High-pressure liquid chromatography of drugs. An evaluation of an octadecylsilane stationary phase

Recent development in liquid chromatography stationary phases for separation of Traditional Chinese Medicine components

Traditional Chinese Medicine (TCM) is an ancient medical practice which has been used to prevent and cure diseases for thousands of years. TCMs are frequently multi-component systems with mainly unidentified constituents. The study of the chemical compositions of TCMs remains a hotspot of research. Different strategies have been developed to manage the significant complexity of TCMs, in an attempt to determine their constituents. Reversed-phase liquid chromatography (RPLC) is still the method of choice for the separation of TCMs, but has many problems related to limited selectivity. Recently, enormous efforts have been concentrated on the development of efficient liquid chromatography (LC) methods for TCMs, based on selective stationary phases. This can improve the resolution and peak capacity considerably. In addition, high-efficiency stationary phases have been applied in the analysis of TCMs since the invention of ultra high-performance liquid chromatography (UHPLC). This review describes the advances in LC methods in TCM research from 2010 to date, and focuses on novel stationary phases. Their potential in the separation of TCMs using relevant applications is also demonstrated.

Mitigation of signal suppression caused by the use of trifluoroacetic acid in liquid chromatography mobile phases during liquid chromatography/mass spectrometry analysis via post-column addition of ammonium hydroxide

A method has been developed to reduce the mass spectrometric ion signal suppression associated with the use of TFA as an additive in LC mobile phases. Through post-column infusion of diluted NH(4)OH solution to LC eluents, the ammonium ion introduced causes the neutral analyte-TFA ion pair to dissociate which consequently releases the protonated analyte as free ions into the gas phase (through regular electrospray ionization mechanisms). An ion signal improvement from 1.2 to 20 times for a variety of compounds had been achieved through the application of this method. The molar ratios of NH(4)OH:TFA which result in a reduction of signal suppression were determined to be between 0.5:1 and 50:1. In addition, it was shown that this NH(4)OH infusion method could reduce the level of doubly-charged species and the product ions formed via in-source collision. The use of diluted NH(4)OH solution is favorable since it is compatible with mass spectrometry analysis, and it is applicable in both positive and negative-ion generation mode.

Optimization of normal-phase chromatographic separation of compounds with primary, secondary and tertiary amino groups

The retention behavior of primary, secondary and tertiary amines was studied using normal-phase-HPLC on silica, diol, and cyano stationary phases. Several classes of amines, including benzylamines, anilines, ephedrines, tryptamines, and azatryptamines were chromatographed using mixtures of hexane and ethoxynonafluorobutane with methylene chloride and methanol. Peak tailing, diminished selectivity and low plate count were minimized by the addition of volatile amines to the mobile phase. The optimal additive was n-propylamine at 0.1% concentration. On diol columns, the elution order of free primary, N-N-methyl, and N,N-dimethylamines was predictable, while the elution order of primary and secondary amines on cyano columns varied depending on the alcohol modifier concentration. The feasibility of preparative normal-phase chromatography was demonstrated by the separation of a mixture of primary, secondary and tertiary amines obtained by direct methylation of norephedrine. The procedures described may provide a practical alternative to traditional methods of analysis and purification of potential drug candidates.

Effect of counter-anion concentration on retention in high-performance liquid chromatography of protonated basic analytes

The influence of acid and salt concentration in the mobile phase on the retention of basic analytes has been studied. An increase in the retention of fully protonated analytes with increasing the concentration of inorganic additives was found. The addition of salt, such as perchlorate, trifluoroacetate, and phosphate, leads to the increase of retention for fully protonated analytes while mobile phase pH remains constant. The observed effect was attributed to the interaction of protonated analytes with the counter-anion of acid or salt, which leads to the disruption of the analyte solvation shell and the increase of its hydrophobicity and corresponding increase of retention. A mathematical model for the description of the influence of counter-anion concentration on analyte retention is proposed.

Rapid reversed-phase high-performance liquid chromatographic determination of the regiospecificity of lipoxygenase products on linoleic acid

A new reversed-phase high-performance liquid chromatography method for the separation of regioisomeric products from lipoxygenase acting on linoleic acid was studied. The addition of salts to the mobile phase improved the retention and separation behaviour of 13-hydroperoxy-9,11-octadecadienoic acid and 9-hydroperoxy-10,12-octadecadienoic acid with respect to the results obtained with other mobile phases reported in the literature. The effect of the pH and ionic strength of the buffer on the retention times, capacity factor and separation factor of these lipoxygenase products were also studied. The pH optimum coincided with the pKa of linoleic acid (close to 7 depending on the fatty acid concentration). Phosphate concentrations close to 100 mM considerably reduced the retention times and led to better separation of the mixture of both products. Finally, this method was applied to the identification and separation of two linoleic acid hydroxides (13-hydroxy-9,11-octadecadienoic acid and 9-hydroxy-10,12-octadecadienoic acid) obtained by the reduction of their corresponding hydroperoxides.

Adenine derivatives as phosphate-activating groups for the regioselective formation of 3',5'-linked oligoadenylates on montmorillonite: possible phosphate-activating groups for the prebiotic synthesis of RNA

Methyladenine and adenine N-phosphoryl derivatives of adenosine 5'-monophosphate (5'-AMP) and uridine 5'-monophosphate (5'-UMP) are synthesized, and their structures are elucidated. The oligomerization reactions of the adenine derivatives of 5'-phosphoramidates of adenosine on montmorillonite are investigated. 1-Methyladenine and 3-methyladenine derivatives on montmorillonite yielded oligoadenylates as long as undecamer, and the 2-methyladenine and adenine derivatives on montmorillonite yielded oligomers up to hexamers and pentamers, respectively. The 1-methyladenine derivative yielded linear, cyclic, and A5'ppA-derived oligonucleotides with a regioselectivity for the 3',5'-phosphodiester linkages averaging 84%. The effect of pKa and amine structure of phosphate-activating groups on the montmorillonite-catalyzed oligomerization of the 5'-phosphoramidate of adenosine are discussed. The binding and reaction of methyladenine and adenine N-phosphoryl derivatives of adenosine are described.

Measurement of naloxone in plasma using high-performance liquid chromatography with electrochemical detection

A new sample preparation technique, solid-phase extraction employing the ion-pair reagent octyl sodium sulfate (OSS), was developed for the selective isolation of nanogram quantities of naloxone from human plasma. Plasma samples containing naloxone, OSS, buffer, and naltrexone (internal standard) were applied to octyldecylsilane (C18) cartridges, and the opiates were eluted with 100% methanol. An extraction efficiency of 90% was achieved. Extracts were then examined by reversed-phase high-performance liquid chromatography with electrochemical detection, with mobile phase composition adjusted to optimize separation and quantitation. An oxidizing potential of +0.93 V resulted in an assay sensitivity of 1.0 ng. Thus, plasma naloxone levels of ca. 20 ng/ml could be readily detected and quantified. The intra-assay coefficient of variation at plasma levels of 80 ng/ml was 7.2%. Plasma naloxone values obtained during a 5-h infusion of the drug (0.24 mg/min) in two normal human subjects were stable and reproducible. Thus, the method herein described is applicable to human studies employing naloxone infusion.

The determination of chlorpromazine, related impurities and degradation products in pharmaceutical dosage forms

The literature relating to the determination of chlorpromazine, related impurities and degradation products in pharmaceutical preparations is reviewed. Related impurities and degradation products are defined and official methods of analysis are described. Analytical methods including gravimetric, titrimetric ultraviolet-visible spectrophotometric, chromatographic and electrochemical techniques are discussed.

Complete separation of urinary metabolites of paracetamol and substituted paracetamols by reversed-phase ion-pair high-performance liquid chromatography

A reversed-phase high-performance liquid chromatographic procedure has been developed for the separation of thirteen urinary metabolites of the analgesic drug paracetamol. The method involved the use of radially compressed columns packed with octadecylsilica with a particle diameter of 5 micron. Metabolites were chromatographed by linear gradient elution using an ion-pair solvent system composed of tetrabutylammonium hydroxide and Tris buffered to pH 5.0 with phosphoric acid, and acetonitrile as the organic solvent. Analyses can be performed at the rate of three per hour. This method enables the direct identification of sulphate and glucuronide conjugates of 3-thiomethylparacetamol and 3-thiomethylparacetamol sulphoxide which have only previously been detected following enzyme hydrolysis of urine samples. The application of this fully optimised separation to the study of the metabolism of substituted paracetamols is also discussed.

Investigation of an alkylamine modified and pH-controlled mobile phase for separation of pyrazolone derivatives by high-performance liquid chromatography

The separation of pyrazolone derivatives with different acid-base properties and polarities has been investigated in view of recent concepts about the retention mechanisms in reversed-phase liquid chromatography. It is shown that both the protonated (BH+) and uncharged (B) bases undergo a dual mechanism of retention due to solvophobic and silanophilic interactions. The latter are easily suppressed by the addition of an alkylamine modifier (dibutylamine) to the mobile phase. An ion-pairing mechanism is found to be valid in the retention of negatively charged (A-) pyrazolone derivatives, since the dibutylamine modifier (BH+) serves as an appropriate counter ion. The behaviour of these solutes can be predicted from the pH dependence of their capacity factors, and a new simple method is proposed for calculating the theoretical sigmoidal curves. It is shown that the elution order of the compounds can be changed by control of both the pH value and the modifier concentration in the mobile phase.

Rapid high-performance liquid chromatography of 3-methylhistidine in human urine

An internally standardized method for the determination of 3-methylhistidine in human urine is presented. This methylated amino acid and the chemically analogous internal standard 3-ethylhistidine were isolated from human urine specimens using small columns of cation-exchange resin. Quantification was accomplished by high-performance liquid chromatography using post-column derivatization with o-phthalicdicarboxaldehyde-2-mercaptoethanol followed by fluorometric detection. Sample-to-sample and day-to-day reproducibility were shown to have respective relative standard deviations of 2 and 5% for a human urine specimen containing 250 nmol/ml 3-methylhistidine when using 250 microliter urine per analysis. The chromatographic separation was evaluated in terms of various peak descriptors (capacity factor and retention time) and "Chromatographic Figures of Merit" (peak symmetry and chromatographic efficiency). The utility of the method was demonstrated by its successful application to 1000 human urine specimens.

Determination of hydrophobic parameters by reversed-phase liquid chromatography: theory, experimental techniques, and application in studies on quantitative structure-activity relationships

The use of RPLC retention parameters as descriptors of the hydrophobic nature of bioactive compounds has been evaluated. The relationship between the capacity factor, measured on n-alkyl-bonded stationary phases using binary eluents, and the n-octanol-water partition coefficient has been illustrated experimentally and theoretically. It is suggested that retention parameters, in particular the capacity factor (log kw) obtained by extrapolation of retention data from binary eluents to 100% water, could successfully replace the n-octanol-water partition coefficient in studies on quantitative structure-activity relationship, and that their use may result in a better correlation with biological data.

Improved high-performance liquid chromatographic method for the determination of 6-N,N,N-trimethyllysine in plasma and urine: biomedical application of chromatographic figures of merit and amine mobile phase modifiers

An internally standardized method for the determination of 6-N,N,N-trimethyllysine in human plasma, human urine, rat plasma, rat urine and hydrolyzed rat urine is described. This methylated amino acid and the procedural internal standard 6-N,N,N-trimethyllysine were isolated from the sample matrices using short ion-exchange columns and detected following high-performance liquid chromatography using a postcolumn reaction (o-phthalic-dicarboxaldehyde-2-mercaptoethanol) and fluorometric detection. The reliable detection limit for 6-N,N,N-trimethyllysine was 0.2 nmol/ml in 200 microliters of human plasma. The chromatographic separation exploits the unique properties of a novel tertiary amine mobile phase modifier, 3-(N,N-dimethylamino)-1,2-propanediol. The capacity factor and "Chromatographic Figures of Merit" (including peak asymmetry and relative system efficiency) were calculated for the chromatographic peak representing 6-N,N,N-trimethyllysine in over 2200 injections made while evaluating 900 biological specimens.

Liquid chromatographic assays for barbiturate injections

Precise, rugged, stability-indicating HPLC assays were developed for phenobarbital, pentobarbital, and secobarbital sodium injections. Degradation products of the three barbiturates were prepared and chromatographed to demonstrate method specificity. The degradation products were also used to demonstrate specificity of official GC methods for pentobarbital and secobarbital sodium capsules. The acetylurea degradation products of pentobarbital and secobarbital were characterized following the separation of their diastereoisomers by HPLC.

Determination of clozapine and its metabolites in serum and urine by reversed phase HPLC

A rapid, specific and sensitive method using reversed phase HPLC for the simultaneous determination of clozapine and its two metabolites in serum and urine has been developed. The mobile phase was a mixture of 67% (v/v) methanol in water containing 0.4% tetramethylethylenediamine and 0.32% acetic acid (pH 5.5). The influence of methanol content, the pH of the mobile phase and the effect of adding alkylammonium ions as peak tailing reducer in the mobile phase have been investigated. The solvent for extracting clozapine from serum and urine was ether. 50 microliters of 0.25 M H2SO4 solution was used to redissolve the dry residue to eliminate the endogenous compounds which could otherwise be eluted together with clozapine from the HPLC column. The analysis of a single sample was accomplished within half an hour. The identities of the chromatographic peaks of clozapine and its N-demethyl metabolite collected from the patient urine sample were confirmed by mass spectrometry. The method is sufficiently sensitive (5 ng/ml) and reproducible (CV 2.9%-6.7%) for clinical and pharmacokinetic studies, and preliminary results in these respects are presented.

Drug level monitoring: sedative hypnotics

Barbiturates and other traditional non-barbiturate sedative hypnotics are still extensively prescribed for the treatment of insomnia. There are a number of situations where identification or quantitative determination of these agents in biological fluids is required. Gas-chromatography offers highly sensitive and specific procedures for the determination of these compounds. The use of a nitrogen-specific detector allows a relatively simple sample preparation for sensitive detection and the use of capillary columns with bonded liquid phase allows separation of barbiturates without derivatization. In recent years liquid chromatography has also been extensively applied to the determination of these compounds. Sensitivity and selectivity of detection of barbiturates have been improved with the use of an alkaline mobile phase. Immunoassays for the determination of therapeutic concentrations of phenobarbital are very popular as the assays can be rapidly performed automatically. Use of these techniques has been extended for emergency detection of barbiturate overdose and for monitoring high-dose pentobarbital therapy.

Paired-ion liquid chromatographic method for the analysis of a phenanthrenemethanol antimalarial in whole blood

A sensitive and specific high-performance liquid chromatographic (HPLC) assay was developed for the determination of the candidate antimalarial (+/-)-(1,3-dichloro-6-trifluoromethyl-9-phenanthryl)-3-di-(n-butyl )aminopropanol hydrochloride in whole blood. A reversed-phase, paired-ion (lauryl sulfate) system achieved separation of the antimalarial and internal standard from interfering constituents with a sensitivity limit of 10 ng/mL by UV detection (254 nm). Chromatographic variables (counterion concentration, pH, and column temperature) were examined to determine their effect on assay characteristics (retention, efficiency, and relative response) in clinical analysis. The antimalarial was isolated from 2.0 mL of whole blood using overnight extraction with 30% ethyl acetate in hexane followed by an acid/base partition sequence to remove major interferences. Overall recovery for the antimalarial was 84% with a CV of 5.0%, and the recovery of the internal standard was 81% (CV = 3.6%). The assay was validated by analysis of both intra- and interlaboratory samples. The assay was applied to the analysis of whole blood samples taken from a 30-year-old healthy human male who had received a single 14.1-mg/kg oral dose. The stability of the antimalarial in whole blood for up to 4 months and in sample extracts for up to 34 d at -17 degrees C was also demonstrated.

Analysis of N-benzoyl-L-tyrosyl-p-aminobenzoic acid (bentiromide) metabolites in urine by ion-pair high-performance liquid chromatography

A high-performance liquid chromatographic method is described for the analysis of bentiromide metabolites in urine. The procedure involves no more than direct injection of the diluted urine sample, obviating the need for an extraction step or an internal standard. A mu Bondapak C18 column is used with a mobile phase of 0.01 M tetrabutylammonium chloride (pH 7.4)--methanol (9:1). A flow-rate of 1.4 ml/min, detection at 254 nm and column temperature of 40 degrees C are employed. These conditions were achieved by investigating the effects of mobile phase pH, and concentrations and types of organic modifiers, buffers and ion-pairing agents on the resolution of the metabolites. The analysis time is 18 min per sample and the coefficient of variation on replicate assays is less than 10% for most concentrations studied. Analytical recoveries were between 95 and 100% throughout the appropriate concentration ranges and no interferences were obtained with the exception of p-acetamidobenzoyl glucuronide which could be eliminated by treatment of the samples with beta-glucuronidase. Concentration profiles of the metabolites were studied in normal subjects, and the method was found to be potentially useful for clinical situations in which the existing bentiromide test leads to ambiguous results because of small bowel and hepatic dysfunctions.

Factors affecting the retention of quaternary ammonium ions in reversed-phase high-performance liquid chromatography

The influence of solute structure (charge and hydrophobic substitution), organic modifier (type and concentration) and ion-pairing agent on the retention of nine quaternary- and bis-quaternary ammonium ions has been investigated in reversed-phase HPLC on ODS-silica. A functional group approach was taken to elucidate the influences of substitution on the charged nitrogen and the addition of a second positive charge to the solute molecule. These and other factors contributing to solute retention are discussed within the context of solvophobic theory. Hydrophobic effects and the solvation of the charged centre are shown to be the major factors contributing to retention in non-ion pair eluents. In addition, various electrostatic interactions in the mobile and stationary phases may contribute to solute retention in eluents containing an ion-pairing agent. It was found that ternary eluents containing hexane sulphonic acid and two organic modifiers offer certain selectivity advantages when compared with binary eluents.

Belladonna alkaloids and phenobarbital combination pharmaceuticals analysis II: High-performance liquid chromatographic determination of phenobarbital

A high-performance liquid chromatographic separation is described for the analysis of phenobarbital in combination pharmaceutical dosage forms containing belladonna alkaloids. A mobile phase of 0.003 M tetramethylammonium chloride in water-methanol (3:2, pH 7.4) was used to separate phenobarbital from belladonna alkaloids on an octadecylsilane column in less than 7 min. The column effluent was monitored at 240 nm, which resulted in a detection limit of 6 ng of phenobarbital. The method is applicable to elixirs, tablets, and capsules with no preliminary extraction procedure. Data from the application of the method to commercial products is also presented.

Reversed-phase chromatography of urinary metabolites of paracetamol using ion suppression and ion pairing

High-performance liquid chromatography (HPLC) has proven particularly useful for the study of paracetamol metabolism. Two alternative methods were developed using reversed-phase C18 columns. A rapid ion suppression technique was used for the analysis of free paracetamol, paracetamol mercapturic acid and cysteine conjugate in urine samples obtained from isolated perfused rat kidney preparations, which has conveniently demonstrated the oxidative metabolic capacity of the kidney towards paracetamol. A somewhat longer, but higher resolution, ion-pair HPLC procedure was developed for the analysis of paracetamol metabolites in urine samples from experimental animals. The ion-pairing solvent was composed of tetrabutylammonium hydroxide, Tris and EDTA buffered to pH 7.2 with phosphoric acid. Gradient programming was further used to enhance resolution. Using this system two new metabolites, the sulphate and glucuronide conjugates of 3-thiomethyl-paracetamol were detected and routinely determined along with other known paracetamol metabolites, viz. free paracetamol, paracetamol sulphate, glucuronide, mercapturic acid, and cysteine conjugates, 3-methoxyparacetamol glucuronide and sulphate, p-aminophenol and its O-glucuronide and O-sulphate conjugates. Phenolic O-substituted glucuronide and sulphate conjugates of N-hydroxyparacetamol were also separated.

Analytical aspects of barbiturate abuse: identification of drugs by the effective combination of gas-liquid, high-performance liquid and thin-layer chromatographic techniques

Chromatographic retention data for a group of barbiturates have been measured on seven chromatographic systems (two gas--liquid chromatographic (GLC), three high-performance liquid chromatographic (HPLC) and two thin-layer chromatographic systems) and the value of these for barbiturate identification has been discussed. The overall correlations observed between pairs of systems are generally low; however, specific groups of barbiturates show very high correlations and this determines the approach to the selection of two or more systems to increase chromatographic discrimination of the barbiturate group. Column chromatographic techniques with lipophilic phases (GLC using SE-30, HPLC using ODS-silica) are most suitable for barbiturate identification. Changes of eluent pH in reversed-phase HPLC proved very effective for the separation of barbiturates with closely related structures.