A new metal emissions measurement and validation system has recently been developed that has the potential to significantly improve our understanding of metal emissions from a wide range of source categories. This system consists of:
- A continuous emission monitoring system (Xact-CEMS)
- An instrumental analyzer procedure (Xact-IAP)
- A sampling and analysis measurement method (XFM)
- A NIST-traceable reference aerosol generator (QAG)
The measurement components of this system are based on collecting a representative sample of stack gas on a chemically reactive filter (solid sorbent) that traps both particulate and vapor phases of metals, followed by non-destructive X-ray fluorescence (XRF) analysis using procedures consistent with ambient measurements. This system is applicable to elements with atomic number ranging from 13 (Al) to 92 (U), and can measure Be after non-destructive XRF analysis if other analytical procedures are used; it can provide detection limits as low as 0.1 ìg/dscm with 15 minute turn around times or lower detection limits with longer sampling and analysis times; and has demonstrated accuracies better than five percent and precisions better than three percent. These generation and measurement components are independently traceable to NIST: the QAG through procedural traceability and solution standards, while the measurement methods are analytically traceable through thin film standards and NIST standard reference materials.
A series of laboratory and field tests have been conducted to demonstrate that the QAG, XFM and Xact-IAP components of this new system meet Method 301 accuracy and precision requirements for conditional method status. Each of the methods met the performance criteria outlined in the Method 301 test plan. These criteria included the following:
- Filter ratios – percent of Cr and Pb trapped on the XFM front PTFE filter. Although this criterion is not required by Method 301, high percentages assure high particle trapping efficiency.
- Metal ratios - ratio of Pb-normalized solution concentration to Pb-normalized aerosol concentration. This criterion is also not required by Method 3011 but ratios close to one assure high relative generation, transport and sampling efficiencies for all metals including vapors.
- Efficiency – percent of NIST-traceable reference aerosol concentration measured. A high percentage assures high absolute generation, transport and sampling efficiencies.
- Bias correction factor –ratio of reference to measured aerosol concentration. A value close to one indicates high method accuracy.
- Precision – percent relative standard deviation determined from sequential measurements. A low percent value indicates a highly precise and stable system.
- Linearity – correlation coefficient (r) based on a regression analysis of a plot of measured versus reference aerosol concentration.