DIFFERENT FIELDS OF APPLICATIONS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 300

Figure5.5. TXRF spectrum of air dust collected from ambient air. A Battelle-type impactor was
used for collection of particles with diameters between 2 and 4 μ m. Germanium was added as
internal standard of 10 ng. The other specifications are also given in nanograms. Figure from
Ref. [3], reproduced with permission. Copyright1996, John Wiley and Sons.
Airborne particulates of wet air or aerosols are reliably deposited and stick
to the flat carriers [26,27]. Particles of dry air, however, are bounced or blown
off. To avoid these effects, the Plexiglas carriers should first be siliconized with
a silicone solution or even coated with a suitable grease. Medical petrolatum
(e.g., Vaseline petroleum jelly) is recommended for its high purity [28]. An
alkaline Ge standard can be pipetted onto the coated carrier and dried by
evaporation. After a blank control, the carriers are inserted into the impactor
and used for sampling. The dust-laden carriers are directly analyzed by TXRF.
Figure 5.5 shows a spectrum of dust from ambient air collected by a two-
stage Battelle impactor [25]. The various elements were found in the nanogram
range, with detection limits below 1 ng down to 0.01 ng [29]—three orders of
magnitude less than with conventional XRF. Consequently, the collection time
can be reduced to 1 h and the sampling volume to 0.5 m 3 , so that pollution in the
course of a day can be observed.
Due to such direct analysis by TXRF, otherwise frequent systematic errors
of digestion or dissolution are avoided. However, the high sensitivity of TXRF
reveals other systematic errors. In a particular study, air was sampled from a
clean bench but nevertheless led to significant contamination of previously
clean carriers [25,28,30]. These blank values were caused by erosion of the
impactor walls made of stainless steel. The error can be avoided by use of a
more suitable impactor material, for example, of an antistatic polymer.
Effective monitoring of air pollution presupposes small sampling volumes
and short collection times, as is offered by TXRF. This advantage has been used