Simple and excellent selective chemiluminescence-based CS2 on-line detection system for rapid analysis of sulfur-containing compounds in complex samples

Article abstract of DOI:10.1021/acs.analchem.5b00722

To study the interesting chemical reaction phenomenon can greatly contribute to the development of an innovative analytical method. In this paper, a simple CL reaction cell was constructed to study the chemiluminescence (CL) emission from the thermal oxidation of carbon disulfide (CS₂). We found that the CL detection of CS₂ exhibits unique characteristics of excellent selectivity and rapid response capacity. Experimental investigations together with theoretical calculation were performed to study the mechanism behind the CL reaction. The results revealed that the main luminous intermediates generated during the thermal degradation of CS₂ are SO₂ and CO₂. Significantly, this CL emission phenomenon has a wide application due to many sulfur-containing compounds that can convert to CS₂ under special conditions. On the basis of this scheme, a CS₂-generating and detection system was developed for rapid measurement of CS₂ or other compounds that can convert to CS₂. The usefulness of the system was demonstrated by measuring dithiocarbamate (DTC) pesticides (selected mancozeb as a representative analyte) based on the evolution of CS₂ in spiked agricultural products. Results showed that the system allows online and large volume detection of CS₂ under nonequilibrium condition, which greatly reduces the analytical time. The concentrations of mancozeb in the spiked samples were well-quantified with satisfied recoveries of 76.9-97.3%. The system not only addresses the urgent need for rapid in-field screening of DTC residues in foodstuffs but also opens a new opportunity for the fast, convenient, and cost-effective detection of CS₂ and some other sulfur-containing compounds in complex samples.

Full text of DOI:10.1021/acs.analchem.5b00722

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Article
Simple and excellent selective chemiluminescence-
based CS2 on-line detection system for rapid analysis
of sulphur-containing compounds in complex samples
Runkun Zhang, Gongke Li, and Yufei Hu
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b00722 • Publication Date (Web): 27 Apr 2015
Downloaded from http://pubs.acs.org on April 27, 2015
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Simple and Excellent Selective ChemiluminescenceꢀBased CS2
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OnꢀLine
Detection
System
for
Rapid
Analysis
of
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SulphurꢀContaining Compounds in Complex Samples
Runkun Zhang, Gongke Li*, Yufei Hu*
School of Chemistry and Chemical Engineering, Sun Yatꢀsen University, Guangzhou 510275, China
* Corresponding Authors: Gongke Li, Yufei Hu
Tel. : +86ꢀ20ꢀ84110922
Fax : +86ꢀ20ꢀ84115107
Eꢀmail: cesgkl@mail.sysu.edu.cn
huyufei@mail.sysu.edu.cn
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ABSTRACT: To study the interesting chemical reaction phenomenon can greatly
contribute to the development of innovative analytical method. In this paper, a simple CL
reaction cell was constructed to study the chemiluminescence (CL) emission from the thermal
oxidation of carbon disulfide (CS2). We found that the CL detection of CS2 exhibits unique
characteristics of excellent selectivity and rapid response capacity. Experimental
investigations together with theoretical calculation were performed to study the mechanism
behind the CL reaction. The results revealed that the main luminous intermediates generated
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*
*
during the thermal degradation of CS2 are SO2 and CO2 . Significantly, this CL emission
phenomenon has a wide application due to many sulphurꢀcontaining compounds can convert
to CS2 under special conditions. On the basis of this scheme, a CS2ꢀgenerating and detection
system was developed for rapid measurement of CS2 or other compounds that can convert to
CS2. The usefulness of the system was demonstrated by measuring dithiocarbamate (DTC)
pesticides (selected mancozeb as representative analyte) based on the evolution of CS2 in
spiked agricultural products. Results showed that the system allows onꢀline and large volume
detection of CS2 under nonꢀequilibrium condition, which greatly reduces the analytical time.
The concentrations of mancozeb in the spiked samples were well quantified with satisfied
recoveries of 76.9ꢀ97.3%. The system not only addresses the urgent need for rapid inꢀfield
screening of DTC residues in foodstuffs, but also opens a new opportunity for the fast,
convenient, and costꢀeffective detection of CS2 and sulfurꢀcontaining compounds in complex
samples.
KEYWORDS: Chemiluminescence; Carbon disulfide; Onꢀline detection; Sulphurꢀcontaining
compound; Dithiocarbamates; Mancozeb; Complex sample.
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INTRODUCTION
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In the history of analytical chemistry, the research on chemical reaction plays an
important role in the development of the innovative analytical methods or instruments.
For example, a number of analytical methods are based on the four general reaction
types, namely: acidꢀbase,1 redox,2 complexation3 and precipitation reactions.4 More
typically, the chemiluminescence (CL) emission from the reaction between ozone and
nitric oxide (NO+O3 system) occupies a prominent place in analytical chemistry.5 The
application of this light emission phenomenon is not limit to direct determine the
concentration of NO, and has been expanded tremendously via directly or indirectly
converting other nitrogenꢀcontaining compounds (e.g. NO2 and Nꢀnitroso compounds)
to NO.6ꢀ8 Significantly, the total nitrogen analyzer and nitrogenꢀspecific gas
chromatography (GC) detector were developed based on this detection scheme.5, 9, 10
Chemiluminescence (CL) has proved to be a powerful analysis method for rapid
analysis due to its high sensitivity, wide dynamic range, simple instrumentation, and
lack of background scattering light interference.11ꢀ13 Many interesting CL phenomena
have been developed into analytical methods for biological assays,14 clinical
diagnoses15 and environmental analysis.13, 16 However, to the best of our knowledge,
methods characterized by coupling sample preparation with CL detection for onꢀline
analysis of objects in complex samples are seldom reported, mainly due to the
unsatisfying selectivity for the CL detection. In addition, the conventional CL analysis
can only detect a limited number of analyses, and are virtually powerless against
many organic gases. In recent years, the research on the CL produced during the
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oxidation processes of combustible compounds on nanomaterial surface—namely,
cataluminescence (CTL)ꢀwhich has greatly expanded the detectable analytes of CL
analysis.17ꢀ19 The luminescence efficiency can be improved greatly when different
catalysts and diverse catalysis conditions were applied to CTL detection, resulting in
many organic gases which were hardly detected previously and then can be detected
successfully.18 Unfortunately, although the selectivity can be improved by selected by
appropriate catalyst, it still remains a major challenge to the CTL detection for the
analysis of complex samples. In this regard, to explore CL reaction with intrinsical
selectivity has promising applications in rapid analysis of complex sample.
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As a highly volatile organic compound (VOC), carbon disulfide (CS2, boiling
point is 46.5 °C; saturated vapor pressure is 53.32 kPa at 28 °C) is a wellꢀknown
occupational hazard in various industrial processes, such as vulcanizing rubber, oil
extraction and viscose rayon fibres manufacturing.21 It is also frequently used as an
insecticide to protect fresh fruit and grains from insects and fungus during shipping, in
adhesives for food packaging.22 However, exposure to CS2 may cause various health
risks such as atherosclerosis, coronary artery disease, encephalopathy and nervous
system diseases etc.23, 24 Aparting from released by anthropogenic activities, CS2 is
also released naturally from volcanoes eruption, marshland, food wastes, coal and
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natural gas.25,
Moreover, many sulphurꢀcontaining compounds also can
quantitatively convert to CS2 under special conditions. For example, CS2 is one of the
degradation products of methyl mercaptan (CH3SH) in the plasma reactor;27 the
thermal decompositions of dimethyl sulfide (CH3S2CH3)28 and ammonium
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thiocyanate (NH4SCN) will form CS2;29 Acid hydrolysis of xanthate and
dithiocarbamates (DTCs) will release CS2.30, 31 Obviously, the development of a
simple and miniaturized analytical system for rapid CS2 detection will has wide
applications in many fields.
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In the present work, we found that the CL emission from the thermal oxidation of
CS2 exhibits intrinsically selectivity for it does not require any catalyst to assist the
light emission, which indicates that this CL reaction has a tremendous application in
complex sample analysis. On the basis of this CL reaction, a CS2ꢀgenerating and
detection system (CS2ꢀgenerating DS) characterized by coupling sample preparation
device with CL reaction cell, was developed for onꢀline rapid determination of CS2 or
other compounds that can convert to CS2. As a proof ofꢀprinciple work, the system
was applied to measure DTC pesticides. DTCs are the most frequently detected
pesticides and they show the highest frequency in exceeding the maximum residue
limits (MRLs). With the global trading of agricultural goods increases, fast and
efficient methods for the monitoring of DTCs in imported/exported foods are urgently
needed. Due to the they are practically insoluble in common organic and inorganic
solvents, the common methods for measuring DTCs are based on the acid digest of
the sample in the presence of stannous chloride (SnCl2) to evolved CS2, which is
further analyzed by absorption spectrometry or gas chromatography (GC).31ꢀ33
However, these methods are tedious and timeꢀconsuming, and have the potential to
increase errors due to they require offꢀline complex operations. The CS2ꢀgenerating
DS allows onꢀline measure CS2 released by DTCs in real samples without separation
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or extraction operation for the excellent selectivity of the CL detection. In addition,
the system also can realize largeꢀvolume detection of CS2 under nonꢀequilibrium
condition for the rapid response speed of CL detection, resulting in its analysis speed
is much faster than the previous methods for DTCs based on the evolution of CS2.
Our system not only addresses the unmet need for inexpensive and simple analytical
instrumentations for rapid analysis of DTCs, but also provides an ingenious approach
for rapid measure of CS2 or sulfurꢀcontaining compounds that can convert to CS2 in
relative fields.
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■ EXPERIMENTAL SECTION
Reagents. Standard solution of 100 µg/mL of mancozeb (chemical formula:
[C4H6MnN2S4]xZny, x:y = 1 : 0.091 ) dissolved in eathylene diamine tetraacetic acid
(EDTA) was purchased from Center of Chinese Standard Material Information
(Beijing, China). Carbon disulfide was purchased from Damao Chemical Reagent
Factory (Tianjin, China). Tin (II) chloride (SnCl2) was obtained from Xilong
Chemical Ind., Co., Ltd (Shantou, China). The information of some other reagents can
be seen in the Supporting Information.
Analysis System Fabrication. As shown in Figure 1, the CS2ꢀgenerating DS mainly
comprises a CL reaction cell using a miniature stainless steel heating tube (SSHT,
length =6 cm, diameter= 0.3 cm) as heating element, and a CS2 generator (a
polytetrafluoroethylene vessel of about 70 mL volume) with two check valves. The
CL reaction cell was simply constructed by placing the miniature SSHT into a quartz
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tube (length=8 cm, diameter=0.6 cm) with a gas inlet and gas outlet. The temperature
of the surface of the SSHT was controlled by a voltage regulator and monitored by a
thermocouple. The inlet of the CS2 generator was connected to a miniature air pump
(Model: FAT200.8, Chengdu Xin Wei Cheng Technology Co. Ltd., Chengdu, China)
by a polytetrafluoroethylene tube (inner diameter= 0.2 mm, outer diameter= 0.3 mm),
and its outlet was connected to the inlet of the CL reaction cell. A computerized
BPCL ultraweak luminescent analyzer with a photomultiplier (PMT) detector
(Institute of Biophysics, Academia Sinica, Beijing, China) was used to measure the
CL intensity. The luminescent analyzer can be easily miniaturized to a very compact
one.34, 35 A high voltage of 860 V was applied to the PMT and the data acquisition
time for each signal point was set as 0.5 s. The detection wavelengths of the system
could be selected by changing the interference optical filters.
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Methods for Mechanism Study. An Agilent HP 6890 gas chromatography (GC)ꢀ973
mass spectra (MS) equipped with an HPꢀVOC fused silica capillary column (60 m
length × 0.320 mm inner diameter× 1.80 µm film thickness) was used to identify the
products from the thermal oxidation of CS2. The instrument operating conditions were
listed in Table S1 in the Supporting Information. The exhaust from thermal oxidation
of CS2 was collected into a sampling bag of 1 L, and 200 µL of the exhaust was
injected into the GCꢀMS. A Shimadzu RFꢀ5301PC spectrofluorophotometer
(Shimadzu Company, Japan) was used to record the CL emission spectrum when the
xenon lamp was turned off. CS2 at the concentration of 4 mg Lꢀ1 was continuously
introduced into the CL reaction cell by the air pump for spectrum scanning. The
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scanning range was 350 to 550 nm, and the scanning speed was set at super grade. In
addition, the CL spectrum was also investigated by the BPCL luminescence analyzer
through changing the bandpass filters of 350, 380, 400, 425, 440, 490, 535 and 555
nm.
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The Gaussian’s implementation of the B3LYP hybrid density functional method
with the 6ꢀ311++G** basis set was adopted to calculate the optimized geometries of
CS2 and its decomposition species. Vibrational frequency were performed at the same
theoretical level to verify reactants and intermediate at the minimum point on the
potential energy surface by no imaginary frequency and the transition states at saddle
point by only one imaginary frequency.
Sample Preparation and Measurement Procedure. Mung bean and barley were
obtained from local market. Because DTC residues are typically located superficially,
it is recommended that samples consisting of bigger units (e.g. apples, melons, tomato,
iceberg lettuce) will require a comminution step, while for samples composed of
small units or particle sizes (e.g. grain, nuts, dry pulses, raisins and even small
berries), sample comminution can be typically omitted as long as the sample portion
employed for analysis is large enough.36 Therefore, 5.0 g of mung bean and barley
were directly placed into the CS2 generator, and then were spiked with mancozeb at
three levels of 1.0, 5.0 and 8.0 mg/kg for the recovery studies. It was reported that
heating, ultrasound or microwave etc. assisted technologies can effectively shorten the
hydrolysis time of DTCs.31 In the present work, ultrasonic bath was used to accelerate
the hydrolysis speed of DTCs.
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After the samples containing DTCs were placed into the generator, 30 mL of
1.5% SnCl2ꢀ4 mol/L HCl (hydrolysis reagent recommended by EU reference
laboratories and European commercial testing laboratories.) was added to digest
DTCs.36 Subsequently, the generator was placed into an ultrasonic bath for 30 min. At
this stage, the valves of the generator were close. After finishing the acid hydrolysis,
the valves were open immediately and a steady air stream was introduced into the
generator to carry the generated CS2 vapor passing through the CL reaction cell. The
data was recorded by a computer and further processed with Origin 8.0. The CL
kinetic curve showing the CL intensity versus time is used for the study of CL
characteristics of CS2, and peak area is used as the indicator for conditions
optimization and quantization.
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■ RESULTS AND DISCUSSION
CL Characteristics of CS2. The CL Characteristics of CS2 were investigated by
offꢀline injection mode. It was reported that many organic gases can be oxidized on
the heated catalysts to produce CL emission, but they cannot produce light emission
without catalysts.37, 38 Therefore, we first tried to use MgO, SrCO3, γꢀAl2O3 and TiO2
as catalysts for CL detections of CS2. As Figure S1 in the Supporting Information
shows, the CL intensities of three continuous determinations of CS2 on these catalyst
surfaces decline significantly in sequence, indicating that CS2 cause the deactivation
of catalysts. Similar phenomenon was also observed during the synthesis process of
ammonia and methanol etc, which was attributed to sulfurꢀpoisoning.39,40
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Unexpectedly but Fortunately, we found that strong CL signal also can be observed
when CS2 was heated in air without any catalysts. Therefore, we used a bare SSHT as
heating element for CL detection of CS2. As the Figure 2A shows, the use of SSHT
for CL detection of CS2 overcomes the adverse effect from catalyst poisoning, which
also endows this CL system with excellent selectivity to CS2. The selectivity of this
CL system was evaluated by injecting in total of 70 compounds (listed in the
Supporting Information) into the CL reaction cell, no CL emission was observed.
Consequently, we employ the miniature SSHT to induce the CL emission of CS2 for
the subsequent experiments.
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Figure 2A also shows that the response speed of the CL detection to CS2 is very
quick. It can be seen that each injection only acquire a single signal point, indicating
that the CL signal of CS2 vanishing just as suddenly as it appears. However, the
signals of the previous reported CL reaction systems generally must spend several
seconds to reach the maximum and dozens of seconds to decay to the background.37,41
In order to give a more concrete comparison, the CL reactions of nꢀpropanol and
acetone on SrCO3 surface were investigated under the same conditions. As Figure 2B
shows, the CL signals of the two reaction systems last over twenty seconds, the
recovery times are much longer than that of CS2.
Mechanism Study. To study the mechanism behind the CL from the thermal
oxidation of CS2, the GCꢀMS experiment was performed to investigate the reaction
products of the dissociation products of CS2. As shown in Figure 3A, the thermal
degradation of CS2 yields carbon monoxide (CO), carbon dioxide (CO2), sulfur
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dioxide (SO2) and carbonyl sulfide (OCS). We also found that CL could not be
observed when replaced clean air with high purity nitrogen, and no any other products
were detected except original CS2. These results fully demonstrate that oxidant is
necessary for the CL emission during the thermal degradation of CS2.
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Gaseous CS2/O2 mixtures were observed to laser in flash photolysis and pulsed
electrical discharge system.42 It was reported that one of the possible channel
reactions for the oxidation of of CS2 is to yield the primary products of OCS and S.
Subsequently, OCS reacts easily with oxygen atoms to form CO and SO radicals, and
*
then SO radicals are immediately transformed to SO2 at excited state (SO2 ), the
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decay of SO2 to its ground state results in photoemissions.43 The SO2 afterglow is a
strong CL continuum emission band which is reported in the literature to be extends
from ultraviolet to blue wavelength with a maximum at about 365 nm, and with some
discrete features between 380 and 469 nm.44, 45 The emission spectrum of our study is
given in Figure 3B, the broad band in the range 350 to 550 nm with a maximum at
368 nm matches well with the SO2 afterglow spectrum. In addition, it was well known
*
that CO also can react with oxygen atoms to form CO2 at excited state (CO2 ),
accompanying with photoemission.46, 47 The CO2* feature which is reported in the
literatures is particularly strong in the CS2 thermal oxidation.
To further study the mechanism, calculations of reaction processes were
performed by using the Gaussian 03 suite of programs. The optimized structures of
reaction species were successfully obtained by Gaussian’s implementation of the
B3LYP hybrid density functional method using the 6ꢀ311++G** basis set for all
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atoms. The thermodynamic energies of the reactants, transition states, intermediate
state and products are shown in Table S2 in the Supporting Information. It can be seen
that the energy of the species in triplet state are lower than they in singlet state,
indicating that the triplet species are more thermodynamically accessible during the
reaction process. Therefore, according to our experimental results and the relevant
data in the literatures, the thermal oxidation mechanism of CS2 is suggested by
reaction paths shown in the Figure 4, and the reactions of 4a and 4b are supposed to
the mainly responsible for the CL emission. According to the suggested reaction paths,
the total reaction of CS2 decomposition can be expressed as:
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CS2 + 3O2 = CO2 + 2SO2 + hv
(1)
According to the Table S2, the enthalpy energy change (ꢁrH°), and Gibbs
freeꢀenergy change (ꢁrG°) of the corresponding reactions can be obtained from the
energy differences between the reactants and the products; the activation energy (ꢁEa)
can be obtained from the energy difference between the reactants and the transition
states. The ꢁrH°, ꢁrG° and ꢁrEa of the corresponding reaction are shown in Table 1,
and the energy profiles are illustrated in Figure S2 in the Supporting Information.
These results show the suggested reaction paths are thermodynamically accessible for
their low activation energies. The path of oxygen dissociation to produce oxygen
atoms requiring large amounts of energy of 79.6 kcal molꢀ1, while the ꢁrH° and ꢁrG°
of the other reaction paths are negative, indicating that these reactions are
considerably exothermic and feasible from the thermodynamic point of view. The
largest negative value of ꢁrH° and ꢁrG° of the total reaction are ꢀ193.4 kcal molꢀ1 and
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ꢀ183.7 kcal molꢀ1, respectively. For a CL reaction, the required energy ( ꢀE
, kcal
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hc
(2)
ꢀE =
N A
4.18 λ
Where h is Planck’s constant (6.626 × 10−34 J·s), c is the speed of light (3 × 108
m s−1), λ is the wavelength of the CL transition, and NA is Avogadro’s number (6.022
× 1023 mol−1). Since the emission spectrum is present at wavelengths between 350 and
550 nm, an average wavelength 450 nm is used for the calculation. The ꢀE is
calculated to be 63.6 kcal molꢀ1. The ꢁrH° (193.4 kcal molꢀ1) of the reaction very
much exceeds the ꢀE , and it is then identified as energetic enough to produce excited
state.
As described above, many sulphurꢀcontaining compounds can release CS2, and
hence we can make full use of the CL emission of CS2 to indirect detect these
compounds. The work mechanism of the methodology can be described as:
Release
[O]
Sulphur ꢀ containing compound →CS2 →CO2 + SO2 + hv (3)
Application. Obliviously, this interesting light emission not only is able to direct
determine the concentration of CS2, but also can be expanded to other
sulfurꢀcontaining compounds via converting them to CS2. We coupled the CL reaction
cell to a CS2 generator to construct CS2ꢀgenerating DS for the demonstration of the
methodology in this dimension. In such system, CS2 is first released by certain
process occurred in the generator, and then is introduced into the CL cell for further
analysis. As a proof ofꢀprinciple work, mancozeb, one of the most widely used DTC
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pesticides in agriculture, was selected as representative analyte. Headspaceꢀgas
chromatography (HSꢀGC) is one of the most commonly used methods for the
determination of the DTC residues.31,33 It involves acid hydrolysis of DTCs to release
CS2, and then a certain volume of CS2 is injected into the instrument after
vaporꢀliquid equilibrium. However, such method is timeꢀconsuming because of it
takes a long time to establish vaporꢀliquid equilibrium, and requires separation
procedure for subsequent analysis. Moreover, they are prone to errors due to the
offꢀline incomplete operations. We expect that the system shown in Figure 1 can
realize onꢀline and largeꢀvolume detection of CS2 released by DTCs under
nonꢀequilibrium condition, and thereby to effectively reduce the analysis time.
1. Feasibility. The precondition of the CS2ꢀgenerating DS to realize onꢀline and
largeꢀvolume detection is that serious trailing in CL response curve must be avoided.
In order to demonstrate the feasibility, 1 mL of CS2 at 4 mg Lꢀ1 was added into the
generator, and then was pretreated as the procedure described in sample preparation
section. The results of three parallel determinations are shown in Figure 5A, we were
pleased to find that the CL response curves show no signs of peak tailing, the signals
decay to background within 15 s. In order to further accentuate the unique CL
response characteristics of CS2, the CL response curves of nꢀpropanol and acetone on
SrCO3 surface were investigated under the same conditions. As shown in Figure 5B,
serious tailing in peak on CL response curves of the two reaction systems are
observed, which will affect the quantitative accuracy. This is the rapid response speed
of the CL detection of CS2 enables the realization of the nonꢀequilibrium and
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largeꢀvolume detection. The CS2ꢀgenerating DS allows the whole CS2 in the generator
to be transferred into the CL cell under nonꢀequilibrium condition, the analysis time is
therefore reduced greatly. In addition, the onꢀline detection also reduce errors because
of it avoids the volatilization loss of CS2.
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2. Conditions Optimization. First, the effect of the time on the acid hydrolysis
efficiency of mancozeb was investigated. As Figure S3 in the Supporting Information
shows, the decomposition of mancozeb can finish in 30 min under ultrasonic bath
condition, while above this value the signal remains constant. Therefore, 30 min was
selected as the optimum time of acid hydrolysis.
Next, the volume of the hydrolysis reagent was optimized. As shown in Figure
S4 in the Supporting Information, the peak area increases with the increase of volume
of the hydrolysis reagent and reaches its maximum at 30 mL. However, the peak area
decreases significantly when the volume is above 30 mL, possibly due to the
headspace of the vessel decreases with the increase of volume of the hydrolysis
reagent.
Finally, the working temperature and flow rate were optimized. The results are
shown in Figure S5 in the Supporting Information. It can be seen that the optimum
working temperature and flow rate are 368 °C and 150 mL minꢀ1, respectively.
Analytical characteristics. Under the optimal detection condition, calibration curves
for CS2 and mancozeb were prepared by plotting the area versus their concentrations,
the results are shown in Figure S6. The linear range of CS2 and mancozeb are 0.32ꢀ82
mg Lꢀ1 and 1.2ꢀ120 mg Lꢀ1, and the detection limits are 0.24 and 0.8 mg Lꢀ1 for CS2
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and mancozeb, respectively. These results fully demonstrate the CS2ꢀgenerating DS is
able to quantify CS2 and other sulphurꢀcontaining compounds that can convert to CS2.
The relative standard deviations (RSDs) for peak height and peak area of five
replicate determinations of 4 mg Lꢀ1 of CS2 are 3.3% and 1.4%, and are 4.3% and
2.8% for five replicate determinations of 4 mg Lꢀ1 of mancozeb, respectively.
Obviously, the peak area is more reproducible than peak height, which is the reason of
the peak area is used for quantization purpose.
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It is worth pointing out that the signal of CS2 evolved from DTCs is not equal to
the same amount of the pure CS2, leading to the regression equation constructed by
the pure CS2 standards is not the same as that constructed by CS2 evolved from DTCs
standards.31,48 In addition, DTCs with different polymeric structures evolve different
stoichiometric amount of CS2 for the different sulphur groups in their chemical
structures (the more detailed state can be seen in the Supporting Information).31,48
Because the current legislation for routine enforcement at international levels is based
on the determination of DTCs as CS2, and the existing maximum residue level for
DTC residues specified worldwide as mg CS2/kg food.31,48,49 Similar with other
reported methods, when we cannot ascertain the identities of DTCs or several kinds of
DTCs coexist in the samples, CS2 standards are preferable for the compliance with
MRLs;31,36 however, it is recommend to use the specific DTCs suitable to be present
in the samples as standard when only one pesticide will be determined and its identity
is well known..48
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Although the CL detection exhibits excellent selectivity, the presence of CS2 not
related to the use of the DTC fungicides will cause positive interference in the
evaluation of DTC residues using the method based on the evolution of CS2.
Lꢀmethionine and Lꢀcysteine are important amino acids containing sulphur in
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foodstuffs,
and
ethylene
thiourea
is
the
main
metabolites
of
ethyleneꢀbisꢀdithiocarbamates. Interferences from these compounds were investigated
by adding 1 mL of these compounds at 100 mg Lꢀ1 into the generator, and then was
analyzed as the same procedure as mancozeb. In addition, interferences from some
organosulfur, organophosphorous and organochlorine pesticides (1 mL of 100 mg Lꢀ1),
including malathion, tolclofosꢀmethyl, iprobenfos, chlorpyrifosꢀmethyl, sulfathiazole,
sulfamethizole, sulfachloropyridazine, sulfadiazine, methamidophos, phorate,
demeton,
phenthoate,
dimethoate,
simazine,
atrazine,
chlorothalonil,
hexachlorobenzene and dichlorvos were also investigated. The results show that all of
these compounds cannot produce CL signal for they cannot decompose into CS2,
which can be possibly attributed to the lack of sulphur–carbon–sulphur bond in their
chemical structure.
Recently, methods based on UV/vis spectroscopy50 and surface enhanced Raman
spectroscopy (SERS)51 were developed for the rapid detection of DTCs. These
methods have advantages of simple instrument, high sensitivity and inꢀfield
application. However, the uses of gold nanomaterials as colorimetric probes or SERS
substrates require expensive and tedious synthetic process to prepare nanomaterials.
Especially, the gold nanomaterials are disposable during the analytical processes,
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which against the reduction of the analytical price. CL methods with lower limits of
detection were also developed,52,53 but they are the same as the SERS methods that
only for the detections of some specific DTCs, but not a generic method for total
DTCs analysis. Our work provides a robust, reversible and inexpensive method for
total DTCs analysis, due to it without using nanometer catalyst, and only consumes
the sample and oxygen from the air during the CL detection process.
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Samples Analysis. In real sample analysis, a lot of water together with other matrix
components will interfere with the detection of CS2. In this account, separation or
extraction steps are necessary for the traditional methods, which leads to these
methods are complex and timeꢀconsuming. The excellent selectivity of the CL
detection suggests that the developed method might be directly applied for detecting
DTC residues in real samples. Considering the MRLs for total DTCs (expressed as
CS2) in mung bean and barley were established at 5.0 mg kgꢀ1 and 2.0 mg kgꢀ1 in EU,
respectively. Therefore, recovery studies were carried out at levels of 1.0, 5.0 and 8.0
mg kgꢀ1 for mancozeb in mung bean and barley to investigate whether our method
could be utilized to analysis the residues of DTCs in real samples. The results are
shown in Table 2, satisfactory recoveries ranging from 76.9 to 97.3% were obtained,
which is an indicator of its good accuracy and practicability for use with real samples.
Significantly, through preparing the 30 samples (two types of crops × three
levels of spiked concentration × five replicate detections) in different generators, the
acid hydrolysis of the samples can be performed as planned in an ultrasonic bath, and
then these samples were analyzed in sequence after 30 min ultrasonic treatment. The
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analytical time of each CL detection is about 15 s (Figure 5A), the batch processing of
sample enables the analysis of 30 samples to be finished in about 45 min, indicating
that our method is particularly promising for customs inspection in where a lot of
agricultural products need analyzing for the evaluation of the residual level of DTCs.
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CONCLUSION
In conclusion, we have studied the CL phenomenon from the thermal oxidation
of CS2 to exploit its analytical application. Experimental investigations together with
theoretical calculation were performed to reveal the mechanism behind the CL
reaction. On the basis of this light emission, a CS2ꢀgenerating DS was developed for
rapid measurement of CS2 or other sulphurꢀcontaining compounds that can convert to
CS2. The usefulness of the system was demonstrated by measuring DTC pesticides
based on the evolution of CS2 in spiked agricultural products. Although further
development is still needed to miniaturize and integrate the system for the better
inꢀfield application, at this stage it could be used as a rapid screening tool in
laboratories to evaluate the need for more detailed analysis of DTCs. In addition, it
also has promising applications in other related fields such as direct monitoring CS2 in
air, monitoring natural process accompanying with CS2 release, or indirectly
determinate xanthate in sulfide ore flotation etc.
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■ ASSOCIATED CONTENT
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Supporting Information
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Additional information as noted in text. This material is available free of charge via
the Internet at http://pubs.acs.org
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■ AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: cesgkl@mail.sysu.edu.cn
*Eꢀmail: huyufei@mail.sysu.edu.cn
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
The authors would like to thank the National Natural Science Foundation of
China for financially supporting this research under grant numbers 21475153,
21127008 and 21105133, and Major National Scientific Instrument and Equipment
Development Project (2011YQ03012409), respectively. We thank Dr. Shen Wei at
Southwest University for the great help of the theoretical calculation.
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partitioning
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Figure 1. The schematic diagram of the CS2ꢀgenerating and detection system.
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Figure 2. (A) The CL kinetic curves of CS2 under offꢀline injection mode using bare
SSHT to induce light emission. (B) The CL kinetic curves of nꢀpropanol and acetone
under offꢀline injection mode using SrCO3 to assist the light emission. Working
temperature, 368 °C; wavelength, 380 nm; air flow rate, 150 mL min−1;
concentrations of CS2, nꢀpropanol and acetone, 4 mg Lꢀ1; volumes of sample injection,
1 mL.
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Figure 3. (A) The GCꢀMS chromatogram of reaction products from the thermal
oxidation of CS2. The red line stands for the result when using high purity nitrogen as
carrier gas, no new product was produced. The black line stands for the result when
using high purity oxygen as carrier gas. (B) The CL spectra of the thermal oxidation
of CS2 measured with spectrofluorophotometer and BPCL ultraweak luminescence
analyzer. The two CL spectra were similar, but the spectrum obtained with the
spectrofluorophotometer showed higher spectral resolution, since only several optical
filters instead of a monochromator were used in the BPCL luminescence analyzer for
the CL analysis. CL reaction condition: working temperature, 368 °C; air flow rate,
150 mL min−1; concentrations of carbon disulfide, 4 mg Lꢀ1.
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Figure 4. Possible reaction processes for thermal oxidation of carbon disulfide.
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Figure 5. (A) The onꢀline and largeꢀvolume detection of CS2 under nonꢀequilibrium
condition. (B) The onꢀline and largeꢀvolume detection nꢀpropanol and acetone under
nonꢀequilibrium condition. Working temperature, 368 °C; wavelength, 380 nm; air
flow rate, 150 mL min−1; concentrations of CS2, nꢀpropanol and acetone, 4 mg Lꢀ1
;
volumes of the three compounds added into the generator, 1mL.
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Table 1. Thermodynamic energy (kCal molꢀ1, 641 K) of possible reactions
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Reactions
ꢁrH°
79.6
ꢁrG°
71.9
ꢁrEa1
—
ꢁrEa2
—
1
2
ꢀ55.7
ꢀ13.5
ꢀ6.5
ꢀ55.9
ꢀ15.4
ꢀ7.1
3.1
16.6
6.0
—
3a
1.2
3b
30.6
30.5
37.1
—
4a
ꢀ127.7
ꢀ132.5
ꢀ193.4
ꢀ118.4
ꢀ124.2
ꢀ183.7
—
4b
—
Total
—
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