Wine Analysis - Fluorescence Spectroscopy for the Non-Destructive Analysis of Wine

Component analysis holds major significance in the microbiology, chemistry, and engineering of food products. It also serves as an important tool in the validation or adulteration of food products in storage and production units.

Fluorescence spectroscopy is a fast, simple, non-destructive and minimally invasive method in comparison to standard food analysis techniques, such as calorimetry and gas or liquid chromatography. It can be directly applied eliminating the need for additional separation/preparation of components. When compared to other spectrophotometric methods Fluorescence spectroscopy has excellent sensitivity, which can be 100 to 1000 times higher.

The fluorescence of wine products can be examined through multidimensional spectral and time resolved methods, such as emission, excitation, lifetime and synchronous scans. This approach allows users to obtain the unique fingerprints relating to fluorescent compounds that occur naturally in food products. Some common fluorescent compounds found in wines are vitamins, flavonoids, polyphenols, stilbenes, amino acids, and tannins.

Methods and Materials

A FLS980 fluorescence spectrometer equipped with double monochromators and a 450W Xe lamp was used to determine both the emission and excitation spectra. Gratings blazed at 400nm wavelength were utilized at the emission and excitation arms. Hamamatsu’s R928P photomultiplier tube detector with a dwell time of 0.2s was also utilized. In addition, long wave-pass filters integrated in the FLS980 fluorescence spectrometer allowed for filtering of higher diffraction orders.

Two white wine samples from different regions (Pinot Grigio, Sicily and Falanghina, Benevento) were measured in quartz cuvettes of 10 mm path-length in right angle geometry. In order to sustain the intact food matrix the samples were used as obtained with 0.5 optical density (OD).

A pico-second pulsed diode laser (EPL-375, 375nm, 5mW) was utilized for performing time-resolved measurements. In such measurements, the sample is excited by a pulse for a short time, shorter than the decay of the sample. To determine the lifetime of individual fluorescent compounds, the exponential decay of the fluorescence can be identified and fitted. However, this should be differentiated from the response function of the instrument (IRF) in order to acquire the pure sample lifetime as discussed in the following section.

Results

The excitation-emission maps (EEM) of the wine samples acquired using the F980 software are illustrated in Figures 2 and 3. The emission maxima can be observed at 456nm upon excitation maxima at 370nm. This is in agreement with the phenol and flavonol fluorescence spectra.

Excitation-emission map of the Falanghina sample, Δλexc = Δλem = 3nm, tint = 0.2s

Figure 1. Excitation-emission map of the Falanghina sample, Δλexc = Δλem = 3nm, tint = 0.2s

Excitation-emission map of the Pinot Grigio sample, Δλexc = Δλem = 4nm, tint = 0.2s.

Figure 2. Excitation-emission map of the Pinot Grigio sample, Δλexc = Δλem = 4nm, tint = 0.2s.

Figure 3 shows the time resolved emission of the Pinot Grigio sample at 450nm fitted by an exponential reconvolution, and Figure 4 shows the time resolved emission map acquired between 400nm and 650nm. From the time resolved emission map, the emission spectra at preferred time intervals can be acquired through the slicing method.

Time-resolved emission of the Pinot Grigio sample at 450nm

Figure 3. Time-resolved emission of the Pinot Grigio sample at 450nm with a four term exponential fit with a χ2 = 1.067, while the three term fit had χ2 = 1.48.

Time-resolved emission map of the Pinot Grigio sample.

Figure 4. Time-resolved emission map of the Pinot Grigio sample. The cross section is at λem = 450nm, while the time calibration of the measurement was 0.025ns.

Normalized emission of the Pinot Grigio sample after slicing the data of Figure 4 at 10 bins between 8 and 55ns.

Figure 6. Normalized emission of the Pinot Grigio sample after slicing the data of Figure 4 at 10 bins between 8 and 55ns.

Figure 5 displays the time resolved emission spectra subsequent to slicing and normalization of data in Figure 4 in the comprehensive F980 software between 8ns and 55ns. A shift in the emission peak from 448nm to 462nm can be seen, in addition to a change in emission ratio with an isoemission point at 520nm.

This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.

For more information on this source, please visit Edinburgh Instruments.

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