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  • Example 1-3) This is a continuation of the calibration curve example 1-1) and 1-2) .

    • This is a continuation of Calibration Curves to Determine Concentration (1-1) and (1-2).

      In (1-1), the signal intensity of the unknown sample was in the middle of the displayed calibration curve, and the concentration could be quantified without any problem.

      In (1-2), we learned how to handle the case when the signal intensity of an unknown sample exceeds the range of signal intensities obtained in the standard sample measurement. We could have analyzed a standard sample with an even higher concentration or diluted the unknown sample with a higher concentration.

      In (1-3), you will learn what to do when the signal intensity of the unknown sample is as low as or lower than the lowest concentration in the standard sample measurement. This can be a difficult process.

       

      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 1-3)

      　An unknown sample (C) was measured and the signal intensity was 3.6 x 103. What should be done next?

      (Concentration of C) = 0.0016079+5.9215×10-7×（3.6・103）=  0.0037 (mg L-1)

      It is about the same as the signal intensity at the lowest concentration in the standard sample. Is this just barely enough?

      When I try to plot it on the calibration curve figure above, it overlaps with the origin (0) and is not visible. We need to zoom in on the low concentration range of the calibration curve to determine its validity.

      Substituting the signal intensity into the regression equation to obtain and report a concentration value leaves one uneasy.


      At any rate, I tried to zoom in.

      Fig. 2


      It looks like it is on the calibration line. I doubt it will be ok, but we need to be very careful.


    • Example of a model answer to question 1-3) ①

      　The peak height of (B) was 3.6・103 and was quantified as 0.0037 (mg L-1) from the regression equation.

      　The peak height in (B) seems reasonable since it is within the range of peak heights of the standard sample. However, even the blank sample, which is expected to have a concentration of zero, has a peak height (1030) that is so-so and large.

      　If a peak appears in a blank sample, there is likely some problem, such as contamination of NO3- in the water used for sample preparation, NO3- from the laboratory air, or a source of NO3- contamination in the analyzer line. To eliminate such concerns, you should always measure a blank sample several times and determine the variation (standard deviation).

      　(Normally, before analyzing an unknown sample, repeat measurements of a blank sample or standard should be performed to confirm its accuracy.)

       

      　Therefore, we prepared 11 blank samples (in this case, pure water), measured them by ion chromatography, and determined their mean and standard deviation.

       

      Measurement results of blank samples (11 samples)

      Peak height of blank sample：1020, 1500, 2000, 800, 1700, 1800, 1030, 1200, 1400, 1800, 1400

      Average of the peak heights of 11 measurements of a blank sample　　     　：1423
      Standard deviation (σ) of peak heights of 11 measurements of blank sample： 382

       

      In analytical chemistry, the limit of quantitation* is often set at 10 times the standard deviation of the blank measurement (10σ).

      However, the determination of the limit of quantitation can be based on various concepts.

      (One way to do this is to set the limit at 10σ of repeated measurements of a blank sample.)

      (* limit of quantitation are explained in detail in the next course.)

       

      　In this case, 10σ = 3820. The concentration corresponding to peak height = 3820 is 0.0039 (mg L-1). Therefore, the limit of quantitation for this analysis is 0.0039 (mg L-1). Concentrations lower than this are considered unreliable for quantitation due to the large variability in the measurement results.

      　Since the peak height of unknown sample (B) is 3600, the concentration of unknown sample (B) should be reported as "below the limit of quantitation (0.0039 mg L-1)".