Simple Preparation of Calibration Curves for Oxygen and Nitrogen Using Air as a Standard Gas

Importance of the Topic

Calibration of oxygen and nitrogen at low concentration levels plays a critical role in environmental monitoring, industrial process control and quality assurance in analytical laboratories. Traditional methods rely on multiple standard gas cylinders and dilution steps that can be time consuming, costly and prone to contamination.

Objectives and Study Overview

This work demonstrates a streamlined approach to prepare multipoint calibration curves for O2 and N2 using atmospheric air as a substitute standard. By adjusting the injection volume of air with an autoinjector, the method aims to eliminate the need for dilution or multiple gas cylinders while maintaining high accuracy and linearity.

Methodology and Instrumentation

Chromatographic analysis was performed on a Nexis GC-2030 coupled to an AOC-30i autoinjector and a barrier discharge ionization detector. Key parameters included a split injection at 250 °C, split ratio 1 to 25, helium carrier gas at 50 cm/s linear velocity and a SH-Rt-Msieve 5A column held at 35 °C. Experiment steps comprised:

• Assessment of syringe air leakage by removing the plunger holder and testing seven syringes
• Variation of septum purge flow rates (3 to 50 mL/min) to minimize air contamination during injection
• Water washing of syringes to form a water film around the plunger and improve gas tightness
• Generation of calibration data by injecting air volumes from 0.05 to 8 μL with both 0.5 μL plunger-in-needle and 10 μL micro syringes

Main Results and Discussion

Air leakage tests confirmed negligible contamination for most syringes, with slight leakage observed in one unit. Increasing septum purge reduced baseline O2 and N2 peaks substantially. Water washing stabilized the response factor for injection volumes above 0.05 μL. Calibration curves for both gases showed excellent linearity (R2 > 0.9995) across the studied volume range. Comparison of the two syringe types at 0.5 μL yielded area ratios close to unity, confirming consistency.

Benefits and Practical Applications

Using air as a standard gas dramatically lowers analysis cost and logistical burden. The method requires no dilution steps or multiple cylinders, reduces gas consumption and supports high injection accuracy via an autoinjector. It is easily implemented in routine gas and dissolved gas analyses.

Future Trends and Potential Applications

Potential developments include extension to other gaseous analytes, integration with dissolved gas analysis in liquids, and automation through advanced digital injectors. Adaptation to field portable GC systems and coupling with mass spectrometry could further broaden applicability.

Conclusion

A simple, low-cost calibration approach for oxygen and nitrogen using atmospheric air and variable injection volumes was demonstrated. The method achieved high linearity and accuracy without standard gas dilution, offering a practical solution for routine gas analysis.

Used Instrumentation

• Nexis GC-2030 gas chromatograph
• AOC-30i autoinjector
• BID-2030 barrier discharge ionization detector
• SH-Rt-Msieve 5A column (30 m × 0.32 mm, 20 μm)