December 21, 2009 

Mr. Kevin Lewis 

Air Sciences 

Re: Barrick Cortez Hydrogen Cyanide (HCN) Results 

Dear Mr. Lewis: 

The purpose of this letter is to comment on the validity of the HCN testing results provided by 

Method 320 and CTM-033 from the recent emissions testing at the Barrick Cortez facility on the 

exhaust stacks of the two Electrowinning (East and West) stacks, and the Preg/Barren Tank 

stack. Method 320 uses a Fourier Transform Infrared (FTIR) spectrometer to provide on-site, 

real-time HCN concentration data. CTM-033 collects HCN in a basic solution for subsequent offsite ion analysis. 

As you are aware, the two Electrowinning stacks and the Preg/Barren Tank were tested for HCN 

in October using FTIR. In November the Electrowinning stacks were re-tested for HCN using 

CTM-033. The results of the November testing were significantly lower than the October results, 

and the question of which data set is correct is of interest. 

Method 320 is known to be susceptible to interference from other gaseous species that absorb 

infrared light. The degree of interference is a function of the concentrations of interfering species

and the absorbance overlap between the interferents and the target analyte. In the current case, 

known and detected interfering species consisted of water vapor (H2O), ammonia (NH3) and 

carbon dioxide (CO2). The FTIR instrument attempts to correct the target analyte concentration 

for known interfering species. The instrument then determines the influence of the interferents by 

calculating an SEC (standard error calculation). This SEC value is statistically determined by the 

FTIR instrument, and is calculated for each stack matrix. A conservative rule of thumb with FTIR 

data is to use two times the SEC as an estimated uncertainty on results. If this approach is used 

on the Cortez data, the results demonstrate very large relative uncertainties on the HCN 

measurements, and consequently low confidence in the FTIR results as shown below. 

Barrick Cortez FTIR Data

 HCN (ppm) CO2 (ppm) NH3 (ppm) H2O (ppm) (1)

Electrowinning West 4 +/- 3 314 +/- 368 95 +/- 2 24,000 

Electrowinning East 5 +/- 9 137 +/- 11 144 +/- 3 37,000 

Preg/Barren Tank 54 +/- 55 103 +/- 9 635 +/- 45 -- (no data) 

1 – H2O data taken from EPA Method 4 measurements – FTIR data are used for spectral 

subtraction, but do not provide quantitative results. 

The figures on the following pages provide various FTIR spectra, including stack gas 

measurements and various standards. The sample spectra are provided to show the differences 

between species with very clear identification (CO2, NH3 and H2O) and less conclusive 

identification (HCN). 
Stack Matrix EW West (red - bottom) Stacked under CO2 Standard (pink - top) 

Close-up of Stack Matrix (red - top) Overlaid with CO2 Standard (pink - bottom). 

For carbon dioxide, elements of the reference spectrum are easily observed in the stack matrix, 

including a close-up view of the region used to quantify the stack levels. 
Stack Matrix EW East (red - bottom) Stacked under NH3 Standard (blue - top) 

Close-up of Stack Matrix (red - top) Overlaid with NH3 Standard (blue - bottom) 

Similarly for ammonia, elements of the reference spectrum are easily observed in the stack 

matrix, including a close-up view of the region used to quantify the stack levels. 
Stack Matrix (red - bottom) Stacked under H2O Standard (pink - top) 

Close-up Stack Matrix (red – shorter peaks) and H2O Standard (pink – taller peaks) 

As with carbon dioxide and ammonia, elements of the water reference spectrum are easily 

observed in the stack matrix, including a close-up view. 
Stack Matrix EW West (red - bottom) and HCN Standard (blue - top) 

Close-up of Stack Matrix (red - top) Overlaid with HCN Standard (blue - bottom) 
Extreme Close-up of Stack Matrix (red – top) Over HCN Standard (green - bottom) 

The final three spectra show the FTIR stack data used to estimate HCN levels. The zoomed in 

figure shows the lack of obvious agreement between the stack data and standard. While there 

are clearly common peaks in the stack data and reference standard, there are obvious nearby 

peaks in the stack spectra indicating some other unidentified species with IR absorption in the 

same region that is used to quantify the HCN. This uncertain agreement is reflected in the high 

SEC for the HCN FTIR data. 

As with Method 320, and any measurement method, CTM-033 is also subject to potential 

interferences. The interferences provided in CTM-033 consist of acid gases which would reduce 

the impinger pH, sulfides, oxidizing agents, and blank contamination. The absence of acid gases 

was confirmed by checking the impinger pH after each test run. No sulfides or oxidizing species 

are suspected, and the reagent and train blanks showed non-detected cyanide for all samples. 

So interferences with the CTM-033 results are not expected. 

Because of the demonstrated interference issues with the Method 320 data, and the apparent 

lack of similar concerns with the CTM-033 data, it is believed that the FTIR results are subject to 

substantially higher uncertainty than the CTM-033 data. Consequently, the CTM 033 data are 

believed to be the more representative results, effectively invalidating dissimilar FTIR results. 

Based on this analysis, APT has prepared a final report for the CTM 033 test data only, which has 

already been delivered to Barrick Cortez. The summary sheets for the Method 320 data are 

attached. 

Respectfully, 

Paul Ottenstein 

Technical Director