Determination of low-level agricultural residues in soft drinks and sports drinks by liquid chromatography/tandem mass spectrometry: single-laboratory validation

In this study, sponsored by PepsiCo Inc., a method was validated for measurement of 11 pesticide residues in soft drinks and sports drinks. The pesticide residues determined in this validation were alachlor, atrazine, butachlor, isoproturon, malaoxon, monocrotophos, paraoxon-methyl, phorate, phorate sulfone, phorate sulfoxide, and 2,4-dichlorophenoxyacetic acid (2,4-D) when spiked at 0.100 microg/L (1.00 microg/L for phorate). Samples were filtered (if particulate matter was present), degassed (if carbonated), and analyzed using liquid chromatography with tandem mass spectrometry. Quantitation was performed with matrix-matched external standard calibration solutions. The standard curve range for this assay was 0.0750 to 10.0 microg/L. The calibration curves for all agricultural residues had coefficient of determination (r2) values greater than or equal to 0.9900 with the exception of 2 values that were 0.9285 and 0.8514. Fortification spikes at 0.100 microg/L (1.00 microg/L for phorate) over the course of 2 days (n=8 each day) for 3 matrixes (7UP, Gatorade, and Diet Pepsi) yielded average percent recoveries (and percent relative standard deviations) as follows (n=48): 94.4 (15.2) for alachlor, 98.2 (13.5) for atrazine, 83.1 (41.6) for butachlor, 89.6 (24.5) for isoproturon, 87.9 (24.4) for malaoxon, 96.1 (9.26) for monocrotophos, 101 (25.7) for paraoxon-methyl, 86.6 (20.4) for phorate, 101 (16.5) for phorate sulfone, 93.6 (25.5) for phorate sulfoxide, and 98.2 (6.02) for 2,4-D.     

Determination of low-level pesticide residues in soft drinks and sports drinks by liquid chromatography with tandem mass spectrometry: collaborative study

A collaborative study was conducted on a method for the measurement of 11 low-level pesticide residues in soft drinks and sports drinks by liquid chromatography with tandem mass spectrometry. The pesticide residues determined in this study were alachlor, atrazine, butachlor, isoproturon, malaoxon, monocrotophos, methyl paraoxon, phorate, phorate sulfone, phorate sulfoxide, and 2,4-dichlorophenoxyacetic acid (2,4-D). Blind fortification solutions containing 3 different levels of pesticide residues were provided to 9 collaborating laboratories to create test samples at concentrations of 0, 0.1, and 0.5 microg/L with a 10-fold concentration for phorate in a total of 6 matrixes (2 colas, 1 diet cola, 1 clear lemon-lime soft drink, 1 orange soft drink, and 1 sports drink). Good qualitative performance of the method was demonstrated for all pesticide residues. Reproducibility relative standard deviation (RSDR) ranged from 7 to 151% for alachlor, atrazine, butachlor, isoproturon, malaoxon, monocrotophos, methyl paraoxon, phorate, phorate sulfone, phorate sulfoxide, and 2,4-D at the 0.1 microg/L level (1.0 microg/L for phorate). At 0.5 microg/L (5.0 microg/L for phorate), RSDR ranged from 9 to 57% for alachlor, atrazine, butachlor isoproturon, malaoxon, monocrotophos, methyl paraoxon, phorate, phorate sulfone, phorate sulfoxide, and 2,4-D in all matrixes. Repeatability relative standard deviation (RSDr), applicable to the diet cola and sports drink, ranged from 0 to 124% for the 11 pesticide residues at the 0.1 microg/L level (1.0 microg/L for phorate). At 0.5 microg/L (5.0 microg/L for phorate), RSDr ranged from 4 to 26%. Recoveries for the 11 pesticide residues in all matrixes ranged from 84 to 300% at the 0.1 microg/L level (1.0 microg/L for phorate) and from 66 to 127% at the 0.5 microg/L (5.0 microg/L for phorate) level. Coefficients of determination (r2) of the matrix-matched calibration curves were > or = 0.95. It is recommended that the method be accepted by AOAC as Official First Action with a limit of quantification of 0.5 microg/L for alachlor, atrazine, butachlor, isoproturon, malaoxon, methyl paraoxon, monocrotophos, phorate sulfone, phorate sulfoxide, and 2,4-D and 5.0 microg/L for phorate.     

Determination of pesticide residues (> 0.5 microg/L) in soft drinks and sports drinks by gas chromatography with mass spectrometry: collaborative study

A collaborative study was conducted on a method for the measurement of 19 low-level pesticide residues in soft drinks and sports drinks by gas chromatography with mass spectrometry (GC/MS). The pesticide residues determined were 2,4'-dichlorodiphenyldichloroethylene (2,4'-DDE); 2,4'-dichlorodiphenyldichloroethane (2,4'-DDD); 4,4'-dichlorodiphenyldichloroethylene (4,4'-DDE); 2,4'-dichlorodiphenyltrichloroethane (2,4'-DDT); 4,4'-dichlorodiphenyltrichloroethane (4,4'-DDT); 4,4'-dichlorodiphenyldichloroethane (4,4'-DDD); alpha-endosulfan; endosulfan-sulfate; dieldrin; aldrin; ethion; chlorpyrifos; beta-endosulfan; malathion; methyl-parathion; alpha-hexachlorocyclohexane (alpha-HCH); beta-HCH; delta-HCH; and gamma-HCH. Blind fortification solutions containing 4 different levels of pesticide residues (0, 0.1, 0.5, and 1.0 microg/L) were provided to 8 collaborating laboratories who used them to create test samples in 6 matrixes (also provided): 2 colas, a diet cola, a clear lemon-lime soft drink, an orange soft drink, and a sports drink. Reproducibility (RSDR) for all 19 pesticide residues in all matrixes ranged from 7 to 151% at the 0.1 microg/L level, 11 to 121% at 0.5 microg/L, and 14 to 67% at 1.0 microg/L. Repeatability (RSDr), applicable to the diet cola and the sports drink, ranged from 1 to 76% for the 19 pesticide residues at the 0.1 microg/L level, 9 to 38% at 0.5 microg/L, and 9 to 38% at 1.0 microg/L. Recoveries for the 19 pesticide residues in all matrixes ranged from 77 to 645% at the 0.1 microg/L level, 60 to 231% at 0.5 microg/L, and 61 to 146% at 1.0 microg/L. It is recommended that the method be accepted by AOAC as Official First Action with a limit of quantification (LOQ) equal to 0.5 microg/L for 4,4'-DDT; 2,4'-DDT; 2,4'-DDD; 4,4'-DDE; 4,4'-DDD; 2,4'-DDE; aldrin; dieldrin; alpha-endosulfan; endosulfan-sulfate; chlorpyrifos; and ethion, and an LOQ equal to 1.0 microg/L for beta-endosulfan; alpha-HCH; beta-HCH; delta-HCH; gamma-HCH; methyl-parathion; and malathion.     

Determination of total cholesterol in meat and poultry by gas chromatography: single-laboratory validation

The primary objective of this study was to determine the intralaboratory performance of a cholesterol determination method that combines direct saponification of a 1 g meat or poultry sample and GC quantification of liberated cholesterol without derivatization. Cholesterol was detected at 11.96 min using a GC-flame ionization detector (FID) system. With a 0.005 mg/mL 5alpha-cholestane internal standard and 0.008 to 0.020 mg/mL cholesterol standard series, the FID response was linearly correlated to standard concentrations with a coefficient of determination of 0.995 and a response factor of 0.66. The LOD and LOQ were 1.24 and 4.00 mg/100 g, respectively. Cholesterol could be analyzed within 6 days of preparation with high precision (CV of 0.92 to 2.69%) and accuracy (recovery of 93.24 to 100.56%). This simplified procedure allows for decreased errors and increased productivity, and the method proved to be reliable and able to withstand practical variation in procedural application. The method has been applied routinely with excellent precision to update data on the cholesterol content of beef, pork, and chicken in the U.S. Department of Agriculture National Nutrient Database for Standard Reference.     

茶叶中有机磷农药残留量的GC-MS测定

利用GC-MS在选择离子模式下对茶叶中几种有机磷农药进行定性和定量分析,在提取过程中,省去过柱操作步骤,缩短了操作时间。结果表明,5种有机磷农药的添加质量浓度为1．0mg／kg时,回收率均在86．5％～101．5％,RSD在4．8％～9．0％之间,方法的检出限在0．0047～0．023mg／kg,符合农药残留量检测要求。     

Determination of fumonisin B1 in botanical roots by liquid chromatography with fluorescence detection: single-laboratory validation

The accuracy, repeatability, and reproducibility characteristics of a published method for measuring levels of fumonisin B1 (FB1) in botanical root products were determined by an AOAC single-laboratory validation procedure. Replicates of 10 test portions of each powdered root product (black cohosh, echinacea, ginger, ginseng, valerian, dong quai, and turmeric) at each spiking level (FB1 at 0, 50, 100, and 200 ng/g) were analyzed on 3 separate days. Test samples were extracted with methanol-acetonitrile-water (25 + 25 + 100, v/v/v). The extracts were centrifuged, the supernatants diluted with phosphate-buffered saline (PBS) containing 1% Tween 20 and filtered, and the filtrates applied to an immunoaffinity column containing antibodies specific for fumonisins. After the column was washed sequentially with PBS and water, the toxin was eluted from the column with 80% methanol, and the eluate dried by lyophilization. The residue was reconstituted with 50% acetonitrile. FB1 was derivatized with a mixture of o-phthaldialdehyde and mercaptoethanol by using an LC autoinjector. Separations were performed with an RP-LC column, and the FB1 derivative was quantified by fluorescence detection. All root products were found to contain FB1 at <10 ng/g. Average within- and between-day recoveries of FB1 from the botanical roots ranged from 67 to 95% and from 68 to 100%, respectively. Total RSD values for within- and between-day repeatability ranged from 5.5 to 26.4%. HorRat values were <1.3 for all of the matrixes examined. The method meets the AOAC method performance criteria at levels of >50 ng/g for the seven botanical roots tested.     

Determination of underivatised efaroxan and idazoxan in blood plasma by capillary gas chromatography with mass-selective detection

Sensitive and specific methods for the determination of efaroxan and idazoxan in blood plasma have been developed based on solvent extraction, chromatographic separation and quantification by selected-ion monitoring using a quadruple mass-selective detector. The use of a short non-polar bonded-phase capillary gas chromatography (GC) column facilitated rapid separation of the compounds of interest from internal standards, metabolites and endogenous material. Of equal significance was the ability to chromatograph these basic compounds without prior derivatisation. The application of bonded-phase capillary GC coupled to selected-ion monitoring resulted in robust analytical procedures with sub-ng ml(-1) sensitivity and high selectivity.     

Determination by glyphosate and aminomethylphosphonic acid in crops by capillary gas chromatography with mass-selective detection: collaborative study.

A collaborative study was conducted to validate a method for the determination of glyphosate and aminomethylphosphonic acid (AMPA) in crops. The analytes are extracted from crops with water, and the crude extracts are then subjected to a cation exchange cleanup. The analytes are derivatized by the direct addition of the aqueous extract into a mixture of heptafluorobutanol and trifluoroacetic anhydride. The derivatized analytes are quantitated by capillary gas chromatography with mass-selective detection (MSD). The collaborative study involved 13 laboratories located in 5 countries; 12 laboratories returned valid data sets. The crops tested were field corn grain, soya forage, and walnut nutmeat at concentrations of 0.050, 0.40, and 2.0 mg/kg. The study used a split-level pair replication scheme with blindly coded laboratory samples. Twelve materials were analyzed, including 1 control and 3 split-level pairs for each matrix, 1 pair at each nominal concentration. For glyphosate, the mean recovery was 91%, the average intralaboratory variance, the repeatability relative standard deviation (RSDr), was 11%, and the interlaboratory variance, the reproducibility relative standard deviation (RSDR), was 16%. For AMPA, the mean recovery was 87%, the RSDr was 16%, and the RSDR was 25% at mg/kg levels.     

Determination of anthocyanins in cranberry fruit and cranberry fruit products by high-performance liquid chromatography with ultraviolet detection: single-laboratory validation

A single-laboratory validation study was conducted on an HPLC method for the detection and quantification of cyanidin-3-O-galactoside (C3Ga), cyanidin-3-O-glucoside (C3GI), cyanidin-3-O-arabinoside (C3Ar), peonidin-3-O-galactoside (P3Ga), and peonidin-3-O-arabinoside (P3Ar) in cranberry fruit (Vaccinium macrocarpon Aiton) raw material and finished products. An extraction procedure using a combination of sonication and shaking with acidified methanol was optimized for all five anthocyanins in freeze-dried cranberry fruit and finished products (commercial extract powder, juice, and juice cocktail). Final extract solutions were analyzed by HPLC using a C18 RP column. Calibration curves for all anthocyanin concentrations had correlation coefficients (r2) of > or = 99.8%. The method detection limits for C3Ga, C3Gl, C3Ar, P3Ga, and P3Ar were estimated to be 0.018, 0.016, 0.006, 0.013, and 0.011 microg/mL, respectively. Separation was achieved with a chromatographic run time of 35 min using a binary mobile phase with gradient elution. Quantitative determination performed in triplicate on four test materials on each of 3 days (n = 12) resulted in RSD(r) from 1.77 to 3.31%. Analytical range, as defined by the calibration curves, was 0.57-36.53 microg/mL for C3Ga, 0.15-9.83 microg/mL for C3GI, 0.28-17.67 microg/mL for C3Ar, 1.01-64.71 microg/mL for P3Ga, and 0.42-27.14 microg/mL for P3Ar. For solid materials prepared by the described method, this translates to 0.06-3.65 mglg for C3Ga, 0.02-0.98 mg/g for C3Gl, 0.03-1.77 mg/g for C3Ar, 0.10-6.47 mg/g for P3Ga, and 0.04-2.71 mg/g for P3Ar.     

Determination of benzoic acid in soft drinks by gas chromatography with on-line pyrolytic methylation technique

A new application of pyrolytic methylation was developed to determine benzoic acid in soft drinks by gas chromatography (GC) without using any pretreatment procedures and special pyrolyzer. With the on-line pyrolytic methylation by tetramethylammonium hydroxide (TMAH), benzoic acid was converted into its corresponding methyl ester in the injector at 280 °C. Thus, samples containing benzoic acid could be well determined by direct-injection in GC on the medium polar stationary phase column. To obtain optimum methylation conditions, important factors were investigated and then applied to the following experiments. The results were obtained as following: 280 °C as reaction temperature, 2:1 as the proportion of TMAH to benzoic acid. The storage time of mixed solution had no obvious effect on the area of benzoic acid methyl ester peak. With the p-xylene as an inter-standard, GC behaviors were investigated under these optimum conditions. The linear range achieved for benzoic acid was 1-10,000 mg/l with the correlation coefficient of 0.9985. The precision was quite high with the R.S.D. of 2.8% and the limit of detection reached 0.1 mg/l. The potential of the proposed method was assessed by applying it to the determination of benzoic acid in soft drinks. The results obtained coincided with the statement on the labels and all of the detected data were below the maximum permitted concentration of the European Union Legislation. This on-line pyrolytic methylation technique was proved to be simple to implement, sensitive and selective.     

Determination of fluvalinate residues in beeswax by gas chromatography with electron-capture detection

A simple, rapid, and accurate method is described for the determination of residual fluvalinate in beeswax. The procedure consists of partitioning on a disposable column of diatomaceous earth (Extrelut), followed by chromatographic cleanup on a Florisil cartridge. The final extract is analyzed by capillary gas chromatography with electron-capture detection (GC-ECD). Briefly, wax samples were dissolved in n-hexane, and the solutions were sonicated and transferred to Extrelut columns. The fluvalinate was extracted with acetonitrile, and a portion of the extract was cleaned up on a Florisil cartridge. The fluvalinate was eluted with diethyl ether-n-hexane (1 + 1) and directly determined by GC-ECD. Recoveries from wax samples spiked at 5 fortification levels (100-1500 microg/kg) ranged from 77.4 to 87.3%, with coefficients of variation of 5.12-8.31%. The overall recovery of the method was 81.4 +/- 3.2%, and the limit of determination was 100 microg/kg.     

Determination of pesticide residues in lettuce by gas chromatography with electron-capture detection

A sensitive, rapid, and simple multiresidue method for the simultaneous determination of 19 pesticides in different varieties of lettuce (Lactuca sativum) was developed. Lettuce samples were extracted by homogenization with acetone and partitioned into ethyl acetate-cyclohexane. Subsequent sample cleanup was not needed. Final determination was made by capillary gas chromatography (GC) with electron-capture detection (ECD). Confirmation analysis of pesticides was performed by GC coupled with mass spectrometry in the selected ion monitoring mode. The average recovery by the GC-ECD method obtained for these compounds varied from 66.4 to 119.2% with relative standard deviations < 7.7%. The GC-ECD method has good linearity, and the detection limit for the pesticides studied varied from 0.1 to 3.8 microg/kg. The proposed method was used to determine pesticide levels in different types of lettuce grown in soils from experimental fields.     

Determination of vitamin B12 in food products by liquid chromatography/UV detection with immunoaffinity extraction: single-laboratory validation

A fast and simple method to determine vitamin B12 in foods is presented. The method allows, in addition to the determination of added cyanocobalamin, the determination of natural vitamin B12 forms, making it also applicable to nonfortified products, especially those that are milk-based. Vitamin B12 is extracted in sodium acetate buffer in the presence of sodium cyanide (100 degrees C, 30 min). After purification and concentration with an immunoaffinity column, vitamin B12 is determined by liquid chromatography with UV detection (361 nm). The method has been validated in analyses of a large range of products: milk- and soy-based infant formulas, cereals, cocoa beverages, health care products, and polyvitamin premixes. The method showed appropriate performance characteristics: linear response over a large range of concentrations, recovery rates of 100.8 +/- 7.5% (average +/- standard deviation), relative standard deviation of repeatability, RSDr, of 2.1%, and intermediate reproducibility, RSDiR, of 4.3%. Limits of detection and quantitation were 0.10 and 0.30 microg/100 g, respectively, and correlation with the reference microbiological assay was good (R2 = 0.9442). The proposed method is suitable for the routine determination of vitamin B12 in fortified foods, as well as in nonfortified dairy products. It can be used as a faster, more selective, and more precise alternative to the classical microbiological determination.     

Multiresidue analysis of seventeen pesticides in wine by gas chromatography with mass-selective detection

We have developed a multiresidue method permitting the simultaneous quantitation of 17 pesticides in wine: dicloran, dimethoate, diazinon, chlorpyrifos-methyl, vinclozolin, carbaryl, methiocarb, dichlofluanid, parathion-ethyl, triadimefon, procymidone, myclobutanil, iprodione, imidan, dicofol, phosalone and azinphos-methyl. Solid-phase extraction of 0.5 ml of wine sample is followed by direct injection of 1 microl of the eluent onto a DB-5 MS gas chromatographic column followed by mass-selective detection using one target and two qualifier ions for each pesticide. The extraction and injection steps are carried out with automatic instrumentation. Good resolution of all compounds was achieved with a run-time approximating 23 min. Detection and quantitation limits were around 2 microg/l and 10 microg/l, respectively, with linear calibration curves up to 3 mg/l for most constituents. Recovery in half the compounds was >90%, and >80% in most of the remainder. Imprecision (relative standard deviation) was <10% for most pesticides and <18% in all. Further analytes can be added to the repertoire without difficulty. The method merits consideration together with four other multiresidue methods now available that offer similar analytical characteristics, slower run-times, and a different selection of analytes.     

Fatty acid profiles of sebaceous triglycerides by capillary gas chromatography with mass-selective detection

Fatty acid methyl esters prepared from the triglyceride fraction of skin surface lipids from six adult human males were chromatographed on a 50-m fused-silica column coated with the highly polar cyanopropylpolysiloxane phase. This permitted the resolution of double-bond positional and geometric isomers. By means of mass-selective detection, 33 saturated and 14 unsaturated fatty acid chain types were analysed. Interpretation of the mass spectra combined with precise calculation of equivalent chain length values permitted the identification of 22 saturated and all of the unsaturated chain types. Quantification by integration of total-ion and selected-ion chromatograms revealed marked variation in the triglyceride fatty acid composition between different subjects. The greatest variation was observed in the concentrations of even-carbon-numbered iso-branched acids, which ranged from 1.5 to 11% of the saturated and from 1.9 to 12.7% of the monounsaturated acids. The anteiso chain structures constituted 4-9% of the saturated and 3-6% of the unsaturated members. Fatty acids with 4-methyl branch showed the least variation, in the range 5.7-7.4%. Other methyl-branched acids made up 4-10% of the saturated group, but were not detected in the unsaturated acids fraction. Two 18:1 fatty acids were identified (a delta 8 and a delta 9), which possibly have different anatomical origins. Similarly, two 18:2 fatty acids (linoleic and a 2,3-dimethyl derivative) were identified. A 2-methyl C17 acid, probably of bacterial origin, was detected.     

Determination of campesterol, stigmasterol, and beta-sitosterol in saw palmetto raw materials and dietary supplements by gas chromatography: single-laboratory validation

In conjunction with an AOAC Presidential Task Force on Dietary Supplements, a method was validated for measurement of 3 plant sterols (phytosterols) in saw palmetto raw materials, extracts, and dietary supplements. AOAC Official Method 994.10, "Cholesterol in Foods," was modified for purposes of this validation. Test samples were saponified at high temperature with ethanolic potassium hydroxide solution. The unsaponifiable fraction containing phytosterols (campesterol, stigmasterol, and beta-sitosterol) was extracted with toluene. Phytosterols were derivatized to trimethylsilyl ethers and then quantified by gas chromatography with a hydrogen flame ionization detector. The presence of the phytosterols was detected at concentrations greater than or equal to 1.00 mg/100 g based on 2-3 g of sample. The standard curve range for this assay was 0.00250 to 0.200 mg/mL. The calibration curves for all phytosterols had correlation coefficients greater than or equal to 0.995. Precision studies produced relative standard deviation values of 1.52 to 7.27% for campesterol, 1.62 to 6.48% for stigmasterol, and 1.39 to 10.5% for beta-sitosterol. Recoveries for samples fortified at 100% of the inherent values averaged 98.5 to 105% for campesterol, 95.0 to 108% for stigmasterol, and 85.0 to 103% for beta-sitosterol.     

Determination of synephrine in bitter orange raw materials, extracts, and dietary supplements by liquid chromatography with ultraviolet detection: single-laboratory validation

A method has been developed to quantify synephrine in bitter orange raw material, extracts, and dietary supplements. Single-laboratory validation has been performed on the method to determine the repeatability, accuracy, selectivity, limit of detection/limit of quantification (LOQ), ruggedness, and linearity for p-synephrine and 5 other biogenic amines: octopamine, phenylephrine (m-synephrine), tyramine, N-methyltyramine, and hordenine, which may be present in bitter orange. p-Synephrine was found to be the primary biogenic amine present in all materials tested, accounting for >80% of the total biogenic amine content in all samples except a finished product. Repeatability precision for synephrine was between 1.48 and 3.55% RSD. Synephrine recovery was between 97.5 and 104%. The minor alkaloids were typically near the LOQ of the method (300-900 microg/g) in the test materials, and between-day precision for the minor compounds was poor because interferences could sometimes be mistakenly identified as one of the minor analytes. Recoveries of the minor components ranged from 99.1 to 103% at approximately 6000 microg/g spike level, to 90.7 to 120% at 300 microg/g spike level.     

Quantitative determination of norethisterone acetate in human plasma by capillary gas chromatography with mass-selective detection

An analytical method for the determination of norethisterone acetate (NETA) in human plasma by capillary gas chromatography-mass-selective detection (GC-MS), with testosterone acetate as internal standard, was developed and validated. After addition of the internal standard, the compounds were extracted from plasma at basic pH into diethyl ether-dichloromethane (3:2 v/v), which was then evaporated to dryness. The compounds were converted into their pentafluoropropionyl derivatives which were determined by gas chromatography using a mass selective detector at m/z 486 for NETA and m/z 476 for the internal standard. Intra-day and inter-day accuracy and precision were found suitable over the range of concentrations between 0.10 to 10 ng/ml. The method was applied to clinical samples.     

Determination of organophosphorus pesticide residues in fruits by gas chromatography with ITD and NPD detection

Summary Several widely used organophosphorus insecticides, diazinon, methyl-parathion, fenitrothion, malathion, fenthion and methidathion were selected for inclusion in this work. A comparative study of the multiresidue determination of these compounds in citrus fruit and grapes has been carried out by gas chromatography with nitrogen-phosphorus and ion trap detection. Samples were spiked with 0.1, 0.5 and 1 mg kg^–1 of each pesticide and blended in a Sorvall homogeniser with ethyl acetate. Column clean-up on Florisil was necessary for citrus peel extracts. The average recoveries varied from 81 to 107% with a relative standard deviation between 0.3 and 9.5% for GC-ITD and from 86 to 104% with a relative standard deviation ranging from 1.4 to 8.0% for GC-NPD. The detection limit of the method was 10 ng g^–1 or less for each organophosphorus insecticide, with both detectors. This method was applied to the analysis of treated lemons and 0.25 mg kg^–1 of fenitrothion in fruit was detected a week after treatment and identified by its mass spectrum. The results obtained showed that the proposed methods are reproducible and sensitive enough for the simultaneous determination of these insecticides in fruits at residue level.