Near complete labeling of antibody by Nd tag was observed as per Maxpar kit specifications. The antibody and APTES/GA co-localized indicating that the attachment was successful and according to the experimental design. A reference-free approach to localized surface quantification was demonstrated. NdO~+ and Nd~+ were detected using positive mode and used to quantify the surface area coverage and density. Surface area coverage of the isotopic metal tag was found to be ～10%. Based on the approximate tagged antibody size and the calculated coverage, the surface density of the tagged antibodies was ～540 molecules per μm~2. Further improvements to detection sensitivity and quantification accuracy may be achieved by labeling with isotopic metal oxide nanoparticles.
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