Single-Injection Analysis of Formic Acid and Free Volatile Fatty Acids Using Flame Ionization Detector with In-jet Methanizer

Significance of the Topic

Volatile fatty acids (VFAs) are key indicators in environmental monitoring, wastewater and sludge treatment, hydraulic fracturing, landfills, and fermentation processes. While most short-chain VFAs (C2–C7) can be directly analyzed by GC-FID, formic acid typically requires derivatization for detection. Developing a method to quantify underivatized formic acid alongside other VFAs enhances analytical efficiency and reduces sample preparation steps.

Objectives and Study Overview

This study aimed to demonstrate the simultaneous analysis of underivatized formic acid with C2–C5 VFAs using a Shimadzu GC-2050 equipped with an in-jet methanizer (Jetanizer) and flame ionization detection (FID). Key goals included evaluating detection performance, signal-to-noise ratios, calibration linearity, and chromatographic behavior under optimized conditions.

Methodology and Instrumentation

A test standard containing acetic, propionic, iso-butyric, butyric, iso-valeric, and valeric acids (1000 ppm each) was prepared in water. Formic acid (1000 ppm) was analyzed both alone and in mixture. Instrumental configuration and conditions:

• Gas chromatograph: Shimadzu Nexis GC-2050 with split/splitless injector and AOC-30i autosampler
• Detector: Flame ionization detector with in-jet methanizer (Jetanizer)
• Column: SH-wax, 30 m × 0.32 mm × 1 µm; connected to a 5 m guard column
• Carrier gas: Helium at 5 mL/min constant flow
• Injection: 0.5 µL splitless
• Oven program: 80 °C hold (2 min), ramp 20 °C/min to 200 °C, hold 3 min
• FID gases: Air 250 mL/min, H2 32 mL/min, N2 makeup 24 mL/min; detector temperature 400 °C

Data were acquired and processed using LabSolutions software. Calibration standards ranged from 75 to 500 ppm for C2–C5 VFAs and 200 to 1000 ppm for formic acid.

Main Results and Discussion

Without the Jetanizer, formic acid was undetectable by FID, while other VFAs yielded strong responses at 100 ppm. Installing the in-jet methanizer enabled conversion of CO/CO2 and small organic acids to methane, making formic acid visible on FID. Initial tests showed reduced S/N for C2–C5 VFAs due to shallower jet insertion. To optimize detection, splitless injection and increased column flow were adopted. Under these conditions, all targeted compounds, including formic acid, were detected at 100 ppm. Signal-to-noise ratios for C3–C5 VFAs ranged from 1142 to 2260. Slight peak tailing was noted but did not compromise chromatographic separation. Calibration curves for all analytes exhibited excellent linearity (r2 > 0.994).

Benefits and Practical Applications

• Eliminates time-consuming and error-prone derivatization steps for formic acid
• Enables single-injection analysis of formic acid and other short-chain VFAs
• Utilizes common GC-FID instrumentation found in environmental and industrial laboratories
• Provides reliable quantitative data with high linearity and good S/N ratios

Future Trends and Possible Applications

Further developments may include extending in-jet methanizer applications to detect other low-molecular-weight acids and permanent gases. Integration with automated sample preparation and advanced data processing could enhance throughput. The approach may also be adapted for online monitoring in bioreactors or environmental surveillance systems.

Conclusion

The use of an in-jet methanizer on a Shimadzu GC-2050 FID system allows rapid, direct analysis of underivatized formic acid alongside C2–C5 VFAs in a single injection. This method overcomes the limitations of traditional derivatization, offering a robust alternative for routine VFA monitoring in environmental, wastewater, and industrial settings.

References

• Henderson M.H., Steedman T.A. Analysis of C2–C6 monocarboxylic acids in aqueous solution using gas chromatography. Journal of Chromatography A, 244 (1982), 337–346.
• Shimadzu Scientific Instruments. Analysis of and Characterization of ARC’s Injet Methanizer for Permanent Gases, Carbon Dioxide, and Light Hydrocarbons. Application News No. GC-2103.