European Union (EU) legislation sets stringent maximum residue limits (MRLs) for pesticides in products of plant origin. In Council Directives 86/362/EEC3 and 90/642/EEC4 maximum residue levels are fixed for pesticide residues in/on products of plant origin. The maximum pesticide residue level in foodstuffs is 0.01 mg/kg. This general level is applicable 'by default', i.e. in all cases where an MRL has not been specifically set for a product or product type.<br> Member States are asked to check regularly the compliance of foodstuffs with these levels. Besides national monitoring programmes, the commission services recommended, via Commission Recommendation 2002/1/EC, the participation of each member state in a specific EU coordinated monitoring programme. The monitoring programmes often carried out, serve as an indicator of the level of compliance with these provisions.<br> The general aim of this thesis is to work towards a system which makes it possible to estimate actual pesticide levels throughout Europe. With all monitoring programmes, analytical data of quality assurance measures have to be massively deployed, otherwise data comparability and thus data based decision making might be compromised. Use of reference materials–where available–for quality control/quality assurance is mandatory under the provisions of ISO 17025, and national accreditation bodies should demand the used of such materials for method validation and other quality assurance/quality control measures. The specific objective of the work presented here is to study the feasibility of producing a Matrix Reference Material (carrot/potato based) for pesticide analysis. The material is intended as a quality assurance tool in support to european policies regarding pesticide residue legislation. This important component of quality control is not possible in the actual scenario since no natural matrix RM is available in the EU. However this approach, can be modified somewhat to account the unavailability of a natural matrix CRM to control the analytical procedure and validation of results: a validated method, with stated certainty. In this case the method replaces the absence of a CRM to asses the verification of the analytical process and spiking experiments are used to demonstrate the accuracy of the method. Isotope dilution mass spectrometry (IDMS) is regarded as such a method. A homogeneity study was carried out for the three candidate reference materials–frozen, freeze-dried and sterilized carrot/potato matrices.Freezing and sterilization were intended to be an alternative to freeze-drying, where a reconstitution step is necessary, to ensure that the matrix format should be as similar as possible to routine laboratory samples. The main reason for the choice of these stabilization techniques is to improve the commutability between real-world samples and CRMs.<br> Based on the method repeatability and the set-up of the study, in average the uncertainty contribution resulting from the homogeneity assessment is 6.1, 2.6 and 6.2 % respectively for the frozen, freeze-dried and sterilized batches of samples.<br> In regard to the short stability studies designed for 4 weeks, stability of all 21 target analytes at -20 °C, in the frozen and dried matrices was proven by analytical measurements via GC-MS, along with the stability of the majority of the target pesticides at +4 °C (except phorate, lambda-cyhalotrin, permethrin and cypermethrin) in the dried matrix. This suggests that transport of such candidate reference material would be feasible at +4 °C for all target analytes, if phorate, lambda-cyhalotrin, permethrin and cypermethrin were not of interest, in a freeze-dried matrix.Moreover the determined average content (ng/g dry matter) is in agreement with the values obtained during homogeneity studies. The long-term stability studies enabled to select the best candidate materials.<br> After conducting homogeneity/stability studies, frozen and freeze-dried materials were elected as the best option for the end-purpose and demonstrated the feasibility of producing a Matrix Reference Material for pesticides in carrots. All studied pesticides remained stable for a period of 5 months in the carrots matrix with an average combined uncertainty contribution of 8.2 % and 10.1 % in the frozen and freeze dried matrix respectively, to the exception of some late elucting compounds in the freeze dried-matrix.<br> Thus, even if a laboratory would not be interested in (international) comparability of its measurements it would have to utilise references to avoid distortion of their measurements results.