Metodo epa 18

1048 METHOD 18 - MEASUREMENT OF GASEOUS ORGANIC COMPOUND EMISSIONS BY GAS CHROMATOGRAPHY NOTE: This method is not inclusive with respect to

specifications (e.g., equipment and supplies) and procedures (e.g., sampling and analytical) essential to its performance. Some material is incorporated by reference Therefore, to obtain

from other methods in this part.

reliable results, personsusing this method should have a thorough knowledge of at least the following additional test methods: Method 1, Method 2, Method 3. NOTE: This method should not be attempted by persons

unfamiliar with the performance characteristics of gas chromatography, nor by those persons who are unfamiliar with source sampling. Particular care should be exercised in the

area of safety concerning choice ofequipment and operation in potentially explosive atmospheres. 1.0 Scope and Application. 1.1 1.2 1.2.1 Analyte. Total gaseous organic compounds.

Applicability. This method is designed to measure gaseous While designed

organics emitted from an industrial source.

for ppm level sources, some detectors are quite capable of detecting compounds at ambient levels, e.g., ECD, ELCD, and heliumionization detectors. Some other types of detectors

1049 are evolving such that the sensitivity and applicability may well be in the ppb range in only a few years. 1.2.2 This method will not determine compounds that

(1) are polymeric (high molecular weight), (2) can polymerize before analysis, or (3) have very low vapor pressures at stack or instrument conditions. 1.3 Range. The lower range ofthis method is

determined by the sampling system; adsorbents may be used to concentrate the sample, thus lowering the limit of detection below the 1 part per million (ppm) typically achievable with direct interface or bag sampling. The upper limit is

governed by GC detector saturation or column overloading; the upper range can be extended by dilution of sample with an inert gas or by usingsmaller volume gas sampling loops. The upper limit can also be governed by condensation of higher boiling compounds. 1.4 Sensitivity. The sensitivity limit for a

compound is defined as the minimum detectable concentration of that compound, or the concentration that produces a signal-to-noise ratio of three to one. The minimum

detectable concentration is determined during the presurveycalibration for each compound. 2.0 Summary of Method.

1050 The major organic components of a gas mixture are separated by gas chromatography (GC) and individually quantified by flame ionization, photoionization, electron capture, or other appropriate detection principles. The

retention times of each separated component are compared with those of known compounds under identical conditions.Therefore, the analyst confirms the identity and approximate concentrations of the organic emission components beforehand. With this information, the analyst then

prepares or purchases commercially available standard mixtures to calibrate the GC under conditions identical to those of the samples. The analyst also determines the need

for sample dilution to avoid detector saturation, gas streamfiltration to eliminate particulate matter, and prevention of moisture condensation. 3.0 4.0 Definitions. Interferences. 4.1 Resolution interferences that may occur can be [Reserved]

eliminated by appropriate GC column and detector choice or by shifting the retention times through changes in the column flow rate and the use of temperature programming. 4.2 The analytical system is demonstrated to beessentially free from contaminants by periodically analyzing blanks that consist of hydrocarbon-free air or nitrogen.

1051 4.3 Sample cross-contamination that occurs when

high-level and low-level samples or standards are analyzed alternately is best dealt with by thorough purging of the GC sample loop between samples. 4.4 To assure consistent detector response, To adjust

calibration...