TM93-06 Raising the method detection limit for gross alpha and gross beta

This technical memorandum is to inform the Districts of the use of a single analytical range used to determine silica (Lab Code 56, WATSTORE 00955, TWRI I-2700-85) utilizing a new Alpkem autoanalyzer, which has been in operation since October 20, 1992. This single analytical range replaces a single range system utilizing an old Technicon autoanalyzer which was recently found not capable of achieving the stated method detection level of 0.1 mg/L. Recently, the National Water Quality Laboratory (NWQL) purchased a new Alpkem autoanalyzer system to replace the aging Technicon colorimetric instrumentation (15 years old) previously used to determine dissolved silica (Lab Code 56, WATSTORE 00955, TWRI I-2700-85). This method calls for a dual analytical range: a low range from 0.1 to 6 mg/L and a high range from 6.0 to 60 mg/L. However, the NWQL has always run this method using a single analytical range from 0.1 to 60 mg/L. A recent precision and method detection limit (MDL) study using the protocol set forth in 40 CFR 136, App. B, revealed that the old single-range Technicon system for silica had an MDL of 0.3 mg/L instead of the stated MDL of 0.1 mg/L. Using the same test criteria, the new single-range Alpkem system showed an MDL of 0.1 mg/L. The Alpkem system, utilizing technical advances, is capable of using a single analytical cartridge to achieve the desired analytical range and MDL. Data comparisons for the two systems are presented in Table 1.