While performing boiler water chemistry testing using a spectrophotometer (iron, silica, etc.) is there a preferred method of blanking the spectrophotometer? I was always taught to use DI water as the zeroing agent, but I've recently come to a new plant where they zero out the spectrophotometer using the sample as the blank. This practice doesn't really make any sense to me but I was hoping that someone could offer some clarification. Thanks.