AL2021 Continuous Hydrogen Peroxide (H2O2) Analyzer for Air and Water Samples

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Fields of application

Climate and environmental research

Trace gas analysis Process control Quality control for the chemical industry

H₂O₂ monitoring during decontamination of isolators

Drinking water quality monitoring

Principle of operation

The detection of peroxides is based on the liquid phase reaction of peroxides with p-hydroxyphenylacetic acid catalyzed by peroxidase. This reaction produces a fluorescent dimer that can be excited at 326 nm and detected between 400 and 420 nm. The reaction is sensitive to all peroxides in the solution, but fastest for H₂O₂. To separate the signal of H₂O₂ from that of the other peroxides, two parallel channels are used. Behind the stripping coil, the solution is distributed to two channels, each having an own reactor and fluorimeter.

In one channel (channel B), the H₂O₂ is destroyed selectively by the enzyme catalase prior to reaction described above. This means that in channel A all peroxides including H₂O₂, and in channel B all peroxides but no H₂O₂ are detected. The concentration of H₂O₂ is then given by the difference between the signals from the two channels, corrected for the destruction efficiency of the catalase solution. 

The above reactions are carried out in aqueous solution of peroxides and other reagents. Therefore, for the measurement of gaseous peroxides, these have to be trapped in aqueous solution first. This is achieved in a stripping coil by pumping air and a stripping solution (H₂O₂) continuously at known flow rates. The air and liquid streams are separated in a glass separator afterwards and the solution is then distributed to two channels.

In the AL2021, the H₂O₂ concentration in air is then calculated from the concentration in solution and the ratio of air and stripping solution flow rates. The coil size and the flow rates of air and stripping solution are optimized for quantitative stripping of H₂O₂. Due to lower solubilities however, the stripping efficiency for other peroxides is lower than that for H₂O₂  and probably varies between the 60% found for methylhydro peroxide and 100% for H₂O₂ . As the composition of other peroxides in air is unknown in most cases, the signal from channel B (after destroying H₂O₂ by catalase) only gives an approximate estimate of the concentration of other peroxides. Therefore, the instrument can not determine the concentration of other peroxides in air absolutely, but provides a relative measure that follows the changes in concentration. For more details see [1].

This effort is necessary for a sound separation of H₂O₂  from the other peroxides in environmental air samples. In the case of gaseous or liquid samples that contain only  H₂O₂  and no other peroxides (decontamination of air or drinking water) one channel without catalase is sufficient. The single channel monitor  H₂O₂  AL2021SC is especially designed for those kind of measurements.

Automatic routine measurements require regular calibrations of the instrument. As liquid phase standards of H₂O₂  are unstable, they have to be prepared freshly directly before use and stored cool and dark even during the calibration process. For automatic calibrations the AL2021 can be equipped with an internal H₂O₂  permeation source (optional). 

The external gas sampling line (6mm) should be made of PTFE or PFA tubing and should not exceed a length of 3m. To avoid inline H₂O₂  losses, it is recommended that the instrument is operated without a filter in the sampling line. Operating the instrument without filter, however, requires frequent inspections of the sampling line to ensure that the tubes and the zero-valve are clean. Soot and aerosol particles in the zero valve can significantly reduce the signal. The instrument is designed to be mounted in a 19" rack, but can also be installed stand-alone. 

The AL2021 requires a cooling box or fridge for storing some of the solutions, necessary for operation at a temperature of ~4°C. In details this are the SCA, SCB and the fluorescent reagent. To ensure a maximum stripping efficiency, the stripping solution should also be kept cool and dark. The other solutions, 0.1n NaOH and 0.1n HCl, can be stored at room temperature outside the cooling box. 

Inside the instrument the used liquid is collected behind the fluorimeter and fed into a waste line, which should be put into a waste bottle stored below the instrument. This bottle should be open to the atmosphere.

 

Specifications of the instrument

Provides absolute concentrations for H₂O₂  for gaseous and liquid samples 

Fast and continuous monitoring of  H₂O₂ concentration. Time resolution: 90 sec (10% – 90%), delay time ~300 sec 

Fully automated operation using micro controller 

Semi-automatic calibration by liquid H₂O₂ -standard or automatic calibration using internal  H₂O₂ permeation source (optional). Automatic zeroing by internal zero trap 

Measurement range is defined by calibration Detection limit: ~100 pptV (gas), ~100 ng/l eq. 3×10-9 molar (liquid) 

Linearity range: ~100 ppt – 3 ppm (gas), ~100 ng/l – 3 mg/l in liquids  Noise: 2% of full scale 

Dimensions: (19") 50cm × 49cm × 13cm. Weight: 20 kg 

Power requirement: 110/220 VAC, 50/60 Hz, 110 W 

RS232 interface and analog output 

Rugged and easy to use 

Interferences from relevant substances to H₂O₂

The enzymatic reaction with peroxidase has only two known significant interferences to other substances: 

O3  < 1 : 3500 NO < 1 : 8000

The following substances show no interferences to H₂O₂ :

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[1] Lazrus et al., Automated fluorometric method for hydrogen peroxide in air, Anal. Chem. 58 (1986) 594