SOLID PHASE EXTRACTION
AND LIQUID CHROMATOGRAPHY ANALYSIS
OF SULFONAMIDE RESIDUES IN HONEY
ANDRZEJ POSYNIAK
,
TOMASZ
Ś
NIEGOCKI
ANDJAN
Ż
MUDZKI
Department of Pharmacology and Toxicology, National Veterinary Research Institute,
24-100 Pulawy, Poland
Sulfonamides were extracted from honey with acetate buffer (pH 5.0), and cleaned up by solid phase extraction (SPE) procedure. The liquid chromatography (LC) separation was carried out on RP C18 column using acetic acid and acetonitrile as the mobile phase in gradient
mode and monitored with fluorescence detector, after pre-column derivatisation. Recoveries from spiked honey were above 80%, and detection limits were 0.1 µg/kg for sulfacetamide and 0.2 µg/g for sulfathiazole and sulfamethazine.
Key words: honey, sulfonamides, residues, method, liquid chromatography.
Honey bee larvae are susceptible to American foulbrood or European foulbrood, disease caused by the organism Bacillus larvae, which can devastate hives. Sulfonamides are relatively stable chemotherapeutics known to control this disease but they are not permitted to use for this purpose in most countries because of the potential of sulfonamide residues to contaminate honey.
Regulatory agencies are responsible for assuring that potentially harmful residues of these drugs are not present in honey or honey products. However, so far, maximum residue limits have been established for sulfonamide compounds in food of animal origin, but not in honey, at level 0.1 mg/kg within the European Union (4) and Poland follows this regulation.
A variety of analytical methods have been used to measure sulfonamide residues in biological materials. Analytical methods available for identification and quantification of sulfonamide residues in honey include: enzyme immunoassay (12), thin-layer chromatography (11), colorimetric (14) and liquid chromatography (3). Liquid chromatography provides a very effective tool since the use of different detection methods like UV, diode array for routine detection, and mass spectrometry as a confirmatory method (1, 2, 17).
Some of the method problems are connected with cost, time or the utility. Liquid chromatography with UV is not a method of choice for sulfonamides in honey, because of interaction with endogenous compounds (13). Although a photodiode-array detection system (7) or post-column derivatisation has been adopted to achieve selectivity for sulfa drugs, they require expensive additional equipment and involve setting up and optimizing the system (6, 9).
We applied a pre-column derivatisation system with fluorescamine (15, 16) to develop a rapid and selective detection of sulfonamides in honey followed by clean-up with solid phase extraction (SPE). The whole procedure was validated for sulfacetamide (SCA), sulfamethazine (SMT) and sulfathiazole (STZ), active compounds of Polisulfamid®, that is used in veterinary practice in Poland.
Material and Methods
Material. Sulfacetamide, sulfamethazine, sulfathiazole and fluorescamine
were obtained from Sigma Chemical Co. (St Louis, MO, USA) and acetonitrile from Mallinckrodt Baker B.V. (Deventer, Holland). Water was freshly distilled, filtered through a 0.45 nm membrane and degassed under vacuum. Acetic acid was from Merck (Darmstadt, Germany). Bakerbond SPE octadecyl (3 ml) columns and SPE manifold were from Mallinckrodt Baker B.V. (Deventer, Holland).
Stock solution and standards. Stock solutions of 1 mg ml-1 were prepared in
acetonitrile. The working solutions for LC and sample spiking were prepared by dilution of 1 ml of each stock solution to serial 10-fold dilutions in mobile phases or in water to concentration of 100, 10, 1 and 0.1 μg ml-1. All the solutions were stored in the dark at 4˚C.
Liquid chromatography. A Shimadzu VP Series liquid chromatograph
(Duisburg, Germany) equipped with a fluorescence detector FR-10AXL with excitation wavelength λ = 405 nm and emission wavelength λ = 495 nm was used to analyse the tested solutions. LC control, data acquisition and peak integration were performed by system controller SCL-10A utilizing RS-232C interface for communication with CLASS-VP chromatography workstation.
The chromatographic analyses were performed on a Phenomenex Luna column (250x4.6 mm, 5 μm) with mobile phase 2% (v/v) acetic acid (AA) – acetonitrile (ACN). Flow 0.9 ml/min was used for the separation of analytes in gradient mode at the following program: 0-2 min, AA+ACN (70:30); 2-5 min, AA+ACN (80:20); 5-12 min, AA+ACN (60:40); 12-19 min, AA+ACN (70:30). Aliquots of 20 µl were injected into the column.
Sample preparation. The honey sample (2.5 g) was diluted with 12.5 ml of
0.1 M acetic buffer (pH 5.0) and then immersed in ultrasonic water bath for 15 min. The whole solution was extracted on an octadecyl phase chemically bound to silica gel disposable column under depression of 0.5 bar.
The SPE-column was preconditioned with 3 ml of methanol, 3 ml water and finally with 3 ml of acetic buffer (pH 5.0). After percolation of the whole solution, the bed of the column was washed with 3 ml of acetic buffer (pH 5.0), 3 ml of water and dried under depression for 5 min. The sulfonamides were eluted with 5 ml of acetonitrile.
This extract was dried under nitrogen stream at 40˚C. The dry residue was dissolved in 900 μl of acetic buffer (pH 3.5), 100 μl of 0.2% fluoroescamine in acetone was added and mixed with a vortex mixer. The sample was ready to analyse after staying for 20 min at room temperature.
Quantitation of sulfonamides. Samples of honey were spiked with
analytes to the concentrations in standard solution. The precision of the assay was measured using the same samples.
Results
In our studies, the choice of pH for the acetate buffer is the result of a compromise that takes into account the differences in a tested concentration of SCA, STZ and SMZ. The extraction of SMZ and STZ is maximal at pH<5.0, and that of SCA at pH>5.0. Ultrasonic treatment of the sample rather than homogenization or shaking was more efficient.
LC conditions for separation of the sulfonamides were examined by varying column temperature and flow mode. The best separation was obtained at 55˚C and at a gradient mode. As shown in Fig. 1 the sulfonamides were well separated when the chromatographic conditions were used. Simple chromatograms with good baselines were achieved in gradient mode case. The presence of chromatographic peaks from the honey matrix, that might resulting errors in the qualitative or quantitative analysis, was minimal. The fluorescent peaks light of the derivative of SCA gave a detection limit of 0.1 ng and 0.2 ng for SMZ and STZ on the chromatogram, respectively. In a practical analysis, the detection limits were set at 0.1 µg/kg for SCA, and 0.5 µg/kgfor SMZ and STZ.
The day–to day reproducibility for several series is shown in Table 1. The variation coefficients are satisfactory, and mean extraction recoveries are above 80%. The study intraserial reproducibility is shown in Table 2. The observed variation coefficients are satisfactory.
Discussion
A critical aspect of drug analysis is the sample extraction step which requires the isolation of residue from a biological material. Traditionally, sulfonamides are extracted by treating the sample with an organic solvent, and clean–up in solid–phase by passage through disposable cartridges (5, 8, 10), or by solid-phase dispersion (18).
Several sample preparation procedures were tested to obtain optimum extraction efficiency for the isolation of sulfonamides from honey matrix. Acetate buffer pH 5.0 proved to be a better extracting solvent than water, acetonitrile or other solvents applied for the isolation of sulfonamide compounds from matrix, as it have been previously described (3, 11, 12).
Twenty commercial honey samples were analysed. The presence of SCA, SMZ and STZ residues were found in 10 samples at levels from 1.0 to 5.6 µg/kg.
In conclusion, the results of the present study show that proposed LC technique is an efficient and reliable method of detection of sulfonamide residues in honey. The use of acetate buffer and SPE on octadecylsilane cartridges makes the isolation of sulfonamides from honey matrix easer and cheaper than commonly used procedures.
a
Minutes 0 2 4 6 8 10 12 14 16 18 20 22 24 % F -0,0002 0,0000 0,0002 0,0004 % F -0,0002 0,0000 0,0002 0,0004 (S T Z ) (S D A ) (S M Z )Detector B (Ex:405nm, Em:495nm) sulfonamidy 078 Name
b
Minutes 0 2 4 6 8 10 12 14 16 18 20 22 24 % F 0,0 0,2 0,4 % F 0,0 0,2 0,4 (S T Z ) (S D A ) (S M Z )Detector B (Ex:405nm, Em:495nm) sulfonamidy 073 Name
c
Minutes 0 2 4 6 8 10 12 14 16 18 20 22 24 % F 0,0 0,2 0,4 % F 0,0 0,2 0,4 (S T Z ) (SD A ) (S M Z )Detector B (Ex:405nm, Em:495nm) sulfonamidy 074 Name
Table 1
Accuracy and day-to-day reproducibility for SCA, SMZ and STZ determination in honey
SCA SMZ STZ Amount added, µg/kg 5.0 10.0 5.0 10.0 5.0 10.0 Amount found, µg/kg 4.3 8.2 4.1 8.1 4.1 8.3 Replicates (n) 6 6 6 6 6 6 Coefficient of variation (%) 6.2 7.8 5.8 7.2 6.7 8.3 Mean recovery (%) 82.6 83.4 81.7 82.3 80.9 82.3 Table 2
Accuracy, precision and within-day reproducibility for SCA, SMZ and STZ determination in honey
SCA SMZ STZ
Amount added, µg/kg 5.0 10.0 5.0 10.0 5.0 10.0
Amount found, µg/kg 4.4 8.6 4.2 8.4 4.3 8.2
Replicates (n) 6 6 6 6 6 6
Acknowledgement. Appreciation is expressed to Mrs A. Grzebalska for an excellent technical assistance.
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