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Internal Quality Assurance Requirements for the Analysis of Dioxins in Environmental Samples

Section 6: Limit of Detection

  1. The Method Detection Limit (MDL) for PCDD/PCDF analysis is defined as the smallest concentration of analyte in a sample extract that will produce clearly defined peaks with an acceptable chlorine isotope ratio, and with a signal-to-noise ratio equal to three for the molecular cluster ion exhibiting the poorer signal-to-noise ratio. Reported MDL must be corrected for surrogate recovery and is calculated as follows:

    MDL Equation: MDL = [3N * (A/H) * Qs] / (As * RRF * S)

    estimated noise level expressed as peak height;
    area/height ratio for the surrogate standard peak;
    amount of surrogate standard added;
    surrogate peak area;
    relative response factor, as defined in Section 5d;
    sample weight or volume.

  2. The noise level for each homologue group should be determined from the actual sample chromatograms. Sample chromatograms (or portions thereof) should be "magnified" if necessary so that noise levels can be assessed and measured. An example of noise determination is provided in Figure 2.

  3. If detection of any potential 2,3,7,8-substituted congeners is rejected solely due to an incorrect isotope ratio, reporting requirements are as follows:
    1. congener presence is reported as NDR, instead of ND, in the concentration column of Figure 3;
    2. a concentration value for the congener is calculated in the same manner as if the isotope ratio had been within specification and this value is reported, in brackets, in the Detection Limit columns (but is not included in the corresponding homologue-total value);
    3. in Figure 4, the ion(s) used for quantitation are bracketed; and
    4. the actual isotope ratio is recorded, in brackets, in the adjacent column.

  4. Recommended Method Detection Limits for various sample matrices are listed in Table 1.

  5. A standard solution at near-detection limit concentrations (i.e., LCS 1 in Table 4 or HCS1 in Table 5) should be analyzed daily to assess and verify instrumental detection limits.

Figure 2: Noise Determination
Noise Determination
Click to enlarge

Figure 3: Sample Data Sheet - Part I
Sample Data Sheet - Part I

Figure 4: Sample Data Sheet - Part II (for isomer-specific analysis)
Sample Data Sheet - Part II (for isomer-specific analysis)

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