Method IO-4.2

Determination of Reactive Acidic and Basic Gases and Strong Acidity of Atmospheric Fine Particles (<2.5um)

This method is from EPA's Inorganic Compendium of methods and is for the determination of reactive acidic and basic gases (as SO2, HNO3 and NH3)and strong acidity of fine particulates (<2.5um) collected  from ambient air sources. Acidity is determined using pH meter and the ions by ion chromatography (IC).  Note that this is a draft method.

A 47mm, 2um pore size Teflon® filter and 47mm, 1um pore size Nylsorb® filter are needed for sample collection.  The sample is collected by using a Annular Denuder System (ADS) per IO-4.1.  The ADS system consists of an inlet with an impactor or cyclone pre-separator to remove all particles with an aerodynamic diameter greater than 2.5um, two annular denuders in series and the filters.  Air is drawn through the sampler at 10 L/min. usally for a collection period of 24 hours.  The placement of the ADS varies according to the site location.  Please see Section 14.1.4 of the method for specifics.

After sample collection, the ends of the ADS are capped with a polyethylene cap and sent to the laboratory for extraction and analysis.  The filters must be unloaded in an ammonia-free atmosphere so use of a glove box is required.  The method states that the samples should be analyzed as soon as possible after extraction.  The pH meter is calibrated using sulfuric acid standards which should be prepared on the day of analysis.  The acidity of the sample extract is measured using a pH meter. 

The denuders are extracted and analyzed with the filters extracts by IC to determine ions (SO2, HNO3 and NH3) in gaseous form from the denuders and ions in particulate form (SO4 and NO3) from the filters.

(EPA 625/R-96/010A)


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Method Data

Hold Times, Preservatives, Preps, Collection, Analytical & Documentation
Holding Time:   Analyze samples as soon as possible after collection per the method and recommended to analyze samples within 30 days of extraction.
Preservatives:   Store extracts at 4°C in the dark.
Required Preps:   47mm, 2um pore size Teflon filter and 47mm, 1um pore size Nylasorb filter
Collection Method:   Annular Denuder system per Compendium Method IO-4.2 procedures.
Analytical Methodology:   pH meter and IC
Documentation:   IO-4.2

Analyte List*

Analyte Formula CAS Number Detection Limit
Sulfur dioxide
Nitric acid
 pH unit

* The analytes and detection limits listed for each method represent the typical detection limits and analytes reported for that particular method. Keep in mind that analyte lists may vary from laboratory to laboratory. Detection limits may also vary from lab to lab and are dependent upon the sample size, matrix, and any interferences that may be present in the sample.