Sentence:  There are two potential problems with this approach: first, the specificity of <WSD>labeling</WSD> depends on the purity of the LPS (if contaminants are present, they may be labeled also); second, removal of the label from the LPS "in vivo" might occur separately from processing of the LPS molecule itself. 
Meaning of the word marked with <WSD> tags:  The introduction of a traceable chemical group (e.g., containing an isotope or a fluorescent dye) into a protein or other biomolecule of interest so it can be tracked or quantified during experimental analysis.