Potentiometric detection of organic acids in liquid chromatography using conducting oligomer electrodes

A conducting oligomer electrode was used for the potentiometric detection of organic acids in reversed phase liquid chromatography (LC). The conducting material consisted of a mixture of a phenylene vinylene trimer with a polycarbonate host polymer and iodine. A glassy carbon electrode was coated with this material by evaporation from a chloroform solution. A theoretical model was given to describe the observed potentiometric responses. The analysis conditions were optimized to obtain both efficient separations, and sensitive potentiometric responses. Detection limits in the nanogram level were attained when a 1 mM phosphoric acid solution was used as the eluent, which were comparable to the values obtained with low-wavelength UV detection. Calibration curves showed a logarithmic dependence on an injected amount for amounts higher than 5 nmol, and a linear dependence for injected amounts below this value. The response times of the electrode were smaller than 1 s at typical LC flow-rates. The reproducibility for consecutive injections was 5%.     

Potentiometric detection of amines in ion chromatography using macrocycle-based liquid membrane electrodes

Potentiometric detection employing solid-state electrodes was applied to the determination of organic amines in cation-exchange, and in ion-interaction chromatography. The amines included nine industrial alkylamines, the biogenic amines putrescine and cadaverine, the neurotransmitter acetylcholine and choline. Three PVC-based liquid membrane electrode coatings were used, incorporating respectively the lipophilic cation-exchanger tetrakis(p-chlorophenyl)borate, a combination of tetrakis(p-chlorophenyl)borate and dibenzo-18-crown-6, or a combination of tetrakis(p-chlorophenyl)borate and a calix[6]arene. The three solid-state electrodes allowed sensitive detection of all amines. The addition of macrocycles with molecular recognition properties resulted in a superior sensitivity when compared to the tetrakis(p-chlorophenyl)borate-based electrode. The effect was most significant for the smaller amines which form the most stable complexes with the macrocycles. Detection limits of the order of 10⁻⁶ M (injected concentration) were measured for mono- and diamines for the macrocycle-based electrodes.     

Potentiometric detection of organic acids in liquid chromatography using polymeric liquid membrane electrodes incorporating macrocyclic hexaamines

Potentiometric detection employing coated-wire electrodes was applied to the determination of organic acids in liquid chromatography (LC). Poly(vinyl chloride)-based liquid membranes, incorporating lipophilic macrocyclic hexaamines as neutral ionophores were used as electrode coatings. The selectivity and sensitivity of the macrocycle-based electrodes were found to be superior to an electrode based on a lipophilic anion exchanger (a quaternary ammonium salt). Sensitive detection was obtained for the di- and tricarboxylic acids tartaric, malonic, malic, citric, fumaric, succinic, pyruvic, 2-oxoglutaric and maleic acids after separation in reversed-phase LC. Detection limits (signal/4sigmanoise=3) of 6 pmol for malonic acid and 2 pmol for maleic acid were attained. The detection was explained using a molecular recognition model. The hexaamine-based potentiometric electrodes had a 1-s response time at 1 ml min(-1) flow-rates. They were stable for at least 4 months, with an intra-electrode variation of 3.2% (n=5).     

Potentiometric detection of halides and pseudohalides in anion chromatography

A small piece of silver wire, coated with an insoluble silver salt, can be used as a selective potentiometric detector for halides in ion chromatography. Several coated electrodes were examined by electron microscopy and their response to various anions evaluated in a flow-injection system. A silver/silver chloride was found to be a selective and reproducible detector for chloride, bromide, iodide, thiocyanate and thiosulfate separated by ion chromatography. Calibration curves were non-linear and had slopes ranging from 40 to 60 mV per concentration decade in the range 0.1–2 mM. A working range of 0.05–2 mM was used. This electrode is also satisfactory when gradient elution is used in ion chromatography.     

Indirect detection in high-performance liquid chromatography

Indirect detection is a technically simple method to follow and quantify compounds without inherent detector response in high-performance liquid chromatography. The underlying principle can be expressed in simple equations. These equations indicate clearly how the detection sensitivity can be optimized and how disturbance effects can be avoided when the mobile phase gives detector response.     

Electronic spectral detection in liquid chromatography

The use of absorbance and fluorescence spectrometers as liquid chromatography detectors is described. These detectors offer sensitive detection that can also identify the peaks in a chromatogram. Many examples, mainly from the separation of polycyclic aromatic hydrocarbons, are given that show the selectivity and usefulness of these detectors.     

Potentiometric detection in ion chromatography using a metallic copper indicator electrode

Summary A metallic copper electrode housed in a suitable flowcell is shown to be a sensitive and versatile potentiometric detector for ion chromatography. This electrode may be used for direct or indirect detection of many inorganic anions and cations and also for organic acids. In the direct detection mode, electrode response is based on either complexation of copper ions at the electrode surface by eluted species, or on oxidation and reduction reactions for eluted species which are strong oxidants or reductants. Direct detection is therefore applicable to such species as amino acids, organic acids, chloride, bromide, iodide, chlorate, bromate and iodate. Indirect detection is possible for anions which do not complex copper ions, provided a copper complexing ligand (such as phthalate) is used in the eluent; cations which complex this ligand are also detectable. Indirect detection may be used for species such as nitrite, nitrate, acetate, formate, succinate, benzoate, alkaline earth ions and transition metal ions. Electrode calibration relationships are discussed and sample separations are presented, together with some typical detection limits attainable in the direct and indirect detection modes.Presented in part at a National Symposium on Ion Chromatography, held at the University of New South Wales, Kensington, Australia, Nov. 21, 1984.     

Computer-aided multichannel detection in high-performance liquid chromatography

Summary The development of a computer-aided rapid-scanning detector for HPLC based on the linear photodiode array UV-visible spectrophotometer is described. Chomatograms monitored at up to three wavelengths, with automatic capture of spectra for eluted peaks and post-run processing of spectral data to generate log₁₀ (A) spectra, second derivative and fourth derivative spectra, are described. Methods are reported for the analysis of forensic samples of diacetylmorphine (heroin) in the presence of the degradation products and potential contaminants caffeine, papaverine, 6-acetylcodeine, thebaine, 6-acetyl-morphine, procaine and morphine separated by HPLC. The novel use of second and higher derivative spectra for the qualitative characterisation of drugs and contaminants separated by HPLC is described.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Indirect conductimetric detection of cyclodextrins in liquid chromatography

Indirect conductimetric detection of cyclodextrins (CDs) was investigated in liquid chromatography (LC). The mobile phase contained an electrolytic substance which maintained high background, and CDs were indirectly detected owing to a depression of the background signal due to dilution of the mobile phase by the analyte molecules. The dynamic reserve, defined as the ratio of the background to its noise level, was (1–2) × 10⁵, and the detection limits at S/N = 3 were 23–40 ng for CDs when a microcolumn LC system was employed.     

Determination of vancomycin related substances by gradient high-performance liquid chromatography

A gradient high-performance liquid chromatographic method for the determination of vancomycin related substances is described. This method was developed to profile vancomycin, co-fermentation products, and degradation products. The resultant chromatograms confirm the multifactored nature of vancomycin, separating a number of peaks from the main component. The development of acceptable chromatographic performance is described, with the final method intended for use as a control assay. Quantitation of total related substances is made by comparing the relative area of the main peak to total peak area in a pair of chromatograms from a stock solution and a twenty-five fold dilution. A rapid-scan UV detector was used to demonstrate the similar spectral characteristics of the vancomycin related substances, confirming the validity of the relative area approach. Statistical method validation data are included, evaluating the use of this method for quantitative applications. Example applications demonstrate the effectiveness of this method.     

Potentiometric detection of organic acids in ion-exclusion chromatography using different types of liquid-membrane electrodes

Four different liquid-membrane electrodes were tested in a potentiometric flow-cell, in combination with an LC ion chromatography system (Aminex HPX-87H column). This setup was used for the determination of weak organic acids. The flow-through detector was of the wall-jet type. Conditions were established to achieve the best separation and detection characteristics. The sensitivity, selectivity and response time of the different electrodes were compared. Calibration curves and detection limits were measured for several organic acids, and compared with conductometric, and with low-wavelength UV detection. The detection limits were improved by inserting a post-column ion-suppressor system between the column and the detector. Several biological samples were analyzed to demonstrate the possibilities of the potentiometric detector.     

Indirect fluorometric detection in micro high-performance liquid chromatography

Summary Indirect fluorometric detection of non-electrolytes and inorganic anions were examined by using a commercially available fluorometer. The dynamic reserve achieved by the present system was (1.0–1.4)×10³, and it was about one-tenth of the value achieved by UV detection. The mass detection limit of 0.2 pmol was achieved for nitrate by using 3×10^–5 M sodium salicylate as the mobile phase. In the case of ion chromatography the mass detection limit was improved by using a low concentration mobile phase.     

Refractive index detection in microbore column liquid chromatography

Summary An existing commercial refractive index detector was modified for use with microbore column LC systems. The detector utilizes the Fresnel method. The effect of band dispersion and dilution at the detector side is of extreme importance in connection with the miniaturized LC system. In the modified model the original heat-exchanger tube was removed and a stainless steel capillary was used for heat-exchanging. Gaskets having different cell volumes were also examined with respect to band broadening and sensitivity. The detection limit was 10ng for di-n-pentyl phthalate. The examples include the detection of phthalates, alcohols, n-paraffins, and kerosine.     

Indirect UV detection of non-ionic substances in liquid chromatography by addition of a UV-absorbing uncharged compound to the mobile phase

Summary Chromatographic systems have been developed, which allow the detection and quantitation of non-ionic substances with low or no UV-absorptive properties. The detector response is obtained by using mobile phases containing an uncharged component with high UV-absorbance. Polar or non-polar bonded stationary phases have been selected, depending on the hydrophobic character of the analytes. The response pattern is compared to that observed in ion-pair reversed-phase systems with a UV-absorbing ion in the eluent. The influence on retention and detection sensitivity of the nature and concentration of the UV-absorbing compound and of the organic modifier added to the mobile phase has been studied. Principles for the optimisation of the response by changing the mobile phase composition are given. The wide application scope of the indirect UV-detection technique in non-ionic systems is demonstrated by examples of separations of various kinds of compounds, such as organic solvents, carbohydrates and polyols. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Electrochemical detection in high-performance liquid chromatography of organic compounds

The selectivity and sensitivity of analytical response in electrochemical (amperometric and conductometric) detection methods in the high-performance liquid chromatography of organic compounds is discussed in comparison with other detection techniques. The importance of electrode materials of different nature for extending the range of analytes is noted. Presented at the V All-Russian Conference with the Participation of CIS Countries on Electrochemical Methods of Analysis (EMA-99), Moscow, December 6–8, 1999.     

Application of long-lived luminescence for detection in liquid chromatography

Summary Quenched and sensitized lanthanide luminescence as detection in liquid chromatography has been investigated. An important advantage in comparison with phosphorescence is that the long-lived luminescence as applied does not require deoxygenation of the samples. In order to obtain a high luminescence intensity Tb(III) complexes with acetylacetonate have been formed, after which indirect excitation of Tb(III) can be realized via the ligands. The potential of Tb(III) luminescence as a detection method in ion chromatography has been shown for chromate, which is an efficient quencher. Sensitizing of the Tb(III) luminescence has been applied for thiol-containing analytes. These compounds are derivatized with maleimidyl salicylic acid to complexes that sensitize the Tb(III) luminescence. From a comparison of the results obtained with normal fluorescence detection and time-resolved sensitized Tb(III) luminescence detection it has become clear that the last method has a higher sensitivity, but in particular a higher selectivity.     

Parallel-current open-tubular liquid chromatography with fluorimetric detection

Some characteristics of the flowing retentive film in parallel-current open-tubular liquid chromatography with a cyclohexanol-water mobile phase and fluorimetric detection are considered. The internal surface of a fused-silica capillari was modified with octamethylcyclotetrasiloxane (D₄ reagent).