Various grades and batches of filter papers were tested for their levels of inorganic nitrogen contamination (measured as ammonium, nitrate and nitrite‐N) by eluting them with 50 mL of 0.5 M potassium sulphate. Significant amounts of ammonium‐N were eluted from all the filter papers, however, most were low in nitrate and nitrite nitrogen. The amounts of ammonium‐N varied with the batch number, among boxes within a batch number and within filter papers from the same box. To minimise the effects of ammonium‐N contained in the filter papers, it is necessary to wash them with a salt solution, such as 0.5 M potassium sulphate, rinse them with deionized water and dry them prior to use