I'm still new working on an ICP machine, but I've also had the same problem, where my result should have been 1000ppm, and instead I got 100ppm. After I re-calibrated the ICP, made up new calibration standard solutions, and getting the same results, it hit me. The problem is quite simple, it's the ICP method settings under the calibration solution values. This is especially a big problem as soon as you dilute the sample, and do a miss-calculation. Just check your settings.