Comparative analysis of nucleosides,nucleobases, and amino acids in different parts of Angelicae Sinensis Radix by ultra high performance liquid chromatography coupled to triple quadrupole tandem mass spectrometry

A rapid and sensitive ultra high performance liquid chromatography coupled to triple quadrupole tandem mass spectrometry method was established and employed to determine 21 nucleosides, nucleobases, and amino acids in 60 samples from different parts of Angelicae Sinensis Radix. The established methods were validated by good linearity (r² > 0.9937), limits of detection (0.12–77.75 ng/mL), limits of quantitation (0.31–272.13 ng/mL), intra‐ and interday precisions (RSD ≤ 4.84%, RSD ≤ 6.26%), stability (RSD ≤ 5.92%), repeatability (RSD ≤ 7.14%), recovery (91.4–103.4%), and matrix effects (0.92–1.03). Chemical comparative analysis revealed that the content of total analytes in four parts of Angelicae Sinensis Radix were different, and exhibited the order: Head (14.89 mg/g) > Body (10.15 mg/g) > All (8.22 mg/g) > Tail (6.23 mg/g). Principal component analysis showed that the samples could be classified into four groups in accord with four different parts of Angelicae Sinensis Radix. The results could provide a scientific basis and reference for the quality control of Angelicae Sinensis Radix, and may be conducive to further research on the pharmacological activities of Angelicae Sinensis Radix.     

Simultaneous determination of eight flavonoids in Sedum sarmentosum Bunge from different areas by UHPLC with triple quadrupole MS/MS

Sedum sarmentosum Bunge (SSB) is a traditional Chinese herbal medicine containing multiple components that has been extensively used clinically to treat chronic viral hepatitis and some inflammatory diseases. Total flavonoids are major pharmacologically active components of SSB. To gain a deeper understanding of SSB resources, we analyzed eight chemical constituents in 33 batches of SSB from 11 regions in China. An accurate, precise and sensitive ultra‐high‐performance liquid chromatography coupled with triple quadrupole electrospray tandem mass spectrometry method was developed for the determination of eight flavonoids in SSB. Under the optimized chromatographic conditions, good separation for the eight target components was obtained on an Agilent Zobax SB C₁₈ (50 × 2.1 mm, 5 μm) column within 4 min. The established methods were validated with good linearity (r ≥ 0.9988), precision (RSD ≤ 2.68%), stability (1.43–3.28%) and repeatability (1.14–2.89%). Moreover, the average recoveries were 95.91–100.68%, and the RSDs were 1.50–3.80%. In addition, the analytical conditions of UPLC–ESI–MS/MS provided better sensitivity with a shorter analysis time when compared with the HPLC–DAD method. Hierarchical clustering analysis and principal component analysis were performed to estimate and classify these samples based on the contents of the eight chemical constituents. This study provided the theoretical basis and scientific evidence for the development and utilization of SSB resources.     

Analysis and evaluation of nucleosides,nucleobases, and amino acids in safflower from different regions based on ultra high performance liquid chromatography coupled with triple‐quadrupole linear ion‐trap tandem mass spectrometry

Safflower has both medicinal and edible values but research on its nutrient composition is still lacking. This study was established for the quantitative determination of 28 nucleosides, nucleobases, and amino acids based on the ultra‐performance liquid chromatography coupled with triple‐quadrupole linear ion‐trap tandem mass spectrometry. Analysis of 30 batches of safflower from different producing areas indicated that the contents of l ‐proline, l ‐asparagine, l (+)‐arginine, l ‐serine, l ‐histidine, uracil, guanosine, and uridine was high in safflower. Principle component analysis and cluster analysis found that samples from different regions could be distinguished well, and samples from the same area could be clustered into one class, different geographical environments may cause the differences of nucleosides, nucleobases, and amino acids in safflower. The analysis of principal component analysis, cluster analysis, and counter propagation artificial neural network show similar results. Then the content of nucleosides, nucleobases, and essential amino acids were compared, and found that the content in safflower from Gansu was higher than those from other regions, and there was a little difference between the samples from Xinjiang, Sichuan, and Yunnan. This research revealed the composition of nucleosides, nucleobases, and amino acids in safflower, and provided a theoretical basis for utilization of safflower.     

Contents variation analysis of free amino acids,nucleosides and nucleobases in semen sojae praeparatum fermentation using UFLC‐QTRAP MS

UFLC–QTRAP MS was used to develop a sensitive and rapid method of evaluating content variation during Semen sojae praeparatum (SSP) fermentation. It did this through the simultaneous quantification of 22 free amino acids and 16 nucleosides and nucleobases in the raw materials and processed products of SSP. The method was shown to be reproducible and accurate. The limits of detection and quantity values were 0.09–168.75 and 0.31–562.50 ng/mL for the 38 analytes, respectively. The data were examined through principal components analysis to compare the content variations. The quantitative results showed that the ingredients were properly determined in most of the samples and were converted regularly throughout the SSP fermentation process. These results correspond to the morphologic changes and principal components analysis results.     

Rapid profiling of polymeric phenolic acids in Salvia miltiorrhiza by hybrid data‐dependent/targeted multistage mass spectrometry acquisition based on expected compounds prediction and fragment ion searching

Phenolic acids are the major water‐soluble components in Salvia miltiorrhiza (>5%). According to previous studies, many of them contribute to the cardiovascular effects and antioxidant effects of S. miltiorrhiza. Polymeric phenolic acids can be considered as the tanshinol derived metabolites, e.g., dimmers, trimers, and tetramers. A strategy combined with tanshinol‐based expected compounds prediction, total ion chromatogram filtering, fragment ion searching, and parent list‐based multistage mass spectrometry acquisition by linear trap quadropole‐orbitrap Velos mass spectrometry was proposed to rapid profile polymeric phenolic acids in S. miltiorrhiza. More than 480 potential polymeric phenolic acids could be screened out by this strategy. Based on the fragment information obtained by parent list‐activated data dependent multistage mass spectrometry acquisition, 190 polymeric phenolic acids were characterized by comparing their mass information with literature data, and 18 of them were firstly detected from S. miltiorrhiza. Seven potential compounds were tentatively characterized as new polymeric phenolic acids from S. miltiorrhiza. This strategy facilitates identification of polymeric phenolic acids in complex matrix with both selectivity and sensitivity, which could be expanded for rapid discovery and identification of compounds from complex matrix.     

Simultaneous quantitation of nucleosides,nucleobases, amino acids,and alkaloids in mulberry leaf by ultra high performance liquid chromatography with triple quadrupole tandem mass spectrometry

An ultra high performance liquid chromatography coupled with a triple quadrupole mass detection and electrospray ionization mass spectrometry method has been established for the simultaneous determination of the 14 nucleosides and nucleobases, 24 amino acids and two main alkaloids in mulberry leaf. In this method, a complicated mobile phase, the flow rate of which was 0.4 mL/min, was applied to the gradient elution, which provided a satisfied separation of the 40 compounds. The present method was validated, and sufficient reproducibility and accuracy were obtained for the quantitative measurement of the 40 compounds. The method was subsequently applied to ten mulberry leaves and the results showed that almost all of these samples were rich in nucleosides, nucleobases, amino acids, and alkaloids. The proposed method, which is convenient and economical, could serve as a prerequisite for the quality control of mulberry leaf herbs and be applied analogously to other Chinese medicines.     

ESI‐MS/MS analysis of protonated N‐methyl amino acids and their immonium ions

Methylation is one of the important posttranslational modifications of biological systems. At the metabolite level, the methylation process is expected to convert bioactive compounds such as amino acids, fatty acids, lipids, sugars, and other organic acids into their methylated forms. A few of the methylated amino acids are identified and have been proved as potential biomarkers for several metabolic disorders by using mass spectrometry–based metabolomics workstation. As it is possible to encounter all the N‐methyl forms of the proteinogenic amino acids in plant/biological systems, it is essential to have analytical data of all N‐methyl amino acids for their detection and identification. In earlier studies, we have reported the ESI‐MS/MS data of all methylated proteinogenic amino acids, except that of mono‐N‐methyl amino acids. In this study, the N‐methyl amino acids of all the amino acids ( 1 ‐ 21 ; including one isomeric pair) were synthesized and characterized by ESI‐MS/MS, LC/MS/MS, and HRMS. These data could be useful for detection and identification of N‐methyl amino acids in biological systems for future metabolomics studies. The MS/MS spectra of [M + H]⁺ ions of most N‐methyl amino acids showed respective immonium ions by the loss of (H₂O, CO). The other most common product ions detected were [MH‐(NH₂CH₃]⁺, [MH‐(RH)]⁺ (where R = side chain group) ions, and the selective structure indicative product ions due to side chain and N‐methyl group. The isomeric/isobaric N‐methyl amino acids could easily be differentiated by their distinct MS/MS spectra. Further, the MS/MS of immonium ions inferred side chain structure and methyl group on α‐nitrogen of the N‐methyl amino acids.     

Simultaneous determination of thirteen kinds of amino acid and eight kinds of acylcarnitine in human serum by LC–MS/MS and its application to measure the serum concentration of lung cancer patients

Easy‐to‐use early cancer detection methods based on metabolomics using serum samples have been developed recently. Among metabolites, amino acids and acylcarnitine are two of the most suitable candidates for diagnosing lung cancer. The purpose of the present study was to develop a novel, sensitive and specific liquid chromatography–tandem mass spectrometry (LC–MS/MS) method to simultaneously determine 13 amino acids and 8 acylcarnitines in lung cancer patients in serum. After derivatization, the 21 analytes were separated using a C₁₈ column with gradient elution program in 14 min, obtaining recovery within 90.4–113.8% and precision within 0.3–14.8%. The method was successfully applied in concentration determination of lung cancer patients and healthy controls. The results showed that the serum concentration of lung cancer patients were significant from those of healthy controls.     

UHPLC–MS/MS quantification combined with chemometrics for the comparative analysis of different batches of raw and wine‐processed Dipsacus asper

A rapid and sensitive ultra‐high performance liquid chromatography with tandem mass spectrometry approach was established for the simultaneous determination of 4‐caffeoylquinic acid, loganic acid, chlorogenic acid, loganin, 3,5‐dicaffeoylquinic acid, dipsacoside B, asperosaponin VI, and sweroside in raw and wine‐processed Dipsacus asper . Chloramphenicol and glycyrrhetinic acid were employed as internal standards. The proposed approach was fully validated in terms of linearity, sensitivity, precision, repeatability as well as recovery. Intra‐ and interassay variability for all analytes were 2.8–4.9 and 1.7–4.8%, respectively. The standard addition method determined recovery rates for each analytes (96.8–104.6%). In addition, the developed approach was applied to 20 batches of raw and wine‐processed samples of Dipsacus asper . Principle component analysis and partial least squares‐discriminate analysis revealed a clear separation between the raw group and wine‐processed group. After wine‐processing, the contents of loganic acid, chlorogenic acid, dipsacoside B, and asperosaponin VI were upregulated, while the contents of 3,5‐dicaffeoylquinic acid, 4‐caffeoylquinic acid, loganin, and sweroside were downregulated. Our results demonstrated that ultra‐high performance liquid chromatography with tandem mass spectrometry quantification combined with chemometrics is a viable method for quality evaluation of the raw Dipsacus asper and its wine‐processed products.     

A rapid,sensitive, and widely applicable method for quantitative analysis of underivatized amino acids in different biological matrices by UHPLC‐MS/MS

A rapid, sensitive, and widely applicable method for the simultaneous quantitative analysis of 20 underivatized amino acids in different biological matrices, including serum, plasma, and tissue homogenates, using ultra high performance liquid chromatography with tandem mass spectrometry was developed and validated. Only 4 µL of serum, plasma, or tissue homogenate was extracted with 996 µL of solution (1.7 mM ammonium formate in 85% acetonitrile containing 0.1% formic acid) containing 100 ng/mL phenylalanine‐d5 as an internal standard without any further derivatization step. In addition, the matrix effects were small because a large volume of extraction solution was used. The total run time including reequilibration was 13 min. The results of linearity, accuracy, repeatability, precision, limits of detection, limits of quantification, and sample stability were sufficient to allow the measurement of the amino acids in different biological matrices. We conclude that our method is rapid, sensitive, and widely applicable and represents an improvement over other currently available technologies.     

液相色谱-质谱联用法测定牛奶中14种青霉素及相应青霉噻唑酸残留量

利用超高效液相色谱-串联质谱法(UPLC-MS/MS)同时测定牛奶中的7种青霉素类抗生素以及7种相应的青霉噻唑酸。样品经乙腈沉淀蛋白,上清液N2吹干后,用水溶解,加入正己烷萃取除去脂肪;提取液经ACQUITY UPLCBEH C18柱分离,乙腈-乙酸铵+甲酸水溶液洗脱。14种物质峰分离良好,定量限范围在5～20μg/kg。在10～50ng/mL质量浓度范围内线性良好,相关系数均大于等于0.999,牛奶中的加标回收率在90%～98%。     

Trace determination of five triazole fungicide residues in traditional Chinese medicine samples by dispersive solid‐phase extraction combined with ultrasound‐assisted dispersive liquid–liquid microextraction and UHPLC–MS/MS

A novel and reliable method for determination of five triazole fungicide residues (triadimenol, tebuconazole, diniconazole, flutriafol, and hexaconazol) in traditional Chinese medicine samples was developed using dispersive solid‐phase extraction combined with ultrasound‐assisted dispersive liquid–liquid microextraction before ultra‐high performance liquid chromatography with tandem mass spectrometry. The clean up of the extract was conducted using dispersive solid‐phase extraction by directly adding sorbents into the extraction solution, followed by shaking and centrifugation. After that, a mixture of 400 μL trichloromethane (extraction solvent) and 0.5 mL of the above supernatant was injected rapidly into water for the dispersive liquid–liquid microextraction procedure. The factors affecting the extraction efficiency were optimized. Under the optimum conditions, the calibration curves showed good linearity in the range of 2.0–400 (tebuconazole, diniconazole, and hexaconazole) and 4.0–800 ng/g (triadimenol and flutriafol) with the regression coefficients higher than 0.9958. The limit of detection and limit of quantification for the present method were 0.5–1.1 and 1.8–4.0 ng/g, respectively. The recoveries of the target analytes ranged from 80.2 to 103.2%. The proposed method has been successfully applied to the analysis of five triazole fungicides in traditional Chinese medicine samples, and satisfactory results were obtained.     

Simultaneous quantification of multiple active components from Xiexin decoction in rat plasma by LC‐ESI‐MS/MS: application in pharmacokinetics

A sensitive and specific liquid chromatography–electrospray ionization–tandem mass spectrometric (LC‐ESI‐MS/MS) method was developed and validated to simultaneously quantify 11 active compounds (coptisine, jatrorrhizine, berberine, palmatine, baicalin, baicalein, wogonoside, wogonin, rhein, emodin and aloeemodin) from Xiexin decoction (XXD) in rat plasma. Plasma samples extracted by a single‐step protein precipitation procedure were separated using the gradient mode on a Dikma ODS‐C₁₈ column. Selected reaction monitoring scanning was employed for quantification with switching electrospray ion source polarity between positive and negative modes in a single run. Calibration curves offered satisfactory linearity (r > 0.995) at linear range of 0.47–60 ng/mL for coptisine, jatrorrhizine, berberine and palmatine, 15–1930 ng/mL for baicalin, 20–2560 ng/mL for baicalein, 14–1790 ng/mL for wogonoside, 0.57–72.8 ng/mL for wogonin, 10–1280 ng/mL for rhein, 0.6–76.8 ng/mL for emodin and 3.0–384 ng/mL for aloeemodin. The intra‐ and interday precisions were less than 10.2% in terms of relative standard deviation (RSD), and the accuracies were within ±10.84% in terms of relative error (RE). It was successfully applied to the evaluation of pharmacokinetics after single oral doses of XXD were administered to rats. Copyright © 2010 John Wiley & Sons, Ltd.     

Pseudotargeted screening and determination of constituents in Qishen granule based on compound biosynthetic correlation using UHPLC coupled with high‐resolution MS

Detection and determination of many known/unknown compounds in traditional Chinese medicines have always been challenging. To comprehensively identify compounds in Qishen granule, which is a widely prescribed herbal formula for treating chronic heart failure, a pseudotargeted screening method was proposed based on compound biosynthetic correlation using ultra high‐performance liquid chromatography coupled with high‐resolution mass spectrometry. Firstly, all possible compounds of Qishen granule were classified into nine types according to their core skeletons, and potential analogue molecular formulas were predicted according to core compound‐related biosynthetic correlations, such as methylation, hydroxylation, and glucosidation. Secondly, nine pseudocompound databases consisting of core compounds, deduced biosynthetic correlations, and predicted analogue molecular formulas were established. Then, compounds of interest were directly located by pseudotargeted screening of high resolution mass spectrometry data and further verified by target tandem mass spectrometry. As a result, 213 constituents were identified and 21 of them were determined as potential new compounds. This demonstrated that pseudotargeted screening based on compound biosynthetic correlations significantly facilitated the processing of extremely large information data and improved the efficiency of compound identification. This research provided essential data for exploration of effective substances in Qishen granule and enriched the methodology for comprehensive characterization of constituents in complex traditional Chinese medicines.     

Simultaneous identification of three pseudoallergic components in Danshen injection by using high‐expression Mas‐related G protein coupled receptor X2 cell membrane chromatography coupled online to HPLC–ESI‐MS/MS

Adverse drug reactions of Danshen injection mainly manifested as pseudoallergic reactions. In the present study, salvianolic acid A and a pair of geometric isomers (isosalvianolic acid C and salvianolic acid C) were identified as pseudoallergic components in Danshen injection by a high‐expression Mas‐related G protein coupled receptor X2 cell membrane chromatography coupled online with high‐performance liquid chromatography with electrospray ionization tandem mass spectrometry. Their pseudoallergic activities were evaluated by in vitro assay, which were consistent with the retention times on the cell membrane chromatography column. Salvianolic acid C, the most outstanding compound, was further found to induce pseudoallergic reaction through Mas‐related G protein coupled receptor X2. All the results above indicated that the system developed in this study is an effective method for simultaneously analyzing pseudoallergic components, even those with similar structures and the microcomponents in complex samples (salvianolic acid C in Danshen injection).     

Metabolic mapping of Schisandra chinensis lignans and their metabolites in rats using a metabolomic approach based on HPLC with quadrupole time‐of‐flight MS/MS spectrometry

Schisandra chinensis lignans are the main active components of the traditional Chinese medicine Schisandra chinensis in East Asia. At present, there are more and more medicines and health foods in which the total S. chinensis lignans extracts are considered as the main active components, but little research has been done on the active components of S. chinensis lignans in the blood and main target organs. In this study, the components of S. chinensis lignans in the blood, liver and brain tissues of rats at different time points after the intragastrical administration of S. chinensis lignans were determined by a metabolomic method based on high‐performance liquid chromatography with quadrupole time‐of‐flight tandem mass spectrometry spectrometry. Twelve Schisandra chinensis lignans and 15 metabolites in the blood, liver, and brain of rats were identified. The results showed that the main metabolic ways of S. chinensis lignans in rats were hydroxylation, demethylation, and demethylation‐hydroxylation, and some of them might undergo demethylation, dehydrogenation, epoxidation, and elimination reaction. The time‐dose characteristics of S. chinensis lignans and their metabolites in the blood and target organs were analyzed, which may be helpful to elucidate the active substances that really exert the pharmacodynamic effects of S. chinensis lignans in organisms.     

Development of an ultra‐high‐performance liquid chromatography coupled with triple quadrupole mass spectrometry method for comparative pharmacokinetics of six triterpenoids in rat plasma and application to different forms of Phytolacca acinosa

Phytolacca acinosa is an herb for treatment of ascites and tumor. Two forms of P. acinosa, i.e. raw and vinegar‐processed herb, have been used in clinic. However, pharmacokinetic difference between the two forms of P. acinosa has not been fully understood. Herein, a comparative pharmacokinetic method based on liquid chromatography with tandem mass spectrometry was developed for quantification of six bioactive triterpenoids, including esculentoside H, esculentoside T, esculentoside A, esculentoside B, phytolaccagenic acid, and phytolaccagenin in rat plasma after oral administration of different forms of P. acinosa. Separation was performed on an Acquity BEH C₁₈ column (1.7 µm, 2.1 mm × 50 mm). The method was validated over a linear range of 2.0–5000 ng/mL. Intraday and interday bias were within ±5%. Besides, all triterpenoids were stable in plasma during different storage conditions. The described method was successfully applied to a comparative pharmacokinetic study of raw and vinegar‐processed P. acinosa in rats. Notably, double peak phenomenon for six triterpenoids of P. acinosa was observed for the first time. AUC_0→t and C_max values of esculentoside H, esculentoside T, phytolaccagenic acid, and phytolaccagenin were significantly lower in vinegar‐processed group than that of raw group, indicating the oral bioavailability of the four triterpenoids was decreased after vinegar processing.     

Validated UHPLC–MS/MS method for simultaneous determination of four triterpene saponins from Akebia trifoliata extract in rat plasma and its application to a pharmacokinetic study

Saponin PH, akemisaponins E, saponin PJ₁ and scheffoleoside A, the main bioactive triterpene saponins of Chinese traditional medicine Akebia trifoliata, contribute to its diuretic pharmacological activity. Because of interactions of the multiple ingredients in vivo, pharmacokinetic studies of multiple triterpenes after administration of A. trifoliata extract are essential to clarify their pharmacological effects. The purpose of this study was to develop an efficient and sensitive UHPLC–MS/MS method for simultaneous determination of these four triterpene saponins in rat plasma. The biosamples were prepared by liquid–liquid extraction with n‐butanol. The chromatographic separation was performed on a Phenomenex Luna^® C₁₈ (150 × 2 mm, 3 μm) with a mobile phase consisting of acetonitrile and water at a flow rate of 0.5 mL/min. The MS/MS system was operated in a negative multiple reaction monitoring mode, and the precursor–product ion transitions were optimized as m/z 941.6 → 471.1 for saponin PH, 941.7 → 471.2 for akemisaponins E, 1089.7 → 601.1 for saponin PJ₁, 957.6 → 487.4 for scheffoleoside A and 799.5 → 637.3 for ginsenoside Rg₁ (Rg₁, internal standard). Method validation parameters (calibration curve linearity, lower limit of detection, recovery, matrix effect, intra‐ and inter‐day precision) were within the acceptable ranges. This is the first reported on the UHPLC–MS/MS detection of saponin PH, akemisaponins E, saponin PJ₁ and scheffoleoside A, and applied to a preclinical pharmacokinetic study after oral administration of A. trifoliata extract in rats. This study provides a basis for clinical application and further development of A. trifoliata extract.     

Simultaneous detection of four flavonoids and two alkaloids in rat plasma by LC–MS/MS and its application to a comparative study of the pharmacokinetics between Abri Herba and Abri mollis Herba extract after oral administration

Abri Herba and Abri mollis Herba both were important members of the Leguminosae family in southwestern China. Abri mollis Herba was often used as Abri Herba due to their proximity, but there are few studies on pharmacokinetics to compare their main identical active compositions. A sensitive and selective high‐performance liquid chromatography with tandem mass spectrometry method in the positive/negative electrospray ionization switching mode was developed and validated for the simultaneous analysis of four flavonoids and two alkaloids in rat plasma. The chromatographic separation was carried out on a C18 column with a gradient mobile phase consisting of methanol and 0.5% acetic acid. The detection of the target compounds was conducted in multiple‐reaction monitoring mode with a hybrid triple quadrupole linear ion trap mass spectrometer equipped with positive/negative ion‐switching electrospray ion source. The differences in pharmacokinetics were discovered, which indicated that the substitution between them is some degree of irrationality. The validated method was successfully applied to pharmacokinetic study of the six components in male rat plasma after oral administration of Abri Herba and Abri mollis Herba extract and the results in the study would provide a useful guide for the clinical application of Abri Herba with those in Abri mollis Herba.     

Sensitive determination of monoamine neurotransmitters,their main metabolites and precursor amino acids in different mouse brain components by liquid chromatography–electrospray tandem mass spectrometry after selective sample clean‐up

For the assessment of diets and supplements formulated for the treatment of phenylketonuria, a highly sensitive and selective method was developed and validated for the quantification of dopamine (DA), serotonin (5‐HT), 3,4‐dihydroxyphenylacetic acid (DOPAC), 5‐hydroxyindoleacetic acid (5‐HIAA), phenylalanine, tyrosine and tryptophan in mouse cerebellum, brain stem, hypothalamus, parietal cortex, anterior piriform cortex and bulbus olfactorius. Samples were extracted by deproteinization with acetonitrile, and the extracts were cleaned up by strong anion exchange and weak cation exchange applied sequentially. The substances were detected by rapid liquid chromatography tandem mass spectrometry. Matrix components were largely removed by the clean‐up, resulting in low matrix effects. The lower limits of quantification for an extracted tissue mass of 100 mg were 0.3, 0.3, 0.2 and 2 ng/g for DA, 5‐HT, 5‐HIAA and DOPAC, respectively. The mean true extraction recoveries were 80–102%. The relative intra‐laboratory reproducibility standard deviations were generally <11% at concentrations of 20–1000 ng/g for DA, 5‐HT, 5‐HIAA and DOPAC and 7% at concentrations of 5–50 μg/g for the amino acids. This method was successfully used in a phenylketonuria mice study including nearly 300 brain tissue samples and for small sample masses (for example, 2 mg of bulbus olfactorius).