1、Designation: E 1683 02 (Reapproved 2007)Standard Practice forTesting the Performance of Scanning RamanSpectrometers1This standard is issued under the fixed designation E 1683; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the yea
2、r of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers routine testing of scanning Ramanspectrometer performance and to assist in locating problem
3、swhen performance has degraded. It is also intended as a guidefor obtaining and reporting Raman spectra.1.2 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
4、health practices and determine the applica-bility of regulatory limitations prior to use. For specificprecautions, see 7.2.1.1.3 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 Standa
5、rds:2E 131 Terminology Relating to Molecular SpectroscopyE 1840 Guide for Raman Shift Standards for SpectrometerCalibration2.2 ANSI Standard:3Z136.1 Safe Use of Lasers3. Terminology3.1 Terminology used in this practice conforms to thedefinitions in Terminology E 131.4. Significance and Use4.1 A scan
6、ning Raman spectrometer should be checkedregularly to determine if its condition is adequate for routinemeasurements or if it has changed. This practice is designed tofacilitate that determination and, if performance is unsatisfac-tory, to identify the part of the system that needs attention.These t
7、ests apply for single-, double-, or triplemonochromatorscanning Raman instruments commercially available. They donot apply for multichannel or Fourier transform instruments, orfor gated integrator systems requiring a pulsed laser source.Use of this practice is intended only for trained opticalspectr
8、oscopists and should be used in conjunction with stan-dard texts.5. Apparatus5.1 LaserA monochromatic, continuous laser source,such as an argon, krypton, or helium-neon laser, is normallyused for Raman measurements. The laser intensity should bemeasured at the sample with a power meter because optic
9、alcomponents between the laser and sample reduce laser inten-sity. A filtering device should also be used to remove non-lasting plasma emission lines from the laser beam before theyreach the sample. Plasma lines can seriously interfere withRaman measurements. Filtering devices include dispersivemono
10、chromators and interference filters.5.2 Sampling OpticsCommercial instruments can be pur-chased with sampling 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 alig
11、n the Raman scattered light.A schematic view of a conventional 90 Raman scatteringgeometry is shown in Fig. 1. 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. Ot
12、her geometries such as180 backscattering are also used. With single monochroma-tors, a filter is normally placed in the optical collection path toblock light at the laser frequency from entering the monochro-mator.5.3 PolarizationFor routine measurements the polariza-tion of the laser at the sample
13、is oriented normal to the planeof the page in Fig. 1. However, measurements using differentpolarizations are sometimes used to determine vibrational1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and Separation Science and is the direct responsibility of Subco
14、m-mittee E13.08 on Raman Spectroscopy.Current edition approved March 1, 2007. Published March 2007. Originallyapproved in 1995. Last previous edition approved in 2002 as E 1683 02.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org
15、. For Annual Book 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.1Copyright ASTM International, 100 Barr Harbor Drive,
16、PO Box C700, West Conshohocken, PA 19428-2959, United States.symmetries as part of molecular 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 simp
17、le measure-ments of randomly-oriented samples (most of the clear liq-uids), an analyzing 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 s
18、ource.Depolarization ratios are calculated using Raman band inten-sities from the two 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 gra
19、ting effects. The scrambler is also frequentlyused during routine measurements and should 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
20、scanning monochromator used forRaman spectroscopy will exhibit high performance require-ments. Double and triple monochromators have particularlystringent performance standards. During the original instru-ment design, features are usually introduced to minimizeoptical aberrations. However, proper ma
21、intenance of opticalalignment is essential. A focused image on the entrance slitshould 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 detecti
22、ng Raman scattered radiation. A tube with goodresponse characteristics at and above the 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 op
23、tions with photomultiplier tubes.6. Guidelines for Obtaining and Reporting RamanSpectra6.1 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 s
24、ample image should becentered on the entrance slit of the monochromator (oftenviewed through 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 sample compart-ment, centered in the laser beam, and t
25、ranslated to theapproximate center of the monochromator optic axis. Themonochromator is 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
26、translatedalong the direction of the beam; (2) the Raman scatteringcollection lens, positioned 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 lensis translated perpendicular to the scatt
27、ered light in order 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 majorFIG. 1 Typical Raman Scattering Measurement GeometryE 1683 02 (2007)2instrument alignment (for
28、example, during initial set-up), butshould not be necessary during routine sample-to-samplealignment. 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 measurem
29、ent.6.2 Calibration:6.2.1 Spectral ResponseThe spectral response of an opti-cal spectrometric 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
30、light from atungsten halogen lamp or other NIST-traceable standard lightsource. A complete 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
31、 a computer is usedto collect spectra.6.2.2 WavenumberThe accuracy of the wavenumber cali-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 la
32、mps are frequentlyused. The non-lasing emission lines of the laser can also beused if the 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 Spect
33、raThe following guidelinesare provided for recording spectra with a rare meter and stripchart 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 Ra
34、te-Meter and Strip ChartRecorderThe range on the rate-meter is set by monitoringthe strongest peak in the spectrum. The relationship betweenthe scan rate, spectral slit width, and time constant of therate-meter, as recommended by IUPAC (10), is:Scan rate, cm21/s! #spectral slit width, cm21!4 3 time
35、constant s!(2)In addition, the time constant of the recorder should beconsiderably faster 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
36、in wavenumbersbetween data points.Aminimum criterion is to collect five datapoints in the 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 wo
37、uld producefive data points within the FWHM in a scan of a line from aplasma emission source. 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 (wavenumb
38、ers), scan rate, laser wavelength and power atthe sample, polarization conditions, integration time, correc-tions for instrumental response, type of spectrometer anddetector, sample information (physical state, concentration,geometry, and so forth), and other important experimentalconditions should
39、always be recorded with the spectra andreproduced for performance testing. A complete record of theparameters to be specified is available in Table 1 of the IUPAC4The boldface numbers in parentheses refer to a list of references at the end ofthe text.FIG. 2 Carbon Tetrachloride Raman Spectrum for Ev
40、aluating Resolution and Scanning AccuracyTABLE 1 Recommended Frequencies from the Spectrum ofIndene for Evaluating Scanning AccuracyBandAFrequency, cm11 730.4 6 0.52 1018.3 6 0.53 1205.6 6 0.54 1552.7 6 0.55 1610.2 6 0.56 2892.2 6 17 3054.7 6 1ABands from Fig. 3.E 1683 02 (2007)3Recommendations for
41、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 is most easilyaccomplished with a test sample such as carbon tetrachloridemeasured under a set
42、 of standard conditions established for theparticular 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 loc
43、ate the source 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 Fi
44、g. 2). (WarningCarbontetrachloride 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
45、at well-known 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
46、 monochromator 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 i
47、nstrument 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-
48、nm argon ion 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
49、for their monochromatorsand measured values should be reasonably close to thosespecified (using the same slit widths and grating). For a double(additive) monochromator demonstrating an overly large band-width, 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 combine
