Mobile-phase pH influence on the retention of some benzoic acid derivatives in reversed-phase chromatography

Five end-capped octadecyl RP stationary phases, among which one was a polar embedded stationary phase, were tested for the analysis of benzoic acid derivatives using two mobile phases with or without addition of formic acid (water pH was measured by a common approach; pH of water with addition of formic acid was 3.0 and without formic acid 5.8). The influence of mobile-phase pH on the retention of benzoic acid derivatives was under study. Consequently, Purospher-STAR and Alltima columns provided symmetrical peaks for benzoic acid derivatives at pH 3.0 and also at pH 5.8. Reprosil and Symmetry stationary phases showed poor peak shapes at higher pH of the mobile phase. Differences between the tested columns may be caused by surface heterogeneity. Another reason may be the presence of some atoms creating additional adsorption sites on the surface of Reprosil and Symmetry stationary phases. This can lead to enhanced silanol activity resulting in peak tailing. The addition of formic acid into the mobile phase improved peak shapes. The polar embedded C18 stationary-phase Synergi-Fusion-RP appeared as not a suitable column for the analysis of benzoic acid derivatives. Synergi-Fusion-RP provided asymmetrical peaks even if formic acid was added into the mobile phase.     

pH柔性膜电极的制作与表征

采用木犀草素为pH值选择性试剂,石墨粉为导电基质,PVC为成膜材料,自制无毒无害的pH柔性膜电极(pH flexible electrode,pH/FE)。该电极有一个氧化峰,它的峰电位对2.00~9.00范围内的pH值有线性响应,并且具有弯曲度好、抗拉伸性强、溶胀性能稳定、准确度高,选择性好等性能。此电极有望作为柔性检测器件用于非平面介质表面液层pH值的测定。     

A.c. polarographic determination of polyethylene glycols — application to analysis of photographic processing solutions

The tensammetric behaviour of several polyethylene glycol (PEG) molecules is described. At neutral pH, PEG gives both a positive and a negative peak. At high pH the positive peak is obscured by the anodic dissolution of mercury and at low pH the negative peak is obscured by the hydrogen evolution reaction. Tensammetry is shown to be applicable to the direct determination of PEG in photographic processing solutions. Despite the complex chemical composition of these solutions the only pre-treatment required was dilution of samples.     

Electrochemical determination of 8-oxoguanine in the presence of uric acid

The electrochemical oxidation of 8-oxoguanine (8-oxoG) in the presence of uric acid (UA) was studied by differential pulse voltammetry over a wide pH range (1-12). The results showed that both compounds follow a pH-dependent oxidation mechanism that involves two electrons and two protons corresponding to reversible charge transfer reactions. The difference between the peak potential for the oxidation of each analyte was found to be always less than 100 mV over the whole pH range, the separation being greater in the pH interval 4-7. In mixtures of both analytes, pH 6 was shown to be the best for 8-oxoG determination in the presence of uric acid, since the peak current is higher and a greater peak separation is achieved.     

Modification and validation of the pyromellitic acid electrolyte for the capillary electrophoretic determination of anions

For the determination of inorganic and organic anions, the pyromellitic acid (PMA) electrolyte is widely used. The pH adjustment of the self-prepared electrolyte was very challenging to satisfy the pH of specification of pH 7.8 +/- 0.1. A modification was proposed to provide a more simple electrolyte by buffering the PMA electrolyte with triethanolamine (TEA) only instead of adjusting the pH by NaOH and TEA. Thus, the proposed electrolyte consisted of 2.25 mmol l(-1) PMA, 0.75 mmol 1(-1) hexamethonium hydroxide and 12 mmol 1(-1) TEA. The performance of the PMA electrolyte buffered by TEA only was compared to a commercial available PMA and statistically validated in accordance with the methodology of Taguchi. No statistically significant difference could be found for both electrolytes assessing the performance and detection limits of hydrodynamic, stacking and electrokinetic injection with transient isotachophoretic preconcentration as well as repeatability of migration times, peak resolutions and peak symmetries.     

Differential pulse polarographic study of the degradation of cephalexin: Determination of Hydrogen Sulphide and other Degradation Products

Differential pulse polarography (d.p.p.) is used to study the degradation of cephalexin. Hydrogen sulphide, evolved during the degradation of cephalexin solutions, was removed continuously in a stream of nitrogen and determined periodically. Other electroactive degradation products were observed by d.p.p. of the degraded sample solutions. The degradation mechanism is highly dependent on pH, the initial concentration of cephalexin, temperature, the particular buffer used, and the presence of dissolved oxygen. The formation and degradation of the diketopiperazine derivative formed by intramolecular aminolysis, particularly at neutral pH, can be followed by means of its polarographic peak at -0.9 V (pH 7.4). Approximately half the total sulphur originally present in cephalexin is liberated as hydrogen sulphide at pH 7.4 at 37°C. Increasing the degradation temperature to 80°C and sweeping out the hydrogen sulphide with nitrogen increases the yield of a major product which gives a peak at -1.26 V. At pH 8.5 (80°C. 100 μg cephalexin ml^-1) the percentage of the sulphur evolved as hydrogen sulphide increases with time, and a peak appears at -0.96 V (probably 2-hydroxy-3-phenyl-6-methylpyrazine) which increases as the peak at -1.26 V becomes smaller. Other products formed under different conditions (concentration, pH, temperature) are reported. At pH 3 (80°C) only 8% conversion via intramolecular aminolysis and 5% evolution of total sulphur is indicated after four hours.     

Retention and column efficiency in reversed phase liquid chromatography as a function of pH for optimization purposes

Summary The effects of pH on both the solute retention and the peak shape of ionogenic compounds are studied in order to propose accurate models for pH optimization purposes. Several mathematical models (theoretical and empirical) for describing the variation of the retention factor versus pH are compared within different pH ranges. Limits of such models used for optimizing the pH by requiring only 3 preliminary experimental runs, are discussed in terms of deviations (≤±5%) of predicted retention times from experimental retention times. An original procedure is developed for selecting the most convenient retention model, from a given set of three retention data. This set is also applied to modeling the variation of both peak width and peak asymmetry with mobile phase pH conditions. Such a procedure is demonstrated as helpful for the separation of ionogenic solutes by considering mobile phase pH as an additional variable that can be useful during optimization procedures.     

The analysis of pharmaceutical bases on a silica stationary phase by capillary electrochromatography using aqueous mobile phases

Summary The capillary electrochromatographic (CEC) separation of a range of pharmaceutical bases was investigated on a commercially available silica stationary phase using aqueous mobile phases. The effects of mobile phase composition, buffer pH, applied voltage, and buffer anion on the retention behaviour of these bases were studied. Promising chromatography was obtained at pH 7.8 but was later found to be irreproducible. However, successful and reproducible chromatography of the bases was achieved at pH 2.3. We have previously demonstrated that the addition of mobile phase additives such as TEA-phosphate at low pH values has resulted in excellent CEC analysis of bases on reversed-phase packing materials. The same approach was applied to the analysis of bases on the silica phase in order to improve peak shape. Excellent chromatography was obtained for the analysis of strong pharmaceutical bases such as benzylamine, nortriptyline and diphenhydramine. The experimental investigations have shown that the CEC separation of a range of pharmaceutical bases can routinely be achieved with excellent peak shapes and peak efficiencies as high as 320,000 plates m⁻¹.     

Cathodic stripping voltammetry of copper-complexed reactive dyes at a hanging mercury drop electrode: Reactive violet 5

Reactive Violet 5 and its hydrolysis product, which is produced as a side product in the dyeing process, can be determined in an admixture at sub-ppb levels by cathodic stripping voltammetry because the potentials of their azo reduction peaks are separated sufficiently. For both dyes, at intermediate pH values the azo peak is preceded by a complexed -copper reduction peak at a less negative potential, which aids the identification of the dyes. The use of pH 6 EDTA buffer removes the complexed-copper peak, as does the use of an acidic buffer (pH < 3). This unusual use of EDTA as a pH buffer facilitates the determination of mixtures of the dye and its hydrolysis product.     

罗丹明6G衍生物基偏酸性“关开型”pH荧光探针

以罗丹明6G和水合肼为原料,先制备罗丹明6G酰肼,接着在乙醇中滴加少量冰醋酸做催化剂后与2,5-二甲氧基苯甲醛反应,合成了一种新型的pH荧光分子探针(RGSBD),进行了结构表征及荧光性能研究。结果表明,原本在氢离子浓度较低,即体系pH较高时(pH≥4.0),探针RGSBD内酰胺螺环闭环导致不显示荧光并且无色,然而在氢离子浓度较大即体系pH较低时(pH<4.0)时,其内酰胺螺环闭环产生了明显的颜色变化,发出强烈的荧光。pH 1.9时,探针的荧光强度达到最大,最大荧光峰发生显著的红移。进一步研究表明,探针RGSBD的荧光峰强度差值与pH在1.9~3.2范围内呈良好的线性关系,探针RGSBD识别H^+的选择性高,稳定性与可逆性强,可发展用作生物体内pH荧光传感材料。     

Studies on the effects of imidazole on the peroxyoxalate chemiluminescence detection system for high performance liquid chromatography

The catalytic effect of bases (imidazole, pyridine, Tris and triethylamine) on the peroxyoxalate chemiluminescence (PO-CL) reaction for high performance liquid chromatography (HPLC) was investigated. Imidazole increased PO-CL intensity extraordinarily, whereas the other bases (pyridine, Tris and triethylamine) did not. The peak heights of dipyridamole (coronary vasodilator) obtained using the eluents containing buffers were largest at pH 7.0, a few times less at pH 6.0 and pH 5.0, 100 times less at pH 4.0 and a few hundred times less at pH 3.0. The eluents containing buffers at pH 3, 4, 5, 6 or 7 each with imidazole increased the peak heights by a few to ten times as compared with those without imidazole, and those peak heights were within one order of magnitude. On the other hand, the eluent containing buffer at pH 2 did not affect the peak heights with or without imidazole. Bis(4-nitro-2-(3,6,9-trioxadecyloxycarbonyl)phenyl) oxalate (TDPO) alone and bis(2,4-dinitrophenyl)oxalate (DNPO) plus TDPO were recommended to be used against eluents containing buffers of pH 5-7 and pH 3-4, respectively. Dipyridamole and benzydamine hydrochloride (anti-inflammatory drug) were separated on the ODS column and detected by the present system. The detection limits of dipyridamole and benzydamine hydrochloride were 40 amol and 270 fmol, respectively.     

Voltammetric determination of 4-nitrophenol at a sodium montmorillonite-anthraquinone chemically modified glassy carbon electrode

A new method for the determination of 4-nitrophenol(4-NP) by differential pulse voltammetry (DPV) based on adsorptive stripping technique was described. Cyclic voltammetry (CV) and linear scan voltammetry (LSV) were used in a comparative investigation into the electrochemical reduction of 4-NP at a Na-montmorillonite(SWy-2) and anthraquione (AQ) modified glassy carbon electrode. With this chemically modified electrode, 4-NP was first irreversibly reduced from phiNO(2) to phiNHOH at -0.78 V. A couple of well-defined redox peaks at +0.22 V (vs. SCE) were responsible for a two-electron redox peak between phiNHOH and phiNO. Studies on the effect of pH on the peak height and peak potential were carried out over the pH range 2.0-9.0 with the phosphate buffer solution. A pH of 3.4 was chosen as the optimum pH. The other experimental parameters, such as film thickness, accumulation time and potential etc. were optimized. Anodic peak currents were found to be linearly related to concentration of 4-NP over the range 0.3-45 mg l(-1), with a detection limit of 0.02 mg l(-1). The interference of organic and inorganic species on the voltammetric response have been studied. This modified electrode can be used to the determination of 4-NP in water samples.     

Effect of graphite and metallic impurities of C60 fullerene on determination of salbutamol in biological fluids

A new method for the determination of salbutamol has been developed using fullerene C60-modified glassy carbon electrode and validated using GC-MS. The presence of graphite and metallic impurities in C60 are found to diminish the peak. The oxidation of salbutamol was observed in a single well-defined, diffusion-controlled process using square wave voltammetry. The peak potential of oxidation peak was dependent on pH and determination was carried out at physiological pH 7.4. The peak current versus concentration plot was linear in the range 100-2000 ng/ml of salbutamol. The detection limit was found to be 40 ng/ml. The determination of salbutamol was carried out in human blood and urine samples and common interferents such as dopamine, ascorbic acid and uric acid do not interfere. The method proved to be specific, rapid, and accurate and can be easily applied for detecting cases of doping. A cross-validation of the observed results with GC-MS indicated a good agreement.     

全固态酸度微电极组的设计与应用

以木犀草素为pH特性敏感物,碳纤维为导电基质,固体石蜡为粘合剂制成了木犀草素碳纤维酸度微电极,将该电极与自制导电凝胶型全固态微型Ag/AgCl电极和铂丝微电极联用,构成全固态酸度微电极组。 采用循环伏安法研究了该电极组对pH值为2.00~10.00的磷酸盐缓冲溶液有效酸度的响应,结果表明,循环伏安曲线上有一对可逆氧化还原峰,两峰的峰电势均随pH值呈线性变化,据此建立了方波伏安法测定溶液酸度的方法,氧化峰的峰电势(E_pa)与pH的线性回归方程为:E_pa(V)=-0.0567pH+0.603(r=0.999),体液中常见离子及蛋白质等不干扰测定。 该电极组微型、无毒、抗干扰能力强,应用于微量体表唾液和汗液酸度的测定,取得了令人满意的结果,为微量体液的实时、在体、现场检测提供了技术基础。     

Electrochemical oxidation of 8-hydroxyquinoline and selective determination of tin(II) at solid electrodes

The electrochemical oxidation of 8-hydroxyquinoline and several structurally related molecules has been investigated by cyclic voltammetry at solid electrodes (GCE, CPE, Pt) as a function of pH in aqueous acetate media. Oxine showed one irreversible oxidation peak whose magnitude decreased in the presence of cations readily complexed by oxine and with concomitent formation of a new oxidation peak at more positive potentials. Tin(II ) was found to exhibit a distinct behavior as its oxidation was catalyzed by the presence of oxine in the analyzed solution. The tin(II )-oxine oxidation peak was found to be linearly related to inorganic tin(II ) concentration allowing its quantitative determination at micromolar levels.     

Effects of pH in reversed-phase liquid chromatography

The effects of concomitant variations in pH and organic modifier concentration on retention, efficiency and peak symmetry are considered for reversed-phase liquid chromatography (RPLC) on octadecyl-modified silica (ODS) columns. A number of factors are discussed, which make the systematic exploitation of pH effects in RPLC more complicated than the optimization of solvent composition. If the pH is varied, a second factor (usually the concentration of organic modifier) will need to be varied simultaneously to maintain retention (capacity factors) in the optimum range. When pH is considered as a parameter in RPLC, not only its effects on retention, but also the variations in efficiency (plate count) and peak shape (asymmetry) need to be considered. These parameters turn out to vary drastically between individual solutes and between different experimental conditions. The results of a study involving a number of acidic, basic and neutral solutes, two different ODS columns and mixtures of either methanol or acetonitrile with aqueous buffers are reported. In the earlier part of the study, using methanol as the organic modifier, reproducible data for retention, peak width and peak symmetry were obtained and these data are reported. In the later part of the study, using acetonitrile, a gradual change in retention as a function of time was observed, this effect coinciding with a decrease in column efficiency. It is concluded that ODS columns are subject to considerable degradation during studies in which the pH is varied. Although this effect can be described mathematically, the preferred solution is thought to be the use of pH-stable columns.     

Peak identification in capillary isoelectric focusing using the concept of relative peak position as determined by two isoelectric point markers

In CIEF analysis, sample peaks can be identified by their relative peak positions (RPP) that are determined using only two internal pI markers. The two internal pI marker peaks should bracket, as close as possible, the sample peaks. The RPP values of the sample peaks are then calculated using the pI values, peak positions of the two pI markers, and peak position of the sample. Use of this method can effectively compensate for pH gradient distortions that often occur as a result of salts. Also, as shown by the results of this paper, regardless of the linearity of the pH gradient established by the given carrier ampholytes, sample peaks can be identified within an SD of 0.1 pH unit in RPP (<2% RSD) as long as the sample is run using the same carrier ampholytes and maintaining salt concentrations in the range of 0-15 mM.     

Cathodic stripping voltammetric determination at a hanging mercury drop electrode of the environmental heavy metal precipitant trimercapto-s-triazine (TMT)

Trimercapto-s-triazine (TMT) is available commercially for precipitating heavy metals in effluents prior to discharge and for recovering silver and copper. The TMT content of an effluent for discharge is normally monitored down to about 2 ppm by means of its UV absorption at 285 nm. Indirect cathodic-stripping voltammetric methods of determining TMT at sub-ppb levels in standard solutions are reported here. These methods might prove suitable for the determination of TMT in effluent at levels lower than is currently possible. TMT can be accumulated and determined indirectly at pH 9.0 as its mercury salt down to sub-ppb levels. Accumulation is made at 0 V and the mercury TMT reduction peak is at -0.47 V. Alternatively, by adding nickel(II), TMT can be determined optimally at pH 7.8, using the catalytic nickel peak at -0.73 V and accumulating between -0.10 and -0.60 V: at this pH the HgTMT peak at -0.47 V is small. At slightly higher pH (pH 8.6) the nickel TMT complex can be accumulated directly at -0.40 V, but at this pH, however, a slightly increased sensitivity can be achieved by accumulating TMT as its mercury salt, at -0.1 V in the presence of nickel(II), the nickel TMT complex being formed during the potential sweep on the release of the TMT when the mercury salt is reduced. Unlike many other thiols TMT is not accumulated as its copper(I) salt on addition of copper(II) to the solution.     

Electrochemical behaviour of tilmicosin at the hanging mercury drop electrode

The electrochemical behaviour of tilmicosin (TIM) was investigated using cyclic voltammetry and square-wave voltammetry in Britton-Robinson buffers (pH 2–12). The voltammograms of TIM showed one-irreversible peak which attributed to reduction of the >C=C< group in the entire pH values. However, an additional peak was shown over the pH range 2.00 to 4.00. Its peak potential is more positive potential from that of >C=C< group, and its potential was not practically dependent on the pH. On the other hand, the peak potential of >C=C< group was shifted to the more negative values with increasing pH. According to the obtained voltammetric data, the probable reaction mechanism for the reduction of TIM was proposed. Moreover, the adsorption of TIM on the mercury electrode surface by means of square-wave adsorptive stripping voltammetry measurements is studied at different pH values. The peak current variations with the deposition time and TIM concentration were observed.     

Variation of the pH of the background electrolyte due to electrode reactions in capillary electrophoresis: Theoretical approach and in situ measurement

Electrode reactions during the electrophoretic process may change the pH of the buffer and subsequently the migration behavior of solutes with resultant loss of reproducibility. A theoretical treatment of pH variations due to electrolytic processes is presented. The choice of buffer appears to have a dramatic influence on the pH variations observed, even if substantial buffer action is expected at the pH chosen. The experimental evaluation of the separation of 4-hydroxy-3-methoxycinnamic acid and 3-hydroxybenzoic acid reveals that the quality of the separation decreases continuously from a baseline separation observed in the first experiment to a comigration of the two solutes (resolution = 0) in the ninth experiment. A pH decrease of about 0.05 pH units accounts for the observed changes in mobility. A novel in situ pH measurement approach is presented, in which the mobility, peak area, and peak height of an indicator dye are related to the pH in the capillary. This enables the identification and quantification of pH variations during electrophoretic runs: the pH decreases at the anodic side already after the first experiment and pH variations as small as 0.02 pH units can be measured. The variations in peak height appear to be less suited. The calculated pH variations are in close agreement with the ones obtained experimentally.