how the internal standard and the blank are applied in the ICP OES

Question

Hello, 
 I am brand new user of Agilent 725 ICP-OES. I'm setting up an ICP-OES method for quantification of As, Al, Zn, Fe, Mg, Ca, K, P, Na, K, Ni, V in veg oils and I used Yttrium (Y) as an internal standard in my method. I noticed that internal standard IS stability ratio plot trends downwards. I am wondering what is leading to the IS decreasing from sample to sample. I am using Agilent ICP Expert II software (version 2.0.4.280), and I hope someone can help to understand how the software is using the internal standard to generate the elements concentration at the end of the run. Also, I am calibrating the instrument with a multi-element std solution at 2, 5 and 10 ppm concentration including a blank. I am wondering how the internal standard and the blank are used in the calculation of the calibration curve. Thanks in advance.

Andrey.D · Accepted Answer

From ICP Expert 7.5 (5xxx instruments) help file: 
 
 How Are Internal Standards Applied? 
 Internal standardization is applied by taking the intensity of the internal standard measured with the first blank (or standard) and referencing that as 1.0. The intensity of the internal standard in subsequent standards and samples is referenced against the first internal standard measurement and calculated as a ratio. This ratio is then applied to the calculated value of the subsequent standards and samples to give a corrected value. That is, the analyte standards and samples are corrected for any changes which may affect both the internal standard and the analytes. 
 
 Should be the same for Expert II, so FOGs22 was right in his explanation.

how the internal standard and the blank are applied in the ICP OES

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