Chemistry Reference
In-Depth Information
Figure4.7.
Dried residues of 1
μ
l sized droplet (top) in comparison to a 10 nl sized droplet (bottom).
The pictures were taken by an X-ray microprobe and show the distribution of copper by means
of the Cu-K
α
peak. (a) 1
μ
l with 10 ng Cu; (b) 1
μ
l with 100 ng Cu; (c) 10 nl with 0.1 ng Cu; (d) 10 nl
with 1 ng Cu. Figure from Ref. [32], reproduced with permission. Copyright1996 by John Wiley
and Sons.
4.1.3.3Picoliter-SizedDropletsbyInkjetPrinting
For even minor deposits, even smaller volumes of liquid samples should be
used; that is, picoliter-sized droplets (10
12
l). Furthermore, they should be
deposited in a well-defined programmable pattern or specific array that
matches the detector's view field. The multifold repeated depositions should
be carried out automatically, not manually, in a short time of a few minutes with
a high repeatability.
For that purpose the technique of inkjet printing was applied by Fittschen
and coworkers [33-36]. Inkjet printers have been developed for computers
since 1970 and represent a fairly mature technique. They either use piezo-
electric pulses to generate droplets of ink and to force them from different
nozzles or they use a tiny heater to rapidly vaporize such droplets propelling
them onto a substrate, usually paper. Because of the high velocity (10-40 m/s)
the distance between head and substrate can be up to 1 cm. The ink filled into a
cartridge is usually an aqueous solution but may have a volatile component.
Commercially available inkjet printers (e.g., the thermal inkjet picofluid
system or TIPS of Hewlett-Packard) can be modified slightly in order to use
pico-sized droplets for TXRF analysis. The droplets of an aqueous standard
solution have a volume of 1-200 pl containing several elements, mostly trace
metals with a mass between 1 pg and 2 ng. They are deposited on different
carriers, such as silicon wafers or quartz glass (coated or uncoated). Residual
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