 and that GC-MS often requires sample preparation which introduces the risk of artefacts,14 and is generally sufficiently technically demanding to perform that expert operators are required, reducing the uptake of the technique for routine use. In response, a number of alternative techniques have been developed, such as matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF)15 and atmospheric pressure ionization (API).16 Since many trace compounds of interest are also volatile, however, the analysis of headspace gases provides an analytical opportunity which does not involve the direct sampling of the beverage or food, with a technique called selected ion flow tube mass spectrometry (SIFT-MS) being one of the leading means to do so. SIFT-MS is a soft ionization technique that allows for the analysis and quantification of trace gases in a sample of air.17 The ionization occurs using three precursor ions, H3O+, NO+ and O2+, which are generated in a microwave discharge, selected by a quadrupole mass filter, and subsequently injected into the helium carrier gas.18 The trace compounds in a sample of gas are introduced to the carrier gas downstream from the precursor ion injection at a known rate, where they react with the precursor ions.19 The product ions resulting from these reactions allow for the identification of the compounds present in the gas sample, and the count rates allow for quantification.18 The advantages of SIFT-MS include that it allows for the real-time analysis of gas samples,19 and that it permits the detection of compounds present even at the single digit ppb level without any form of pre-concentration.18 Previous studies have demonstrated the effectiveness of the SIFT-MS instrument for the analysis of flavorant compounds in Ocimum basilicum and the pasta sauce “Pesto all Genovese”,20 as well as dry fermented sausages21 and the identification of volatile compounds released from Atlantic cod fillets.22 In addition to studies analyzing the volatile flavorant compounds in foodstuffs, SIFT-MS has also been used for quality control purposes, such as for the detection of oxidation in olive oil.13 These previous experiments indicate that there is significant potential for the use of SIFT-MS for the analysis of flavorant and contaminant compounds in alcoholic beverages. With respect to the analysis of alcoholic beverages, however, SIFT-MS is not ideal since the large concentration of ethanol in the headspace reacts with the precursor ions to the extent that they are mostly or wholly consumed by that reaction leaving none to react with other lower concentration gases such as methanol.23 Previous work using SIFT-MS23 and proton transfer reaction (PTR)-MS,24 however, has indicated that the product ions produced by the reaction between ethanol and H3O+ can themselves be used as precursor ions in the analysis of other trace gases. In this study we have investigated whether such an analysis can be used to quantify headspace methanol in the presence of methanol. Quantification of Methanol in the Presence of Ethanol by SIFT-MS Methods Materials Methanol was obtained from Fisher Chemical (Whitby, Canada), and had a purity of 99.9%. Food-grade ethanol was obtained from the Liquor Control Board of Ontario (LCBO) in Thunder Bay, Canada and was 94% pure. Alcoholic beverages, two samples of beer, two of wine and two of spirits, were purchased from an LCBO outlet in Thunder Bay, Ontario. SIFT-MS analysis SIFT-MS analysis was completed using a Profile 3 SIFT mass spectrometer (Instrument Science, Crewe, UK) as previously described.20 The flow tube pressure was maintained at 1 Torr, and the temperature at 300 K, with an inlet temperature of 150°C. Before each use the instrument was allowed to stabilise for between 60 min and 90 min. Precursor ions were also evaluated before each use and determined to have a purity of at least 99%, and ion counts of 500,000 counts per second or greater for H3O+ (including hydrates). Headspace analysis The openings of the 250 mL flasks containing the samples were covered in aluminum foil, and a needle was inserted through the foil into the flask to sample the headspace. The needle was connected to the instrument by a section of 1/16² (OD) Teflon tubing and headspace gases sampled by negative pressure using a flow rate of approximately 0.4 Torr L s–1. While headspace analysis was performed, samples were stirred at a speed of 200 rpm using a magnetic stirrer to aid re-equilibration of the headspace during sampling. Mass spectra Mass spectra were recorded over the range of m/z 10 to m/z 200 using the full scan mode, with each scan occurring over a time period of 200 s. Count rates were corrected for ion diffusion using a mathematical procedure as described25 and for mass discrimination.26 Results and discussion Ion chemistry in a mixture of water vapor and/or ethanol vapor SIFT-MS was used to identify the reactions between the precursors H 3O +, NO +, O 2+• and ethanol vapor. Full scan spectra were generated to identify the major product ions. Protonated ethanol ions and their hydrates were identified, as well as protonated ethanol ions with ethanol clusters. Reactions with H3O+ As expected26 the major reaction products at low ethanol concentrations were the [MH]+ product ion and its hydrates C 2 H 5 OH 2 + (H 2 O) 0,1,2 . As the ethanol concentration was increased, the concentrations of H3O+ precursor ions and protonated ethanol, along with their hydrates, decreased and C. Chambers-Bédard and B.M. Ross, Eur. J. Mass Spectrom. 22, 159–164 (2016)161 Figure 1. Mass spectra of ionized products formed in the reaction between the headspace of 50% (v/v) ethanol and H3O+ or NO+ as indicated. The m/z of the major products ions are indicated. were replaced by protonated ethanol ions with ethanol clusters C2H5OH2+(C2H5OH)0,1,2,3. As indicated in Figures 1 and 2, the major ion at mid to high ethanol concentrations (>2%) was C2H5OH2+(C2H5OH)2 (m/z 139), though C2H5OH2+(C2H5OH)3 (m/z 185), C2H5OH2+C2H5OH (m/z 93). C2H5OH2+(C2H5OH) (H2O) (m/z 111) and C2H5OH2+(C2H5OH)2(H2O) (m/z 157) were also present. A previous study by Dryahina et al. characterized the major product ions produced by ethanol vapor and H3O+ precursors,23 and their results are in agreement with those in this study. Reactions with NO+ When NO+ was reacted with a low concentration of ethanol, the major reaction mechanism was hydride ion (H–) loss as has been reported previously.26–28 The hydride ion transfer resulted in the production of the C2H5O+ ion (m/z 45), and its monohydrate (m/z 63). As the ethanol concentration was increased, the major reaction mechanism became proton transfer, which produced products very similar to those resulting from reactions between ethanol and H3O+ (Figure 2) with the exception that the major product ion (m/z 157) formed in the H3O+ reaction was absent in the NO+ reaction. Proton transfer products are typically attributed to self-chemical ionization, which occurs when an M+ ion protonates a neutral molecule.29 As such, the self-chemical ionization products are not produced by reactions between NO + and ethanol, but rather by a secondary reaction between M+ and ethanol. As increasing ethanol concentrations resulted in increased protonation products, and therefore increasing similarity to the H3O+ products, the decision was made to only use H3O+ Figure 2. Count rates of precursor ions and ethanol product ions formed in the reaction between the headspace of aqueous solutions containing increasing concentrations of ethanol and H3O+. for the remainder of the study. The O2+ precursor produced similar results to NO+ (data not shown). H3O+ reactions with methanol in the presence of ethanol In the absence of ethanol, methanol reacts with H 3O + to produce the expected protonated product ion CH 3 OH 2 + (m/z 33), along with its monohydrate (m/z 51) and dihydrate (m/z 69).30 The ions are still produced with ethanol present [illustrated for 5% (v/v) ethanol in Figure 3] but are accompanied by a number of other reaction products deriving Figure 3. Mass spectra of ionized product formed in the reaction between the headspace of 5% ethanol and 0.5% methanol (v/v) and H3O+. Ions deriving from ethanol alone are indicated by an “E” and those formed when methanol is present are shown by an “M”. 162 Quantification of Methanol in the Presence of Ethanol by SIFT-MS Table 1. Product ions produced by methanol, propanol and acetic acid when reacted with H3O+ precursor ions in the presence (grey background) or absence (clear background) of ethanol. Compound Methanol Product ions (m/z) 33 51 Propanol Acetic acid 43 69 79 97 65 79 97 107 125 135 153 65 79 97 107 125 135 153 from ethanol cluster ions which become predominant at ethanol concentrations exceeding 2% (v/v) (Figure 4). These include C 2 H 5 OH 2 + (CH 3 OH)(H 2 O) 0,1,2 (m/z 79, 97 and 115), C2H5OH2+C2H5OH(CH3OH) (m/z 125), C2H5+(C2H5OH)2(CH3OH) (m/z 153) and C2H5OH2+(C2H5OH)2(CH3OH) (m/z 171). The dominant methanol ion at 5% ethanol in water was the adduct C2H5OH2+(C2H5OH)(CH3OH) (m/z 125), produced by an association reaction31 between the C2H5OH2+(C2H5OH) precursor ion (m/z 93) and methanol, CH3OH: C2H5OH2+(C2H5OH) + CH3OH ® C2H5OH2+(C2H5OH)(CH3OH) The ion C 2 H 5 OH 2 + (CH 3 OH) (m/z 79) is also formed by an association reaction, but the precursor ion is C2H5OH2+ (m/z 47). The hydration of this ion produced the monohydrate C2H5OH2+(CH3OH)(H2O) (m/z 97), as well as a dihydrate, C2H5OH2+(CH3OH)(H2O)2 (m/z 115). Similarly, the methanol product ion C2H5OH2+(C2H5OH)2(CH3OH) (m/z 171) was also the product of an association reaction, with this reaction occurring between the ethanol cluster ion (m/z 139) and methanol. The product ion C2H5+(C2H5OH)2(CH3OH) (m/z 153) is the result of a combination of association and water elimination reactions from the m/z association product. Water elimination reactions have previously been observed in saturated alcohols32 and some terpenoids.33 As ethanol concentration increased the relative concentrations of each product ion shifted towards the larger m/z 171, 153 and 125 ions, concomitant with the increased abundance of the larger ethanol cluster ions, while the overall rate of product ion formation declined suggesting the reaction rate(s) which results in these ions is lower that for that for the smaller m/z products (Figure 4). Figure 4. Count rate of product ions deriving from methanol formed in the reaction between the headspace of 0.5% methanol (v/v) and increasing concentrations of ethanol, and H3O+. 115 125 153 171 Quantification of methanol in the presence of ethanol Knowledge of the product ions produced in the methanol/ ethanol/H3O+ reactions forms the basis of a quantification method. Quantification with SIFT-MS relies on the assumption that the product ions utilized in the calculation derive only from the analyte of interest, in this case methanol. Although we have not carried out an extensive investigation using SIFT-MS of the many trace compounds in alcoholic beverages by using knowledge of the reaction chemistry described herein and by Dryahina and colleagues23 we expected that propanol and/ or acetic acid could react with H3O+ ions in the presence of ethanol to produce ions of the same m/z as some of those produced in the methanol reaction. These experiments will be described in detail elsewhere (manuscript in preparation) but preliminary data (Table 1) suggests that such an overlap in the m/z of certain product ions does occur. As a result two methanol product ions, m/z 115 and 171, were selected to use for methanol quantification that are not produced in the reaction with the other two compounds. Quantification was accomplished using a standard curve with methanol being dissolved in a solution containing the same concentration of ethanol as that of the beverage being analyzed given that the total product ion count rate varies with ethanol concentration (Figure 4). Methanol standard curves were therefore prepared in aqueous solutions containing 4%, 13% and 40% ethanol, concentrations which matched those of the beverages being assayed, with the standard curve for 13% ethanol presented in Figure 5. It should be noted that in some cases of methanol adulteration the methanol concentration actually exceeds Figure 5. Relationship between rate of formation of methanol derived product ions (m/z 115 and 171) in the reaction between H3O+ ions and the headspace of various concentrations of methanol dissolved in a 13% (v/v) ethanol solution. The “bestfit” regression line is shown (r2p = 0.99). C. Chambers-Bédard and B.M. Ross, Eur. J. Mass Spectrom. 22, 159–164 (2016)163 Table 2. Quantification of methanol in some alcoholic beverages using SIFT-MS. Values are the mean ± SD of triplicate determinations. LOD: limit of detection. Beverage Ethanol content (% v/v) Methanol content (mg L–1) Lager 4 Below LOD Stout beer 4 24 ± 4 White wine 13 95 ± 14 Red wine 13 154 ± 28 Scotch whisky 40 191 ± 30 Gin 40 249 ± 38 that of ethanol.12 In such cases our preliminary experiments suggest that while the product ion count rates are large the relationship between count rates and methanol concentration ceases to be linear making accurate quantification difficult although still serving as a semi-quantitative indicator of the presence of large quantities of methanol. The limits of detection for methanol in various concentrations of ethanol were calculated, the LOD being defined to be methanol ion count rates three times greater than the count rates produced before the addition of methanol to the ethanol solution. In 5%, 13% and 40% ethanol solutions the limits of detection for methanol were found to be approximately 0.0005% (5 mg L–1), 0.0008% (8 mg L–1) or 0.0015% (15 mg L–1) methanol respectively. Although we used product ion count rates as the measured quantity, Dryahina and colleagues23 have used precursor ion to product ion ratios to construct standard curves for their measurement of methylamine in the presence of ethanol. Their method is likely to result in more stable and accurate measurements given that such ratios account for varying product ion count rates. Using the standard curves, methanol levels were measured in samples of red wine and white wine, spirits and beer (Table 2). 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