Nitrocellulose

Nitrocellulose, also called cellulose nitrate, is a chemically modified cellulose in which some or all of the available hydroxyl groups are converted into nitrate esters. The relevant connection is $\mathrm{R}-\mathrm{O}-\mathrm{N}{\mathrm{O}}_2$: the nitrogen is attached through oxygen, rather than directly to the carbon backbone as in a nitro group.^[ref]

A cellulose repeat has three substitutable hydroxyls. For original atom bookkeeping, write its unmodified internal repeat as ${\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_5$. Each replacement of $-\mathrm{OH}$ by $-\mathrm{ON}{\mathrm{O}}_2$ removes one hydrogen and adds one nitrogen and two oxygens. With $d$ substitutions per repeat, the corresponding composition isAt $d=3$ this gives ${\mathrm{C}}_6{\mathrm{H}}_7{\mathrm{N}}_3{\mathrm{O}}_{11}$. A fractional $d$ describes an average over repeat units, not fractional atoms in an individual unit; finite-chain end groups are omitted. “Nitrocellulose” consequently need not denote one unique substitution pattern or chain length.

A scientifically useful application is the nitrocellulose Western blot membrane. In a classic experiment, an electric field moved separated proteins from a gel onto a nitrocellulose sheet, preserving a spatial pattern accessible to antibodies. Transfer was quantitative for the tested urea-gel ribosomal proteins but not for the tested SDS gels.^[ref]

Thus a retained band can locate a protein without proving that all of that protein was transferred. The receiving material, its retention and the detection method are separate parts of the measurement.