1、Designation: E2529 061Standard Guide forTesting the Resolution of a Raman Spectrometer1This standard is issued under the fixed designation E2529; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number i
2、n parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1NOTEAdded title to Table 1 in August 2013.1. Scope1.1 This guide is designed for routine testing and assess-ment of the spectral resolution of Raman spect
3、rometers usingeither a low-pressure arc lamp emission lines or a calibratedRaman band of calcite.1.2 The values given in SI units are to be regarded as thestandard.1.3 Because of the significant dangers associated with theuse of lasers, ANSI Z136.1 shall be followed in conjunctionwith this practice.
4、1.4 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.2. Referenced D
5、ocuments2.1 ASTM Standards:2E131 Terminology Relating to Molecular SpectroscopyE1683 Practice for Testing the Performance of ScanningRaman SpectrometersE1840 Guide for Raman Shift Standards for SpectrometerCalibration2.2 ANSI Standard:3ANSI Z136.1 Safe Use of Lasers3. Terminology3.1 DefinitionsTermi
6、nology used in this guide conformsto the definitions in Terminology E131.4. Significance and Use4.1 Assessment of the spectrometer resolution and instru-ment line shape (ILS) function of a Raman spectrometer isimportant for intercomparability of spectra obtained amongwidely varying spectrometer syst
7、ems, if spectra are to betransferred among systems, if various sampling accessories areto be used, or if the spectrometer can be operated at more thanone laser excitation wavelength.4.2 Low-pressure discharge lamps (pen lamps such asmercury, argon, or neon) provide a low-cost means to provideboth re
8、solution and wave number calibration for a variety ofRaman systems over an extended wavelength range.4.3 There are several disadvantages in the use of emissionlines for this purpose, however.4.3.1 First, it may be difficult to align the lamps properlywith the sample position leading to distortion of
9、 the line,especially if the entrance slit of the spectrometer is underfilledor not symmetrically illuminated.4.3.2 Second, many of the emission sources have highlydense spectra that may complicate both resolution and wavenumber calibration, especially on low-resolution systems.4.3.3 Third, a signifi
10、cant contributor to line broadening ofRaman spectral features may be the excitation laser line widthitself, a component that is not assessed when evaluating thespectrometer resolution with pen lamps.4.3.4 An alternative would use a Raman active compound inplace of the emission source. This compound
11、should bechemically inert, stable, and safe and ideally should provideRaman bands that are evenly distributed from 0 cm-1(Ramanshift) to the C-H stretching region 3000 cm-1and above. TheseRaman bands should be of varying bandwidth.4.4 To date, no such ideal sample has been identified;however carbon
12、tetrachloride (see Practice E1683) and naph-thalene (see Guide E1840) have been used previously for bothresolution and Raman shift calibration.4.5 The use of calcite to assess the resolution of a Ramansystem will be addressed in this guide. Calcite is a naturallyoccurring mineral that possesses many
13、 of the desired opticalproperties for a Raman resolution standard and is inexpensive,safe, and readily available.1This guide 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.
14、Current edition approved Dec. 1, 2006. Published December 2006. Originallyapproved in 2006. Last previous edition approved in 2006 as E2529 06. DOI:10.1520/E2529-06E01.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual
15、 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.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700,
16、West Conshohocken, PA 19428-2959. United States14.6 The spectral bandwidth of dispersive Raman spectrom-eters is determined primarily by the focal length of thespectrometer, the dispersion of the grating, and the slit width.Field portable systems typically operate with fixed slits andgratings and th
17、us operate with a fixed spectral bandwidth,while in many laboratory systems the slit widths and gratingsare variable. The spectral bandwidth of Fourier-Transform(FT)-Raman systems is continuously variable by altering theoptical path difference of the interferometer and furthermore iscapable of obtai
18、ning much lower spectral bandwidth than mostpractical dispersive systems. Therefore, data obtained of anarrow Raman band on a FT-Raman system can be used todetermine the resolution of a dispersive Raman system. Acalibration curve of the full width at half height (FWHH) forthe 1085-cm-1band of calcit
19、e as a function of spectralresolution has been reported for this purpose.4Measurement ofthis calcite band on a test dispersive instrument enables anestimation of the spectrometer resolution.4.7 This guide will describe the use of calcite and pen lampsfor the evaluation of Raman spectrometer resoluti
20、on fordispersive (grating based) Raman systems operating with a785-nm laser wavelength.5. Reagents5.1 Calcite and calcium carbonate (CaCO3) come in manyforms. Iceland spar, from Iceland and, more commonly,Mexico, is easily cleavable into a rhombohedron and is theclear crystal commonly found in retai
21、l stores. It is readilyavailable and inexpensive but may fluoresce under blue exci-tation. In addition, it is birefringent.5.2 Low-pressure discharge emission (pen) lamps arewidely available from optical supply companies. They aretypically made with noble gases or a metal vapor. Argon,krypton, and x
22、enon pen lamps are applicable as resolutioncalibration sources for Raman spectrometers operating with785-nm excitation. These pen lamps cover a wide wavenumber range but have reasonably sparse spectra.6. Procedure6.1 Calcite Calibration:6.1.1 Measure the Raman spectrum of calcite using thevendors re
23、commended procedure for producing a Ramanspectrum of a sample with good signal to noise. The Ramanspectrum of calcite is shown in Fig. 1. Because the Ramanscattering of the 1085-cm-1band is polarized, the peak heightwill depend upon the polarization of the laser and the locationof the sample with re
24、spect to the excitation laser. Rotate thesample under excitation laser beam to obtain the maximumsignal from the 1085-cm-1band. The calibration relationdetermined in 4.6 is:Bw1085cm21! 5 1.0209*Sresolution10.684 (1)Where:Bw1085= the measured bandwidth of the 1085-cm-1CaCO3Raman band, andSresolution=
25、 the nominal resolution of the reference FT-Raman spectrometer described in 4.6.6.1.2 After acquiring the Raman spectrum of the calcitesample, determine the FWHH of the 1085-cm-1band, Bw1085,by using the spectral analysis feature commonly found in thecontrol software provided with the spectrometer.
26、These pro-grams typically use a Levenburg-Marquardt nonlinear leastsquares to determine the line shape of the Raman band.5Thecalibration equation (Eq 1) was determined using a fit to amixed Gaussian and Lorentzian function. Solve for the nomi-nal resolution of the spectrometer under test by rearrang
27、ing Eq1 to:Sresolution5 Bw10852 0.684!/1.0209 (2)6.1.3 This fit is reported to be good to approximately 20 %accuracy, which is adequate for validation purposes. The1085-cm-1band is a good approximation for system resolution4B.T. Bowie and P.R. Griffiths, “Determination of the Resolution of a Multi-c
28、hannel Raman Spectrometer Using Fourier Transform Raman Spectra,” AppliedSpectroscopy, Vol 57, No 2., 2003, pp 190-196.5D. W. Marquardt, J. Soc. Ind. Appl. Math.,Vol 11, 1963, pp. 431-441.FIG. 1 Calcite Raman SpectrumE2529 0612estimation as it is centered in the Raman spectra for fixedgrating system
29、s that typically operate from 200 to approx. 2000cm-1. This material is suitable for use with all laser wave-lengths; however, many samples have been observed to fluo-resce with excitation wavelengths below 532-nm excitation.6.2 Pen Lamp CalibrationThe spectra of xenon, argon,and krypton in Raman sh
30、ift from 12 739 cm-1(785-nmexcitation) are shown in Figs. 2-4. The associated emissionlines from each source is listed in Table 1 in air wavelength,absolute cm-1(air), and Raman shift from 12739 cm-1.Ifafiber-probe-based Raman system is to be calibrated, a conve-nient source of an argon spectra is t
31、he light emitted from olderbacklit laptop computer screens or overhead fluorescent lights.Place a translucent target at the focal point of the fibercollection system. An example would be several thicknesses ofscotch tape placed on a glass slide. Otherwise, illuminate theslit as evenly as possible. C
32、heck for symmetric lines in thecollected spectrum and use integration times that preventsaturation of the detector. This is especially true for the xenonsource in which the 881.9-nm line is very intense. Determinethe FWHH of bands in the low, middle, and long Raman shiftregion of the spectra. The re
33、solution (FWHH) shall not beconstant, but vary from the low to high Raman shift region.Gratings disperse light nearly linearly in wavelength andtherefore the reciprocal linear dispersion in wavelength units(nm) will be nearly constant. The reciprocal linear dispersionin wave number (cm-1) units will
34、 increase at higher Raman(Stokes) shift due to the inverse relation between wavelengthand wave number. For Raman systems based upon the com-monly used spectrometer designs, the resolution will theoreti-cally increase (FWHH decreases) on the Stokes-shifted (longerwavelength) side of the excitation la
35、ser line. It is not unusual,however, to observe the center of the spectra of fixed gratingsystems to have the smallest FWHH (highest resolution) whilethe edges (low and high Raman shift region) exhibit lowerresolution. This effect is due to error incurred by the curvatureof the focal plane for low f
36、number spectrometers.7. Keywords7.1 calcite; low-pressure arc lamp calibration; Raman spec-troscopy; resolution calibrationFIG. 2 Emission Spectra of Argon Plotted in Shift Units from 12 738.85 cm-1(785 nm)E2529 0613FIG. 3 Emission Spectra of Krypton Plotted in Shift Units from 12 738.85 cm-1(785 nm
37、)FIG. 4 Emission Spectra of Xenon Plotted in Shift Units from 12 738.85 cm-1(785 nm)E2529 0614ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determinat
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42、(www.astm.org/COPYRIGHT/).TABLE 1 Emission Lines of Argon, Krypton, and XenonArgon Krypton Xenonnm Abs cm-1(Air) ShiftAnm Abs cm-1(Air) ShiftAnm Abs cm-1(Air) ShiftA794.818 12 581.50 157.4 805.95 12 407.72 331.1 823.163 12 148.26 590.6800.616 12 490.38 248.5 810.436 12 339.04 399.8 828.012 12 077.12
43、 661.7801.479 12 476.93 261.9 819.005 12 209.94 528.9 834.7 11 980.35 758.5810.369 12 340.06 398.8 826.324 12 101.79 637.1 840.919 11 891.75 847.1811.531 12 322.39 416.5 828.105 12 075.76 663.1 881.941 11 338.63 1400.2826.453 12 099.90 639.0 829.811 12 050.94 687.9 895.2 11 170.69 1568.2840.821 11 8
44、93.14 845.7 850.887 11 752.44 986.4 904.54 11 055.34 1683.5842.145 11 869.93 868.9 877.675 11 393.74 1345.1 916.265 10 913.87 1825.0852.145 11 735.09 1003.8 892.869 11 199.85 1539.0912.297 10 961.34 1777.5 975.176 10 254.56 2484.3922.45 10 840.70 1898.2 985.624 10 145.86 2593.0AShift from 785 nm or 12 738.85 cm-1, Absolute cm-1number are invariant. Shift numbers will vary depending upon the actual wavelength of the excitation laser.E2529 0615
