A Portable Multiexcitation Dispersive Raman Spectrometer for Environmental Analysis

Chemical examination and investigation for environmental analysis often demand portable, versatile, and real-time analytical techniques with high specificity and accuracy, and direct and noncontact measurement with little or no sample preparation. Raman spectroscopy is one of the most suitable techniques that satisfy these essential criteria.1 Coupled with optical microscopes, Raman spectrometers are further capable of identifying trace evidence in the micro scale. This article describes recent technical advances in the development of portable Raman spectrometers and microscopes that combine multiple laser excitation wavelengths covering from the visible to the infrared with extended detection ranges. Since each sample is sensitive to a certain excitation wavelength, having multiple wavelength excitations in one unit offers great flexibility and convenience for investigating a wide variety of real-world samples in environmental analysis.

In particular, infrared laser (e.g., 1064 nm) excitation for dispersive Raman spectroscopy is extremely advantageous due to its superior capability in fluorescence suppression; in real-world Raman measurements, fluorescence interference (which is thousands of times stronger than Raman scattering) is the largest obstacle, because it completely masks or significantly reduces the quality of the Raman spectra. With the advances in lasers, fast detectors, and its highly efficient transmissive VPG® (volume phase gratings), BaySpec (San Jose, CA) has significantly improved the Raman technique by offering ease of operation, fast speed, stability, and especially portability (as small as 6 in.). Ultimately, the instruments are an effective solution for the most complex environmental analysis with minimal sample handling.

Instrumentation

A key component that allows miniaturization of Raman systems is the volume phase grating. VPGs are periodic transmissive phase structures used to diffract different wavelengths of light from a common input into different angular output paths. VPGs are economical and are designed and manufactured to provide the highest diffracting efficiency (up to 99%) and largest angular dispersion.2 Compared with bulky traditional reflective gratings, the spectrograph with VPG gratings can be made very compact without any moving parts (Figure 1). This will greatly enhance the sensitivity and reliability of the system. Together with narrow-line, compact solid-state lasers, a single-excitation Raman system can be miniaturized in a single unit as small as a cellular phone. Thus, multiple excitation wavelengths and detectors can be simultaneously equipped, while the entire unit remains compact and offers a versatile platform for all sampling situations.

Figure 1 a) Compact, efficient VPG design, which has no moving parts in the system. b) A new generation of battery-powered miniature Raman systems, the BaySpec Agility™, features dual wavelengths (785 and 1064 nm) and portability (310 mm × 380 mm × 170 mm, 6 kg).

In real-world Raman measurements, noisy fluorescence emission from the sample and its background remains the biggest challenge in producing Raman spectra with sufficient SNR (signal-to-noise ratio). The best way to eliminate fluorescence is to use a longer excitation wavelength, such as infrared (IR), which usually does not excite much fluorescence. However, IR excitation typically results in a much lower signal because the Raman efficiency is low (inversely proportional to λ4), and conventional high-sensitivity charge-coupled device (CCD) detectors do not work for the IR range. All current Raman spectrometers designed to accommodate 1064-nm excitation are either incapable of delivering signal strength that is high enough; are extremely costly and bulky in laboratory settings, making it impossible for them to be packed together and commercialized; or are interferometer-based FT-Raman, which are nonportable, slow, and require special handling of samples.

Therefore, there was a need to develop a miniaturized 1064-nm dispersive Raman sensor that is capable of providing high-quality, fluorescence-free Raman spectra for high-throughput and efficient detection and characterization quickly and conveniently. Specifically, three technology advances in this decade helped to realize such a system: 1) an ultracompact, high coupling efficiency, high-power (up to 1000 mW), narrow-line (up to 100 kHz) 1064-nm laser; 2) high-performance deep-cooled InGaAs detector architectures covering the spectral region of 1064–1700 nm (0–3500 cm–1 in Raman shift for 1064-nm excitation); and 3) compact and efficient VPG gratings.

Furthermore, to advance from a single-point measurement to microscopic samples and full sample chemical imaging with great details, Raman spectroscopy is coupled with confocal microscopy. The confocal geometry allows additional background removal and higher specificity since only the signals from the sampling point can reach the detector. The software-controlled sample stage can scan the sample in three dimensions to generate Raman images. These advances in lasers, VPGs, and detectors have made multiwavelength (532, 785, and 1064 nm) excitation dispersive Raman microscopy a reality (Nomadic™ [BaySpec]). Its spatial resolution can go up to 0.2 μm (for a 532-nm laser with a 1.4-NA objective). This allows scanning with extremely high SNR for chemical mapping of samples. The company has developed a full line of Raman systems, from miniature Raman systems with dual-laser excitations including high-resolution 1064-nm Raman systems, to Raman microscopes equipped with multiple laser excitations.

Portable Raman spectrometer for mineral analysis

Mineral samples are Raman sensitive, but most have a native color (not white or transparent). They usually display high fluorescence when excited by visible lasers. Thus, it is difficult to measure them using traditional Raman measurements. With 1064-nm Raman spectrometers, their Raman spectra can be measured with high SNR without fluorescence interference.

Figure 2Mineral samples calcite, alunite, ulexite, and kaolinite measured by BaySpec’s portable Raman systems using different excitations: a) 532 nm, b) 785 nm, and c) 1064 nm. Although slightly longer integration times (5 sec to 10 sec) were used than with the 532 and 785 nm (1 sec to 5 sec), the 1064-nm Raman spectrometer produces the best-quality Raman spectra required for identification.

Demonstrated in Figure 2 are four minerals: calcite, alunite, ulexite, and kaolinite. The 1064-nm Raman spectrometer can acquire significantly enhanced Raman spectra for all four samples, in comparison to those spectra excited by 532- or 785-nm lasers. With BaySpec portable 1064 Raman spectrometers, Spec20/20 software, and Raman database, minerals—even those heavily colored—can be easily measured and automatically identified.

Quantitative chemical analysis in aqueous samples

Aqueous samples are common in environmental analysis. Typical chemical analysis of these samples requires multiple chemistry procedures, large amounts of samples, and days to process. With portable Raman spectrometers, analysis can be done easily and quickly on site. Figure 3 shows an example in which a portable 785-nm Raman spectrometer was used to determine H2O2 quantity in solution. The Raman feature centered at 875 cm–1 from the O–O stretching in H2O2 molecules is used for quantification. With the increase in H2O2 concentration, the intensity increases linearly. H2O2 concentration can easily be determined with a calibration curve established between the spectral intensity at 875 cm–1 and H2O2 standards. The detection limit of H2O2 is better than 0.1%. With the ease of use and accuracy of the portable Raman spectrometers, many similar applications can be developed for aqueous samples in the field for environmental analysis.

Figure 3a) Raman spectra of pure water, measured using the BaySpec portable 785-nm Raman spectrometer. b) Raman spectra of samples with different H2O2 concentrations. c) Calibration curve for the concentration of H2O2, based on the Raman intensity at 875 cm–1.

1064-nm Raman for fuel analysis

The production and use of liquid fuels, including petroleum products and those derived from plant biomass (biofuels), have a huge impact on the environment. Raman spectroscopy would be ideal for high-throughput and real-time analysis for these samples. However, traditional Raman instruments based on visible and NIR (e.g., 785- or 810-nm) lasers induce strong fluorescent background due to their abundance of pigments, thus rendering the method useless. BaySpec 1064-nm Raman systems offer the means with which to minimize interference from fluorescence for those samples (Figure 4). The battery-powered portable system can measure samples easily in the field.

Figure 4a) Raman spectra of gasoline and methanol mixtures. b) Quantitative analysis of the methanol percentage in blended gasoline, based on the ratio of their characteristic spectra peaks of 1030 cm–1 and 1465 cm–1. c) 785-nm and 1064-nm dispersive Raman spectra of olive oil; 785-nm Raman excites a high level of fluorescence from olive oil, degrading its Raman spectrum and analyzing power. In contrast, 1064-nm Raman produces a high-quality spectrum. d) 1064-nm dispersive Raman spectrum of diesel. Characteristic peaks such as 1450 cm–1 (C–H2 bend), 1650 cm–1 (C=C stretch), and 1745 cm–1 (triglyceride C=O stretch) can be used to quantitatively derive liquid fuel’s parameters.3

Chemical imaging for solid samples

Integration of Raman spectroscopy with confocal microscopy enables nondestructive chemical imaging without any sample preparation. Microscopic differences in chemical composition and structure on a sample can be vividly revealed in its chemical image; these features are often completely invisible in optical imaging. The Nomadic (see Figure 5) is a dispersive 3-wavelength 532/785/1064-nm confocal Raman microscope that integrates all of the benefits of 1064-nm Raman spectroscopy and the convenience of diagnosing samples with different lasers by the push of a button. The true confocality with powerful 3-D chemical mapping capability combined with chemometrics such as principal component analysis (PCA) and multivariate curve resolution (MCR)4 enable both easy measurements and practical data display/manipulation in seconds.

Figure 5a) Nomadic dispersive 3-wavelength 532/785/1064-nm confocal Raman microscope. b) Chemical mapping (3-D rendering) of a cement sample. c) The portable Raman spectrometer can be coupled with a microscope via optical fibers for micro-Raman measurements.

Conclusion

This article demonstrates representative applications of portable, field-deployable Raman spectrometers, which can be powerful quantitative tools for environmental analysis in many real-world sampling conditions. These Raman spectrometers with multiexcitation wavelengths, including 1064 nm, are now mature techniques that offer tremendous benefit in producing background-free, high-quality Raman spectra.

References

  1. Ferraro, J. Introductory Raman Spectroscopy, 2nd ed.; Elsevier: Amsterdam, The Netherlands, 2002.
  2. Zhang, S.; Yang, W. Compact Double-Pass Wavelength Multiplexer- Demultiplexer Having an Increased Number of Channels. U.S. Patent 6,108,471; Aug 2000.
  3. Wu, H.; Volponi, J.V. et al. In vivo lipidomics using single-cell Raman spectroscopy. PNAS2011, 108, 3809–14.
  4. McCreery, R. Raman Spectroscopy for Chemical Analysis; Wiley: New York, NY, 2000.

Also see "A New Player in Forensic Analysis: 1064-nm Dispersive Raman."

The authors are with BaySpec, Inc., 1101 McKay Dr., San Jose, CA 95131, U.S.A.; tel.: 408-512-5928; e-mail: lchandler@bayspec.com;www.bayspec.com.

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