Isotonic dilution is a specific forn of internal standard. With mass spectrometry, we can distinguish molecules that are chemically identical, except that certain elements are isotope variations (eg Carbon 13, instead of Carbon 12). But internal standards do not need to be isotonic standards and can be employed with other detection methods. I do have a video on internal standards which shows the general process

Thank you for your question. In this example, I showed a "hypothetical" calibration curve made in distilled water, and another curve for those standards in plasma. In that case, the slope of the curve (S) would be different. However, this is not what we are actually plotting. In the standard addition experiment, we never generate a calibration curve in distilled water. What we do instead is combine the standard with the plasma. And we do this in such a way that the background solution is nearly constant (typically by adding a very small volume of the distilled water to the plasma). Therefore, since both samples (plasma alone; plasma + standard) are measured in a plasma matrix, each equation will have the same value for S, which therefore can be cancelled out. Sorry I didn't make that completely clear in the video!

​@@shortchemistry7927Hi, for determination of K percentage in a fertilizer, I make calibration curve mixing standards and unknown. As 10 ml of unknown to all balloons and different amounts of standard, 1 balloon only has unknown without any standard. Why is that? Shouldn't the b from y=ax+b and signal of the balloon without standard be the same? If they are not same what should I do? I'm confused, please help. I mean why we determine signal of balloon that doesn't have any standard added? The b must be same as it! If b and the signal of sample without any standard aren't same numbers what must be done?