A method for performing an acidic hydrolysis of a liquid mixture comprising a biological matrix and a polyethyleneimine -based transfection reagent, wherein the biological matrix comprises a recombinant virus or virus-like particles produced using the PEI-based transfection reagent, wherein the method comprises the step of incubating the liquid mixture comprising the biological matrix in an aqueous solution comprising from 0.1% to 10% (v/v) hydrochloric acid (HCl) at a temperature ranging from 60°C to 110°C for a time period ranging from 2 hours to 24 hours, preferably an aqueous solution comprising 0.1% (v/v) HCl at a temperature of 110°C for 2 hours or in an aqueous solution comprising 1% (v/v) HCl at a temperature ranging from 60°C to 80°C for 2 hours, wherein said acidic hydrolysis does not degrade the PEI-based transfection reagent, and wherein the PEI-based transfection reagent is of general formula or an acceptable salt thereof: wherein: - m represents an integer between 27 to 1200 and n represents an integer between 3 to 600, with the proviso that n is lower than m and the sum of m+n ranges from 30 to 1200, - X represents H or a group of formula: in which q represents an integer between 10 and 800, - p represents an integer between 1 and 4, - Z represents a group of formula: - Y 0, Y 1, Y 2, Y 3 and Y 4, which may be identical or different, represent C or N, with the proviso that at least two, but no more than three, of Y 0, Y 1, Y 2, Y 3 and Y 4, are N, - W 1, W 2, W 3 and W 4, which may be identical or different, represent H, a linear or branched, saturated or unsaturated C 1 -C 18 alkyl, C 6 -C 18 aryl, a linear or branched, saturated or unsaturated C 6 -C 18 aryl-C 1 -C 18 alkyl, C 5 -C 10 heteroaryl, a linear or branched, saturated or unsaturated C 2 -C 18 heteroalkyl, an amine, a linear or branched, saturated or unsaturated C 1 -C 18 alkylamine, a C 1- C 12 alkoxy; or (i) W 1 and W 2 or (ii) W 2 and W 3 or (iii) W 3 and W 4 together form a fused, optionally substituted six-membered aryl; or a fused, optionally substituted six-membered heteroaryl containing no more than 1 N atom, with the proviso that at least one, but no more than two, of W 1, W 2, W 3 and W 4 is, or are, absent. wherein the biological matrix comprises a recombinant virus or virus-like particles produced using the PEI-based transfection reagent, wherein the method comprises the step of incubating the liquid mixture comprising the biological matrix in an aqueous solution comprising from 0.1% to 10% (v/v) hydrochloric acid (HCl) at a temperature ranging from 60°C to 110°C for a time period ranging from 2 hours to 24 hours, preferably an aqueous solution comprising 0.1% (v/v) HCl at a temperature of 110°C for 2 hours or in an aqueous solution comprising 1% (v/v) HCl at a temperature ranging from 60°C to 80°C for 2 hours, wherein said acidic hydrolysis does not degrade the PEI-based transfection reagent, and wherein the PEI-based transfection reagent is of general formula or an acceptable salt thereof: wherein: - m represents an integer between 27 to 1200 and n represents an integer between 3 to 600, with the proviso that n is lower than m and the sum of m+n ranges from 30 to 1200, - X represents H or a group of formula: in which q represents an integer between 10 and 800, - p represents an integer between 1 and 4, - Z represents a group of formula: - Y 0, Y 1, Y 2, Y 3 and Y 4, which may be identical or different, represent C or N, with the proviso that at least two, but no more than three, of Y 0, Y 1, Y 2, Y 3 and Y 4, are N, - W 1, W 2, W 3 and W 4, which may be identical or different, represent H, a linear or branched, saturated or unsaturated C 1 -C 18 alkyl, C 6 -C 18 aryl, a linear or branched, saturated or unsaturated C 6 -C 18 aryl-C 1 -C 18 alkyl, C 5 -C 10 heteroaryl, a linear or branched, saturated or unsaturated C 2 -C 18 heteroalkyl, an amine, a linear or branched, saturated or unsaturated C 1 -C 18 alkylamine, a C 1- C 12 alkoxy; or (i) W 1 and W 2 or (ii) W 2 and W 3 or (iii) W 3 and W 4 together form a fused, optionally substituted six-membered aryl; or a fused, optionally substituted six-membered heteroaryl containing no more than 1 N atom, - m represents an integer between 27 to 1200 and n represents an integer between 3 to 600, with the proviso that n is lower than m and the sum of m+n ranges from 30 to 1200, - X represents H or a group of formula: in which q represents an integer between 10 and 800, - p represents an integer between 1 and 4, - Z represents a group of formula: - Y 0, Y 1, Y 2, Y 3 and Y 4, which may be identical or different, represent C or N, with the proviso that at least two, but no more than three, of Y 0, Y 1, Y 2, Y 3 and Y 4, are N, - W 1, W 2, W 3 and W 4, which may be identical or different, represent H, a linear or branched, saturated or unsaturated C 1 -C 18 alkyl, C 6 -C 18 aryl, a linear or branched, saturated or unsaturated C 6 -C 18 aryl-C 1 -C 18 alkyl, C 5 -C 10 heteroaryl, a linear or branched, saturated or unsaturated C 2 -C 18 heteroalkyl, an amine, a linear or branched, saturated or unsaturated C 1 -C 18 alkylamine, a C 1- C 12 alkoxy; or (i) W 1 and W 2 or (ii) W 2 and W 3 or (iii) W 3 and W 4 together form a fused, optionally substituted six-membered aryl; or a fused, optionally substituted six-membered heteroaryl containing no more than 1 N atom, with the proviso that at least one, but no more than two, of W 1, W 2, W 3 and W 4 is, or are, absent.