Effect of pH and mobile phase additives on the chromatographic behaviour of an amide‐embedded stationary phase: Cyanocobalamin and its diaminemonochloro‐platinum(II) conjugate as a case study

Several mobile phase additives (i.e., organic acids and their ammonium salts) were used to modulate the chromatographic retention of cyanocobalamin and its cis‐diaminemonochloroplatinum(II) conjugate, depending on the specific nature of the stationary phase. Regardless of the mobile phase additive, the positively charged cyanocobalamin‐cis‐diaminemonochloroplatinum(II) conjugate was systematically less retained than cyanocobalamin on a conventional octadecyl‐silica column. In contrast, the amide‐embedded C18 column exhibited a progressive increase in the conjugate retention time upon changing the mobile phase additive from organic (acetic, formic and trifluoroacetic) acids to ammonium salts, ultimately leading to an inversion of the elution order. This change of retention was interpreted by invoking the interplay between hydrophobic interactions, hydrogen bonding between the conjugate and the polar amide groups and the ion‐pairing ability of the lyophilic counterions, whereby the acetate anion was found to be the most suitable to control the solute retention.     

Effect of ionic additives on the elution of sodium aryl sulfonates in supercritical fluid chromatography

Addition of a small amount of polar solvent (i.e., modifier) to CO2 in packed column supercritical fluid chromatography (SFC) has shown major improvements in both polar analyte solubility and interaction of the polar analyte with the stationary phase. Recently, the addition of an ionic component (i.e., additive) to the primary modifier by one of us has been shown to extend even further the application of SFC to polar analytes. In this work, the effect of various ionic additives on the elution of ionic compounds, such as sodium 4-dodecylbenzene sulfonate and sodium 4-octylbenene sulfonate, has been studied. The additives were lithium acetate, ammonium acetate, tetramethylammonium acetate, tetrabutylammonium acetate, and ammonium chloride dissolved in methanol. Three stationary phases with different degrees of deactivation were considered: conventional cyanopropyl, deltabond cyanopropyl, and bare silica. The effect of additive concentration and additive functionality on analyte retention was investigated. Sodium 4-dodecylbenzene sulfonate was successfully eluted using all the additives with good peak shape under isocratic/isobaric/isothermal conditions. Different additives, however, yielded different retention times and in some cases different peak shapes.     

A Base‐Free Neutral Phase‐Transfer Reaction System

Although phase‐transfer reactions catalyzed by using quaternary ammonium salts are generally believed to require base additives, we discovered that, even without any base additives, conjugate additions of 3‐substituted oxindoles to nitroolefins proceeded smoothly in the presence of lipophilic quaternary ammonium bromide under water–organic biphasic conditions. The mechanism of this novel base‐free neutral phase‐transfer reaction system is investigated and the assumed catalytic cycle is presented together with interesting effects of water and lipophilicity of the phase‐transfer catalyst. The base‐free neutral phase‐transfer reaction system can be applied to highly enantioselective conjugate addition and aldol reactions under the influence of chiral bifunctional ammonium bromides as key catalysts. The structure of the chiral ammonium enolate intermediate is discussed based on the single‐crystal X‐ray structures of relevant ammonium salts and the importance of bifunctional design of catalyst is clearly explained in the model of intermediate.     

Chromatographic retention behaviour, modelling and separation optimisation of the quaternary ammonium salt isometamidium chloride and related compounds on a range of reversed-phase liquid chromatographic stationary phases

This paper describes the reversed-phase liquid chromatographic behaviour of the trypanocidal quaternary ammonium salt isometamidium chloride and its related compounds on a range of liquid chromatographic phases possessing alkyl and phenyl ligands on the same inert silica. In a parallel study with various extended polar selectivity phases which possessed different hydrophobic/silanophilic (hydrogen bonding) activity ratios, the chromatographic retention/selectivities of the quaternary ammonium salts was shown to be due to a co-operative mechanism between hydrophobic and silanophilic interactions. The highly aromatic and planar isometamidium compounds were found to be substantially retained on stationary phases containing aromatic functionality via strong π-π interactions. The chemometric approach of principal component analysis was used to characterise the chromatographic behaviour of the isometamidium compounds on the differing phases and to help identify the dominant retention mechanism(s). Two-dimensional (temperature/gradient) retention modelling was employed to develop and optimise a rapid liquid chromatography method for the separation of the six quaternary ammonium salts within 2.5 min which would be suitable for bioanalysis using liquid chromatography-mass spectrometry. This is the first reported systematic study of the relationship between stationary phase chemistries and retention/selectivity for a group of quaternary ammonium salts.     

Effects of ion-pairing reagents on the electrospray signal suppression of sulphonated dyes and intermediates

High-performance liquid chromatography/mass spectrometry (HPLC/MS) analysis of anionic species such as sulphonic acid dyes and intermediates requires volatile ion-pairing mobile phase additives. Six di- and trialkylammonium acetates were compared with tetraalkylammonium salts and ammonium acetate in the concentration range 0-20 mmol l(-1) as mobile phase additives for HPLC/MS of polysulphonated compounds. The effects of the structure and concentration of the ion-pairing reagents on the electrospray response of mono-, di- and tetrasulphonic aromatic acids and acid dyes were studied in detail. Further, five different mass analysers and instrument geometries were compared. A higher signal decrease is observed with linear geometry instruments in comparison to orthogonal or even Z-spray geometry mass spectrometers. The concentration of mobile phase additives has a significant influence on the abundance ratios of multiply charged ions in the mass spectra of polysulphonated compounds. The competing ions of sulphonic acids may also cause significant signal suppression.     

Tuning and calibration in thermospray liquid chromatography/mass spectrometry using perfluorinated alkyl acids and their ammonium salts

Of the several approaches used to interface liquid chromatography with mass spectrometry, the thermospray interface has evolved as the most popular and applicable for the analysis of polar and labile organic compounds. Since a separate ion source must be used for thermospray, separate tuning and calibration are required. Unfortunately, current approaches to tuning and calibration suffer from serious shortcomings, most notably rapid contamination of the ion source. Recent reports have shown that this shortcoming can be overcome by using acetic acid-ammonia cluster ions or trifluoroacetic acid–ammonia cluster ions for tuning and calibration. In the latter approach, the tuning solution could also be used in the negative ion mode of operation but suffers from concentration of most of the ion current in one ion. The use of perfluorinated alkyl acids and their ammonium salts to generate intense high-mass negative ions for tuning and calibration to m/z 2000 is reported. Additionally, the ammonium salts of the longer chain perfluorinated acids offer an enhanced high-mass response in the positive ion mode of operation.     

The gas-liquid chromatographic stationary phase properties of liquid organic salts: Anomalous selectivity variation when employing the Rohrschneider/McReynolds system

The liquid organic salts studied here have wide stable liquid temperature ranges and act as efficient, highly selective gas-liquid chromatographic stationary phases. The effect of carbon number on the gas liquid chromatographic stationary phase properties of this series of tetra-n-butylammonium n-alkylsulfonate salts was evaluated by the well-known Rohrschneider/McReynolds system. Several problems arose when attempting to characterize these polar liquid organic salts employing this system. The specific retention volumes for the polar selectivity probes were generally not affected by an increase in the carbon number for the series studied here. However, the specific retention volumes for the n-alkane retention index markers increased dramatically as the anion carbon number was increased. The overall effect was a net decrease in the calculated McReynolds constants with increasing carbon number, although the true selectivity of the different stationary phases remained constant. Additionally, the specific retention volumes of the basic test probe, pyridine, showed large erratic variations and, in some cases, was not recovered from the columns. The results suggest the possibility of on-column chemical reactions occurring with some of these salts, and an alternative test probe, 2,6-dimethylpyridine (lutidine) is proposed to eliminate this problem. As McReynolds constants are presently the most commonly used parameters for predicting retention and gas chromatographic stationary phase selection, it is important that workers are aware of the inherent limitations of this scheme.     

Effect of hydrophobic and hydrophilic organic salts on charge transfer polymerisation of styrene

Conventional charge transfer (CT) polymerisation often involves an interaction of a monomer with an electron donor and an acceptor in additives such as Dimethyl Sulfoxide (DMSO). The conventional method suffers from the drawback of poor yields. An attempt has been made in this study to synthesise the polymers by employing different organic salts as an ionisation enhancing additive. The use of a hydrophobic organic salt enhanced the polymer yields significantly and also increased thermal stability. Hydrophilic salts, based on organo-borate anions and quaternary ammonium cations also increased the polymer yields compared to the conventional system.     

Analysis of polar peptides using a silica hydride column and high aqueous content mobile phases

The retention behavior of a set of polar peptides separated on a silica hydride stationary phase was examined with a capillary HPLC system coupled to ESI‐MS detection. The mobile phases consisted of formic acid or acetic acid/acetonitrile/water mixtures with the acetonitrile content ranging from 5 to 80% v/v. The effects on peptide retention of these two acidic buffer additives and their concentrations in the mobile phase were systematically investigated. Strong retention of the peptides on the silica hydride phase was observed with relatively high‐organic low‐aqueous mobile phases (i.e. under aqueous normal‐phase conditions). However, when low concentrations of acetic acid were employed as the buffer additive, strong retention of the peptides was also observed even when high aqueous content mobile phases were employed. This unique feature of the stationary phase therefore provides an opportunity for chromatographic analysis of polar peptides with water‐rich eluents, a feature usually not feasible with traditional RP sorbents, and thus under conditions more compatible with analytical green chemistry criteria. In addition, both isocratic and gradient elution procedures can be employed to optimize peptide separations with excellent reproducibility and resolution under these high aqueous mobile phase conditions with this silica hydride stationary phase.     

Duplicating the Retention of Cationic Analytes Obtained with Ammonium Formate Mobile Phases when Switching to UV Transparent Mobile Phase Additives in RP-LC

This study explored feasibility of utilizing sodium phosphate and mixtures of sodium phosphate and sodium perchlorate salts in mobile phases as UV transparent alternatives to the ammonium formate salts commonly used in LC–MS mobile phases. Chromatography experiments were run at pH 3.5 in 25% acetonitrile mobile phase, using several model cationic analytes to evaluate cation retention on two different C18 columns as the type or amount of salt was varied. For both columns, phosphate consistently showed less cation retention than formate. In other respects, the two columns showed very different behavior. The study suggests that it is feasible to use UV transparent mobile phase additives to provide comparable cation retention of formate mobile phases, but that the exact composition needed for optimal retention agreement is column dependent.     

缓冲盐类型和离子对试剂非对离子对强离解酸性化合物离子对反相液相色谱保留行为的影响

在离子对反相液相色谱(IP-RPLC)分析中,溶质保留受对离子(counter ion)的影响比较受人关注,但鲜有研究流动相中缓冲盐类型和离子对试剂中非对离子(non-counter ion)对溶质保留行为的影响。鉴于此,该文以14种磺酸化合物为研究对象,甲醇为有机调节剂,分别考察了3种缓冲盐体系(磷酸二氢铵、氯化铵和乙酸铵)和5种离子对试剂体系(四丁基溴化铵、四丁基磷酸二氢铵、四丁基硫酸氢铵、四丁基硝酸铵和四丁基乙酸铵)下强离解酸性化合物的IP-RPLC保留行为,通过比较不同流动相条件下得到的溶质log k_w(100%水相作流动相时的保留因子)、S(线型溶剂强度模型线性回归得到的常数),以及CHI(色谱疏水指数,log k_w/S),寻找保留行为规律。研究表明,流动相中的缓冲盐类型和离子对试剂非对离子均会影响化合物的log k_w和S值,所有化合物在氯化铵缓冲盐体系下具有最大的log k_w值。相对于无机阴离子,离子对试剂中弱离解性有机阴离子(乙酸根)的存在有利于增加磺酸化合物的S值。通过对比不同条件下的保留行为,推测磺酸化合物的IP-RPLC保留机理中同时存在着离子对模型和动态离子交换模型。与log k_w和S值不同,化合物的CHI值受缓冲盐类型以及离子对试剂非对离子的影响较弱。此外,研究发现化合物的表观正辛醇/水分配系数(log D)与log k_w、S、CHI之间均具有良好的线性相关性。不同缓冲溶液和不同离子对试剂非对离子条件下获得的log k_w和S值存在着一定的差异,而CHI值相对稳定,因此,CHI更适用于IP-RPLC中定量结构-保留行为关系模型的建立。     

Electrodeposition of manganese dioxide: effect of quaternary amines

The effect of quaternary ammonium salts (tetraethyl ammonium bromide, tetrapropyl ammonium bromide, and tetrabutyl ammonium bromide) on the structural, morphological, and electrochemical characteristics of electrolytic manganese dioxide (EMD) obtained from acidic aqueous sulfate solution has been investigated. Physical characterization of the EMD was achieved by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, differential thermal analysis, and Fourier transform infrared spectroscopy. The charge–discharge profile of the materials was determined to evaluate their potential for alkaline battery applications. The presence of these quaternary ammonium salts as organic additives in the solution increased the current efficiency while decreasing energy consumption during electrochemical deposition of manganese dioxide (MnO₂). All the additives influenced the discharge characteristics of the EMD samples significantly, producing a cathode material with increased cumulative discharge capacity relative to EMD prepared in the absence of additives. This is attributed to the ability of the additives to affect the particle size and morphology, and therefore electrochemical activity, of electrodeposited materials; the effects in the case of the additives investigated in this work were positive, producing a material with potential application to battery technology.     

High performance liquid chromatography-mass spectrometric analysis of sulphonated dyes and intermediates

This paper reports our results in the analysis of polysulphonated anionic dyes and their intermediates using high-performance liquid chromatography-mass spectrometry (HPLC-MS). Negative-ion electrospray ionization is the most suitable ionization technique for the molecular mass determination of polysulphonated dyes or other dyes carrying a negative charge. From the series of [M-xH]x- ions and their sodiated adducts [M-(x + y)H+yNa]x-, the molecular mass and the number of sulphonic and carboxylic groups can be determined. The mobile phase should be compatible with the mass spectrometric detection, which rules out non-volatile tetraalkylammonium salts usually used as ion-pair mobile phase additives for the HPLC of sulphonated compounds. Some mono- and disulphonated dyes and intermediates can be separated with aqueous-organic mobile phases containing 5 mM ammonium acetate, which is the most suitable additive as far as compatibility with MS detection is concerned. However, the retention of compounds with two or more sulphonic groups is too low for a successful separation both with this mobile phase additive and with ion-pair additives with short alkyl chains. The dihexylammonium acetate ion-pairing reagent offers a reasonable compromise in terms of sufficient volatility and adequate retention and separation selectivity for the HPLC-MS analysis of polysulphonated dyes.     

烷基吡啶盐和芳基季铵盐作检测和改性剂的羟基酸间接光度高效液相色谱研究

研究了苄基三甲基溴化铵动态改性、十四烷基吡啶盐等“永久性”涂渍的两种反相色谱,系统分离和间接光度检测了强极性、弱紫外吸收羟基酸。测定这些检测和改性试剂在YWG-CH反相填料和甲醇-磷酸缓冲液色谱体系的吸附等温线,探讨试剂在固定相的吸附量和其它因素对羟基酸保留、检测灵敏度的影响及分离检测机理。     

Analysis of sulfobutyl ether-beta-cyclodextrin mixtures by ion-spray mass spectrometry and liquid chromatography-ion-spray mass spectrometry

A comparison is made of the retention properties of additives applied as positively charged pseudo-stationary phases for electrokinetic chromatography of neutral analytes. All additives have a quaternary ammonium as functional group. The polymeric additive [poly(N,N,N',N'-tetramethyl-N-trimethylenehexamethylenediammonium), Polybrene] has a concentration of 2% (w/w) in the background electrolyte (acetate, pH 5.2). Monomeric octyltrimethylammonium (OTMA) was used at a concentration below or above its critical micelle concentration (CMC) (140 mmol/l). At a concentration (259 mmol/l) above the CMC the system is that normally used for micellar electrokinetic chromatography with cationic micelles. However, even below the CMC, where OTMA is present as monomer, retention of the neutral analytes is observed as well. In all systems coating of the capillary wall with Polybrene establishes an electroosmotic flow directed towards the anode, counter-migrating to the electrophoretic movement of the additive. Based on the measurement of the mobility of the analytes (15 small, monofunctional aromatic compounds with different functional groups), their capacity factors, k(i), were determined in all systems. Low correlation of the k(i) values is observed between the particular systems, indicating their different selectivity at least for individual pairs of analytes. Based on the log k(i) values, a linear free energy relationship was applied to elucidate the main types of chemical interaction responsible for retention. As a result, cavity formation and n or pi electron interactions were found being significant for the micellar OTMA system, which agrees with findings described in the literature for other (cationic and anionic) micellar systems. For the polymeric system and for the monomeric OTMA system, the significant retention parameter is indicating n and pi electron interactions.     

Liberating Native Mass Spectrometry from Dependence on Volatile Salt Buffers by Use of Gábor Transform

Volatile salts, such as ammonium acetate, are commonly used in buffers for the analysis of intact proteins and protein complexes in native electrospray ionization mass spectrometry. Although these solutions are not technically buffers near pH 7, the volatile nature of the salt minimizes ion adduction to proteins upon transfer to vacuum. Conversely, common biochemical salt buffers, such as Tris/NaCl, are not traditionally used in native mass spectrometry because of the tendency of sodium and other ions to adduct to proteins or form large cluster ions, severely frustrating accurate mass assignment. Here, we demonstrate a Gábor transform method for extracting signal from native-like protein ions even in the presence of a large salt-cluster background. We further show the utility of this method in characterizing polymers and show that the measured average mass of long-chain polyethylene glycol ions from a commercial polymer sample is ∼30 % higher than the manufacturer-estimated average mass. It is expected that this method will enable more widespread use of conventional biochemical buffers in native mass spectrometry and decrease dependence on volatile salts.     

Separation of Free Amiimo Acids on Reverse Stationary Phases Using an Alkyl Sulfonate Salt as a Mobile Phase Additive

Abstract

Alkylsulfonate (RSO₃ ^?) salts were evaluated as mobile phase additives for the separation of free amino acids on reverse stationary phases using an acidic mobile phase where the amino acids are cations. The enhanced amino acid retention is the result of two major interactions, one being retention of the RSO₃ ^? salt on the stationary phase and the other an ion exchange selectivity between the amino acid analyte cation and the RSO₃ ^? countercation, or other countercations in the mobile phase. Major mobile phase variables are: type and concentration of RSO₃ ^? salt (the studies focused on C₈SO₃ ^? salts), presence of organic modifier, type of countercation present, and mobile phase pH and ionic strength. Alkyl modified silica and polystyrenedivinyl-benzene copolymeric reverse stationary phases were compared. A mobile phase gradient, increasing per cent organic modifier was shown to be best, is necessary for separating complex mixtures of polar and nonpolar or basic amino acids. The procedure is applicable to the identification and/or determination of amino acids in mixtures or in peptides after hydrolysis.     

Abtrennung quartärer Ammoniumverbindungen über zerlegbare Ionenpaarextrakte mit nachfolgender Identifizierung

Ion-pairs of quaternary ammonium salts with lipophilic anions may be extracted into an organic layer. By using such anions capable to loose their negative charge by protonization (pH shiftment), the ion-pair separated with the organic solvent will be split by treatment with aqueous acids. Of the substances investigated, 6-phenyl-2,4-dinitrophenol seems the most promising. Surplus volatile acid may be removed in vacuo, leaving the quaternary salt as residue. Identification is accomplished by UV spectra of the aqueos solutions and TLC. Interesting observations in the search for spray reagents were: the probable ion-pair nature of the “Dragendorff” complexes, colour reactions of quaternary pyridinium derivatives with potassium hexacyanoferrat(II), and dark spots given by quaternary salts with mercury(I) nitrate on alumina layers.     

Negative matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry fragmentation of synthetic analogs of the O-specific polysaccharide of Vibrio cholerae O:1 in the presence of anionic dopants

Oligosaccharides (tri- to hexamers) that represent terminal epitopes of O-antigens of Vibrio cholerae O:1, serotypes Ogawa and Inaba, have been studied by negative matrix-assisted laser desorption/ionization time of flight/time-of-flight mass spectrometry (MALDI ToF/ToF MS). The [M - H(+)](-) ions are formed after expulsion of a proton from molecules studied under condition of MALDI MS analysis in the negative mode. Several ammonium salts (chloride, nitrate, hydrogencarbonate and hydrogensulfate) were used as additives to increase the formation of negative ions from saccharides. The most efficient was the addition of ammonium hydrogencarbonate, which increased the amount of [M - H(+)](-) ions more than six times. Between three fragmentation pathways, the new conjugated transfer of electrons within the second downstream unit of oligosaccharides was discovered. Production of these ions, which has not been observed in any other kinds of measurement, distinguishes substances belonging to Ogawa and Inaba serotypes. The negative MALDI ToF/ToF mass spectra are simpler and, at the same time, more informative, as compared with positive and negative electrospray ionization ion trap as well as with positive MALDI ToF/ToF analysis.     

强极性中药组分的亲水作用色谱保留行为

采用亲水作用色谱模式,以二醇基硅胶为固定相,水-有机溶剂为流动相,通过改变流动相中有机溶剂种类及浓度、缓冲盐、有机酸种类及其浓度、柱温等条件,研究了强极性中药组分在亲水作用色谱中的保留行为。结果表明:流动相中水的比例在0～100%(V/V)变化时,溶质保留呈U型曲线,属于亲水色谱和反相色谱的混合保留机理。水含量在0～50%(V/V)范围时,亲水作用控制溶质的保留。溶质保留随流动相中缓冲盐浓度的增大而增强。对于酸性溶质,其保留随有机酸三氟乙酸、甲酸、乙酸的酸性降低而增大。