AL4021 Continuous Formaldehyde in Air and Water Monitor

Front view of a AL4021 formaldehyde analyzer. The 1/16'' inlet in the front is used for calibration and liquid measurements.

Automated HCHO analyzer for very low concentrations

The AL4021 is a completely automated and continuously working formaldehyde (HCHO) analyzer for gaseous and liquid samples based on the Hantzsch reaction, which is sensitive to very low concentrations of HCHO (ppbV for gaseous samples and µg/L for liquid samples). All functions of the AL4021 are controlled via a 7" touch screen on the front panel. The AL4021 is easy to use, reliable and has a low need for maintenance. It has an built-in HCHO source for automatic calibration (optional). The AL4021 provides exact and reliable results in free programmable concentration ranges. The AL4021 can store data continuosly on an USB stick.

Further information


Fields of application 

  • Climate and environmental research / Trace gas analysis
  • Online monitoring of HCHO emission / Emission limit control
  • Process control / Quality control for the chemical industry
  • Chamber testing of wood-, plastic-, fabric-based products using up to 12 chambers simultaneously
  • Measurement of indoor air quality
  • Replacement for the wet chemistry in the acetyl-acetone method

Principle of operation

The detection of formaldehyde is based on the liquid phase reaction of formaldehyde with acetyl-acetone (2,4-pentadione) and ammonia. This reaction produces 3,5-diacetly-1,4-dihydrolutidine (DDL), which is absorbing light at 410 nm and shows a strong fluorescence around 510 nm. To gain maximum sensitivity the fluorescence light is measured by a photomultiplier. Compared to the colorimetric measurement of DDL, the continuous fluorimetric measurement is much more sensitive, faster, and due to the short reaction time, also less cross-sensitive to other aldehydes and ketones. Since the Hantzsch reaction works in aqueous solution, gaseous formaldehyde has to be trapped in aqueous solution first. This is achieved in a stripping coil where air and a stripping solution are brought into contact continuously at defined flow rates and contact surfaces. The air and liquid streams are separated afterwards and the solution is then analyzed for formaldehyde.

Hantzsch-reaction: In the presence of ammonia, formaldehyde reacts with acetyl-acetone to 3,5-diactyl-1,4-dihydrolutidine (DDL).

For gaseous samples the HCHO mixing ratio in air is calculated from the concentration in the solution and from the ratio of air and stripping solution flow rates. Size and flow rate of the stripping coil are optimized for a quantitative reaction of HCHO within short times. Formaldehydes in liquid solutions can be measured directly. In this mode, stripping is not necessary and, consequently, either zero air has to be applied to the stripping solution or the internal zero trap has to be switched on.

Block diagram of the AL4021. Formaldehyde from the sample gas is brought into aqueous solution in a stripping coil. The temperature and the flow speed of the sample gas and the stripping solution is digitally controlled. The Hantzsch reaction takes place in a heated reactor. The reaction product DDL is measured fluorimetrically by using a photomultiplier.

The instrument is capable to sample ambient air by an internal air pump, or water probes by the internal peristaltic pump. When measuring gaseous samples, formaldehyde is dissolved into aqueous solution in a stripping coil. All parameters, like temperature and the flows of stripping solution and sample air are controlled by a microprocessor. When measuring liquid samples, an internal zero trap is switched in the gas line, providing zero air free of HCHO. 

Scheme of tubing between the AL4021 and the cooled reagents (Hantzsch, Stripper). A zero trap in the pressure compensation line of the bottles prevents a contamination of both solutions with formaldehyde from the ambient. The liquid waste is fed into a waste bottle, which needs to be open to atmosphere.

The AL4021 requires a cooling box or fridge for storing the Hantzsch reagent and the stripping solution at a temperature of ~4°C. Our standard bottles have a volume of 5 liters which is sufficient for stripper and reagent for one week of continuous 24h operation. In order to ensure a steady flow of the reagent and stripper, the AL4021 should be installed above the cooling box containing the solutions. Inside the cooling box a zero trap is mounted in the pressure compensation line. This prevents a contamination of both solutions with ambient formaldehyde. 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 that is stored below the instrument. 

Multi-chamber measurements 

Diagram of the simultaneous measurement of formaldehyde using 4 test chambers. It is possible to employ up to 16 separate test chambers with one AL4021 unit.

Specifications of the instrument

  • Provides absolute concentrations of formaldehyde for gaseous and liquid samples
  • Fast and continuous monitoring of HCHO concentration. Time resolution: 90 sec (10% – 90%), delay time ~300 sec
  • Fully automated operation using an ARM microprocessor, 16bit digitalisation
  • Semi-automatic calibration by liquid HCHO-standard or automatic calibration using internal HCHO permeation source (optional). Automatic zeroing by internal zero trap
  • Measurement range is defined by calibration
  • Detection limit: ~100 ppt (gas), ~150 ng/l eq. 5×10-9 molar (liquid)
  • Linearity range: ~100 ppt – 3 ppm (gas), ~150 ng/l – 5 mg/l in liquids
  • Detection range: ~100 ppt – 50 ppm (gas), ~150 ng/l – 70 mg/l in liquids
  • Noise: 2% of full scale within linearity range
  • Dimensions: (19’’) 50cm × 49cm × 13cm. Weight: 20 kg
  • Power requirement: 110/220 VAC, 50/60 Hz, 110 W
  • RS232 interface and analog output (optional)
  • Automatic data storage on USB stick
  • Free programmable AutoSequences for scheduling calibrations, zeroings, valve switching, etc.
  • Rugged and easy to use
  • Simultaneous measurement with up to 16 test chambers using an optional valve controller (AL-V08/AL-V12/AL-V16)


Interferences from relevant substances to HCHO

The Hantzsch method with fluorimetric measurement of DDL is only minimal sensitive for other aldehydes and ketones due to its short reaction times. Furthermore, the AL4021 shows only minor interferences from other relevant substances:


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