超高效液相色谱-串联质谱法测定草鱼肉中阿苯达唑及其代谢物残留

建立了同时测定草鱼肉中阿苯达唑及其3种代谢物阿苯达唑亚砜、阿苯达唑砜、阿苯达唑-2-氨基砜的超高效液相色谱-串联质谱(UPLC-MS/MS)分析方法。草鱼肉样品通过碱性乙酸乙酯提取,正己烷净化,超高效液相色谱分离,串联质谱检测,氘代同位素内标定量。本方法分析时间短,在4 min内即可完成4种被分析物的液质联用分析。本方法在0.1～10μg/kg范围内各物质线性良好,线性系数在0.9991～0.9996之间。阿苯达唑、阿苯达唑亚砜、阿苯达唑砜、阿苯达唑-2-氨基砜在0.5～5.0μg/kg添加水平下的平均回收率分别为98.4%～102.6%,97.1%～103.4%,98.8%～103.7%和101.1%～104.2%检出限分别为0.2,0.1,0.2和0.1μg/kg;日内精密度小于4.77%,日间精密度小于3.03%。本方法为阿苯达唑及其代谢物的检测及监控提供了一种快速、灵敏度高、重现性好的定量分析方法。     

Simultaneous determination of albendazole and its metabolites in fish muscle tissue by stable isotope dilution ultra-performance liquid chromatography tandem mass spectrometry

A rapid, specific, and sensitive method utilizing ultra-performance liquid chromatography tandem mass spectrometry was developed and validated to determine albendazole, albendazole sulfoxide, albendazole sulfone, and albendazole 2-aminosulfone in fish muscle tissue. The fish samples were extracted with ethyl acetate, then the organic phase was evaporated to dryness, and the residue was reconstituted in methanol–water solution and cleaned up by n-hexane. Reversed-phase separation of target compounds was achieved using a BEH C18 column and a gradient consisting of 0.2% (v/v) formic acid and methanol. Tandem mass spectrometry analyses were performed on a triple–quadrupole tandem mass spectrometer. In the whole procedure, the isotope-labeled internal standards were used to correct the matrix effect and variations associated with the analysis. The method was validated with respect to linearity, specificity, accuracy, and precision. The method exhibited a linear response from 0.1 to 20 ng mL^-1 (r ² > 0.9985). The limit of quantitation for albendazole (ABZ), albendazole sulfoxide (ABZSO), albendazole sulfone (ABZSO₂), and albendazole 2-aminosulfone (ABZ-2-NH₂SO₂) was 0.1, 0.1, 0.1, and 0.2 ng g^-1, respectively. The mean recoveries of ABZ, ABZSO, ABZSO₂, and ABZ-2-NH₂SO₂ spiked at a level of 0.2–5.0 ng g^-1 were 95.3–113.7%, and the relative standard deviations of intra- and inter-day measurements were less than 6.38%. The method was later successfully applied to the determination of albendazole and its three metabolites in 60 fish samples collected from local markets.     

Determination of albendazole and its metabolites in the muscle tissue of hybrid striped and largemouth bass using liquid chromatography with fluorescence detection

A liquid chromatographic method was developed for the determination of albendazole and its metabolites albendazole sulfoxide, albendazole sulfone, and albendazole-2-aminosulfone from largemouth and hybrid striped bass muscle tissue with adhering skin. The muscle tissue samples were made alkaline with potassium carbonate and extracted with ethyl acetate. The extracts were further subjected to cleanup by using a series of liquid-liquid extractions. After solvent evaporation, the residue was reconstituted in mobile phase and chromatographed. The chromatography was carried out on a reversed-phase Luna C18 column, using acetonitrile-methanol buffer as the mobile phase. The analytes were detected by fluorescence with excitation and emission wavelengths of 290 and 330 nm, respectively. The average recoveries from the fortified muscle tissue of the 2 fish species for albendazole (25-100 ppb), albendazole sulfoxide (8.75-52.5 ppb), albendazole sulfone (1-10 ppb), and albendazole-2-aminosulfone (10-100 ppb) were 89, 82, 99, and 74%, respectively. The coefficient of variation for each compound was <20% in all cases. The procedure was applied to the determination of albendazole and its 3 metabolites in the muscle tissue of the 2 fish species after orally dosing them with albendazole.     

Therapeutic drug monitoring of albendazole: determination of albendazole, albendazole sulfoxide, and albendazole sulfone in human plasma using nonaqueous capillary electrophoresis

A nonaqueous capillary electrophoretic method (NACE) for the fast determination of plasma levels of albendazole (ABZ), albendazole sulfoxide (ABZSO), and albendazole sulfone (ABZSO2) is described. The assay is based upon liquid/liquid extraction of these compounds using dichloromethane at pH 10.2 (recovery between 63 and 98%), followed by a NACE separation performed within 8 min employing a 0.036 M borate buffer (apparent pH 9.9) in a mixture of methanol and N-methylformamide (1:3) and on-column absorbance detection at 280 nm. Using 0.5 mL of plasma and extract reconstitution in 200 microL N-methylformamide, drug levels between 1.0-10 microM were found to provide linear calibration graphs. Intraday and interday imprecisions evaluated from peak area ratios (n = 5) were <10% and <12%, respectively. Corresponding imprecisions of detection times (n = 5) were <1% and <6%, respectively. The limit of detection (LOD) for ABZ, ABZSO and ABZSO2 was 8 x 10(-7) M. The reliability of the method developed was verified via analysis of 45 plasma samples obtained from patients treated with ABZ. Good agreement was obtained between the levels of ABZSO and those determined by routine HPLC. ABZ was found to be undetectable in all patient samples, whereas the levels of ABZSO2 were below or close to LOD.     

Residue depletion of albendazole and its metabolites in the muscle tissue of large mouth and hybrid striped bass after oral administration

The residue depletion profiles of albendazole (ABZ) and its major metabolites: albendazole sulfoxide (ABZ-SO), albendazole sulfone (ABZ-SO₂) and albendazole aminosulfone (ABZ-2-NH₂SO₂) were studied in the muscle tissues of large mouth (LMB) and hybrid striped bass (HSB). A single oral dose of 10 mg/kg albendazole was given to the two fish species via intra-gastric tube. The muscle tissues with adhering skin were collected at 8, 16, 24, 48, 72, 96 and 120 h post dose from both species. The samples were homogenized in dry ice and subjected to extraction and cleanup procedures. The final sample extracts were analyzed by high performance liquid chromatography. The results indicate that both ABZ and its pharmacologically active metabolite ABZ-SO were retained longer in LMB than in HSB after oral treatment. Albendazole was detectable until 8 h or 6.7 degree days (°D) and 48 h (40 °D) in HSB and LMB, respectively. However, ABZ-SO was detectable up to 48 h (40 °D) and 96 h (80 °D) in HSB and LMB, respectively. Among the inactive metabolites, ABZ-SO₂ was present in both fish species; however, ABZ-2-NH₂SO₂ was detected only in LMB.     

Enantioselective analysis of albendazole sulfoxide in cerebrospinal fluid by capillary electrophoresis

Albendazole (ABZ) is a benzimidazole anthelmintic drug used in the treatment of neurocysticercosis. After oral administration, ABZ is rapidly oxidized to albendazole sulfoxide (ABZSO), which has an asymmetric sulfur center, and later to albendazole sulfone (ABZSO2). ABZSO is the active metabolite responsible for the therapeutic effect of the drug. Previous studies have demonstrated pharmacokinetic differences between the two enantiomers, with the predominance of (+)-ABZSO in human biological fluids. This article describes for the first time the enantioselective analysis of ABZSO in cerebrospinal fluid (CSF) using capillary electrophoresis. The samples were prepared by liquid-liquid extraction using chloroform:isopropanol (8:2 v/v). The resolution of ABZSO enantiomers was obtained with a fused-silica capillary (60 cm x 75 microm ID) using 20 mmol/L Tris, pH 7.0, with 3.0% w/w sulfated beta-cyclodextrin as running buffer. The coefficient of variations and % relative error obtained for both within-day and between-days assays were lower than 15%. The method was linear over the concentration range of 100 to 2,500 ng/mL for each enantiomer, indicating that it is suitable for the analysis of ABZSO enantiomers in CSF from patients medicated with ABZ.     

Determination of pyrimethamine in animal tissue and egg by liquid chromatography with fluorescence detection

A sensitive and selective liquid chromatographic (LC) assay was developed to determine the concentration of pyrimethamine in animal tissue and egg by fluorescent derivative. Animal samples were extracted with acetonitrile, centrifuged, and purified by hexane. Fluorescent derivatization was performed by reacting pyrimethamine with chloroacetaldehyde and subjected to LC with fluorescence detection (excitation wavelength 300 nm, emission wavelength 420 nm). The limit of detection was 10 ng/g (10 ppb) and the standard calibration curve was linear in the range of 1-100 ppb (0.01-1 ng/10 microL). Recoveries from samples fortified at levels of 0.1 and 1 ppm (microg/g) were 61.0-77.4 and 65.5-81.2%, respectively. The method was applied to the monitoring of marketed samples. Pyrimethamine was not determined in any of the 70 samples: 20 swine muscle; 20 chicken muscle; 10 chicken liver; and 20 egg.     

Highly sensitive LC-MS/MS-ESI method for simultaneous quantitation of albendazole and ricobendazole in rat plasma and its application to a rat pharmacokinetic study

A highly sensitive and specific LC-MS/MS-ESI method was developed for simultaneous quantification of albenadazole (ABZ) and ricobendazole (RBZ) in rat plasma (50 μL) using phenacetin as an internal standard (IS). Simple protein precipitation was used to extract ABZ and RBZ from rat plasma. The chromatographic resolution of ABZ, RBZ and IS was achieved with a mobile phase consisting of 5 m m ammonium acetate (pH 6) and acetonitrile (20:80, v/v) at a flow rate of 1 mL/min on a Chromolith RP-18e column. The total chromatographic run time was 3.5 min and the elution of ABZ, RBZ and IS occurred at 1.66, 1.50 and 1.59 min, respectively. A linear response function was established for the ranges of concentrations 2.01-2007 and 6.02-6020 ng/mL for ABZ and RBZ, respectively. The intra- and inter-day precision values for ABZ and RBZ met the acceptance as per FDA guidelines. ABZ and RBZ were stable in battery of stability studies, viz. bench-top, auto-sampler and freeze-thaw cycles. The developed assay was applied to a pharmacokinetic study in rats.     

LC–MS–MS identification of albendazole and flubendazole metabolites formed <Emphasis Type="Italic">ex vivo</Emphasis> by <Emphasis Type="Italic">Haemonchus contortus</Emphasis>

Resistance of helminth parasites to common anthelminthics is a problem of increasing importance. The full mechanism of resistance development is still not thoroughly elucidated. There is also limited information about helminth enzymes involved in metabolism of anthelminthics. Identification of the metabolites formed by parasitic helminths can serve to specify which enzymes take part in biotransformation of anthelminthics and may participate in resistance development. The aim of our work was to identify the metabolic pathways of the anthelminthic drugs albendazole (ABZ) and flubendazole (FLU) in Haemonchus contortus, a world-wide distributed helminth parasite of ruminants. ABZ and FLU are benzimidazole anthelminthics commonly used in parasitoses treatment. In our ex vivo study one hundred living adults of H. contortus, obtained from the abomasum of an experimentally infected lamb, were incubated in 5 mL RPMI-1640 medium with 10 μmol L⁻¹ benzimidazole drug (10% CO₂, 38 °C) for 24 h. The parasite bodies were then removed from the medium. After homogenization of the parasites, both parasite homogenates and medium from the incubation were separately extracted using solid-phase extraction. The extracts were analyzed by liquid chromatography–mass spectrometry (LC–MS) with electrospray ionization (ESI) in positive-ion mode. The acquired data showed that H. contortus can metabolize ABZ via sulfoxidation and FLU via reduction of a carbonyl group. Albendazole sulfoxide (ABZSO) and reduced flubendazole (FLUR) were the only phase I metabolites detected. Concerning phase II of biotransformation, the formation of glucose conjugates of ABZ, FLU, and FLUR was observed. All metabolites mentioned were found in both parasite homogenates and medium from the incubation.     

Monoclonal-based enzyme-linked immunosorbent assay and immunochromatographic assay for enrofloxacin in biological matrices

Enrofloxacin has been increasingly used in veterinary medicine to treat microbial infections. A simple and reliable analytical method for this drug is required. The current determination by high performance liquid chromatography (HPLC) is sensitive but labor-intensive. This paper reports an enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody (MAb) and the development of a rapid test kit based on immunochromatography. The detection limits using the ELISA were 10 ppb for chicken liver and muscle, and 1 ppb for cattle milk, respectively. The mean recovery values were 77.3-96.0% for chicken liver, 72.4-92.0% for chicken muscle and 84.0-99.0% for cattle milk. The detection limits using the kit were ca. 100 ppb for chicken muscle and ca. 10 ppb for cattle milk, respectively. All ELISA results for assay of chicken liver, chicken muscle and cattle milk were confirmed using HPLC which is used as the routine assay. The HPLC (x) and ELISA (y) results showed close correlation for chicken liver (y = 8.7 + 0.85x, r2 = 0.99, n = 25), chicken muscle (y = -3.9 + 0.94x, r2 = 0.98, n = 25) and cattle milk (y = 18.4 + 0.92x, r2 = 0.99, n = 25).     

Simultaneous determination of residual antiparasitic lactones in bovine muscle and liver by liquid chromatography with fluorescence detection

Abamectin, doramectin, eprinomectin, ivermectin, milbemectin, and moxidectin in bovine muscle and liver were extracted with acetonitrile. The extracts were partitioned with n-hexane and then evaporated to dryness. The residue was cleaned up on Bond Elute NH2 cartridge, and the drugs were eluted from the cartridge with methanol-ethyl acetate (3 + 7). The eluate was evaporated to dryness, and residues were derivatized with N,N-dimethylformamide-acetic anhyride-1-methylimidazole. The derivatives were determined by liquid chromatography with fluorescence detection. Recoveries of the 6 drugs were 79.6-63.8% in muscle and 71.6-60.6% in liver at 0.01 ppm levels. The quantitation limits were 5 ppb for each drug.     

A simplified LC–DAD method with an RP-C12 column for routine monitoring of three sulfonamides in edible calf and pig tissue

Use of an RP-C₁₂ analytical column with a mobile phase at pH 4.5 enabled excellent chromatographic separation and quantification of sulfadiazine (SDZ), sulfamerazine (SMR), and sulfamethazine (SMZ) in edible calf and pig tissue (muscle and liver). Tissue samples were extracted with acetonitrile, centrifuged without further clean-up, and analyzed by LC–DAD. The proposed conditions are useful for a rapid and reliable screening of the SDZ, SMR, and SMZ content of calf and pig tissue at concentrations lower than the maximum residue level (MRL) (LOQ between 27 and 55 ppb). Separation of some possible SMZ metabolites should be further investigated.     

固相萃取/液相色谱-串联质谱法测定食品中二甲基黄

建立了液相色谱-串联质谱(LC-MS/MS)测定食品中二甲基黄(DMY)的分析方法。样品经乙酸乙酯提取,二甲基黄专用固相萃取小柱(ProElut DMY SPE)净化,XDB-C18色谱柱(50 mm×4.6 mm,1.8μm)分离,并以5mmol/L乙酸铵水溶液(含0.1%(v/v)甲酸)-乙腈(含0.1%(v/v)甲酸)为流动相,梯度洗脱,电喷雾正离子模式(ESI~+)电离,多反应监测模式(MRM)检测,内标法定量。结果表明,DMY在0~50μg/L范围内线性关系良好,相关系数(r~2)均大于0.999。方法的检出限(LOD,S/N3)和定量限(LOQ,S/N10)分别为2μg/kg和10μg/kg。不同食品基质中,DMY在10、20和100μg/kg的添加水平下的平均加标回收率为93.3%~98.9%,相对标准偏差为1.6%~3.9%(n=6)。该方法有效补偿了液相色谱-串联质谱检测过程中的离子化抑制效应,灵敏度和准确度高,适用于腐乳、辣椒酱、禽蛋、豆干、糖果和火腿中DMY的测定。     

Determination of flumequine in channel catfish by liquid chromatography with fluorescence detection.

Rapid methods are described for determination of flumequine (FLU) residues in muscle and plasma of farm-raised channel catfish (Ictalurus punctatus). FLU residues were extracted from tissues with an acidified methanol solution, and extracts were cleaned up on C18 solid-phase extraction cartridges. FLU concentrations were determined by liquid chromatography (LC) using a C18 analytical column and fluorescence detection (excitation, 325 nm; emission, 360 nm). Mean recoveries of FLU from fortified muscle were 87-94% at 5 levels ranging from 10 to 160 ppb (5 replicates per level). FLU recoveries from fortified plasma were 92-97% at 5 levels ranging from 20 to 320 ppb. Limits of detection (signal-to-noise ratio, 3:1) for the method as described were 3 and 6 ppb for muscle and plasma, respectively. Relative standard deviations (RSDs) for recoveries were < or = 12%. Live catfish were dosed with 14C-labeled or unlabeled FLU to generate incurred residues. Recoveries of 14C residues throughout extraction and cleanup were 90 and 94% for muscle and plasma, respectively. RSDs for incurred FLU at 2 levels in muscle and plasma ranged from 2 to 6%. The identity of FLU in incurred tissues was confirmed by LC/mass spectrometry.     

High-performance liquid chromatographic determination of pyridostigmine bromide, nicotine, and their metabolites in rat plasma and urine

This study reports on the development of a rapid and simple method for the determination of the antinerve agent drug pyridostigmine bromide (3-dimethylaminocarbonyloxy-N-methyl pyridinium bromide) (PB), its metabolite N-methyl-3-hydroxypyridinium bromide, nicotine (S-1-methyl-5-(3-pyridyl)-2-pyrrolidine), and its metabolites nornicotine (2-(3-pyridyl)pyrrolidine) and cotinine (S-1-methyl-5-(3-pyridyl)-2-pyrrolidone) in rat plasma and urine. The compounds are extracted and eluted by methanol and acetonitrile using C18 Sep-Pak cartridges and separated using high-performance liquid chromatography by a gradient of methanol, acetonitrile, and water (pH 3.2) at a flow rate of 0.8 mL/min in a period of 14 min. UV detection was at 260 nm for nicotine and its metabolites and at 280 nm for PB and its metabolite. The limits of detection ranged between 20 and 70 ng/mL, and the limits of quantitation were 50-100 ng/mL. The average percent recovery of five spiked plasma samples were 85.7 +/- 7.3%, 80.4 +/- 5.8%, 78.9 +/- 5.4%, 76.7 +/- 6.4%, and 79.7 +/- 5.7% and for urine were 85.9 +/- 5.9%, 75.5 +/- 6.9%, 82.6 +/- 7.9%, 73.6 +/- 5.9%, and 77.7 +/- 6.3% for nicotine, nornicotine, cotinine, PB, and N-methyl-3-hydroxypyridinium bromide, respectively. The calibration curves for standard solutions of the compounds of peak areas and concentration are linear for a range between 100 and 1,000 ng/mL. This method is applied in order to analyze the previously mentioned chemicals and metabolites following their oral administration in rats.     

Determination of benzimidazole anthelmintics in animal-derived biological matrices

This paper describes an easy multiresidue procedure for the determination of 8 benzimidazole anthelmintics (Albendazole, Albendazole sulphone, Albendazole sulphoxide, Ox-bendazole, Ossibendazole, p-OH-Fenbendazole, Fenbendazole, Flubendazole) in foodstuffs of animal origin for human consumption. According to the proposed procedure, 10 g of homogenised sample were extracted with acetonitrile in ultrasonic bath, the organic layer was evaporated to dryness and the residue dissolved in HCl 0.005 M. After an initial washing with hexane to remove fats, the aqueous layer was purified on SPE C2 column and the analytes were eluted with methanol, evaporated to dryness and reconstituted with phosphate buffer/acetonitrile (50/50). The solution obtained was analysed by HPLC with diode array detector (DAD); this detector provides useful information about the peaks present in routine samples, because it gains not only retention times of analytes, as well as their UV-spectra. Benzthiazuron was used as internal standard and the method was validated in the range 10-100 ppb, as requested by the imposed limits. The mean recoveries in spiked samples ranged between 70% and 95%, for 10 and 100 ppb respectively. The developed method resulted sensitive, simple and fast. It is particularly suitable for laboratories that execute screening analysis on a great number of samples, observing the EC regulations that establish the benzimidazole Maximum Residue Limits (MRLs) in food.     

Determination of fenbendazole and its metabolites in trout by a high-performance liquid chromatographic method.

A high-performance liquid chromatographic (HPLC) method based on solid phase extraction was developed for the simultaneous determination of fenbendazole (FBZ), fenbendazole sulfoxide (FBZ-SO) and fenbendazole sulfone (FBZ-SO2) in trout muscle and skin tissues. The compounds were extracted with acetonitrile and the extract was concentrated and cleaned up by solid phase extraction on C18 and CN cartridges. The extract was analysed by reversed-phase gradient HPLC on a C18 column followed by ultraviolet detection at 290 nm. The method's detection limits were 4.0 micrograms kg-1 for FBZ, 4.5 micrograms kg-1 for FBZ-SO and 3.8 micrograms kg-1 for FBZ-SO2. The mean recovery in muscle was 88% for FBZ, 94% for FBZ-SO and 92% for FBZ-SO2 at a level of 5-150 micrograms kg-1. The corresponding mean recoveries in skin tissue were 88, 81 and 86% at a level of 10-100 micrograms kg-1. The mean relative repeatability standard deviation was 9.2% at a level of 5 micrograms kg-1, 5.9% at a level of 10-100 micrograms kg-1 and 2.3% at a level of 150 micrograms kg-1. The method was applied to trout given feed containing FBZ in an aquaculture pilot plant. The three compounds FBZ, FBZ-SO and FBZ-SO2 were all detected in muscle and skin tissues shortly after administration. The concentrations were generally highest in skin tissue.     

Determination of acetylsalicylic acid and its major metabolite, salicylic acid, in human plasma using liquid chromatography-tandem mass spectrometry: application to pharmacokinetic study of Astrix in Korean healthy volunteers

The first liquid chromatography-tandem mass spectrometry (LC/MS/MS) method for determination of acetylsalicylic acid (aspirin, ASA) and one of its major metabolites, salicylic acid (SA), in human plasma using simvastatin as an internal standard has been developed and validated. For ASA analysis, a plasma sample containing potassium fluoride was extracted using a mixture of ethyl acetate and diethyl ether in the presence of 0.5% formic acid. SA, a major metabolite of ASA, was extracted from plasma using protein precipitation with acetonitrile. The compounds were separated on a reversed-phase column with an isocratic mobile phase consisting of acetonitrile and water containing 0.1% formic acid (8:2, v/v). The ion transitions recorded in multiple reaction monitoring mode were m/z 179 --> 137, 137 --> 93 and 435 --> 319 for ASA, SA and IS, respectively. The coefficient of variation of the assay precision was less than 9.3%, and the accuracy exceeded 86.5%. The lower limits of quantification for ASA and SA were 5 and 50 ng/mL, respectively. The developed assay method was successfully applied for the evaluation of pharmacokinetics of ASA and SA after single oral administration of Astrix (entero-coated pellet, 100 mg of aspirin) to 10 Korean healthy male volunteers.     

Confirmation of avermectin residues in food matrices with negative-ion atmospheric pressure chemical ionization liquid chromatography/mass spectrometry

A multi-residue LC/MS method has been developed to confirm avermectin drug residues in several food matrices. Ivermectin (IVR), doramectin (DOR), eprinomectin (EPR) and moxidectin (MOX) are confirmed using atmospheric pressure chemical ionization (APCI) with negative ion detection and selected ion monitoring of three to four ions for each compound. The drug residues are extracted from tissue or milk using previously published procedures. IVR and DOR are confirmed at 20 ppb levels in fortified salmon muscle; IVR is also confirmed in tissue from salmon dosed with the drug. Residues of DOR, IVR, and EPR are confirmed in fortified milk at the 20 ppb level and in fortified beef liver at 40 ppb. Residues of MOX can also be confirmed in these matrices, but at slightly higher levels (40-80 ppb).     

Simultaneous determination of residues of chloramphenicol, florfenicol, florfenicol amine, and thiamphenicol in shrimp tissue by gas chromatography with electron capture detection

A gas chromatographic (GC) method is presented for determining residues of chloramphenicol (CAP), florfenicol (FF), florfenicol amine (FFa), and thiamphenicol (TAP) in shrimp tissues, with meta-nitrochloramphenicol (mCAP) as the internal standard. The composited shrimp is extracted with basic ethyl acetate, followed by an acetonitrile-basic ethyl acetate mixture. This extract is centrifuged, filtered, evaporated, and reconstituted in water; the reconstituted extract is acidified, defatted with hexane, and passed through a propylsulfonic acid (PRS) and C18 solid-phase extraction (SPE) system. The C18 SPE column is eluted with methanol, and the PRS SPE column is eluted with basic MeOH plus counter ion. The combined eluates are evaporated, reconstituted in acetonitrile, and derivatized with Sylon BFT. After derivatization, the addition of toluene directly to the sample, followed by the addition of basic water, quenches the derivatization process. After centrifugation, the organic layer is carefully removed, and the analytes are determined by GC with electron capture detection. Shrimp tissues were fortified with fenicols (i.e., CAP, FF, FFa, and TAP) at 5, 10, 20, 40, and 80 ng/mL. Overall recoveries were 88, 101, 91, and 84% with overall interassay (between-day) variabilities (i.e., relative standard deviations) of 5.3, 9.4, 12.8, and 7.4% for CAP, FF, FFa, and TAP, respectively. The method detection limits were calculated as 0.7, 1.4, 2.4, and 1.3 ng/g (ppb) for CAP, FF, FFa, and TAP, respectively, based on a 10 g sample. The quantitation limit as determined empirically by this method is the lower limit of the standard curve, which is about 5 ng/g (ppb) for each analyte.