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Robust Method for Analyzing Organic Chemicals in a Wide Array of Samples

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Confidence intervals

The confidence intervals that are reported with results have been shown to be reliable for water and soil.  In these studies those quality controls that are to be met to ensure the viability of confidence intervals were found experimentally.
 

Internal standards friend or foe?

The article, The Role of Internal Standards and their Interaction with Soils Impact Accuracy of Volatile Organics Determinations found that the addition of internal standards to soils described in SW-846 as being "immediately before analysis" could lead to understating the concentrations of analytes. This negative bias is a result of an analyte being in equilibrium with a matrix due to a previous exposure and the internal standard being just added to the soil (plus water). Depending on the compound and the organic content of the sample this bias can vary from minimal to severe.

It was also found in the publication "Internal standards: A source of analytical bias for volatile organic analyte determinations" that calibration errors and analyte result bias are related to the internal standards identified in Method 8260. Both calibration error and analyte bias increase as the differences between internal standard and analyte increase (relative volatility and boiling point).

Introduction
This field is an area that has been studied as a means to extract volatile chemicals from most media. This approach was initially investigated for a group of analytes that are commonly known as volatile organic compounds (VOCs) or sometimes as volatile organic analytes (VOAs). The VOCs include some of the most common organic compounds that are used by industry and include common solvents and petroleum constituents.

Vacuum distillation is a technique that uses a vacuum to extract analytes from a sample. A condensing column is used in the technique to condense water that may also be present in the sample. A cryogenically cooled trap (cryotrap) is used to condense the compounds of interest. During a vacuum distillation procedure a sample is evacuated to a low pressure vaporizing volatile compounds, including water. The condenser column retains vaporized water and those vaporized compounds that are not condensed in the column are frozen in the cryotrap. The extract retained in the cryotrap are then transferred (sweeping the cryotrap with a carrier gas while heating the cryotrap) to another instrument for detection and quantification of the components in the extract. An overview of the vacuum distillation chemistry is in the training presentation, An Overview of SW-846 Method 8261 Chemistry [PDF, 46 pp., 557K], and more elaborate explanation of the technique is available in publications.

Apparatus

Vacuum distillation uses a specialized apparatus. This apparatus has been developed and patented by the EPA. Through the Federal Technology Transfer Act this invention has been made available for commercialization. Available vendors Exiting EPA Disclaimer for this instrumentation are being evaluated.

Vacuum distillation Methods
’s methods manual SW-846.  Method 5032 describes the use of vacuum distillation as a sample preparation procedure for use with GC/MS Method 8260 for quantitation.

It has been shown that the recovery of compounds during a vacuum distillation relates to a compound’s vapor pressure (we use boiling point) and how it partitions between a matrix and the vapor state (relative volatility) for water, soil, and oil, fish and vegetation.  A second SW-846 method, Method 8261A, combines vacuum distillation and unique GC/MS quantitation steps.  The quantitation incorporates boiling point and relative volatility to assess the recovery of compounds improving the determination of compounds in matrices.   The calculations described in Method 8261A are automatically performed using software provided by the EPA.

Method 8261A quantitation reports presents data not available in other protocols.  This data includes compound results with confidence intervals (includes option to propagate calibration curve error) and matrix effects shown graphically.  The data include all information necessary to validate results as shown in the report Verification of Analytical Reports produced by SMCReporter for Method 8261 Determinations.  This data is so complete in describing how a matrix affects a compound’s recovery that matrix spikes/matrix spike duplicates that are normally required for RCRA methods are not required for Method 8261A.

The use of boiling point and relative volatility to determine analyte recovery for quantitation differs from Method 8260A where the recovery of a compound is assumed to be the same as its assigned internal standard.  The publication Internal Standards: A source of analytical bias for volatile organic analyte determinations describes the bias associated with using a single internal standard to quantify analytes for a variety of matrices.

 


 

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Environmental Sciences | Research & Development
National Exposure Research Laboratory
Author:  Mike Hiatt
Email:  Hiatt.Mike@epa.gov
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