4.3.2. Analytical Operation
-Samples
After exposure to the test environment, the stainless screens and cellulose fiber
filter are put into a 25 ml glass vial, containing 8ml water. Use separate glass
vials for NO2 and NOx elements. Vials should be stoppered, per accepted laboratory
practice.
Over the next 30minutes, occasional shaking of the vials is recommended. Do not
use ultrasonic bath.
At the end of the first 30 minute period, vials are cooled to 2-6' C, and 2 ml of
color producing reagent are added. SHAKE QUICKLY, after adding the color producing
reagent, and continue to keep vials cool for an additional 30 minutes.
Vials are then allowed to equilibrate at room temperature, for about 20 minutes,
and the amount of colored derivative is determined with a spectrophotometer at a
wavelength of 545 nm.
Unexposed elements are put through the same procedure to obtain a blank value
determination.
-Standard Solutions
From the previously prepared nitrite working standard solutions, (those containing
0 - 1.0/rg nitrite/ml), mix 8 ml of each of the nitrite working standards with 2
ml of color reagent to prepare a standard curve. Chill standard solution, like
samples, before adding color reagent.
-Supplement
It is strongly recommended that separate pipettes be used for nitrite standard
stock solution from those used for preparing nitrite working standard solutions.
When the exposed NOx element is placed in a vial with 8 ml of water, nitrite ion
is produced. When 2 ml of the color producing reagent is added, and the reaction
is allowed to take place at 2-6' C, the amount of colored derivative produced is
proportional to the amount of original nitrite. If the reaction takes place at
warmer temperatures, the amount of color derivative produced is not consistent.
It is recommended to use at least 3 blanks for each lot analyzed.
-Procedures which help insure accurate analysis results.
1. Screens and/or filter pads should be added to vials already containing a
solution, never into a dry vial.
2. Vials must be shaken immediately after adding color reagent.
3. Cooling must be done according to the protocol.
-Flow Injection Method
This method is very useful where the analysis of large numbers of samples is
required. It is a very simple and accurate method, minimizing deviations caused
by elapsed time, temperature, and other variables. The results, using this method,
have high precision.
4.3.3. Calculation Of Concentration
NO concentration (ppb) =alpha NO × (WNOx-WNO2)/ t
NO2 concentration (ppb) =alpha NO2 × WNO2/ t
Where :
WNOx and WNO2 are the nitrite quantities (in micro g)
collected on the NOx
and NO2 elements respectively, determined using the standard curve.alpha NO and alpha NO2 are the ppb concentration coversion coefficients (ppb min/micro g), assuming 20deg.C, 70% RH, with values respectively of 60 and 56.
t = Sample collection time (min.)
To compensate for sampling conditions different from 20deg.C and 70% RH.
