Chemistry Reference
In-Depth Information
was deposited by means of a Sonoplot printing device and dried. The camera was
positioned directly above the sample in order to monitor the shading of the dried
residues at usual small angles of incidence. By a rotation of the sample, the
shading could be minimized and the fluorescence intensity be maximized.
4.1.3.4MicrodispensingofLiquidsbyTriple-JetTechnology
A new technique for automatically dispensing a liquid sample on a flat carrier
was developed by the company M2 Automation in Berlin, Germany, and is
commercially available. The instrument, TWO, is a 2D imaging system for
droplet detection and imaging and combines three different microdispensers:
(1) a piezo-driven dispenser for picoliter droplets; (2) an M2 dispenser with
shockwave generator for low nanoliter drops; and (3) a solenoid-valve-driven
dispenser for midnanoliter drops. The first produces single droplets between 20
and 300 pl, the second produces droplets between 5 and 200 nl, and the third
produces drops between 30 nl to 1
μ
l. Dispensing tips are made of glass,
stainless steel, and plastic material. The latter tips are cheap and can simply
be disposed after use instead of washing them. The maximum driven range is
20 cm in the
xy
directions and 5 cm in
z
direction. The speed is about 0.1 m/s,
and the accuracy is
±
20
μ
min
xy
directions. Dispensing is carried out with a
maximum velocity of five depositions per second. A substrate designer assists in
arraying patterns of some 10 to 400 dots via a mouse click.
This instrument was tested by Mages
etal
. for TXRF analysis [38]. Biological
samples (benthic invertebrates) were analyzed after cold-plasma ashing and
digestion by HNO
3
and H
2
O
2
. A manual pipetting of 10
μ
l droplets lead to the
notorious ring-shaped residues after drying and to a reduced precision and
accuracy of analytical determinations. By means of the microdispensing tech-
nique, 80 single droplets with only 10 nl volume were pipetted in six concentric
circles on a carrier automatically. After drying at 50
°
C, all the single dots were
mutually analyzed by TXRF. The relative standard deviation of all 13 investi-
gated elements determined by three replicates could be reduced to about 1 to
2.5%. Five elements K, Mn, Fe, Cu, and Zn were determined with a concen-
tration between 11
μ
g/ml (ppm) for potassium and 45 ng/ml (ppb) for copper.
For these elements, the relative standard deviation, resp. the precision, was
improved by a factor between 3 and 12 (geometric mean 6.3). The deviation of
the mean values determined after manual pipetting of one drop and automatic
dispensing of 80 droplets was below 3% for seven elements.
4.1.3.5SolidMatterofDifferentKinds
Solid samples in the form of fine powders with a grain size of
<
1
μ
m can be
applied directly in a most simple way. A spatula made of PTFE can be used to
take a small amount of the powder and dust it on a carrier. The small grains
usually stick to the carrier but can be removed by knocking the carrier when too
large an amount has been taken.
Search WWH ::
Custom Search