1 m aqueous acetic acid solution

Toluene (HPLC grade, > 99.8%), tetramethoxysilane (TMOS), 1 M aqueous acetic acid solution, and polyethylene glycol (PEG) of molecular weight (MW) = 10 000 g/mol were obtained from Sigma-Aldrich Co. (Diegem, Belgium). Methanol (HPLC super-gradient grade) was obtained from Biosolve B.V.
(Valkenswaard, NL). Deionized water was produced in-house with a Milli-Q water purification system Merck Millipore (Billerica, MA, USA). Coumarin 440 (C440: 7-amino-4-methyl-2H-1-benzopyran-2-one), coumarin 460 (C460: 7-(diethylamino)-4-methyl-2H-1-benzopyran-2-one), and coumarin 480 (C480: 2,3,6,7-tetrahydro-9-methyl-1H,5H,11H- [1] benzopyrano- [6,7,8-ij] quinolizin-11-one) were purchased from Vadeno Optical Solutions (Apeldoorn, NL). PTFE filters (0.20 μm × 25 mm) were purchased from Macherey-Nagel (Düren, DE). Fused-silica capillaries with an inner diameter (i.d.) of 5 µm, possessing an outer diameter of 375 µm were purchased from Polymicro Technologies (Phoenix, AZ, USA).

Toluene (HPLC grade, > 99.8%), tetramethoxysilane (TMOS), 1 M aqueous acetic acid solution, and polyethylene glycol (PEG) of molecular weight (MW) = 10,000 g/mol were obtained from Sigma-Aldrich Co. (Diegem, Belgium). Octadecyldimethyl-N,N-dimethylaminosilane was purchased from ChemPur Feinchemikalien und Forschungsbedarf GmbH (Karlsrule, DE). Methanol (HPLC super-gradient grade) was obtained from Biosolve B.V. (Valkenswaard, NL). Deionized water was produced in-house with a Milli-Q water purification system Merck Millipore (Billerica, MA, USA). Coumarin 440 (C440: 7-amino-4-methyl-2H-1-benzopyran-2-one), coumarin 450 (C450: 7-(ethylamino)-4,6-dimethyl-2H,-1-benzopyran-2-one), coumarin 460 (C460: 7-(diethylamino)-4-methyl-2H-1-benzopyran-2-one), and coumarin 480 (C480: 2,3,6,7-tetrahydro-9methyl-1H,5H,11H-[1]benzopyrano-[6,7,8-ij]quinolizin-11-one) were purchased from Vadeno Optical Solutions (Apeldoorn, NL). Samples were prepared by dissolving these coumarins in the mobile phase to a concentration of 2 mM per component. PTFE filters (0.20 μm × 25 mm) were purchased from Macherey-Nagel (Düren, DE). Fused-silica capillaries with an inner diameter (i.d.) of 5 µm and 10 µm, possessing an outer diameter of 375 µm were purchased from Polymicro Technologies (Phoenix, AZ, USA).

Toluene (HPLC grade, ≥99.8%), urea, tetramethoxysilane (TMOS), metyltrimethoxysilane (MTMS), 1 M aqueous acetic acid solution, polyethylene glycol (PEG) of molecular weight (MW) = 10,000 g/mol, thiourea, acetophenone, propiophenone, butyrophenone, valerophenone, hexanophenone, heptanophenone and octanophenone were purchased from Sigma-Aldrich (Overijse, Belgium). Trifluoroacetic acid (TFA, LC-MS Ultra grade) was purchased from Fluka (Buchs, Switzerland). Octadecyldimethyl-N,Ndimethylaminosilane (ODS-DMA) was purchased from ChemPur Feinchemikalien und Forschungsbedarf GmbH (Karlsruhe, Germany). Acetonitrile (ACN, HPLC supra-gradient grade) was purchased from Biosolve B.V. (Valkenswaard, the Netherlands). N-(Trimethylsilyl)-imidazole (TMSI) was obtained from Merck Schuchardt OHG (Hohenbrunn, Germany). Deionized water was produced in-house with a Milli-Q water purification system of Merck Millipore (Billerica, MA, USA). PTFE filters (0.20 µm × 25 mm) were purchased from Macherey-Nagel (Düren, Germany). MassPREP Peptide Mixture was purchased from Waters (Milford, MA, USA). Fused-silica capillaries of 20 µm i.d. (90 µm outer diameter (o.d.)) and 40 µm i.d. (105 µm o.d.) were purchased from Polymicro Technologies (Phoenix, AZ, USA).