Heinar Schmidt

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.

Preliminary investigation on the relationship of Raman spectra of sheep meat with shear force and cooking loss

A prototype handheld Raman system was used as a rapid non-invasive optical device to measure raw sheep meat to estimate cooked meat tenderness and cooking loss. Raman measurements were conducted on m. longissimus thoracis et lumborum samples from two sheep flocks from two different origins which had been aged for five days at 3-4°C before deep freezing and further analysis. The Raman data of 140 samples were correlated with shear force and cooking loss data using PLS regression. Both sample origins could be discriminated and separate correlation models yielded better correlations than the joint correlation model. For shear force, R(2)=0.79 and R(2)=0.86 were obtained for the two sites. Results for cooking loss were comparable: separate models yielded R(2)=0.79 and R(2)=0.83 for the two sites. The results show the potential usefulness of Raman spectra which can be recorded during meat processing for the prediction of quality traits such as tenderness and cooking loss.

Surface-enhanced Raman scattering (SERS) system for continuous measurements of chemicals in sea-water

A laboratory-based system for measurement of chemicals in sea-water with sol–gel-derived surface-enhanced Raman scattering (SERS) substrates is presented. The motivation behind this work was the development of a marinized prototype sensor. A long-term spectral stability of better than 1 cm−1 over a period of several months was observed. Photodegradation of the SERS layer was avoided by the choice of optical components and layout. Dielectric edge filters and holographic notch filters were compared. Coated fibres were tested and results are discussed in terms of fibre choice, reduction of background signal, sensitivity and time response. Continuous analysis was performed with two flow-through cells, the first a modification to a standard glass cuvette and the second an improved in-house-constructed aluminium cell. SERS investigations on samples whose turbidity ranged from 0 to 400 NTU were performed with both cells. These tests show the suitability of the developed system for continuous monitoring of real samples and possible application in on-line process control. Copyright

Microsystem 671 nm light source for shifted excitation Raman difference spectroscopy

We present a 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). The laser system consists of two broad-area gain media in separate laser cavities using two reflection Bragg gratings with slightly different center wavelengths. A spectral width below 100 pm and a constant wavelength shift of 0.57 +/- 0.06 nm 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 the fluorescent contaminant.

Optimized recipe for sol–gel-based SERS substrates

The aim of this study was to develop further surface-enhanced Raman scattering (SERS) substrates appropriate for the in situ environmental determination of chemicals in sea-water important for global environmental problems. By encapsulating silver colloids in a sol–gel-derived xerogel, SERS-active coatings were produced with the high mechanical and chemical stability required for underwater field measurements. A flow-through system was used which simulates measurements under real flow conditions. Substrates were optimized regarding sensitivity to different analytes and long-term stability by varying solvent type, pH and sol–gel precursors. The substrates were characterized by their SERS activity and calibration functions of the Langmuir isotherm type were found for a variety of aromatic hydrocarbons. Limits of detection as low as 9 ppb for phenylacetylene and 135 ppb for biphenyl were achieved. Detailed tests on long-term stability in sea-water yielded a substrate half-life of 13 days, which resulted in a substrate lifetime of at least 5 weeks. Copyright

Fluorimetric detection of protoporphyrins as an indicator for quality monitoring of fresh intact pork meat

In fresh meat production fast and non-destructive quality monitoring along the distribution chain is a key aspect to guaranteeing high quality and safe products for consumption. The applicability of fluorescence spectroscopy using protoporphyrins as indicators for meat ageing was investigated. Porcine musculus longissimus dorsi (MLD) was stored in slices over 20 days at 5 and 12°C and measured every day with an excitation of 420nm and an emission range of 550-750nm. Additionally, pH, drip loss and colour were examined to assess possible correlations. The obtained spectra of the MLD showed an increase in three peaks at 592, 638 and 705nm which could be reconstructed using the spectra of standard solutions of protoporphyrin IX (PP) and zinc protoporphyrin IX (ZnPP) or magnesium protoporphyrin (MgPP), respectively. Using principal component analysis (PCA) on the fluorescence spectral data, the meat slices stored at 5°C showed differences in the fluorescence signal after the 10th day and 5th day when stored at 12°C. An interrelationship between the additional analyses and the fluorescence intensities on these relevant days could not be established. In conclusion, the increase of ZnPP fluorescence due to temperature related changes of physiological meat properties is capable of serving as a quality indicator with regards to inadequate conditioning (e.g. during transportation and/or storage) of pork meat.

Raman Spectroscopy compared against traditional predictors of shear force in lamb m. longissimus lumborum

A Raman spectroscopic hand held device was used to predict shear force (SF) of 80 fresh lamb m. longissimus lumborum (LL) at 1 and 5days post mortem (PM). Traditional predictors of SF including sarcomere length (SL), particle size (PS), cooking loss (CL), percentage myofibrillar breaks and pH were also measured. SF values were regressed against Raman spectra using partial least squares regression and against the traditional predictors using linear regression. The best prediction of shear force values used spectra at 1day PM to predict shear force at 1day which gave a root mean square error of prediction (RMSEP) of 13.6 (Null=14.0) and the R(2) between observed and cross validated predicted values was 0.06 (R(2)cv). Overall, for fresh LL, the predictability SF, by either the Raman hand held probe or traditional predictors was low.

Microsystem light source at 488 nm for shifted excitation resonance Raman difference spectroscopy.

A microsystem light source emitting at 488 nm was tested and applied as a light source for shifted excitation resonance Raman difference spectroscopy (SERRDS). A nonlinear frequency conversion using a distributed feedback (DFB) diode laser emission at 976 nm and a periodically poled lithium niobate (PPLN) waveguide crystal was realized on a micro-optical bench with a footprint of 25 mm × 5 mm. Joint temperature management via the microbench is used for wavelength tuning. Two emission lines at 487.61 nm and 487.91 nm are used for the SERRDS experiments. The Raman spectra of the test sample polystyrene demonstrate that a laser bandpass filter did not need to be implemented. Resonance Raman spectra of Tartrazine (FD&C Yellow 5, E 102) in distilled water are presented to demonstrate the suitability of this light source for SERRDS in, e.g., food safety control.

The use of surface-enhanced Raman scattering (SERS) for detection of PAHs in the Gulf of Gdańsk (Baltic Sea)

A field operable surface enhanced Raman scattering (SERS) sensor system was applied for the first time under real conditions for the detection of polycyclic aromatic hydrocarbons (PAHs) as markers for petroleum hydrocarbons in the Gulf of Gdańsk (Baltic Sea). At six stations, seawater samples were taken, and the sensor system was applied in situ simultaneously. These measurements were compared to the results of conventional GC/MS laboratory analysis of the PAH concentrations in the seawater samples. For a PAH concentration above 150 ng(12PAH)l(-1), there was agreement between the SERS sensor and the GC/MS determinations. A standard addition experiment yielded a PAH concentration of 900 ng l(-1) at the Gdańsk Harbor, which was of the same order as the GC/MS determinations of 12PAHs (200 ng(12PAH)l(-1)). The high SERS detection limit for seawater samples is explained by the competition for PAHs between the sensor membrane and particulate matter surfaces. Thus, the SERS sensor can be applied, e.g., as a non-quantitative alarm sensor for relatively high PAH concentrations in heavily polluted waters. The spectral unmixing procedure applied for Gdańsk Harbor water confirmed the presence of phenanthrene at the highest concentration ([Phe]=140 ngl(-1)) and of Chr (2.7 ng l(-1)), but it did not detect the other PAHs present in the Gdańsk Harbor water, as determined by GC/MS. When compared to the past literature and databases, the SERS spectra indicated the presence of a mixture of molecules consisting of carotenoids, n-alkanes, amines or fatty acids, and benzimidazoles at the coastal station ZN2. The spectra in the offshore direction indicated carboxylic acids. Interpretation of the farthest offshore in situ SERS measurements is difficult, principally due to the limited availability of reference spectra. The detection of the lower PAH concentrations commonly found in Baltic coastal water needs further research and development to obtain better sensitivity of the SERS sensor. However, the high analytical specificity of the SERS sensor also allows the detection of other chemical species that require the development of a SERS/Raman library for specific in situ spectral interpretation.

Gas phase structure and vibrational spectra of dimethoxysulfane (CH3O)2S

The vapor phase structure of (CH3O)2S [1] has been investigated by electron diffraction and ab initio MO calculations, which both result in a C2 symmetry for the most stable geometry, the Cs conformer being less stable by about 12 kJ/mol. The torsional barrier for rotation about one SO bond was calculated as 37 kJ/mol (trans barrier). The geometrical parameters (electron diffraction) of the C2 conformer are: dSO = 162.5(2), dCO = 142.6(3), dCH = 110.5(7) pm angles OSO = 103(1)°, SOC = 115.9(4)°, HCH = 109(1)°, torsional angle COSO = 84(3)°. Geometrical data calculated with 6-31G* basis set agree well with the diffraction data; calculated dipole moments 1.1 D (C2) and 3.3 D (Cs). The infrared spectrum of gaseous (CH3O)2S and the Raman spectra of liquid and solid (CH3O)2S are reported and have been almost fully assigned to the 27 fundamental vibrations.