1、Designation: E1683 02 (Reapproved 2014)1Standard Practice forTesting the Performance of Scanning RamanSpectrometers1This standard is issued under the fixed designation E1683; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、 of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1NOTEUnits statement was inserted in Section 1.2 editorially in June 2014.1. Scope1.1 This practice covers routine testing of
3、scanning Ramanspectrometer performance and to assist in locating problemswhen performance has degraded. It is also intended as a guidefor obtaining and reporting Raman spectra.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.
4、1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. For specificpr
5、ecautions, see 7.2.1.1.4 Because of the significant dangers associated with theuse of lasers, ANSI Z136.1 should be followed in conjunctionwith this practice.2. Referenced Documents2.1 ASTM Standards:2E131 Terminology Relating to Molecular SpectroscopyE1840 Guide for Raman Shift Standards for Spectr
6、ometerCalibration2.2 ANSI Standard:3Z136.1 Safe Use of Lasers3. Terminology3.1 Terminology used in this practice conforms to thedefinitions in Terminology E131.4. Significance and Use4.1 A scanning Raman spectrometer should be checkedregularly to determine if its condition is adequate for routinemea
7、surements or if it has changed. This practice is designed tofacilitate that determination and, if performance isunsatisfactory, to identify the part of the system that needsattention. These tests apply for single-, double-, or triplemono-chromator scanning Raman instruments commercially avail-able.
8、They do not apply for multichannel or Fourier transforminstruments, or for gated integrator systems requiring a pulsedlaser source. Use of this practice is intended only for trainedoptical spectroscopists and should be used in conjunction withstandard texts.5. Apparatus5.1 LaserAmonochromatic, conti
9、nuous laser source, suchas an argon, krypton, or helium-neon laser, is normally used forRaman measurements. The laser intensity should be measuredat the sample with a power meter because optical componentsbetween the laser and sample reduce laser intensity. A filteringdevice should also be used to r
10、emove non-lasting plasmaemission lines from the laser beam before they reach thesample. Plasma lines can seriously interfere with Ramanmeasurements. Filtering devices include dispersive monochro-mators and interference filters.5.2 Sampling OpticsCommercial instruments can be pur-chased with sampling
11、 optics to focus the laser beam onto asample and to image the Raman scattering onto the monochro-mator entrance slit. Sample chamber adjustments are used tocenter the sample properly and align the Raman scattered light.A schematic view of a conventional 90 Raman scatteringgeometry is shown in Fig. 1
12、. The laser beam propagates at aright angle to the direction in which scattered light is collected.It is focused on the sample at the same position as themonochromator entrance slit image. Other geometries such as180 backscattering are also used. With singlemonochromators, a filter is normally place
13、d in the opticalcollection path to block light at the laser frequency fromentering the monochromator.1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and Separation Science and is the direct responsibility of Subcom-mittee E13.08 on Raman Spectroscopy.Current e
14、dition approved May 1, 2014. Published June 2014. Originallyapproved in 1995. Last previous edition approved in 2007 as E1683 02(2007).DOI: 10.1520/E1683-02R14E01.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book
15、 of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West
16、Conshohocken, PA 19428-2959. United States15.3 PolarizationFor routine measurements the polariza-tion of the laser at the sample is oriented normal to the planeof the page in Fig. 1. However, measurements using differentpolarizations are sometimes used to determine vibrationalsymmetries as part of m
17、olecular structure determinations. Avariety of optical configurations can be used to make polariza-tion measurements; a detailed discussion of these is beyond thescope of this practice. Briefly, for polarization simple measure-ments of randomly-oriented samples (most of the clearliquids), an analyzi
18、ng element such as a polaroid filter oranalyzing prism is added to the optical system and Ramanspectra are collected for light scattered in (1) the same directionas the source (parallel), (2) perpendicular to the source.Depolarization ratios are calculated using Raman band inten-sities from the two
19、spectra as follows:Depolarization ratio 5Intensity parallelIntensity perpendicular(1)5.3.1 A polarization scrambler is shown in Fig. 1. Thiselement is used to avoid making corrections for polarization-dependent grating effects. The scrambler is also frequentlyused during routine measurements and sho
20、uld be placedbetween the sample and entrance slit, close to the collectionlens. A polaroid filter placed between the scrambler andcollection lens provides a simple polarization measurementsystem.5.4 MonochromatorA scanning monochromator used forRaman spectroscopy will exhibit high performance requir
21、e-ments. Double and triple monochromators have particularlystringent performance standards. During the original instru-ment design, features are usually introduced to minimizeoptical aberrations. However, proper maintenance of opticalalignment is essential. A focused image on the entrance slitshould
22、 be optically transferred to and matched with the otherslits. If the monochromator is not functioning properly contactthe manufacturer for assistance.5.5 Photomultiplier TubeA photomultiplier can be usedfor detecting Raman scattered radiation. A tube with goodresponse characteristics at and above th
23、e laser wavelengthshould be selected. Dark signal can be reduced with thermo-electric cooling for improved detection of weak signals.Current and voltage amplification or photon counting arecommercially available options with photomultiplier tubes.6. Guidelines for Obtaining and Reporting RamanSpectr
24、a6.1 Alignment of Optical ElementsRefer to the manufac-turer for detailed sample chamber alignment instructions. Uponinstallation, each optical component should be aligned indi-vidually. For optimal alignment the sample image should becentered on the entrance slit of the monochromator (oftenviewed t
25、hrough a periscope accessory or with the aid of ahighly scattering sample or a white card at the slit). To performthe alignment a test sample is mounted in the samplecompartment, centered in the laser beam, and translated to theapproximate center of the monochromator optic axis. Themonochromator is
26、set to monitor a strong Raman band and itssignal is maximized by adjusting the sample stage, lenses, or acombination of the two. Normally three orthogonal lensadjustments are used: (1) the laser focusing lens is translatedalong the direction of the beam; (2) the Raman scatteringcollection lens, posi
27、tioned between the sample and the entranceslit, is translated along the direction of the propagating scat-tered light in order to provide focus; and (3) the collection lensFIG. 1 Typical Raman Scattering Measurement GeometryE1683 02 (2014)12is translated perpendicular to the scattered light in order
28、 to scanthe image of the laser-excited scattering volume across thewidth of the monochromator entrance slit. (Refer to Fig. 1.)This collection lens adjustment should be made during majorinstrument alignment (for example, during initial set-up), butshould not be necessary during routine sample-to-sam
29、plealignment. Sample and lens adjustments should be repeated asnecessary while the slits are narrowed from a relatively largeinitial width down to the size determined by the resolutionrequirements of the measurement.6.2 Calibration:6.2.1 Spectral ResponseThe spectral response of an opti-cal spectrom
30、etric system will depend on the efficiency of thegratings (which is both wavelength and polarization depen-dent) and the spectral response of the photomultiplier tube.This can be measured routinely by collecting light from atungsten halogen lamp or other NIST-traceable standard lightsource. A comple
31、te procedure for performing spectral responsecorrections has been published by Scherer and Kint (1).4It isstrongly recommended that corrections for spectral response beincorporated directly into the software when a computer is usedto collect spectra.6.2.2 WavenumberThe accuracy of the wavenumber cal
32、i-bration over a large region should be determined using astandard low-pressure emission source with enough lines tomake many measurements over the range of the instrument.Low-pressure mercury, argon, and neon lamps are frequentlyused. The non-lasing emission lines of the laser can also beused if th
33、e laser filtering device is removed. Accurate wave-number values are available (2-9). For measurement at resolu-tions 0.5 wavenumbers a more rigorous calibration methodshould be employed.6.3 Recording Raman SpectraThe following guidelinesare provided for recording spectra with a rare meter and strip
34、chart recorder or with a computer or digital signal averager. Inboth cases it is important to record a spectrum so that spectralfeatures are not distorted by the mode of data acquisition.6.3.1 Recording With a Rate-Meter and Strip ChartRecorderThe range on the rate-meter is set by monitoring thestro
35、ngest peak in the spectrum. The relationship between thescan rate, spectral slit width, and time constant of the rate-meter, as recommended by IUPAC (10), is:Scan rate, cm21/s! #spectral slit width, cm21!4 3time constant s!(2)In addition, the time constant of the recorder should beconsiderably faste
36、r than the rate-meters time constant, and thespeed of the paper should be adequate to measure the spectralfeatures.6.3.2 Recording With a Computer or Signal AveragerInthis case one needs to define the increments in wavenumbersbetween data points.Aminimum criterion is to collect five datapoints in th
37、e full width at half the maximum intensity (FWHM)of the narrowest spectral band. For example, if the slits wereset to provide a measured band width at half maximum of 4wavenumbers, then 1-wavenumber increments would producefive data points within the FWHM in a scan of a line from aplasma emission so
38、urce. To better define peak shape decreasethe size of the increments. This is especially important forbands that deviate from Lorentzian shape.6.4 Reporting Experimental ConditionsThe spectral slitwidth (wavenumbers), scan rate, laser wavelength and power atthe sample, polarization conditions, integ
39、ration time, correc-tions for instrumental response, type of spectrometer anddetector, sample information (physical state, concentration,geometry, and so forth), and other important experimentalconditions should always be recorded with the spectra andreproduced for performance testing. A complete re
40、cord of theparameters to be specified is available in Table 1 of the IUPACRecommendations for the Presentation of Raman Spectra inData Collections (10).7. Evaluation of Raman Instrument Parameters7.1 The performance of an instrument should be evaluatedregularly to determine if it has degraded. This
41、is most easilyaccomplished with a test sample such as carbon tetrachloridemeasured under a set of standard conditions established for the4The boldface numbers in parentheses refer to a list of references at the end ofthe text.FIG. 2 Carbon Tetrachloride Raman Spectrum for Evaluating Resolution and S
42、canning AccuracyE1683 02 (2014)13particular instrument. Signal intensity and wavelength accu-racy are the two spectral features to check. If peak signal levelshave diminished or are shifted from accepted wavenumbervalues, the components of the system should be evaluatedindependently to locate the so
43、urce of performance degradation.Guidelines for such an evaluation are as follows:7.2 Test SamplesThe following readily available materialsare commonly used for evaluating the performance of Ramanspectrometers:7.2.1 Carbon TetrachlorideThe major Raman bands are218, 314, and 459 cm-1 (see Fig. 2). (Wa
44、rningCarbontetrachloride is toxic and a suspected carcinogen. It is recom-mended that carbon tetrachloride be used in closed containersto avoid inhalation of harmful vapors.)7.2.2 CyclohexaneThe major Raman bands are at 384.1,801.3, 1444.4, and 2852.9 cm-1.7.2.3 IndeneThere are many bands at well-kn
45、own RamanShift (2, 11, 12). Samples should be vacuum-distilled, sealed,and stored in the dark. A reference spectrum is shown in Fig.3 (2).7.3 MonochromatorThere is a trade-off between spectralresolution of a monochromator and the intensity throughput.The following five characteristics of a monochrom
46、ator can beevaluated independently:7.3.1 Spectral BandwidthThe minimum spectral band-width that can be measured with a Raman spectrometer isdetermined by the focal length of the mirrors, the groovedensity of the gratings, and its optical alignment. Mirror focallength is determined during instrument
47、design. Usually grat-ings with several groove densities are available from themanufacturer. The spectral bandwidth may be checked bymeasuring the FWHM intensity of a sharp plasma line emittedfrom a low-pressure atomic source. The mercury line at 546.07nm (1122.5 cm-1 shift from the 514.53-nm argon i
48、on laser line)is often used. If a lamp is not available the laser emission linescan be used. The spectral bandwidth of a double (additive)dispersing monochromator should ideally be one half that of asingle monochromator with the same slits and grating. Mostmanufacturers specify bandwidths for their
49、monochromatorsand measured values should be reasonably close to thosespecified (using the same slit widths and grating). For a double(additive) monochromator demonstrating an overly largebandwidth, each of the monochromator stages can be checkedseparately by closing its slits to a relatively narrow width (forexample 50 m), opening the slits of the other monochromatorstage wider (for example, 300 m), and measuring the emissionline FWHM. The bandwidths of the individual stages should bethe same and equal to twice the bandwidth of the combinedstage
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