**2. Materials and Methods**

*2.1. Samples and Reagents*

2.1.1. Oil Mixture Preparation

Samples were supplied by a local company, which collects and reuses fishing surplus and fish industry by-products from different industrial activities. Eight fish oil samples (named with consecutive letters from A to H) obtained from fish by-products were used to make 269 different mixtures. The origin of the fish species of the oils, as well as the industries and processes from which they came, were unknown. These samples were divided in two sets: calibration (172 mixtures) and validation (97 mixtures). For external validation purposes and to ensure the robustness of the calibration, 29 new mixtures were made. The set of mixtures used for this aim was composed of three out of eight of the previous fish oils, together with a new oil (I) and two additional commercial fish oil supplements (named Supplement A and Supplement B) (Table 1).

**Table 1.** Samples of oil mixtures used in each dataset.

The volume of the prepared oil mixtures was at least 3 mL. Therefore, different volumes of the initial oils were taken and mixed using automatic pipettes. The minimum amount of oil used for the mixtures was 0.1 mL, and the maximum was 2.9 mL. For some mixtures, only 2 oils were used, and the maximum number of oils used in a mixture was 6. The percentage of oil in each mixture was formulated so that the range of the mixtures covered all possible variability. All samples were filtered with Whatman grade 1 filter paper before analysis.