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yet commonly used for trace metal analysis. Therefore, prior to the
implementation of either of these methods, the new method must be
rigorously compared with a proven technique for validation.
7. The linear range for palladium quantification must be as broad as possible -
An API may contain palladium in the 2-2000 ppm range. Therefore, the
lower and upper limits of quantification need to span at least three
orders of magnitude.
16.4 Fluorimetric and Colorimetric Methods for
Quantifying Trace Amounts of Palladium
16.4.1 Chemosensors for Palladium
Chemosensors are molecules that reversibly bind to an analyte and change
the optical properties. The generally quick binding event is beneficial,
allowing rapid determination of palladium concentration. A major limi-
tation is that one molecule of palladium can only generate one molecule of a
fluorescent complex at best, providing only moderately sensitive methods. In
other words, chemosensors do not utilize catalysis to generate a fluorescent
signal. Furthermore, APIs may interfere with the resulting signal, as these
methods generally rely on metal-heteroatom bindings.
Nielsen's group exploited the anity of sulfur atoms towards palladium,
as depicted in Figure 16.4(a); the thiourea derivative 11 bound to palladium
to form the complex 12 that exhibited a unique UV-visible spectrum.
43
With
this method, the product of a Sonogashira coupling 15 [Figure 16.4(b)] was
analyzed for trace palladium. Their colorimetric method and inductively
coupled plasma sector field mass spectrometry indicated 376 and 610 ppm
of palladium, respectively. Notably, Pd, Pt, Cu and Ni showed different ab-
sorption spectra when bound to 11.
(a)
11
12
(b)
13
14
15
Figure 16.4
(a) Compound 11 binds to palladium to form complex 12.(b)A
Sonogashira coupling was performed between 13 and 14 and product
15 was analyzed for its trace palladium content using 11.
Reproduced with permission from Ref. 43. Copyright 2006 Georg
Thieme Verlag KG.
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