where : 



Cone. = Concentration in parts-per-billion (ng/g) of 

 target isomer or congener class. 

 Ac = Sum of integrated areas for the target 

 isomer or congener class; 



Qis = Quantity of internal standard in ng; 



Ais = Total integrated areas for the internal standard; 



W = Sample weight in g; and 



Rf = Response factor. 



Each pair of resolved peaks in the selected-ion-current 

 chromatograms was evaluated manually to determine if it met the 

 criteria for a PCDD or PCDF isomer. By examining each pair of 

 peaks separately, quantitative accuracy was improved over what is 

 obtained when all of the peaks in a selected chromatographic 

 window are averaged. When averaged data are used, it is possible 

 for pairs of peaks with high and low chlorine isotope ratios to 

 produce averaged data that meets the isotope ratio criterion. 

 For example, two pairs of peaks having chlorine isotope ratios of 

 0.56 and 0.96, both outside of the acceptable range, would have 

 an average ratio of 0.76. 



The criteria that were used to identify PCDD and PCDF 

 isomers were: 



(1) Simultaneous responses at both ion 

 masses; 



(2) Chlorine isotope ratio within ± 15% of the 

 theoretical value; 



(3) Chromatographic retention times within 

 windows determined from analyses of standard 

 mixtures ; 



(4) Signal-to-noise ratio equal to or 

 greater than 2.5 to 1. 



The 2,3,7,8-substituted PCDD/PCDF isomers and the octa- 

 CDD included the additional criterion that they coeluted within 

 ±2 seconds of their isotopically labelled analogs. 



A limit of detection (LOD) was calculated for samples 

 in which isomers of a particular chlorine congener class were not 

 detected. The formula used for calculating the LOD was: 



LOD (ng/g) = He x Qis x 2.5 

 His X W X Rf 



