Evaluation of the Mercury Concentration Accessory for US EPA Methodology

Importance of the Topic

Atomic absorption determination of mercury at ultratrace levels is crucial due to mercury’s toxicity and stringent environmental regulations requiring sub-ppb monitoring. The Agilent MCA-90 enhancement accessory extends the capabilities of the VGA-76 vapor generation system, enabling detection limits an order of magnitude lower. This methodology supports compliance with regulatory thresholds (e.g., 50 ppt in Europe) and enhances laboratory performance in environmental, industrial, and quality-control analyses.

Study Objectives and Overview

The application note evaluates the Agilent Mercury Concentration Accessory (MCA-90) integrated with the SpectrAA-400 AAS and VGA-76. Key goals include:

• Assessing detection limits and sensitivity enhancements via mercury amalgamation on a gold trap
• Validating method performance against US EPA Method 245.1 digestion protocols
• Comparing results with conventional VGA-76 measurements

Methodology and Instrumentation

Sample Preparation and Digestion:
EPA-approved acid digestion converts all mercury species to inorganic form. The procedure involves sulfuric and nitric acid oxidation, potassium permanganate and persulfate treatment, heating, and reduction with hydroxylamine or chloride, yielding samples ready for cold-vapor analysis.
MCA-90 Operation and Parameters:
The microprocessor-controlled MCA-90 sequence includes:

1. Delay (flush) time: 35 s
2. Collection time: 40 s on the gold amalgamation trap (MAT)
3. Heat time: 45 s to desorb mercury

Instrumentation:

• SpectrAA-400 AAS (253.7 nm, 0.5 nm slit, peak-height measurement)
• VGA-76 vapor generator with 10 % SnCl2 reductant channel and hydroxylamine HCl acid channel

Main Results and Discussion

Calibration and Detection Limits:
A five-point calibration (0.25–5.00 µg/L) yielded a linear response. With a 40 s trap time, the detection limit reached 0.01 µg/L (10 ppt), estimated at 20 ppt after sample dilution. Extended collection (up to 10 min) achieved low single-digit ppt sensitivity.
Precision and Stability:
Replicate stability tests over four hours showed relative standard deviations below 4 %, confirming digested standard stability. Carryover was minimal (< 1 %), managed by reagent blanks.
Sample Analysis:
Three environmental water samples analyzed with MCA-90 and VGA-76 showed close agreement. Reference standards (EPA and ERA-CLP) fell within certified ± 2σ ranges, validating accuracy.

Benefits and Practical Applications

• Enhanced sensitivity for ultratrace mercury monitoring in environmental and industrial settings
• Compliance with stringent regulatory limits (e.g., 50 ppt in Europe)
• Low sample consumption and high throughput with optimized trap times
• Minimal background interference and robust precision

Future Trends and Applications

• Adapting longer collection times and larger traps for sub-ppt quantification
• Integration with automation for high-throughput regulatory testing
• Expanding to speciation studies by coupling with chromatographic separation
• Development of improved anti-contamination protocols for ppt-level analysis

Conclusion

The Agilent MCA-90 accessory significantly improves mercury detection capabilities of the VGA-76/SpectrAA-400 system, achieving sub-ppb and ppt-level sensitivity in accordance with US EPA methodology. The approach combines robust performance, regulatory compliance, and operational flexibility for advanced mercury monitoring applications.

References

1. Varian AA at Work No. 104, Varian Australia, 1991
2. EPA Method 245.1, Methods for Chemical Analysis of Water & Wastes, EPA-600/4-79-020, 1979
3. MCA-90 Operation Manual, Varian Australia, Publication 85-100973-00
4. VGA-76 Service Manual, Varian Australia, Publication 85-100568-06