[ICP-OES] Method concentration vs calculated concentration - large errors

Question

Hello everyone,

I am a new ICP-OES user, and trying to learn more about this instrument.

Can someone help explain how the "calculated concentration" is obtained?

I am asking because sometimes, there is a huge difference in method concentration and calculated concentration that lead to large error margin.

Thanks in advance.

Admin · Accepted Answer

This question has been marked as assumed answered.

blhahesy · Answer

When a calibration is run a line of best fit (or calibration curve) is generated, based on the calibration parameters (fit type, weighting, through blank), for the standard solution measured intensities and defined (or "method") concentrations. This results in an equation for the line that can be used to calculate concentrations from intensity measurements. Once the calibration is complete the measured intensities for each of the calibration solutions are then used to calculate a concentration based on the equation for the line, resulting in the "calculated" concentration in the software. In an ideal world the measured points will exactly match the line of fit and the calculated concentration will be the same as the method or defined concentration, but this will rarely (if ever) be the case when the number of calibration points is greater than the minimum required for the particular fit type for a range of reasons. The difference between the "method" and "calculated" concentrations gives an indication of how well the line of fit matches each measured point. Assuming that there are no issues in the standard preparation and sample introduction, small differences or errors indicate that the line of fit is a good representation of the instrument response, while large errors would indicate that the line of fit is not a good representation of the response. Different fit types and parameters will produce different calculated concentrations and associated errors. Other issues in standard preparation, sample introduction components, selection of inappropriate concentrations and a number of other factors will also affect how well the calculated concentrations match the method concentrations.

blhahesy · Answer

A quadratic fit requires a minimum of 3 points, just as a linear fit requires a minimum of 2 points. In each of these cases the line of fit would pass through all of the points, so there would be no error in the calculated concentration and parameters such as weighting and forcing through blank would have no effect. For a quadratic fit the three points may be a blank and two standards, but the blank could be omitted of the fit is forced through the origin, i.e. the zero concentration and intensity point on the curve (which would be used as the 3rd point), and the same would apply to a linear fit. From memory, if only two points are supplied when a quadratic fit is selected in ICP Expert II it will automatically switch back to calculating a linear fit.

[ICP-OES] Method concentration vs calculated concentration - large errors

Top Replies