= Nitrocellulose
\b[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 $\mathrm{R-O-NO_2}$: the nitrogen is attached through oxygen, rather than directly to the carbon backbone as in a <nitro group>.<Nitrocellulose in ChEBI>[CHEBI:53325, definition and structure]{ref}
A <cellulose> repeat has three substitutable hydroxyls. For original atom bookkeeping, write its unmodified internal repeat as $\mathrm{C_6H_{10}O_5}$. Each replacement of $\mathrm{-OH}$ by $\mathrm{-ONO_2}$ removes one hydrogen and adds one nitrogen and two oxygens. With $d$ substitutions per repeat, the corresponding composition is
$$
\mathrm{C_6H}_{10-d}\mathrm{N}_{d}\mathrm{O}_{5+2d},\qquad 0\leq d\leq3.
$$
At $d=3$ this gives $\mathrm{C_6H_7N_3O_{11}}$. 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.<Electrophoretic transfer of proteins from polyacrylamide gels to nitrocellulose sheets: procedure and some applications by Harry Towbin and others>[Abstract and pp.4350–4352, transfer and recovery comparisons]{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.
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