CEMs - Tekran Instrument Corp. Series 3300 and Thermo Electron's Mercury Freedom System Continuous Emission Monitors (CEMs) for mercury are designed to determine total and/or chemically speciated vapor-phase mercury in combustion emissions. Performance for mercury CEMs are contained in Chapter 40 of the Code of Federal Regulations, Part 75 and Part 60 (40 CFR Parts 75 and 60) and require assessment of the performance of newly installed mercury CEMs only for their determination of Hg T . This total is the sum of vapor-phase mercury in all chemical forms in the combustion gas, including Hg 0 and Hg OX . The CEMs tested claim to measure Hg T , Hg 0 , and Hg OX. They both use atomic fluorescence for detection. Sorbent-based Sampling System-Apex Instruments and Environmental Supply Co.HG-324K Appendix K of 40CFR Part 75 established sorbent-based sampling systems as an acceptable approach for determining mercury in the stack of utility generating stations. Sorbent-based systems collect integrated samples of mercury from stack gas onto selected sorbent material over an extended time period (from a few hours to several days). The collected mercury samples are then analyzed and the stack gas mercury concentrations are then calculated. Appendix K defines procedures for use of such systems to collect total vapor-phase mercury in combustion source emissions and require the use of multi-stage sorbent traps pre-spiked with mercury as a quality assurance measure. Apex used atomic absorption for final analysis, while ESC uses cold vapor atomic fluorescence spectrometry.