Benchtop electrospray ionization mass spectrometry (ESI-MS)

The Benchtop ESI-MS was used for all online measurements during the platform experiments and for the Mass Index calculation. Data are presented in the paper in Figs. 4–6 and in the SI in Figs. 4–6, 10, 18 + 19. The ESI-MS analysis was performed on a Benchtop ‘expression L-CMS’ system from Advion. After the sample passed the HPLC-DAD, it went through a split valve resulting into a flow of 0.2 mL/min injected into the ESI system. The mass spec was run in positive and negative mode, switching between both modes during analysis with a switching speed of 50 ms. The positive mode turned out to be more useful and only the results of this mode were used for the algorithm. The m/z range was set from 10 to 2000 m/z and the scan time was 3345 ms at a scan speed of 595 m/z/sec. The ion source parameters were as follows: capillary temperature 300 V, capillary voltage 120 V, source voltage offset 20 V, source voltage span 30 V, and the source gas temperature 200 °C. All settings were the same in both positive and negative modes, except for the ESI voltage, which was 3500 V for the positive mode and 2500 V in the negative mode. A calibration was performed regularly with a MS tuning mix (Agilent). Data were analyzed using the Advion Data Express software.