A method for performing size exclusion chromatography separation, i.e. SEC, or desalting of a target from a sample in a system comprising: i) the step of providing a system with at least three columns (A, B, C, D) at least two of which are connectable in series at the same time; ii) the step of connecting in series a column (A, B, C, D) containing the target with another of the columns, iii) the step of washing at least one further column with a washing fluid at the same time as the target is being transported by a flow of buffer from said column containing the target to said another of the columns, wherein said at least three columns comprise four columns (A, B, C, D) for separating a target from a sample, wherein the target passes through three or four columns before being collected, wherein each column has an inlet (Ai, Bi, Ci, Di) and an outlet (Ao, Bo, Co, Do) fluidly connected to a valve arrangement for selectively simultaneously connecting said at least two chromatography columns in series and one of the other chromatography columns to flow of washing fluid, wherein said valve arrangement comprises a first inlet for inputting a buffer solution, a second inlet for inputting a washing fluid, an inlet for receiving fluid from said chromatography columns, and a first outlet leading to valve means for diverting fluid to a waste arrangement or a target collection arrangement; the system further comprising: a first detector for detecting the presence of the target substance; a second detector for detecting the presence of the target substance; one or more pumps (49A, 49B) for pumping buffer and washing fluid (43, 45); a supply of a first buffer solution; a supply of said washing fluid; and a means for injecting a sample into a SEC column. i) the step of providing a system with at least three columns (A, B, C, D) at least two of which are connectable in series at the same time; ii) the step of connecting in series a column (A, B, C, D) containing the target with another of the columns, iii) the step of washing at least one further column with a washing fluid at the same time as the target is being transported by a flow of buffer from said column containing the target to said another of the columns, wherein said at least three columns comprise four columns (A, B, C, D) for separating a target from a sample, wherein the target passes through three or four columns before being collected, wherein each column has an inlet (Ai, Bi, Ci, Di) and an outlet (Ao, Bo, Co, Do) fluidly connected to a valve arrangement for selectively simultaneously connecting said at least two chromatography columns in series and one of the other chromatography columns to flow of washing fluid, wherein said valve arrangement comprises a first inlet for inputting a buffer solution, a second inlet for inputting a washing fluid, an inlet for receiving fluid from said chromatography columns, and a first outlet leading to valve means for diverting fluid to a waste arrangement or a target collection arrangement; the system further comprising: a first detector for detecting the presence of the target substance; a second detector for detecting the presence of the target substance; one or more pumps (49A, 49B) for pumping buffer and washing fluid (43, 45); a supply of a first buffer solution; a supply of said washing fluid; and a means for injecting a sample into a SEC column. a first detector for detecting the presence of the target substance; a second detector for detecting the presence of the target substance; one or more pumps (49A, 49B) for pumping buffer and washing fluid (43, 45); a supply of a first buffer solution; a supply of said washing fluid; and a means for injecting a sample into a SEC column.