Delta vs mass spectrometer

All beaks were dried at 60 °C for 24–48 hours and ground into a fine powder. Powder sub-samples were weighed (to the nearest 0.3 mg) with a micro-balance and sterile-packed in tin containers. Stable isotope values were determined by a Flash EA 1112 Series elemental analyser coupled online via a Finnigan ConFlo II interface to a Delta VS mass spectrometer (Thermo Scientific) and expressed as: δ¹³C and δ¹⁵N = [(R_sample/R_standard) − 1] * 1000, where R = ¹³C/¹²C and ¹⁵N/¹⁴N, respectively. The carbon and nitrogen isotope ratios were expressed in delta (δ) notation relative to Vienna-PeeDee Belemnite limestone (V-PDB) for δ¹³C and atmospheric nitrogen (AIR) for δ¹⁵N. Replicate measurements of internal laboratory standards (acetanilide STD: Thermo Scientific PN 338 36700) in every batch (n = 14) indicated precision <0.2‰ for both δ¹³C and δ¹⁵N values. Mean mass C:N ratio of all samples was 3.19 ± 0.02.

All beak subsections were dried at 60 °C for 24–48 h, weighed (to the closest value to 0.3 mg) with a micro-balance and cracked if needed, and sterile-packed in tin containers. Stable isotopic signatures were determined by a Flash EA 1112 Series elemental analyser (Thermo Scientific Inc., Waltham, MA, USA) coupled online via a Finnigan ConFlo II interface to a Delta vs. mass spectrometer (Thermo Scientific, Bremen, Germany) and expressed as: δ¹³C and δ¹⁵N = [(R_sample/R_standard) − 1] * 1000, where R = ¹³C/¹²C and ¹⁵N/¹⁴N, respectively. The carbon and nitrogen isotope ratios were expressed in delta (δ) notation relative to Vienna-PeeDee Belemnite limestone (V-PDB) for δ¹³C and atmospheric nitrogen (AIR) for δ¹⁵N. Replicate measurements of internal laboratory standards (acetanilide STD: Thermo Scientific PN 338 36700) in every batch (n = 14) indicated a precision <0.2‰ for both δ¹³C and δ¹⁵N signatures. The mass C:N ratio for each sample, where it was obtained, ranges from 3.05 to 3.86 (Table S2). The analyses were carried out at the Marine and Environmental Science Centre, University of Coimbra (Coimbra, Portugal).