VOCs analytical methods sampling

Automated analyzers may be used for continuous monitoring of ambient poUutants and EPA has developed continuous procedures (23) as alternatives to the referenced methods. Eor source sampling, EPA has specified extractive sampling trains and analytical methods for poUutants such as SO2 and SO [7446-11-9] sulfuric acid [7664-93-9] mists, NO, mercury [7439-97-6], beryUium [7440-41-7], vinyl chloride, and VOCs (volatile organic compounds). Some EPA New Source Performance Standards requite continuous monitors on specified sources. 

Diffusive samplers originally developed for workplace monitoring have been applied to a wide range of studies of VOCs in non-industrial indoor air through a modification of sampling times and analytical methods. The main types of... 

The methods that generally are used to remove volatile organic chemicals (VOCs) from biological samples for analysis are applicable to chlorobenzene. These include headspace analysis, purge-and-trap (gas stripping) collection from aqueous solutions or slurry samples, solvent extraction, and direct collection on resins. Headspace analysis offers speed, simplicity, and good reproducibility for a particular type of sample. However, partitioning of the analyte between the headspace and the sample matrix is dependent upon the nature of the matrix and must be determined separately for different kinds of matrices (Walters 1986). 

Soil gas surveys are widely used for delineating sites with VOC contamination. Even though soil gas data may be generated with the most definitive and accurate analytical method available, such as gas chromatography/mass spectrometry (GC/MS), they will always remain screening data due to uncertainty associated with the sample matrix. 

Soil samples with aromatic and halogenated hydrocarbon concentrations above 200 pg/kg or with gasoline concentrations above 5000 pg/kg are defined as samples with high VOC concentration levels. Because these concentrations exceed the calibration ranges of most analytical instruments, sample extracts must be diluted for proper quantitative analysis. EPA Method 5035 describes two sampling techniques for soil with high VOC concentrations ... 

Sixteen groundwater samples were collected and analyzed for VOCs (EPA Method 8260), lead (EPA Method 6010), and nitrite (EPA Method 354.1). The intended use of all data is quarterly monitoring. The SAP states the sampling and holding time completeness goal of 100 percent analytical completeness goal is 90 percent. 

Thermal Desorption Thermal desorption is an alternative GC inlet system particularly used for VOC analysis. However, the analytes subjected to thermal desorption must be thermally stable to achieve successful analysis. Otherwise, decomposition occurs. This technique is mainly used for determination of volatiles in the air. Such a methodology requires sample collection onto sohd sorbents, then desorption of analytes and GC analysis. Traditionally, activated charcoal was used as a sorbent followed by extraction with carbon disulfide. However, solvent desorption involves re-dilution of the VOCs, thus partially negating the enrichment effect. Therefore, the sampling method is to pump a sample of gas (air) through the sorbent tube containing certain sorbents in order to concentrate the VOC. Afterwards, the sample tube is placed in thermal desorber oven and the analytes are released from the sorbent by application of high temperature and a flow of carrier gas. Additionally, desorbed compounds are refocused in a cold trap and then released into the GC column. Such a two-step thermal desorption process provides a narrow chromatographic band at the head of the column. 

The analysis of VOCs in water and solid samples is complex due to the large number of compounds to be analyzed and because precaution has to be taken to provide accurate and reliable results. The use of P T-GC-MS is the most adequate for trace analysis of VOCs because limits of detection at the ng/L levels can be achieved and confirmatory analysis can be performed. Although P T is a well-established technique, several analytical parameters can be optimized to obtain high sensitivity and selectivity. Eor the specific target analytes, method optimization and quality assurance are necessary. The main drawbacks of this technique result from the fact that high purity gases are required and that the system can... 

The direct headspace (HS) technique has been used to determine VOCs in water samples. This method overcomes comphcations associated with the sample matrix and can be applied to a wide range of concentrations. HS requires little sample preparation. Salt is usually added to improve the partitioning into the gas phase and the sample is heated to temperatures of about 50-60 °C, which enhances the volatilisation of the analyte, increasing the efficiency of the extraction process and consequently the sensitivity. Typically, the HS is directly sampled with a p,L-lock valve-gastight syringe and injected into the GC. This method has been used for the determination of the isotope composition of MTBE, ETBE and TAME, reporting detection limits of 3-6 mgL for 5 C and 8-20 mgL for the 3 H [37-40]. 

In seawater systems, the oxidation method chosen for analysis of the organic carbon depends on the fraction (DOC, POC or VOC) of interest, the origin of the sample, and the type of information required. However, analytical results from different methods cannot be compared readily. By examining these various analytical methods for the measurement of the dissolved, particulate, and volatile fractions of the organic matter, the sources of the differences in the reported results may be better understood. 

The P T technique is recommended as an extraction technique for VOCs in several standard methods (Table 23.3). The US EPA has proposed different standard protocols for the analysis of volatiles in water using P T. These methods can be used for most of VOCs that have boiling points below 200°C and are insoluble or slightly soluble in water. The type of sample matrix being analyzed determines the implemented configuration of the extraction technique. The 500 series EPA methods are addressed to potable waters, whereas the 600 series refer to analysis of wastewaters. The analytical methods for determining hazardous waste are known as the 8000 series methods (US EPA SW-846). 

Given all of the problems associated with analytical methods for breath sampling and analysis, it is not surprising that to date the majority of research associated with breath analysis and PTR-MS has been limited to proof-of-principle trials which may not have been using ideal breath sampling procedures. Often these studies have also involved an insufficient number of patients and provided little detailed analysis. In-depth clinical trials to substantiate the effectiveness of VOCs as selective biological indicators of disease are required, but these are expensive and time-consuming. 

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