Download Quantification of methanol in the presence of

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). All samples
were determined to have a methanol content below 2% v/v,
the methanol content previously associated with potential
harm.9 Our measured methanol concentrations are similar to
those in the literature indicating the utility of our technique.34
Moreover, our findings conform to previous studies indicating
a greater amount of volatile compounds, including methanol,
in red wines compared to white wines.35
In summary, we have developed a method for the near real
time quantification of methanol in the headspace of alcoholic
beverages using SIFT-MS. The technique is suited to situations where disturbing the liquid of the beverage is undesirable such as in quality control applications.
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