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

Front view of an AL2021 hydrogen peroxide analyzer. The supply with reagents, liquid calibration standard and liquid sample is realized by 1/16'' tubing in the front of the instrument.

Automated H2O2 analyzer for very low concentrations 

The hydrogen peroxide (H2O2) monitor AL2021 has an extraordinary high sensitivity and a unique low detection limit of <100ppt (parts per trillion) for gaseous samples and <100 ng/liter (eq. 3×10-9 molar) for liquid samples, respectively. The complete chemical processing, including gas stripping, is integrated into the instrument. The measurement process is fully automated, all necessary reagents are provided by a digitally controlled high precision peristaltic pump. The standard dual-channel instrument AL2021 provides absolute concentrations for H2O2 and relative concentrations for all other peroxides. There is a special single-channel version (AL2021SC) for decontamination control and drinking water quality monitoring.

Further information

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.  

(a): Enzymatic reaction between p-hydroxyphenylacetic acid and H2O2 to a fluorescent dimer.
(b): destruction of H2O2 by catalase in channel B.

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 (pH-Buffered water free of 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].

Simplified block diagram of the AL2021. H2O2 from the sample gas is brought into aqueous solution in a stripping coil. After stripping, the solution is distributed to two channels. In the reactor A an enzymatic reaction sensitive to all peroxides takes place while in reactor B H2O2 is destroyed selectively by catalase. The signals of the reaction products are measured with maximum sensitivity by a pair of photomultipliers. The concentration of H2O2 is calculated subsequently from the difference of both signals.

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.

Logical flow chart for the calculation of absolute concentrations of H2O2 by the AL2021.

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. 

Scheme of tubing between the AL2021 and the reagents. SCA, SCB, and the reagent need to be cooled. Additionally, the reagent needs to be kept in the dark. For a better stripping efficiency the stripping solution should also be stored in the fridge. NaOH and HCl do not need to be cooled. To prevent confusion all tubes are color coded. The liquid waste is fed into a waste bottle, which must be open to atmosphere.

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
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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:

SO2

PAN

NO2

glyoxal

isobutane

1-butane

formaldehyde

benzene

toluene

methanol

acetone

methylamine

dimethylamine

n-butane

cis-2-butene

trans-2-butene

Ions, such as: J-, Cl-, Br-, NO3-, PO4-, benzoate 


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