Gas-phase acid-base properties of melamine and cyanuric acid

The thermochemical properties of melamine and cyanuric acid were characterized using mass spectrometry measurements along with computational studies. A triple-quadrupole mass spectrometer was employed with the application of the extended Cooks kinetic method. The proton affinity (PA), gas-phase basicity (GB), and protonation entropy (Δ_pS) of melamine were determined to be 226.2 ± 2.0 kcal/mol, 218.4 ± 2.0 kcal/mol, and 26.2 ± 2.0 cal/mol K, respectively. The deprotonation enthalpy (Δ_acidH), gas-phase acidity (Δ_acidG), and deprotonation entropy (Δ_acidS) of cyanuric acid were determined to be 330.7 ± 2.0 kcal/mol, 322.9 ± 2.0 kcal/mol, and 26.1 ± 2.0 cal/mol K, respectively. The geometries and energetics of melamine, cyanuric acid, and related ionic species were calculated at the B3LYP/6-31+G(d) level of theory. The computationally predicted proton affinity of melamine (225.9 kcal/mol) and gas-phase deprotonation enthalpy of cyanuric acid (328.4 kcal/mol) agree well with the experimental results. Melamine is best represented as the imide-like triazine-triamine form and the triazine nitrogen is more basic than the amino group nitrogen. Cyanuric acid is best represented as the keto-like tautomer and the N-H group is the most probable proton donor.     

Chiral recognition capabilities of melamine and cyanuric acid supramolecular structures

Currently, there are numerous papers that discuss local chiral domains in supramolecular structures of achiral molecules established using the STM method, and by using DFT calculations. However, there are no data regarding the obtainment of macroscopically chiral 2D-supramolecular structures from achiral molecules. In this study, melamine and cyanuric acid supramolecular structures were self-assembled on a graphitized carbon black surface, which had a surface structure that was identical to HOPG, and also on the surface of an inert solid support for chromatography. Chirality induction according to the Kondepudi effect was used. For the supramolecular structures, MD calculations showed the possibility of obtaining a chiral structure. To establish macroscopic chirality, we proposed the use of the difference in enantiomer adsorption on the modified adsorbents. For this, two indirect methods were used: static adsorption with a polarimetric control and gas chromatography. Both methods indicated the chiral recognition ability of the adsorbents used.     

气相色谱-质谱法快速测定牛奶中的三聚氰胺和三聚氰酸

建立了气相色谱-质谱法测定牛奶中三聚氰胺和三聚氰酸含量的分析方法。样品经二乙胺-乙腈-水溶液超声提取,离心,滤液用氮气吹干后加入硅烷化衍生试剂衍生,气相色谱-质谱测定,选择离子模式监测,外标法定量。在0.025～2 mg/kg范围内,目标物的峰面积与其质量浓度的线性关系良好(r>0.98);在0.5,1.0和2.5 mg/kg的添加水平,三聚氰胺和三聚氰酸的平均回收率分别为84%～87%和75%～102%,相对标准偏差(RSD)分别为5.7%～11.7%和4.9%～7.8%;三聚氰胺和三聚氰酸的检出限(LOD)分别为0.05 mg/kg和0.10 mg/kg。结果表明:该方法简便、快速、灵敏、准确,适合牛奶中三聚氰胺和三聚氰酸的确证和定量测定。     

HPLC同时检测食品接触材料中三聚氰胺与三聚氰酸单体迁移量

建立了一种用液相色谱技术检测分析食品接触材料中三聚氰胺与三聚氰酸单体迁移量的方法.样品采用水、4%乙酸和10%乙醇3种模拟物进行浸泡.色谱分离采用Phenomenex Luna NH2柱,柱温:30 ℃,流动相为乙腈-5 mmol/L,pH=6.5 PBS缓冲溶液(75∶25,V/V),流速1.0 mL/min,DAD...     

反相高效液相色谱法同时分析三聚氰酸和三聚氰胺

建立了同时分析三聚氰酸和三聚氰胺的反相高效液相色谱法．发现三聚氰酸和三聚氰胺在C8柱上的保留比在C18柱上明显要强,这一结果表明三聚氰酸和三聚氰胺在反相固定相中的保留并非以通常的疏水分配作用为主导．方法的紫外检测下限为0．034～0．31mg／L,标准曲线线性范围为0．5-100mg／L．方法不经特殊的样品前处理即可用于奶粉和游泳池水等样品中三聚氰酸和三聚氰胺的同时分析．     

Dynamic interaction between melamine and cyanuric acid in artificial urine investigated by quartz crystal microbalance

The dynamic interaction between melamine (M) and cyanuric acid (CA) in artificial urine was investigated by a flow injection analysis-quartz crystal microbalance (FIA-QCM) system. Melamine was used as the recognition element and immobilized onto the QCM gold surface. The process of M and CA interaction was recorded by FIA-QCM in real-time. The multilayer complex of M and CA was successfully formed on the crystal surface of the QCM when CA and M were introduced into the FIA-QCM system alternately. The influence of pH on the M and CA interaction indicated that the M-CA multilayer complex possesses high stability over a wide range of pH values, especially in physiological urine condition. The association and dissociation rate constants were determined under artificial urine condition and the association constant for M and CA was calculated as 10(2) L mol(-1). The M-CA complex was further characterized by Fourier transform infrared spectroscopy (FT-IR) and atomic force microscopy (AFM). Mechanism study suggested that the formation of the multilayer complex in artificial urine was mainly attributed to the intermolecular hydrogen bonding between M and CA. The work provides direct insight into the dynamic interaction between M and CA under physiological conditions and would be helpful for the research of kidney stones induced by melamine contamination.     

A novel type of hydrogen-bonded assemblies based on the melamine.cyanuric acid motif

This paper reports the formation of novel hydrogen-bonded assemblies 1(3).CA obtained upon mixing cyanuric acid (CA) with melamine derivatives 1, in which two of the three possible H-bonding arrays have been blocked. The four components are held together by 9 hydrogen bonds and form a rigid planar structure in which a central CA (three ADA motifs: A = acceptor, D = donor) is hydrogen bonded to three peripheral melamine derivatives (DAD motif). Furthermore, the synthesis and assembly studies are described of hydrogen-bonded assemblies 2-4.CA, comprised of three melamine derivatives that are covalently connected, and CA. The overall thermodynamic stability of assemblies 2-4.CA is superior to 1(3).CA (I(Tm) = 9 vs 3.6). The presence of the 2.CA complex in chloroform was confirmed by (1)H NMR spectroscopy and MALDI-TOF mass spectrometry. Substitution of the trimelamines with chiral or fluorescent groups (R(3)) enabled the study of the assemblies by CD and fluorescence spectroscopy. Titration experiments revealed strongly enhanced stabilities even in the presence of polar solvents, such as THF and CH(3)OH. Depending on the polarity of the solvent, stacking between the planar assembly units was observed.     

Colorimetric and visual read-out determination of cyanuric acid exploiting the interaction between melamine and silver nanoparticles

We present two colorimetric procedures for the determination of cyanuric acid, using silver nanoparticle-based (AgNPs) probes. The first is making use of melamine-modified AgNPs which bind to cyanuric acid through hydrogen bonding to form a large conjugate network that enhances the aggregation of AgNPs to produce an absorbance peak at 640 nm and a green coloration. In the second assay, melamine is directly added to the sample in order to form a stable complex with cyanuric acid. AgNPs are then added, resulting in the formation of an absorbance peaking at 525 nm and a color change from green (blank sample) to purple or orange-red as a function of cyanuric acid concentration. Matrix effects, that originate from the interaction of alkaline earth metals with the charged surface of the AgNPs, are mitigated through a matrix-matched calibration. In this manner, spectral transitions can be selectively attributed to the concentration of cyanuric acid, which can be even visually quantified at low mg L^?1 levels with minimum sample pre-treatment and without sophisticated instrumentation.

固相萃取-亲水相互作用色谱/串联质谱法同时测定食品中三聚氰胺和三聚氰酸

建立了固相萃取-亲水相互作用色谱/串联质谱同时测定食品中三聚氰胺和三聚氰酸残留量的方法。采用乙腈和水提取试样中残留的三聚氰胺和三聚氰酸,正己烷脱脂,提取液经亲水性键合硅胶和阳离子交换树脂复合填料柱(MCT柱)净化。采用亲水相互作用色谱柱进行分离,质谱采用正、负离子切换模式电离,多反应监测模式检测,同位素内标法定量。三聚氰胺和三聚氰酸在10～2500 μg/L范围内呈线性相关,相关系数(r)均大于0.99,定量限分别为25和50 μg/kg。本方法在动物源性食品、植物源性食品、乳及乳制品等不同样品中的三聚氰胺和三聚氰酸高、中、低3个添加水平的回收率分别在70.0%～129.6%和70.0%～128.6%之间,相对标准偏差分别在1.4%～23.3%和2.8%～18.7%之间。该方法可满足食品中三聚氰胺和三聚氰酸同时定量测定的需要。     

Rapid determination of melamine and cyanuric acid in milk powder using direct analysis in real time-time-of-flight mass spectrometry

The use of fast semi-automated method employing direct analysis in real time (DART) ion source coupled to time-of-flight mass spectrometry (TOFMS) for analysis of melamine (MEL) and cyanuric acid (CYA) in milk powder and milk based products has been demonstrated in this study. Simple sample extraction procedure employing methanol–5% aqueous formic acid mixture, which enabled disruption of melamine–cyanurate complex, was followed by direct, high-throughput (30 s per run) examination of sample extracts spread on a glass rod by mass spectrometry under ambient conditions, without any prior chromatographic separation. After optimization of instrument parameter settings, limits of detection (LODs) 170 and 450 μg kg⁻¹ were achieved for MEL and CYA, respectively. In the final phase of study, the possibility of minimizing spectral interference, thus improving method performance characteristics through the use of ultrahigh resolving power offered by Orbitrap based mass analyzer is demonstrated.     

亲水作用色谱-串联质谱法同时测定液态奶中三聚氰酸和三聚氰胺

建立了亲水作用色谱-串联质谱同时测定液态奶中三聚氰酸和三聚氰胺的方法。液态奶样品经体积分数2.5%甲酸溶液提取、离心后乙腈稀释,亲水作用色谱柱分离,电喷雾串联四极杆质谱检测器检测,分别在负、正离子模式下测定三聚氰酸和三聚氰胺。三聚氰酸和三聚氰胺分别在0.5～100μg/L、0.1～50μg/L范围内线性关系良好。在0.25～15mg/kg、0.1～7.5mg/kg添加水平范围内,三聚氰酸平均回收率为84.5%～98.0%(RSD为2.1%～6.1%),三聚氰胺平均回收率为85.5%～88.9%(RSD为3.2～5.8%)。三聚氰酸、三聚氰胺定量限分别为0.25mg/kg、0.1mg/kg。     

Determination of melamine and related triazine by-products ammeline, ammelide, and cyanuric acid by micellar electrokinetic chromatography

In this study, micellar electrokinetic chromatographic (MEKC) methods were developed for the detection of traces of melamine and its related by-products (ammeline, ammelide, and cyanuric acid). Two on-line sample concentration steps namely reversed electrode polarity stacking mode (REPSM) and cation-selective injection (CSI) were used for improving the detection sensitivity. For REPSM, a borate-NaOH buffer (pH 10, 35 mM) composed of 60 mM SDS and 10% (v/v) methanol, was used as carrier electrolyte, and samples were prepared in an aqueous solution of 10 mM NaOH. In CSI, a phosphate buffer (pH 2, 50 mM) containing 41 mM SDS was used as the carrier electrolyte, and samples were prepared with an aqueous solution of 10 mM NaOH and a phosphate buffer (pH 2.0, 25 mM) in a volume ratio of 1:9. The results indicated that REPSM enhanced all analyte signals except for melamine, which could be concentrated only by the CSI. The detection limit was reduced from 1.7 mg L⁻¹ to 2.8 μg L⁻¹ for melamine by the optimal CSI step, and from 0.23-1.2 mg L⁻¹ to 2.4-5.0 μg L⁻¹ for the other three analytes by the optimal REPSM step. Tableware made of melamine and samples of flour were used as test samples, and the results indicated that the proposed MEKC methods can successfully determine contaminations from melamine. The study also indicated that when the plastic made of melamine was exposed only once to an acidic solution (acetic or phosphoric acid) at 80 °C for 30 min, melamine continuously leached out from the test sample even without any further treatment with an acidic solution.     

Using an on-line microdialysis/HPLC system for the simultaneous determination of melamine and cyanuric acid in non-dairy creamer

The recent revelation of melamine (MEL) contamination in foodstuffs in China has rocked the international public health community. Many food categories have been involved in this scandal, including non-dairy creamer (NDC). In this study, we investigated the use of hollow-fiber microdialysis (MD) sampling coupled on-line with high-performance liquid chromatography (HPLC) as an alternative to sample pretreatment for the direct determination of MEL and its analogue cyanuric acid (CYA) in NDC. After MD sampling, the dialysate was injected on-line into the chromatographic system for analysis of MEL and CYA with UV detection at 203 nm. We monitored the effects of various parameters affecting the MD efficiency, namely the characteristics of the MD probe membrane, the flow-rate and the nature of the polarity modifier in the perfusion stream, and the addition of salt in the sample solution. The optimal enrichment efficiency for collecting MEL and CYA from aqueous NDC samples occurred with MD sampling using a hollow polysulfone MD fiber and MeOH as the perfusate at a flow rate of 10 μL min⁻¹. The optimized chromatographic conditions involved using a reversed-phase phenyl column and a mobile phase of 5 mM phosphate buffer in 10% (v/v) MeOH, buffered at pH 6.5. Detection was linear in the concentration range from 0.02 to 5 ppm for MEL and from 2 to 100 ppm for CYA, with detection limits of 1 ppb for MEL and 30 ppb for CYA. The volume of perfusate required to extract MEL and CYA from the NDC solution was only 21 μL. The total MD sampling time was 2.1 min. This method allows the sensitive, eco-friendly, and rapid determination of MEL and CYA in NDC—a risk food for economically motivated adulteration.     

超高效液相色谱-串联质谱法同时测定牛奶和奶粉中的二氰二胺、三聚氰胺和三聚氰酸

建立了一种超高效液相色谱-串联质谱法测定牛奶和奶粉中二氰二胺(DCD)、三聚氰胺(MEL)和三聚氰酸(CY)的分析方法。样品用5%(质量分数)三氯乙酸水溶液提取,无需净化和浓缩,用Acquity UPLC HSS T3色谱柱(50 mm×2.1 mm, 1.8 μm)分离,以乙腈和乙酸铵作为流动相进行梯度洗脱,电喷雾正、负离子(ESI^±)模式同时电离,多反应监测模式进行检测。结果表明:在5.0～200.0 μg/L范围内线性关系良好(r≥0.995)。牛奶中3种化合物在0.02、0.10、0.20 mg/kg的添加回收率为60.0%～105.8%,相对标准偏差(RSD, n=6)为4.2%～13.6%;奶粉中3种化合物在0.05、0.10、0.20 mg/kg的添加回收率为78.0%～115.0%, RSD (n=6)为2.7%～7.5%。牛奶中方法的定量限(信噪比为10)为0.02 mg/kg,奶粉中方法的定量限为0.05 mg/kg。该方法简单、快速、灵敏、准确,适合于牛奶、奶粉等乳及乳制品中DCD、MEL和CY的快速、高灵敏的分析检测。     

Molecules to supermolecules and self assembly: a study of some cocrystals of cyanuric acid

The preparation and structure elucidation of cocrystals 1a, 1b, 2a-4a, formed from cyanuric acid (CA) and the aza-donor compounds 4,7-phenanthroline, 1,7-phenanthroline, phenazine and 1,3-bis(4-pyridyl)propane, respectively, have been reported. While CA forms different types of self-assembling modes—monomers (1a), dimers (1b and 4a) and infinite tapes (2a and 3a)—the recognition of the constituents, however, is through a triple hydrogen-bonding pattern, consisting of an N-H?N and two C-H?O hydrogen bonds, except in 4a.     

Determination of cyanuric acid in swimming pool water by differential pulse polarography

A reliable differential pulse polarographic method is described for the determination of cyanuric acid (1,3,5-triazine-2,4,6-triol) in pool water. Cyanuric acid in the range 10^?5–10^?3 M is determined by means of the peak at ca. –60 mV (vs. Ag/AgCl/3 M NAcl). The high sensitivity of the polarographic technique allows ten-fold dilution of samples, thus avoiding matrix effects. It it shown that the peak can be attributed to formation of insoluble mercury(I) cyanurate, Hg₂(HC₃N₃O₃), at the mercury electrode.     

Simultaneous determination and confirmation of melamine and cyanuric acid in animal feed by zwitterionic hydrophilic interaction chromatography and tandem mass spectrometry

We developed a new method to analyze animal feed and feed ingredients for melamine and cyanuric acid. The method is capable of extracting and detecting both melamine and cyanuric acid in a single procedure, whether present as free compounds or bound together as the melamine:cyanurate complex. A novel chromatographic system based on zwitterionic hydrophilic interaction chromatography (ZIC-HILIC) columns enables separation and detection of both compounds in one run. Samples are extracted with a strong aqueous acid which is then diluted to bring the concentration within the working range of the method. The method is applicable over the range of 0.5 to 50 micrograms/gram (microg/g). Samples at higher concentrations may be diluted into this range, which is equivalent to 3.6-360 ng/mL in the injection solvent. Analytes are detected using liquid chromatography/tandem mass spectrometry. The data confirm the presence of both compounds according to criteria recommended by the US FDA Center for Veterinary Medicine. The LC/MS/MS method provides an alternative to derivatization and gas chromatography-mass spectrometry for regulatory analysis of feed samples. Published in 2008 by John Wiley & Sons, Ltd.     

Photoresponsive melamine barbiturate hydrogen-bonded assembly

UV-irradiation of hydrogen-bonded assembly between trans-azobenzene-incorporated melamine and barbiturate in chloroform drastically suppressed transformation from soluble cyclic rosette into insoluble tape-like polymers.     

FTIR spectroscopy coupled with multivariate classification methods to identify different powdered infant formulas adulterated with melamine and cyanuric acid

Fourier transform infrared spectroscopy (FTIR) is a nondestructive, simple, rapid, and cheap measurement technique for analysis of many multicomponent chemical systems, e.g., detection of adulterants in food samples. In this respect, this study proposes combining FTIR spectroscopy with multivariate classification methods for classification and discrimination of different samples of infant formulas adulterated by melamine or/and cyanuric acid. Different parametric and non-parametric multivariate classification methods including the linear discriminant analysis (LDA), partial least squares-discriminant analysis (PLS-DA), soft independent modeling of class analogy (SIMCA), K-nearest neighbors (KNN), and classification and regression tree (CART) approaches were used to classify the recorded FTIR data. Assessing the performance of the multivariate methods according to their sensitivity, specificity and percent of correct prediction results demonstrated that coupling FTIR spectroscopy with multivariate classification can be applied as a rapid and powerful technique to the simultaneous detection of melamine and cyanuric acid in powdered infant formulas. This combinatorial method is efficient for adulterant concentrations as low as 0.0001 w/w%.     

Membrane sampling with microdialysis coupled to HPLC/UV for on-line simultaneous determination of melamine and cyanuric acid in non-dairy coffee creamer

An on-line microdialysis/high-performance liquid chromatography method was developed for the simultaneous determination of melamine and cyanuric acid in non-dairy coffee creamer. To collect these analytes from aqueous samples, the microdialysis system featured a microdialysis probe incorporating a polyarylethersulfone membrane and employed 0.05 M HCl in 0.1% (v/v) MeOH as the perfusate, with optimal efficiency obtained at a flow rate of 1 μL min(-1). The chromatographic conditions were optimized when using a reverse-phase phenyl column and a mobile phase of phosphate buffer solution in 10% (v/v) MeOH, buffered at pH 3.0. Good linearity relationship (r(2) > 0.9987), intra- and inter-day precisions (RSDs < 6.6%), recoveries (96.9 - 105.0%), and limits of detection (melamine, 3 ppb; cyanuric acid, 150 ppb) were observed for the two analytes. This method has been successfully applied to simultaneous determination of melamine and cyanuric acid in commercial creamers with the recoveries in the range of 97.5 to 102.6%.