This study addresses the use of epistemic criteria related to the scientific practice of inquiry in the context of environmental chemistry. In particular, it analyses the type of criteria that are used by pre-service teachers when assessing the adequacy of several scientific procedures for identifying microplastics in beach sand, as well as determining the ways in which these participants make use of said criteria. The participants were 22 pre-service primary teachers who were divided into small groups of 3-4 participants who were given the task of assessing the scientific quality of three different procedures before selecting which they considered to be the best option. The data collected includes audio recordings of the participants' small group conversations and their written comments. The data analysis is framed in qualitative content analysis, in which the participants' conversations were transcribed and coded using the ATLAS.ti software. The coding frameworks that were used to address each research question were developed by taking into consideration both the literature and the collected data. The main results indicate different patterns in terms of the types of criteria that were used in the participants' assessments, as well as the different uses of criteria within each of the small groups. These results could have been influenced by the participant's limited knowledge of both scientific inquiry and chemistry.