Raman spectroscopy is an idealtool for chemical analysis due toits unique advantages over otheranalytical techniques. It is a nondestruc-tive and noncontacting method ofobtaining the fingerprint spectrum ofmaterials, requiring no special samplepreparation. Raman can analyze sam-ples via sample plastic bags, glass vials,and in aqueous solution. It does notrequire a large amount of a sample to doanalysis; the sample can be as small as12 m across. A short amount of meas-uring time, normally a few seconds, isrequired to obtain a Raman spectrum.Thus, it can be used to monitor chemi-cal reactions in real time.

Despite the fact that Raman spec-troscopy has so many advantages, it stillis not in widespread use. The main rea-son is the high cost typically associatedwith Raman analyzer systems.Currently, a high-resolution and highsignal-to-noise ratio Raman analyzercan be priced from $40,000 and above.On the other hand, there is a marketneed for low-cost Raman analyzer sys-tems as general laboratory tools.However, those systems usually are

equipped with low-resolution, low-power visible lasers, and low signal-to-noise ratios, which is not adequateenough to perform any high perform-ance chemical analysis. Therefore, bet-ter-resolution and lower-cost Ramansystems are key to increasing andenabling greater acceptance and usageof Raman spectroscopy.

This article presents a few keyadvancements in Raman componentsthat will enable low costs and high per-formance. It also establishes the figureof merit to identify and characterizeRaman systems in terms of high resolu-tion and low cost. Finally, Raman appli-cations and sample Raman spectra inpharmaceutical, petrochemical process-ing, and plastic recycling applicationswill be presented.

Modern Raman SystemsA typical laboratory Raman analyzersystem consists of four major compo-nents: the excitation source, the collec-tion device, the spectrograph, and thepost-processing software. The excita-tion source must be strong enough togenerate sufficient Raman signals, and

30 Raman Technology For Todays Spectroscopists June 2004

A New Era in Affordable Raman SpectroscopyThis article overviews several recent advancements in Raman components that are enabling high analytical performance in low-cost Raman instruments.

Tony Lam

Tony Lam is a senior system engineer at EnWave Optronics (Irvine, CA). He can bereached at: tlam@enwaveopt.com.

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Recent developments indiode laser wavelength stabi-lization and linewidth nar-rowing have been very dra-matic. Current wavelengthstabilization designs includeLittrow with traditionalgratings, Littmann with tra-ditional gratings, fiber Bragggratings, volume Bragg grat-ings, distributed feedbacklasers (DFBs), distributedBragg reflector lasers(DBRs), and master oscilla-tor power amplifier lasers(MOPAs). Some designsalso integrate a bandpass fil-ter or amplified sponta-neous emission (ASE)reduction technique toremove the unwanted ASEinterference that often isseen in a high-power diodelaser. All these lasers shouldat least provide 150 mW in opticalpower, a stabilized center wavelengthwithin a tenth of a wavenumber, and aspectral linewidth of less than 1 cm-1 fora high-resolution Raman system. A laserlinewidth (~34 cm-1) close to the spec-trometers resolution is not ideal becausethe systems overall spectral profile orresponse will be the convolution of thetwo responses (laser linewidth and spec-trometers resolution). Thus, any laserspectral profile changes due to tempera-ture, current, or any other nonlinearitieswill affect the overall system spectralprofile and cause inaccuracies in chemo-metric prediction. Figure 1 depicts a costsurvey over the past eight years for wave-length-stabilized, 785-nm, 300-mW, 1-cm-1 linewidth Raman spectroscopylaser systems on the market.

Due to the recent development of asilicon CCD camera, the CCD sensor hasreplaced the single channel detector,photomultiplier tube (PMT) and ava-lanche photodiodes (APD), as a multi-channel photon detector in the spectro-graph. It can detect radiation from 400to 1100 nm (2). In addition, the CCDsensor will reduce integration timebecause it allows spectrum multiplexing.A typical integration time of the CCDspectrograph is less than 30 s.

A general laboratory Raman probe iscomposed of two fibers. The excitationfiber is used to deliver the excitationsource to a sample. The collection fiberis used to collect Raman signals anddeliver it to the spectrograph.Normally, the bandpass filter andband-reject filter are integrated into

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Cost in US dollars (x104)

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Figure 2. Comparison of diode-based Raman lasersources on the current market (figure of merit =power/linewidth).

monochromatic light toprovide clean and uncom-plicated spectra. A lasertypically is used as the exci-tation source because it canprovide a coherent beam ofmonochromatic light withhigh intensity. The collec-tion device is the fiber-optic probe. It collects thescattered photons, filtersout the Rayleigh scattering,and sends the Raman sig-nals to the spectrograph.The spectrograph then sep-arates the Raman signals bytheir wavelengths andtransfers them to the lightdetector, which records theintensity of the accumulat-ed photons at its ownwavelength. These record-ed data then are passed tothe postprocessing soft-ware to display as a Raman spectrum.

Several types of lasers can be used asthe excitation source for Raman spec-troscopy. They are Ar+ ion (488.0 and514.5 nm), Kr+ ion (530.9 and 647.1nm), He:Ne (632.8 nm), Nd:YAG (1064nm), and diode (630 and 980 nm)lasers (1). Due to recent advancementsin diode laser technology, it is the bestcandidate to be utilized in low-cost,portable Raman analyzer systemsbecause of its relatively low cost, com-pact size, high reliability (>10,000-hlifetime), and near-infrared (NIR)emission wavelength for fluorescencereduction. A typical diode laser used ina modern Raman analyzer system is ahigh power laser diode (>300 mW),with wavelengths ranging from 630 to980 nm.

For any high resolution (

June 2004 Raman Technology For Todays Spectroscopists 35

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of laser sources is comparedbecause the laser still is themajor cost component for aRaman system. The figure ofmerit of the laser is definedas the laser power inversedproportionally with the laserlinewidth:

Figure of merit =power/linewidth [1]

Using Equation 1, thefigures of merit of lasers(AE) are calculated andmapped in Figure 2.

Observing the figure-of-merit chart (Figure 2), thelaser excitation sources aredivided into three groups.One group is the high per-formance and high costgroup, including A and Claser sources. The second group is thelow-performance and low-cost group,including B and D laser sources. Thelast group, E laser sources, is the highperformance and low cost group.

The figure of merit of the Ramansystem is defined as the product of thespectral range (coverage) and the laserpower divided by the system resolution:

Figure of merit = (coverage xpower)/(resolution) [2]

Using Equation 2, the figures ofmerit of Raman systems (AE) are cal-culated and mapped in Figure 3.

Observing the figure-of-merit chartof the Raman system (Figure 3), the Aand E Raman systems are considered tobe the medium-performance and low-cost systems. The B and C systems areconsidered to be low-performance andhigh-cost systems. The D system is con-sidered a high-end system, high per-formance versus high cost. It is the

Raman spectroscopy applications thatdetermine which Raman systems shouldbe used, but for most of the applications,a Raman system that has a 6-cm-1 spec-tral resolution is sufficient (1).

Applications of Affordable Raman SystemsWith high-resolution and low-costRaman systems, many Raman spec-troscopy applications in chemicalanalysis become feasible. To name a few,Raman spectroscopy is useful in drugidentification for the pharmaceuticalindustry, the control of octane level ingasoline, and the classification of plasticin the plastic recycling industry. Todemonstrate the feasibility of using alow-cost Raman instrument in theseapplications, the HRC-10 Raman ana-lyzer with less than 6-cm-1 system reso-lution, manufactured by EnWaveOptronics, was chosen to collect Raman

Figure 4. Raman spectra of OTC medicines.

the fiber-optic probe headto remove the laserRayleigh scattering, silicaRaman interference fromthe fiber, and ASE from thelaser excitation source.

The data collected fromthe spectrograph then aresent to the software. Thesoftware then converts thedata into the Raman spec-trum and displays it on anxy chart. The x axis of thechart is the wavenumber,while the y axis is the inten-sity of the spectrum.

Resolution of RamanSystemsThree basic bandpass filtersare used in a typical Ramanmeasuring system to deter-mine the overall Ramanspectrum quality (that is, resolution).These three bandpass filters are: laserlinewidth bandpass, spectrometer band-pass, and sample bandpass (naturallinewidth) filters.

Ideally, a laser linewidth bandpassfilter should be much less than thespectrometer bandpass filter. In thiscase, the spectrometer will be the onlyfactor in determining the Raman sys-tems bandpass filter. If the bandpassfilters of the two (that is, laser andspectrometer) are close, the systemsbandpass filter will be a convolutionalbandpass filter of the two components.Generally, a spectrometer bandpass fil-ters profile is much more stable thanthe laser bandpass filter profile, thus amuch narrower laser linewidth band-pass filter is preferred and could makea system much more stable.

For most Raman applications insolid and liquid samples, a system reso-lution of 6 cm-1 or less is sufficient.Unfortunately, any high-resolution (300 mW), wavelength-stabilized,and linewidth narrowed (

June 2004 Raman Technology For Todays Spectroscopists 37

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lene (PP, 5); polystyrene(PS, 6); and other,labeled as 7 (5). In theplastics industry, Ramanspectroscopy systems findapplication in real-timemonitoring of polymer-ization reactions to con-trol the processing time, inquantitative composition-al analysis of polymermelt streams, and in plas-tic identification for recy-cling purposes. Figure 6shows an example of plas-tic Raman spectra.

ConclusionWith the currentadvancements in diodelaser technology, Ramansystems have beenimproved on two fronts:cost and resolution. Raman excitationsources now can be made with a lowercost. In addition, laser wavelength sta-bilizing and linewidth narrowing haveimproved the resolution of Raman sys-tems. Consequently, these advance-ments have eliminated the high costsassociated with the high-resolutionsystem and brought this novel technol-ogy to every corner of life. It signifies anew era in Raman spectroscopy.Perhaps, within five years, it will finditself within many potential applica-tions that have not been consideredpreviously for chemical analysis.

References1. Richard L. McCreery, Raman

Spectroscopy for Chemical Analysis(John Wiley & Sons, New York, 2000).

2. Benjamin A. DeGraff and Mandy

Hennip, Chem. Educator, 2, 1518(2002).

3. Ian R. Lewis and Howell G.M. Edwards,Eds., Handbook of RamanSpectroscopy: From the ResearchLaboratory to the Process Line(Marcel Dekker, New York, 2001).

4. Susanne Brunsgaard Hansens, TheApplication of Raman Spectroscopyfor Analysis of Multi-ComponentSystems (Department of Chemistry,Technical University of Denmark,Denmark, 2000).

5. Resin Identification Codes, AmericanPlastics Council(http://americanplasticscouncil.org;accessed 4/29/04).

6. CHEM*3440 Raman Spectroscopy:How it Works the Spectrum,(http://www.chembio.uoguelph.ca;accessed 4/29/04).

Figure 6. Raman spectra of polypropylene and polystyrene plastics.

spectra of some over-the-counter medicines, unlead-ed isooctane levels ofChevron gasoline, andsome plastic samples.

Pharmaceutical. One ofthe advantages of theRaman system is its abilityto measure down to 50 cm-1 in spectral rangecompared to the mid-IRsystem that can only reachapproximately 400 cm-1.The feature is very attractiveto the pharmaceuticalindustry because most ofthe spectra of drug latticesare below 200 cm-1 (3).Raman spectroscopy findsits application in latticevibration study for the char-acterization of the solidstate of a drug and especial-ly for the investigation of polymor-phism and amorphous systems. In addi-tion, it finds application in analysis andquantification of drug blends, and in theexamination of drug formulations anddrug excipient incompatibilities.

Petrochemical. The gasoline pur-chased from any local gas station hasthe methyl-tert-butyl ether (MTBE)(4) content added to improve theoctane number. The octane number isan experimentally determined proper-ty of petroleum fuel that defines thequality of engine antiknocking. Ramanspectroscopy can be used to identifythe octane level within gasoline. Anexample of the octane level in Chevronunleaded gasoline is shown in Figure 5.Also, Raman systems can be used tomeasure oxygenate material in thegasoline. According to the U.S.

Environmental Protection Agency(EPA) (3), to reduce the levels of car-bon monoxide in the air, the govern-ment entity requires the use of oxy-genate additives to the gasoline. Thepetroleum manufacturers have beenmaintaining and monitoring the oxy-genate level accurately in the gasolineduring refinery process. Raman spec-troscopy systems have been a favoredapproach for qualitative identificationof oxygenate materials.

Plastics. Plastics are used extensivelyand vastly in many common materials.Plastics consist of seven different kindsof polymers. They are polyethylene-terephthalate (labeled as PET, andnumber 1 under recycling symbol);high density polyethylene (HDPE, 2);polyvinyl chloride (PVC, 3); low densi-ty polyethylene (LDPE, 4); polypropy-

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Figure 5. Raman spectra of Chevron unleaded gasolinewith various isooctane levels.