Biomedical Engineering Reference
In-Depth Information
suitable for miniaturization using microfluidic components compared with chro-
matography (e.g., [
91
]). There is also no need for probably tedious column
equilibration, and the columns themselves are less expensive. Sample preparation
can be omitted, as shown during degradation studies of phenols with Rhodococcus
[
92
]. However, monitoring of organic acids from lactic acid bacteria had to use a
membrane interface to the bioreactor prior to loading the samples into CE [
93
]; the
time resolution could be reduced to 2 min in this case.
CE comprises a system in which fluids are transported (by either micropumps or
air pressure, or electroosmotically) while separation of dissolved components takes
place in an electrical field. Since this ranges into the 10-kV domain, the materials
used for construction must sustain this high voltage: good electrical insulators are
needed, and PEEK, PTFE or glass are materials of choice.
The method is versatile and enables the separation of small ionic species (e.g.,
trace elements [
94
]) over organic acids up to (charged) biopolymers. The vari-
ability (or heterogeneity) of the substances that can be separated in one run is
usually broader than in chromatographic techniques. Applicable detectors are
either electrochemical or optical, e.g., UV/Vis or laser-induced fluorescence
detectors. The degree of downsizing (miniaturization) is nowadays limited by the
sensitivity of the detectors.
3.3.4 Mass Spectrometry
Mass spectrometers may be the detectors at the end of the analytical line (for
instance as a LC-MS chain) or may be the analytical instrument of choice per se.
If more than two gasses, or gasses other than O
2
and CO
2
, should be analyzed,
or if one needs to follow aroma components or simply volatiles, a mass spec-
trometer would be the ideal instrument (see also
Sect. 2.2.5
). A capillary inlet is
appropriate for this goal. A mass spectrometer is essential even for CO
2
(and other
gases or volatiles) when its isotopic distribution is of interest; for instance, in the
case of tracing the fate of a
13
C-labeled substrate, the mass spectrometer can
distinguish between
12
CO
2
and
13
CO
2
, whereas other instruments cannot. This
may be of significant importance in metabolomic, regulomic, or fluxomic studies
[
95
,
96
].
Sampling directly from the gassed biosuspension is possible and has been done
using membrane interfaces in which the membrane serves as (a) a sterile barrier,
(b) a pressure barrier, and (c) a selective gate to avoid or permit entry of various
species. However, one has to order a new mass spectrometer (at least the high-
vacuum parts) if the membrane happens to fail, which is not attractive for pro-
duction facilities. Chemical ionization MS (CIMS) was used by Custer et al. [
24
]
to follow many volatiles produced during a B. subtilis cultivation, namely acet-
aldehyde, ethanol, acetone, butanol, acetoin, diacetyl, and isoprene, with simul-
taneous gas chromatography as reference. This technique is also being applied in
the food industry, for instance, laser-ionization time-of-flight mass spectroscopy in
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