EPA Method 26

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EPA Method 26:

An integrated sample is extracted from the source and passed through a prepurged heated probe and filter into dilute sulfuric acid and dilute sodium hydroxide solutions which collect the gaseous hydrogen halides and halogens, respectively. The filter collects particulate matter including halide salts but is not routinely recovered and analyzed. The hydrogen halides are solubilized in the acidic solution and form chloride (Cl-), bromide (Br-), and fluoride (F-) ions. The halogens have a very low solubility in the acidic solution and pass through to the alkaline solution where they are hydrolyzed to form a proton (H+), the halide ion, and the hypohalous acid (HClO or HBrO). Sodium thiosulfate is added in excess to the alkaline solution to assure reaction with the hypohalous acid to form a second halide ion such that 2 halide ions are formed for each molecule of halogen gas. The halide ions in the separate solutions are measured by ion chromatography (IC).

This method is applicable for determining emissions of hydrogen halides (HX) [HCl, HBr, and HF] and halogens (X2) [Cl2 and Br2] from stationary sources when specified by the applicable subpart. Sources, such as those controlled by wet scrubbers, that emit acid particulate matter must be sampled using Method 26A.

The official copy of this method is available at 40 Code of Federal Regulations (CFR) 60, Appendix A. The CFR can be accessed on-line at http://www.gpoaccess.gov/cfr/index.html

(A) Volatiles. Volatile materials, such as chlorine dioxide (ClO2) and ammonium chloride (NH4Cl), which produce halide ions upon dissolution during sampling are potential interferents. Interferents for the halide measurements are the halogen gases which disproportionate to a hydrogen halide and a hydrohalous acid upon dissolution in water. However, the use of acidic rather than neutral or basic solutions for collection of the hydrogen halides greatly reduces the dissolution of any halogens passing through this solution.(B) Cross-analyte bias. The simultaneous presence of HBr and Cl2 may cause a positive bias in the HCL result with a corresponding negative bias in the Cl2 result as well as affecting the HBr/Br2 split.(C) Nitrogen Oxide. High concentrations of nitrogen oxides (NOx) may produce sufficient nitrate (NO3-) to interfere with measurements of very low Br- levels.(D) Filtering. A glass wool plug should not be used to remove particulate matter since a negative bias in the data could result.(E) Teflon. There is anecdotal evidence that HF may be outgassed from new teflon components. If HF is a target analyte, then preconditioning of new teflon components, by heating should be considered.

QC Requirements:
(A) Audit sample analysis -- This is done to evaluate analytical technique, preparation of standards.(B) Leak Check: A leak-check prior to the sampling run is optional; however, a leak-check after the sampling run is mandatory. Also, it is suggested (not mandatory) that the pump be leak-checked separately, either prior to or after the sampling run. Instructions on leak checks are provided in Section 8.1.3 of the method.Temperature: Adjust the probe temperature and the temperature of the filter and the stopcock, i.e., the heated area in Figure 26-1 to a temperature sufficient to prevent water condensation. This temperature should be at least 20?C (68?F) above the source temperature, and greater than 120?C (248?F). The temperature should be monitored throughout a sampling run to ensure that the desired temperature is maintained. It is important to maintain a temperature around the probe and filter of greater than 120?C (248?F) since it is extremely difficult to purge acid gases off these components.(D) Duplicate Injection Agreement: Between injections of the appropriate series of calibration standards, inject in duplicate the reagent blanks, quality control sample, and the field samples. Measure the areas or heights of the Cl-, Br-, and F- peaks. Use the mean response of the duplicate injections to determine the concentrations of the field samples and reagent blanks using the linear calibration curve. The values from duplicate injections should agree within 5 percent of their mean for the analysis to be valid. If the values of duplicate injections are not within 5 percent of the mean, the duplicate injections shall be repeated and all four values used to determine the average response. Dilute any sample and the blank with equal volumes of water if the concentration exceeds that of the highest standard.

Maximum Holding Time:
4 weeks (Cl- samples)



> 0.1 ppm (HCl and Cl2)

The within-laboratory relative standard deviations are 6.2 and 3.2 percent at HCl concentrations of 3.9 and 15.3 ppm, respectively. The method does not exhibit a bias to Cl2 when sampling at concentrations less than 50 ppm.

A typical IC instrumental detection limit for Cl- is 0.2 ug/ml. Detection limits for the other analyses should be similar. Assuming 50 ml liquid recovered from both the acidified impingers, and the basic impingers, and 0.06 dscm of stack gas sampled, then the analytical detection limit in the stack gas would be about 0.1 ppm for HCl and Cl2, respectively.

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