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An EMPC is defined in section 11.9.5.2 of Method 8290 as the following;

11.9.5.2 Estimated Maximum Possible Concentration -- An estimated maximum possible concentration (EMPC) is calculated for 2,3,7,8-substituted isomers that are characterized by a response with an S/N of at least 2.5 for both the quantitation ions, and meet all of the identification criteria in Sec. 11.8.4 except the ion abundance ratio criteria or when a peak representing a PCDPE has been detected. An EMPC is a worst-case estimate of the concentration.

1) The retention time must be within -1 to +3 seconds of the corresponding isotopically labelled internal standard.

2) All ion current intensities must be > to 2.5 times the background noise.

3) The integrated ion currents for the two quantitation ions must have a ratio to each other that is between the upper and lower established limits.

For example;

Natural Abundance refers to the abundance of isotopes of a chemical element as naturally found on the planet. In our dioxin/furan analysis we use the two naturally occurring isotopes of chlorine, those being ³⁵Cl and ³⁷Cl. ³⁵Cl has a mass of 34.9688 and a natural abundance of 77.8%. ³⁷Cl has a mass of 36.9659 and a natural abundance of 24.2%.

If we use 2,3,7,8-TCDF as an example – the ³⁵Cl isomer of 2,3,7,8-TCDF (C₁₂H₄³⁵Cl₄O) has a mass of 303.9016 and the ³⁷Cl isomer of 2,3,7,8-TCDF (C₁₂H₄³⁵Cl₃³⁷ClO) has a mass of 305.8987.  Identification criteria #3 above translates to a theoretical ion abundance ratio of 0.77 for the two 2,3,7,8-TCDF isotopes (C₁₂H₄³⁵Cl₄O & C₁₂H₄³⁵Cl₃³⁷ClO). The ratio of these two ions must fall between 0.65 and 0.89 in order for the peak to be considered a “positive”. Keeping in mind that the other two criteria must also be met. 

As a result we identify the analyte as EMPC / NDR – Peak detected does not meet ratio criteria and has resulted in an elevated detection limit where NDR stands for Non Detect Ratio. This translates into the worst-case scenario of the concentration.

DPE is defined as Polychlorinated Diphenyl Ethers.

11.8.4.4 Polychlorinated diphenyl ether interferences 

In addition to the above criteria, the identification of a GC peak as a Furan can only be made if no signal having a S/N > 2.5 is detected at the same retention time (± 2 sec) in the corresponding polychlorinated

diphenyl ether (PCDPE) channel.

For example, if the Hexachlorodiphenyl ether (C₁₂H₄³⁵Cl₅³⁷ClO) loses 2 chlorine atoms it will have the same molecular mass as the 2,3,7,8-Tetrachlorodibenzofuran (C₁₂H₄³⁵Cl₃³⁷ClO). 

As such the de-chlorinated DPE interferes with the quantitation of the 2,3,7,8-TCD Furan and will result in a raised detection limit.

Note that both NDR or DPE are used in conjunction with EMPC to signify the reason for the Estimated Maximum Possible Concentration as either elevated due to a ratio failure or a DPE interference.

EMCL is defined as Estimate Maximum Concentration Level. This is when a sample is, for the case of Dioxin and Furans, diluted beyond 20 times and the analyte is still above the concentration of the highest calibration standard. Since the analysis is done by isotope dilution no further dilution of the sample can be done as the level of the internal standard is so low that it cannot be used in the isotope dilution scheme.