6 Replies Latest reply on Aug 2, 2018 10:45 PM by to.a.stranger

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

    to.a.stranger

      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.

        • Re: [ICP-OES] Method concentration vs calculated concentration - large errors
          tmcsweeney

          It depends on the values that you have in the "weight", "volume", and "dilution" columns on the Sequence page. I'm assuming you have a 5110 ICPOES. Unadjusted concentration values are typically in ppm in the solution or ug/ml. This is then multiplied by the value for the sample in the "volume" column (ml), and divided by what is in the "weight" column (g). If there is a dilution factor, this value is multiplied by that factor. This results in an adjusted concentration in ppm in the original solid sample (ug/g). It also depends on the fit you have selected on the "Standards" page, whether linear, rational, or quadratic and whether you have selected weighted fit and whether you have forced through the blank or origin.

            • Re: [ICP-OES] Method concentration vs calculated concentration - large errors
              to.a.stranger

              Thank you for your reply. I’m using 700 series with ICP Expert II software. I kept the “weight” “volume” and “dilution” columns in the “Sequence” page blank. It appears that deleting outliers in the calibration curve decreases the discrepancy between the “method concentration” and “calculated concentration.” That, as well as changing the conditions in the “Standard” page.

                • Re: [ICP-OES] Method concentration vs calculated concentration - large errors
                  blhahesy

                  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.