Combined HPLC methods

To meet the speed and high efficiencies in separations demanded by the pharmaceutical industry, combined HPLC methods have been frequently used to simultaneously determine combination products [67]. A stability-indicating method for the simultaneous determination of aspirin and warfarin in warfarin sodium/aspirin combination tablets has been recently developed and validated [68]. In another example [69], the simultaneous determination of enalapril (2) and its two degradants, enapril-DKP (3) and enalapril-diacid (4), and felodipine (5) and its degradant, named HI52/37 (6) was achieved using combined method approach. 

The chromatogram in Fig. 2.5 demonstrates that the retention of (3), (5) and (6) is governed by reversed-phase partitioning processes, whereas for (2) and (4), both reverse-phase and cation-exchange mechanism are operating. Again, this method was validated to be precise, accurate, specific and robust. 

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A simple, specific, and accurate reversed-phase HPLC method for the assay of niclosamide and several anthelmintics in veterinary products was reported [73], The method afforded rapid and efficient separation, good resolution, and identification of the examined compounds, alone or combined. The method was used to quantify these drugs, alone or in combination, in tablet, powder, and liquid formulations. 

The near-IR technique has been used very successfully for moisture determination, whole tablet assay, and blending validation [23]. These methods are typically easy to develop and validate, and far easier to run than more traditional assay methods. Using the overtone and combination bands of water, it was possible to develop near-IR methods whose accuracy was equivalent to that obtained using Karl-Fischer titration. The distinction among tablets of differing potencies could be performed very easily and, unlike HPLC methods, did not require destruction of the analyte materials to obtain a result. 

The same group reported in 1986 a sensitive and selective HPLC method employing CL detection utilizing immobilized enzymes for simultaneous determination of acetylcholine and choline [187], Both compounds were separated on a reversed-phase column, passed through an immobilized enzyme column (acetylcholine esterase and choline oxidase), and converted to hydrogen peroxide, which was subsequently detected by the PO-CL reaction. In this period, other advances in this area were carried out such as the combination of solid-state PO CL detection and postcolumn chemical reaction systems in LC [188] or the development of a new low-dispersion system for narrow-bore LC [189],... 

Because of the good validation parameters, the RP-HPLC method combined with SPE has been proposed for the investigation of the pharmacokinetics of RCA in plasma and brain [108],... 

Pluym et al. used a combination of CZE and HPLC methods to develop the impurity profile of domperidone. Each individual method was unable to resolve all impurities. 

A rather sensitive RP-HPLC method combined with UV (270 nm) and ESI-MS detection has been established for the determination of flavonoids and other phenolic compounds in various biological matrices.LODs based on UV data of flavonoids in onion and soybean were 6 to 42 pmol injected, which corresponded with analyte concentrations of 0.08 to 0.63 mg/... 

The prototype of the tobacco specific nitrosamines, NNN, has been detected in both tobacco smoke and unburned tobacco. Various analytical methods have been used including gas chromatography (GLC) (13,14,15,16) combined GLC-mass spectrometry (17), thin layer chromatography (18) high pressure liquid chromatography (HPLC) (19,20), and combined HPLC-thermal energy analysis (21). 

Since APCI and ESI interfaces operate at atmospheric pressure and do not depend upon vacuum pumps to remove solvent vapor, they are compatible with a wide range of HPLC flow rates. HPLC methods that have been developed using conventional detectors such as UV/VIS, IR, or fluorescence are usually transferable to LC/MS systems without adjustment. However, the solvent system should contain only volatile solvents, buffers, or ion-pair agents to reduce fouling of the mass spectrometer ion source. In the case of chlorophyll solvent systems, isocratic and gradient combinations of methanol, acetonitrile, water, acetone, and/or ethyl acetate have been used for APCI or ESI LC/MS. Unlike continuous-flow FAB/LSIMS, no sample matrix is necessary. 

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