Jianwei Qin

Simultaneous detection of multiple adulterants in dry milk using macro-scale Raman chemical imaging.

The potential of Raman chemical imaging for simultaneously detecting multiple adulterants in milk powder was investigated. Potential chemical adulterants, including ammonium sulphate, dicyandiamide, melamine, and urea, were mixed together into skim dry milk in the concentration range of 0.1-5.0% for each adulterant. Using a 785-nm laser, a Raman imaging system acquired hyperspectral images in the wavenumber range of 102-2538 cm(-1) for a 25 × 25 mm(2) area of each mixture sample, with a spatial resolution of 0.25 mm. Self-modelling mixture analysis (SMA) was used to extract pure component spectra, by which the four types of the adulterants were identified at all concentration levels based on their spectral information divergence values to the reference spectra. Raman chemical images were created using the contribution images from SMA, and their use to effectively visualise identification and spatial distribution of the multiple adulterant particles in the dry milk was demonstrated.

A Line-Scan Hyperspectral System for High-Throughput Raman Chemical Imaging

A line-scan hyperspectral system was developed to enable Raman chemical imaging for large sample areas. A custom-designed 785 nm line laser based on a scanning mirror serves as an excitation source. A 45° dichroic beam splitter reflects the laser light to form a 24 cm × 1 mm excitation line normally incident on the sample surface. Raman signals along the laser line are collected by a detection module consisting of a dispersive imaging spectrograph and a CCD camera. A hypercube is accumulated line by line as a motorized table moves the samples transversely through the laser line. The system covers a Raman shift range of −648.7 to 2889.0 cm−1 and a 23 cm wide area. An example application for authenticating milk powder is presented to demonstrate the system performance. In 4 min the system acquired a 512 × 110 × 1024 hypercube (56 320 spectra) from four, 47 mm diameter Petri dishes containing four powder samples. Chemical images were created for detecting two adulterants (melamine and dicyandiamide) that had been mixed into the milk powder.

Continuous temperature-dependent Raman spectroscopy of melamine and structural analog detection in milk powder.

Hyperspectral Raman imaging has the potential for rapid screening of solid-phase samples for potential adulterants. We can improve mixture analysis algorithms by defining a temperature range in which the contaminant spectrum changes dramatically and uniquely compared with unadulterated material. Raman spectra were acquired for urea, biuret, cyanuric acid, and melamine (pure and at 1% in dried milk powder) from 50 to 310 °C with a gradient of 1 °C min−1. Adulterants were clearly indentified in the milk powder. Specific frequencies that were mainly associated with ring breathing, stretching, and in-plane deformation shifted with respect to temperature up to 12 cm−1 in all four molecules. Specific frequencies significantly increased/decreased in intensity within narrow temperature ranges independent of whether the amine was mixed in milk. Correlation of Raman and differential scanning calorimetry data identified structural components and vibrational modes, which concur with or trigger phase transitions.

Continuous Gradient Temperature Raman Spectroscopy of Oleic and Linoleic Acids from -100 to 50 °C.

We analyzed the unsaturated fatty acids oleic (OA, 18:1n-9) and linoleic (LA, 18:2n-3), and a 3:1 LA:OA mixture from −100 to 50xa0°C with continuous gradient temperature Raman spectroscopy (GTRS). The 20xa0Mb three-dimensional data arrays with 0.2xa0°C increments and first/second derivatives allowed rapid, complete assignment of solid, liquid, and transition state vibrational modes. For OA, large spectral and line width changes occurred in the solid state γ to α transition near −4xa0°C, and the melt (13xa0°C) over a range of only 1xa0°C. For LA, major intensity reductions from 200 to 1750xa0cm−1 and some peak shifts marked one solid state phase transition at −50xa0°C. A second solid state transition (−33xa0°C) had minor spectral changes. Large spectral and line width changes occurred at the melt transition (−7xa0°C) over a narrow temperature range. For both molecules, melting initiates at the diene structure, then progresses towards the ends. In the 3:1 LA:OA mixture, some less intense and lower frequencies present in the individual lipids are weaker or absent. For example, modes assignable to C8 rocking, C9H–C10H wagging, C10H–C11H wagging, and CH3 rocking are present in OA but absent in LA:OA. Our data quantify the concept of lipid premelting and identify the flexible structures within OA and LA, which have characteristic vibrational modes beginning at cryogenic temperatures.

Continuous gradient temperature Raman spectroscopy of N-6DPA and DHA from -100 to 20°C.

One of the great unanswered questions with respect to biological science in general is the absolute necessity of docosahexaenoic acid (DHA, 22:6n-3) in fast signal processing tissues. N-6 docosapentaenoic acid (n-6DPA, 22:5n-6), with just one less double bond, group, is fairly abundant in terrestrial food chains yet cannot substitute for DHA. Gradient temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements that occur near and at phase transitions. Herein we apply GTRS and DSC to n-6DPA and DHA from -100 to 20°C. 20Mb three-dimensional data arrays with 0.2°C increments and first/second derivatives allowed complete assignment of solid, liquid and transition state vibrational modes, including low intensity/frequency vibrations that cannot be readily analyzed with conventional Raman. N-6DPA and DHA show significant spectral changes with premelting (-33 and -60°C, respectively) and melting (-27 and -44°C, respectively). The CH2(HCCH)CH2 moieties are not identical in the second half of the DHA and DPA structures. DPA has bending (1450cm-1) over almost the entire temperature range. In contrast, DHA contains major CH2 twisting (1265cm-1) with no noticeable CH2 bending, consistent with a flat helical structure with a small pitch. Further modeling of neuronal membrane phospholipids must take into account torsion present in the DHA structure, which essential in determining whether the lipid chain is configured more parallel or perpendicular to the hydrophilic head group.

Continuous gradient temperature Raman spectroscopy and differential scanning calorimetry of N-3DPA and DHA from −100 to 10 °C

Docosahexaenoic acid (DHA, 22:6n-3) is exclusively utilized in fast signal processing tissues such as retinal, neural and cardiac. N-3 docosapentaenoic acid (n-3DPA, 22:5n-3), with just one less double bond, is also found in the marine food chain yet cannot substitute for DHA. Gradient temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements that occur near and at phase transitions. Herein we apply GTRS and both conventional and modulated DSC to n-3DPA and DHA from -100 to 20°C. Three-dimensional data arrays with 0.2°C increments and first derivatives allowed complete assignment of solid, liquid and transition state vibrational modes. Melting temperatures n-3DPA (-45°C) and DHA (-46°C) are similar and show evidence for solid-state phase transitions not seen in n-6DPA (-27°C melt). The C6H2 site is an elastic marker for temperature perturbation of all three lipids, each of which has a distinct three dimensional structure. N-3 DPA shows the spectroscopic signature of saturated fatty acids from C1 to C6. DHA does not have three aliphatic carbons in sequence; n-6DPA does but they occur at the methyl end, and do not yield the characteristic signal. DHA appears to have uniform twisting from C6H2 to C12H2 to C18H2 whereas n-6DPA bends from C12 to C18, centered at C15H2. For n-3DPA, twisting is centered at C6H2 adjacent to the C2-C3-C4-C5 aliphatic moiety. These molecular sites are the most elastic in the solid phase and during premelting.

Development of a Raman Chemical Imaging System for Food Safety Inspection

Raman chemical imaging technique combines Raman spectroscopy and digital imaging to visualize composition and structure of the target, and it offers great potential for food safety and quality research. In this study, a laboratory-based Raman chemical imaging platform was designed and developed. The imaging system utilizes a 785 nm spectrum stabilized laser as an excitation source to generate Raman scattering. The detection module mainly consists of a Raman signal detection probe, a reflection grating-based Raman imaging spectrometer, and a high performance spectroscopic CCD camera. The imaging system works in a point scanning mode. A Raman spectrum is obtained at a time for a single position in the scene. The specimens are carried by a two-axis motorized positioning table, and hyperspectral image data are accumulated as the samples move along two spatial dimensions. The parameterization and data-transfer interface software was developed using LabVIEW. Spectral and spatial calibration procedures were presented. The system covers a Raman shift range of 102.2-2538.1 cm-1 with a spectral resolution of 2.9 cm-1 and a spatial resolution as high as 0.1 mm. Performance of the system was demonstrated by an application example on detection of melamine in dry milk. Melamine was mixed into dry milk with concentrations (w/w) ranging from 0.2% to 10.0%. The system was able to create Raman chemical images that can be used to visualize quantity and spatial distribution of melamine particles in the mixtures. The developed system is versatile, and it will be used for safety and quality inspection of food and agricultural products.

Continuous gradient temperature Raman spectroscopy from −100 to 40 °C yields new molecular models of arachidonic acid and 2-Arachidonoyl-1-stearoyl-sn-glycero-3-phosphocholine

Despite its biochemical importance, a complete Raman analysis of arachidonic acid (AA, 20:4n-6) has never been reported. Gradient temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements that occur near and at phase transitions. Herein we utilize the GTRS technique for AA and 1-18:0, 2-20:4n-6 phosphatidyl choline (AAPC) from cryogenic to mammalian body temperatures. 20Mb three-dimensional data arrays with 0.2°C increments and first/second derivatives allowed complete assignment of solid, liquid and transition state vibrational modes. The AA DSC shows a large exothermic peak at -60°C indicating crystallization or a similar major structural change. No exothermic peak of this magnitude was observed in six other unsaturated lipids (DHA, n-3DPA, n-6DPA, LA, ALA, OA). Melting in AA occurs over a large range: (-60 to -35°C): very large frequency offsets and intensity changes correlate with premelting initiating circa -60°C, followed by melting (-37°C). Novel, unique 3D structures for both molecules reveal that AA is not symmetric as a free fatty acid, and it changes significantly when in the sn-2 phospholipid position. Further, different CH and CH2 sites are unequally elastic and nonequivalent.

Detection of color dye contamination in spice powder using 1064 nm Raman chemical imaging system

Spice powders are used as food additives for flavor and color. Economically motivated adulteration of spice powders by color dyes is hazardous to human health. This study explored the potential of a 1064 nm Raman chemical imaging system for identification of azo color contamination in spice powders. Metanil yellow and Sudan-I, both azo compounds, were mixed separately with store-bought turmeric and curry powder at the concentration ranging from 1 % to 10 % (w/w). Each mixture sample was packed in a shallow nickel-plated sample container (25 mm x 25 mm x 1 mm). One Raman chemical image of each sample was acquired across the 25 mm x 25 mm surface area using a 0.25 mm step size. A threshold value was applied to the spectral images of metanil yellow mixtures (at 1147 cm-1) and Sudan-I mixtures (at 1593 cm-1) to obtain binary detection images by converting adulterant pixels into white pixels and spice powder pixels into the black (background) pixels. The detected number of pixels of each contaminant is linearly correlated with sample’s concentration (R2 = 0.99). This study demonstrates the 1064 nm Raman chemical imaging system as a potential tool for food safety and quality evaluation.

Non-targeted and targeted Raman imaging detection of chemical contaminants in food powders

Economically motivated adulteration and fraud to food powders are emerging food safety risks that threaten the health of the general public. In this study, targeted and non-targeted methods were developed to detect adulterants based on macro-scale Raman chemical imaging technique. Detection of potassium bromate (PB) (a flour improver banned in many countries) mixed in wheat flour was used as a case study to demonstrate the developed methods. A line-scan Raman imaging system with a 785 nm line laser was used to acquire hyperspectral image from the flour-PB mixture. Raman data analysis algorithms were developed to fulfill targeted and non-targeted contaminant detection. The targeted detection was performed using a single-band Raman image method. An image classification algorithm was developed based on single-band image at a Raman peak uniquely selected for the PB. On the other hand, a mixture analysis and spectral matching method was used for the non-targeted detection. The adulterant was identified by comparing resolved spectrum with reference spectra stored in a pre-established Raman library of the flour adulterants. For both methods, chemical images were created to show the PB particles mixed in the flour powder.