Due to its analytical ability and sensitivity to molecular vibrations, Raman spectroscopy provides valuable information of
the secondary structure of proteins. Moreover, polarized Raman spectroscopy is shown to be a useful instrument to
investigate the structural changes resulting from the aging and spoilage process of meat.
In this work, polarized Raman spectra were measured on oriented cuts of pork and turkey. Fresh meat slices were stored
at 5 °C and measured for a consecutive time period of 10 days. A 671 nm microsystem diode laser was used as excitation
light source. The laser power at the sample was 50 mW and the integration time of each Raman spectrum was set to
5 seconds. Measurements were performed with a laser beam orientation perpendicular to the long axis of the muscle
fibers. In that arrangement, the fibers were aligned either parallel or perpendicular to the polarization direction of the
laser source.
By using the statistical method of principal components analysis (PCA), a clear separation of the meat samples can be
found for fresh meat according to the orientation (parallel or perpendicular) using the first two principal components.
During the storage period, this separation subsequently vanishes due to the aging process and due to an increase of the
microbial spoilage of the meat surface. For the latter effect, a time-dependent distinction of the Raman spectra is
presented as well. Furthermore, specific changes of conformation-sensitive Raman bands were recognized, notably a
decrease of the intensities of α-helical protein conformation.
Based on a miniaturized optical bench with attached 671 nm microsystem diode laser we present a portable Raman
system for the rapid in-situ characterization of meat spoilage. It consists of a handheld sensor head (dimensions: 210 x
240 x 60 mm3) for Raman signal excitation and collection including the Raman optical bench, a laser driver, and a
battery pack. The backscattered Raman radiation from the sample is analyzed by means of a custom-designed miniature
spectrometer (dimensions: 200 x 190 x 70 mm3) with a resolution of 8 cm-1 which is fiber-optically coupled to the sensor
head. A netbook is used to control the detector and for data recording.
Selected cuts from pork (musculus longissimus dorsi and ham) stored refrigerated at 5 °C were investigated in timedependent
measurement series up to three weeks to assess the suitability of the system for the rapid detection of meat
spoilage. Using a laser power of 100 mW at the sample meat spectra can be obtained with typical integration times of 5 -
10 seconds.
The complex spectra were analyzed by the multivariate statistical tool PCA (principal components analysis) to determine
the spectral changes occurring during the storage period. Additionally, the Raman data were correlated with reference
analyses performed in parallel. In that way, a distinction between fresh and spoiled meat can be found in the time slot of
7 - 8 days after slaughter. The applicability of the system for the rapid spoilage detection of meat and other food products
will be discussed.
Experimental results in shifted excitation resonance Raman difference spectroscopy (SERRDS) at 488 nm will be
presented. A novel compact diode laser system was used as excitation light source. The device is based on a distributed
feedback (DFB) diode laser as a pump light source and a nonlinear frequency doubling using a periodically poled lithium
niobate (PPLN) waveguide crystal. All elements including micro-optics are fixed on a micro-optical bench with a
footprint of 25 mm × 5 mm. An easy temperature management of the DFB laser and the crystal was used for wavelength
tuning. The second harmonic generation (SHG) provides an additional suppression of the spontaneous emission. Raman
spectra of polystyrene demonstrate that no laser bandpass filter is needed for the Raman experiments. Resonance-Raman
spectra of the restricted food colorant Tartrazine (FD&C Yellow 5, E 102) in distilled water excited at 488 nm
demonstrate the suitability of this light source for SERRDS. A limit of detection (LOD) of 0.4 μmol·l-1 of E102 enables
SERRDS at 488 nm for trace detection in e.g. food safety control as an appropriate contactless spectroscopic technique.
Raman spectroscopy is a powerful tool to obtain "fingerprint" spectra from substances in numerous applications. In-situ
and/or real time measurements are interesting for the detection of minerals, organic compounds, gas hydrates or methane
in the deep-ocean. In this work we present Raman measurements of selected minerals. A DFB diode laser at 785 nm was
used as excitation light source with an output power up to 1.5 W. An excitation fiber of 1000 m length was used to
demonstrate the suitability of Raman spectroscopy for deep-sea applications.
A novel compact wavelength stabilized diode laser system at 671 nm on a micro-optical bench as a light source for
shifted excitation Raman difference spectroscopy (SERDS) is presented. Two broad-area gain media in separate laser
cavities are used with two reflection Bragg-gratings with slightly different center wavelengths. A constant wavenumber
difference of 13 cm-1 ± 1.3 cm-1 together with a spectral width below 100 pm is obtained up to output powers of
250 mW. The suitability of this light source for SERDS is demonstrated using Raman spectra of ethanol with increasing
concentrations of cresyl violet as fluorescent contaminant.
A hand-held Raman sensor head was developed for the in-situ characterization of meat quality. As light source, a
microsystem based external cavity diode laser module (ECDL) emitting at 671 nm was integrated in the sensor head and
attached to a miniaturized optical bench which contains lens optics for excitation and signal collection as well as a
Raman filter stage for Rayleigh rejection. The signal is transported with an optical fiber to the detection unit which was
in the initial phase a laboratory spectrometer with CCD detector.
All elements of the ECDL are aligned on a micro optical bench with 13 x 4 mm2 footprint. The wavelength stability is
provided by a reflection Bragg grating and the laser has an optical power of up to 200 mW. However, for the Raman
measurements of meat only 35 mW are needed to obtain Raman spectra within 1 - 5 seconds. Short measuring times are
essential for the hand-held device.
The laser and the sensor head are characterized in terms of stability and performance for in-situ Raman investigations.
The function is demonstrated in a series of measurements with raw and packaged pork meat as samples. The suitability
of the Raman sensor head for the quality control of meat and other products will be discussed.
A microsystem based Raman sensor system for the in situ control of meat was realized. As excitation laser source a
compact external cavity diode laser (ECDL) emitting at 671.0 nm mounted on a micro optical bench with a total
dimension of (13 x 4 x 1) mm3 is implemented. An output power of 200 mW, a stable emission at 671.0 nm, and a
narrow spectral width of about 80 pm, i.e. 2 cm-1, were measured. The device is well suited for Raman measurements
of liquid and solid samples. The devices parameters and the stability will be reviewed. The micro-system laser device
is implemented into a specifically laboratory prototype, including an optical bench with a diameter of 25 mm and a
length of 170 mm. The probe is coupled fiber-optically to a polychromator with CCD detector for rapid spectral
analysis. The Raman probe is characterized and first Raman measurements of porcine musculus longissimus dorsi
through the package will be presented. The usefulness of Raman spectroscopy will be discussed with a view of
integrating the sensor in a handheld laser scanner for food control.
Due to the narrow linewidth signals and its fingerprinting nature, Raman spectra provide information about the
molecular structure and composition of the samples. In this paper, the applicability of Raman spectroscopy is shown for
the in-situ characterization of the aging of meat. Miniaturized diode lasers are utilized as light sources with excitation
wavelengths of 671 nm and 785 nm with a view to the development of a portable field device for meat. As test sample,
musculus longissimus dorsi from pork was taken. The chops were stored refrigerated at 5 °C and Raman spectra were
measured daily from slaughter up to three weeks.
Throughout the entire period of one month, the Raman spectra preserve the basic spectral features identifying the
samples as meat. More specific, the spectra exhibit gradual changes of the Raman signals and they show a time-dependent
modification of the background signal which arises from a laser-induced fluorescence (LIF). To analyze the
time-correlation of the complex spectra, multivariate statistical methods are employed. By means of principal components
analysis (PCA) a distinction of spectra is found on the time scale between day 8 and 10. This corresponds to the
transition from ripened meat to meat at and beyond the limit of inedibility. After ca. 10 days of storage at 5 °C the
microbial load is overwhelming and LIF increases.
The results of the Raman measurements depending on the storage time of meat are discussed in the context of reference
analyses which have been performed in parallel.
Experimental results on RW and BA DFB lasers emitting at 785 nm suitable for Raman spectroscopy are presented.
Optical spectra of the RW DFB laser reveal single mode operation with a side-mode suppression ratio of more than 45 dB at optical output powers up to 163 mW. A reliable operation of more than 8800 h of these devices is demonstrated. Within a spectral width of 0.6 nm, more than 99.9% of 1.1 W optical power of the BA DFB laser emitting at 785 nm are included. Raman measurements with RW and BA DFB lasers as excitation light sources and polystyrene as a test sample are presented. At an output power of 1.1 W of the BA device the integration time for the Raman measurement could be reduced to 50 ms.
In this contribution, we present new tailor-made substrates for surface-enhanced Raman scattering (SERS). They are based on precise control of the mean shape and the average diameter of nanoparticles prepared by self-assembly of atoms on dielectric supports. Tailoring of the SERS substrates have been achieved by precise tuning of the localized surface plasmon polariton resonance of silver nanoparticles to the vicinity of the laser wavelength used for SERS excitation. The
underlying method relies on control of the growth kinetics of supported metal nanoparticles which causes a pronounced shape change as a function of particle size. Additionally, the strong dependence of the energetic position of surface plasmon excitation on the shape of the particles is exploited. With this preparation method, SERS substrates with optimized plasmon resonances and field enhancement can be easily produced for specific excitation wavelengths and detection ranges. The nanoparticles have been characterized by optical spectroscopy and atomic force microscopy (AFM). Silver nanoparticles with a plasmon resonance at about 2.4 eV were prepared with and without a protective CaF2 coating. SERS spectra of pyrene were obtained with excitation at 514.5 nm. They exhibit a good reproducibility. Furthermore, the substrates did not show degradation during the measurements and those with protective coating still yielded 70% of the SERS intensity of uncoated substrates, indicating their potential usefulness for an analytical detection of specific molecules. Further tailoring of supported metal nanoparticles for SERS applications by laser irradiation will be discussed.
A distributed feedback (DFB) laser diode emitting at 785 nm was used as light source for Raman spectroscopy. An in situ surface enhanced Raman spectroscopic (SERS) sensor was developed to detect polycyclic aromatic hydrocarbons (PAHs) in marine environment at concentration levels down to nmol/l. The DFB diode used in our SERS experiments had an output power of 150 mW at 785 nm and a spectral linewidth of 3 MHz. The wavelength stability of this laser was ± 0.004 nm over 12 hours. A side mode suppression ratio up to 45 dB was determined. For trace detection of chemicals in marine environment, SERS active substrates were employed based on silver colloids encapsulated in a sol-gel derived matrix. The resulting spectra and a limit of detection (LOD) of phenathrene (34 nmol/l) as an example for PAHs in seawater are presented. A concentration of 1 nmol/l of pyrene was detected.
The purpose of this work is the development of a method for the in-situ detection of chemicals in sea water. Raman scattering was chosen as it is a non-intrusive method yielding finger-printing spectra of the analytes. Surface-enhanced Raman scattering (SERS) is used to provide the sensitivity necessary for trace analysis in the environment. SERS active coatings are produced by encapsulating a silver colloid in sol-gel derived xerogel films. The sol is coated onto silicon plates, microscope slides, quartz windows or optical fibers allowing a variety of measurement configurations. Results are presented for a range of aromatic hydrocarbons in sea water measured with methyl triethoxy silane (MTEOS) and ethyl triethoxy silane (ETEOS) based substrates. The suitability for operation in the marine environment is discussed in terms of selectivity, response time, limits of detection, and long term stability. Two compact marinised optodes are presented with a lensed head and a fiber head allowing for in-situ measurements in sea water with exchangeable SERS substrates or coated fibers. The optode is linked via optical fibers to an underwater core instrument containing light source and spectrograph. This is discussed in the context of a multi- disciplinary field-operable measuring device intended for operation depths up to 300 m.
The development of surface-enhanced Raman scattering substrates suitable for in situ environmental analysis is presented. Sol-gel techniques are introduced to improve the mechanical stability of an existing active substrate type, to develop a new type of SERS active medium and finally to improve the sensitivity of substrates by providing a controllable coating. Two substrate types are discussed. The first consists of a glass slide covered with aluminium oxide particles which is then coated with gold.In the second type, a gold metal colloid is encapsulated within a porous thin film sol-gel structure. The sol-gel substrates were optically characterized and both substrates were tested in the laboratory. The mechanical stability of the new substrates is compared with that of conventionally prepared substrates and results are presented for pyridine, toluene and chlorobenzene in artificial sea-water.
The measurement of actinic UV-radiation will be most precisely realized by the spectroradiometric method. The spectral measurement quantities are evaluated numerically with weighting functions. But laboratory devices are not transportable. Outdoor measurements and measurements at working places are possible with the developed equipment with an acceptable accuracy.
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