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  • This is a continuation of the calibration curve example (1-1).

    • This is a continuation of Calibration Curves to Determine Concentration (1-1).

      In (1-1), the signal intensity of the unknown sample was located in the middle of the displayed calibration curve, and the concentration was obtained by substituting the signal intensity into the regression equation. This was done without any problem.

      In this example (1-2), you will learn how to handle the case where the signal intensity of an unknown sample exceeds the range of signal intensities obtained from a standard sample measurement.

       

      Use the same data as in (1-1) below.

      　　Standard sample   　    　Adjusted concentration　                  　Measurement results　

      ①　blank　　　　　 （NO3- concentration = 0）                          →　peak height =    1030

      ②　Standard sample（NO3- concentration = 0.002 mg L-1）      →　peak height =    3500

      ③　Standard sample（NO3- concentration = 0.02 mg L-1）        →　peak height =   36150

      ④　Standard sample（NO3- concentration = 0.10 mg L-1）        →　peak height =  163223

      ⑤　Standard sample（NO3- concentration = 0.20 mg L-1）        →　peak height =  330409

      ⑥　Standard sample（NO3- concentration = 0.50 mg L-1）        →　peak height =  814093

    • Fig. 1

    • Question 1-2)

      　An unknown sample (B) was measured and the signal intensity was 9.3 x 105. What should be done next? (The text of the question is at the bottom of this page. Until then, the text is an explanation of the process.)

       

      unknown sample (B)：signal intensity（peak height）of 9.3・105

      Substituting this signal intensity into the regression equation noted in the figure above,

      (Concentration of B) = 0.0016079+5.9215×（9.3・105）=  0.55 (mg L-1)


      　The concentration range of the signal intensity measured for the standard sample is exceeded. Is it OK to extend the regression line to the high concentration side? I am quite worried.


    • 【What to do when signal intensity beyond the range of the standard sample is obtained from an unknown sample (1)】

      ①　To ensure that the peak heights of the unknown samples fall within the range of the peak heights of the standard samples, we prepared standard samples ⑦ and ⑧ with even higher concentrations in an attempt to expand the concentration range covered by the calibration curve (straight line).

      The results of standard sample ⑦: 0.7 (mg L-1) and ⑧: 0.8 (mg L-1) were added to ⑦ and ⑧ in the figure below.

      Fig. 2


      　We expected that measuring the high concentration standard samples ⑦ and ⑧ would result in a plot on the extension of the previous calibration curve (gray dashed line), but as shown by the solid gray line in the above figure, the peak height was smaller than expected. This means that the peak height does not increase in proportion to the NO3- concentration in the range above 0.5 (mg L-1), no matter how much the concentration increases. In other words, it means that the signal intensity has reached a plateau.

      【Reasons why the signal intensity in ion chromatography reaches a plateau.】

      　In ion chromatography, there is an upper limit to the amount of ions that a column can hold, and the upper limit of quantitation is reached when the column saturates with ions. In such cases, it is necessary to dilute the analyzed sample so that the concentration does not exceed the upper limit that the column can retain. Unknown sample (B): The signal intensity (peak height) was 9.3・105, so the regression curve (solid gray line) in the figure indicates that the concentration is in the range of approximately 0.6 to 0.7 mg/L. If this unknown sample (B) is diluted twice, the concentration of the diluted solution will be approximately 0.3 to 0.35 mg/L, which falls within the range of the calibration curve (straight line). Thus, the experiment should proceed successfully by guessing the optimal dilution rate.

    • (Here is the text of Example (1-2))


      【What to do when signal intensity beyond the range of a standard sample is obtained from an unknown sample (2)】

      　If a signal intensity greater than the range of the standard is obtained from the unknown sample, dilute the unknown sample by an appropriate factor.

      　The unknown sample (B) was aspirated with a 25 mL whole pipette, placed in a 50 mL volumetric flask, and scalped up with water. The diluted sample was analyzed and a signal intensity of 527933 was obtained. Find the concentration of the unknown sample before dilution.

    • In the next course, you will learn what to do when the signal intensity of an unknown sample is small. This is often a more difficult decision to make.