1、Designation: E2911 13Standard Guide forRelative Intensity Correction of Raman Spectrometers1This standard is issued under the fixed designation E2911; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A num
2、ber in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide is designed to enable the user to correct aRaman spectrometer for its relative spectral-intensity responsefunction using NIST S
3、tandard Reference Materials2in the224X series (currently SRMs 2241, 2242, 2243, 2244, 2245,2246), or a calibrated irradiance source. This relative intensitycorrection procedure will enable the intercomparison of Ramanspectra acquired from differing instruments, excitationwavelengths, and laboratorie
4、s.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 Because of the significant dangers associated with theuse of lasers, ANSI Z136.1 or suitable regional standardsshould be followed in conjunction with this practice.1.4 Th
5、is 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 Documen
6、ts2.1 ASTM Standards:3E131 Terminology Relating to Molecular SpectroscopyE1840 Guide for Raman Shift Standards for SpectrometerCalibrationE2529 Guide for Testing the Resolution of a Raman Spec-trometer2.2 ANSI Standard:4Z136.1 Safe Use of Lasers3. Terminology3.1 DefinitionsTerminology used in this p
7、ractice con-forms to the definitions in Terminology E131.4. Significance and Use4.1 Generally, Raman spectra measured using grating-baseddispersive or Fourier transform Raman spectrometers have notbeen corrected for the instrumental response (spectral respon-sivity of the detection system). Raman sp
8、ectra obtained withdifferent instruments may show significant variations in themeasured relative peak intensities of a sample compound. Thisis mainly as a result of differences in their wavelength-dependent optical transmission and detector efficiencies. Thesevariations can be particularly large whe
9、n widely different laserexcitation wavelengths are used, but can occur when the samelaser excitation is used and spectra of the same compound arecompared between instruments. This is illustrated in Fig. 1,which shows the uncorrected luminescence spectrum of SRM2241, acquired upon four different comm
10、ercially availableRaman spectrometers operating with 785 nm laser excitation.Instrumental response variations can also occur on the sameinstrument after a component change or service work has beenperformed. Each spectrometer, due to its unique combinationof filters, grating, collection optics and de
11、tector response, has avery unique spectral response. The spectrometer dependentspectral response will of course also affect the shape of Ramanspectra acquired upon these systems. The shape of this re-sponse is not to be construed as either “good or bad” but is theresult of design considerations by t
12、he spectrometer manufac-turer. For instance, as shown in Fig. 1, spectral coverage canvary considerably between spectrometer systems. This istypically a deliberate tradeoff in spectrometer design, wherespectral coverage is sacrificed for enhanced spectral resolution.4.2 Variations in spectral peak i
13、ntensities can be mostlycorrected through calibration of the Raman intensity (y) axis.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.Current edition approved Ju
14、ly 1, 2013. Published July 2013. DOI: 10.1520/E2911132Trademark of and available from NIST Office of Reference Materials, 100Bureau Drive, Stop 2300, Gaithersburg, MD 20899-2300. http:/www.nist.gov/srm.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Serv
15、ice at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.Copyright ASTM International, 10
16、0 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1The conventional method of calibration of the spectral re-sponse of a Raman spectrometer is through the use of aNational Metrology Institute (NMI), for example, NIST, trace-able calibrated irradiance source. Such lamps
17、 have a definedspectral output of intensity versus wavelength and proceduresfor their use have been published (1)5. However, intensitycalibration using a white-light source can present experimentaldifficulties, especially for routine analytical work. Calibratedtungsten halogen lamps have a limited l
18、ifetime and requireperiodic recalibration. These lamps are often mounted in anintegrating sphere to eliminate polarization effects and provideuniform source irradiance. In practice, these sources can bedifficult to align with the variety of sampling arrangements thatare now typical with Raman spectr
19、ometers, especially micro-scope based systems and process Raman analyzers whereelectrical safety concerns persist in hazardous areas. Theadvantage of a standard lamp is that it can be used for multipleexcitation wavelengths.4.3 The spectra of materials that luminesce with irradiationcan be corrected
20、 for relative luminescence intensity as afunction of emission wavelength using a calibrated Ramanspectrometer. An irradiance source, traceable to the SI, can beused to calibrate the spectrometer. Several groups have pro-posed these transfer standards to calibrate both Raman andfluorescence spectrome
21、ters (1-6). The use of a luminescentglass material has the advantage that the Raman excitationlaser is used to excite the luminescence emission and thisemission is measured in the same position as the sample. Theseglasses can be used in a variety of sampling configurations andthey require no additio
22、nal instrumentation. The glasses arephotostable and unlike primary calibration sources, may notrequire periodic recalibration. NIST provides a series offluorescent glasses that may be used to calibrate the intensityaxis of Raman spectrometers. A mathematical equation, whichis a description of the co
23、rrected emission, is provided with eachglass. The operator uses this mathematical relation with ameasurement of the glass on their spectrometer to produce asystem correction curve.4.4 This guide describes the steps required to produce arelative intensity correction curve for a Raman spectrometerusin
24、g a calibrated standard source or a NIST SRM and a meansto validate the correction.5. Reagents5.1 Standard Reference Materials, SRM 2241, SRM 2242,SRM 2243, SRM 2244, SRM 2245, and SRM 2246 areluminescent glass standards designed and calibrated at NISTfor the relative intensity correction of Raman s
25、pectrometersoperating with excitation laser wavelengths of 785 nm, 532nm, 488 nm/514.5 nm, 1064 nm, 632.8 nm and 830 nm,respectively (3-5). The corrected luminescence spectra of eachis shown in Fig. 2.5.2 Raman shift reagents (see Guide E1840).6. Raman Shift Verification (X-Axis)6.1 Verification of
26、the calibration of the spectrometersx-axis in Raman shift wavenumbers (glyph507 cm-1) is necessarybefore intensity correction of the y-axis is performed. TheRaman shift axis is calculated from Eq 1: 5 (02 s) (1)5The boldface numbers in parentheses refer to the list of references at the end ofthis st
27、andard.FIG. 1 SRM 2241 Measured on Four Commercial Raman Spectrometers Utilizing 785 nm ExcitationE2911 132where:glyph507 = the wavenumber in units of Raman shift (cm-1),0= the laser frequency in wavenumbers (cm-1),s= the wavelength axis of the spectrometer expressed inwavenumbers (cm-1),6.2 The las
28、er frequency can be measured using a wavemeterwhile the absolute wavenumber axis of the spectrometer iscalibrated with emission pen lamps. Several references (7-10)have detailed the use of the appropriate emission lamps for therelevant Raman frequency range. Users should defer to thevendors instruct
29、ions for the purpose of Raman shift axiscalibration or verification. However, independent validation ofthe Raman shift axis may be performed by referring to GuideE1840-96(2007).7. Relative Instrument Response Function Calibration(Y-Axis):7.1 General Procedure for Relative Response Calibration:7.1.1
30、The most practical approach to calibrating a relativeinstrumental response function (IRF) involves the use of astandard of known spectral flux (intensity versus wavelength).The standard source is aligned to the spectrometer such that theemitted optical radiation is directed into the optical path toe
31、mulate Raman scattered radiation collected by the spectrom-eter from the sample position. The best accuracy is achievedwhen the calibration source radiation and Raman scatter of thesample travel the same illumination path through the collectionoptics of the spectrometer. The standard source spectrum
32、 ismeasured using, as nearly as possible, the same instrumentalparameters (for example, spectral coverage, slit width, filters,or other optical elements) as used for sample data collection.Excitation laser power, however, is sample dependent and therelative response correction of the spectrometer wi
33、ll be inde-pendent of this parameter. Acquisition time should be adjustedto optimize the signal-to-noise ratio (SNR).7.1.2 The relative IRF is defined as the ratio of the measuredspectrum of the standard source, SL(glyph507), to the known standardoutput, IL(glyph507). The inverse of this relation is
34、 used to calculatea relative intensity correction curve as in Eq 2:ICORR() 5 IL()SL() (2)where:ICORR(glyph507) = the relative intensity correction curve,IL(glyph507) = the known standard output,SL(glyph507) = the measured spectrum of the standard source(see 7.1.4).7.1.3 Once determined, this correct
35、ion curve is used tocorrect the measured Raman spectrum of a sample, SMEAS(glyph507),for the system dependent response according to Eq 3:SCORR() 5 ICORR() 3 SMEAS() (3)where:SCORR(glyph507) = the corrected Raman spectrum of the sample,ICORR(glyph507) = the relative intensity correction curve,SMEAS(g
36、lyph507) = the measured Raman spectrum of the sample(see 7.1.4).7.1.4 Prior to calculating the relative intensity correctioncurve or corrected sample Raman spectrum, the measuredspectra should be corrected by removing contributions to thesignal not originating from the sample or calibration sourcebe
37、ing measured. Background signal can arise from processesindependent of light incident on the detector, such as detectorFIG. 2 Certified Models of the Corrected Luminescence Spectra of SRMs 2241 through 2246 as a Function of Raman Shiftfrom the Specified Laser Excitation WavelengthE2911 133bias and t
38、hermal charge generation. Correction for theseprocesses is often referred to as dark correction or darksubtraction. Corrections for other sources of backgroundinterference, such as ambient lighting or luminescence fromoptical components in a system, can also be performed. Theseprocedures may be comb
39、ined into a single measurement andautomated during the spectral acquisition. Throughout theremainder of this guide, the term “background” will be usedgenerally to refer to spectral background contributions bothdependent and independent of light incident on the detector.The suitability of a particula
40、r approach to background correc-tion depends on instrumentation as well as application and,therefore, a specific procedure cannot be universally prescribed(see 7.4.4.1). For the calibration source, the measured spectrumis typically corrected by subtraction of a background spectrumrecorded by blockin
41、g or removing the calibration source orleaving the detector shutter closed and measuring a spectrumusing the same acquisition time used to measure the calibrationsource. The measured sample spectrum is similarly correctedby subtraction of a background spectrum; however, since thespectrum depends on
42、integration time, this will typically bedifferent from the background spectrum used for the calibrationsource. Furthermore, the procedure for measuring a back-ground spectrum may differ between a calibration source andsample spectrum. Other variables, such as environmentalconditions, can also be imp
43、ortant (for example, correction atone temperature may not be universal).7.1.5 Due to polarization biases that can be present inRaman instrumentation, typically due to the diffraction gratingand sample orientation dependent components of the Ramantensor, a polarization scrambler is recommended in the
44、 Ramanlight-collection optics, most preferably in a region of colli-mated light. Raman spectral bands that exhibit various degreesof polarization will not be properly intensity-corrected withoutthe use of a scrambler (see 7.4.1).7.1.6 Calibration data for light sources are typically pro-vided in ene
45、rgy output versus wavelength. While the SI unit forspectral irradiance has units of W m-3, numerous other units arein common use with energy in terms of mW or W, area interms of cm2, and spectral bandpass in terms of cm, m or nm.Modern Raman spectrometer systems generally count photonsand the wavele
46、ngth axis is expressed in Raman shift wave-numbers (glyph507,cm-1). More appropriate units in this case are interms of photon flux as a function of wavenumber (cm-1). Ageneralized relationship between spectral irradiance in energyversus wavelength and photons versus wavenumbers can beexpressed as Eq
47、 4:p() 5 C3e() (4)where:p() = a photon flux in terms of absolute wavenumbers(cm-1),e() = an energy flux in terms of wavelength,C = a constant that depends on the energy and wave-length calibration units.7.1.7 Conventional units used for Raman spectrometersemploying photon counting detectors are phot
48、ons sec-1(cm-1)-1on a relative scale. Additional units relating to area and solidangle may also be included. With the excitation laser wave-length known, the photon flux in terms of Raman shift, p(glyph507),may be calculated for a spectrometer system. This approach isutilized for the corrected lumin
49、escence spectra of the NISTSRM series for relative Raman intensity correction.7.2 Relative Response Calibration using NIST SRMs:7.2.1 SRMs 2241 through 2246 are glass artifact standardsthat transfer a relative irradiance calibration from a NISTcertified spectral irradiance source to a users spectrometer.Under laser excitation at the specified wavelength the SRMluminescence provides a source of known relative spectralintensity, described by a certified mathematical model, over thespectral range of certification. Shown in Fig. 2 are
