Chemical preservation of milk samples is a common

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1 NOA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, RESIDUES AND TRACE ELEMENTS Stability of Sulfonamides, Nitrofurans, and Chloramphenicol Residues in Preserved Raw Milk Samples Measured by Liquid Chromatography MARIO NOA Universidad Agraria de La Habana, Departamento de Prevención, Facultad de Medicina Veterinaria, Autopista Nacional y Carretera de Tapaste, San José de las Lajas, Havana, Cuba NORMA PEREZ, 1 REY GUTIERREZ,IRMA ESCOBAR,GILBERTO DIAZ,SALVADOR VEGA,GUADALUPE PRADO, and GEORGINA URBAN Universidad Autónoma Metropolitana-Xochimilco, Departamento de Producción Agrícola y Animal, Laboratorio de Análisis Instrumental, Calzada del Hueso 1100, México Distrito Federal, CP 04960, Mexico A stability study was made of 10 antimicrobials: 6 sulfonamides, 3 nitrofurans, and chloramphenicol residues in raw milk samples preserved with 0.1% potassium dichromate (K 2 Cr 2 O 7 ) and 0.05% mercuric bichloride (HgCl 2 ) during cold storage for 7 days. Preserved milk samples fortified with 50 ppb of each antimicrobial were analyzed by liquid chromatography (modified AOAC Method ). Drugs were extracted with chloroform acetone after solvent evaporation residues were dissolved with aqueous sodium acetate buffer solution (0.02M, ph 4.8), and fat was removed with hexane. Sulfonamides and chloramphenicol were detected at 275 nm (UV) by using a gradient system of sodium acetate buffer solution acetonitrile starting at (v/v) and finishing at (v/v). Nitrofurans were detected at 375 nm (UV) isocratically with sodium acetate buffer solution acetonitrile ( , v/v). Residues stability was measured through recovery data. Sulfamethoxazole, sulfachloropyridazine, nitrofurazone, furazolidone, and furaltadone residues remained stable in the presence of either preservative for 7 days. Sulfamethazine and chloramphenicol were not affected by K 2 Cr 2 O 7, but had significant losses (p <0.05) when HgCl 2 was used: 26.2 and 13.4%, respectively. Average recoveries of sulfamonomethoxine, sulfamerazine, and sulfathiazole significantly decreased by Day 7, with losses of 17.1, 17.2, and 23.2% for K 2 Cr 2 O 7, and 23.3, 20.7, and 48.0% for HgCl 2, respectively. During 5 days of cold storage all antimicrobials tested, except sulfathiazole, remained stable in milk samples preserved with 0.1% K 2 Cr 2 O 7 or 0.05% HgCl 2. Received January 30, Accepted by JS May 29, Author to whom correspondence should be addressed; normaaperez@hotmail.com. Chemical preservation of milk samples is a common practice in milk analysis. Preservatives such as potassium dichromate (K 2 Cr 2 O 7 ) and mercuric bichloride (HgCl 2 ) increase milk sample storage time when sampling and analyses may not be done immediately (1, 2). However, such preservatives should not interfere with compounds of interest. Although K 2 Cr 2 O 7 and HgCl 2 do not interfere with protein (Kjeldhal, infrared) or fat (Gerber, infrared) determinations in milk (3), these substances may affect some determinations such as lactose (3, 4), added urea detection (5), enzymatic determination of antimicrobials with the commercial Penzyme R test (6), and bacteriological determinations (2). No references were found concerning the influence of K 2 Cr 2 O 7 or HgCl 2 on the liquid chromatographic (LC) determination of sulfonamides, nitrofurans, and chloramphenicol residues in milk samples. The aim of this study was to measure the stability (through recovery coefficients) of 6 sulfonamides: sulfathiazole (STZ), sulfamerazine (SMR), sulfamethazine (SMT), sulfachloropyridazine (SCP), sulfamonomethoxine (SMM), and sulfamethoxazole (SMX); 3 nitrofurans: nitrofurazone (NF), furaltadone (FD), and furazolidone (FZ); and chloramphenicol (CAP) residues in raw milk samples preserved with 0.1% K 2 Cr 2 O 7 and 0.05% during 7 days of storage at refrigeration temperatures (4 7 C), and analyzed by LC. METHOD Apparatus (a) LC system. Merck Hitachi (Mexico City, Mexico). Variable UV detector, mm reversed-phase C 18 column, 5 µm particle size (LiChroCART 250-4, Merck ); 2 cm guard column. Operating conditions: flow rate, 1.0 ml/min (sulfonamides), 1.2 ml/min (nitrofurans); standard injection, 5 µl; sample injection, 20 µl. Run time, 45 min for sulfonamides and chloramphenicol, and 15 min for nitrofurans; chart speed, 0.5 cm/min; detector set at 275 nm for sulfonamides and chloramphenicol, and 370 nm for nitrofurans. (b) Rotary evaporator

2 1416 NOA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, 2002 (c) Pipettors (d) Filter paper cm diameter, fast speed (coarse precipitates). (e) Nylon filters µm porosity, 47 mm; and 0.45 µm porosity, 13 mm. (f) Vortex mixer (g) Volumetric pipets. 1, 5, and 10 ml. (h) Volumetric flasks. 10 and 25 ml. (i) Funnels. Short stem, 75 mm id. (j) Separatory funnels. 125 ml. (k) Round-bottom flasks. For rotoevaporator; 100 ml. (l) Conical tubes. 15 ml. (m) Disposable syringes. 3 ml. (n) Pasteur pipets Reagents (a) Chemicals. K 2 Cr 2 O 7 and mercuric bichloride HgCl 2, anhydrous sodium acetate, glacial acetic acid (ACS, reagents). (b) Water, acetonitrile, hexane, and methanol. LC grade. (c) Acetone and chloroform. Residue grade. (d) Sodium acetate solution. 0.02M; in water. Filter through 0.45 µm nylon filter. (e) Acetic acid solution. 0.02M; in water. Filter through 0.45 µm nylon filter. (f) Sodium acetate buffer solution. 0.02M, ph 4.8. Adjust sodium acetate solution, (d), to ph 4.8 with acetic acid solution, (e). (g) Mobile phases. (1) Sodium acetate buffer solution 0.02M acetonitrile (95 + 5, v/v). (2) Sodium acetate buffer solution 0.02M acetonitrile ( , v/v). Table 1. Gradient program: linear Time, min %B (h) Extraction solution. Chloroform acetone (2 + 1, v/v). Prepare daily 100 ml/sample. Measure chloroform and acetone in separate graduated cylinders, add acetone to chloroform, and mix thoroughly. Let mixture equilibrate ca min, to room temperature before using. (i) Sulfonamide standards. STZ (Aldrich chemicals); SMR, SMT, SCP, and SMM (Sigma, St. Louis, MO); SMX (Mexican Pharmaceutical Standard, Mexico City, Mexico). (j) Nitrofuran standards. NF and FD (Sigma); FZ (Mexican Pharmaceutical Standard). (k) CAP standard. Mexican Pharmaceutical Standard. Preparation of Standard Solutions (a) Stock solutions. Accurately weigh 25 mg, to nearest 0.1 mg, STZ, SMR, SMT, SCP, SMX, SMM, NF, FZ, FD, and CAP standards into separate weighing vessels. Transfer to separate 25 ml volumetric flasks (1 mg/ml). Dissolve in methanol. Figure 1. Stability of sulfonamides in raw milk samples preserved with 0.1% K 2 Cr 2 O 7 and 0.05% HgCl 2 during cold storage for 7 days. Sulfonamides were spiked at 50 ppb level.

3 NOA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, Figure 2. Stability of nitrofurans in raw milk samples preserved with 0.1% K 2 Cr 2 O 7 and 0.05% HgCl 2 during cold storage for 7 days. Nitrofurans were spiked at 50 ppb level. (b) Fortification solution (10 g/ml of each substance under study). Transfer 250 µl of each standard solution (1 mg/ml) into single 25 ml volumetric flask, dilute to volume with methanol, and mix thoroughly. Prepare daily. Preparation of Preserved Raw Milk Samples Raw cow s milk was obtained directly from local dairy cow herds within 6hofmilking. Raw milk (1 L) was separated in two 500 ml fractions (A and B). K 2 Cr 2 O 7 was added to fraction A to a final concentration of 0.1% (1) and mixed thoroughly. HgCl 2 was added to fraction B to a final concentration of 0.05% (1) and mixed thoroughly. Preserved raw milk sample controls (50 ml aliquots from each fraction, A and B) were placed in stoppered flasks. Fortified preserved raw milk samples (350 ml of each fraction, A and B) were spiked with standard mixture to a final concentration of 0.05 µg/ml (50 ppb) and mixed thoroughly. Six 50 ml aliquots from each fraction were placed in 6 stoppered flasks. Sample Storage Fortified preserved milk samples (A and B) were stored under refrigerated conditions (4 7 C) until day of analysis. Sample Analysis Preserved milk controls (A and B) and one of 6 fortified preserved milk samples (A and B) were analyzed on the day of preparation (Day 0). The other 5 fortified milk samples (A and B) were analyzed on Days 1 3, 5, and 7 of cold storage. Preserved milk controls and fortified samples were analyzed with 3 replicates. LC Methodology LC analysis of 6 sulfonamides, 3 nitrofurans, and chloramphenicol residues in milk has been described previously (7) and is based on AOAC Method for multiple sulfonamide residues (8), with some modifications in the chromatographic system. Method validation (7) showed that the detector response was linear over the range of ng/ml for all drugs under study, with correlation coefficients >0.99. Method detection limits ranged from 4 ppb (NF) to 16 ppb (SMT). Average recoveries, ranged from 65.5% for STZ to 104.2% for CAP, and precision [coefficients of variation (CV) <13%] was acceptable on the basis of Codex Committee on Veterinary Drug Residues guidelines, which regards CV <20% as acceptable (9). Sample Extraction Place 10 ml milk sample in 125 ml separatory funnel. Add 50 ml extraction solution and stopper. Shake vigorously 1 min; carefully vent through stopper. (Do not vent through stopcock because milk solids clogging it may result in sample loss.) Shake for 1 additional min, vent, and let phases separate 1 min. Repeat shaking sequence and let phases separate 5 min. Draw off extraction solution, filtering through washed (with extraction solution) filter paper, and collect in 100 ml round-bottom flask (take extreme care to prevent milk from entering stopcock). Add 25 ml extraction solution to separatory funnel and repeat exactly as before. Add this extract to the other. Rinse filter with extraction solution (2 5 ml) and add to the other extracts. Figure 3. Stability of chloramphenicol in raw milk samples preserved with 0.1% K 2 Cr 2 O 7 and 0.05% HgCl 2 during cold storage for 7 days. Chloramphenicol was spiked at 50 ppb level.

4 1418 NOA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, 2002 Figure 4. Liquid chromatograms of milk sample preserved with 0.1% K 2 Cr 2 O 7 spiked with the standard mixture of sulfonamides, chloramphenicol, and nitrofurans at a level of 50 ppb. A: 1) Sulfathiazole; 2) sulfamerazine; 3) sulfamethazine; 4) sulfamonomethoxine; 5) sulfachloropyridazine; 6) sulfamethoxazole; 7) chloramphenicol (gradient system, 275 nm). B: 1) Nitrofurazone; 2) furazolidone; 3) furaltadone (isocratic system, 370 nm). Sample Preparation Carefully evaporate extracts collected in 100 ml round-bottom flask, just to dryness on rotary evaporator at 32 C. Dissolve residue in 1 ml 0.02M sodium acetate buffer solution, agitating 1 min on Vortex mixer. Add 5 ml hexane, and mix on a Vortex mixer 1 min. Using Pasteur pipet, transfer phases to 15 ml conical tubes; let phases separate 2 min; and immediately mix on a Vortex mixer 1 more min. Let phases separate > 15 min. Remove aqueous layer with a Pasteur pipet and place in 2 ml vial. (Note: Aqueous layer will be homogeneous; therefore, it is not necessary to remove 100% of layer; 50 75% of aqueous layer is adequate for 2 injections.) Using 3 ml syringe, filter aqueous layer through nylon filter 0.45 µm,13 mm. Store in sealed vials. Chromatographic Systems Two chromatographic systems were used: (1) Gradient system for determination of 6 sulfonamides and chloramphenicol (Table 1). Detection at 275 nm; run, time 45 min. Mobile phases: (A) 0.02M sodium acetate buffer solution acetonitrile (95 + 5, v/v); (B) 0.02M sodium acetate buffer solution acetonitrile ( , v/v). (2) Isocratic system for nitrofurans determination. Detection at 370 nm; run time, 15 min. Mobile phase B. Samples were first analyzed with the isocratic system and then with the gradient system, after equilibration time of 1 h. Statistical Analysis Differences between Days 0 7 and treatments with K 2 Cr 2 O 7 or HgCl 2 were determined by analysis of variance (ANOVA) and Tukey s multiple comparison test. Statistical software SPSS was used. Results and Discussion Average recoveries of sulfonamide residues in fortified preserved raw milk samples during 7 days of cold storage are shown in Figure 1. There were no significant differences (p > 0.05) in the recoveries of the 6 sulfonamides between Days 0 and 5 in milk samples preserved with K 2 Cr 2 O 7. On Day 7 of cold storage there was a significant loss (p < 0.05) of 17.1, 17.2, and 23.2% in the average recoveries of SMM, SMR, and STZ, respectively. However, SMT, SCP, and SMX residues remained stable during the 7 days of experimentation. Average recoveries of the 6 sulfonamides under study ranged from 68.4 to 96.8% (CV < 13). These results were similar to those obtained previously (7). In fortified raw milk samples preserved with 0.05% HgCl 2, STZ recoveries dropped significantly (almost 50%) by Day 3 of storage. STZ recoveries decreased from 73.6 (Day 0) to 39.6% (Day 3). SMR, SMM, and SMT recoveries remained constant, at a 95% confidence level, through Day 5. However, significant (p < 0.05) losses of 20.7, 23.3, and 26.2%, respectively, were observed on Day 7 of storage. Only SCP and SMT residues remained stable, with average recoveries of 74.3 and 89.2%, respectively, during the 7 days of cold storage. To verify the apparent degradation of STZ in the presence of HgCl 2, a stability test was performed by using only solvents. STZ was dissolved in water and methanol at a concentration of 0.05 µg/ml, respectively. Aqueous and methanolic mercuric bichloride solutions (0.05%) of STZ were also prepared (0.05 µg/ml). These solutions were kept at 4 C and analyzed by LC at Days 0 2 and 7. STZ remained stable in methanol and methanolic mercuric bichloride during 7 days of cold storage. A 5.8% loss of STZ recovery was observed in

5 NOA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 85, NO. 6, aqueous solution by Day 7. Moreover, there was an important loss (73.5%) of STZ in aqueous HgCl 2 solution after 1 day of cold storage. These results confirmed the influence of HgCl 2 on STZ residue stability in aqueous media such as ocurred in milk. Hg may react with some thiazole ring derivatives causing ring cleavage (10) or may form mercuric derivatives of the thiazole ring (11), which may explain the STZ losses. Addition of K 2 Cr 2 O 7 or HgCl 2 to raw milk samples did not affect the stability of the nitrofuran residues throughout the analysis (Figure 2). In the first case, average recoveries ranged from 75.1 to 101.6% (CV < 13%) and in the second case, from 88.3 to 107.2% (CV < 10%). Significantly higher recoveries (p < 0.05) were obtained for FD in milk samples preserved with HgCl 2. In general, nitrofuran recoveries were similar to those reported previously (7). Chloramphenicol recoveries in milk samples preserved with K 2 Cr 2 O 7 showed no significant differences during all of the experiments (Figure 3); total mean recovery was 101.8% (CV < 8%). In contrast, a significant (p < 0.05) loss of 13.4% was observed on Day 7 when HgCl 2 was used, with average recoveries ranging from (Day 0) to 95.55% (Day 7), CV < 6.2%. Figure 4 illustrates the liquid chromatograms of a milk sample preserved with 0.1% K 2 Cr 2 O 7 spiked with the standard mixture of sulfonamides, chloramphenicol, and nitrofurans at a level of 50 ppb. Stability studies of veterinary drug residues in milk samples under storage have been recommended (12) to determine how long samples may be stored before compounds of interest may be degraded. The recovery data in the present work demonstrated that the 10 antimicrobials studied may be analyzed by LC in raw milk samples preserved with 0.1% K 2 Cr 2 O 7 during cold storage of 5 7 days, depending on the type of compound, without significant losses. Similar results were obtained for 0.05% HgCl 2, except for STZ residues, which apparently degraded in the presence of this preservative. References (1) FIL/IDF (1988) International IDF Standard , International Dairy Federation, Brussels, Belgium (2) Official Methods of Analysis of AOAC INTERNATIONAL (1995) 16th Ed., AOAC INTERNATIONAL, Gaithersburg, MD (3) Pinto, M., Vega, S., & Pérez, N. (1996) Métodos de análisis de la leche y derivados (in Spanish), UAM-X/UACH, Valdivia, Chile, pp (4) Kamakar, B., & Gathak, P.K. (1997) Cheiron 26, (5) Bector, B.S., Singhal, O.P., & Ram, M. (1998) Indian Dairyman 50, (6) Molina, M.P., Altahus, R.L., Fernández, N., Peris, C., & Rodríguez, M. (1999) Producción Animal (in Spanish) 20, (7) Pérez, N., Gutiérrez, R., Noa, M., Díaz, G., Luna, H., Escobar, I., & Munive, Z. (2002) J. AOAC Int. 85, (8) Official Methods of Analysis of AOAC INTERNATIONAL (1995) 16th Ed., AOAC INTERNATIONAL, Gaithersburg, MD, Method (9) CODEX (1995) Residuos de medicamentos veterinarios en los alimentos (in Spanish), FAO/OMS, Rome, Italy, Sec. 3, p. 70 (10) Acheson, R.M. (1960) An Introduction to the Chemistry of Heterocyclic Compounds, 2nd Ed., Wiley Interscience, New York, NY, p. 322 (11) Barton, D., & Ollis, D. (1979) Comprehensive Organic Chemistry, Vol. 4, Pergamon Press, New York, NY, p. 991 (12) CODEX (1995) Residuos de medicamentos veterinarios en los alimentos (in Spanish), FAO/OMS, Rome, Italy, Sec. 3, p. 76