radiello™ Diffusive Air Sampling Applications - Aldehydes

radiello™ Components Used:

Blue diffusive body, Product No. RAD1201
Supporting plate, Product No. RAD121
Vertical adapter, Product No. RAD122 (optional)
Chemiadsorbing cartridge, Product No. RAD165
Filtration kit, Product No. RAD174 (only for analysis)

Principle

Cartridge (Product No. RAD165) is a stainless steel net cartridge filled with 2,4-dinitrophenylhydrazine (2,4-DNPH) coated Florisil^®. Aldehydes react with 2,4-DNPH to give the corresponding 2,4-dinitrophenylhydrazones

The 2,4-dinitrophenylhydrazones are then extracted with acetronitrile and analyzed by reverse-phase HPLC and UV detection.

Sampling Rates

Sampling rates values Q at 298 K (25 °C) and 1013 hPa are listed below:

Effect of  Temperature, Humidity, and Wind Speed

Sampling rate varies from the value at 298 K on the effect of temperature (in Kelvin) as expressed by the following equation

where Q_K is the sampling rate at the temperature K and Q₂₉₈ is the reference value at 298 K. This produces a variation of ±1% for 10 °C variation (upwards or downwards) from 25 °C. Sampling rate is invariant with humidity in the range 15-90% and with wind speed between 0.1 and 10 m·s^-1.

Calculations

The average concentration C over the whole sampling time (in μg·m^-3) is calculated according to the expression:

where:
m = mass of aldehyde in μg
t = exposure time in minutes

Exposure

The optimum exposure duration varies with the expected concentration. Taking formaldehyde as an example, concentration values of 5-30 μg·m^-3 are usually found in outdoor urban measurements while 20-200 μg·m^-3 are expected in workplace environments. In workplace environments concentrations may be as high as 2,000-3,000 μg·m^-3 for short time intervals: it can therefore be interesting to evaluate the peak value (usually referred to by STEL). The corresponding advised exposure time is shown in the table below:

Do not expose all of the cartridges belonging to the same lot: keep at least two cartridges as blanks.

Storage

The cartridges need to be kept in a dark place at 4 °C. Closed in their bags, they are stable at least six months from the production date. Each cartridge has a blank value of formaldehyde less than 0.1 μg, corresponding to a concentration in air less than 0.1 μg·m^-3 over one week of exposure or 2 μg·m^-3 over 8 hours. The blank value may increase with time.

The expiration date (printed onto the plastic bag) indicates when the expected content of formaldehyde (for a correctly stored cartridge) will probably exceed 0.2 μg.

Formaldehyde stability in the cartridge after the sampling (on top) and in solution (left). The stability tests were performed upon cartridges exposed for one week in a standard atmosphere chamber at 25 °C and with 50% relative humidity and at two different concentration levels. Each bar in the plot represents the average and error from the analysis of six samples.

After exposure keep the cartridges well capped at 4 °C, they are stable for 60 days. After solvent desorption (see
Analysis) and extraction of the cartridge from the tube, the resulting solution, well capped and stored at 4 °C, is stable for at least 42 days (see on page C2).

USER TIP
For extracting the cartridges use Acetonitrile (Product No. 34921) with low aldehyde content.
For a reliable and rapid filtration use the filtration kit (Product No. RAD174).
To obtain an accurate calibration curve, we offer the calibration solution (Product No. RAD302).

Analysis

Desorption

Materials
- HPLC- or spectroscopy-grade acetonitrile
- class A volumetric pipette, capacity 2 mL
- micropore filter membranes, porosity 0.45 μm, solvent resistant

Procedure

Introduce 2 mL acetonitrile directly in the cartridge tube, recap and stir from time to time for 30 minutes. Discard the cartridge. Filter the resulting solution and keep it capped until analysis. If analysis must be delayed, store the solution at 4 °C.

IMPORTANT: verify the presence and the abundance of the 2,4-DNPH chromatographic peak: otherwise, the cartridge could be saturated.

Instrumental Analysis

The method suggested below may be altered as needed.

Materials

• Reverse-phase C18 HPLC column, length 150 mm, 4.6 mm diameter, 5 μm packing particle size (e.g., Ascentis C18, Product No. 581324-U)
• HPLC apparatus capable of elution gradient and UV detection

Procedure

Set the detector at the wavelength of 365 nm. Inject between 10 and 50 μL of solution and elute as follows:
- flow: 1.9 mL·min^-1
- Isocratic elution with acetonitrile/water 38:62 v/v for 10 minutes, up to acetonitrile/ water 75:25 v/v in 10 minutes, reverse gradient to acetonitrile/water 38:62 v/v in 5 minutes.
Below: the chromatogram of a real sample analyzed under the described conditions.

IMPORTANT

Acrolein gives place to three chromatographic peaks, two of them are unresolved. Calculate the concentration basing onto this most abundant peak and ignore the others.
Isopentanal appears as two unresolved peaks: its concentration should be obtained by integration of both peaks as a sum.

USER TIP

If you perform several analyses, a barcode reader will substantially improve productivity and will minimize the possibility of errors in the copying of sample labels. Please contact us for help in the implementation of the reader. We have software solutions for the analytical data processing and automated production of analysis reports.

Interferences

Other Carbonyl Compounds

All carbonyl compounds, ketones included, react with 2,4-DNPH but do not interfere in the analysis if proper chromatographic parameters are selected. In the described chromatographic conditions acetone-2,4-DNPH peak is well resolved from acrolein-2,4-DNPH. Neverthless, if acetone concentration is higher than 50,000 μg·m^-3, acrolein-2,4-DNPH peak intensity is depressed by 25%.

Ozone

Examples of ozonolysis of dinitrophenylhydrazones on active supporting materials as silica gel are found in the literature.

Cartridge (Product No. RAD165), packed with coated Florisil^®, ozonolysis is much less important than on any other commercial aldehyde sampling device, either diffusive or pumped, and becomes appreciable only if ozone concentration, averaged over the whole exposure time interval, is higher than 100 ppb. Generally no correction is needed to account for ozone concentration. If there is evidence that ozone concentration is equal or higher than 100 ppb over the whole exposure time, make use of the corrected sampling rate values shown in the table below, where [O₃] is ozone concentration in ppb.

Sampling rate as a function of ozone concentration normalized to 100 for [O₃] equal to zero. Apart from acetaldehyde, ozone effect becomes relevant only at concentration levels higher than 100 ppb as an average over the whole exposure time interval.

The listed values are for 298 K (25 °C). For deviations greater than ± 10 °C, substitute the base value (e.g., 99 mL·min^-1 for formaldehyde) with the corrected value calculated according to equation on page C1. No experimental data is available for butanal and glutaric aldehyde.

Sampling rate for ozone concentration [O₃] in ppb (apply only if [O₃] >100 ; for acetaldehyde [O₃] >50ppb)