Sensitive and Reproducible Determination of Nitrosamines by GC/MSD

Importance of the topic

Nitrosamines are a class of potent genotoxic and carcinogenic contaminants that can form as disinfection byproducts during water treatment and occur in other environmental and industrial matrices. Regulatory agencies worldwide are setting increasingly strict guidance values for key nitrosamines (notably NDMA). Analytical methods capable of reliable, trace‑level quantitation with good selectivity and reproducibility are therefore essential for monitoring, compliance and risk management.

Objectives and overview of the study

This application note evaluates the analytical performance of an Agilent 5977 GC/MSD operated in positive chemical ionization (PCI) mode using ammonia as the reagent gas for the quantitation of seven nitrosamines defined in US EPA Method 521: NDMA, NMEA, NDEA, NPYR, NDPA, NPIR and NDBA. The study aimed to demonstrate linearity, repeatability and instrument detection limits (IDLs) for standards prepared in dichloromethane and to illustrate the suitability of ammonia PCI for trace nitrosamine analysis.

Methodology

The study used ten‑level calibration series prepared in dichloromethane over 0.1–20 ppb for six compounds and 0.5–20 ppb for NPYR. Analysis was performed in selected ion monitoring (SIM) mode. Repeatability was assessed by six replicate injections at the lowest calibration levels. IDLs were calculated at 99% confidence from the replicate measurements using the Student t multiplier for five degrees of freedom.

Instrumentation used

• Gas chromatograph: Agilent configuration with Agilent HP‑5 ms UI capillary column (30 m × 0.25 mm, 0.25 µm).
• Mass spectrometer: Agilent 5977 GC/MSD operated in positive chemical ionization (PCI) using ammonia (20%) as reagent gas; acquisition in SIM mode to monitor target and qualifier ions for each analyte.
• Injection: Pulsed splitless, 1.0 µL injection volume, inert fritted splitless liner.
• GC conditions: constant helium flow (~1.5 mL/min), low initial inlet temperature ramp to 280 °C, and a temperature program starting at 40 °C with staged ramps to 270 °C for chromatographic separation.
• MS conditions: source and transfer line heated to minimize condensation; SIM gain and ion selection tuned to maximize [M+H]+ response from ammonia PCI.

Main results and discussion

• Linearity and dynamic range: Excellent linear calibration was observed for NDMA, NMEA, NDEA, NDPA, NPIR and NDBA across 0.1–20 ppb with R2 ≥ 0.9992. NPYR showed strong linearity across 0.5–20 ppb with R2 = 0.9987.
• Repeatability: Six replicate injections at the lowest calibration levels produced peak area relative standard deviations (RSD) < 4.8% and calculated concentration RSD < 3.7%, indicating stable instrument performance at trace concentrations.
• Instrument detection limits: IDLs calculated at 99% confidence ranged from 0.0063 to 0.0655 ppb (6.3–65.5 parts per trillion, ppt). NDMA achieved the lowest IDL (0.0063 ppb), while NPYR had the highest IDL (0.0655 ppb), consistent with its higher lowest calibration level.
• Role of ammonia PCI: Ammonia PCI produced dominant protonated molecular ions [M+H]+ with limited fragmentation, concentrating signal into a single high‑abundance ion and improving signal‑to‑noise for trace quantitation. The clean spectral background of ammonia and efficient proton transfer contributed to sensitivity gains versus conventional EI.
• Limitations noted: The evaluation was performed on standards in dichloromethane rather than on environmental matrices; consequently, matrix effects, extraction recoveries (SPE), large volume injection or sample cleanup were not assessed here. EPA Method 521 typically includes SPE and large‑volume injection for real water samples, steps not demonstrated in this solvent‑based performance study.

Benefits and practical applications

• The demonstrated IDLs are well below many regulatory guidance levels (for example, national guidance values reported in the literature are on the order of tens of ng/L), enabling reliable monitoring of NDMA and related nitrosamines at concentrations relevant to public health limits.
• The combination of ammonia PCI and SIM on a 5977 GC/MSD provides a sensitive, selective and reproducible platform that laboratories can deploy for routine screening and quantitation when paired with appropriate sample preparation protocols.
• Operational advantages include simplified spectral interpretation because of reduced fragmentation, and potential for improved quantitation using molecular‑ion monitoring and isotope dilution strategies.

Future trends and applications

• Extension to environmental matrices: Integration of SPE, on‑line preconcentration, or large‑volume injection will be required to translate solvent‑phase sensitivity into routine environmental monitoring workflows; validation with natural matrices should address recovery and matrix suppression effects.
• Enhanced selectivity: Tandem mass spectrometry (MS/MS) or high‑resolution accurate mass spectrometry can provide confirmatory identification and lower false positives in complex matrices.
• Isotope dilution quantitation: Use of isotopically labeled internal standards (e.g., NDMA‑d6) will further improve accuracy and correct for matrix and extraction variability.
• Automation and method standardization: Automated sample preparation, standardized SPE cartridges and harmonized protocols will support regulatory monitoring and inter‑laboratory comparability.
• Broader analyte scope: Expanding target lists to include nitrosamine precursors and related transformation products will improve source identification and treatment optimization efforts.

Conclusion

The Agilent 5977 GC/MSD operated in ammonia PCI mode achieves sub‑50 ppt instrument detection capability for most EPA Method 521 nitrosamines, with robust linearity and excellent repeatability on solvent standards. Ammonia PCI effectively concentrates signal into protonated molecular ions, improving sensitivity and simplifying SIM‑based quantitation. To support routine environmental monitoring, the platform should be combined with validated sample preparation and matrix‑specific validation. Overall, the instrument and ionization approach provide a sensitive and practical analytical foundation for trace nitrosamine surveillance and regulatory programs.

References

1. Magee, P. N. Toxicity of Nitrosamines: Their Possible Human Health Hazards. Food and Cosmetic Toxicology 1971, 9(2), 207–218.
2. Munch, J. W. Method 521: Determination of Nitrosamines in Drinking Water by Solid Phase Extraction and Capillary Column Gas Chromatography with Large Volume Injection and Chemical Ionization Tandem Mass Spectrometry (MS/MS). U.S. Environmental Protection Agency, Washington D.C., 2005.
3. Guidelines for Canadian Drinking Water Quality: Guideline Technical Document — N‑Nitrosodimethylamine (NDMA), Health Canada, 2010 (guidance value reported: 40 ng/L).
4. Maria, J. F.; et al. Determination of 15 N‑Nitrosodimethylamine Precursors in Different Water Matrices by Automated On‑Line Solid‑Phase Extraction Ultra‑High‑Performance‑Liquid Chromatography Tandem Mass Spectrometry. Journal of Chromatography A 2016, 1458, 99–111.