3.8  Determination of aldehydes and ketones in ambient air

3.8.1  Introduction

Recent years several methods for simultaneous determining of aldehydes and ketones in air have been investigated. The most popular method so far utilizes a solid adsorbent coated with 2,4-dinitrophenylhydrazine (DNPH), and the resulting derivatives are subsequent analysed by high performance liquid chromatography and UV detection, e.g. Slemr (1991). This method has been modified for use in EMEP, and the procedure is described below.

The range of concentrations of the individual aldehydes and ketones which may be determined is typically from 0.1 µg/m3 to 10 µg/m3.

The detection limit of a typically sample (sample volume 750 litres) lies in the range 0.01 µg/m3–0.05 µg/m3. Preliminary results from parallel sampling at Birkenes give a relative standard deviation of 6% for methanal (mean 0.30 µg/m3) and propanone (mean 0.99 µg/m3, and 12% for ethanal (mean 0.42 µg/m3).

3.8.2  Principle

The air sample is drawn through a cartridge which contains 2,4-dinitrophenyl­hydrazine (2,4-DNPH)-coated silica packed in a polyethylene tube. Aldehydes and ketones react with the acified 2,4-DNPH to form the corresponding hydrazones.



Figure 3.8.1:   Reaction scheme.

After exposure the cartridge is eluated with acetonitrile, and the sample extract is analysed by reversed phase high-performance liquid chromatography, using UV-detection (diode array detector). The detection and quantification is carried out at 369 nm (band width 22 nm) using 474 nm (band width 50 nm) as the reference wavelength. The detection and quantification of dicarbonyls is carried out at 440 nm (band width 22 nm) using 337 nm (band width 50 nm).

3.8.3  Sampling equipment

3.8.4  Commercial supply

Absorption tubes:
Sep.-PAK DNPH-silica, cartridges, No. 37500
Waters, Millipore Corporation
Waters Chromatography
Millford, MA, USA.

Membrane pump:
GAST, Model DOA-P101-BN
MFG Corp.,
Benton Harbor, Mich. USA

Gas meter:
FLONIDAN
Gallus 2000 G1.6
Islandsvej 29
DK-8700 Horsens, Denmark

The sampling equipment set-up is shown in Figure 3.8.2. The threads in the pump inlet, pump outlet, and gasmeter inlet require use of teflon tape to get the connections leakproof. Be careful and do not overtighten the connections!

Before use, the equipment should be leakproof-tested and tested for appropriate air flow rate (typically 1.5 litres/min–2 litres/min).

 Leakproof-test: Plug the air inlet PE-tube with a male Luer plug (or your thumb) and start the pump. Keep the sampling line plugged for 2 minutes. During this time period the gas meter reading should be constant.

Air flow rate test: Connect the DNPH-silica cartridge (connect a cartridge which has been used and dried ) to the air inlet tube (as shown in Figure 3.8.3) and start the pump. Measure the "sample" volume over a period of 5 to 10 minutes and calculate the flow rate in litres/min. If necessary adjust the needle valve until you have a flow rate of 2 litres/min or as high flowrate as possible not exceeding 2 litres/min (sometimes the cartridge-restriction makes it impossible to reach 2 litres/min).

3.8.5  Preparation of ozone-scrubber

Form a coil from a copper tube (1 meter, 0.46 cm i.d.). Fill the coil with a potassium iodide solution (dilute a saturated aqueous potassium iodide solution 1:1 with water) for 5-10 minutes. Drain the coil and dry it completely by passing nitrogen through the coil.


 
Figure 3.8.2: Sampling equipment set-up.

 

 

 

 

 

 

Figure 3.8.3: The figure shows how DNPH-silica cartridges and the ozone-scrubber are connected to the pump inlet polyethylene (PE) tube.


3.8.6  Sampling procedure

The "cookbook":

  1. Record the following in the sampling form: Batch number (= Lot No.) of the sample cartridge, ozone scrubber number, sample name, outdoor temperature, indoor temperature, sampling technique and signature.
  2. Remove the endcaps from the ozone scrubber.
  3. Open the sealed pouch which contains the cartridge. Remove the Luer plug and cap and keep them in a place where they do not disappear (you need the plug and cap for sealing the cartridge after completed sampling).
  4. Connect the ozonescrubber, cartridge, and the PE-tube in the manner shown in Figure 3.8.3. (The ozone scrubber should be mounted upstream of the cartridge.) The male Luer tip of the cartridge has to enter the PE-tube (“edge-to-edge”), in such a way that the silicon tube becomes a support and not the main seal (then there will be no contact between the exhaust and the silicon tube).
  5. Install the sampling equipment in sampling position.
  6. Record the following in the sampling form: gasmeter reading and sampling start time. Then turn on the sampling equipment.
  7. After a run time of 5-10 minutes optimize the flow rate.
  8. Seal the cartridge with Luer cap and plug after completed sampling.
  9. Record the following in the sampling form: gasmeter reading and sampling end time.
  10. Seal the ozone scrubber with end caps.

In order to avoid the potassium iodide film being dissolved by water, a weather-protection "device" has to be mounted on the scrubber (teflon funnel or the upper half of a teflon bottle).

3.8.7  Sampling handling

3.8.8  Procedure for blank sample preparation

  1. After preparing the sampling equipment for sampling, open a sealed pouch with same batch number as the sample batch number.
  2. Record the following in the sampling form: Batch number (= Lot No.) of the blank sample cartridge, starting time (= opening time for the pouch), sample name (= same as "real" sample name), indoor temperature and signature. Instead of recording gasmeter readings and sampling end time "BLANK" should be noted in the sampling form.
  3. Place the cartridge nearby the sampling equipment during sampling. Do not remove the end cap and plug.
  4. After completion of the real sampling the blank sample cartridge should be treated in same manner as the real sample. Label the glass container with "sample name" and BLANK.
  5. The blank sample cartridge has to be mailed together with the real sample in the same envelope. Please do remember that the sampling forms have to be mailed together with samples.

3.8.9  References

Dye, C. and Oehme, M. (1992) Comments concerning the HPLC separation of acrolein from other C3 carbonylcompounds as 2.4-Dimitrophenylhydrazones: A proposal for improvement. J. High Resolut. Chromatogr., 15, 5.

Millipore Corp. (1992) Waters Sep-Pak DNPH-Silica Cartridge; Care and Use Manual. Milford, USA, Waters Chromatography Publications.

Slemr, J. (1991) Determination of volatile carbonylcompounds in clean air. Fresenius J. Anal. Chem., 340, 672.


Last revision: November 2001