Determination of phenylalanine and tyrosine in dried blood specimens by ion-exchange chromatography using the Hitachi L-8800 analyzer

Manganese tetraphenylporphyrin and carbon nanocoil interface-based electrochemical sensing of tyrosine

Tyrosine is one of the essential metabolites present in the human body for nutritional maintenance and normal physiological functioning. Its concentration in the body is crucial in predicting various hereditary, emotional, and physiological disorders. Therefore, quantitative monitoring of tyrosine in clinical samples is indispensable. We state the use of carbon nanocoils/manganese tetraphenylporphyrin convened glassy carbon electrode (CNC/MnTPP/GC) for the streamlined electrochemical sensing of tyrosine. Cutting-edge analytical techniques were employed to perform a comprehensive physicochemical analysis of the synthesized materials. To investigate the electrochemical properties, various techniques such as cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical impedance spectroscopy, and chronocoulometry were employed. CNC/MnTPP/GC displayed an optimal response at pH 5 and exhibited remarkable linearity within the concentration range of 0.05 to 100 μM for tyrosine. Using DPV, it demonstrated a low limit of detection (21 nM ± 1.17) and a sensitivity of 0.12 μA μM-1 cm-2. CNC/MnTPP/GC displayed excellent performance in terms of repeatability, reproducibility, and stability for up to 30 days, making it suitable for real-time analysis, particularly in the analysis of tyrosine in blood serum. Notably, CNC/MnTPP/GC showcased a superior detection limit compared to previously reported methods.

Investigation of the relationship between phenylalanine in venous plasma and capillary blood using volumetric blood collection devices

Measurement of plasma and dried blood spot (DBS) phenylalanine (Phe) is key to monitoring patients with phenylketonuria (PKU). The relationship between plasma and capillary DBS Phe concentrations has been investigated previously, however, differences in methodology, calibration approach and assumptions about the volume of blood in a DBS sub-punch has complicated this. Volumetric blood collection devices (VBCDs) provide an opportunity to re-evaluate this relationship. Paired venous and capillary samples were collected from patients with PKU (n = 51). Capillary blood was collected onto both conventional newborn screening (NBS) cards and VBCDs. Specimens were analysed by liquid-chromatography tandem mass-spectrometry (LC-MS/MS) using a common calibrator. Use of VBCDs was evaluated qualitatively by patients. Mean bias between plasma and volumetrically collected capillary DBS Phe was -13%. Mean recovery (SD) of Phe from DBS was 89.4% (4.6). VBCDs confirmed that the volume of blood typically assumed to be present in a 3.2 mm sub-punch is over-estimated by 9.7%. Determination of the relationship between plasma and capillary DBS Phe, using a single analytical method, common calibration and VBCDs, demonstrated that once the under-recovery of Phe from DBS has been taken into account, there is no significant difference in the concentration of Phe in plasma and capillary blood. Conversely, comparison of plasma Phe with capillary DBS Phe collected on a NBS card highlighted the limitations of this approach. Introducing VBCDs for the routine monitoring of patients with PKU would provide a simple, acceptable specimen collection technique that ensures consistent sample quality and produces accurate and precise blood Phe results which are interchangeable with plasma Phe.

Validation and application of volumetric absorptive microsampling (VAMS) dried blood method for phenylalanine measurement in patients with phenylketonuria

BACKGROUND

Frequent blood phenylalanine (Phe) measurement is required for phenylketonuria (PKU) patients for diagnosis and disease status monitoring. Though various methods are available for blood Phe measurement, there is a lack of validated quantitative methods for measuring Phe with less than 15% variability. A method to allow at home blood sample collection for the PKU community is in high demand.

METHODS

A volumetric absorptive microsampling (VAMS) dried blood collection high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method was developed and fully validated for blood Phe measurement in compliance with regulatory guidances. The method accuracy, precision, stability, selectivity, matrix and hematocrit effects were assessed. A venous plasma collection HPLC-MS/MS method was developed and validated as a reference method. 311 matching VAMS and plasma samples were collected from 24 PKU subjects in a Phase 2 clinical study. Phe measurements using the two methods were compared.

RESULTS

Both VAMS and the plasma sample collection methods met the acceptance criteria for Good Laboratory Practice (GLP) bioanalytical analysis. Comparisons showed a high Pearson's correlation of 0.9813. The Passing-Bablok analysis showed that the difference was estimated to be less than 5% and Bland Altman analysis indicated that the difference was proportional with Phe concentration and for the majority of samples (88.85%) the measurement was within ±20% difference. Following 7 days treatment with 60 or 20 mg/kg/day PTC923 (Sepiapterin) or 20 mg/kg/day sapropterin, PKU patients exhibited respectively -206.4, -146.9, and -91.5 µmol/L reductions of blood Phe as measured by the VAMS method.

CONCLUSIONS

Concordant results were obtained using VAMS and plasma methods, which demonstrated that VAMS is a reliable method for clinical applications to monitor blood Phe for PKU patients.

Comparison of Tandem Mass Spectrometry and the Fluorometric Method-Parallel Phenylalanine Measurement on a Large Fresh Sample Series and Implications for Newborn Screening for Phenylketonuria

Phenylketonuria (PKU) was the first disease to be identified by the newborn screening (NBS) program. Currently, there are various methods for determining phenylalanine (Phe) values, with tandem mass spectrometry (MS/MS) being the most widely used method worldwide. We aimed to compare the MS/MS method with the fluorometric method (FM) for measuring Phe in the dried blood spot (DBS) and the efficacy of both methods in the NBS program. The FM was performed using a neonatal phenylalanine kit and a VICTOR2^TM D fluorometer. The MS/MS method was performed using a NeoBase^TM 2 kit and a Waters Xevo TQD mass spectrometer. The Phe values measured with the MS/MS method were compared to those determined by the FM. The cut-off value for the NBS program was set at 120 µmol/L for FM and 85 µmol/L for MS/MS. We analyzed 54,934 DBS. The measured Phe values varied from 12 to 664 µmol/L, with a median of 46 µmol/L for the MS/MS method and from 10 to 710 µmol/L, with a median of 70 µmol/L for the FM. The Bland-Altman analysis indicated a bias of -38.9% (-23.61 µmol/L) with an SD of 21.3% (13.89 µmol/L) when comparing the MS/MS method to the FM. The Phe value exceeded the cut-off in 187 samples measured with FM and 112 samples measured with MS/MS. The FM had 181 false positives, while the MS/MS method had 106 false positives. Our study showed that the MS/MS method gives lower results compared to the FM. Despite that, none of the true positives would be missed, and the number of false-positive results would be significantly lower compared to the FM.

An ECL sensor combined with a paper electrode for the determination of phenylalanine

An electrochemiluminescence (ECL) sensor combined with a paper electrode was developed for the detection of phenylalanine (l-Phe) in blood samples.

A highly accurate mass spectrometry method for the quantification of phenylalanine and tyrosine on dried blood spots: Combination of liquid chromatography, phenylalanine/tyrosine-free blood calibrators and multi-point/dynamic calibration

BACKGROUND

Measurement of quantitative levels of phenylalanine and tyrosine in blood is an essential test for the diagnosis of and monitoring genetic disorders associated with phenylalanine metabolism, such as phenylketonuria (PKU), tyrosinemia, and defects of tetrahydrobiopterin synthesis and recycling. We developed a highly accurate and fast liquid chromatography with tandem mass spectrometry (LC-MS/MS) method for the quantification of phenylalanine and tyrosine on dried blood spot (DBS). We also designed a performance score system to evaluate various calibration methods in matrix matched material.

METHODS

Phenylalanine/tyrosine-free whole blood was used to make accurate and stable DBS calibrators. Six calibrators cover the range of 0-1000 µmol/L. Underivatized phenylalanine and tyrosine were extracted and measured by LC-MS/MS. Precision, accuracy, limit of quantification, recovery and carryover were validated. External quality assurance materials were also used to evaluate performance of multi-point calibrations and single-point calibrations.

RESULTS

The run time was 4.5-minute. Accuracy analysis showed good agreement with reference materials. Precision, recovery, and the lower and upper limit of quantification met the criteria. When phenylalanine and tyrosine concentrations were less than 150 µmol/L, the 5-point calibration without the calibrator of 1000 µmol/L had the best performance. When the concentrations were > 250 µmol/L, the single-point calibration of 500 µmol/L had the best performance.

CONCLUSION

We developed a simple, fast and highly accurate method for the detection of phenylalanine and tyrosine on DBS, with chromatographic separation and underivatized analysis. Based on the calibration performance, a 6-point calibration method is satisfying for this test. An optional dynamic calibration method, which includes 6-point calibration, 5-point calibration and single-point calibration, can further increase test reliability.

Dried blood spot versus venous blood sampling for phenylalanine and tyrosine

BACKGROUND

This study investigated the agreement between various dried blood spot (DBS) and venous blood sample measurements of phenylalanine and tyrosine concentrations in Phenylketonuria (PKU) and Tyrosinemia type 1 (TT1) patients.

STUDY DESIGN

Phenylalanine and tyrosine concentrations were studied in 45 PKU/TT1 patients in plasma from venous blood in lithium heparin (LH) and EDTA tubes; venous blood from LH and EDTA tubes on a DBS card; venous blood directly on a DBS card; and capillary blood on a DBS card. Plasma was analyzed with an amino acid analyzer and DBS were analyzed with liquid chromatography-mass spectrometry. Agreement between different methods was assessed using Passing and Bablok fit and Bland Altman analyses.

RESULTS

In general, phenylalanine concentrations in LH plasma were comparable to capillary DBS, whereas tyrosine concentrations were slightly higher in LH plasma (constant bias of 6.4 μmol/L). However, in the low phenylalanine range, most samples had higher phenylalanine concentrations in DBS compared to LH plasma. Remarkably, phenylalanine and tyrosine in EDTA plasma were higher compared to all other samples (slopes ranging from 7 to 12%). No differences were observed when comparing capillary DBS to other DBS.

CONCLUSIONS

Overall agreement between plasma and DBS is good. However, bias is specimen- (LH vs EDTA), and possibly concentration- (low phenylalanine) dependent. Because of the overall good agreement, we recommend the use of a DBS-plasma correction factor for DBS measurement. Each laboratory should determine their own factor dependent on filter card type, extraction and calibration protocols taking the LH plasma values as gold standard.

Highly sensitive determination of L-tyrosine in pig serum based on ultrathin CuS nanosheets composite electrode

Nanometer-sized copper sulfide has remarkable properties such as metal like electrical conductivity and electrocatalytic activity. In this work, ultrathin copper sulfide nanosheets (CuS NS) were synthesized and employed to modify on surface of glassy carbon electrode (GCE) combining with chitosan (CS) and acidified multi-walled carbon nanotubes (F-MWCNTs). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the shape of CuS NS was hexagon with side length of 13.33 ± 0.67 nm and thickness of 4.50 ± 0.58 nm. The electrochemical characteristics of different nanocomposite modified electrodes were examined by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV), indicating that the modified electrode of CuS NS-CS/F-MWCNTs/GCE possessed good electrocatalytic activity towards oxidation of L-tyrosine (L-Tyr). Under the optimal condition, the modified electrode exhibited a wide linear response range for L-Tyr (0.08-1.0 μM) with a detection limit of 4.9 nM. No obvious interferences from coexisted two-fold of L-tryptophan and 50-fold of other amino acids could be observed, indicating its relatively good selectivity. The electrode also had good repeatability, reproducibility and stability. Compared with a commercial instrument analytical method, HPLC, the electrode can be successfully applied to the determination of L-Tyr in pig serums with a recovery rate of 95.7%-102.6%, and its test results are in good agreement with that of HPLC, showing its promising application value.

Clinical relevance of the discrepancy in phenylalanine concentrations analyzed using tandem mass spectrometry compared with ion-exchange chromatography in phenylketonuria

Introduction

Metabolic control of phenylketonuria (PKU) and compliance with the low-phenylalanine (phe) diet are frequently assessed by measuring blood phe concentrations in dried blood spots (DBS) collected by patients instead of plasma phe concentrations.

Objective

Our objective was to investigate the difference in blood phe concentrations in DBS collected by subjects and analyzed using either a validated newborn screening tandem mass spectrometry (MS/MS) protocol or ion-exchange chromatography (IEC) compared to plasma phe concentrations obtained simultaneously and analyzed using IEC.

Design

Three to four fasting blood samples were obtained from 29 subjects with PKU, ages 15–49 years. Capillary blood was spotted on filter paper by each subject and the DBS analyzed using both MS/MS and IEC. Plasma was isolated from venous blood and analyzed using IEC.

Results

Blood phe concentrations in DBS analyzed using MS/MS are 28% ± 1% (n = 110, p < 0.0001) lower than plasma phe concentrations analyzed using IEC resulting in a blood phe concentration of 514 ± 23 μmol/L and a plasma phe concentration of 731 ± 32 μmol/L (mean ± SEM). This discrepancy is larger when plasma phe is > 600 μmol/L. Due to the large variability across subjects of 13.2%, a calibration factor to adjust blood phe concentrations is not recommended. Analysis of DBS using IEC reduced the discrepancy to 15 ± 2% lower phe concentrations compared to plasma analyzed using IEC (n = 38, p = 0.0001). This suggests that a major contributor to the discrepancy in phe concentrations is the analytical method.

Conclusion

Use of DBS analyzed using MS/MS to monitor blood phe concentrations in individuals with PKU yields significantly lower phe levels compared to plasma phe levels analyzed using IEC. Optimization of current testing methodologies for measuring phe in DBS, along with patient education regarding the appropriate technique for spotting blood on filter paper is needed to improve the accuracy of using DBS to measure phe concentrations in PKU management.

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Phe concentration in dried blood spots is significantly lower than plasma phe.

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Blood phe concentration cannot be adjusted due to large variability across subjects.

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Analysis of dried blood spots using IEC instead of MS/MS improves accuracy.

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Plasma phe concentration using IEC is the most accurate for metabolic monitoring in PKU.

Effects of Titanium Dioxide Nanoparticles on the Synthesis of Fibroin in Silkworm (Bombyx mori)

Silkworm (Bombyx mori) is an economically important insect, and its silk production capacity largely depends on its ability to synthesize fibroin. While breeding of B. mori varieties has been a key strategy to improve silk production, little improvement of B. mori silk production has been achieved to date. As a result, the development of sericulture economy has not progressed well, pointing to the need of new ways for improvement of B. mori silk production. Titanium dioxide nanoparticles (TiO2 NPs), a food additive widely used for livestock, have been shown to promote animal growth and increase the protein synthesis in animals. However, no studies on effect of TiO2 NPs on fibroin synthesis in B. mori have been available. In this study, the differential expression profiles of genes and proteins in the silk gland of B. mori fed without or with TiO2 NPs (5 μg ml(-1)) were analyzed and compared using digital gene expression (DGE), reverse transcription quantitative polymerase chain reaction (RT-qPCR), semi-qPCR, and Western blot analysis. The effects of TiO2 NPs feeding on the activity of proteases in the midgut and the synthesis and transportation of amino acids in hemolymph were also investigated. DGE analyses showed that among a total of 4,741 genes detected, 306 genes were differentially expressed after the TiO2 NPs feeding, of which 137 genes were upregulated whereas 169 genes were downregulated. 106 genes were shown to be involved in fibroin synthesis, of which 97 genes, including those encoding cuticular protein glycine-rich 10, serine protease inhibitor 28, aspartate aminotransferase, lysyl-tRNA synthetase, and splicing factor arginine/serine-rich 6, and silk gland factor-1 (SGF-1), were upregulated with the maximum induction of 8.52-folds, whereas nine genes, including those encoding aspartylglucosaminidase, the cathepsin L in Tribolium castaneum, and similar to SPRY domain-containing SOCS box protein 3, were downregulated with the maximum reduction of 8.11-folds. Transcription levels of nine genes were further verified by RT-qPCR, and the results were consistent with those with DGE. Transcription and expression levels of fibroin light chain (Fib-L) gene were increased after TiO2 NPs feeding, indicating that TiO2 NPs improves fibroin synthesis. Compared with that of control, the mean protease activity was increased by 56.67% in the B. mori fed with TiO2 NPs, and the transport of four key amino acids used for fibroin synthesis in hemolymph was also increased. These findings indicated that TiO2 NPs feeding can improve the absorption and utilization of amino acids from the feed and could be a new way to increase the fibroin synthesis in B. mori.

Simplifying sample pretreatment: application of dried blood spot (DBS) method to blood samples, including postmortem, for UHPLC-MS/MS analysis of drugs of abuse

The complexity of biological matrices, such as blood, requires the development of suitably selective and reliable sample pretreatment procedures prior to their instrumental analysis. A method has been developed for the analysis of drugs of abuse and their metabolites from different chemical classes (opiates, methadone, fentanyl and analogues, cocaine, amphetamines and amphetamine-like substances, ketamine, LSD) in human blood using dried blood spot (DBS) and subsequent UHPLC-MS/MS analysis. DBS extraction required only 100μL of sample, added with the internal standards and then three droplets (30μL each) of this solution were spotted on the card, let dry for 1h, punched and extracted with methanol with 0.1% of formic acid. The supernatant was evaporated and the residue was then reconstituted in 100μL of water with 0.1% of formic acid and injected in the UHPLC-MS/MS system. The method was validated considering the following parameters: LOD and LOQ, linearity, precision, accuracy, matrix effect and dilution integrity. LODs were 0.05-1ng/mL and LOQs were 0.2-2ng/mL. The method showed satisfactory linearity for all substances, with determination coefficients always higher than 0.99. Intra and inter day precision, accuracy, matrix effect and dilution integrity were acceptable for all the studied substances. The addition of internal standards before DBS extraction and the deposition of a fixed volume of blood on the filter cards ensured the accurate quantification of the analytes. The validated method was then applied to authentic postmortem blood samples.

Changes in free amino acid, protein, and flavonoid content in jujube (Ziziphus jujube) fruit during eight stages of growth and antioxidative and cancer cell inhibitory effects by extracts

Jujube (Ziziphus jujube) was analyzed at eight stages of ripeness (S1-8) for protein, by HPLC and mass spectroscopy for free amino acids and flavonoids, and by colorimetry for total flavonoids and antioxidative activity. The ripe fruit had lower levels of protein, flavonoids, and antioxidative activity than that of the unripe fruit. Free amino acids levels peaked at S5, due mainly to an increase in free asparagine. Extracts were also tested against four cell lines using the MTT cell viability assay. All growth stages dose-dependently inhibited HeLa cervical cancer cells, whereas the inhibition of Hel299 normal lung and A549 lung cancer cells decreased as the fruit matured and was well correlated with the flavonoid content and antioxidative activity. Chang normal liver cells were inhibited by only the S5 extract. U937 lymphoma cells were unaffected by the extracts. These results show the effect of fruit maturity on nutritional and health-promoting components.

Distribution of free amino acids, flavonoids, total phenolics, and antioxidative activities of Jujube (Ziziphus jujuba) fruits and seeds harvested from plants grown in Korea

Fruit pulp and seeds from the jujube plant possess nutritional and medicinal properties. The bioactive components have been shown to vary both with cultivar and with growing conditions. Most studies report the components of varieties from China. We measured free amino acid, individual phenolic, and total phenolic content, and antioxidative activities in three jujube fruit pulp extracts from Boeun-deachu, Mechu, and Sanzoin cultivars and two seed extracts (Mechu and Sanzoin) from plants grown in Korea. In g/100 g dry weight, total free amino acid content measured by ion-exchange chromatography ranged from 5.2 to 9.8 in the pulp and from 4.0 to 5.3 in the seed. Total phenolic content measured by Folin-Ciocalteu ranged from 1.1 to 2.4 in the pulp and from 3.6 to 4.6 in the seed. Flavonoids were measured by HPLC and ranged from 0.7 to 1.8 in the pulp and from 3.2 to 4.0 in the seed. Flavonoids were identified by HPLC elution position and UV/vis and mass spectra. Fruits contained the following flavonoids: procyanidin B2, epicatechin, quercetin-3-O-rutinoside (Q-3-R), quercetin-3-O-galactoside (Q-3-G), kaempferol-glucosyl-rhamnoside (K-G-R), and two unidentified compounds. Seeds contained the following flavonoids: saponarin, spinosin, vitexin, swertish, 6'''-hydroxybenzoylspinosin (6'''-HBS), 6'''-feruloylspinosin (6'''-FS), and one unidentified substance. Dimensions and weights of the fresh fruit samples affected phenolic content. The distribution of the individual flavonoids among the different samples varied widely. Data determined by the FRAP antioxidative assay were well correlated with total phenolic content. In a departure from other studies, data from the DPPH free radical assay were not correlated with FRAP or with any of the measured compositional parameters. Because individual jujube flavonoids are reported to exhibit different health-promoting effects, knowledge of the composition and concentration of bioactive compounds of jujube products can benefit consumers.

Determination of phenylalanine and tyrosine in plasma and dried blood samples using HPLC with fluorescence detection

The determination of phenylalanine and tyrosine is presently the most reliable direct approach to the diagnosis of phenylketonuria. An HPLC method for the simultaneous measurement of phenylalanine and tyrosine in samples of dried blood spots and plasma has been developed and evaluated. We have used an inherent fluorescence of both phenylalanine and tyrosine. For the separation, a reverse-phase column LiChroCart 125-4, Purospher RP-18e, 5microm, was used. The mixture of ethanol and deionized water (5:95, v/v) was used as a mobile phase. Analytical performance of this method is satisfactory for both phenylalanine and tyrosine: the intra-assay and inter-assay coefficients of variation were below 10%. Quantitative recoveries from spiked plasma and blood samples were between 92.0 and 102.9%. The limit of detection was 10.0 and 5.0micromol/L, respectively. The preliminary reference ranges of phenylalanine and tyrosine in a group of newborns are 69.3+/-13.1 and 42.7+/-12.9micromol/L, in a group of blood donors are 68.4+/-9.9 and 52.1+/-10.9micromol/L. The presented method is inexpensive and suitable for diagnosis of phenylketonuria.

Blood phenylalanine monitoring for dietary compliance among patients with phenylketonuria: comparison of methods

PURPOSE

Blood phenylalanine monitoring is critical for the management of phenylketonuria. We compared three methods for measuring blood phenylalanine concentration: the amino acid analyzer, high-performance liquid chromatography with fluorometric detection, and tandem mass spectrometry.

METHODS

We studied 22 female patients with phenylketonuria, ages 12-48 years, who attended our Metabolic Camp. Blood was collected into heparinized tubes (for analysis by the amino acid analyzer) or filter paper (for analysis by high-performance liquid chromatography with fluorometric detection and tandem mass spectrometry).

RESULTS

Blood phenylalanine concentrations of plasma measured by the amino acid analyzer were significantly higher than those obtained from whole blood on filter paper by high-performance liquid chromatography (difference: 102 microM; 95% confidence interval: 23, 181) and tandem mass spectrometry (difference: 137 microM; 95% confidence interval: 58, 216). Phenylalanine concentrations from high-performance liquid chromatography and tandem mass spectrometry were not significantly different (P = 0.5).

CONCLUSIONS

When monitoring blood phenylalanine concentrations for dietary compliance, clinicians should be mindful of the method being used; analyses of whole blood on filter paper were consistently approximately 15% lower than analyses of plasma.

Development of micellar electro kinetic chromatography for the separation and quantitation of L-valine, L-leucine, L-isoleucin and L-phenylalanine in human plasma and comparison with HPLC

Phenylketonuria (PKU) and Maple Syrup Urine Disease (MSUD) are two inborn metabolic diseases which are carried by autosomal recessive genes in man. These genetic errors result in accumulation of phenylalanine (in PKU) or valine, leucine and isoluecin (in MSUD). At high concentrations, amongst other problems, these amino acids cause mental retardation. However if detected early after birth, using special diets and other forms of therapy, mental abnormalities can be prevented. As a result in many countries screening of infants for MSUD and PKU, by measuring plasma amino acids has become a routine neonatal test. Capillary Electrophoresis (CE) assays have a number of advantages over the traditional chromatography techniques (such as GC or HPLC). These include low cost, high speed of analysis and high resolution. These characteristics, make CE an ideal method for the screening of inborn errors of metabolism. We developed a CE assay based on pre-column derivatisation of amino acids with phenylisothiocyanate. This conjugate has strong absorbance at 254 nm. CE was carried out using a Spectraphoresis 1000 instrument, fitted with 40 cm of a 25 microm capillary, at 17 degrees C. A running voltage of 18KV was used to separate the amino acid mixture in an electrophoretic buffer containing 45 mM imidazole, 6 mM borate and 208 mM SDS, fixed at pH 9 with 2-N-morpholino ethane sulfonic acid. The assay was calibrated using various concentrations of amino acid standards. LOD, LOQ, recovery, inter-day and intra-day variations of the assay were determined. Also, levels of the 4 amino acids in normal and abnormal plasma were determined and compared with HPLC.