4.16  Determination of aldehydes and ketones in ambient air

Aldehydes and ketones sampled as 2.4-dinitrophenylhydrazones in impregnated tubes can be analysed in extracts with high performance liquid chromatography (HPLC).

4.16.1  Instrumentation

The following instruments and equipment may be used:

4.16.1.1  Chemicals

4.16.2  Analytical procedure

Fill a 5 ml syringe with acetonitrile. (Collect the sample extract in a 3 ml narrow neck flask). Eluate the derivatives by slowly (approximate 1.5 ml/min) pushing acetonitrile through the cartridge. Stop the elution when the 3 ml mark is reached. Transfer approximately 0.5 ml of the sample solution to a 2 ml autosampler vial and seal the vial. The sample is now ready for HPLC analysis.

10 µl of a sample or standard solution is separated by using a quaternary mixture of methanol/water/acetonitrile/tetrahydrofuran. Table 4.16.1 shows the gradient profile which is used.

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).

Following carbonylcompounds should be measured: methanal, ethanal, propenal, propanal, propanone, 2-methyl-propenal, butanal, 2-butanone, 3-buten-2-one, pentanal, hexanal, benzenecarbaldehyde, ethandial, oxopropanal.

Table 4.16.1:  Quaternary gradient which separates the carbonylhydrazones of interest (EMEP) at a flowrate of 0.8 ml/min.

Time

% Tetrahydrofuran

% Acetonitrile

% Water

% Methanol

  0.0

18.0

22.0

60.0

0.0

  0.5

18.0

22.0

60.0

0.0

20.0

8.4

37.4

54.2

0.0

24.0

0.0

0.0

34.0

66.0

40.0

0.0

0.0

15.0

85.0

41.0

0.0

0.0

15.0

85.0

45.0

0.0

100.0

0.0

0.0

48.0

18.0

22.0

60.0

0.0


4.16.3  Blanks

Each day analyses of carbonylcompounds are performed, a laboratory blank should be prepared. Periodically field blanks should be obtained once every week. The blank levels of methanal, ethanal, and propanone will probably change with cartridge batchnumber and the batchnumber of acetonitrile. The blank level should not exceed 
0.05 µg/m3 of the carbonylcompound in a sample volume of 750 litres.

4.16.4  Preparation of hydrazones

  1. Dissolve 400 mg 2,4-dinitrophenylhydrazine in 2 ml 96% sulphuric acid. This solution is then added, with stirring, to 13 ml 75% ethanol. Any undissolved solid is removed by aid of a pasteurpipette.
  2. A volume corresponding to 500 mg of the carbonylcompound is transferred to 20 ml ethanol.
  3. The carbonylsolution (step 2) is than transferred to the DNPH solution (step 1) with stirring. Let the solution stand for 15 minutes to complete the reaction.
  4. Filter the solution in step 3, and recrystallize the hydrazone from aqueous ethanol. (Hydrazones from unsaturated/aromatic carbonyls should be recrystallized from aqueous acetonitrile.)
  5. Dry the hydrazone and do a purity test (HPLC-UV).

4.16.5  Calibration

Prepare a stock solution from each carbonylhydrazone by dissolving approximately 5 mg (+/- 1%) in 100 ml acetonitrile. (These stock solutions will be ready for use.) Calibration solutions are prepared by dilution of the stock solutions (1 µg/ml to 2 µg/ml is suitable for most analyses).

4.16.6  Quantification

The concentration of carbonylcompounds in the air sample expressed as g/m3 , is given by:

C     : Concentration of the carbonylcompound in the air sample [µg/m3]
c     : Concentration of the carbonylcompound in the standard [µg/ml]
H(s)  : Peak height/area of the carbonylcompound in the standard [counts]
H(p)  : Peak height/area of the carbonylcompound in the sample [counts]
k     :  Conversion factor (e.g. from hydrazon to carbonyl) methanal:0.1429, ethanal:0.1964, propenal:0.2373 etc.
V     : Sample volume [m3]
v     : Volume of the prepared sample [ml]


4.16.7  Interferences

Failure to remove ozone by the ozone scrubber will result in serious under­estimating of some carbonylhydrazones.

4.16.8  References

Vairavamurthy, A., Roberts, J.M. and Newman, L. (1992) Methods for determination of low molecular weight carbonyl compounds in the atmosphere: a review. Atmos. Environ., 26A, 1965-1993.

Slemr, J. (1991) Determination of volatile carbonyl compounds in clean air. Fresenius J. Anal. Chem., 340, 672-677.

Dye, C. and Oehme, M. (1992). Comments concerning the HPLC separation of acrolein from other C3 carbonyl compounds as 2,4-dinitrophenylhydrazones: a proposal for improvement. J. High Res. Chrom., 15, 5-8.


Last revision: November 2001