1、Designation: E 1944 98 (Reapproved 2007)Standard Practice forDescribing and Measuring Performance of LaboratoryFourier Transform Near-Infrared (FT-NIR) Spectrometers:Level Zero and Level One Tests1This standard is issued under the fixed designation E 1944; the number immediately following the design
2、ation indicates the year oforiginal adoption or, in the case of revision, the year 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 two levels
3、 of tests to measure theperformance of laboratory Fourier transform near infrared(FT-NIR) spectrometers. This practice applies to the short-wave near infrared region, approximately 800 nm (12 500cm-1) to 1100 nm (9090.91 cm-1); and the long-wavelengthnear infrared region, approximately 1100 nm (9090
4、.91 cm-1)to2500 nm (4000 cm-1). This practice is intended mainly fortransmittance measurements of gases and liquids, although it isbroadly applicable for reflectance measurements.1.2 The values stated in SI units are to be regarded asstandard.1.3 This standard does not purport to address all of thes
5、afety 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 Documents2.1 ASTM Standards:2E 131 Terminology Relating to
6、 Molecular SpectroscopyE 168 Practices for General Techniques of Infrared Quanti-tative AnalysisE 932 Practice for Describing and Measuring Performanceof Dispersive Infrared SpectrometersE 1252 Practice for General Techniques for Obtaining In-frared Spectra for Qualitative AnalysisE 1421 Practice fo
7、r Describing and Measuring Performanceof Fourier Transform Mid-Infrared (FT-MIR) Spectrom-eters: Level Zero and Level One Tests3. Terminology3.1 For definitions of terms used in this practice, refer toTerminology E 131. All identifications of spectral regions andabsorbance band positions are given i
8、n nanometers (nm), andwavenumbers (cm-1); and spectral energy, transmittance, re-flectance, and absorbance are signified by the letters E, T, Rand A respectively. A subscripted number signifies a spectralposition in nanometers, with wavenumbers in parenthesis (forexample, A1940(5154.64), denotes the
9、 absorbance at 1940 nm or5154.64 cm-1).4. Significance and Use4.1 This practice permits an analyst to compare the generalperformance of a laboratory instrument on any given day withthe prior performance of that instrument. This practice is notintended for comparison of different instruments with eac
10、hother, nor is it directly applicable to dedicated process FT-NIRanalyzers. This practice requires the use of a check samplecompatible with the instrument under test as described in 5.3.5. Test Conditions5.1 Operating ConditionsIn obtaining spectrophotometricdata for the check sample, the analyst mu
11、st select the properinstrumental operating conditions in order to realize satisfac-tory instrument performance. Operating conditions for indi-vidual instruments are best obtained from the manufacturersinstructional literature due to the variations with instrumentdesign. It should be noted that many
12、FT-NIR instruments aredesigned to work best if left in standby mode when they are notin use. A record should be kept to document the operatingconditions selected during a test so that they can be duplicatedfor future tests. Note that spectrometers are to be tested onlywithin their respective recomme
13、nded measurement wavelength(wavenumber) ranges.5.2 Instrumental characteristics can influence these mea-surements in several ways. Vignetting of the beam (that is, theaperture of the sample cell is smaller than the diameter of thenear infrared beam at the focus) reduces the transmittancevalue measur
14、ed in nonabsorbing regions, and on most instru-ments can change the apparent wavelength (or wavenumber)1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and Separation Science and is the direct responsibility of Subcom-mittee E13.03 on Infrared and Near Infrared
15、 Spectroscopy.Current edition approved Dec. 1, 2007. Published December 2007. Originallyapproved in 1998. Last previous edition approved in 2002 as E 1944 - 98 (2002).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual
16、Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.scale by a small amount, usually less than 0.01 nm (0.1 cm-1).Focus changes can
17、 also change transmittance values, so thesample should be positioned in the same location in the samplecompartment for each measurement. The angle of acceptance(established by the f number) of the optics between the sampleand detector significantly affects apparent transmittance. Heat-ing of the sam
18、ple by the beam or by the higher temperatureswhich exist inside most spectrometers changes absorbancessomewhat, and even changes band ratios and locations slightly.Allow the sample to come to thermal equilibrium prior tomeasurement.5.3 The recommended check sample should meet the fol-lowing requirem
19、ents: the check sample should be fully com-patible with the requirements for repeatable sample presenta-tion to the measuring spectrophotometer. The check sampleshould consist of a single pure compound or precisely knownmixture of compounds which is spectroscopically stable overmonths or years. The
20、spectra obtained from such a checksample should be known to indicate changes in the spectro-photometer, not the check sample itself. It is recommended thatindependent verification of the integrity of the check sample beused prior to test measurement. The check sample should bemeasured under precisel
21、y the sample measurement conditionsof temperature, humidity, and instrument set up configuration.Suggested check samples may include, but are not limited tothe following: for gases, water vapor at 5.89 Torr and 1atmosphere ina2mgascell, or methane at 18 psig pressure ina 10 cm gas cell; for liquids,
22、 pure spectroscopic gradehydrocarbon compounds (for example, toluene, decane, isooc-tane, etc.), or precise mixtures of these pure compounds; forreflectance measurements of solids, rare earth oxides mixedwith white halon powder, or Spectralon3-based rare earth oxidereflectance standards. Reference r
23、eflectance standards yieldinga featureless, near 100 % reflectance spectrum are pure pow-dered sulfur, halon, or Spectralon.6. Level Zero Tests6.1 Nature of TestsRoutine checks of instrument perfor-mance can be performed within a few minutes. They aredesigned to uncover malfunctions or other changes
24、 in instru-ment operation but not to specifically diagnose or quantita-tively assess any malfunction. For Level Zero tests, a resolu-tion of 4 cm-1and a nominal measurement time of 30 s isrecommended. Resolution and measurement times can bespecified to match conditions used for routine measurementap
25、plications. The exact measurement time, along with the date,time, sample identification, number of scans, and operatorsname, should always be recorded.6.2 PhilosophyThe philosophy of the tests is to usepreviously stored test results as bases for comparison and thevisual display screen or plotter to
26、overlay the current testresults with the reference results (known to be good). If the oldand new results agree, they are simply reported as no change.Level Zero consists of three tests. Run the tests under the sameconditions that you would normally use to run a sample (that is,sample temperature, pu
27、rge time, warm-up time, beam splittertype, detector configuration, etc.)6.3 Variations in Operating Procedure for DifferentInstrumentsMost of the existing FT-NIR instruments shouldbe able to use the tests in this procedure without modification.However, a few instruments may not be able to perform th
28、etests exactly as they were written. In these cases, it should bepossible to obtain the same final data using a slightly differentprocedure. The FT-NIR manufacturer should be consulted forappropriate alternative procedures.6.4 SampleThe check sample used for performance testsis described in 5.3. The
29、 same sample should be used for all testcomparisons (note serial number, or other identifying informa-tion, of sample) as well as orientation of the sample within thesample compartment during test measurements.6.5 Reference SpectraTwo spectra acquired and storedduring the last major instrument calib
30、ration are used asreferences. These spectra will be identified as Reference 1 andReference 2.6.5.1 Reference 1 is a Fourier-transformed single-beamenergy spectrum of an empty beam. (in this and all later usage,empty beam means that nothing is in the sample path exceptdry air or the purge gas normall
31、y present within the spectrom-eter sample compartment). For reflectance measurements thisspectrum is a spectrum of a flat, pure reflectance standardapproximating 100 % R.6.5.2 Reference 2 is a transmittance spectrum of the checksample. For reflectance measurements this spectrum is areflectance spect
32、rum of the check sample.6.6 Repeatability of ProceduresCare should be taken thateach of the spectral measurements is made in a consistent andrepeatable manner, including sample orientation (although,different spectral measurements do not necessarily use theidentical procedure). In particular, for th
33、ose instruments havingmore than one sample beam or path in the main samplecompartment, all of the test spectra always should be measuredusing the same optical path.6.7 MeasurementsThree test spectra will be acquired andstored. The test spectra will be identified hereafter as Spectrum1, Spectrum 2, a
34、nd Spectrum 3.6.7.1 Spectrum 1An empty-beam spectrum stored as aFourier-transformed single beam energy spectrum (or as aninterferogram). If stored as an interferogram, it must betransformed before use in the ensuing tests.6.7.2 Spectrum 2An empty-beam spectrum taken imme-diately after Spectrum 1. Th
35、is spectrum should be stored aseither a Fourier-transformed single-beam energy spectrum oras a transmittance spectrum ratioed against Spectrum 1.6.7.3 Spectrum 3A spectrum of the check sample ob-tained reasonably soon after Spectrum 2. This spectrum shouldbe stored as a transmittance spectrum (or re
36、flectance spectrum,when applicable) ratioed against either Spectrum 1 or Spec-trum 2, or as a single-beam energy spectrum. To reproduciblyinsert the sample, the serial number (or other identifyinginformation) should be right side up facing the instrumentdetector (or aligned in a manner that allows r
37、epeatable mea-surements each time the check sample is measured).3Spectralon, available from Labsphere, Inc., P.O. Box 70, Shaker St., NorthSutton, NH 03260-0070, has been found satisfactory for this purpose.E 1944 98 (2007)27. Level Zero Test Procedures7.1 Energy Spectrum TestOverlay Spectrum 1 and
38、Refer-ence 1. Note any changes in energy level across the spectrum.Ratio Spectrum 1 to Reference 1. Video display resolution maylimit the accuracy to which this test can be interpreted if thecomparison is made on-screen. In addition, if the interferogramwas saved, it may be displayed or plotted and
39、the center burstheight recorded. Changes in the interferogram height aredifficult to interpret since minor decreases in source tempera-ture that only affect high frequencies can result in changes ininterferogram height. These changes do not affect photometricaccuracy.7.1.1 ReportageReport by making
40、an overlay plot ofSpectrum 1 energy ratioed against Reference 1 energy over therange of 95 to 105 % T, and by reporting the following energyratios:For short2wave near infrared:RATIO800/100012 500/10 000!5 E800/100012 500/10 000!(1)For long2wave near infrared:RATIO1500/20006666.67/5000!5 E1500/200066
41、66.67/5000!RATIO2000/25005000/4000!5 E2000/25005000/4000!Report the date and time of both spectra used, and the actualnumbers of scans and measurement times, as well as details ofthe instrument set up conditions.7.1.2 InterpretationAn overall drop in the energy level inwhich the largest percentage o
42、f change occurs at higherwavenumbers usually indicates interferometer misalignment ora reduction in source temperature. An overall drop in theenergy level without wavelength (wavenumber) dependencesuggests beam obstruction (vignetting) or misalignment ofnon-interferometer optical components. The app
43、earance ofbands or other features indicates purge gas contributions, beamobstruction by a partially transmitting object, oil or smokedeposition on mirrors or windows, or a forgotten sample withinthe beam. With cooled detectors (for example InSb), theappearance of a broad band around 1940 nm (5154.64
44、 cm-1)indicates ice deposition on the detector surface. Non-zeroenergy levels below the detector cut-off (more than 0.2 % ofthe maximum energy-level in the single beam spectrum)indicate system nonlinearities or detector saturation. On manyinstruments anomalous increases in the actual measurementtime
45、 for a set number of scans indicate instrument problems(mis-triggering, white light misalignment, excessive purgerate, or interferometer drive-problems).7.2 One Hundred Percent Line TestRatio Spectrum 2 toSpectrum 1. Note the noise level and any variations from100 % transmittance (or reflectance) ac
46、ross the spectrum.7.2.1 ReportageMake an overlay plot of Spectra 1 and 2.Then ratio the two and plot the 100 % transmittance (orreflectance) line. The ordinate range should be 99 to 101 %T/R. If the noise or baseline drift exceeds these bounds, makea plot from 90 to 110 % T/R and consider performing
47、 LevelOne tests. Report the RMS (preferred) or peak-to-peak noiselevels at over a ;8-18 nm (100 cm-1) range centered at 800 nm(12 500 cm-1), 1000 nm (10 000 cm-1), 1500 nm (6666.67cm-1), 2000 nm (5000 cm-1), 2500 nm (4000 cm-1). If theinstrument wavelength (wavenumber) range does not includesome of
48、these, substitute the nearest measurable wavelength(frequency).7.2.2 InterpretationExcessive noise may result from mis-alignment or source malfunction (refer to the energy spectrumtest) or from a malfunction in the detector or the electronics.Repetitive noise patterns (for example, spikes or sinusoi
49、ds)sometimes indicate digital problems. Isolated noise spikes maybe digital malfunctions or they can indicate electromagneticinterference. Positive or negative bands often indicate a rapidchange in purge quality. Simultaneously positive and negativesharp bands in the water region may indicate instrumentalproblems or excessive water vapor within the spectrometer.Deviations from the 100 % level (usually at lower wavelengths(higher wavenumbers) indicate interferometer, detector, orsource instability (see Practice E 1421).7.3 Check Sample TestRatio Spectrum 3 to S
