Nitrogen and oxygen analysers verify the purity of these process gases at the point of use — confirming that the gas supply meets the specification required for the application, whether that is food packaging modified atmosphere, inert blanket for electronics soldering, or medical-grade oxygen for clinical use. In Singapore, gas purity verification is required across a wide range of industries regulated by the Health Sciences Authority (HSA), Singapore Food Agency (SFA), NEA, and various international quality management standards. Using an off-specification gas supply can cause product spoilage, process failures, or serious patient harm.
Why Gas Purity Testing is Necessary
Industrial gases are not inherently pure at the point of use. Several failure modes can degrade purity:
- Cylinder cross-contamination — cylinders misidentified or improperly cleaned in the supply chain contain residual gas from previous use
- On-site generation impurity — nitrogen generators (PSA or membrane types) produce gas of varying purity depending on operating condition and filter condition; purity degrades as filters age or if the generator is overloaded
- Distribution system leaks — air ingress through fittings, valve stems, or flexible hoses in nitrogen systems reduces purity
- Moisture contamination — water vapour entering the system from ambient humidity or condensation changes effective gas composition
- Cylinder depletion effects — as high-pressure cylinders empty, some impurities can concentrate in the residual gas
Nitrogen Purity Grades and Applications
Nitrogen is one of the most widely used industrial gases in Singapore. Common purity grades and their specifications:
| Grade | Purity (%) | O2 Content (max) | Typical Application |
|---|---|---|---|
| Industrial | 99.5% | 5,000 ppm | Purging, blanketing non-critical |
| Technical | 99.9% | 1,000 ppm | Heat treatment, HVAC purge |
| High purity | 99.99% (4N) | 100 ppm | Electronics, food packaging MAP |
| Ultra high purity | 99.999% (5N) | 10 ppm | Semiconductor, analytical lab |
| Research grade | 99.9999% (6N) | 1 ppm | Reference gas, mass spectrometry |
Oxygen Purity Grades and Applications
Oxygen is used in medical care, water treatment, combustion enhancement, and as a process gas in various chemical and pharmaceutical applications:
| Grade | Purity (%) | Typical Application |
|---|---|---|
| Industrial | 99.5% | Combustion support, water treatment, welding |
| Medical | 99.5% min (BP/USP) | Clinical use, anaesthesia, respiratory therapy |
| High purity | 99.9% | Research, semiconductor oxidation |
| Electronic grade | 99.999% | Semiconductor thermal oxidation |
Medical oxygen used in Singapore's hospitals and clinics must meet the specifications of the British Pharmacopoeia (BP) or United States Pharmacopeia (USP) and is regulated by HSA as a medicinal product. Oxygen purity must be verified through documented testing as part of the medical gas supply chain quality system.
Nitrogen Analyser Technologies
Electrochemical Oxygen Sensors (for Nitrogen Purity by Oxygen Trace)
The most common method for verifying nitrogen purity is measuring the residual oxygen content. Electrochemical oxygen sensors (trace oxygen analysers) are the standard tool for this — they measure oxygen concentration down to single-digit ppm levels. The nitrogen purity is then derived: purity (%) = 100% minus O2 content (%) minus other impurities (typically neglected for standard purity checking).
Electrochemical trace O2 analysers are compact, portable, and relatively low-cost. They are used at the point of cylinder connection, at the nitrogen generator outlet, and at critical point-of-use applications. Sensor life is typically 12–24 months and calibration is required at regular intervals using certified calibration gas.
Galvanic (Paramagnetic) Oxygen Analysers
Paramagnetic oxygen analysers exploit the unique paramagnetic property of oxygen molecules — oxygen is attracted to strong magnetic fields, unlike most other gases. A dumbbell-shaped sensor body deflects in proportion to the oxygen concentration. Paramagnetic analysers are highly accurate and stable, immune to most cross-interference, and are the standard technology for permanent laboratory and process oxygen analysers. They are used where the highest accuracy is required and electrochemical sensor replacement cost or downtime is unacceptable.
Zirconia Oxygen Sensors
Zirconia sensors operate at high temperature (600–800°C) and produce a Nernst voltage proportional to the ratio of oxygen partial pressure between the sample gas and a reference gas. They are used for very low oxygen measurement (ppm to ppb range) in inert gas and process applications, and for high-temperature in-situ measurements. They require careful temperature control and are more complex to operate than electrochemical sensors.
Thermal Conductivity Detectors (TCD)
Thermal conductivity detectors measure the difference in thermal conductivity between the sample gas and a reference gas. Since nitrogen has significantly different thermal conductivity from oxygen, argon, or other impurity gases, TCDs can be used to monitor nitrogen purity directly. They are particularly useful in applications where multiple impurities need monitoring or where a single-component approach (oxygen only) is insufficient.
Oxygen Analyser Technologies
For verifying oxygen purity, the same electrochemical, paramagnetic, and zirconia technologies apply, but the measurement range shifts from trace (ppm) to percent (%) oxygen:
- Electrochemical sensors (percent range) — suitable for ambient and elevated oxygen measurement; widely used in medical oxygen purity checking and general process monitoring
- Paramagnetic analysers — the reference technology for accurate percent-range oxygen measurement; used in medical gas testing laboratories and process control
- Fuel cell sensors — similar to electrochemical but use a galvanic cell; limited to percent-range oxygen; compact and portable
Applications in Singapore Industry
Food Packaging — Modified Atmosphere Packaging (MAP)
MAP extends the shelf life of fresh food products by replacing the air in the package with a controlled mixture of gases — typically nitrogen, CO2, and sometimes oxygen. The exact mixture depends on the product: high-CO2/zero-O2 for red meat packed under nitrogen, higher O2 for intact-muscle products to maintain colour. Singapore's SFA food safety requirements and international food safety standards (BRC, FSSC 22000) require that MAP gas compositions be verified using calibrated analysers.
Electronics and Semiconductor Manufacturing
Singapore's world-class semiconductor and electronics manufacturing sector uses high-purity nitrogen extensively — for inert atmosphere reflow soldering, wafer storage, die bonding, and conveyor oven inerting. The nitrogen supply (typically generated on-site from a PSA generator) must be continuously or periodically monitored to verify purity at the point of use. Trace oxygen analysers at ppm level are the standard monitoring tool.
Medical Gas Supply
Singapore hospitals use piped medical oxygen, nitrous oxide, medical air, and nitrogen. HSA regulates medical gases as medicinal products — every batch of medical oxygen cylinder supply must have a certificate of analysis showing purity, and hospital gas systems must be maintained and periodically tested. Portable paramagnetic or electrochemical oxygen analysers are used by medical gas engineers during commissioning and maintenance.
Laboratory and Analytical Instrument Supply
Gas chromatographs, mass spectrometers, atomic absorption spectrometers, and other analytical instruments use high-purity carrier and reagent gases. Any impurity in the carrier gas introduces baseline noise, ghost peaks, or systematic errors. Laboratories typically verify the purity of gas cylinders at receipt and when analytical results are anomalous. View analytical and measurement instruments available from Unitest Instruments.
Calibration of Gas Purity Analysers
Nitrogen and oxygen purity analysers require regular calibration to maintain measurement accuracy. Calibration involves exposing the sensor to certified reference gases of known purity and adjusting the analyser output. The calibration gas must have a certified concentration with an uncertainty statement traceable to national standards.
For medical gas applications under HSA requirements, calibration records for analytical instruments form part of the quality documentation reviewed at GMP inspections. For food safety certified facilities, calibration records for MAP gas analysers are reviewed at BRC/FSSC audits.
Unitest Instruments provides gas instrument calibration services under SAC-SINGLAS accreditation (LA-2023-0845-C). Our calibration is traceable through the ILAC-MRA network to international standards. Calibration certificates include as-found and as-left readings with measurement uncertainty statements. Contact our team for calibration scheduling.
For related gas quality measurement, see our guide on compressed air quality testing to ISO 8573 and our article on industrial gas leak detection methods.