Vacancy ion-exclusion chromatography of carboxylic acids on a weakly acidic cation-exchange resin

In this preliminary study, a new approach to ion-exclusion chromatography is proposed to overcome the relatively poor conductivity detection response which occurs in ion-exclusion chromatography when acids are added to the eluent in order to improve peak shape. This approach, termed vacancy ion-exclusion chromatography, requires the sample to be used as eluent and a sample of water to be injected onto a weakly acidic cation-exchange column (TSKgel OApak-A). Vacancy peaks for each of the analytes appear at the retention times of these analytes. Highly sensitive conductivity detection is possible and sharp, well-shaped peaks are produced, leading to efficient separations. Retention times were found to be affected by the concentration of the analytes in the eluent, and also by the presence of an organic modifier such as methanol in the eluent. Detection limits for oxalic, formic, acetic, propionic, butyric and valeric acids were 0.1, 0.2, 0.3, 0.3, 0.4 and 0.5 microM, respectively, and linear ranges for some acids extended over two orders of magnitude. Precision values for retention times were 0.21% and for peak areas were <1.90%. The vacancy ion-exclusion chromatography method was found to give detection responses four to 10 times higher than conventional ion-exclusion chromatography using sulfuric acid eluent and two to five times higher than conventional ion-exclusion chromatography using benzoic acid eluent.     

Vacancy ion-exclusion chromatography of aromatic carboxylic acids on a weakly acidic cation-exchange resin

Determination of aromatic carboxylic acids by conventional ion-exclusion chromatography is relatively difficult and methods generally rely on hydrophobic interaction between the solute and the resin. To overcome the difficulties in determining aromatic carboxylic acids a new approach is presented, termed vacancy ion-exclusion chromatography, which is based on use of the sample as mobile phase and an injection of aqueous 10% methanol onto a weakly acidic cation-exchange column (TSKgel OApak-A). Highly sensitive conductivity detection occurred with sharp and well-shaped peaks, leading to very efficient separations. The effects of sulfuric acid concentration added to the mobile phase, flow-rate, and column temperature on the retention volume of tested aromatic carboxylic acids was investigated. Retention times were found to be affected by the concentration of the analytes in the mobile phase and to some extent also by the addition of an organic modifier such as methanol to the injected water sample. Separation of sulfuric acid (SA), naphthalenetetracarboxylic acid (NTCA), phthalic acid (PA) and benzoic acid (BA) was satisfactory using this new approach. Detection limits were 0.66, 0.67, 0.42 and 0.86 microM and detector responses were linear in the range 1-100, 1-80, 2.5-100 and 10-40 microM, for SA, NTCA, PA and BA, respectively. Precision for retention times was 0.36% and for peak areas was 1.5%.     

Complexation in Benzo-15-Crown-5–Chloroform–Substituted Acetic Acid Extraction Systems

To study the complexing processes occurring in the extraction systems, the extraction of acetic, propionic (methylacetic), monochloroacetic, dichloroacetic and trichloroacetic acids using benzo-15- crown-5 as an extractant is considered. The extraction isotherms of acids with the pure solvent and 1 M benzo-15-crown-5 in chloroform are determined. The dependences of the distribution ratios of the considered acids on the extractant concentration in the organic phase are presented. Based on the experimental data and the “blank” extraction isotherm equations, the acid: benzo-15-crown-5 ratio in the complexes formed in the extract have been calculated (minus the contribution of extraction with the pure solvent). No complexation of methylacetic acid with benzo-15-crown-5 occurs, and the stability of the complexes of chloro-substituted acetic acids rises in the order acetic acid < monochloroacetic acid < dichloroacetic acid < trichloroacetic acid.     

Non-suppressed conductivity and indirect UV detection of carboxylic acids in environmental samples by ion-exclusion chromatography using 2,6-pyridinedicarboxylic acidic eluent

2,6-Pyridinedicarboxylic acid (PDCA) was evaluated as an eluent for indirect UV and non-suppressed conductivity detection of carboxylic acids in ion-exclusion chromatography. The effect of PDCA concentration on the separation and detection sensitivity was investigated. The reasonable resolutions between carboxylic acids were achieved using 1 mM PDCA eluent. Detection limits were 1.0-7.0 microM for conductivity detection and 8-30 microM for UV detection. Compared to the eluent containing 1 mM sulfuric acid, the method offers a high resolution and high detection sensitivity for both detectors due to its high molar absorptivity and low background conductance. The proposed method was demonstrated to be useful for the determination of carboxylic acids in environmental samples with direct sample injection.     

Qualitative analysis of some carboxylic acids by ion-exclusion chromatography with atmospheric pressure chemical ionization mass spectrometric detection

A simple, selective and sensitive method for the determination of carboxylic acids has been developed. A mixture of formic, acetic, propionic, valeric, isovaleric, isobutyric, and isocaproic acids has been separated on a polymethacrylate-based weak acidic cation-exchange resin (TSK gel OA pak-A) based on an ion-exclusion chromatographic mechanism with detection using UV-photodiode array, conductivity and atmospheric pressure chemical ionization mass spectrometry (APCI-MS). A mobile phase consisting of 0.85 mM benzoic acid in 10% aqueous methanol (pH 3.89) was used to separate the above carboxylic acids in about 40 min. For LC-MS, the APCI interface was used in the negative ionization mode. Linear plots of peak area versus concentration were obtained over the range 1-30 mM (r2=0.9982) and 1-30 mM (r2=0.9958) for conductimetric and MS detection, respectively. The detection limits of the target carboxylic acids calculated at S/N=3 ranged from 0.078 to 2.3 microM for conductimetric and photometric detection and from 0.66 to 3.82 microM for ion-exclusion chromatography-APCI-MS. The reproducibility of retention times was 0.12-0.16% relative standard deviation for ion-exclusion chromatography and 1.21-2.5% for ion-exclusion chromatography-APCI-MS. The method was applied to the determination of carboxylic acids in red wine, white wine, apple vinegar, and Japanese rice wine.     

固相萃取-离子色谱法测定饮用水中的痕量卤代乙酸

建立了固相萃取-离子色谱(SPE-IC)测定饮用水中痕量卤代乙酸(HAAs)(包括一氯乙酸、二氯乙酸、三氯乙酸、一溴乙酸和二溴乙酸)的方法。固相萃取采用LiChrolut EN SPE柱来进行痕量待测物的预浓缩（25倍）和基体杂质的消除,用NaOH(10 mmol/L)洗脱;色谱分离采用亲水性、高容量、氢氧化物选择型阴离子交换柱Dionex IonPac AS16(250 mm×4 mm i.d.),以NaOH为流动相进行浓度梯度淋洗,淋洗速度为0.8 mL/min,电导检测,进样量为500 μL。结果表明,用SPE-IC法测定HAAs,一溴乙酸的检测限为12.5 μg/L,其余4种HAAs的检测限为0.38~1.69　μg/L。该法可实现对饮用水中痕量卤代乙酸的测定。     

Ion-exclusion chromatographic behavior of aliphatic carboxylic acids and benzenecarboxylic acids on a sulfonated styrene--divinylbenzene co-polymer resin column with sulfuric acid containing various alcohols as eluent

The addition of C1-C7 alcohols (methanol, ethanol, propanol, butanol, heptanol, hexanol and heptanol) to dilute sulfuric acid as eluent in ion-exclusion chromatography using a highly sulfonated styrene-divinylbenzene co-polymer resin (TSKgel SCX) in the H+ form as the stationary phase was carried out for the simultaneous separations of both (a) C1-C7 aliphatic carboxylic acids (formic, acetic, propionic, isobutyric, butyric, isovaleric, valeric, 2-methylvaleric, isocaproic, caproic, 2,2-dimethyl-n-valeric, 2-methylhexanoic, 5-methylhexanoic and heptanoic acids) and (b) benzenecarboxylic acids (pyromellitic, hemimellitic, trimellitic, o-phthalic, m-phthalic, p-phthalic, benzoic and salicylic acids and phenol). Heptanol was the most effective modifier in ion-exclusion chromatography for the improvement of peak shapes and a reduction in retention volumes for higher aliphatic carboxylic acids and benzenecarboxylic acids. Excellent simultaneous separation and relatively highly sensitive conductimetric detection for these C1-C7 aliphatic carboxylic acids were achieved on the TSKgel SCX column (150 x 6 mm I.D.) in 30 min using 0.5 mM sulfuric acid containing 0.025% heptanol as eluent. Excellent simultaneous separation and highly sensitive UV detection at 200 nm for these benzenecarboxylic acids were also achieved on the TSKgel SCX column in 30 min using 5 mM sulfuric acid containing 0.075% heptanol as eluent.     

Electrocatalytic dechlorination of chloroacetic acids by silver nanoparticles modified glassy carbon electrode

A simple potentiostatic method was employed to prepare silver nanoparticles deposited on glassy carbon electrode. The silver nanoparticles exhibit extraordinary electrocatalytic activities toward the reduction process of chloroacetic acids. The electrochemical behavior of trichloroacetic acid, dichloroacetic acid, and monochloroacetic acid has been investigated by cyclic voltammetry at the silver nanoparticles-modified glassy carbon electrode in 0.1 M LiClO₄ solution; each compound exhibits a series of reduction peaks which represent sequential dechlorination steps up to acetic acid. The electrocatalytic dechlorination mechanism for chloroacetic acids was also discussed in this work.     

离子排斥色谱用于厌氧消化处理水中脂肪族羧酸的分析（英文）

The analysis of seven aliphatic carboxylic acids(formic,acetic,propionic,iso-butyric,n-butyric,iso-valeric and n-valeric acid) in anaerobic digestion process waters for biogas production was examined by ion-exclusion chromatography with dilute acidic eluents(benzoic acid,perfluorobutyric acid(PFBA) and sulfuric acid) and non-suppressed conductivity/ultraviolet(UV) detection.The columns used were a styrene/divinylbenzene-based strongly acidic cation-exchange resin column(TSKgel SCX) and a polymethacrylate-based weakly acidic cation-exchange resin column(TSKgel Super IC-A/C).Good separation was performed on the TSKgel SCX in shorter retention times.For the TSKgel Super IC-A/C,peak shape of the acids was sharp and symmetrical in spite of longer retention times.In addition,the mutual separation of the acids was good except for iso-and n-butyric acids.The better separation and good detection was achieved by using the two columns(TSKgel SCX and TSKgel Super IC-A/C connected in series),lower concentrations of PFBA and sulfuric acid as eluents,non-suppressed conductivity detection and UV detection at 210 nm.This analysis was applied to anaerobic digestion process waters.The chromatograms with conductivity detection were relatively simpler compared with those of UV detection.The use of two columns with different selectivities for the aliphatic carboxylic acids and the two detection modes was effective for the determination and identification of the analytes in anaerobic digestion process waters containing complex matrices.     

Sensitive Determination of Five Priority Haloacetic Acids by Electromembrane Extraction with Capillary Electrophoresis

A method for sensitive determination of five priority haloacetic acids in drinking water has been developed for the first time based on electromembrane extraction (EME) prior to CZE with capacitively coupled contactless conductivity detection (CZE‐C⁴D). The target analytes were extracted from 10 mL of the sample solution (donor phase), through the supported liquid membrane (using a polypropylene membrane supporting 1‐octanol), and into 10 µL of 50 mmol/L NaAc solution (acceptor phase). The extracted solution was directly analyzed by CZE‐C⁴D without derivatization. Several factors that affect separation, detection and extraction efficiency were investigated. Under the optimum conditions, five haloacetic acids (monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid) could be well separated from other components coexisting in water samples within 23 min, exhibiting a linear calibration over two orders of magnitude (r?0.9943); the enrichment factors at 430–671 were obtained in a 30 min of extraction, and the limits of detection were in the range of 0.17–0.61 ng/mL. The intraday relative standard deviations for peak areas investigated at 10 ng/mL were between 1.2% and 9.7% for the combined EME‐CZE‐C⁴D procedure. This approach offers an attractive alternative to the officially proposed method for purified drinking water analysis, which requires derivatization procedure prior to gas chromatography analysis.     

Determination of Monochloroacetic Acid and Dichloroacetic Acid for Quality Control of Acetic Acid Chlorination Industry by Ion Chromatography

An ion chromatographic method is described for the purpose of quality control in the process of monochloroacetic acid production. Using 2.5 mM NaOH–10% methanol as eluent, the simultaneous determination of acetic acid, monochloroacetic acid, dichloroacetic acid, and Cl⁻ was obtained in a single run. Monochloroacetic acid and dichloroacetic acid showed good linearity in the range 0.1–20 and 0.15–20 μg/ml and correlation coefficients were 0.9999 and 0.9998, respectively. The detection limits (signal-to-noise ratio 3:1) of monochloroacetic acid and dichloroacetic acid were 17 and 25 ng/ml. This simple, sensitive, and time-saving method can be applied for composition analysis in acetic acid chlorination production.     

Ion-exclusion chromatography with conductimetric detection of aliphatic carboxylic acids on a weakly acidic cation-exchange resin by elution with benzoic acid-beta-cyclodextrin

In this study, an aqueous solution consisting of benzoic acid with low background conductivity and beta-cyclodextrin (beta-CD) of hydrophilic nature and the inclusion effect to benzoic acid were used as eluent for the ion-exclusion chromatographic separation of aliphatic carboxylic acids with different pKa values and hydrophobicity on a polymethacrylate-based weakly acidic cation-exchange resin in the H+ form. With increasing concentration of beta-cyclodextrin in the eluent, the retention times of the carboxylic acids decreased due to the increased hydrophilicity of the polymethacrylate-based cation-exchange resin surface from the adsorption of OH groups of beta-cyclodextrin. Moreover, the eluent background conductivity decreased with increasing concentration of beta-cyclodextrin in 1 mM benzoic acid, which could result in higher sensitivity for conductimetric detection. The ion-exclusion chromatographic separation of carboxylic acids with high resolution and sensitivity was accomplished successfully by elution with a 1 mM benzoic acid-10 mM cyclodextrin solution without chemical suppression.     

离子色谱法测定饮用水中的5种卤代乙酸

利用IonPac AS19大容量阴离子交换色谱柱对水中的一氯乙酸、二氯乙酸、三氯乙酸、一溴乙酸、二溴乙酸5种卤代乙酸(HAAs)进行了分离,优化了分离条件.通过控制分离温度实现了二氯乙酸(DCAA)与NO2-的分离;通过梯度洗脱使三氯乙酸(TCAA)与SO42-得到较好、较快的分离;通过中和脱气法解决了在大量CO32-(HCO3-)存在时对实验的干扰.DCAA、TCAA的检出限(以3倍信噪比计)分别达到了2.50 μg/L和3.75 μg/L.5种HAAs在10.0～2 000.0 μg/L线性范围内线性相关系数在0.999以上.     

乙酸和一氯乙酸电离常数的高效毛细管区带电泳法测定

首次建立了测定一氯乙酸和乙酸的电离常数的高效毛细管区带电泳新方法.该方法利用中性标记物和电流突跃两种方法来标记电渗流,通过测定乙酸和一氯乙酸在一定pH的缓冲溶液中的电泳淌度,结合数据回归分析拟合,求得乙酸和一氯乙酸的电离常数;所得数据和文献报道值较为接近.总体而言,毛细管区带电泳法可简单、快速、可靠地用于测定待测化合物的电离常数.     

Vacancy ion-exclusion chromatography of inorganic acids on a weakly acidic cation-exchange resin column

Vacancy ion-exclusion chromatography (VIEC) for inorganic acids such as H(2)SO(4), HCl, H(3)PO(4), HNO(3), HI and HF is tested on a polymethacrylate-based weakly acidic cation-exchange resin column in the H(+)-form. That is, mixture of inorganic acids in the mobile phase is adsorbed to the resin phase passing through the separation column, and each vacant peak induced by injecting water is determined. Retention times are dependent on the degrees of retention for each analyte in the resin phase. In VIEC, well-shaped peaks of inorganic acids are produced, leading to efficient separations. However, retention behaviors of inorganic acids were strongly affected by the concentrations of the acids in the mobile phase. Sulfosalicylic acid was mixed with inorganic acids in the mobile phase prior to the introduction of a separation column in order to obtain the well-resolutions in the lower concentrations of the acids. By using this method, the separations of inorganic acids could be achieved in the range of 0.01-1 mM, and the linear ranges could be extended over two-orders of magnitude. This is considered since the protonated carboxylic groups fixed on the resin phase were increased with increasing the acid concentrations in the mobile phase, and the penetration effects for the acids to the resin phase were thus enhanced. The detection limits (S/N=3) were below 1.0 microM for all analyte acids. Precision values for retention times were below 0.32% and for peak area were below 0.91%.     

Modeling of ion-exclusion and vacancy ion-exclusion chromatography in analytical and concentration overload conditions

Ion-exclusion chromatography (IEC) finds applications in various different analytical separations of weak acids. Pure, deionized water or a diluted, aqueous solution of a strong mineral acid (such as, e.g., sulphuric acid) is used as the mobile phase, whereas a typical stationary phase is a strongly acidic resin in the H(+) form (e.g., the sulfonated polystyrene-divinylbenzene resin with a high ion-exchange capacity, provided by the sulfonic acid groups). When pure water is used as the mobile phase, then the characteristic leading (i.e., frontally tailing) peaks are obtained, and the retention depends mainly on the concentration of the analyte. An alternative technique is vacancy ion-exclusion chromatography (v-IEC), in which the column is equilibrated with the sample solution, flowing as the mobile phase through the system, and pure water is injected as the sample. In this case, the symmetrical vacant peaks are obtained. The aim of this paper is to describe the retention mechanism in IEC and v-IEC for the adsorptive and nonadsorptive acids in analytical and concentration overload conditions, with pure water and the diluted sulphuric acid solution as the two different mobile phases. The retention times and the peak shapes predicted by the derived equations remain in a good qualitative and quantitative agreement with the experimental data. The model proposed in this paper predicts the new features characteristic of IEC for the adsorptive acids. These are, namely, an increase in the retention time of the peak apexes (up to a certain level and concurring with an increase in the acid concentration), followed by a subsequent decrease of the retention time (with the further growth of the acid concentration in the eluent). Similar changes in the retention time observed for v-IEC in the specific adsorption conditions were also correctly predicted by the model.     

Determination of some aliphatic carboxylic acids in anaerobic digestion process waters by ion-exclusion chromatography with conductimetric detection on a weakly acidic cation-exchange resin column

The determination of seven aliphatic carboxylic acids, formic, acetic, propionic, isobutyric, n-butyric, isovaleric and n-valeric acids in anaerobic digestion process waters was examined using ion-exclusion chromatography with conductimetric detection. The analysis of these biologically important carboxylic acids is necessary as a measure for evaluating and controlling the process. The ion-exclusion chromatography system employed consisted of polymethacrylate-based weakly acidic cation-exchange resin columns (TSKgel OApak-A or TSKgel Super IC-A/C). weakly acidic eluent (benzoic acid), and conductimetric detection. Particle size and cation-exchange capacity were 5 microm and 0.1 meq./ml for TSKgel OApak-A and 3 microm and 0.2 meq./ml for TSKgel Super IC-A/C, respectively. A dilute eluent (1.0-2.0 mM) of benzoic acid was effective for the high resolution and highly conductimetric detection of the carboxylic acids. The good separation of isobutyric and n-butyric acids was performed using the TSKgel Super IC-A/C column (150 mm x 6.0 mm i.d. x 2). The simple and good chromatograms were obtained by the optimized ion-exclusion chromatography conditions for real samples from mesophilic anaerobic digestors, thus the aliphatic carboxylic acids were successfully determined without any interferences.     

A convenient and sensitive method for haloacetic acid analysis in tap water by on‐line field‐amplified sample‐stacking CE–ESI‐MS

In this study, we propose a simple strategy based on flow injection and field‐amplified sample‐stacking CE–ESI‐MS/MS to analyze haloacetic acids (HAAs) in tap water. Tap water was passed through a desalination cartridge before field‐amplified sample‐stacking CE–ESI‐MS/MS analysis to reduce sample salinity. With this treatment, the signals of the HAAs increased 300‐ to 1400‐fold. The LODs for tap water analysis were in the range of 10 to 100 ng/L, except for the LOD of monochloroacetic acid (1 μg/L in selected‐ion monitoring mode detection). The proposed method is fast, convenient, and sensitive enough to perform on‐line analysis of five HAAs in the tap water of Taipei City. Four HAAs, including trichloroacetic acid, dichloroacetic acid, dibromoacetic acid, and monobromoacetic acid, were detected at concentrations of approximately 1.74, 1.15, 0.16, and 0.15 ppb, respectively.     

Degradation and toxicity changes in aqueous solutions of chloroacetic acids by Fenton-like treatment using zero-valent iron

Three priority pollutants, i.e. mono-, di-, and trichloroacetic acids, were degraded by the conventional Fenton AOP system (Fe²⁺ and H₂O₂). The results obtained suggest that the degradation decreased in the order: monochloroacetic, dichloroacetic, and trichloroacetic acid. The best of advanced oxidation processes (AOPs) for the degradation of trichloroacetic acid was reductive dechlorination with the use of zero-valent iron (Fe°). The results of Escherichia coli toxicity tests revealed that the reagents’ toxicity after the Fenton treatment process was decreased.     

Characterization of the properties of stationary phases for liquid chromatography in aqueous mobile phases using aromatic sulphonic acids as the test compounds

We investigated the effects of the concentration of naphthalene sulphonic acids (NSAs) as anionic test compounds in the injected sample and of the salt additives to the mobile phase on ion-exclusion. The retention behaviour of NSAs sensitively reflects even minor changes in the ionic and hydrophobic interactions and can be useful for predicting the effects of the stationary phases in reversed-phase chromatography of polar and ionic compounds, both small ones and biopolymers, e.g., oligonucleotides. We studied chromatographic properties of several stationary phases intended for separations in aqueous mobile phases: a C18 column end-capped with polar hydrophilic groups, a densely bonded C8 column doubly end-capped with short alkyl groups, a short alkyl stationary phase designed to keep full pore accessibility in highly-aqueous mobile phases and a Bidentate column with “bridged” C18 groups attached to the silica hydride support. The chemistry and pore structure of various types of column packing materials and of the salt additives to the mobile phase affect the proportion of the pore volume non-accessible to anions due to ion-exclusion and consequently the peak asymmetry and hydrophobic selectivity in reversed-phase chromatography of organic acids. We also addressed the problems connected with the determination of column hold-up volume in aqueous mobile phases. The accessibility of the stationary phase for anionic compounds in contact with the sample zone is affected by ion-exclusion due to repulsive interactions with the negatively charged surface in the pores of the stationary phase. The accessible part of the stationary phase increases and consequently the migration velocity along the column decreases with increasing concentration of the sample in the zone moving along the column. Because of a limited access to the stationary phase, its capacity can be easily overloaded. The combination of the column overload and ion-exclusion effects may result in fronting or tailing peak asymmetry. To explain this behaviour, we proposed a modified Langmuir model, respecting the variation of the column capacity due to the effects of sample concentration on ion-exclusion.