Aleksandr V. Mikhonin

Recent growth in commercial carbon nanotube (CNT) production resulted in a strong demand for analytical techniques capable of rapid characterization of these nanomaterials. The latest developments in portable Raman spectroscopy made this technique one of the most viable options for the task. This paper compares the relative performance of 532- and 785-nm portable Raman systems, as well as demonstrates an automated analytical methodology suitable for CNT characterization and quality control applications. Both 532- and 785-nm Raman spectra were used to directly analyze and compare important CNT structural parameters and properties including CNT diameters, diameter distributions, CNT structural quality (% of defects), CNT types, and other properties. The data indicate advantages in a number of areas for using 532- versus 785-nm excitation for CNT Raman measurements.

	Carbon nanotubes (CNTs) are a relatively new class of nanomaterials first discovered by Iijima (1). The unique electrical and mechanical properties of CNTs make them one of the most attractive building blocks for multiple nanomaterials designed for various applications. The typical applications include but are not limited to nanoelectronics and nanomechanics, ultrastrong yarns, interconnects, batteries and capacitors, and aerospace and defense (2,3). As a result, the global CNTs and CNT-based product market is projected to reach 5.64 billion US dollars by 2020, with an annual growth rate (CAGR) of 20.1% (4).

	Manufacturing, modification, functionalization, and customization of the CNT-based materials require bulk quantities of well-defined CNTs, with a high degree of structural purity. Since tight control of the CNT sizes, chirality, and structural quality still remains one of the major challenges during CNT fabrication (3), analytical techniques capable of thorough CNT characterization have become a critical part of the manufacturing process.

	Several techniques are currently used for CNT characterization, including electron microscopy, Raman spectroscopy, thermogravimetric analysis (TGA), and optical absorption spectroscopy. However, Raman spectroscopy has become increasingly attractive for CNT characterization because of its intrinsic ability to detect even small changes in CNT structure, its low analysis cost, little or no sample preparation required, and dramatically improved analysis efficiency due to recent engineering breakthroughs in Raman instrumentation.

	CNT Raman spectra can provide information about the CNT diameter, chirality, structural quality, conductor or semiconductor character, crystallinity, and degree of functionalization (5–8). A brief introduction to major Raman features of carbon nanotubes is given below.

	The radial breathing mode (RBM) band represents radial expansion–contraction of a nanotube. This is a very important band because it is a marker of the presence of single-wall carbon nanotubes (absent in the spectra of essentially all other graphite-like materials). The RBM band frequency can also be directly related to CNT diameter (7,8).

	The G-band is a major Raman band in graphite-like materials representing a tangential shear mode of carbon atoms. In CNTs, this mode has two major contributions, G+ and G- (7). The shape of the G-band and its frequency profile can be used to estimate whether the CNT is “metallic” or semi-conducting; or roughly estimate the nanotube diameter.

	Because of Raman selection rules, the D-band is a forbidden Raman mode in ideal nanotubes and requires a defect to show up in the Raman spectra (7). Thus, the D/G band ratio can directly serve as a measure of CNT structural quality.

	The G′-band is the first overtone of the D-band. However, unlike the D-band, it does not require a defect to show up in Raman spectra. The G′-band can be used, in particular, to probe CNT electronic structure and independently verify CNT chirality (7).

	In this article, the relative performance of 532- and 785-nm portable Raman spectrometers was compared for CNT characterization and quality control applications. In addition, an analytical methodology is presented that enables the calculation of CNT structural parameters directly from measured Raman spectra.

	Carbon nanotube samples were examined using the RamTest-NANO and Mobile µ-Raman analyzers with Raman technology (both from BioTools, Inc.). The RamTest-NANO analyzer is a handheld unit with 532-nm laser excitation, ~120–4000 cm

 spectral resolution, and a maximum laser power of 30 mW. The Mobile-µ-Raman analyzer is a portable unit with 785-nm excitation and variable laser power within a 0–100 mW range. A high resolution (HR) configuration of the Mobile µ-Raman analyzer was used in this study, with ~200–2000 cm

 resolution to match the resolution of the RamTest unit.

	The RamTest unit was run in automated mode (requiring no prior knowledge of Raman spectroscopy), where all measurement parameters are automatically adjusted by the instrument to optimize signal-to-noise ratio and minimize fluorescence, with the remaining fluorescence background (if any) automatically subtracted.

	The Mobile µ-Raman analyzer was run in a mode where all measurement parameters were manually set before the measurement. The laser power was set to 30 mW to match that of the RamTest unit.

	BioTools’ Ram-NANO software add-on is capable of calculating the following CNT parameters or properties: structural quality (percent of structural defects), CNT diameter distribution, type of nanotube, and several other parameters. This software add-on is directly applicable to both RamTest-NANO and Mobile µ-Raman data to enable automated CNT Raman data processing, and a PDF or Excel report generation.

	Figures 1 and 2 show 532- and 785-nm Raman spectra of CNTs recorded at identical laser powers of 30 mW at both wavelengths, respectively. There are four major Raman bands observed for the CNT samples measured during this study: RBM bands around 190–320 cm

 (out of instrument spectral range for the 785-nm Raman spectrum shown in Figure 2). The meaning of these CNT Raman bands has been extensively described in the literature (5–8).

 532-nm Raman spectrum of carbon nanotubes measured at 30-mW laser power. This spectrum contains all of the important CNT Raman bands, which can be used for CNT characterization (see text for detailed discussion).

 785-nm Raman spectrum of carbon nanotubes measured at 30-mW laser power. Low RBM/G-band intensity ratio combined with high D/G band intensity ratio may indicate that the 785-nm laser partially destroys CNTs at the laser power density used in this study (see the text for detailed discussion).

	There are three major observations from Figures 1 and 2. First, the 532-nm Raman spectrum shows a much greater RBM/G band intensity ratio than the 785-nm Raman spectrum. Second, the D/G band intensity ratio is much smaller for the 532-nm Raman spectrum than that for the 785-nm Raman spectrum. Third, the G-band of the 532-nm Raman is fairly symmetric, whereas the 785-nm G-band shows a distinct high-frequency shoulder.

	Since the RBM band is indicative of CNT presence and the D-band is a measure of fraction of structural defects in CNTs, these observations may indicate that a 785-nm laser partially destroys CNTs, at least at the laser power density used in this study. The increased heating or destruction of the CNTs by 785-nm light (versus 532-nm light at an identical power of 30 mW) can likely be explained by the increased absorption of the 785-nm light by these particular CNT samples. A further decrease of laser power helps to reduce the 785-nm laser-induced CNT damage (data not shown). However, it also results in reduction of the 785-nm Raman signal and, therefore, the necessity to increase measurement time.

	Thus, the preliminary conclusion is that both 532- and 785-nm Raman can be used for CNT characterization. However, care must be taken to ensure that relatively delicate CNT samples are not being overheated or even destroyed during analysis.

Homogeneous Linewidth of the RBM Raman Band of an Isolated CNT

	The RBM band profile of CNT samples measured in this study is fairly broad and spans the ~190–320 cm

 spectral region (Figures 3a and 3b). Since the RBM frequency is related to CNT diameter (7,8), the observed RBM frequency span is likely to result from a subset of CNTs with different diameters. Thus, if we knew the homogeneous linewidth of the CNT RBM band, we could directly estimate the CNT diameter population distributions directly from the RBM band profile.

	Fortunately, Raman spectra of isolated single-wall CNTs were reported in the literature (8), and their RBM bandwidths are in the range from 9 to 14 cm

 full width at half height (FWHH). These RBM bandwidths are wider than the resolution of both the 532- and 785-nm portable Raman instruments used in this study (~4 cm

). Thus, for the sake of simplicity, we can take a “rough average” of the reported values, and estimate the RBM band homogeneous linewidth of an individual CNT with a particular diameter, Γ, as ~6 cm

 homogeneous linewidth of a single isolated CNT with a particular diameter, and assuming that Raman cross-sections of all CNT conformations are independent of their frequencies, we can estimate the population distribution of each CNT conformation directly from the RBM band profiles using a method similar to that reported by Asher and colleagues (9). We assume that an inhomogeneously broadened RBM band profile, RBM(ν) is a sum of N Lorentzians with identical intensities and homogeneous linewidths Γ = 6 cm

, occurring at different central frequencies ν = ν

 is a probability for a band to occur at frequency ν

	The resulted histograms of frequency populations, 

, are shown in Figures 3c and 3d for 532- and 785-nm Raman RBM bands, respectively.

	CNT diameter can be estimated from RBM band frequency using the following relationship (8):

	Using equation 2, we can convert the RBM frequency population distributions into CNT diameter population distributions, as shown in Figures 3e and 3f. These diameter distributions reflect the percentage of CNTs with their diameters falling into a specified diameter interval and, thus, can be directly utilized for quality control purposes. 

	Figure 3 demonstrates that RBM band profiles and the resulting CNT diameter distributions obtained from the 532-nm Raman spectra differ from those obtained from the 785-nm Raman spectra. This is despite the fact that both 532- and 785-nm Raman measurements were taken on the identical CNT samples, and using the same laser power of 30 mW on both 532- and 785-nm instruments.

	Theoretically, the difference in diameter distributions obtained from the 532- and 785-nm Raman spectra could have been explained by selective resonance enhancement of different subsets of CNTs by 532- and 785-nm light, because of the different resonance Raman conditions (5). However, as it was already discussed above, comparison of 532- and 785-nm relative Raman intensity ratios for RBM, D, and G bands indicates that, most likely, the 785-nm laser at 30-mW power partially destroys the CNTs (in contrast to 532-nm laser used at the same power). If this conclusion is correct, then Figure 3 may indicate that the 785-nm laser selectively destroys CNTs with higher diameters.  

	Both 532- and 785-nm portable Raman instruments can be used for CNT characterization and quality control applications. Specifically, CNT diameters, diameter distributions, and structural quality (percent of structural defects), other important parameters, and multiple controls plots can be directly obtained or generated from 532- and 785-nm Raman spectra using the automated analytical methodology demonstrated in this paper.

	The differences observed between 532- and 785-nm Raman spectra of identical CNT samples can likely be explained by partial destruction of CNT samples by the 785-nm laser, at the laser power used in this study. Alternatively, these differences can be explained by selective resonance Raman enhancement of different subsets of CNTs by 532- and 785-nm lasers, respectively (5).

	Even though multiple Raman excitations are still preferable for complete characterization of CNTs, the following techno-economic benefits are identified for 532-nm as opposed to 785-nm portable Raman, if, for any reason, an end-user decides to use only one Raman wavelength for CNT analysis:

		A 532-nm laser at 30-mW power does not readily destroy CNTs examined here, whereas the 785-nm laser must only be used at a significantly lower power. This can likely be explained by increased absorption of the 785-nm light by these particular CNT samples.

		The Raman signal at 532-nm excitation is approximately fivefold stronger than that at 785 nm, per unit laser power. This is because Raman signal is ~(1/λ

 = 532 nm, 785 nm, and so forth (10). Thus, 532-nm portable Raman units are fundamentally capable of approximately five times faster analysis or improved analysis quality. In addition, in the case of especially delicate samples, the 532-nm Raman analyzers can be used at up to approximately fivefold lower laser power than the 785-nm based units, respectively, without compromising spectral analysis quality.

		The 532-nm Raman technique provides a superior combination of spectral resolution and spectral range (compared to the 785-nm Raman technique) to enable superior CNT structural characterization. This is because the use of 532-nm light provides an engineering advantage in space-limited portable instruments.

		Quantum efficiencies of best in class charge-coupled devices (CCD) detectors and optics are greater at 532 nm (compared to 785 nm) to even further increase the observed Raman signal in favor of 532-nm excitation.

		The 532-nm Raman analyzer has a superior performance not only for the analysis of CNTs “as is,” but also for the analysis of CNT suspensions in aqueous solutions, because of the reduced absorption of 532-nm light by water versus that of 785-nm light.

		Data obtained from www.marketsandmarkets.com.

Carbon Nanotubes: Synthesis, Structure, Properties, and Applications

, M.S. Dresselhaus, G. Dresselhaus, and Ph. Avouris, Eds. (Springer-Verlag, 2001), pp. 213–246.

 J.M. Chalmers and P.R. Griffiths, Eds. (John Wiley & Sons, Chichester, 2002).

Articles

In this paper, we examine the relative performance of 532 and 785 nm portable Raman systems, as well as demonstrate an automated analytical methodology applicable for carbon nanotube (CNT) characterization and quality control applications. Both 532 and 785 nm Raman spectra were used to directly analyze and compare important CNT structural parameters and properties including CNT diameters, diameter distributions, CNT structural quality (% of defects), CNT types, and other properties. The data indicate advantages in a number of areas for using 532 versus 785 nm excitation for CNT Raman measurements.

Carbon Nanotube Characterization and Quality Control Using Portable Raman: 532-nm Versus 785-nm Laser Excitation

Recent advances in Raman instrumentation have resulted in the development of easy-to-use and efficient handheld Raman analyzers. Most of the commercially available handheld Raman devices use 785- or 1064-nm excitation. This article directly demonstrates the performance of 532-nm handheld Raman (versus 785- and 1064-nm excitation) for the analysis of biopharmaceuticals for structure and counterfeit testing as well as explosive detection. The results presented here will contribute to recognition of 532-nm Raman excitation as a highly attractive option for rapid and effortless analysis in the field.

Raman spectroscopy is well known for its high sensitivity to chemical structure and composition, noncontact, noninvasive, and nondestructive nature of its measurements, with little or no sample preparation required (1). Furthermore, recent engineering and analytical methodology breakthroughs, including fluorescence compensation, allow full use of the intrinsically broad applicability of Raman spectroscopy. These include medical diagnostics (2), pharmaceutical analysis (3), counterfeit medicine testing (4), explosive detection (5), cultural heritage (6), and many others.

Arguably, one of the most important recent advances in Raman spectroscopy is the development of efficient and easy-to-use miniature portable or handheld Raman systems. In addition, most of these handheld systems are capable of analyzing substances directly through containers and packages to eliminate any need for sample preparation or handling and exposure to the chemicals.

These recent developments have made Raman directly suitable for a variety of field applications, where bringing samples to a laboratory is impractical. At this time, there are more than 20 commercial handheld Raman analyzers from more than 10 manufacturers on the market. The key factors to consider when choosing the best available unit for a particular application are briefly presented and discussed below.

The first goal of this study is to present semitheoretical, semiempirical considerations to help an end user choose a handheld Raman unit that best suits his or her intended field application. Performance data for comparison are summarized in Table I and Figure 1.

Figure 1: Relative performance (per unit laser power) of 532-, 785-, and 1064-nm handheld Raman for analytes in common sample matrices. Shown are results for (a) optical transmittance, (b) pure analyte, (c) analyte in water, and (d) analyte in ethanol.

The second goal is to revisit or explain why the excitation at 532 nm is the best choice or business case for many practical applications. Briefly, 532-nm excitation enables twofold reduction of instrument costs and up to 5â16 times improved analysis speed and accuracy (Figure 1). In this article, the superior performance of 532-nm handheld Raman is directly demonstrated for counterfeit biologics testing and fastest-in-class explosive detection.

532-, 785-, or 1064-nm Excitation for Handheld?

The excitation wavelength of a handheld Raman analyzer largely determines the strength of Raman signal (analysis speed and accuracy); as well as a propensity for unwanted fluorescence background to occur. In addition, efficiency of all optical elements and quantum efficiency of a relevant detector (charge-coupled device [CCD] for ultravioletâvisible [UVâvis] or InGaAs for visânear infrared [NIR]), as well as spectral resolution limits are also impacted by the choice of laser excitation wavelength.

Most of the commercially available handheld Raman units use 785- or 1064-nm excitations. Only a few of the most recently produced handheld Raman systems use other excitation wavelengths, including 532 nm. The introduction of these new units was aided by recent advances in analytical methodology to significantly reduce the impact of fluorescence on Raman measurements.

Even though more than one of the commercially available excitations might be technically feasible for a given field application, usually only one of these excitations can provide the best business case and analytical solution for a particular application. To select the best excitation wavelength, all of the following factors must be carefully considered:

Analysis speed and accuracy at each excitation wavelength

Fluorescence background of intended samples at each excitation wavelength

Sample matrix transparency: container walls, solvent, analyte, and so forth, around each excitation wavelength.

As for analysis speed and accuracy, 532-nm excitation results in 5- and 16-fold stronger observed Raman signal intensity per unit laser power than those at 785 and 1064 nm, respectively. This is because Raman intensity is inversely proportional to the fourth power of laser excitation wavelength (7), 

. In addition, the best-in-class light detectors and optics have higher quantum efficiencies around 532 nm than those available for 785- and 1064-nm systems, to further improve observed Raman signal-to-noise ratios with 532-nm excitation.

In contrast, 1064 nm is a preferred excitation wavelength to minimize fluorescence background. However, the advantage in the unwanted fluorescence reduction is significantly handicapped by ~16 and ~3 times slower analysis compared to 532- and 785-nm systems (per unit laser power), respectively. Thus, 1064-nm excitation can be a preferred analytical solution only in case of the most strongly fluorescent samples (typically dyes or compounds with a significant number of conjugated, double, or triple bonds), whereas in all the other cases 785- and especially 532-nm excitations provide faster analysis.

To account for the impact of sample matrix transparencies on Raman signals, Figure 1a provides measured optical transmittances for a few sample matrixes typical for field analysis. Transmittances for transparent glass (laboratory vial or bottle), amber glass (vial or bottle), transparent plastic (Petri dish, plastic bottle, evidence bag, or blister pack), amber plastic (medical prescription bottle), water, and ethanol are presented.

Using the data in Figure 1a, Figures 1bâd provide a comparative chart for observed relative Raman signal intensities (normalized to 532 nm) to cover several typical situations for field analysis: pure analyte with or without a listed container (Figure 1b); analyte in water solution with or without a container (Figure 1c); and analyte in ethanol solution with or without a container (Figure 1d).

Figure 1 shows that Raman signal intensity at 532 nm is ~25â1600% stronger for all typical field analysis cases considered. The data in Figure 1 are further reduced and summarized in Table I to provide an easy, at-a-glance performance comparison for 532-, 785-, and 1064-nm systems.

The comparison indicates that 532-nm excitation clearly outperforms the 785- and 1064-nm systems in seven out of nine cases as shown in Table I. These cases include all nonfluorescent and weakly fluorescent samples, some fraction of moderately fluorescent samples; measuring through most common glass and plastic containers including amber; as well as analyte detection and quantitation in water (normalization against water OH-stretching band only available with 532 nm) and most organic solvents.

All samples were analyzed using a RamTest handheld Raman identifier (BioTools, Inc.), with a 532-nm laser excitation, 120â4000 cm

 spectral resolution. All tests were run in automated mode, where all measurement parameters are automatically adjusted to optimize signal-to-noise ratio and minimize fluorescence, with remaining fluorescence background (if any) automatically subtracted.

532-nm Handheld Raman for Counterfeit Biologics Detection

One of the most promising new applications for 532-nm handheld Raman is counterfeit biologics detection. The superior performance of 532-nm handheld systems can be explained by the stronger Raman signal and lower absorption of 532-nm light by water compared to 785- and 1064-nm systems (Figure 1). The combination of these two factors enables unmatched analytical power of 532-nm handheld Raman spectroscopy for discrimination and quantitation of various peptides or proteins in aqueous solution.

The growing counterfeit medication problem not only affects pills mainly for cholesterol and erectile dysfunction, but also biologics (cancer drugs) that are administered in doctor`s offices (8). This latter group is an ideal target for counterfeiting as they are high in cost and are parenterally administered medications packaged in vials.

In 2002, counterfeit erythropoietin vials were produced by the original manufacturer, Amgen, but were later up-labeled on the gray market denoting 40,000 U/mL instead of their real content 2000 U/mL. Bogus Avastin came to the United States from abroad in 2012 with obvious defects in labeling (the Roche logo instead of Genentech) and contained cornstarch, acetone, and other chemicals, but no active ingredient to fight cancer. Only portions of the counterfeit vials were discovered in both cases, concluding that the majority of the ineffective drugs must have been administered to patients. Yet, a quick and simple check with a low-cost 532-nm handheld Raman analyzer would have determined the falsely labeled drugs at any stage of the distribution network.

Figure 2 imitates both of the above mentioned incidents, counterfeit biologics with diluted active pharmaceutical ingredients (API) or with no API. We selected two biologics from the top 10 list of best-selling biologic drugs in the world. Three samples were measured in each case: authentic product, diluted API (10% for mimicking up-labeled drug), and formulation buffer (placebo imitating no API case) using a 532 nm handheld Raman analyzer.

Figure 2: Two of the top 10 bestselling biologics measured with 532-nm handheld Raman: (a) biologics sample 1; (b) biologics sample 2. Green = original drug, red = counterfeit drug, and black = buffer or placebo. Data are normalized to the water 3200â3400 cm

 OH-stretching bands for quantitative API determination.

All specimens were tested using the automated sampling method that requires no previous Raman knowledge. In both cases it was quick, easy, and unambiguous to differentiate the counterfeit drugs from the original ones. These results directly demonstrate that 532 nm handheld Raman could serve as a powerful, low-cost solution for pharmaceutical companies, receiving pharmacies and for end-user doctors’ offices.

532-nm Handheld Raman for Detection of Explosives

Data from the Global Terrorism Database shows that the number of terrorist attacks using explosive devices has dramatically increased within the last decade (9). Numerous analytical methods, including portable Raman, have been developed to enable detection of explosives, and their precursors or breakdown products in the field (5).

Figure 3 directly demonstrates superior performance of 532-nm handheld Raman to enable fastest-in-class, reliable and safe detection, identification, and quantitation of explosives. It should be noted that some of the shown explosives or precursors are considered to be either “strongly fluorescent” (dinitronaphthalene) or “hard to detect” using a handheld Raman (such as cyclotrimethylenetrinitramine [RDX], ammonia, and ammonium nitrate).

Figure 3a shows 532-nm handheld Raman spectra of powdered explosives: trinitrotoluene (TNT), RDX, pentaerithrityl tetranitrate (PETN), ethyleneglycole dinitrate (EGDN), dinitronaphtalene (DNN), tetranitroteraazocyclooctane (TNTACO), and ammonium nitrate powder. Figure 3b shows 532-nm handheld Raman spectra of typical liquid explosives or precursors: acetone, ethanol, isopropanol, cyclohexane, toluene, ammonia, and aqueous solution of ammonium nitrate. It should be noted that 532-nm excitation enables reliable identification and detection of all these substances within 1â5 s. Such a fast and reliable identification is a direct result of the stronger Raman signal at 532-nm excitation compared to those at 785 and 1064 nm.

Figure 3: Spectra of explosives measured by 532-nm handheld Raman spectroscopy: (a) powdered explosives, (b) liquid explosives-precursors, (c) hydrogen peroxide solutions in water, (d) Automated hydrogen peroxide quantitation using 3200â3400 cm

 OH-s water bands. Insets in (c) show the magnified ~874 cm

One more attractive feature of 532-nm excitation is a stronger dispersion of 532-nm light compared to those at 785 and 1064 nm. This increased light dispersion enables the production of handheld Raman systems with a widest-in-class spectral range of ~100â4000 cm

, and still the best-in-class spectral resolution of 4â6 cm

. This broader spectral range directly enables new applications for handheld Raman, including automated quantitation of analytes in aqueous solutions. Figure 3d directly demonstrates automated hydrogen peroxide quantitation in water down to <0.1%. Specifically, 3200â3400 cm

 OH-stretch bands of water, which are unattainable with today’s commercial 785- and 1064-nm handheld systems, were used as internal intensity standards (Figure 3c).

Our analysis indicates that 532-nm excitation should be revisited as a highly attractive option for handheld Raman spectroscopy. Identified benefits include ~twofold reduced instrument cost, up to 5â16 times faster analysis for a great deal of practical field applications, reduced laser power (enabling safe detection of explosives, reduced laser safety concerns, or laser-induced sample degradation, as well as extending battery continuous operation time), superior performance in water and most organic solvents, the ability to analyze through a large variety of glass and plastic containers (including amber), improved combination of spectral range and spectral resolution, as well as reduced detection limits with improved analysis accuracy.

As a result, 532-nm handheld Raman spectroscopy can dramatically improve business cases for a significant fraction of practical field applications or extend the applicability scope of handheld Raman to new fields. The best applications include, but are not limited to, counterfeit biologics, rapid detection of explosives, identification of individual components in complex mixtures, automated quantitation of analytes in aqueous solutions, diluted analytes in water or organic solvents, and reliable detection of several compounds previously considered “hard to detect” using handheld Raman devices.

, J.M. Chalmers and P.R. Griffiths, Eds. (John Wiley & Sons, Chichester, UK, 2002).

Nanomedicine: Nanotechnology, Biology, and Medicine

		T. Vankeirsbilck, A. Vercauteren, W. Baeyens, and G. Van der Weken, 

		K.L. Gares, K.T. Hufziger, S.V. Bykov, and S.A. Asher, 

 (John Wiley & Sons, New York, New York, 2002).

 are with BioTools Inc., in Jupiter, Florida. 

 is with BioTools Inc., and the Department of Chemistry at Syracuse University in Syracuse, New York. Direct correspondence to: 

Recent advances in Raman instrumentation have resulted in the development of easy-to-use and efficient handheld Raman analyzers. Most of the commercially available handheld Raman devices utilize 785 or 1064 nm excitation. This paper directly demonstrates the performance of 532 nm handheld Raman (versus 785 and 1064 nm) for the analysis of biopharmaceuticals for structure and counterfeit testing as well as explosive detection (TSA screening and CSI applications). The results presented here will contribute to recognition of 532 nm Raman excitation as a highly attractive option for a rapid “in-place” analysis in the field.

Aleksandr V. Mikhonin

Articles

Carbon Nanotube Characterization and Quality Control Using Portable Raman: 532-nm Versus 785-nm Laser Excitation

Recent Developments in Handheld Raman Spectroscopy for Industry, Pharma, Forensics, and Homeland Security: 532-nm Excitation Revisited