ASTM E1683-02(2007)
(Practice)Standard Practice for Testing the Performance of Scanning Raman Spectrometers
Standard Practice for Testing the Performance of Scanning Raman Spectrometers
SIGNIFICANCE AND USE
A scanning Raman spectrometer should be checked regularly to determine if its condition is adequate for routine measurements or if it has changed. This practice is designed to facilitate that determination and, if performance is unsatisfactory, to identify the part of the system that needs attention. These tests apply for single-, double-, or triplemonochromator scanning Raman instruments commercially available. They do not apply for multichannel or Fourier transform instruments, or for gated integrator systems requiring a pulsed laser source. Use of this practice is intended only for trained optical spectroscopists and should be used in conjunction with standard texts.
SCOPE
1.1 This practice covers routine testing of scanning Raman spectrometer performance and to assist in locating problems when performance has degraded. It is also intended as a guide for obtaining and reporting Raman spectra.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific precautions, see 7.2.1.
1.2 Because of the significant dangers associated with the use of lasers, ANSI Z136.1 should be followed in conjunction with this practice.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E1683 − 02 (Reapproved2007)
Standard Practice for
Testing the Performance of Scanning Raman
Spectrometers
This standard is issued under the fixed designation E1683; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope facilitate that determination and, if performance is
unsatisfactory, to identify the part of the system that needs
1.1 This practice covers routine testing of scanning Raman
attention. These tests apply for single-, double-, or triplemono-
spectrometer performance and to assist in locating problems
chromator scanning Raman instruments commercially avail-
when performance has degraded. It is also intended as a guide
able. They do not apply for multichannel or Fourier transform
for obtaining and reporting Raman spectra.
instruments, or for gated integrator systems requiring a pulsed
1.2 This standard does not purport to address all of the
laser source. Use of this practice is intended only for trained
safety concerns, if any, associated with its use. It is the
optical spectroscopists and should be used in conjunction with
responsibility of the user of this standard to establish appro-
standard texts.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. For specific
5. Apparatus
precautions, see 7.2.1.
5.1 Laser—Amonochromatic,continuouslasersource,such
1.3 Because of the significant dangers associated with the
asanargon,krypton,orhelium-neonlaser,isnormallyusedfor
use of lasers, ANSI Z136.1 should be followed in conjunction
Raman measurements. The laser intensity should be measured
with this practice.
at the sample with a power meter because optical components
between the laser and sample reduce laser intensity.Afiltering
2. Referenced Documents
device should also be used to remove non-lasting plasma
2.1 ASTM Standards:
emission lines from the laser beam before they reach the
E131 Terminology Relating to Molecular Spectroscopy
sample. Plasma lines can seriously interfere with Raman
E1840 Guide for Raman Shift Standards for Spectrometer
measurements. Filtering devices include dispersive monochro-
Calibration
mators and interference filters.
2.2 ANSI Standard:
5.2 Sampling Optics—Commercial instruments can be pur-
Z136.1 Safe Use of Lasers
chased with sampling optics to focus the laser beam onto a
3. Terminology
sample and to image the Raman scattering onto the monochro-
mator entrance slit. Sample chamber adjustments are used to
3.1 Terminology used in this practice conforms to the
centerthesampleproperlyandaligntheRamanscatteredlight.
definitions in Terminology E131.
A schematic view of a conventional 90° Raman scattering
4. Significance and Use
geometry is shown in Fig. 1. The laser beam propagates at a
right angle to the direction in which scattered light is collected.
4.1 A scanning Raman spectrometer should be checked
It is focused on the sample at the same position as the
regularly to determine if its condition is adequate for routine
monochromator entrance slit image. Other geometries such as
measurements or if it has changed. This practice is designed to
180° backscattering are also used. With single
monochromators, a filter is normally placed in the optical
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
collection path to block light at the laser frequency from
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
entering the monochromator.
mittee E13.08 on Raman Spectroscopy.
Current edition approved March 1, 2007. Published March 2007. Originally
5.3 Polarization—For routine measurements the polariza-
approved in 1995. Last previous edition approved in 2002 as E1683 – 02. DOI:
tion of the laser at the sample is oriented normal to the plane
10.1520/E1683-02R07.
of the page in Fig. 1. However, measurements using different
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
polarizations are sometimes used to determine vibrational
Standards volume information, refer to the standard’s Document Summary page on
symmetries as part of molecular structure determinations. A
the ASTM website.
variety of optical configurations can be used to make polariza-
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. tionmeasurements;adetaileddiscussionoftheseisbeyondthe
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1683 − 02 (2007)
FIG. 1 Typical Raman Scattering Measurement Geometry
scope of this practice. Briefly, for polarization simple measure- response characteristics at and above the laser wavelength
ments of randomly-oriented samples (most of the clear should be selected. Dark signal can be reduced with thermo-
liquids), an analyzing element such as a polaroid filter or electric cooling for improved detection of weak signals.
analyzing prism is added to the optical system and Raman Current and voltage amplification or photon counting are
spectraarecollectedforlightscatteredin(1)thesamedirection commercially available options with photomultiplier tubes.
as the source (parallel), (2) perpendicular to the source.
Depolarization ratios are calculated using Raman band inten- 6. Guidelines for Obtaining and Reporting Raman
sities from the two spectra as follows:
Spectra
Intensityparallel
6.1 Alignment of Optical Elements—Refer to the manufac-
Depolarizationratio 5 (1)
Intensityperpendicular
turerfordetailedsamplechamberalignmentinstructions.Upon
installation, each optical component should be aligned indi-
vidually. For optimal alignment the sample image should be
5.3.1 A polarization scrambler is shown in Fig. 1. This
centered on the entrance slit of the monochromator (often
element is used to avoid making corrections for polarization-
viewed through a periscope accessory or with the aid of a
dependent grating effects. The scrambler is also frequently
highly scattering sample or a white card at the slit).To perform
used during routine measurements and should be placed
the alignment a test sample is mounted in the sample
between the sample and entrance slit, close to the collection
compartment, centered in the laser beam, and translated to the
lens. A polaroid filter placed between the scrambler and
approximate center of the monochromator optic axis. The
collection lens provides a simple polarization measurement
monochromator is set to monitor a strong Raman band and its
system.
signal is maximized by adjusting the sample stage, lenses, or a
5.4 Monochromator—A scanning monochromator used for
combination of the two. Normally three orthogonal lens
Raman spectroscopy will exhibit high performance require-
adjustments are used: (1) the laser focusing lens is translated
ments. Double and triple monochromators have particularly
along the direction of the beam; (2) the Raman scattering
stringent performance standards. During the original instru-
collectionlens,positionedbetweenthesampleandtheentrance
ment design, features are usually introduced to minimize
slit, is translated along the direction of the propagating scat-
optical aberrations. However, proper maintenance of optical
tered light in order to provide focus; and (3) the collection lens
alignment is essential. A focused image on the entrance slit
istranslatedperpendiculartothescatteredlightinordertoscan
should be optically transferred to and matched with the other
the image of the laser-excited scattering volume across the
slits. If the monochromator is not functioning properly contact
width of the monochromator entrance slit. (Refer to Fig. 1.)
the manufacturer for assistance.
This collection lens adjustment should be made during major
5.5 Photomultiplier Tube—A photomultiplier can be used instrument alignment (for example, during initial set-up), but
for detecting Raman scattered radiation. A tube with good should not be necessary during routine sample-to-sample
E1683 − 02 (2007)
FIG. 2 Carbon Tetrachloride Raman Spectrum for Evaluating Resolution and Scanning Accuracy
21 21
alignment. Sample and lens adjustments should be repeated as Scan rate, ~cm /s!# spectral slit width, ~cm !
(2)
necessary while the slits are narrowed from a relatively large 4 3time constant s
~ ~ !!
initial width down to the size determined by the resolution
In addition, the time constant of the recorder should be
requirements of the measurement.
considerably faster than the rate-meter’s time constant, and the
6.2 Calibration:
speed of the paper should be adequate to measure the spectral
6.2.1 Spectral Response—The spectral response of an opti-
features.
cal spectrometric system will depend on the efficiency of the
6.3.2 Recording With a Computer or Signal Averager—In
gratings (which is both wavelength and polarization depen-
this case one needs to define the increments in wavenumbers
dent) and the spectral response of the photomultiplier tube.
betweendatapoints.Aminimumcriterionistocollectfivedata
This can be measured routinely by collecting light from a
pointsinthefullwidthathalfthemaximumintensity(FWHM)
tungsten halogen lamp or other NIST-traceable standard light
of the narrowest spectral band. For example, if the slits were
source.Acomplete procedure for performing spectral response
set to provide a measured band width at half maximum of 4
corrections has been published by Scherer and Kint (1). It is
wavenumbers, then 1-wavenumber increments would produce
stronglyrecommendedthatcorrectionsforspectralresponsebe
five data points within the FWHM in a scan of a line from a
incorporateddirectlyintothesoftwarewhenacomputerisused
plasma emission source. To better define peak shape decrease
to collect spectra.
the size of the increments. This is especially important for
6.2.2 Wavenumber—The accuracy of the wavenumber cali-
bands that deviate from Lorentzian shape.
bration over a large region should be determined using a
6.4 Reporting Experimental Conditions—The spectral slit
standard low-pressure emission source with enough lines to
width(wavenumbers),scanrate,laserwavelengthandpowerat
make many measurements over the range of the instrument.
the sample, polarization conditions, integration time, correc-
Low-pressure mercury, argon, and neon lamps are frequently
tions for instrumental response, type of spectrometer and
used. The non-lasing emission lines of the laser can also be
detector, sample information (physical state, concentration,
used if the laser filtering device is removed. Accurate wave-
geometry, and so forth), and other important experimental
number values are available (2-9). For measurement at resolu-
conditions should always be recorded with the spectra and
tions <0.5 wavenumbers a more rigorous calibration method
reproduced for performance testing. A complete record of the
should be employed.
parameters to be specified is available in Table 1 of the IUPAC
6.3 Recording Raman Spectra—The following guidelines
Recommendations for the Presentation of Raman Spectra in
are provided for recording spectra with a rare meter and strip
Data Collections (10).
chart recorder or with a computer or digital signal averager. In
both cases it is important to record a spectrum so that spectral
TABLE 1 Recommended Frequencies from the Spectrum of
features are not distorted by the mode of data acquisition.
Indene for Evaluating Scanning Accuracy
6.3.1 Recording With a Rate-Meter and Strip Chart
A 1
Band Frequency, cm
Recorder—Therangeontherate-meterissetbymonitoringthe
1 730.4 ± 0.5
strongest peak in the spectrum. The relationship between the
2 1018.3 ± 0.5
scan rate, spectral slit width, and time constant of the rate-
3 1205.6 ± 0.5
4 1552.7 ± 0.5
meter, as recommended by IUPAC (10), is:
5 1610.2 ± 0.5
6 2892.2 ± 1
7 3054.7 ± 1
A
The boldface numbers in parentheses refer to a list of references at the end of
Bands from Fig. 3.
the text.
E1683 − 02 (2007)
7. Evaluation of Raman Instrument Parameters single monochromator with the same slits and grating. Most
manufacturers specify bandwidths for their monochromators
7.1 The performance of an instrument should be evaluated
and measured values should be reasonably close to those
regularly to determine if it has degraded. This is most easily
specified (using the same slit widths and grating). For a double
accomplished with a test sample such as carbon tetrachloride
(additive) monochromator demonstrating an overly large
measured under a set of standard conditions established for the
bandwidth, each of the monochromator stages can be checked
particular instrument. Signal intensity and wavelength accu-
separately by closing its slits to a relatively narrow width (for
racyarethetwospectralfeaturestocheck.Ifpeaksignallevels
example 50 µm), opening the slits of the other monochromator
have diminished or are shifted from accepted wavenumber
stagewider(forexample,300µm),andmeasuringtheemission
values, the components of the system should be evaluated
lineFWHM.Thebandwidthsoftheindividualstagesshouldbe
independentlytolocatethesourceofperformancedegradation.
the same and equal to twice the bandwidth of the combined
Guidelines for such an evaluation are as follows:
stages. If one stage has a significantly larger bandpass than the
7.2 TestSamples—Thefollowingreadilyavailablematerials
other, and that bandpass is much larger than twice that
are commonly used for evaluating the performance of Raman
expected for the entire monochromator, then it has a problem.
spectrometers:
Also, if combining the stages does not reduce the bandwidth as
7.2.1 Carbon Tetrachloride—The major Raman bands are
expected, then there is a problem and the manufacturer should
218, 314, and 459 cm-1 (see Fig. 2). (Warning—Carbon
be consulted.
tetrachloride is toxic and a suspected carcinogen. It is recom-
7.3.2 Resolution—A test frequently used to check the reso-
mended that carbon tetrachloride be used in closed containers
lution of Raman spectrometers is illustrated in Fig. 4. The four
to avoid inhalation of harmful vapors.)
components of the mercury 579.1-nm emission line are dis-
7.2.2 Cyclohexane—The major Raman bands are at 384.1,
tinctly visible. Alternately, the components of the carbon
801.3, 1444.4, and 2852.9 cm-1.
tetrachloride 459 cm-1 Raman scattering band can be used, as
7.2.3 Indene—There are many bands at well-known Raman
shown i
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.