The article deals with the current issue of evaluation of qualitative results uncertainty of chemical analyses in the example of chromatographic analyses of specimens of accelerants from a fire place with the SPME-GC/MS technique. The suitability of this technique demonstrates with the results of alidation measurements of analytical characteristics: sensitivity, detection limits, linearity, selectivity and repeatability. It characterizes briefly the current state of knowledge in the area and applies the Bayesian theory for the unconditional “a-priori” probability of an analyte presence in a sample, P (H) of 0.5 and 0.95 for calculation estimates the “a-posteriori” conditional probability of the presence of the analyte in the sample relative to the experimental evidence, P (H / E) with the results of 0.97 and 0.98. This result can be interpreted as a high level of assurance of the hypothesis of presence of the analyte in the sample. In conclusion it is stated that the knowledge can also be applied in the field of construction not only in qualitative analyses of building materials / products, e. g, by FTIR, but also in the probability risk assessment.