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The determination of peak purity in chromatographic methods used in pharmaceutical analysis is a vital step in evaluating specificity to ensure the safe production of medicines. Current peak purity assessments accomplished through diode array detectors (DAD) and mass spectrometry (MS) have some challenges that could lead to misinterpreted purity values when the sample under evaluation is susceptible to the limitations of these techniques. Two-dimensional liquid chromatography (2D -LC) has the ability to resolve difficult-to-separate mixtures, and has been widely adapted to biological compounds. This study proposes a 2D-LC screening method as a means of determining peak purity of active pharmaceutical ingredients (API) and their related substances. Results confirm that the 2D-LC screen was successful in separating API/impurity mixtures in all 10 test cases studied. Selection of screening conditions including mobile phase choice and column selection is discussed along with the effectiveness and idiosyncrasies of peak purity analysis by 2D-LC., The possible pitfalls of diode array detectors-ultraviolet region (DAD-UV) absorbance based peak purity determinations have been well documented (1,2). Some possible issues with DAD-UV based peak purity include: i) co-eluted [...]

In two recent installments of 'LC Troubleshooting,' I discussed the roles of mixers in liquid chromatography (LC) systems in a general sense (1,2). Mixers are often used in pumping systems [...]

The article offers information on the roles of mixers in liquid chromatography (LC) systems. Topics discussed include information on the use of mixers in pumping systems to remove irregularities in mobile phase composition; discussions on the effect of diluent and eluent mismatch on hydrophilic interaction liquid chromatography separations; and the analyte recovery and plugging of LC components and columns.

A new decarbonylation reaction is observed for [(K2-acetate)Pd(K2-diphosphine)]+ complexes. Gas-phase IR experiments identify the product as [CH3Pd(OP(Ph2)CH2PPh2)]+. DFT calculations uncovered a plausible mechanism involving O atom abstraction by the diphosphine ligand within the coordination sphere to yield the acetyl complex, [CH3COPd(OP(Ph2)CH2PPh2)]+, which then undergoes decarbonylation. [ABSTRACT FROM AUTHOR]

The formation and investigation of sulfur-based cysteine radicals cationized by a group 1A metal ion or Ag+ in the gas phase are reported. Gas-phase ion-molecule reactions (IMR) and infrared multiple-photon dissociation (IRMPD) spectroscopy revealed that the Li+, Na+, and K+ adducts of the cysteine radical remain S-based radicals as initially formed. Theoretical calculations for the three alkali metal ions found that the lowest-energy isomers are Cα-based radicals, but they are not observed experimentally owing to the barriers associated with the hydrogen-atom transfer. A mechanism for the S-to-Cα radical rearrangement in the metal ion complexes was proposed, and the relative energies of the associated energy barriers were found to be Li+>Na+>K+ at all levels of theory. Relative to the B3LYP functional, other levels of calculation gave significantly higher barriers (by 35-40 kJ mol−1 at MP2 and 44-47 kJ mol−1 at the CCSD level) using the same basis set. Unlike the alkali metal adducts, the cysteine radical/Ag+ complex rearranged from the S-based radical to an unreactive species as indicated by IMRs and IRMPD spectroscopy. This is consistent with the Ag+/cysteine radical complex having a lower S-to-Cα radical conversion barrier, as predicted by the MP2 and CCSD levels of theory. [ABSTRACT FROM AUTHOR]

Intramolecular hydrogen atom transfer (HAT) was examined in homocysteine (Hcy) thiyl radical/alkali metal ion complexes in the gas phase by combination of experimental techniques (ion-molecule reactions and infrared multiple photon dissociation spectroscopy) and theoretical calculations. The experimental results unequivocally show that metal ion complexation (as opposed to protonation) of the regiospecifically generated Hcy thiyl radical promotes its rapid isomerisation into an α-carbon radical via HAT. Theoretical calculations were employed to calculate the most probable HAT pathway and found that in alkali metal ion complexes the activation barrier is significantly lower, in full agreement with the experimental data. This is, to our knowledge, the first example of a gas-phase thiyl radical thermal rearrangement into an α-carbon species within the same amino acid residue and is consistent with the solution phase behaviour of Hcy radical. [ABSTRACT FROM AUTHOR]

We report on the carrier-rotor coupling effect in perovskite organic-inorganic hybrid lead iodide (CH3NH3PbI3) compounds discovered by isotope effects. Deuterated organic-inorganic perovskite compounds including CH3ND3PbI3, CD3NH3PbI3, and CD3ND3PbI3 were synthesized. Devices made from regular CH3NH3PbI3 and deuterated CH3ND3PbI3 exhibit comparable performance in band gap, current-voltage, carrier mobility, and power conversion efficiency. However, a time-resolved photoluminescence (TRPL) study reveals that CH3NH3PbI3 exhibits notably longer carrier lifetime than that of CH3ND3PbI3, in both thin-film and single-crystal formats. Furthermore, the comparison in carrier lifetime between CD3NH3PbI3 and CH3ND3PbI3 single crystals suggests that vibrational modes in methylammonium (MA(+)) have little impact on carrier lifetime. In contrast, the fully deuterated compound CD3ND3PbI3 reconfirmed the trend of decreasing carrier lifetime upon the increasing moment of inertia of cationic MA(+). Polaron model elucidates the electron-rotor interaction.

Background: Quantitation of CAM H4 in biological matrix has been a challenge due to low concentrations, instability and the existence of four CAM diastereomers. Historically, either inactive clopidogrel acid or CAM diastereomers without separation were measured for exposure and PK parameters., Results: This study presents a sensitive and fast UHPLC-MS/MS method for simultaneous quantitation of clopidogrel, clopidogrel acid and active metabolite H4 in human plasma. The method demonstrated the separation of H4 from other isomers and yet retained clopidogrel acid. Matrix stabilities, accuracy and precision, recovery and matrix effect were evaluated during development., Conclusion: With LLOQ of 0.05 ng/ml for clopidogrel and H4, and 5.5 min LC gradient, it is the most sensitive and fastest method to our knowledge.

Radical migration, both intramolecular and intermolecular, from the tyrosine phenoxyl radical Tyr(O(∙)) to the cysteine radical Cys(S(∙)) in model peptide systems was observed in the gas phase. Ion-molecule reactions (IMRs) between the radical cation of homotyrosine and propyl thiol resulted in a fast hydrogen atom transfer. In addition, radical cations of the peptide LysTyrCys were formed via two different methods, affording regiospecific production of Tyr(O(∙)) or Cys(S(∙)) radicals. Collision-induced dissociation of these isomeric species displayed evidence of radical migration from the oxygen to sulfur, but not for the reverse process. This was supported by theoretical calculations, which showed the Cys(S(∙)) radical slightly lower in energy than the Tyr(O(∙)) isomer. IMRs of the LysTyrCys radical cation with allyl iodide further confirmed these findings. A mechanism for radical migration involving a proton shuttle by the C-terminal carboxylic group is proposed.