1、Designation: E1944 98 (Reapproved 2013)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 E1944; the number immediately following the designat
2、ion 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 () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers two levels of
3、 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 (12 500 cm-1) to 1100 nm (9090.91 cm-1); and the long-wavelength nearinfrared region, approximately 1100 nm (9090.9
4、1 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. No other units of measurement are included in thisstandard
5、.1.3 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
6、Documents2.1 ASTM Standards:2E131 Terminology Relating to Molecular SpectroscopyE168 Practices for General Techniques of Infrared Quanti-tative AnalysisE932 Practice for Describing and Measuring Performance ofDispersive Infrared SpectrometersE1252 Practice for General Techniques for Obtaining Infra-
7、red Spectra for Qualitative AnalysisE1421 Practice for 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 E131. All identifications of spect
8、ral regions andabsorbance band positions are given in nanometers (nm), andwavenumbers (cm-1); and spectral energy, transmittance,reflectance, and absorbance are signified by the letters E, T, Rand A respectively. A subscripted number signifies a spectralposition in nanometers, with wavenumbers in pa
9、renthesis (forexample, A1940(5154.64), denotes the 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 notintende
10、d for comparison of different instruments with eachother, 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 spectroph
11、otometricdata for the check sample, the analyst must 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 wi
12、th instrumentdesign. It should be noted that many 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 ar
13、e to be tested onlywithin their respective recommended 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 a
14、t the focus) reduces the transmittancevalue measured in nonabsorbing regions, and on most instru-ments can change the apparent wavelength (or wavenumber)scale by a small amount, usually less than 0.01 nm (0.1 cm-1).1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectrosco
15、py and Separation Science and is the direct responsibility of Subcom-mittee E13.03 on Infrared and Near Infrared Spectroscopy.Current edition approved Jan. 1, 2013. Published January 2013. Originallyapproved in 1998. Last previous edition approved in 2007 as E1944 98 (2007).DOI: 10.1520/E1944-98R13.
16、2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C
17、700, West Conshohocken, PA 19428-2959. United States1Focus changes can 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 dete
18、ctor significantly affects apparent transmittance. Heat-ing of the sample 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 tomeasuremen
19、t.5.3 The recommended check sample should meet the fol-lowing requirements: 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
20、 compounds which is spectroscopically stable overmonths or years. The spectra obtained from such a checksample should be known to indicate changes in thespectrophotometer, not the check sample itself. It is recom-mended that independent verification of the integrity of thecheck sample be used prior
21、to test measurement. The checksample should be measured under precisely the sample mea-surement conditions of temperature, humidity, and instrumentset up configuration. Suggested check samples may include,but are not limited to the following: for gases, water vapor at5.89 Torr and 1 atmosphere ina2m
22、gascell, or methane at 18psig pressure in a 10 cm gas cell; for liquids, pure spectro-scopic grade hydrocarbon compounds (for example, toluene,decane, isooctane, etc.), or precise mixtures of these purecompounds; for reflectance measurements of solids, rare earthoxides mixed with white halon powder,
23、 or Spectralon3-basedrare earth oxide reflectance standards. Reference reflectancestandards yielding a featureless, near 100 % reflectance spec-trum are pure powdered sulfur, halon, or Spectralon.6. Level Zero Tests6.1 Nature of TestsRoutine checks of instrument perfor-mance can be performed within
24、a few minutes. They aredesigned to uncover malfunctions or other changes 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
25、times can bespecified to match conditions used for routine measurementapplications. 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 res
26、ults as bases for comparison and thevisual display screen or plotter to 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 tha
27、t you would normally use to run a sample (that is,sample temperature, purge 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 witho
28、ut modification.However, a few instruments may not be able to perform thetests 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 S
29、ampleThe check sample used for performance testsis described in 5.3. The same sample should be used for all testcomparisons (note serial number, or other identifyinginformation, of sample) as well as orientation of the samplewithin the sample compartment during test measurements.6.5 Reference Spectr
30、aTwo spectra acquired and storedduring the last major instrument calibration 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 tha
31、t nothing is in the sample path exceptdry air or the purge gas normally 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 checksa
32、mple. For reflectance measurements this spectrum is areflectance spectrum 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
33、 do not necessarily use theidentical procedure). In particular, for those 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
34、 test spectra will be identified hereafter as Spectrum1, Spectrum 2, and 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 Sp
35、ectrum 2An empty-beam spectrum taken imme-diately after Spectrum 1. This spectrum should be stored aseither a Fourier-transformed single-beam energy spectrum oras a transmittance spectrum ratioed against Spectrum 1.6.7.3 Spectrum 3Aspectrum of the check sample obtainedreasonably soon after Spectrum
36、2. This spectrum should bestored as a transmittance spectrum (or reflectance 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
37、facing the instrumentdetector (or aligned in a manner that allows repeatable 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.E1944 98 (2013)27. Level Z
38、ero Test Procedures7.1 Energy Spectrum TestOverlay Spectrum 1 and 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,
39、if the interferogramwas saved, it may be displayed or plotted and 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
40、do not affect photometricaccuracy.7.1.1 ReportageReport by making 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 short 2 wave near infrared: (1)RATIO800/100012 500/10 000!5 E800/100012 500/10 000!
41、For long 2 wave near infrared:RATIO1500/20006666.67/5000!5 E1500/20006666.67/5000!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 Interpretat
42、ionAn overall drop in the energy level inwhich the largest percentage of 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 (vigne
43、tting) or misalignment ofnon-interferometer optical components. The appearance 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 (f
44、or example InSb), theappearance of a broad band around 1940 nm (5154.64 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 saturatio
45、n. On manyinstruments anomalous increases in the actual measurementtime 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 noi
46、se level and any variations from100 % transmittance (or reflectance) across 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 exceed
47、s these bounds, makea plot from 90 to 110 % T/R and consider performing 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).
48、If theinstrument wavelength (wavenumber) range does not includesome of 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 th
49、e electronics.Repetitive noise patterns (for example, spikes or sinusoids)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 ins
