Chad Lieber

Owing to technological improvements spurred on by the telecommunications boom of the last decade, Raman spectroscopy has become much more accessible to users in all application areas, including agricultural, forensic, pharmaceutical, biomedical, and others.  However, there remains a struggle to extract useful Raman spectra from fluorescent and luminescent samples. 

Most users with fluorescent samples switched to near-infrared wavelengths such as 785 nm and 830 nm to avoid fluorescence. But now, BaySpec's new dispersive 1064 nm Raman spectrometer family offers a turn-key solution combines the speed, sensitivity, and ruggedness of the dispersive Raman instruments with superior fluorescence avoidance of traditional FT-Raman.  

Any new method for routine determination of chemical composition in beverage manufacturing processes should be non-invasive, non-destructive, and rapid. Raman spectroscopy offers the advantages of minimal sample preparation and fast spectral collection. However, the pigments in red wines are usually too fluorescent to allow Raman to be measured, even at 785 nm. But at 1064 nm, a clear Raman spectrum is generated while florescence background is almost totally avoided. 

Figure 1: Identification of methanol in red wine based on its unique Raman features different from ethanol.

Further, glass is mostly transparent to NIR wavelengths, compared to 532 nm and 785 nm, which makes 1064 nm Raman even more suitable for non-destructive measurement of red wine through the bottle. As in Figure 1, comparing the red wine spectrum in a secondary vial with that collected through the bottle, despite a small attenuation in the signal intensity, the Raman features are maintained and distinguishable for further identification and quantification. 

Further component analysis was performed by adding methanol into the wine and measuring as before. As seen in Figure 2, most of the Raman features from pure red wine could be easily identified as ethanol. For the Raman spectrum of red wine with methanol added, a Raman peak centered around 1020 cm

  is apparent, and this peak can be utilized for the quantification of the methanol concentration profile in different red wine samples as seen in Figure 3. Slight variances in spectral intensity on different measuring spots were found due the heterogeneity of the bottle glass. 

Figure 2: Identification of methanol in red wine based on its unique Raman features different from ethanol.			

Based on these experiments, 1064 nm dispersive Raman is demonstrated as a viable new option to determine wine composition and contamination in the bottle. Chemometrics tools such as principal component analysis (PCA) and partial least squares regression (PLS) can significantly increase the accuracy and precision. It can be expected that future development of such applications may provide the wine industry a fast and non-destructive quality control and assurance tool for composition monitoring. This methodology can be easily applied to distilled spirits, beer, and others. 

Figure 3: Quantification of methanol concentrations in red wine based on the intensity of one major Raman feature.			

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Articles

Owing to technological improvements spurred on by the telecommunications boom of the last decade, Raman spectroscopy has become much more accessible to users in all application areas, including agricultural, forensic, pharmaceutical, biomedical, and others.

Non-Destructive Red Wine Measurement with Dispersive 1064 nm Raman Spectroscopy

Biological tissues and other materials often autofluoresce at near-infrared wavelengths, prohibiting Raman acquisition. New, dispersive Raman systems at 1064 nm allow fluorescence-free measurement in similar integration times.

Fluorescence is much more likely and intense at short wavelengths where energies are more apt to cause an electronic transition. Yet even at near-IR wavelengths like 785 or 830 nm, many substances still fluoresce, sometimes prohibiting Raman spectral acquisition. For those users who require longer wavelengths such as 1064 nm, the only available option has been FT-Raman, which is typically noisier and slower than dispersive  Raman systems. But now, BaySpec's new dispersive 1064 nm Raman spectrometer family of instruments offers a turn-key solution that combines the speed, sensitivity, and rugged design of traditional dispersive Raman instruments with the fluorescence avoidance of traditional FT-Raman instruments. 

A variety of animal tissues obtained from a local market were interrogated using two BaySpec benchtop systems: the RamSpec™ 785 and the RamSpec™ 1064. Both systems utilized filtered fiber probes designed for each respective wavelength. Acquisition times were 30 s for both systems. Power was set at 50 mW for the 785 nm measurements, and 150 mW for the 1064 nm measurements. 

Pigmented and porphyrin-rich tissues such as kidney are often too fluorescent to be measured, even at 785 nm; see Figure 1. But using 1064 nm, a clear Raman spectrum relatively devoid of fluorescence background is generated using the same integration time. Additionally, because of the extended quantum efficiency of the InGaAs detector, high wavenumber features (C–H, O–H, and N–H stretching modes) are also simultaneously captured with the same laser.  

Figure 1: Kidney (porcine) is highly fluorescent at 785 nm, preventing extraction of a usable Raman signal. At 1064 nm, however, this fluorescence interference is largely avoided and clear Raman bands are evident. 			

In addition to allowing users the ability to measure Raman spectra from highly fluorescent samples, the dispersive 1064 nm Raman systems also reduce the stringent sampling conditions necessary at shorter wavelengths. For example, 785-nm Raman measurement through vials, cuvettes, or under cover slips often requires that these materials be made of fused silica, quartz, or calcium fluoride for their reduced fluorescence, all of which cost considerably more than bulk glass materials. However, even inexpensive glass sample vials can be used for 1064-nm measurement of weak Raman scatterers such as tissue, owing to the fluorescence avoidance of the longer wavelength approach. 

One of the lingering concerns about the use of 1064-nm Raman is the reduced Raman scattering cross-section. As compared to 785-nm Raman, this cross-section, indeed, reduces approximately 3.4×. However, according to permissible standards for human tissue (skin) exposure, the maximum power level that can be tolerated at 1064 nm is approximately 3.4× higher than the power permissible at 785 nm (1). So, even in photosensitive samples such as biological tissue, the physical reduction in Raman efficiency can be totally compensated by increased laser power.  

Based on these experiments, 1064-nm dispersive Raman will provide a viable new option for those users who are studying highly fluorescent tissues, desire to measure multiple samples in simple glass containers, or those users who are interested in simultaneous fingerprint and high wavenumber spectral acquisition. Future studies will certainly evidence further advantages of this approach, as compared to shorter wavelength (785 or 830 nm) Raman approaches or FT-Raman systems. 

(1) National Institute of Standards and Technology, Z136.1, 76–77, (2007). 

tel. (408) 512-5928, fax: (408) 512-5929  

Chad Lieber

Articles

Non-Destructive Red Wine Measurement with Dispersive 1064 nm Raman Spectroscopy

Tissue Raman Measurement at 1064 nm