There is an SP7 switching valve on our 5110 ICP-OES The first set of triplicate aspirations of Std always ride really close to the precision limit of 2%RSD and then std#2 and Std3 are typical 0.5 or less. (ISTD Method) All of these standards are in the PPM range and nowhere close to the LOD for the elements we are monitoring. We could run the method 10x in one day and this happens every single time. I am wondering if the sample loop is not getting flushed enough with standard on the first set of aspirations. Individual aspirations of the first std appear to climb from the first to the third aspiration but usually at about 2%+/-.5% I even switched the order of the order of the stds and when I aspirate the highest concentration first it fails or comes close to failing and the standards that follow all drop to 0.5% RSD or below including the low std that used to be std1 in this case.
Does the sample loop need rinsed with sample longer or is it possible that the valve is having an issue?