The determination of 2beta-(3-hydroxypropoxy)-1alpha,25-dihydroxy vitamin D3 (ED-71) in human serum by high-performance liquid chromatography-electrospray tandem mass spectrometry

A liquid chromatography-electrospray ionization tandem mass spectrometric (LC/ESI-MS/MS) method for the determination of 2beta-(3-hydroxypropoxy)-1alpha,25-dihydroxy vitamin D3 (ED-71) in human serum has been developed. ED-71 in human serum was extracted using two solid-phase extraction steps on Bond Elut C18 and NH2 cartridge. The separation of ED-71 and preED-71 isomer was attained by LC using 2 mmol/L ammonium acetate-methanol (15:85, v/v) as a mobile phase on a Symmetry C18 column (5 microm, 150 mm x 2.1mm i.d.). ESI-MS/MS analysis was operated using selected reaction monitoring (SRM) in positive ion mode. The method achieved a lower limit of quantitation of 25 pg/mL. The calibration curve (25-3200 pg/mL) gave acceptable linearity (r>0.9964). Intra-assay precision ranged from 2.3 to 9.7%. Inter-assay precision ranged from 1.0 to 3.4%. The accuracy was within 90.8-107.0%. This highly sensitive and reproducible method is able to determine only biologically active ED-71 by separating it from preED-71, which is considered to be applicable for the determination of serum samples from pharmacokinetic studies in human.     

Sensitive and specific LC-ESI-MS/MS method for the determination of a styrylquinoline, BA011FZ041, a potent HIV anti-integrase agent, in rat plasma

A LC-MS/MS method was validated for the determination of BA011FZ041, a styrylquinoline derivative. After addition of BA011FZ055 as internal standard (IS), the method involved solid phase extraction (SPE), LC separation with an ether-phenyl column and quantification by MS/MS after positive ESI. The calibration curve, ranging from 1 to 500 ng/mL was fitted to a 1/x-weighted quadratic regression model. Lower limit of quantification (LLOQ) was 1 ng/mL using 100 microL of plasma. Intra- and inter-assay precision and accuracy values were within the regulatory limits. The method was successfully applied to the determination of BA011FZ041 in rat plasma and PBMCs after i.v. dosing.     

The novel sensitive and high throughput determination of cefepime in mouse plasma by SCX-LC/MS/MS method following off-line μElution 96-well solid-phase extraction to support systemic antibiotic programs

A sensitive and high throughput off-line μElution 96-well solid-phase extraction (SPE) followed by strong cation exchange (SCX) liquid chromatography with tandem mass spectrometry (LC/MS/MS) quantification for determination of cefepime has been developed and validated in mouse plasma. Using the chemical analog, ceftazidime as an internal standard (IS), the linear range of the method for the determination of cefepime in mouse plasma was 4–2048 ng/mL with the lower limit of quantitation level (LLOQ) of 4 ng/mL. The inter- and intra-assay precision and accuracy of the method were below 9.05% and ranged from 95.6 to 113%, respectively, determined by quality control (QC) samples at five concentration levels including LLOQ. After μElution SPE, 71.1% of cefepime was recovered. The application of the validated assay for the determination of cefepime in mouse pharmacokinetics (PK) samples after intravenous (IV) and subcutaneous (SC) doses was demonstrated.     

Ultra sensitive quantitation of endogenous oxytocin in rat and human plasma using a two-dimensional liquid chromatography-tandem mass spectrometry assay

Oxytocin (OT) is a neuropeptide with an extremely low endogenous level (low pg/ml) in human plasma. It is very challenging to develop a highly sensitive assay to measure endogenous OT, including radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA). Electrospray ionization (ESI) liquid chromatography–tandem mass spectrometry (LC–MS/MS) can provide high-throughput and selective methods for quantification of peptides in biological samples. A novel and highly sensitive two-dimensional LC–MS/MS (2D-LC–MS/MS) assay combining solid-phase extraction (SPE) has been developed and validated for the determination of endogenous OT in both human and rat plasma. The lower limit of quantification (LLOQ) was 1.00 pg/ml for human and 50.0 pg/ml for rat. Human plasma diluted with water (1:6, v/v) was successfully optimized as a surrogate matrix for human to prepare standard curves without endogenous interference. The extraction efficiency and absolute recovery were above 65.8% using the HLB SPE procedure, and matrix effects were lower than 12%. The method was validated in the range of 1.00–250 pg/ml for human plasma and 50.0–10,000 pg/ml for rat plasma with precision less than 12.7% and accuracy less than 7%.     

Determination of a neuroprotective agent (S)-(+)-BMS-204352 in human, rat and dog plasma by enantioselective liquid chromatography-tandem mass spectrometry

A sensitive liquid chromatography-electrospray ionization tandem mass spectrometry method (LC/ESI/MS/MS) for the enantioselective determination of (S)-(+)-BMS-204352, a potent and specific maxi-K channel opener, in human, rat and dog plasma was developed. (S)-(+)-BMS-204352, its enantiomer (R)-(--)-BMS-204353 and the internal standard (13C-deuterated racemate of (S)-(+)-BMS-204352) were extracted from plasma using toluene. Chromatographic separation for the enantiomers was achieved on a Chiralcel OD-H analytical column with a run time of 8 min. An aqueous mobile phase modifier was added post column to enhance the mass spectrometer sensitivity. ESI mass spectra were acquired in the negative mode with selected reaction monitoring. The limit of quantitation (LLOQ) is 0.10 ng/mL for human plasma assay. Samples from a clinical study and two animal studies were processed using these procedures. Based on the in vivo data, lack of inversion of (S)-(+)-BMS-204352 to (R)-(--)-BMS-204353 was demonstrated in human, rat and dog after administration of the drug. A sensitive non-enantioselective LC/ESI/MS/MS assay has also been developed for (S)-(+)-BMS-204352 which uses a similar extraction procedure with a C18 column with a limit of quantitation at 0.05 ng/mL. Human study samples were analyzed by both methods and the correlation coefficient between both sets of data is greater than 0.99.     

Enantioselective analyses of chloroquine and hydroxychloroquine in rat liver microsomes through chiral liquid chromatography–tandem mass spectrometry

An efficient, sensitive and selective liquid chromatography–tandem mass spectrometry (LC–MS/MS) chiral analysis method was established for determination of chloroquine and hydroxychloroquine enantiomers in rat liver microsomes. Effects of polysaccharide chiral stationary phases and basic additives on chiral separations of two analytes were discussed in detail. Amylose tris(3, 5-dimethylphenylcarbamate)-coated chiral stationary phase showed the best separation performance for them with acetonitrile-diethylamine-ethanol-diethylamine mixture (90:0.1:10:0.1, v/v/v/v) among four chiral stationary phases. Then, multiple reaction monitoring mode was selected as the data acquisition for determination of two pairs of enantiomers. The proposed LC–MS/MS chiral analysis method was validated in terms of linearity, accuracy, precision, and specificity. Good linearity with correlation coefficient over 0.998 was obtained in the concentration range of 0.05–5 μM. Limits of quantification for chloroquine and hydroxychloroquine enantiomers were 5.0 and 1.0 nM, respectively. The recoveries ranged from 81.14% to 111.09%. The intra-day and inter-day relative standard deviation were less than 6.5%. Moreover, concentrations of chloroquine and hydroxychloroquine enantiomers in rat liver microsomes were determined through the proposed LC–MS/MS analysis method. After incubated with rat liver microsomes for 10 min, the enantiomeric factor of hydroxychloroquine decreased from 0.50 to 0.45 (p < 0.001). In brief, our developed determination method for chloroquine and hydroxychloroquine enantiomers through LC–MS/MS spectrometry showed the characteristics of high-efficiency, fast speed, and very low detection limit, and would be greatly beneficial for screening and quantitation of them in biological matrices.     

Simultaneous determination of 18 tetrahydrocorticosteroid sulfates in human urine by liquid chromatography/electrospray ionization-tandem mass spectrometry

A liquid chromatography (LC)/electrospray ionization (ESI)-mass spectrometry (MS) method for the direct determination of eighteen tetrahydrocorticosteroid sulfates in human urine has been developed. The analytes were 3- and 21-monosulfates and 3,21-disulfates of tetrahydrocortisol (THF), tetrahydrocortisone (THE), tetrahydro-11-deoxycortisol (THS), and their corresponding 5α-H stereoisomers. The mass spectrometric behavior of these sulfates in negative-ion ESI-MS/MS revealed the production of intense structure specific product ions within the same group of sulfates and permitted distinction between regioisomeric sulfates by collision-induced fragmentation with the MS/MS technique using a linear ion-trap instrument. For the quantitative analysis, selected reaction monitoring analysis in the negative-ion detection mode using triple-stage quadrupole mass spectrometer was performed by monitoring transitions from [M−H]⁻ to the most abundant product ion of each tetrahydrocorticosteroid sulfate. After addition of 3- and 21-monosulfates of [2,2,3β,4,4-d₅]-THF, -THE, and -THS as internal standards, urine sample was applied to a solid phase extraction using a lipophilic-weak anion exchange cartridge column, and then analyzed by LC/ESI-MS/MS. The method had satisfactory performance in terms of intra- and inter-assay precision (less than 9.7% and 9.6%, respectively), and accuracy (91.2–108.2%). The limit of quantification was lower than 2.5 ng/mL for all sulfates examined. We applied this method to determine the concentration of eighteen tetrahydrocorticosteroid sulfates in the urine of healthy subjects. Thus, we have developed a sensitive, precise and accurate assay for urinary tetrahydrocorticosteroid sulfates that should be useful for clinical and biological studies.     

Determination of rimantadine in rat plasma by liquid chromatography/electrospray mass spectrometry and its application in a pharmacokinetic study

A rapid and sensitive liquid chromatography/mass spectrometry (LC/MS) method was developed and validated for the determination of rimantadine in rat plasma. Rimantadine was extracted by protein precipitation with methanol, and the chromatographic separation was performed on a C(18) column. The total analytical run time was relatively short (4.6 min), and the limit of assay quantification (LLOQ) was 2 ng/mL using 50 microL of rat plasma. Rimantadine and the internal standard (amantadine) were monitored in selected ion monitoring (SIM) mode at m/z 180.2 and 152.1, respectively. The standard curve was linear over a concentration range from 2 to 750 ng/mL, and the correlation coefficients were greater than 0.999. The mean intra- and inter-day assay accuracy ranged from 100.1-105.0% to 100.3-104.0%, respectively, and the mean intra- and inter-day precision was between 1.3-2.3% and 1.8-3.0%, respectively. The developed assay method was successfully applied to a pharmacokinetic study in rats after oral administration of rimantadine hydrochloride at the dose of 20 mg/kg.     

Determination of Simvastatin in human plasma by liquid chromatography-mass spectrometry

A simple, sensitive and selective liquid chromatography coupled with electrospray ionization mass spectrometry (LC/ESI/MS) method for the determination of simvastatin (I) has been developed. After extraction by ethyl acetate, using lovastatin (II) as internal standard, solutes are separated on a C(18) column with a mobile phase consisting of methanol-water (9:1). Detection is performed on an atmospheric pressure ionization single quadruple mass spectrometer equipped with an ESI interface and operates in positive ionization mode. Simvastatin quantification was realized by computing peak area ratio (I/II) of the extracts analyzed in SIM mode (m/z: 441 and m/z: 427 for I and II, respectively) and comparing them with calibration curve (r=0.9997). Accuracy and precision for the assay were determined by calculating the intra-batch and inter-batch variation at three concentrations 0.1, 5.0, 10.0 ng/ml; the intra batch relative standard deviation (RSD) was less than 10% and ranged from 1.8 to 8.5%, respectively; the inter-batch RSD was less than 20% and ranged from 4.1 to 16.5%. The limit of detection was 0.05 ng/ml.     

Determination of imidapril and imidaprilat in human plasma by high-performance liquid chromatography–electrospray ionization tandem mass spectrometry

A sensitive and specific assay of imidapril and its active metabolite, imidaprilat, in human plasma has been developed. This method is based on rapid isolation and high-performance liquid chromatography (HPLC)–electrospray ionization (ESI)-tandem mass spectrometry (MS–MS). Imidapril and imidaprilat were isolated from human plasma using OASIS HLB (solid-phase extraction cartridge), after deproteinization. The eluent from the cartridge was evaporated to dryness, and the residue was reconstituted in mobile phase and injected into the HPLC–ESI-MS–MS system. Each compound was separated on a semi-micro ODS column in acetonitrile–0.05% (v/v) formic acid (1:3, v/v). The selected ion monitoring using precursor→product ion combinations of m/z 406→234 and 378→206, was used for determination of imidapril and imidaprilat, respectively. The linearity was confirmed in the concentration range of 0.2 to 50 ng/ml in human plasma, and the precision of this assay, expressed as a relative standard deviation, was less than 13.2% over the entire concentration range with adequate assay accuracy. The HPLC–ESI-MS–MS method correlates well with the radioimmunoassay method, therefore, it is useful for the determination of imidapril and imidaprilat with sufficient sensitivity and specificity in clinical studies.     

Investigation of high-throughput ultrafiltration for the determination of an unbound compound in human plasma using liquid chromatography and tandem mass spectrometry with electrospray ionization

A high-throughput ultrafiltration method with a direct injection assay has been developed to determine unbound concentrations of a high-protein binding compound, an alpha(v)beta(3) bone integrin antagonist (I), in human plasma for a clinical pharmacokinetic study. The 96-well MultiScreen filter plate with Ultracel-PPB membrane was evaluated for the separation of unbound from protein-bound compound I by ultrafiltration. The sample preparation was automated using a Packard MultiPROBE II EX liquid handling system to transfer the plasma samples to the 96-well PPB plate for centrifugation and to prepare ultrafiltrate samples for analysis. Using on-line extraction with a column-switching setup for sample clean-up and separation, the ultrafiltrate samples were directly injected onto a reversed-phase HPLC system and analyzed using a mass spectrometer interfaced with an electrospray ionization (ESI) source in the positive ionization mode (LC/ESI-MS/MS). The performance of the ultrafiltration using Ultracel-PPB 96-well plate for unbound I analysis was evaluated and optimized with respect to sample volume, centrifugation temperature, speed and time, and the relationship of the well positions of the PPB plate versus filtrate volumes and concentrations. The assay intraday accuracy and precision were between 93.9 and 104.8 and <7.3% (CV), respectively. The linear range of the calibration curve for the assay was 0.1-500 ng/mL on a Finnigan TSQ Quantum LC/ESI-MS/MS system. Evaluation and validation of the unbound plasma assay demonstrated it to be rapid, sensitive and reproducible.     

Determination of lumefantrine in rat plasma by liquid–liquid extraction using LC–MS/MS with electrospray ionization: Assay development,validation and application to a pharmacokinetic study

A simple, sensitive and rapid method for the analysis of lumefantrine in rat plasma using liquid chromatography coupled to tandem mass spectrometry (LC–MS/MS) was developed. Detection was performed by positive ion electrospray ionization (ESI) in multiple reaction monitoring (MRM) mode. The method included a chromatographic run of 5 min using a C₁₈ analytical column and the calibration curve was linear over the concentration range of 2–500 ng/mL with a correlation coefficient (r) of 0.996 or better. The intra- and inter-day assay precision ranged from 1.5 to 7.5% and 5.5 to 7.7%, respectively, and intra- and inter-day assay accuracy was between 91.3–109.7% and 97.0–104.7%, respectively. The method was successfully applied for the pharmacokinetic study in rats.     

A high-throughput platform for the rapid screening of vitamin D status by direct infusion-MS/MS

Vitamin D is an important fat-soluble prohormone with pleiotropic effects on human health, such as immunomodulation of the innate and adaptive immune system. There is an unmet clinical need for a rapid screening platform for 25-hydroxyvitamin D (25OH-D) determination without chromatographic separation that offers better precision and accuracy than immunoassays. Here, we introduce a high-throughput method for assessing vitamin D status from blood specimens based on direct infusion-MS/MS (DI-MS/MS) following click derivatization using 2-nitrosopyridine. We developed an optimized liquid-phase extraction protocol to minimize ion suppression when directly infusing serum or plasma extracts via a capillary electrophoresis system for quantitative determination of 25OH-D. Acceptable reproducibility (mean coefficient of variation = 10.9%, n = 412), recovery (mean = 102% at 15, 30, and 45 nmol/l), and linearity (R² > 0.998) were achieved for 25OH-D with lower detection limits (limit of detection ～1.2 nmol/l, S/N ～ 3), greater throughput (～3 min/sample), and less bias than a commercial chemiluminescence immunoassay prone to batch effects. There was mutual agreement in 25OH-D concentrations from reference blood samples measured by DI-MS/MS as compared with LC-MS/MS (mean bias = 7.8%, n = 18). We also demonstrate that this method could reduce immunoassay misclassification of vitamin D deficiency in a cohort of critically ill children (n = 30). In conclusion, DI-MS/MS offers a viable alternative to LC-MS/MS for assessment of vitamin D status in support of large-scale studies in nutritional epidemiology as well as clinical trials to rapidly screen individual patients who may benefit from vitamin D supplementation.     

Application of liquid chromatography-tandem mass spectrometry method for the analysis of new nonselective beta-adrenergic blocker 1-(1-H-indol-4-yloxy)-3-{[2-(2-methoxy phenoxy)ethylo]amino}propan-2-ol (2F109) in rat plasma

A sensitive and specific liquid chromatography electrospray ionization-tandem mass spectrometry method for the enantioselective determination of the novel beta-adrenolytic compound, 1-(1-H-indol-4-yloxy)-3-{[2-(2-methoxyphenoxy)ethylo]amino} propan-2-ol, in rat plasma has been developed and validated. Chromatography was performed on a reversed-phase Chiralcel OD-RH analytical column (150x4.6 mm, 5 microm, Daicel Chemical Industries, Tokyo, Japan) with isocratic elution using a mobile phase containing acetonitrile and water with 0.01% formic acid. Detection was achieved by an Applied Biosystems MDS Sciex (Concord, Ontario, Canada) API 2000 triple quadrupole mass spectrometer. Electrospray ionization (ESI) was used for ion production. The limit of detection in the MRM mode was found to be 1.25 ng/ml. The limit of quantification of both enantiomers was 2.5 ng/ml. The precision and accuracy for both intra- and inter-day determination of 2F109 enantiomers ranged from 2.6 to 12% and from 89.1 to 107.1%. This analytical method allowed us to carry out pharmacokinetic studies in rats. Our findings demonstrate that 2F109 shows stereoselective disposition in rat plasma after i.v. administration. The terminal half-lives of (+)-(R)-2F109 and (-)-(S)-2F109 were 33.5 and 42.6 min, respectively. The AUC0-inf of (+)-(R)-2F109 exceeded that of (-)-(S)-2F109.     

Sensitive assay for determining plasma tenofovir concentrations by LC/MS/MS

An LC/MS/MS assay for the determination of tenofovir (TNF) was developed and validated for use with the EDTA anticoagulated human plasma matrix. Heparin-treated plasma and serum matrices were also validated. After addition of adefovir as an internal standard, trifluoroacetic acid was used to produce a protein-free extract. Chromatographic separation was achieved with a Polar-RP Synergi, 2.0 mm x 150 mm, reversed-phase analytical column. The mobile phase was 3% acetonitrile/1% acetic acid, aq. Detection of TNF and the internal standard was achieved by ESI MS/MS in the positive ion mode using 288/176 and 274/162 transitions, respectively. The method was linear from 10 to 750 ng/ml with a minimum quantifiable limit of 10 ng/ml when 250 microl aliquots were analyzed. The usefulness of this LC/MS/MS method to routinely monitor plasma concentrations of TNF was demonstrated along with its ability to assist in the performance of pharmacokinetic studies.     

Determination of testosterone concentrations in rat plasma using liquid chromatography-atmospheric pressure chemical ionization mass spectrometry combined with ethyl oxime and acetyl ester derivatization

A quantitative method for measuring testosterone (T) concentrations in rat plasma was developed using ethyl oxime and acetyl ester derivatization and liquid chromatography-atmosphere pressure chemical ionization tandem mass spectrometry (LC-APCI-MS/MS). The method utilizes a solid phase extraction with Varian Bond Elut C18, a derivatization process to form testosterone ethoxime acetate and LC-APCI-MS/MS with a reversed phase LC and a C8 column. This method is capable of detecting testosterone concentrations as low as 0.2 ng/ml in a 0.05 ml sample of rat plasma. This method can be used as a sensitive chromatography-based assay for small sample volumes of rat blood.     

Determination of 13-O-demethyl tacrolimus in human liver microsomal incubates using liquid chromatography-mass spectrometric assay (LC-MS)

A simple, sensitive and specific liquid chromatography coupled electrospray ionization mass spectrometric (LC/ESI/MS) method for the determination of 13-O-demethylated metabolite (MI), one of the major metabolites of tacrolimus has been developed. The assay uses 32-demethoxyrapamycin (IS) as the internal standard; ethyl acetate as extraction solvent; a Hypersil-Keystone Beta Basic-18 reversed-phase column; and a gradient mobile phase of consisting 0.1% formic acid in water and methanol-acetonitrile (3:49, v/v). Mass detection is performed on a single quadrupole mass spectrometer equipped with an electrospray ionization (ESI) interface and operated in a positive ionization mode. MI in the microsomal incubates was quantitated by computing the peak area ratio (MI/IS) analyzed in single ion monitoring (SIM) mode (m/z: 804 and m/z: 901 for MI and IS, respectively). Precision of the assay was determined by calculating the intra-run and inter-run variation at three concentrations (15, 25, 80 ng/ml); the intra run relative standard deviation (R.S.D.) was less than 10% and ranged from 5.0 to 8.3%; and the inter-run R.S.D. was less than 10% and ranged from 4.6 to 9.6%. The limits of detection was 2 ng/ml. This assay has been used to evaluate the effect of three human immunodeficiency virus (HIV) protease inhibitors on the metabolism of tacrolimus in human liver microsomes.     

Simultaneous determination of hypericin and hyperforin in human plasma with liquid chromatography-tandem mass spectrometry

A selective and sensitive method for the simultaneous determination of hypericin and hyperforin--the two main active ingredients of St. John's Wort (SJW) extract--in human plasma depending on liquid/liquid-extraction and LC/MS/MS detection has been developed, validated after specifying the stability of the photosensitive hypericin in plasma samples during light exposure and applied to samples of a patient. After extraction with ethyl acetate/n-hexane in the darkness, sample extracts were chromatographed isocratically within 6 min on a Kromasil RP-18 column. The analytes were detected with tandem mass spectrometry in the selected reaction monitoring mode using an electrospray ion source. The limit of quantification was 0.05 ng/mL for hypericin and 0.035 ng/mL for hyperforin. The accuracy of the method varied between 101.9 and 114.2% and the precision ranged from 4.7 to 15.4% (S.D., batch-to-batch) for both analytes. The method was linear at least between 0.05 and 10 ng/mL for hypericin and between 0.035 and 100 ng/mL for hyperforin. Using this method hypericin and hyperforin were determined successfully in a patient over seven days following discontinuation of exposure with therapeutic doses of St. John's Wort extract.     

Measurement of free and total sialic acid by isotopic dilution liquid chromatography tandem mass spectrometry method

The measurement of urine sialic acid (N Acetylneuraminic Acid: Neu5Ac) is useful for screening sialic acid storage disorders. We developed a new LC MS/MS method for the determination of a sialic acid. Urine samples were analyzed, after an HCl n-Butanol derivatization step, by a reverse phase based high-performance liquid chromatography method using 1,2,3-(13)C(3) N-Acetyl-D-neuraminic Acid ((13)C-Neu5Ac) as an internal standard. Selective detection was performed by tandem mass spectrometry using an electrospray source operating in positive ionization mode employing multiple reactions monitoring to monitor N-Acetylneuraminic Acid and the internal standard. The transitions m/z 366→330 and 369→333 for Neu5Ac and (13)C-Neu5Ac were respectively monitored. The limit of the method quantification was 1.40 μM of N-Acetylneuraminic Acid and the calibration curve showed a good linearity up to 1000 μM. The within assay precision and accuracy of the method ranged from 3.22 to 5.95% and 98.69 to 109.18%, respectively and the between assay precision and accuracy ranged, respectively, from 5.15 to 7.65% and 96.14 to 102.30%. The method can be applied for the determination of N-Acetylneuraminic Acid concentrations in urine and other biological fluids (e.g., amniotic and peritoneal fluids).     

Stable-isotope dilution LC–ESI-MS/MS techniques for the quantification of total homocysteine in human plasma

Homocysteine is an endogenous sulphydryl aminoacid irreversibly catabolized by transsulfuration to cysteine or remethylated to methionine. Increased plasma levels of homocysteine are an independent risk factor for atherosclerosis and cardiovascular disease. Accurate and reliable quantification of this amino acid in plasma samples is essential in clinical practice to explore the presence of a hyperhomocysteinemia, for instance after an ischemic event, or to control a possible adjunctive risk factor in patients at higher risk. In this review, LC–ESI-MS/MS methods are discussed and compared with other analytical methods for plasma homocysteine. LC–ESI-MS/MS is a technique combining the physicochemical separation of liquid chromatography with the analysis of mass spectrometry. It is based on stable-isotope dilution and possesses inherent accuracy and precision. Quantitative analysis is achieved by using commercially available homocystine-d₈ as an internal standard. Taking advantage of the high sensitivity and specificity, approaches involving LC–ESI-MS/MS require less laborious sample preparation, no derivatization and produce reliable results.