Table 2. November '97 South

 

SEA FTIR Summary Table for November '97 South Fenceline

 

 Target Compound  

 # confirmed detects

 max conc. (ppm)

 avg conc. (ppm)

 min conc. detected

 avg. detect limit

 min detect limit

 max detect limit

 # false positives
 MTBE    862  0.059  0.032  0.015  0.032  0.008  0.050  0
 MTBE (Tosco's #s)    3  0.076  0.074  0.072  0.072  0.039  0.115  0
 2-methyl pentane    341  0.167  0.047  0.015  0.039  0.009  0.199  0
 3-methyl pentane    63  0.181  0.132  0.025  0.052  0.015  0.166  0
 n-heptane    6  0.037  0.025  0.017  0.026  0.010  0.084  0
 propane    19  0.051  0.039  0.028  0.144  0.009  0.512  0
 propane (Tosco's #s)    0  0  0  0  0.322  0.162  3.576  0
 carbonyl sulfide    0  0  0  0  0.017  0.003  0.237  0
 carbonyl sulfide (Tosco)    0  0  0  0  0.030  0.010  0.213  0
             
               
                   

 

 

About The Detection Limits

In FTIR spectroscopy of this type, there are no "set" detection limits for any particular chemical. In each absorbance spectrum (i.e. each 5-minute data point) a Minimum Detection Limit (MDL) is calculated for every target compound which is not detected. The figures in the above table represent the best, average, and worst detection limits reported for each compound during the month. (min. = best) Many factors affect these reported detection limits. Poor detection limits can be the result of inclement weather conditions, poor instrument performance, or problems with the analytical method used to detect any given chemical ( i.e., problems with the "script file").

One of the advantages of the FTIR method, is that it allows for post-analysis of the raw spectral data which is generated during the "real-time" monitoring. In the process of analyzing these infrared spectra, we at SEA do not face many of the constraints inherent in working with this data in real-time. It is both typical and expected that we would be able to achieve better detection limits (lower MDLs) in this type of post-analysis of the raw FTIR data. By comparing the MDL figures reported in Tosco's FTIR data summaries to those we report, we can begin to make determinations as to how good a job Tosco is doing with the real-time monitoring of various compounds.