Volume 1, Number 12
A Gossman Consulting, Inc. Publication
December 1995


Craig Cape

GCI has recently become aware of a potentially controversial interpretation in data reporting for dioxin results. This interpretation involves the way report managers treat certain analytical data in the calculation of dioxin TEQ values. The question is, what is the correct way of handling a reported estimated maximum possible concentration (EMPC) value for a TEQ calculation. One interpretation is to zero the EMPC or, in essence, not include it for calculating the TEQ. This practice would occur presumably because the EMPC value falls below the normal QA/QC quantitative reporting limits. This interpretation is consistent with a recent (January, 1995) draft of SW-846 Method 8280A. However, GCI has had recent (November, 1995) verbal discussions with USEPA personnel who believe that not including the actual EMPC value when calculating a TEQ value is wrong. The draft Method 8280A was not specifically discussed with the EPA personnel at that time. But, the interpretation given by the EPA staff person indicated that the EMPC value as given should be used for TEQ calculation. This obviously differs with the current draft 8280A method.

There are several ways that dioxin analysis results can be reported. One method of PCDD/PCDF (D/F) reporting is the use of a toxic equivalent concentration (TEQ) calculation. This method assigns a 2,3,7,8-TCDD toxicity equivalence factor (TEF) to each of the seventeen 2,3,7,8- substituted TCDD/TCDFs shown in table 1.

Table 1

2,3,7,8-TCDD Toxicity Equivalent Factors (TEFs) for the Polychlorinated Dibenzodioxins and Dibenzofurans



1 2,3,7,8-TCDD 1.00
2 1,2,3,7,8-PeCDD 0.50
3 1,2,3,6,7,8-HxCDD 0.10
4 1,2,3,7,8,9-HxCDD 0.10
5 1,2,3,4,7,8-HxCDD 0.10
6 1,2,3,4,6,7,8-HpCDD 0.01
7 1,2,3,4,6,7,8,9-OCDD 0.001
8 2,3,7,8-TCDF 0.1
9 1,2,3,7,8-PeCDF 0.05
10 2,3,4,7,8-PeCDF 0.5
11 1,2,3,6,7,8-HxCDF 0.1
12 1,2,3,7,8,9-HxCDF 0.1
13 1,2,3,4,7,8-HxCDF 0.1
14 2,3,4,6,7,8-HxCDF 0.1
15 1,2,3,4,6,7,8-HpCDF 0.01
16 1,2,3,4,7,8,9-HpCDF 0.01
17 1,2,3,4,6,7,8,9-OCDF 0.001

The TEQ is calculated by summing for all seventeen, the product of the reported concentration for each compound and its TEF listed in the table. The purpose of this calculation is to provide a single value normalized to the toxicity of 2,3,7,8-TCDD that is easier to discuss and use in regulatory decision making.

A confusion in applying this relatively simple procedure comes about when deciding what to do about reported concentrations above detection limits, yet below quantitation levels or not meeting QA/QC. In SW-846 Method 8280A revision 1, January 1995, the method states, "Do not include EMPC or EDL values in the TEQ calculation" which seems very straight forward. However, in the same section the method also states, "... include only those 2,3,7,8 substituted isomers that were detected in the sample and met all qualitative identification criteria...." Possible D/F isomers reported at EMPC levels may meet some qualitative criteria, but not all. In essence, they may or may not be there, but from a method stand point, they are not counted for the calculation of TEQs.

GCI contacted USEPA concerning the treatment of EMPC values. What GCI learned is that the term EMPC is not an official term, at least under Method 23. The term was reportedly created by Triangle Labs to indicate the detected presence of a compound above zero but not meeting QA/QC reporting level criteria. EPA confirmed the EMPC may be conservatively high because it does not meet the usual high degree of QA/QC. However, the EPA staff person concluded that the compound is none-the-less there at a level above zero. The discussion with the EPA representative elicited the opinion that counting an EMPC value as zero would be technically incorrect and could not be justified. They offered that another option would be to reanalyze the sample to eliminate interferences and improve reporting limits.

GCI contacted Triangle Labs to discuss this issue further. They confirmed that EMPC was a term created by them to describe situations where a gas chromatographic (GC) peak had a similar identification to a D/F compound peak. However, at least one of the definitive QA/QC identification criteria from the method was not met. Therefore, the peak could not be confirmed as a D/F isomer as per the method. Since the peak was still there, Triangle Labs felt it should be reported and thus the term, EMPC, was created. For this method, the most common criteria which is not met for a peak in question is that of the required signal to noise ratio. A simpler analysis example of this is when a result is above the detection limit, but below the quantitation limit. There are other causes of criteria failure such as interfering peaks or high background noise. Whatever the cause, the peak in question did not meet all of the method identification criteria and was thus listed as an EMPC.

GCI would like to caution those who seek to use or explain dioxin TEQ results calculated with EMPC values. The verbal opinions received from EPA differ from the draft method. From a more conservative perspective, it may be argued to not correct EMPC values to zero in the calculation of TEQs. However, it should be noted that evaluations using TEQs usually contain many conservative assumptions in their models already. If you have TEQs calculated using this method, it may be beneficial to verify how EMPC values were handled. For future analysis, GCI suggests that working with your analytical lab to try to resolve the EMPC values analytically does make sense. Reanalysis with efforts to reduce interferences may help improve results falling into the reporting category of EMPC. It is also advisable to carefully consider how these results are to be used when reported. If EMPC values were zero for the TEQ calculation, an EPA perception that the calculations were incorrect would be important even if calculations are correct by the method. An EPA opinion of incorrectness may not only affect the EPA acceptance, but also the public perception of the results. The last communication GCI had with EPA on this issue was to provide a copy of the revised Method 8280A to them. To date, no further discussion or response has been received.

As a final note, GCI has published a useful guidance manual titled "Understanding Dioxin Testing". It is available here.