Kay Sowoidnich

A Prototype Hand-Held Raman Sensor for the in Situ Characterization of Meat Quality

As a tool for the in situ characterization of meat quality, a hand-held Raman sensor head using an excitation wavelength of 671 nm was developed. A microsystem-based external cavity diode laser module was integrated into the sensor head and attached to a Raman probe, which is equipped with lens optics for excitation and signal collection as well as a Raman filter stage for Rayleigh rejection. The Raman signal was guided by an optical fiber to the detection unit, which was in the initial phase a laboratory spectrometer with a charge-coupled device (CCD) detector. The laser and the sensor head were characterized in terms of stability and performance for in situ Raman investigations. Raman spectra of meat were obtained with 35 mW within 5 seconds or less, ensuring short measuring times for the hand-held device. In a series of measurements with raw and packaged pork meat, the Raman sensor head was shown to detect microbial spoilage on the meat surface, even through the packaging foil.

Noninvasive analysis of thin turbid layers using microscale spatially offset Raman spectroscopy

Here, we demonstrate, for the first time, the extension of applicability of recently developed microscale spatially offset Raman spectroscopy (SORS), micro-SORS, from the area of cultural heritage to a wider range of analytical problems involving thin, tens of micrometers thick diffusely scattering turbid layers. The method can be applied in situations where a high turbidity of layers prevents the deployment of conventional confocal Raman microscopy with its depth resolving capability. The method was applied successfully to detect noninvasively the presence of thin, highly turbid layers within polymers, wheat seeds, and paper. An invasive, cross sectional analysis confirmed the micro-SORS findings. Micro-SORS represents a new Raman imaging modality expanding the portfolio of noninvasive, chemically specific analytical tools.

Fluorescence Rejection by Shifted Excitation Raman Difference Spectroscopy at Multiple Wavelengths for the Investigation of Biological Samples

Shifted excitation Raman difference spectroscopy (SERDS) was applied for an effective fluorescence removal in the Raman spectra of meat, fat, connective tissue, and bone from pork and beef. As excitation light sources, microsystem diode lasers emitting at 783 nm, 671 nm, and 488 nm each incorporating two slightly shifted excitation wavelengths with a spectral difference of about 10 cm−1 necessary for SERDS operation were used. The moderate fluorescence interference for 783 nm excitation as well as the increased background level at 671 nm was efficiently rejected using SERDS resulting in a straight horizontal baseline. This allows for identification of all characteristic Raman signals including weak bands which are clearly visible and overlapping signals that are resolved in the SERDS spectra. At 488 nm excitation, the spectra contain an overwhelming fluorescence interference masking nearly all Raman signals of the probed tissue samples. However, the essentially background-free SERDS spectra enable determining the majority of characteristic Raman bands of the samples under investigation. Furthermore, 488 nm excitation reveals prominent carotenoid signals enhanced due to resonance Raman scattering which are present in the beef samples but absent in pork tissue enabling a rapid meat species differentiation.

Hand-held Raman sensor head for in-situ characterization of meat quality applying a microsystem 671 nm diode laser

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.

In-situ Characterization of Meat Aging with Diode-Laser Raman Spectroscopy

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.

Spatially offset Raman spectroscopy for photon migration investigations in long bone

Raman Spectroscopy has become an important technique for assessing the composition of excised sections of bone, and is currently being developed as an in vivo tool for transcutaneous detection of bone disease using spatially offset Raman spectroscopy (SORS). The sampling volume of the Raman technique (and thus the amount of bone material interrogated by SORS) depends on the nature of the photon scattering in the probed tissue. Bone is a complex hierarchical material and to date little is known regarding its diffuse scattering properties which are important for the development and optimization of SORS as a diagnostic tool for characterizing bone disease in vivo. SORS measurements at 830 nm excitation wavelength are carried out on stratified samples to determine the depth from which the Raman signal originates within bone tissue. The measurements are made using a 0.38 mm thin Teflon slice, to give a pronounced and defined spectral signature, inserted in between layers of stacked 0.60 mm thin equine bone slices. Comparing the stack of bone slices with and without underlying bone section below the Teflon slice illustrated that thin sections of bone can lose appreciable number of photons through the unilluminated back surface. The results show that larger SORS offsets lead to progressively larger penetration depth into the sample; different Raman spectral signatures could be retrieved through up to 3.9 mm of overlying bone material with a 7 mm offset. These findings have direct impact on potential diagnostic medical applications; for instance on the detection of bone tumors or areas of infected bone.

Miniaturized diode laser-based light sources for in-situ shifted excitation Raman difference spectroscopy

Weak Raman bands are often covered by pronounced background signals due to fluorescence or Rayleigh scattering. Several techniques to separate Raman lines from the background are known. In this paper, diode laser based light sources will be presented suitable for shifted excitation Raman difference spectroscopy (SERDS). The two wavelengths are realized by varying the injection current, by addressing two micro-integrated ECLs or by temperature tuning. Due to the freedom of choice in the wavelengths using diode lasers, the emission wavelength can be selected with respect to the addressed application (e.g. the required penetration depth) or the plasmonic resonances of the substrates for surface enhanced Raman spectroscopy. Devices were developed for the wavelengths 488 nm, 671 nm, and 785 nm. The two emission wavelengths each were selected to have a spectral distance of 10 cm-1 according to the typical width of Raman lines of solid or liquid samples. Output powers between 20 mW for the shorter wavelength devices and 200 mW for the red emitting lasers were achieved at electrical power consumptions below 1 W. With a footprint of only 25 x 25 mm2 including all collimation and filter elements, these devices are well suited for portable applications. The diode lasers were implemented into Raman measurement systems. The SERDS signal-to-background ratio was improved by several orders of magnitude.

High sensitivity calixarene SERS substrates for the continuous in-situ detection of PAHs in seawater

In-situ monitoring of pollutant chemicals in sea-water is of worldwide interest. For that purpose, fast response sensors based on Raman spectroscopy are suitable for a rapid identification and quantification of these substances. Surface-enhanced Raman scattering (SERS) was applied to achieve the high sensitivity necessary for trace detection. In the project SENSEnet, funded by the European Commission, a SERS sensor based on calixarene-functionalized silver nanoparticles embedded in a sol-gel matrix was developed and adapted for the in-situ detection of polycyclic aromatic hydrocarbons (PAHs). The laboratory set-up contains a microsystem Raman diode laser with two slightly different emission wavelengths (670.8 nm and 671.3 nm) suitable also for shifted excitation Raman difference spectroscopy (SERDS). The output power at each of both wavelengths is up to 200 mW. For the detection of the SERS spectra integration times of typically 1 - 10 seconds were chosen. The SERS substrate is located inside a flow-through cell which provides continuous flow conditions of the analyte. The spectra were recorded using a laboratory spectrograph with a back-illuminated deep depletion CCD-detector. We present scanning electron microscope images of the developed calixarene-functionalized Ag colloid based SERS substrates as well as results for the SERS adsorption properties of major PAHs (pyrene, fluoranthene, and anthracene) in artificial sea-water and their limits of detection (e. g. 0.1 nM for pyrene). The suitability of the presented device as an in-situ SERS sensor for application on a mooring or buoy will be discussed.

Polarized Raman investigations of oriented animal muscle fibers affected by storage time applying a 671 nm diode laser

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.

Assessment of photon migration for subsurface probing in selected types of bone using spatially offset Raman spectroscopy

Bone diseases and disorders are a growing challenge in aging populations; so effective diagnostic and therapeutic solutions are now essential to manage the demands of healthcare sectors effectively. Spatially offset Raman spectroscopy (SORS) allows for chemically specific sub-surface probing and has a great potential to become an in vivo tool for early non-invasive detection of bone conditions. Bone is a complex hierarchical material and the volume probed by SORS is dependent on its optical properties. Understanding and taking into account the variations in diffuse scattering properties of light in various bone types is essential for the effective development and optimization of SORS as a diagnostic in vivo tool for characterizing bone disease. This study presents SORS investigations at 830 nm excitation on two specific types of bone with differing mineralization levels. Thin slices of bone from horse metacarpal cortex (0.6 mm thick) and whale bulla (1.0 mm thick) were cut and stacked on top of each other (4-7 layers with a total thickness of 4.1 mm). To investigate the depth origin of the detected Raman signal inside the bone a 0.38 mm thin Teflon slice was used as test sample and inserted in between the layers of stacked bone slices. For both types of bone it could be demonstrated that chemically specific Raman signatures different from those of normal bone can be retrieved through 3.8-4.0 mm of overlying bone material with a spatial offset of 7-8 mm. The determined penetration depths can be correlated with the mechanical and optical properties of the specimens. The findings of this study increase our understanding of SORS analysis of bone and thus have impact for medical diagnostic applications e.g. enabling the non-invasive detection of spectral changes caused by degeneration, infection or cancer deep inside the bone matrix.