The Northwest and West Regions of Cameroon fall under the Western Highlands’ Ecological Zone, also known as the Grass Field Zone. These locations are situated within the Western High Plateau, an area defined by altitudes ranging from 1,000 to over 2500 meters above sea level [3][9]. This zone is characterized by high relief, cool temperatures, heavy rainfall and Savanna vegetation [10]. It consists of mountain ranges and volcanoes made of crystalline and igneous rocks. This zone is boarded by the South Cameroon Plateau to the Southeast, the Adamawa Plateau to the Northeast and the Cameroon Coastal plain to the South. The map of the western Highlands is shown in Figure 1.

Vegetables: A total of twenty-four vegetable samples, comprising Daucus carota(6), Solanium lycopersicum(6), Solanum tuberosum(6), and Apium graveolens(6), were procured from six distinct agricultural sites: Tubah, Bamenda III, Santa, Babadjou, Foumbot, and Banyangam. At each location, one kilogram of each vegetable species was harvested from two independent farmers and subsequently pooled to create a representative composite sample. From these composites, 500 g aliquots were sealed in Ziploc polyethylene bags, labeled, and transported under refrigerated conditions (4˚C) to the Directorate of Regulations and Quality Control (DRCQ) laboratory in Yaoundé, Cameroon. Upon receipt, all samples were inspected, coded, and stored at 4˚C to maintain analytical stability.

Source: Adapted from NIC (2023). Map of the Western Highlands of Cameroon. Yaoundé: National Institute of Cartography.

Figure 1. The Map of the Western Highlands.

Soil: To ensure a representative assessment of soil characteristics, six composite samples were collected across the study area. At each site, topsoil (0 - 15 cm depth) was extracted from eight distinct locations using a clean soil auger. These eight increments were thoroughly homogenized to form a single composite. From each composite, a 500 g subsample was weighed, transferred to a sterile container, and immediately stored in a portable cooler at approximately 4˚C to minimize microbial activity and chemical degradation during transport.

Water: Six water samples were collected using 1.5 L high-density polythene containers. To prevent cross-contamination, each vessel was rinsed three times with the source water prior to final collection. Samples were obtained by gently immersing the container below the surface, ensuring sufficient headspace remained for subsequent laboratory homogenization. Immediately following collection, samples were stored in a portable cooler maintained at 4˚C to inhibit microbial activity and chemical degradation. This protocol was replicated across all designated sampling sites, and samples were transported to the laboratory for immediate processing and preservation.

The samples were analyzed at the Directorate of Regulations and Quality Control (DRCQ) of the Ministry of Agriculture and Rural Development (MINADER) in Yaoundé, Cameroon using the Multi-Residue QuEChERS AOAC official 2007.01 method. Extraction was performed by adding acetonitrile to each homogenized sample. The extract was purified using a dispersive solid-phase extraction (d-SPE) cleanup, employing anhydrous magnesium sulphate (MgSO₄) to induce phase separation by removing water, sodium acetate (CH₃COONa) to maintain an optimal pH buffer, and Primary/Secondary Amine (PSA) to remove polar matrix interferences, such as organic acids and sugars. Octadecylsilane (C₁₈) was also added to remove lipids and non-polar interferences.

The pure extract was analyzed using an Agilent 7890A Gas Chromatograph coupled with an Agilent 5975C VL Mass Selective Detector (MSD) (Agilent Technologies, Palo Alto, CA, USA). Separation was achieved using a HP-5MS capillary column (30 m × 0.250 mm inner diameter, 0.25 µm film thickness) (Agilent Technologies). The GC-MS system was operated in electronic impact (EI) ionization mode (70 eV) with Single Ion Monitoring (SIM) acquisition.

Helium was employed as the carrier gas at a flow rate of 1.0 - 1.2 mL/min. A 1.5 µl sample volume was injected in split less mode, with the inlet temperature maintained at 280˚C. The GC oven temperature program was initiated at 150˚C (held for 1 min), increased to 260˚C at a rate of 45˚C/min, and finally ramped to 280˚C at a rate of 20˚C/min, holding for 9 min, resulting in a total run time of 13.44 min. The MS heat transfer line temperature was set to 250˚C. Chromatograms displaying pesticide retention times and abundances were produced, allowing for identification by matching observed retention times with the instrument’s database.

The analytical method was validated according to EU SANTE/11312/2021 guideline for specificity, linearity, accuracy, precision and sensitivity. The method demonstrated robust performance, with recoveries consistently ranging from 70% - 120% and relative standard deviations (RSDs) within 20% across the validated range. Limits of detection (LOD) and quantification (LOQ) were established to ensure compliance with regulatory Maximum Residue Limits (MRLs), with the LOQ set at 0.01 mg/kg.

The concentrations of the detected pesticides in the extracts were determined using standard calibration solutions prepared from analytical-grade pesticide standards. The calibration curves demonstrated high linearity with a correlation coefficient greater than 0.9950 (R² > 0.9950). The calibration graphs are shown in Figure 2 and Figure 3.

Figure 2. Calibration graph of chlorothalonil.

Figure 3. Calibration graph of chlorpyrifos-methyl.

Concentrations obtained from the calibration graphs were substituted into Equation (1) to determine the corresponding pesticide residue concentrations in the vegetables. These results were then compared with MRLs to determine the level of contamination

The concentrations were compared with Maximum Residue limits (MRLs) to determine the level of contamination.

Sample chromatograms showing the detected pesticides residues in the samples are presented in Figures 4-7.

Figure 4. Chromatogram of celery sample from Babadjou, containing chlorothalonil with a retention time of 2.987 minutes.

Figure 5. Chromatogram of celery sample from Foumbot, containing chlorpyrifos-methyl with a retention time of 3.433 minute.

Figure 6. Chromatogram of celery sample from Bamenda III, containing chlorothalonil with a retention time of 2.981 minutes.

Figure 7. Chromatogram of soil sample from Santa, containing phenol-2,4-bis (1,1-dimethyl) with a retention time of 6.683 minutes.

As seen on the chromatograms, chlorothalonil was detected in celery samples from Bayangam, Babadjou, and Bamenda III, while chlorpyrifos-methyl was found only in the celery sample from Foumbot. The summary of vegetable samples containing pesticide residues is presented in Table 1.

Table 1. Summary of vegetable samples containing pesticide residues.

The results of the quantified pesticide detections, including concentrations and corresponding maximum residue limits (MRLs), are presented in Table 2.

Table 2. Concentration of pesticide residues and maximum residue limits (MRLs).

The concentration of chlorpyrifos-methyl in Foumbot celery and chlorothalonil in Babadjou celery exceeded established MRLs, whereas the rest complied with regulatory standards. The frequency of sample contamination by pesticide residues is presented in Table 3.

Table 3. Frequency of sample contamination by pesticide residues.

The high frequency of negative samples (83.3%) suggests that the majority of crops were either produced without the application of synthetic pesticides, were harvested in strict compliance with established Pre-Harvest Intervals (PHIs), or contained residues below the analytical Limit of Detection (LOD). Adherence to PHIs allows for the natural degradation of active ingredients via hydrolysis, photolysis, oxidation, and microbial metabolism, often reducing concentrations below the 0.01 mg/kg threshold. While Gas Chromatography-Mass Spectrometry (GC-MS) is highly effective for volatile, non-polar compounds such as organochlorines and organophosphates, it possesses inherent limitations regarding polar or systemic residues. These substances are more accurately quantified using Liquid Chromatography-Mass Spectrometry (LC-MS). Therefore, samples categorized as negative may represent instances where residues were present but fell below the specific sensitivity range of the GC-MS instrumentation.

Quantification results indicate that celery samples from Babadjou exhibited the highest concentration of chlorothalonil (62.1 mg/kg), which significantly exceeds the 20 mg/kg Maximum Residue Limit (MRL) established, by the Codex Alimentarius and Japanese regulatory standards. Samples from Bamenda III (18.3 mg/kg) remained within the established MRL, while samples from Bayangam (6.6 mg/kg) were also compliant. Conversely, the concentration of chlorpyrifos-methyl in celery from Foumbot (2.5 mg/kg) represents a substantial exceedance of the MRLs established by the Codex Alimentarius (0.05 mg/kg), and the European Union (0.01 mg/kg). The detection of these residues at levels exceeding regulatory thresholds suggests non-compliance with Good Agricultural Practices (GAP), specifically regarding dosage, application timing, or failure to observe the mandatory Pre-Harvest Interval (PHI).

The findings of this study regarding pesticide exceedance align with previous investigations conducted across Cameroon. Research in the Western Highlands (Bamenda) by Sonchieu etal. [11] identified high levels of organophosphate residues, specifically chlorpyrifos that exceeded Codex Maximum Residue Limits (MRLs) in market garden crops. Furthermore, a 2017 pilot study by Tarla etal. [12] documented the application of banned substances and the persistent presence of chlorothalonil residues in vegetables within the Northwest and West regions. Complementing these findings, Galani etal. [13] reported that 100% of analyzed samples contained at least one pesticide residue, with 38% surpassing European Union MRLs. Collectively, these investigations underscore that non-compliant residue levels of pesticides, including chlorpyrifos-methyl and chlorothalonil, represent a critical food safety challenge for agricultural products in Cameroon.

Limitations

This study was limited by a small sample size and reliance on a single analytical method (GC-MS), which restricts the ability to detect polar or thermally labile pesticides. Additionally, the results do not account for the substantial reduction in pesticide residue levels that generally occurs through household processing like washing or cooking. Finally, the study was unable to definitively determine the source of phenol, 2,4-bis (1,1-dimethylethyl) in the soil samples.