interferent
/ in-ter-FEER-ent /
You are trying to weigh a letter on a kitchen scale, but a fly keeps landing on the pan. The fly is not the letter, yet it changes the number you read. In a chemical measurement, a substance that is not the analyte but that nonetheless alters the signal you attribute to the analyte is called an interferent. It quietly pushes your answer up or down.
An interferent is any component of the sample, other than the analyte, that affects the measured signal and so distorts the result. It may add its own signal (looking like extra analyte), suppress the analyte's signal (looking like less), or react chemically with the analyte. Whether something counts as an interferent depends on the method: the same molecule may be harmless in one technique and troublesome in another.
Interferents matter because they are a leading cause of wrong results that still look perfectly believable. The good news is that analysts have many tools against them: improving selectivity, separating the analyte first, adding a masking agent, or using standard addition. The caveat is that you can only defend against interferents you have thought of — unsuspected ones are the most dangerous.
When measuring calcium by titration, magnesium ions in the same water can react with the titrant too, inflating the apparent calcium result. Here magnesium is the interferent, and a masking agent is often used to keep it out of the reaction.
An interferent is a non-analyte that nonetheless changes the result.
An interferent is not the same as the matrix as a whole. The matrix is all the non-analyte material; an interferent is specifically the part of it that actually distorts your measurement.