Introduction

Food safety is an increasing public health concern around the world. Food of animal origin has been identified as a major source of transmitting pathogens, toxic substances, and residues of chemicals to humans (Begum 2008).

Antimicrobials are used in food-producing animals as treatments, prophylaxis, and to improve growth. During life, any individual may contact with antibiotics directly as treatments or indirectly through ingestion of contaminated food and water. Sulfonamides are a group of antibiotics used for treatment and prevention of bacterial diseases in humans as well as in animals (Giguère et al. 2006). In veterinary practice, sulfonamides are extensively used due to its broad spectrum of activity and low cost (Yi et al. 2006). Sulfonamides are also used as a feed additive to promote growth in animals (Roybal et al. 2003). Improper use of these antibiotics may lead to the development of microorganisms which are resistant to antibiotics and occurrence of residues in food of animal origin (Yi et al. 2006).

The poultry is a vital animal husbandry sector in Sri Lanka and contributes to 70% of the total animal husbandry production in the country. Within the country, poultry is the only well-stabilized livestock industry which achieved self-sufficiency (Ministry of Livestock and Rural Community Development 2011). During the past 10 years, there has been 57% growth in the local poultry production (Kothalawala et al. 2011). Based on this background, the need has arisen to establish the analytical technique to detect sulfonamide residues in chicken food commodities. Among the many techniques already available to detect the food contaminants, TLC is one of the most uncomplicated and inexpensive techniques that can be used to screen for the presence of chemical contaminants in food (Bergwerff 2005; Croubels et al. 2004).

The TLC methods are comparatively economical than the HPLC methods, which makes them suitable for routine residue analysis (Sherma and Fried 2003). Therefore, the objective of this study was to establish a TLC method to screen sulfonamides in chicken tissues for residue monitoring.

Materials and Method

Chemicals and Equipment

All chemicals, ethyl acetate, petroleum spirit, methanol, acetone, and acetic acid, were of analytical grade. Aluminum-backed flexible silica gel 250-μm-layer TLC plates obtained from Whatman were used for separation of SAs, while fluorescamine was used for the derivatisation of SAs. The TLC plates were viewed using the CAMAG TLC viewing chamber and scanned using the CAMAG TLC scanner.

Standard and Working Solutions

Sulfonamide standards, sulfadiazine (SDZ), sulfadoxine (SD), sulfamethazine (SMZ), sulfathiazole (STZ), and sulfaquinoxaline (SQ), were obtained from Sigma-Aldrich, St. Louis, MO, USA.

A separate stock solution of sulfonamides (1 mg/ml) was prepared by dissolving in methanol except for SDZ and SQ, used 0.1 M NaOH as the diluent. The stock solution was diluted in methanol to obtain the intermediate standard solution of 100 μg/ml. The stock and intermediate standard solutions were stored at − 20 and 4 °C respectively. The prepared intermediate solution was further diluted with methanol to obtain the working standards of 10 μg/ml.

Solvent Systems

Silica gel plates were activated in methanol and dried overnight at 60 °C. Different mobile phases and wavelengths were studied for the separation of analytes. The following mobile phase solvent systems were studied for better separation of analytes:

  1. 1.

    Solvent system 01: diethyl ether: methanol solvent system with a second solvent system of dichloromethane: acetic acid

  2. 2.

    Solvent system 02: ethyl acetate: n-butanol: methanol: aqueous ammonia solvent system

  3. 3.

    Solvent system 03: chloroform and n-butanol solvent system

Selection of Analytical Wavelength

Different wavelengths, 275, 366, and 410 nm, were examined for optimum detection of the analytes.

Samples

Muscle samples of poultry were directly obtained from reliable farms that did not use antimicrobials during grower stage, and such samples were frozen until used as blank matrices. Accurately weighed 3 g of homogenized samples were treated with 150 μl of 2 μg/ml mixed standard solution to spike samples at 100 μg/kg. All samples were kept for 20 min at room temperature before being subjected to extraction.

Extraction Procedure

Two extraction procedures were evaluated for sulfonamide analysis.

Extraction Method 1

For each sample, 0.3 ml of 0.1 M HCl and 3 ml of distilled water was added and vortexed for 1 min. Samples were extracted with 4.5 ml ethyl acetate. The tube was then centrifuged at 524×g for 10 min. The supernatant was collected into a glass tube. For the second time, the remaining pellet was treated with ethyl acetate and repeated the same procedure. The extraction procedure was repeated, and supernatants obtained in both instances were mixed and evaporated under a mild nitrogen flow at 55 °C. The resultant dry residue was dissolved in 1 ml of methanol—water in 75:25 (v/v) mixture. The prepared solution was treated twice with 1 ml of petroleum spirit to remove fat. The resulting solution was evaporated under a mild stream of nitrogen at 90 °C, reconstituted with 100 μl of methanol and used for TLC analysis.

Extraction Method 2

For the extraction, 4 ml of dichloromethane was added to 3 g of homogenate and vortexed for 1 min. The tube was then centrifuged at 524×g for 10 min. The supernatant was collected into a glass tube. Again, the sample was extracted with 4 ml of dichloromethane and repeated the same procedure. The supernatant was evaporated under a mild nitrogen flow at 55 °C. Finally, the dry residue was dissolved in 1 ml of methanol—water in 75:25 v/v mixture. The prepared solution was defatted twice with 1 ml petroleum spirit. The resulting solution was evaporated under a mild stream of nitrogen at 90 °C then reconstituted with 100 μl of methanol and was used for TLC analysis.

Experimental Procedure

A 25 μl of standards and samples were spotted using graduated micro capillaries on activated silica plates (10 × 10 cm). The space between two spots was 10 mm. Then, plates were dried using a dryer. The TLC chamber was kept at room temperature (25 ± 2 °C) for 30 min until it was saturated with the mobile phase. The loaded TLC plates were developed in one-dimensional linear ascending mode in a 10 × 10 cm TLC trough glass chamber (Camag, Muttenz, Switzerland). The TLC plate was kept in the chamber until the solvent front has reached 8 cm from the sample application point. Then, the developed plate was removed and dried using a dryer.

The samples were derivatised by spraying with freshly prepared 0.1 mg/ml fluorescamine in acetone solution. The developed TLC plates were scanned using different wavelengths in a Camag TLC scanner operated with winCATS software (V 3.15, Camag).

Validation of the Method

Linearity

The calibration curves were prepared by diluting the working standard solution in methanol to obtain final concentrations of 200, 150, 100, 50, 30, and 25 ng/g. Calibration curves were constructed by plotting a peak area against the concentration of the analyte. The least-squares method was used for the regression analysis of calibration curves.

Precision

Inter-day and intra-day repeatability evaluated the precision of the method. The inter-day repeatability was assessed by analyzing standards at three different concentrations 100, 150, and 200 ng/g and repeating the assay for six times. The relative standard deviation (RSD %) was calculated for the replicates. The inter-day precision was assessed through the same procedure and conditions by repeating in different days.

Specificity

The specificity of the method was determined by analyzing ten blank samples of each chicken muscle in order to verify the absence of potentially interfering compounds at Rf values of SAs.

The Limit of Detection and Limit of Quantification

The limit of detection (LOD) was calculated by ten independent sample blanks fortified at a lowest acceptable concentration measured once for each type of meat. Calculation of limit of quantification (LOQ) was conducted by fortifying sample blank at various analyte concentrations close to the LOD.

Results

Optimization of TLC Analysis

The experimental conditions of TLC including wavelength, mobile phase, and extraction procedure were optimized in order to obtain more accurate, reliable, and reproducible analysis of sulfonamides in chicken tissues.

Only two separated peaks were visible in chromatograms obtained by scanning TLC plates developed with the solvent system 01 (diethyl ether: methanol solvent system followed by dichloromethane: acetic acid). The second solvent system was able to separate sulfonamide mixture into two separate peaks. The third solvent system with chloroform and n-butanol at 80:20 and 85:15 ratios were able to separate the five sulfonamide mixture into four peaks (Fig. 1). The solvent combination with 90:10 was able to separate all five analytes into five separate peaks (Fig. 1). However, chloroform and n-butanol at 95:5 did not improve the separation of the compounds (Fig. 1).

Fig. 1
figure 1

Chromatograms of mixture of five sulfonamide standards, STZ, SDZ, SMZ, SQ and SD eluted with chloroform: n-butanol ratio of (a) 80:20 v/v, (b) 85:15 v/v, (c) 90:10 v/v, and (d) 95:5 v/v and visualized under UV light at 366 nm

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The separation of individual sulfonamides from a mixture at different concentrations and extracts of chicken meat treated with sulfonamides mixture at a concentration of 50 ng/g concentration were evaluated under present conditions using TLC in order to determine the sensitivity and the specificity. Further, the present method was able to extract an adequate amount of sulfonamides from chicken treated at a concentration as low as 50 ng/g so that individual sulfonamides can be separated and visualized under the present TLC method.

All the chromatograms obtained from scanning at three different wavelengths for triplicates of samples were given in Fig. 2. The baseline was not stable, and peaks of the chromatogram representing individual sulfonamides spots were not very clear when the spots representing individual sulfonamides in TLC plates were scanned at 275 and 410 nm. However, when the same sulfonamides spots were scanned at 366 nm, clear chromatographic peaks representing individual sulfonamides were observed with a stable baseline.

Fig. 2
figure 2

Three-dimensional views of chromatograms of sulfonamide standards, STZ, SDZ, SMZ, SQ, and SD eluted with chloroform: n-butanol (90:10 v/v) and scanned under different wavelengths of (a) 275, (b) 366, and (c) 410 nm

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Optimization of Extraction Procedure

The mean recovery values of SAs obtained with the two different extraction methods are given in Table 1. The recovery of SAs under the first extraction method ranged from 80 to 120% while under the second method, it ranged from 37 to 170%. In the 1st extraction method, STZ had the lowest recovery value of 81%. In the 2nd extraction method, the lowest recovery, i.e., 37%, was observed for STZ while the highest recovery, i.e., 170%, was observed for SQ. In this context, extraction method one found to be the most suitable extraction method for SAs and recoveries of SAs under extraction method one complies with EU regulations.

Table 1 The recovery values of sulfonamides extracted under extraction method 01 and 02 from chicken meat fortified with SAs at a concentration of 100 ng/g
Full size table

Validation

The calibration curves were established for sulfonamides by plotting a peak area against the concentrations. The correlation coefficients for all the analytes were above 0.99. The peak areas of all sulfonamides were linearly correlated with the concentration range of 12.5 to 100 ng/spot.

Figure 3a illustrates the matrix effect of chicken meat while Fig. 3b showed the well-separated symmetrical sharp peaks of each sulfonamide with the matrix effect. There were no interfering matrix peaks in the same Rf of the sulfonamide standards.

Fig. 3
figure 3

Chromatograms of (a) blank chicken sample and (b) spiked chicken sample with the mixture of five sulfonamide standards, STZ, SDZ, SMZ, SQ, and SD at 100 μg/kg, eluted with chloroform: n-butanol (90:10 v/v) and visualized under UV light at 366 nm

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The relative standard deviations of intra-day precision were in the range of 3–22% for all the sulfonamides (Table 2). The highest and lowest intra-day precision of 3% at 150 ng/g and 22% at 100 ng/g were reported for STZ. The highest inter-day precision of 42% was reported with SMZ. However, the lowest inter-day precision value of 6% resulted in STZ at 150 ng/g.

Table 2 The inter-day and intra-day precision of sulfonamide standards at 100 ng/g, 150 ng/g and 200 ng/g eluted with chloroform: n-butanol (90:10) and visualized under UV light 366 nm
Full size table

The SD had the highest LOD and LOQ values of 60 and 100 ng/g respectively. All the other analytes’ LOD value was 40 ng/g. The STZ had the lowest LOQ values of 50 ng/g. The Rf values of the sulfonamides were given in Table 3.

Table 3 The LOD, LOQ, and retention factor of different sulfonamides in chicken tissue extracted with method 01 and eluted with chloroform: n-butanol (90:10), visualized under UV light 366 nm
Full size table

Discussion

TLC is one of the essential standard method used in screening of sulfonamides in the food of animal origin. In this context, there are many TLC methods that have been developed for the detection of sulfonamides in different types of food of animal origin.

A number of sulfonamide analytes can be separated from the TLC method. The methods developed by Unruh et al. (1990) and Bevill et al. (1978) were able to analyze only SMZ in milk and plasma respectively. Reimer and Suarez (1991) have developed a method which can analyze five sulfonamides, namely SDZ, SMRZ, SMTZ, SDMX, and SP, in salmon tissue. The method established by Thomas et al. (1983) was able to detect SMZ, STZ, SQ, sulfadimethoxine, and sulfabromomethazine in beef, pork, turkey, and duck meat. Five sulfonamides, sulfanilamide, SDZ, SMTZ, SQ, and SD could be detected in the muscle and kidney of pig, poultry, and cattle (Haagsma et al. 1984). Most of these sulfonamides are not commonly imported or used in Sri Lanka (VDCA 2012). The modified method presented in this study was capable of analysis of SDZ, SD, SMZ, STZ, and SQ in chicken meat. Several HPLC methods have been developed to analyze sulfonamides (Xia et al. 2017; Premarathne et al. 2017; Liu et al. 2016; Wang et al. 2016). The method reported by Xia et al. (2017) analyzed SDZ, STZ, sulfamerazine (SMR), SMZ, and sulfamethoxypyridazine (SMP) while the method developed by Wang et al. (2016) could detect 15 different sulfonamides in milk, and seven sulfonamides SDZ, STZ, sulfapyridine (SPD), SMR, sulfamethazine (SMT), SMZ, and SMP in chicken and eggs were able to be detected by Premarathne et al. (2017).

Different types of mobile phases have been developed for separation of sulfonamides. Ethyl acetate: n-butanol: methanol: aqueous ammonia (35:45:15:2) was used for separation of five sulfonamides—SDZ, sulfamerazine (SMRZ), sulfamethazine (SMTZ), sulfadimethoxine (SDMX), and sulfapyridine (SP) in salmon muscle (Reimer and Suarez 1991). For analysis of sulfamethazine chloroform, tetra butanol mobile phases have been used (Fried and Sherma 1996). Haagsma et al. (1984) have been able to replace tetra butanol with n-butanol and obtained separation between five sulfonamides: sulfamethazine, sulfadiazine, sulfaquinoxaline, sulfanilamide, and sulfadoxine. In this current method, n-butanol was used in the place of tetra butanol. In general, the first two solvent systems were unable to separate all the five sulfonamides, but the third solvent system with chloroform: n-butanol was able to demonstrate improved separation. Better separation was observed with increasing the ratio of chloroform. However, when the percentage of chloroform was increased beyond 95%, there was no clear separation among the analytes. Haagsma et al. (1984) have used chloroform: n-butanol in 4:1 ratio for the separation of four sulfanilamides: SDZ, SMTZ, SQ, and SD. However, in the present study, the solvent system with chloroform: n-butanol at 9:1 was able to separate of all five analytes.

The methods used for the extraction of sulfonamides from different matrices and even from the same matrices were varying from the method to method. Reimer and Suarez (1991) have applied Matrix solid-phase dispersion for extraction using C18-derivatised silica gel and eluted with dichloromethane. In Unruh et al. (1990) method, it has employed C18 solid-phase extraction (SPE) columns for cleaning up the samples. Even Haagsma et al. (1984) employed Sep-Pak silica disposable columns for the extraction. This type of extraction method is not affordable in developing countries like Sri Lanka due to the high cost of the columns. The use of these columns may provide good recovery values; however, have to be vigilant about the application of these methods for routine residue analysis due to the associated high cost per analysis and non-availability of columns regularly in developing countries. In some methods, they have utilized toxic chemicals such as dichloromethane for the extraction procedure. In this study, of the two extraction methods tested, method one was selected due to the high recovery of analytes compared to the second extraction method. Further, the current method used only ethyl acetate which is considered as a safe chemical compared with dichloromethane for the extraction. The HPLC detection methods recently utilized nanocomposite-based extraction and clean up methods for analysis of sulfonamide residues in food samples (Xia et al. 2017; Wang et al. 2016). Premarathne et al. (2017) method used liquid-liquid extraction.

Different techniques have been used for visualization of SAs on TLC plates. Some methods have used fluorescamine for derivatisation (Sigel et al. 1975; Haagsma et al. 1984; Reimer and Suarez 1991; Unruh et al. 1990). However, in some methods, sulfonamides were detected without derivatisation (Thomas et al. 1983). The method developed in this study also used fluorescamine for derivatisation. Usage of a derivatising agent improves the selectivity and sensitivity of the analysis (Jork et al. 1990).

The TLC analysis of sulfonamides has been conducted in various commodities of animal origin with a different limit of detection. Four sulfonamides, SDZ, SMTZ, SDMX, and SP, have been analyzed at 0.04 ppm and SMRZ at 0.10 ppm in salmon tissue (Reimer and Suarez 1991). The method developed by Sigel et al. (1975) can only detect SDZ at 0.1 ppm but can be detected in milk, eggs, liver, kidneys, muscle, skin, and fat. Thomas et al. (1983) have an LOD more than 0.1 ppm in tissues of cattle, swine, turkey, and duck. Bevill et al. (1978) and Unruh et al. (1990) method have an LOQ at 510 ng/ml. The method by Haagsma et al. (1984) could detect sulfonamides at 0.05 mg/kg level. In this current method, the LOD was at 40 ng/g for all the SAs except for SD at 60 ng/g. The HPLC methods were also developed to detect sulfonamides in various samples including meat (Xia et al. 2017), fruits (Liu et al. 2016), and milk (Wang et al. 2016) with lower LOD including 1.73–5.23 ng/g, 0.21–0.57 ng/mL, and 0.02 and 0.13 μg L−1 respectively.

The recovery percentages of the SDZ, SMRZ, SMTZ, SDMX, and SP in the method by Reimer and Suarez (1991) were 61, 63, 60, 63, and 57% respectively. There is no record of recovery values in the methods developed by Haagsma et al. (1984). According to Unruh (1990), recoveries for SMZ have ranged from 88.36 to 103.15% in the analysis range 0.51–15.34 μg/L. The HPLC methods also reported 76.1–102.6% (Xia et al. 2017) and between 73.4 and 97.4% (Wang et al. 2016) recoveries. The recovery values of this method ranged between 81 and 113% for five sulfonamides, which indicates the present method is efficient to the other TLC methods and was in parallel with the recoveries of HPLC method which used nanocomposite-based extraction.

In the TLC method, it can analyze several samples without application of pressure. Therefore, TLC methods are comparatively economical than the HPLC methods, which makes them suitable for routine residue analysis. One of the most frequent problems encountered by the TLC method is inconsistency in the Rf value. Even in the same laboratory settings, it will not reproduce the same Rf value. Factors like the chamber, mobile phase conditions, stationary phase conditions, and environmental conditions like humidity can affect the Rf value. Therefore, to confirm the analytes, one has to scrape the layer and elute with following a detection method like infrared (IR) spectrometry, nuclear magnetic resonance (NMR) spectrometry, or mass spectrometry (MS) (Sherma and Fried 2003). Screening using a TLC method and confirmation of positive samples with a HPLC method is more economical and efficient. Every time in TLC analysis, samples are applied and separated in a new stationary phase; therefore, carry-over effects and cross-contamination will be minimal. Amount of mobile phase required for analysis is comparatively lower than the other chromatographic methods. However, the sensitivity of the TLC method is comparatively lower than the HPLC method which resulted in a higher LOD value.

Further, the present method can detect levels lower than the MRL value and therefore this method could be applied to certify whether chicken samples are free of sulfonamide residues. According to EU guidelines of commission decision 2002/657/EC, recovery of the confirmatory methods at a concentration ≥ 10 μg/kg should be within the range of − 20 to + 10%, and still, there are no guidelines for screening methods. Even though this method is a screening method, recovery values are within the guidelines for confirmatory methods except for SMZ. The current method sensitivity and specificity also can be improved with the incorporation of an automated applicator and high-performance thin-layer chromatography plates.