Rhamnolipids Characteristics, Production, Applications, and Analysis - Biosurfactants: Research Trends and Applications - page 89

near-IR spectral regions (250-2500 nm) (Allen et al., 2012), the intensity of the light
passing through the solution can be an accurate indicator of the concentration. For
measuring the rhamnolipids using spectrophotometer, some researchers introduced
a method consisting of using orcinol as a solvent.
The orcinol method consists of extracting and separating the extract from the cells
and the heating the mixture of supernatant, solvent, and sulfuric acid and orcinol
(1,3-dihydroxy-5-methylbenzene) until a blue-green color is observed. According to
Abdel-Mawgoud et al. (2011), this blue-green color is the result of the hydrolyzing
of rhamnose groups into methyl furfural. After the appearance of this green-blue
color, the intensity of the color is measured by using a spectrophotometer at 421 nm
(Abdel-Mawgoud et al., 2011), then comparing the result with the standard curve at
various concentrations of the rhamnolipids for calibration (Abdel-Mawgoud et al.,
2011; Ballot, 2009; Wang et al., 2007).
Mass Spectrometry
MS can be used to measure both the quantity and the quality of the materials. It can
be used to determine the composition of the materials, to detect and to identify the
unidentified compounds, and to identify bonds and the structure of the material.
Mass spectrometers are made of three main parts: ion source, mass analyzer, and
detector (Downard, 2004). In this method, the mass to charge ratio of the material
is measured, and this is done through ionizing the chemical compounds in order to
create charged particles. For this method, the sample loaded onto mass spectrom-
eter is vaporized; volatile samples can be introduced directly. However, nonvolatile
samples are solved in a volatile solvent.
After the vaporization of the samples, they go through an ionizing chamber, where
an electron beam is focused and the particles bombarded to ionize them. Next, these ion-
ized particles are accelerated through an electric field toward an electromagnetic field
and finally a detector. These charged particles can easily react with other gases on their
path. Therefore, to prevent the collision of these ionized particles with gaseous contami-
nants and air molecules, the operation takes place under vacuum. When these vaporized
ions go through electromagnetic fields in the analyzer, they are separated according to
their mass and charge. After separation, ions pass onto an ion detector. Depending on their
mass and charge, these ions are producing some electrical currents. The electronic signal
is amplified and recorded; then the detected values are sent to a computer. By studying the
peaks of the generated graphs, quantity of each ion could be measured. Mass spectroscopy
(MS) can detect as low as 10 −12 mol (Downard, 2004; Verbeck et al., 2002). By coupling
gas chromatography with mass spectroscopy (GC-MS) and coupling inductively coupled
plasma source with mass spectrometer (ICP-MS), the detection level could be lowered
and the need for using carrier gases (for GC) eliminated.
The investigations of Pereira et al. (2012) on rhamnolipids, produced by P. aerugi-
nosa strains isolated from Brazilian crude oil, showed that MS coupled with electro-
spray ionization (ESI-MS) analysis provided a rapid and accurate characterization
of biosurfactants. Likewise, tandem mass spectrometry (MS/MS) provided accurate
information about the structure of the biosurfactants. Furthermore, GC-ICP-MS
and HPLC-ICP-MS have been introduced as new generation of analyzers that could
be easily used for speciation of molecules (Bird, 1998; Hata et al., 2007).