Compare the function that matters
PEG4 provides the shorter spacer. If purified labeling density is adequate, check that target binding or activity is retained. If crowding near the site is suspected, compare PEG12 at matched molar input and record recovery and background.
Match molar input
The products have different molecular weights, so equal masses give different molar inputs. Calculate each input separately and match sample lot, volume, solvent fraction, time and purification before comparing recovery, background and function.
Product solvents and solubility
The current instructions support water, PBS or anhydrous DMSO for AF405, AF488, AF555, AF594, AF700 and AF750 PEG12-DBCO. A PEG spacer alone does not establish aqueous solubility for other PEG4/PEG12 products; confirm the solvent and usable concentration for the catalog number. Check for cloudiness or precipitation after addition to the sample and match final solvent fractions when comparing products.