Amoxicillin In Plasma Research Articles

Simultaneous analysis of amoxicillin and sulbactam in human plasma by HPLC-DAD for assessment of bioequivalence

A simple and accurate high-performance liquid chromatography with diode array detection-based (HPLC-DAD) method has been developed and validated for simultaneous determination of amoxicillin and sulbactam in human plasma. Sample preparation was involved in protein precipitation with acetonitrile followed by one-step extraction procedure. Chromatographic separation was achieved on a C18 column with an isocratic mobile phase consisting of water (containing 30 mM potassium dihydrogen phosphate, pH 2.8) and acetonitrile. The detection wavelengths of a diode array detector were set at 210 nm for amoxicillin and sulbactam, and 263 nm for the internal standard (cefadroxil). The method was validated for linearity, accuracy, precision, and stability. The calibration curve was linear from 0.163 to 14.7 μg/mL with correlation coefficient squared of 0.9991 for amoxicillin and 0.250–15.0 μg/mL with correlation coefficient squared of 0.9988 for sulbactam using 500 μL plasma samples. The lower limit of quantification was 0.163 and 0.250 μg/mL for amoxicillin and sulbactam, respectively. The imprecisions of intra- and inter-day validations for amoxicillin and sulbactam were <11% and their accuracies (%) were within the range of 95.4–105.7%. Mean recoveries were 75.9, 72.8, and 70.0% for amoxicillin, sulbactam, and cefadroxil, respectively. The established method was successfully applied to a bioequivalence study of two combination formulations of amoxicillin and sulbactam pivoxil in healthy male volunteers.

A novel approach to the rapid determination of amoxicillin in human plasma by solid phase microextraction and liquid chromatography

A new approach to the rapid determination of amoxicillin (AMO) in human plasma followed by solid phase microextraction (SPME) fiber coatings based on conducting polymers (polypyrrole and polythiophene) and high performance liquid chromatography (HPLC) has been described. The porous structures of the electrochemically deposited polymer coatings have been characterized by scanning electron microscopy (SEM). The experimental parameters relating to the extraction efficiency of the SPME fibers such as pH, extraction time and desorption conditions (solvents, time) were studied and selected. The SPME/HPLC-UV method was linear over a working range of 1-50 μg ml(-1). The inter-day accuracy (expressed as coefficients of variations, CVs) was less than 15% and precision (expressed as the relative standard deviations, RSDs) with percentage values was less than 5.9%. Amoxicillin was found to be stable in the human plasma at room temperature (20 °C) within 8 hours. The developed method was successfully applied to the analysis of real human plasma samples. The limit of detection and limit of quantification for amoxicillin in plasma were 1.21 μg ml(-1) and 3.48 μg ml(-1), respectively.

Simple, Economical, and Reproducible LC--MS Method for the Determination of Amoxicillin in Human Plasma and its Application to a Pharmacokinetic Study

A simple, economical, and reproducible high-performance liquid chromatography mass spectrometric (MS) method is developed and validated for the determination of amoxicillin in human plasma. The present method has been successfully used to determine bioequivalence between a test and innovator formulation of amoxicillin 500 mg capsules. The method is validated in terms of selectivity, precision/accuracy, recovery, dilution integrity, matrix effect, effect of anti-coagulant, and stability studies. Sample preparation is carried out by solid-phase extraction (HLB Oasis cartridges). The processed sample is chromatographed on Hypersil Gold (4.6 x 50 mm); 3 microm C18 column, using 10mM ammonium formate buffer (pH 5.0) and acetonitrile, (10:90, v/v) as mobile phase. Amoxicillin is detected by MS-MS detection with turbo-ion spray in positive ion mode. The weighed (1/X2) calibration curves were linear over the range of 0.17 to 17.0 microg/mL. The intra day precision is from 1.3% to 8.8% and intra day accuracy is 94.1% to 108.5%. The inter day precision is from 1.8% to 6.2% and inter-day accuracy is 95.1% to 105.9%. Mean recovery of 66.3% is observed for amoxicillin and 71.6% for internal standard (ampicillin). The stability of amoxicillin is studied at -15 degrees C and -50 degrees C using human plasma with different anti-coagulants (citrate, monobasic sodium phosphate, dextrose, and adenine-citrate, monobasic sodium phosphate, dextrose, and adenine and ethylene diamine tetraacetic acid-ethylene diamine tetraacetic acid). No significant degradation is observed for 60 days.

Simultaneous determination of amoxicillin and ambroxol in human plasma by LC–MS/MS: Validation and application to pharmacokinetic study

A rapid, simple and sensitive LC–MS/MS method was developed for simultaneous determination of amoxicillin and ambroxol in human plasma using clenbuterol as internal standard (IS). The plasma samples were subjected to a simple protein precipitation with methanol. Separation was achieved on a Lichrospher C 18 column (150 mm × 4.6 mm ID, dp 5 μm) using methanol (containing 0.2% of formic acid) and water (containing 0.2% of formic acid) as a mobile phase by gradient elution at a flow rate of 1.0 mL/min. Detection was performed using electrospray ionization in positive ion multiple reaction monitoring (MRM) mode by monitoring the ion transitions from m/ z 365.9 → 348.9 (amoxicillin), m/ z 378.9 → 263.6 (ambroxol) and m/ z 277.0 → 203.0 (IS). Calibration curves were linear in the concentration range of 5–20,000 ng/mL for amoxicillin, and 1–200 ng/mL for ambroxol, with the intra- and inter-run precisions of <9% and the accuracies of 100 ± 7%. The method has been validated and applied to pharmacokinetic studies of compound amoxicillin and ambroxol hydrochloride tablets in healthy Chinese volunteers.

Simultaneous determination of amoxicillin and clavulanic acid in human plasma by isocratic reversed-phase HPLC using UV detection

A simple, rapid and sensitive isocratic reversed phase HPLC method with UV detection using internal standard has been developed and validated for simultaneous determination of amoxicillin and clavulanic acid in human plasma. The assay enables the measurement of amoxicillin and clavulanic acid for therapeutic drug monitoring with a minimum quantification limit of 15 and 30 ng ml −1, respectively. The method involves simple, one-step extraction procedure and analytical recovery was complete. The separation was carried out in reversed-phase conditions using a Chromolith Performance (RP-18e, 100 mm × 4.6 mm) column with an isocratic mobile phase consisting of 0.02 M disodium hydrogen phosphate buffer–methanol (96:4, v/v) adjusted to pH 3.0. The wavelength was set at 228 nm. The coefficients of variation for inter-day and intra-day assay were found to be less than 9.0%.

Microanalysis of amoxicillin, flucloxacillin, and rifampicin in neonatal plasma

Simple and rapid reversed-phase high-performance liquid chromatographic assays with ultraviolet detection have been developed and validated for the determination of amoxicillin, flucloxacillin and rifampicin in neonatal plasma. Plasma samples were either precipitated with perchloric acid (amoxicillin) or methanol (rifampicin) or extracted with methylene chloride (flucloxacillin). Precision coefficients of variation and inaccuracy were less than 15% for all three assays. Only small sample volumes (20-40 microL) were required, making the assays suitable for therapeutic drug monitoring and pharmacokinetic studies in preterm and term neonates. The assays have successfully been applied to analysis of amoxicillin, flucloxacillin and rifampicin in previously published pharmacokinetic studies in neonates.

Simultaneous determination and validation of antimicrobials in plasma and tissue of actinomycetoma by high-performance liquid chromatography with diode array and fluorescence detection

A simple, precise, and reliable chromatographic method was developed for the simultaneous determination in plasma and infected tissue of five antimicrobials proposed for the treatment of actinomycotic mycetoma: amoxicillin, trimethoprim, linezolid, sulfamethoxazole and garenoxacin. Separation of the analytes was achieved on an Atlantis dC18 column (150 mm × 4.6 mm, ID 5 μm) with a mobile phase composed of acetonitrile and trifluoroacetic acid (ATF) 0.1% (v/v) using a gradient program. The detection was carried out using a diode array detector at 254 nm and in a fluorescence detector at wavelengths of excitation and emission of 292 nm and 392 nm for linezolid and sulfamethoxazole, and 292 nm and 408 nm for garenoxacin, respectively. The intraday precision was in the range of 0.7–15% of relative standard deviations (%R.S.D.) for plasma and 1–18% for tissue. Linearity range was from 2.4 to 20 μg/ml for amoxicillin, 0.3 to 20 μg/ml for trimethoprim, sulfamethoxazole and linezolid, and 0.3 to 10 μg/ml for garenoxacin. Acetonitrile was used to precipitate proteins from plasma. Recoveries in plasma ranged from 71% to 118% and in infected tissue from 78% to 122%. Limits of detection (LODs) were 1.2 and 0.5 μg/ml for amoxicillin in plasma and tissue, respectively and 0.15 and 1.2 μg/ml in plasma and tissue, respectively for the other antimicrobials. The method can be applied for individual or simultaneous determination of the antimicrobials in plasma and tissue of mouse infected with actinomycetoma.

Simple and Rapid LC Method for the Determination of Amoxicillin in Plasma

A rapid, simple, accurate, sensitive and reproducible high performance liquid chromatographic (HPLC) method for the quantitation of amoxicillin in human plasma using cefadroxil as an internal standard (IS) has been developed and validated. The procedure involves an ultrafiltration step prior to a reversed-phase liquid chromatography. The drug and the IS were eluted from Symmetry® C18 stainless steel column (5 μm, 150 × 4.6 mm I.D.) at room temperature with a mobile phase consisting of methanol: 75 mM potassium dihydrogen phosphate buffer solution (10:90,v/v) (pH adjusted to 3.0 with phosphoric acid), at a flow rate of 1.5 mL min−1. The effluent was monitored using a UV detector set at 228 nm. Each analysis required no longer than 10 min. Quantitation was achieved by measurement of the peak area ratio of the drug to the internal standard, and the limit of quantification of amoxicillin in plasma was 0.5 μg mL−1 (RSD% and DEVs% were <15%). The method showed good precision: the intraday RSD% values for amoxicillin were in the range of 1.41–6.86% whereas the values for interday were in the range of 1.13–8.16% at four different concentrations. The method is rapid (total run time <12 min) and accurate (DEVs,% <10%). The mean relative recovery was 99.67% and the mean absolute recovery was 86.68%. Stability testing shows that amoxicillin is stable in plasma for at least 4 weeks when stored at −70 °C. The method was successfully applied in a pharmacokinetic study involving healthy rabbits.

Simultaneous determination of amoxicillin and ranitidine in rat plasma by high-performance liquid chromatography

A high-performance liquid chromatography method using ultraviolet detection at 230 nm for the simultaneous determination of amoxicillin and ranitidine in rat plasma has been validated. Plasma samples after pretreatment with acetonitrile to effect deproteinization were dried under N 2 and reconstituted with water. The standard calibration curves for amoxicillin and ranitidine were linear ( r 2 = 0.9999) over the concentration range of 0.2–20 μg ml −1 and 0.03–6 μg ml −1 in rat plasma, respectively. The intra- and inter-day assay variability range for amoxicillin was 2.4–8.5% and 3.2–11.7%, and for ranitidine was 1.7–9.0% and 4.5–10.1%, respectively. This method has been successfully applied to a pharmacokinetic study after oral coadministration of amoxicillin and ranitidine to rats.

Development and Validation of a Solid-Phase Extraction-Liquid Chromatographic Method for Determination of Amoxicillin in Plasma

A bioanalytic method for the determination of amoxicillin in plasma by hydrophilic interaction solid-phase extraction and liquid chromatography has been developed and validated. Plasma was precipitated with acetonitrile before samples were loaded onto a zwitterionic hydrophilic interaction liquid chromatography (ZIC-HILIC) solid-phase extraction column. Amoxicillin was analyzed by liquid chromatography on an Aquasil (150 x 4.6 mm) LC column with mobile-phase acetonitrile: phosphate buffer (pH 2.5; 0.1 mol/L) (7:93, v/v) and UV detection at 230 nm. A regression model using 1/concentration weighting was found the most appropriate for quantification. The intraassay precision for plasma was 3.3% at 15.0 microg/mL and 10.9% at 0.200 microg/mL. The interassay precision for plasma was 1.8% at 15.0 microg/mL and 7.5% at 0.200 microg/mL. The total-assay precision for plasma over 4 days using a total of 20 replicates was 13.2%, 5.5%, and 3.8% at 0.200 microg/mL, 3.00 microg/mL, and 15.0 microg/mL, respectively. The lower limit of quantification and the limit of detection were 0.050 microg/mL and 0.025 microg/mL, respectively, for 100 microL plasma. Long-term storage stability studies of amoxicillin in plasma indicate that a temperature of -80 degrees C is necessary to prevent degradation of amoxicillin.

Simultaneous determination of amoxicillin and clavulanic acid in human plasma by HPLC–ESI mass spectrometry

A simple, fast and sensitive high-performance liquid chromatography (HPLC)–mass spectrometric (MS) method has been developed for simultaneous determination of amoxicillin and clavulanic acid in human plasma using terbutaline as internal standard. After precipitation of the plasma proteins with acetonitrile, the analytes were separated on a C 8 reversed-phase column with formic acid–water–acetonirile (2:1000:100) and detected using electrospray ionization (ESI) mass spectrometry in negative selected ion monitoring (SIM) mode. The method was validated and successfully applied to analysis of amoxicillin and clavulanic acid in clinical studies. The limit of quantitation, 0.12 μg/ml for amoxicillin and 0.062 μg/ml for clavulanic acid, was five times lower than that of the published HPLC–UV method.

Comparing Pharmacokinetics of Amoxicillin Given Twice or Three Times per Day to Children Older than 3 Months with Pneumonia

For children with ambulatory pneumonia, the World Health Organization (WHO) recommends oral amoxicillin (15 mg/kg of body weight/dose) thrice daily (t.i.d.) or oral cotrimoxazole (4 mg of trimethoprim/kg/dose) twice daily (b.i.d.). The more frequent amoxicillin dosing may lead to compliance problems. To compare the pharmacokinetics and levels of amoxicillin in plasma in the current WHO acute respiratory infection recommendations with the 25-mg/kg/dose b.i.d. regimen, we performed a two-group parallel study of 66 children ages 3 to 59 months with pneumonia. Amoxicillin was given orally at 25 mg/kg/dose b.i.d. or 15 mg/kg/dose t.i.d. Amoxicillin concentrations were determined by high-performance liquid chromatography after the first dose on days 1 and 3. After the first dose on day 1, the mean area under the concentration-time curve (AUC) for amoxicillin after the 25-mg/kg dose was 54.7 versus 24.9 micro g. h/ml after the 15-mg/kg dose. After the first dose on day 3, the mean AUC was 44.1 versus 28.5 micro g. h/ml. All but two children had plasma amoxicillin concentrations above 0.5 micro g/ml for >50% of the dose interval on both days. Six children on day 1 and five children on day 3 had concentrations above 1.0 micro g/ml for <50% of the dose interval. On day 1, 16 of 27 children in the b.i.d. group and 11 of 26 children in the t.i.d. group had concentrations that were above 2.0 micro g/ml for <50% of the dose interval, and on day 3, 18 of 31 children in the b.i.d. group and 8 of 31 children in the t.i.d. group had concentrations that were above 2.0 micro g/ml for <50% of the dose interval. Amoxicillin b.i.d. is a feasible alternative for t.i.d. dosing. To lengthen the time above the MIC at higher concentration levels, a 30- to 40-mg/kg/dose b.i.d. should be considered instead of the 25 mg/kg/dose used in this study.

DEVELOPMENT OF A REVERSED-PHASE LIQUID CHROMATOGRAPHIC METHOD FOR THE ANALYSIS OF AMOXICILLIN, METRONIDAZOLE, AND PANTOPRAZOLE IN HUMAN PLASMA USING SOLID-PHASE EXTRACTION

ABSTRACT A high-performance liquid chromatography method has been developed and validated for the simultaneous determination of amoxicillin, metronidazole, and pantoprazole in human plasma. Solid-phase extraction (SPE) was utilized to extract the analytes along with the internal standard, tinidazole. Baseline resolution was achieved using a 30 : 70 v/v acetonitrile–25 mM potassium phosphate buffer containing 0.25% triethylamine (pH 6.5) mobile phase, at a flow rate of 0.5 mL/min. A 5 μm phenyl column, equipped with a guard column, with detection at 230 nm constituted the HPLC system. The method yields retention times of 3.7, 4.8, 6.9, and 16.2 min for amoxicillin, metronidazole, tinidazole (internal standard), and pantoprazole, respectively. Limits of detection were 200 ng/mL for amoxicillin and metronidazole and 100 ng/mL for pantoprazole. Recoveries from human plasma ranged from 83–92% for amoxicillin, 81–89% for metronidazole, 85–94% for tinidazole, and 93–101% for pantoprazole. Intra-day (n=5) and inter-day (n=15) precision (% RSD) and accuracy (% error) for all analytes ranged from 1.11–5.97% and 0.72–13.5%, respectively.

Simultaneous determination of amoxicillin and clavulanic acid in human plasma by HPLC with UV detection

A simple and accurate high-performance liquid chromatographic method with ultraviolet detection at 220 nm has been validated for the simultaneous determination of amoxicillin and clavulanic acid in human plasma. Plasma samples were pretreated by direct deproteinization with methanol. A good chromatographic separation between both compounds was achieved using a reversed phase C8 column and a mobile phase, consisting of acetonitrile-phosphate solution-tetramethyl ammonium chloride solution. The calibration curves were linear over the concentration range of 0.625–20 mg l −1 for amoxicillin and 0.3125–10 mg l −1 for clavulanic acid with determination coefficients >0.998. The method is accurate (bias <7%) and reproducible (intra- and inter-day R.S.D. <15%), with a quantitation limit of 0.625 and 0.3125 mg l −1 for amoxicillin and clavulanic acid, respectively. Analytical recoveries from human plasma ranged from 91 to 102% for both components. This fully validated method, which allows the simultaneous measurement of amoxicillin and clavulanic acid in biological samples, is rapid (total run time <10 min) and requires only a 100 μl sample. This assay is suitable for biomedical applications and was successfully applied to a pilot pharmacokinetics study in healthy volunteers after a single-oral administration of amoxicillin/clavulanic acid combination (500/125 mg).

Measurement of amoxicillin in plasma and gastric samples using high-performance liquid chromatography with fluorimetric detection

A rapid, selective and sensitive HPLC assay has been developed for the routine analysis of amoxicillin in rat plasma, gastric juice aspirate and gastric tissue which is applicable to low concentrations of amoxicillin (<1 μg mL −1) or small sample volumes. Amoxicillin was converted, via an internal rearrangement, to form a fluorescent product which was subsequently recovered using liquid–liquid extraction. A Kromasil ODS 3 μm (150×3.2 mm I.D.) column was maintained at 40 °C and used with a mobile phase consisting of methanol–water (55:45, v/v). Fluorimetric detection was at an λ ex of 365 nm and an λ em of 445 nm. The limits of quantitation for amoxicillin were 0.1 μg mL −1 for gastric juice aspirate (500 μL), 0.5 μg mL −1 for plasma (50 μL) and 0.075 μg g −1 for gastric tissue (250 mg). The method was linear up to at least 15 μg mL −1 in gastric juice aspirate, up to 200 μg mL −1 in plasma and up to 100 μg g −1 in gastric tissue, with inter- and intra-day RSDs being less than 19%. The assay has been applied to the measurement of amoxicillin in rat plasma, gastric juice aspirate and gastric tissue for pharmacokinetic studies in individual rats.

Co-amoxiclav pharmacokinetics during posttraumatic hemorrhagic shock.

To determine the effects of severe trauma with hemorrhagic shock on amoxicillin and clavulanate concentrations in plasma and their pharmacokinetics. A prospective, open, descriptive study. A 12-bed, adult surgical intensive care unit in a university-affiliated hospital in France. Subjects were 12 patients (10 men, 2 women) with severe trauma: median (range) Injury Severity Score, 38 (17-48); Acute Physiology and Chronic Health Evaluation II, 16 (7-38); Simplified Acute Physiology Score II, 41 (23-77). Also enrolled were 12 healthy volunteers who were matched on age (+/-5 yrs), gender, and body-surface area (+/-20 cm2). All the trauma patients suffered hemorrhagic shock defined as the association of at least one episode of systolic blood pressure <90 mm Hg and an intravascular volume expansion >2000 mL between trauma and surgery. Prophylactic perioperative administration of 2 g of amoxicillin and 0.2 g of clavulanate in combination during the first 12 hrs posttrauma in patients, and at the start of the pharmacokinetic study in volunteers. Serial plasma samples (n = 13) were obtained after the first antibiotic administration to measure antibiotic levels by using high-performance liquid chromatography assays. Compared with volunteers, trauma patients had higher plasma amoxicillin and clavulanate concentrations, attributed to a reduction of the volume of distribution (p =.001 and p =.06, respectively) and, to a lesser extent, of the total body clearance (p =.09 and p =.20, respectively). Consequently, amoxicillin and clavulanate elimination half-lives were similar for the two groups of subjects. The interindividual variabilities for all the amoxicillin pharmacokinetic parameters were higher in patients. In trauma patients with hemorrhagic shock requiring surgery, the administration of 2 g of amoxicillin and 0.2 g of clavulanate seems adequate, according to the antibiotic concentrations observed in plasma for both drugs. However, further studies exploring antibiotic concentrations in tissues are warranted.

Pharmacokinetics and tissue distribution of amoxicillin in healthy and Salmonella Typhimurium-inoculated pigs.

To determine pharmacokinetics and tissue distribution of amoxicillin in healthy and Salmonella Typhimurium-inoculated pigs. 12 healthy pigs and 12 S Typhimurium-inoculated pigs. Concentration of amoxicillin in tissue was measured by use of high-performance liquid chromatography 4, 8, 12, and 24 hours after IM administration. Pharmacokinetic values of amoxicillin in plasma were assessed by use of a 1-compartment model with first-order absorption. Inoculation caused diarrhea and increased rectal temperature and WBC count. Absorption half-life was shorter in inoculated pigs (0.26 hours) than in healthy pigs (0.71 hours), and inoculated pigs had longer elimination half-life. Distribution ratios in healthy pigs ranged from 0.31 to 0.56 and in inoculated pigs ranged from 0.14 to 0.48. Ratios for distribution to intestinal mucosa ranged from 0.34 to 1.16 in healthy pigs and from 0.22 to 0.36 in inoculated pigs. Salmonella Typhimurium inoculation altered absorption of amoxicillin from the injection site and prolonged elimination half-life. However, distribution of amoxicillin to intestinal tract tissue was only affected to a minor degree.

Determination of amoxicillin in human serum and plasma by high-performance liquid chromatography and on-line postcolumn derivatisation

A highly selective, sensitive and fast HPLC method was developed for the determination of amoxicillin in human serum and plasma. After protein precipitation with perchloric acid, an aliquot of the supernatant was neutralized by mixing it with sodium acetate solution and injected into a C 18 HPLC column. Detection was done by a fluorescence detector after on-line postcolumn derivatisation with fluorescamine. The practical limit of quantification was 0.1 μg/ml using 0.3 ml of plasma. Linearity was given in the tested range of 0.1 to 15 μg/ml plasma. Inter-day precision (relative standard deviation) over 7 days for 0.42 μg/ml was ±7.27%; for 4.54 μg/ml, ±5.24% and for 13.18 μg/ml, ±5.25%. Stability over 50 days in serum and plasma occurs at −70°C but not at −20°C (−25 to −35% reduction). This method was used for thousands of human serum and plasma samples.

Improved high-performance liquid chromatographic determination of amoxicillin in human plasma by means of column switching

A highly sensitive and selective HPLC method was developed for the determination of amoxicillin in human plasma. After addition of buffer and internal standard, the sample was ultrafiltered and injected on to a precolumn to remove polar plasma interferences. Detection was effected with a UV detector set at 230 nm. The limit of quantification for amoxicillin was 50.1 ng/ml with an imprecision of 4.2% using 0.25 ml of plasma. Linearity was confirmed over the whole calibration range (25.4-0.0501 μg/ml) and the inter-day variation ranged from 2.0 to 4.5%. The method was validated according to GLP guidelines and its suitability was demonstrated by the analysis of several hundred samples in a bioequivalence study. The method can be used to determine pharmacokinetic parameters of amoxicillin in humans after a single oral dose of 500 mg.

Sensitive assay for measuring amoxicillin in human plasma and middle ear fluid using solid-phase extraction and reversed-phase high-performance liquid chromatography

We developed a sensitive assay to measure amoxicillin in human plasma and midle ear fluid (MEF) using solid-phase extraction and reversed-phase HPLC. Amoxicillin and cefadroxil, the internal standard, were extracted from 50–200 μl of sample with Bond Elut C 18 cartridges. The exact was analyzed on a 15 cm × 2 mm, 5μm Keystone MOS Hypersil-1 (C 8) column with UV detection at 210 nm. The mobile phase was 6% acetonitrile in 5 m M phosphate buffer (pH = 6.5) and 5 m M tetrabutylammonium. The average absolute recovery of amoxicillin and cefadroxil were 91.2 ± 16.6% and 91.0 ± 6.8%, respectively. The limit of quantitation was 0.125 μg/ml with 200 μl sample size. The linear range was from 0.125 to 35.0 μg/ml with correlation coefficients greater than 0.999. These analytic conditions produced a highly sensitive amoxicillin assay in human body fluids without derivatization.