Pharmaceutical antibiotic compounds in soils - a review - Susane.info

148 Thiele-Bruhn J. Plant Nutr. Soil Sci. 2003, 166, 145±167
The TCs strongly absorb light and thus, are susceptible to
photodegradation (Mitscher, 1978).
Sulfonamides
Sulfonamides (SAs) are relatively insoluble in water. They are
characterized by two pK a values indicating protonation of the
amino group at a pH of 2 to 3 and deprotonation of the
R 1 SO 2 NHR 2 moiety at a pH of 5 to 11 (Ingerslev and Halling-
Sùrensen, 2000). In general, the amphoteric SAs behave as
weak acids and form salts in strongly acidic or basic
solutions. Mostly, SAs, substituted at the amino-N, have
greatly reduced antibacterial activity.
Aminoglycosides
Antibiotics from the class of aminoglycosides are basic,
strongly polar polycationic compounds. Their molecular
structure is characterized by two or more amino sugars that
are glycosidically bound to aminocyclitol. They are water
soluble, mostly hydrophilic, and susceptible to photodegradation.
b-Lactams
Penicillins and cephalosporins are the two major sub-classes
of the b-lactams. The antibiotic effect of penicillins is directly
connected to the b-lactam ring. This ring is easily cleaved in
acidic and basic media. Cephalosporins are derivatives of 7amino-cephalosporanic
acid, condensed with a six-membered
heterocycle in contrast to the five-membered heterocycle
of penicillins.
Macrolides
Macrolides are defined as lactone structures with cycles of
more than 10 C-atoms. Many macrolides are weak bases and
are unstable in acids. Their water solubility varies considerably
between the different derivatives.
Fluorquinolones
Most fluorquinolones (FQs), also known as quinolones,
exhibit large chemical stability. They are insensitive to
hydrolysis and increased temperatures, but are degraded
by UV light. Their antibiotic potency depends mostly on the
aromatic fluorine substituent at the C-6 position (Wetzstein,
2001).
3 Extraction and determination
Numerous antibiotics are comprised of a non-polar core and
polar functional groups (Juhel-Gaugain et al., 2000). They are
sensitive to bases and strong acids and dissociate or
protonate depending on the pH of the medium. Thereby,
their distribution behavior changes considerably (Holten
Lützhùft et al., 2000). Consequently, incomplete extraction
of antibiotics with very polar and non-polar extractants and
strong adsorption to polar and non-polar solid phase
extractants (SPE) pose serious analytical problems. Thus,
for the extraction of most antibiotics, the use of weakly acidic
buffers in combination with organic solvents is recommended
(Tab. 3). In food analysis, a 0.1 M EDTA-McIlvaine buffer (pH
4.0) is often used (Weimann and Bojesen, 1999; Juhel-
Gaugain et al., 2000). Cooper et al. (1998) and Kühne et al.
(2000) suggested citric ethylacetate (pH 5.0) for the
extraction of TCs. This method was adopted for soil samples
by Hamscher et al. (2002a). At least in the case of TCs, this
extractant yields better recovery rates from soil samples,
although the solubility of numerous antibiotics of different
structural classes is considerably smaller in pure ethylacetate
as compared to methanol or DMSO (Salvatore and Katz,
1993). Oka et al. (2000) reviewed techniques for the
extraction and analysis of TCs and a compilation of methods
can be found in Juhel-Gaugain et al. (2000). The use of 0.01
M CaCl 2 to extract the mobile, non-adsorbed fraction of
xenobiotics as prescribed by OECD (1997), cannot be
recommended for TCs. These compounds form sparingly
soluble complexes with Ca 2+ (Wessels et al., 1998). As an
alternative to CaCl 2 , 0.1 M NH 4 NO 3 was successfully used
(Thiele-Bruhn, unpublished data).
In general, the sample clean-up is done by SPE or 0.45 lm
filtration; extracts from centrifugation and liquid/liquid separation
are usually concentrated by evaporation (Tab. 3). For
SPE of SAs, reversed phases are very effective. In contrast,
the usage of reversed phase materials for SPE of TCs
requires pre-treatment of the solid phase with EDTAor
silylating agents whereas for conditioning and elution, acidic
buffered solvents are recommended (Oka et al., 1991; Zhu et
al., 2001; Loke et al., 2002). The TCs bind so strongly to free
silanol groups that they cannot be eluted by the usual organic
solvents. Lindsey et al. (2001) suggested a macroporous
copolymer combined with Na 2 EDTAas the chelating agent.
To overcome the problems resulting from the amphoteric
character of many antibiotics, new functionalized SPE
materials that separate analytes by their hydrophobic as
well as polar properties, e.g. hydrophilic-lipophilic balance
cartridges (HLB) and mixed-mode HLB-cation exchange
cartridges (MCX), appear to be advantageous (e.g. Kolpin
et al., 2002). Golet et al. (2001) extracted FQs from
wastewater with a mixed-mode silica based sorbent consisting
of a non-polar phase and a strong cation exchanger
interacting with the aromatic moiety of the core and the
charged amino groups of the substituents.
Antibiotics are usually separated by chromatographic techniques
and subsequently detected. To avoid dissociation of
the compounds or binding to free silanol groups from the
widely used silica based chromatographic columns, dilute
acids or weakly acidic buffers, e.g. phosphoric, citric, formic,
and oxalic acid or EDTA, are commonly used (Oka et al.,
2000). Alternatively ion pair-chromatography is applied.
Currently, the time and solvent consuming thin layer
chromatography has been mostly replaced by high performance
liquid chromatography combined with UV and Diode-
Array Detection (HPLC-UV, -DAD). Increasingly, liquid
chromatography with mass spectrometry (LC-MS) or tandem
mass spectrometry (LC-MS/MS) (Thomashow et al., 1997;
Oka et al., 2000; Hamscher et al., 2002a) with chemical