A process for the preparation of a composition comprising one or more compounds of formula I wherein R 1, R 2 and R 3 each independently represent H or -NO, wherein n is 0 or 1; wherein when n is 0, R 1 is H; and wherein when n is 1, R 2 is H, provided that at least one of R 1 R 2 and R 3 represents -NO, said process comprising the step of: (i) reacting a corresponding compound of formula I but wherein R 1, R 2 and R 3 represent H with a source of nitrite, optionally in the presence of a suitable acid, wherein: (a) when the source of nitrite is an organic nitrite, step (i) is performed in a suitable organic solvent; and (b) when the source of nitrite is an inorganic nitrite, step (i) is performed in a bi-phasic solvent mixture comprising an aqueous phase and a non-aqueous phase wherein the process further comprises the step of adding a further amount of a compound of formula I but wherein R 1, R 2 and R 3 represent H, such that the combined mixture of the one or more compounds of formula I and a compound of formula I but wherein R 1, R 2 and R 3 represent H comprises from about 0.01% to about 9% by weight of the one or more compounds of formula I, wherein the term "about" means that the defined value may deviate by ±10%. wherein R 1, R 2 and R 3 each independently represent H or -NO, wherein n is 0 or 1; wherein when n is 0, R 1 is H; and wherein when n is 1, R 2 is H, provided that at least one of R 1 R 2 and R 3 represents -NO, said process comprising the step of: (i) reacting a corresponding compound of formula I but wherein R 1, R 2 and R 3 represent H with a source of nitrite, optionally in the presence of a suitable acid, wherein: (a) when the source of nitrite is an organic nitrite, step (i) is performed in a suitable organic solvent; and (b) when the source of nitrite is an inorganic nitrite, step (i) is performed in a bi-phasic solvent mixture comprising an aqueous phase and a non-aqueous phase wherein the process further comprises the step of adding a further amount of a compound of formula I but wherein R 1, R 2 and R 3 represent H, such that the combined mixture of the one or more compounds of formula I and a compound of formula I but wherein R 1, R 2 and R 3 represent H comprises from about 0.01% to about 9% by weight of the one or more compounds of formula I, wherein the term "about" means that the defined value may deviate by ±10%. (i) reacting a corresponding compound of formula I but wherein R 1, R 2 and R 3 represent H with a source of nitrite, optionally in the presence of a suitable acid, wherein: (a) when the source of nitrite is an organic nitrite, step (i) is performed in a suitable organic solvent; and (b) when the source of nitrite is an inorganic nitrite, step (i) is performed in a bi-phasic solvent mixture comprising an aqueous phase and a non-aqueous phase wherein the process further comprises the step of adding a further amount of a compound of formula I but wherein R 1, R 2 and R 3 represent H, such that the combined mixture of the one or more compounds of formula I and a compound of formula I but wherein R 1, R 2 and R 3 represent H comprises from about 0.01% to about 9% by weight of the one or more compounds of formula I, wherein the term "about" means that the defined value may deviate by ±10%. (a) when the source of nitrite is an organic nitrite, step (i) is performed in a suitable organic solvent; and (b) when the source of nitrite is an inorganic nitrite, step (i) is performed in a bi-phasic solvent mixture comprising an aqueous phase and a non-aqueous phase wherein the process further comprises the step of adding a further amount of a compound of formula I but wherein R 1, R 2 and R 3 represent H, such that the combined mixture of the one or more compounds of formula I and a compound of formula I but wherein R 1, R 2 and R 3 represent H comprises from about 0.01% to about 9% by weight of the one or more compounds of formula I, wherein the term "about" means that the defined value may deviate by ±10%.