PERFORMANCE OF TXRF and GI-XRF ANALYSES - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 215

The addition of an internal standard is a further aid to eliminating certain
systematic errors. With reference to a standard element, various nonspecific
errors of preparation are compensated for, for example, nonspecific losses. In
order to compensate for a maximum of errors, the standard should be added to
the sample as early as possible. This step, called spiking , should be done after
dissolution or suspension of the sample but before a matrix separation or trace
concentration if possible.
Different types of complex samples can be prepared for TXRF analysis by
ashing, pasting, and plotting [27]. Consequences can be summarized by the
following statements:
• Edible oils (fatty acids) can be prepared by fast microwave ashing and
analyzed down to sub-ppb.
• Solar cells can be pasted onto an aluminum disc with hot glue and contami-
nants of the cells that get stuck on the disc can be determined by TXRF. Iron
and copper can be detected with amounts of some ng down to 35 pg.
• Seawater can be diluted with PVA (polyvinyl solution) in order to avoid
so-called coffee rings after drying. By means of the automatic dispenser
TWO described in Section 4.1.3.4, a grid of 12 × 12 droplets of 16 nl each
can be plotted on a quartz carrier. Plotting can be repeated several times
and the dried residue can be analyzed by TXRF.
• A tutorial review on sample pretreatment strategies was given by De La
Calle etal . [28]. Acid digestion, extraction, slurry preparation, insitu
microdigestion, microflow online preconcentration, and lab-on-a-chip are
represented. Their advantages as well as drawbacks are considered.
Statistical revision of the different strategies published in papers between
2008 and 2013 is also given.
4.1.3PresentationofaSpecimen
Small amounts of a sample should be placed on a cleaned carrier and presented
as a specimen. As already mentioned in Section 3.5.1, quartz glass is the most
frequently used carrier for aqueous solutions and the most acidic and basic
solutions. Although Plexiglas is a cheaper carrier, it is only suitable for aqueous
solutions. After an electrostatically protective foil is stripped off, the clean
carriers can be used directly.
As an upper limit, micrograms of solids (1-200 μ g) and microliters of liquids
(1-200 μ l) yielding micrograms of dry residue can be used as specimens. The
appropriate amount can easily be determined by means of the detector.
The specimens should be restricted to an amount keeping the dead time
loss of the detector below 63% (see Section 4.2.3). As a lower limit, even
picograms of a solid and picoliters of a liquid can be sufficient for analysis. A
more detailed consideration needed for quantitative analyses of three typical
sample matrices is given in Section 4.4.3.