Quantitative NMR assay for aspirin, phenacetin, and caffeine mixtures with 1,3,5-trioxane as internal standard

Automated Parallel Dialysis for Purification of Polymers

The implementation of a dialysis method for the simultaneous purification of different polymer materials in a commercially available automated parallel synthesizer (APS) is discussed. The efficiency of this "unattended" automated parallel dialysis (APD) method was investigated by means of proton nuclear magnetic resonance (¹H-NMR) measurements, which confirmed that the method enables the removal of up to 99% of the unreacted monomer derived from the synthesis of the corresponding polymers in the APS. Size-exclusion chromatography (SEC) revealed that the molar mass and molar mass distribution of the investigated polymers did not undergo significant changes after the application of the APD method. The method discussed herein can be regarded as a good alternative to the "unattended" and reliable purification of polymer libraries prepared in APS. This method may be useful for overcoming current limitations of high-throughput/-output (HT/O) synthesis of polymer libraries, where purification of the generated materials currently represents a significant constraint for establishing fully automated experimental workflows necessary to advance towards a full digitalization of research and development of new polymers for diverse applications.

Quantitative NMR Interpretation without Reference

As has been documented numerous times over the years, nuclear magnetic resonance (NMR) experiments are intrinsically quantitative. Still, quantitative NMR methods have not been widely adopted or largely introduced into pharmacopoeias. Here, we describe the quantitative interpretation of the 1D proton NMR experiment using only absolute signal intensities with the variation of common experimental parameters and their application.

Anodic Determination of Acetylsalicylic Acid at a Mildly Oxidized Boron-Doped Diamond Electrode in Sodium Sulphate Medium

Differential pulse voltammetry (DPV) and chronoamperometry (CA) were used to detect and determine acetylsalicylic acid (ASA) at a mildly oxidized boron-doped diamond (BDD) electrode in a neutral sodium sulphate solution as supporting electrolyte. ASA determination in unbuffered medium was achieved using neutralized standard and real samples. Over the concentration range of 0.01 mM–0.1 mM, linear calibration plots of anodic current peaks in DPV and anodic currents in CA experiments versus concentration were obtained with very high correlation coefficients and good sensitivity values. The limits of detection were situated around 1 μM. The association of DPV and CA techniques with standard addition method represented a suitable option for the determination of ASA in real samples such as pharmaceutical formulations.

Monitoring of ampicillin and its related substances by NMR

A 1H NMR procedure for the monitoring of ampicillin (Amp) and its main related substance in different media, has been developed. The characteristics peak of Amp, 6-aminopenicillanic acid (6-APA), phenylglycine (PhG), and penicilloic acid in the range of 0.5-0.9 and 3.0-4.5 ppm were used for their identification in drugs and serum samples. The quantitative works were performed based on the intensity of protons of the methyl group link to the beta-lactam cyclic of Amp and 6-APA and the aromatic protons of PhG relative to the protons of methylene group of maleic acid, as internal standard, at constant temperature. The resulting data are compared with those obtained with an HPLC method proposed by British Pharmacopoeia. Statistical studies show that, at a confidence limit of %95, there is no significant difference between the two methods. In comparison with the HPLC method, the proposed NMR method does not require any sample pretreatment, standard solution preparation, long analysis time and use of any carcinogenic solvent. The method was applied to the determination of Amp and its related substances in synthetic mixtures, drug powders and serum samples.

Quantitation of combinatorial libraries of small organic molecules by normal-phase HPLC with evaporative light-scattering detection

The advantages of evaporative light-scattering detection over UV detection for the quantitation of combinatorial libraries composed of small organic compounds by HPLC are described. The detector's response is independent of the sample chromophore, which makes it well-suited to chromatographic analyses of mixtures of dissimilar solutes. Thus, HPLC with evaporative light-scattering detection offers to potential for reducing false positive or false negative results in screening assays, because of its ability to detect the presence of impurities that absorb poorly in the UV (e.g., those impurities originating from the polymeric support). Furthermore, the evaporative light-scattering detector exhibits a nearly equivalent response to compounds of similar structural class. Hence, rapid quantitation of compound libraries may be carried out with the use of a single external standard. For example, the quantitation errors, based on a single external standard, for a series of steroids, hydantoins, and BOC- and Fmoc-protected amino acids by normal-phase HPLC with evaporative light-scattering detection average approximately +/-10%. The application of the evaporative light-scattering detector to the quantitation of low-level sample impurities and the detector's compatibility with gradient elution are also described.

Applications of quantitative 1H- and 13C-NMR spectroscopy in drug analysis

The usefulness of 1H and 13C Fourier transform (FT) nuclear magnetic resonance spectroscopy (1H- and 13C-NMR) as quantitative methods stems from the potential direct relationship between the area under an NMR peak and the number of the particular type of nuclei that give rise to the signal, though it is necessary, especially for quantitative 13C-NMR, to take some precautions. The experimental limitations that have to be overcome in order to obtain quantitative 13C-NMR spectra are associated with the relaxation time, the nuclear Overhauser effect (NOE), and the NMR instrument itself (filter characteristics, power level of the exciting pulse, dynamic range, digital resolution). Practical problems aside, 13C-NMR has a greater potential than 1H-NMR for the study of organic systems. The sensitivity of 13C chemical shifts to small differences in molecular environment, coupled with a large chemical shift range, gives a "chromatographic" separation of resonances of interest, and has made 13C-NMR an attractive method for analysing complex mixtures. Some applications of quantitative 1H- and 13C-NMR spectroscopy in drug analysis are discussed.

Simultaneous spectrophotometric analysis of a ternary mixture of pharmaceuticals--assay for meclozine hydrochloride, pyridoxine hydrochloride and caffeine

Based on an extension of Vierordt's method and in continuation of earlier work, the simultaneous analysis of a ternary mixture of meclozine hydrochloride, pyridoxine hydrochloride and caffeine is discussed. Using 0.01 M methanolic sodium hydroxide as the solvent, the absorbance of the solution of the mixture is recorded at 230, 273 and 307 nm. The concentration of each component is determined by the solution of three simultaneous equations.

Determination of aspirin by pre-column transacetylation reaction of 3-aminophenol and reversed-phase high-performance liquid chromatography: simultaneous determination of aspirin, acetaminophen, and caffeine

The accuracy of the measurement of aspirin by high-performance liquid chromatography (HPLC) is reduced by its hydrolysis into salicylic and acetic acids during sample preparation. The ready and quantitative transacetylation of 3-aminophenol by aspirin, giving 3-hydroxyacetanilide (which is stable), has been utilized as a pre-column reaction for the determination of aspirin either alone or in the presence of acetaminophen and caffeine by reversed-phase HPLC.