The main objective of the World Coal Quality Inventory (WoCQI) was to collect and analyze a global set of samples of mined coal during a time period from about 1995 to 2006 (Finkelman and Lovern, 2001). Coal samples were collected by foreign collaborators and submitted to country specialists in the U.S. Geological Survey (USGS) Energy Program. However, samples from certain countries, such as Afghanistan, India, and Kyrgyzstan, were collected collaboratively in the field with USGS personnel. Samples were subsequently analyzed at two laboratories: the USGS Inorganic Geochemistry Laboratory located in Denver, CO and a commercial laboratory (Geochemical Testing, Inc.) located in Somerset, PA. Thus the dataset, which is in Excel (2003) format and includes 1,580 samples from 57 countries, does not have the inter-laboratory variability that is present in many compilations. Major-, minor-, and trace-element analyses from the USGS laboratory, calculated to a consistent analytical basis (dry, whole-coal) and presented with available sample identification information, are sorted alphabetically by country name. About 70 percent of the samples also have data from the commercial laboratory, which are presented on an as-received analytical basis.
The USGS initiated a laboratory review of quality assurance in 2008, covering quality control and methodology used in inorganic chemical analyses of coal, coal power plant ash, water, and sediment samples. This quality control review found that data generated by the USGS Inorganic Geochemistry Laboratory from 1996 through 2006 were characterized by quality practices that did not meet USGS requirements commonly in use at the time. The most serious shortcomings were (1) the adjustment of raw sample data to standards when the instrument values for those standards exceeded acceptable limits or (2) the insufficient use of multiple standards to provide adequate quality assurance.
In general, adjustment of raw data to account for instrument 'drift' is an acceptable practice within strictly defined limits. During the denoted period, USGS required that the maximum adjustment of instrument values, guided by calibration standards, was not allowed to exceed 10 percent. However, in some cases, the Inorganic Geochemistry Laboratory released data that were adjusted by more than 10 percent and (or) were not constrained by an adequate number of control standards. Original instrument values no longer exist for about 80 percent of the analyses during this period; therefore, the acceptability of drift corrections for most of the samples analyzed cannot be determined. For these reasons, the WoCQI data from the USGS Inorganic Geochemistry Laboratory should be used with care. For more information, individuals may contact laboratory management at EnergyLabs@usgs.gov with specific questions about particular datasets or analytical attributes.
Standard USGS sampling methods were provided and recommended to collaborators, but the analyzed samples may or may not be representative of their locale; for some samples, only limited information is available concerning sample provenance. Single samples cannot represent spatial or temporal variability within a coal area.
Geochemical datasets of U.S. coals can be found in the COALQUAL database (Bragg and others, 1997) and the National Coal Quality Inventory (Hatch and others, 2006), as only non-U.S. sample data are presented in the WoCQI. Although the WoCQI does not contain worldwide coverage of coal deposits, it is truly a unique and valuable compilation. The information in the WoCQI should prove useful for identifying possible areas for future global coal research.