Photodegradation kinetics and byproducts identification of the Aflatoxin B1 in aqueous medium by ultra-performance liquid chromatography- quadrupole time-of-flight mass spectrometry

Article abstract of DOI:10.1002/jms.1741

A photodegradation study of Aflatoxin B₁ (AFB₁) in water solution was performed under UV irradiation at different AFB₁ initial concentrations and UV irradiation intensities. The effect of UV intensity on the AFB₁ photodegradation ratio isdominative, when compared with AFB₁ initial concentration. The photodegradation of AFB₁ was proved to follow first-order reaction kinetics (R ² ≥ 0.99). Three photodegradation products, i.e. P₁ (C₁₇H₁₄O₇), P₂ (C₁₆H ₁₄O₆) and P₃ (C₁₆H ₁₂O₇), were identified on the basis of low mass error and high matching property by ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF MS), and the degradation pathway was proposed. This study first reports the appearance of these photodegradation products and the proposed degradation pathway in aqueous media. Copyright

Full text of DOI:10.1002/jms.1741

Research Article
Received: 20 December 2009
Accepted: 15 March 2010
Published online in Wiley Interscience: 9 April 2010
(www.interscience.com) DOI 10.1002/jms.1741
Photodegradation kinetics and byproducts
identification of the Aflatoxin B1 in aqueous
medium by ultra-performance liquid
chromatography–quadrupole time-of-flight
mass spectrometry
Ruijie Liu,a Qingzhe Jin,a Guanjun Tao,a Liang Shan,a Jianhua Huang,a
Yuanfa Liu,a Xingguo Wang,a∗ Wenyue Maob and Shanshan Wangb
A photodegradation study of Aflatoxin B1 (AFB1) in water solution was performed under UV irradiation at different AFB1 initial
concentrationsandUVirradiationintensities. TheeffectofUVintensityontheAFB1 photodegradationratioisdominative, when
compared with AFB1 initial concentration. The photodegradation of AFB1 was proved to follow first-order reaction kinetics
(R2 ≥ 0.99). Three photodegradation products, i.e. P1 (C17H14O7), P2 (C16H14O6) and P3 (C16H12O7), were identified on the basis
of low mass error and high matching property by ultra-performance liquid chromatography–quadrupole time-of-flight mass
spectrometry (UPLC–Q-TOF MS), and the degradation pathway was proposed. This study first reports the appearance of these
c
photodegradation products and the proposed degradation pathway in aqueous media. Copyright ꢀ 2010 John Wiley & Sons,
Ltd.
Keywords: AFB1; photodegradation; UV; aqueous media; UPLC–Q-TOF MS
Introduction
has revealed the appearance of new photodegradation products,
andUVandIRspectroscopyhaverevealeddecreasesinthecharac-
teristic absorption of the aflatoxins in analysis performed after the
UVirradiationofaflatoxins,[10–13] knowledgeofthenatureofthese
UV-induced aflatoxin degradation products was previously rather
limited, and the exact nature of the resulting photodegradation
products of AFB1 remains unknown. Quadruple time-of-flight
(Q-TOF) mass spectrometers have been suggested as suitable
analytical tools for the identification of metabolites[14,15] and
degradation products of harmful trace materials present in
drinking water[16] or pesticide residues in foods.[17]
In our earlier work, the photodegradation products of AFB1 in
acetonitrile solution were identified.[18] The aim of this article is to
study the photodegradation behavior and the products in water
solution, provides clues to the study of the photodegradation
mechanism of AFB1 and the assessment of safety issues of UV
method applied in AFB1 decontamination. Q-TOF MS was used
here to identify the photodegradation products and the pathway
of UV degradation of AFB1.
Aflatoxins, a group of highly toxic, mutagenic and carcinogenic
compounds, are secondary metabolites of Aspergillus flavus and
A. parasiticus, which are found worldwide in air and soil to infest
both living and dead plants and animals.[1] Among the various
aflatoxin species, Aflatoxin B1 (AFB1) is the most potent teratogen,
mutagen and hepatocarcinogen, which is classified as a Group
1 carcinogen by the International Agency for Research in Cancer
(IARC).[2,3] Aflatoxins occur in many countries, especially in tropical
and subtropical regions, where conditions of temperature and
humidity are optimum for the growth of the molds and for
the production of the toxin. Many agricultural commodities and
important crops, especially peanuts and peanut-based foods, are
susceptible to such contamination. Preventing the contamination
of food by the toxigenic fungi, the most rational and economical
approach to avoid the potential hazards, is not always possible
under certain agronomic storage practices. Therefore, removal or
inactivation in salvaging food and feedstuff already contaminated
with toxic fungal metabolites is a major concern.[1]
Various physical, chemical and biological approaches are avail-
able for the detoxification of aflatoxins.[1,4,5] UV irradiation has
been discovered for many years as an effective physical method
to destroy aflatoxins for its photosensitive property.[6] Recently,
advanced photochemical transformation has emerged as a pow-
erful method for degrading and transforming the photosensitive
materialsintoharmlesssubstances.[7–9] But, whatthephotodegra-
dation products are and whether they are harmless substances
becomethemajorapprehensivenessofthisadvancedphotochem-
ical detoxification method. Although thin-layer chromatography
∗
Correspondence to: Xingguo Wang, State Key Laboratory of Food Science and
Safety, School of Food Science and Technology, Jiangnan University, 1800 Lihu
Road,Wuxi214122,JiangsuProvince,PRChina.E-mail: wxg1002@hotmail.com
a
State Key Laboratory of Food Science and Safety, School of Food Science
and Technology, Jiangnan University, 1800 Lihu Road, Wuxi 214122, Jiangsu
Province, PR China
b
Shandong Luhua Group Co., Ltd., 39 Longmen East Road, Laiyang 265200,
Shandong Province, PR China
c
J. Mass. Spectrom. 2010, 45, 553–559
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

R. Liu et al.
Experimental
To ensure accuracy and reproducibility, all analyses were acquired
using an independent reference spray via the Lock Spray interface;
Tyr-Gly-Gly-Phe-Leu was used as lock mass (m/z 556.2771) under
positive-ion conditions. The Lock Spray frequency was set at 6 s,
meaning that every 5 s flow from the Lock Spray was introduced
into the mass spectrometer for 1 s, thus giving the software the
possibility of performing ongoing correction of the exact mass of
the analyte. Data for the reference compound were averaged over
ten spectra per minute. The accurate mass and composition of the
precursor and fragment ions were calculated using the MassLynx
4.1 software supplied with the instrument. The software has a
feature that calculates all possible elemental composition from
the accurate mass; then, by using previous knowledge, such as
low i-FIT (Norm), type and number of atoms, various impossible
formulae can be further ruled out. Therefore, Q-TOF system is a
powerful tool for forming hypotheses about the identity of an
unknown compound. Final identification can then be performed
on the basis of accurate measurement of the mass of the parent
ions and the fragments obtained in MS–MS experiments.
Materials
AFB1 (2,3,6α,9α-tetrahydro-4-methoxycyclopenta [c] furo [2,3 : 45]
furo [2,3-h] chromene-1,11-dione; purity >98%) was obtained
from Fermentek (Jerusalem, Israel). High-performance liquid
chromatography (HPLC)-grade acetonitrile and benzene were
purchased from Sigma (St Louis, MO, USA).
For the UPLC–Q-TOF MS studies, deionized water (18-Mꢀ cm−1
resistivity) was obtained from a Milli-Q SP Reagent Water system
(Millipore, Bedford, MA, USA) and prefiltered through a 0.2-µm
membrane. Acetonitrile was Optima LC–MS-grade from Fisher
Scientific (Fremont, CA, USA).
Standard stock solutions (200 mg/l) of AFB1 were prepared
in benzene–acetonitrile (98:2 v/v) and stored at 4 2 ◦C in a
refrigerated dark room (stability of stock and standard solutions
under these conditions was checked and demonstrated for at
least 3 months). Immediately before use, 0.5 ml of standard stock
solutions were placed in glass tubes and dried under a jet of
nitrogen. Working solutions of AFB1 with initial concentrations of
0.2, 2 and 5 ppm were prepared by adding 500, 50 and 20 ml of
water solution (pH 7.0), respectively.
Calculation methods and reproducibility
The experiments were made at least in triplicate and the analytical
methods were applied at least in triplicate. The calculation and
statistical methods used are available in the program Origin8.0.
Photodegradation procedure
For degradation experiments, water solutions of pure AFB1
were placed in quartz vessels and irradiated at 4 2 ◦C under
an ultraviolet lamp (NatureGene Corp., Medford, NJ, USA) at
different intensities. The intensity reaching the sample surface
was measured by means of a UV intensity detector (Beijing Normal
University Photoelectric Instrument Factory, Beijing, China).
Meanwhile, controlexperimentsinthedark(blankexperiments)
under the same conditions were carried out in parallel for
comparison without the application of light or AFB1. At selected
time intervals, samples were collected and quantitatively analyzed
directly by UPLC–Q-TOF MS for the amount of the compound
of interest remaining in the solution after irradiation based on
external calibration.
Results and Discussion
Effect of initial concentration on degradation of AFB1
The effect of initial concentration on the degradation of AFB1 by
UV irradiation is presented in Fig. 1(a). The initial concentrations
of AFB1 were 0.2, 2.0 and 5.0 ppm, and there are no detectable
changes shown in blank experiment with different concentrations
(results exemplified by that pertaining to an initial concentration
of 0.2 ppm shown in Fig. 1(a)). Therefore, the declines observed in
the degradation curve arise from the photodegradation process.
It can be seen that there is no significant difference in the
three degradation curves, indicating that the effect of the initial
concentrationintheselectedrangeontheAFB1 photodegradation
is nearly inexistent, which is in agreement with the feature of the
first-order kinetics model.[19]
Instrumental analysis and quantification
UPLC was performed on a Waters Acquity UPLC system (Milford,
MA, USA) equipped with a binary solvent-delivery system and
an autosampler. Chromatography was performed on a 10 cm ×
2.1 mm, 1.7-µm particle, Waters Acquity C18 column. The injection
volume was 2 µl. The mobile phase was a gradient prepared from
acetonitrile (component A) and 0.1% formic acid aqueous solution
(component B). Elution started with 8% A for 0.1 min and then the
proportion of A was increased linearly to 30% at 10 min and then
to 100% at 15 min and brought back to 8% A at 15.1 min. Total
run time, including conditioning of the column prior to the initial
Effect of UV intensity on degradation of AFB1
Significant changes among each degradation curves of AFB1 at
different UV intensities are presented in Fig. 1(b). The initial con-
centration of AFB1 was 2 ppm, and UV intensities corresponding to
thecurvesareshowninFig. 1(b)asaninset.Quantitativerecoveries
from blank experiments sampled over the entire exposure period
of simulated UV irradiation showed that AFB1 did not undergo
dark reaction; thus, the decline observed in the degradation curve
should be attributed to the photodegradation process. Therefore,
photodegradation rate of AFB1 is strongly affected by UV intensity.
conditions, was 17 min. The flow rate was 500 µl min−1
.
Mass spectrometry was performed on a Waters Synapt
Q-TOF system (Milford, Massachusetts, USA). Compounds were
analyzed in the positive-ion (PI) mode. The optimized conditions
were: desolvation gas 500 l/h at a temperature of 420 ◦C, cone
gas 50 l/h, source temperature 120 ◦C and capillary and cone
potentials 3000 and 30 V, respectively. The Q-TOF instrument was
operated in the wide pass quadrupole (V) mode, and data were
collectedbetweenm/z 50and1000, withascanaccumulationtime
of 0.2 s. The MS–MS experiments were performed using a collision
energy of 25 or 30 eV, which was optimized for each compound.
Photodegradation kinetic of AFB1
The plot of ln(Ct/C0) versus irradiation time for AFB1 at different
initial concentrations and UV irradiation intensity is given in
Fig. 2, and the deduced parameters are listed in Table 1. The
linear relationship between ln(Ct/C0) and irradiation time indicate
that the degradation followed a first-order kinetics (R2 ≥ 0.99),
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2010, 45, 553–559

Photodegradation kinetics and byproducts identification of the Aflatoxin B1
Figure 2. Kinetics of the photodegradation of AFB1 at (a) various initial
concentrations and (b) different UV irradiation intensities.
Figure 1. Photodegradation curve of AFB1 at (a) various initial concentra-
tions and (b) different UV irradiation intensities.
given by the equation Ct = C0e−kt where C0 and Ct are the
concentrations at times 0 and t, t is the irradiation time and k is
the first-order rate constant. The half-life t1/2 of AFB1, the time
required for its concentration to fall to half its initial value, is
related to the rate constant by the equation: t1/2 = ln 2/k. It
can be observed from Table 1 that the values of k and t1/2 are
nearly the same for different initial AFB1 concentrations and they
have no connection with the AFB1 initial concentration. However,
the values of k and t1/2 in Table 1 show obvious changes under
different UV intensities, and the AFB1 photodegradation rate was
lower following the order: 800 > 400 > 200 µw/cm2 illustrating
a strong dependence on the intensity of UV irradiation.
first, second and the third new photoproducts, P1, P2 and P3, can
be detected at 10, 20 and 40 min, respectively, and the response
of P3 is much lower than that of P1 and P2.
Identification of photodegradation products of AFB1
P1, P2 and P3 had a retention time of 6.9, 7.5 and 3.8, respectively,
showing an order of polar character P3 > P1 > P2 > AFB1
(shown in Fig. 3). There were no available standards with which
to match UPLC-diode array detector (DAD) retention times, so the
photodegradation products were identified by UPLC–Q-TOF MS.
UPLC–MS analysis suggested that P1 (molecular mass m/z
331.0812), P2 (m/z 303.0868) and P3 (m/z 317.0659) were the
products of photodegradation of AFB1; for component AFB1,
m/z = 313.0709 was in a good agreement with the molecular
mass of the parent compound (AFB1). Accurate masses, molecular
formulas, mass error (mDa and ppm), double-bond equivalents
(DBE) and i-FIT (Norm) value (the likelihood that the isotopical
pattern of the elemental composition matches a cluster of peaks
in the spectrum) of these four protonated molecules are shown
in Table 2 using the elemental composition tool incorporated
in the MassLynx 4.1 software. The specifications of the Synapt
Q-TOF state a maximum mass error of 5 ppm for the range of
masses discussed in this article. In this work, the accurate mass and
molecularformulaoftheparentionwereusedasanothermasslock
to verify the precision of the Synapt Q-TOF system. As is apparent
from Table 2, the experimental mass of the parent (AFB1) ion given
Time development of the formation of photodegradation
products of AFB1
To study the time development of the formation of AFB1
photodegradation products and simultaneously identify their
structure, the solution irradiation at the intensity of 400 µw/cm2
was sampled periodically and the samples were analyzed by
UPLC–Q-TOF MS. It is apparent from Fig. 3(a) that AFB1 is de-
graded within 100 min, leading to the formation of the three new
compounds denoted ‘P1’, ‘P2’ and ‘P3’, which were not detected
in the blank experiment and the ultraviolet absorption spectra
of these three compounds are similar to that of AFB1. Thus, these
three new compounds were considered as photodegradation
products of AFB1. It can also be noted that the presence of the
c
J. Mass. Spectrom. 2010, 45, 553–559
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
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R. Liu et al.
Table 1. Kinetic parameters of AFB1 at different initial concentrations and UV intensities
Initial concentration (ppm)/UV
intensity (µw/cm2)
Equation
R2
k (min−1
)
t1/2 (min)
0.2 ppm
2.0 ppm
5.0 ppm
ln(Ct/C0) = −0.03391t
ln(Ct/C0) = −0.03274t
ln(Ct/C0) = −0.03336t
ln(Ct/C0) = −0.03291t
ln(Ct/C0) = −0.04745t
ln(Ct/C0) = −0.07325t
0.99680
0.99695
0.99885
−0.03391 0.0006
−0.03274 0.0006
−0.03336 0.0003
−0.03291 0.0005
−0.04745 0.0014
−0.07325 0.0018
20.5
21.2
20.8
200 µw/cm2
400 µw/cm2
800 µw/cm2
0.99693
0.99001
0.99316
21.1
14.6
9.5
Figure 3. (a) Kinetics of formation of the photodegradation products of AFB1. (b) Curves showing the evolution of AFB1 photodegradation products P1,
P2 and P3 with increasing UV irradiation time.
by the Synapt Q-TOF system is 313.0709, which is 0.3 mDa less
than the theoretical mass, 313.0712, and the molecular formula
proposed, C17H13O6, is the same as the theoretical molecular
formula. The mass errors of the three photodegradation products
were all below 5 ppm and their i-FIT (Norm) values were all 0,
which indicated at least 95% confidence in the accuracy of the
suggested composition.
In separate MS–MS in Q-TOF mode, the ions of these four
compounds were used as precursor compounds, and the ion-
filtering function of the quadrupole permitted only ions of m/z
331, 303, 317 and 313 to pass to the collision cell, where 30 eV
was applied for 331, 303 and 317, and 25 eV was applied for 313.
Thus, the MS–MS spectra, the fragmentation formation and the
corresponding parameters (mass error, DBE, i-FIT, etc.) of these
precursor compounds were obtained and shown in Fig. 4 and
Table 2, respectively. The structural formulas of these degradation
products were proposed based on the molecular formula
calculated from the accurate mass and the MS–MS fragmentation
formation supplied by the Q-TOF mode and elucidated by the tool
Massfragment incorporated in the MassLynx 4.1 software to verify
howmuchtheymatchedthecorresponding fragmentinformation
(matching information shown in Fig. 4 as an inset).
The photodegradation pathway of AFB1 in water solution was
proposed based on the identified structural formulas of these
photodegradationproductsandshowninFig. 5. Itcanbededuced
that a photoaddition reaction induced by water might occur in the
C(8)–C(9) bond that produces P1 from AFB1, and a photoreduction
reaction in the O(1)–C(14) bond and a photoelimination reaction
in the C(4)–O(12) bond combined lead to the reaction from P1
to P2, while a photoelimination reaction alone in the C(12)–O(13)
bond forms P3 from P1. Nevertheless, the quantity of P2 is much
more than P3, which may be due to the competition between the
reaction from P1 to P2 and the reaction from P1 to P3.[20,21] The
C(8)–C(9) bond and the O(1)–C(14) bond are two active sites of
AFB1[4] and are easy to be reduced to more stable saturated bonds
in photochemistry reaction; thus, these three photoreduction
or photoaddition products would be more stable than their
parent compound, which is in agreement with Fig. 3(a).[20,22–24]
These photochemical principles have been used here to propose
the photodegradation pathway of AFB1, but the photochemical
elucidation of the proposed photodegradation pathway of AFB1
needs to be proved further in future study.
This photodegradation study on AFB1, a trace toxin in food,
points out the good possibilities of Q-TOF MS to identify
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J. Mass. Spectrom. 2010, 45, 553–559

Photodegradation kinetics and byproducts identification of the Aflatoxin B1
Figure 4. TOF MS–MS spectra and proposed fragmentation of degradation products in the presence of UV irradiation. Proposed fragmentations are
shown as an inset.
c
J. Mass. Spectrom. 2010, 45, 553–559
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R. Liu et al.
Table 2. Q-TOF MS accurate mass elemental composition of the AFB1 photodegradation productsa
Mass error
Compound
Experimental
Theoretical
precursorion
mass (m/z)
mass (m/z)
mDa
ppm
DBE
Molecular formula
C17H15O7
i-FIT (Norm)
P1
331.0812
313.0715
285.0764
270.0527
241.0507
303.0868
259.0602
231.0657
203.0706
317.0659
299.0550
289.0345
271.0612
229.0508
313.0709
285.0759
241.0504
214.0629
331.0818
313.0712
285.0763
270.0528
241.0501
303.0869
259.0606
231.0657
203.0708
317.0661
299.0556
289.0348
271.0606
229.0501
313.0712
285.0763
241.0501
214.0630
−0.6
0.3
−1.8
1.0
10.5
11.5
10.5
11.0
10.5
9.5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
C17H13O6
C16H13O5
C15H10O5
0.1
0.4
−0.1
0.6
−0.4
2.5
C
14H9O4
P2
−0.1
−0.4
0.0
−0.3
−1.5
0.0
C16H15O6
9.5
C
C
14H11O5
13H11O4
8.5
−0.2
−0.2
−0.6
−0.3
0.6
−1.0
−0.6
−2.0
−1.0
2.2
7.5
C12H11O3
C16H13O7
P3
10.5
11.5
10.5
10.5
9.5
C
16H11O6
C14H9O7
15H11O5
C
0.7
3.1
C
13H9O4
AFB1
−0.3
−0.4
0.3
−0.9
−1.4
1.2
11.5
10.5
10.5
9.0
C17H13O6
C16H13O5
C
14H9O4
−0.1
−0.5
C
13H10O3
a The elemental composition report was obtained using single-mass analysis of 5 mDa with DBE between −1.5 and 50.0, monoisotopic masses, odd
and even electron ions and element limits from 0 to 20 for C, H and O.
initial concentration. The photodegradation of AFB1 was proved
to follow the first-order reaction kinetics well.
Three photodegradation products of AFB1 were observed and
identified by UPLC–Q-TOF MS for the first time. The photodegra-
dation pathway of AFB1 was proposed, which is supported by the
elementary reaction mechanisms of photochemistry.
The power of UPLC–Q-TOF MS is displayed here another time
for the identification of the trace materials and their degradation
products. Measurement of an accurate mass for product ions
and Elemental Composition tool and MassFragment tool software
incorporated in the software MassLynx 4.1 supplied with the
instrument offer easier and more precise identification and
elucidation of the structures of photodegradation products.
Figure 5. Photodegradation pathway of AFB1 proposed on the basis of
the results from this study.
Acknowledgements
The work is supported by the National Key Technology R&D
Program in the 11th Five year Plan of China (Contract No:
2009BADB9B08) and the National High Technology Research
and Development Program (863 Program) of China (Contract
No: 2010AA101504).
non-target molecules such as photodegradation products of
photosensitive trace materials another time. Published literature
on the subject, chemical database and websites are very useful to
unambiguously identify compounds, combined with the Q-TOF
MS. The main problem of this technique is the lack of libraries
that allow us to search a possible structure for a given elemental
composition within the equipment software. That would be a
very good feature for the identification work.
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J. Mass. Spectrom. 2010, 45, 553–559

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