Page 170 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

Page 155

homogeneous. However, this can be an advantage if a spatial map of the analyte within the sample is
required.

Slurry nebulization has also proved very popular. In this technique, sample (typically 0.25 g) is placed
in a 30 ml plastic bottle and 10 g of expanded zirconia beads are added. A dispersant is added and the
bottle is sealed and then placed on a mechanical shaker for several hours. During the shaking, the
zirconia beads grind the sample into very fine particles. After dilution to a known volume, the slurry
may be aspirated directly into an atomic spectrometric instrument. Other methods of slurry preparation
also exist, e.g. using a micronizer, but the 'bottle and bead' method is the most common.

There are several advantages to slurry nebulization:

(i) samples may be analysed against aqueous calibrants;

(ii) it avoids the use of hazardous acids that are required for digestion /dissolution;

(iii) it is relatively contamination free;

(iv) it avoids possible losses of volatile analytes.

The main disadvantage of slurry nebulization is that a very small particle size is required. If the
particles are too large, then once diluted to volume, the particles sink to the bottom of the flask. The
slurry aspirated may therefore not be representative of the original sample. Another problem that arises
when the particle size exceeds approximately 2 µm, is that the slurry no longer behaves in a similar way
to aqueous solution, and can therefore not be determined against aqueous standards. In the spray
chamber of the spectrometer, larger particles pass directly to waste, and therefore a non-representative
sample is presented to the plasma. For sample preparation, several dispersants may be used. For
inorganic samples, e.g. glasses, firebricks, dispersants such as sodium hexametaphosphate or sodium
pyrophosphate may be used. For samples that are more organic or biological in nature, Triton X-100
or Aerosol-OT are available. Slurries have also been analysed using ETAAS. The preparation is
similar, but the particle size is less critical. The slurries have to be stabilized for analysis using an
autosampler, i.e. the slurry has to be homogeneous when it is analysed. To ensure this, an ultrasonic
probe has been developed that mixes the slurry just prior to injection.

7.8 Nebulizers

In addition to concentric pneumatic nebulizers that are used most frequently for the majority of atomic
spectrometric systems, a range of other nebulizers also exist. Some nebulizers that are used for plasma
instrumentation, e.g. direct injection nebulizers (DIN) or ultrasonic nebulizers (USN), increase