TM98-08 Change in the Analytical Method Used for the Determination of Inorganic Carbon in Sediment

The National Water Quality Laboratory (NWQL) has historically determined inorganic carbon in sediment by method O-5102-83 as described by Wershaw and others (1987). There are two lab codes, and two WATSTORE and method codes for this method. Lab code 503, WATSTORE code 00686, and method Code C are for unsieved samples. Lab code 5051, WATSTORE code 49270, and method code D are for sieved samples. This method is a classic gasometric technique, but improved techniques are currently (1998) available. The USGS Geologic Division (GD) uses a coulometric method for the determination of inorganic carbon (carbonate carbon) in geologic materials. Brandt and others (1990) described this method. The NWQL will implement this method to determine inorganic carbon in sediment beginning October 1, 1998. The new lab code for unsieved samples will be 2321, and the new method code will be E. The new lab code for sieved samples will be 2319, and the new method code will be E. The WATSTORE codes will remain unchanged. The preliminary method detection limit (MDL), which was determined by the procedure described by the U.S. Environmental Protection Agency (1992), is 0.1 g/kg and the method report limit is 0.2 g/kg. Organic carbon in sediment is calculated as the difference between total carbon and inorganic carbon. The lab code and method code for organic carbon must also change because the method for inorganic carbon is changing. The new lab code for sieved sample organic carbon in sediment is 2320, and the new method code is C. Organic carbon in unsieved samples is not reported by NWQL because the subtraction is performed by the customer. The method for total carbon in sediment will not be changed. An expanded version of this tech memo, including all the data used in this work, will be made available on the NWQL Home Page as a technical paper at http://wwwnwql.cr.usgs.gov/USGS. This paper should be available by January 1, 1999.