ASTM E1944-1998(2002) Standard Practice for Describing and Measuring Performance of Laboratory Fourier Transform Near-Infrared (FT-NIR) Spectrometers Level Zero and Level One Tests.pdf

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1、Designation: E 1944 98 (Reapproved 2002)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 as thestandard.1.3 This standard does not purport to address all of

5、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 Documents2.1 ASTM Standards:E 131 Terminology Relating

6、 to Molecular Spectroscopy2E 168 Practices for General Techniques of Infrared Quanti-tative Analysis2E 932 Practices for Describing and Measuring Performanceof Dispersive Infrared Spectrometers2E 1252 Practice for General Techniques for QualitativeAnalysis2E 1421 Practice for Describing and Measurin

7、g Performanceof Fourier Transform Infrared (FT-IR) Spectrometers:Level Zero and Level One Tests23. Terminology3.1 For definitions of terms used in this practice, refer toTerminology E 131. All identifications of spectral regions andabsorbance band positions are given in nanometers (nm), andwavenumbe

8、rs (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 absorbance at 1940 nm or 5154.6

9、4cm-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 eachother, nor is it directly appli

10、cable 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 must select the properinstrumental

11、 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 FT-NIR instruments aredesigned t

12、o 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 recommended measurement wavelength(wave

13、number) 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 measured in nonabsorbing regions, and

14、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).Focus changes can also change transmittance values, so thesample should be positioned in the same location in the samplecompartment for each measurement. The angle of

15、acceptance1This practice is under the jurisdiction of ASTM Committee E-13 on MolecularSpectroscopy and is the direct responsibility of Subcommittee E 13.03 on InfraredSpectroscopy.Current edition approved March 10, 1998. Published August 1998.2Annual Book of ASTM Standards, Vol 03.06.1Copyright ASTM

16、 International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.(established by the f number) of the optics between the sampleand detector significantly affects apparent transmittance. Heat-ing of the sample by the beam or by the higher temperatureswhich exist ins

17、ide 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 requirements: the check sample should be fully com-patible with the

18、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 spectra obtained from such a checksample should be known to

19、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 precisely the sample measurement conditionsof temperature, humidity,

20、 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, pure spectroscopic gradehydrocarbon compounds (for example,

21、 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 reflectance standards yieldinga featureless, near 100 % refle

22、ctance spectrum are purepowdered 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 in instru-ment operation but not to specifically diagnose or

23、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 measurementapplications. The exact measurement time, along with the date,ti

24、me, 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 overlay the current testresults with the reference results (kn

25、own 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, purge time, warm-up time, beam splittertype, detector configurat

26、ion, 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 thetests exactly as they were written. In these cases, it should

27、 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 same sample should be used for all testcomparisons (note seri

28、al 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 calibration are used asreferences. These spectra will be identified

29、 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 normally present within the spectrom-eter sample compartment). For re

30、flectance 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 spectrum of the check sample.6.6 Repeatability of ProceduresCare sh

31、ould 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 those instruments havingmore than one sample beam or path in the

32、 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, and Spectrum 3.6.7.1 Spectrum 1An empty-beam spectrum stored as

33、 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. This spectrum should be stored aseither a Fourier-transformed si

34、ngle-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 reflectance spectrum,when applicable) ratioed against either Spe

35、ctrum 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 repeatable mea-surements each time the check sample is measured

36、).7. Level Zero 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 the3Spectralon, available from Lab

37、sphere, Inc., P.O. Box 70, Shaker St., NorthSutton, NH 03260-0070, has been found satisfactory for this purpose.E 1944 98 (2002)2comparison is made on-screen. In addition, if the interferogramwas saved, it may be displayed or plotted and the center burstheight recorded. Changes in the interferogram

38、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 an overlay plot ofSpectrum 1 energy ratioed against Reference

39、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 E1500/20006666.67/5000!RATIO2000/25005000/4000!5 E2000/25005000/4000!Repor

40、t 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 of change occurs at higherwavenumbers usually indicates interfe

41、rometer 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 appearance ofbands or other features indicates purge gas contribu

42、tions, 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 cm-1)indicates ice deposition on the detector surface. Non-ze

43、roenergy 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 for a set number of scans indicate instrument problems(mis-tr

44、iggering, 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 from 100% transmittance (or reflectance) across the spectrum.7.2.1 ReportageMake an overlay plot of Spect

45、ra 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 LevelOne tests. Report the RMS (preferred) or peak-to-peak no

46、iselevels 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 these, substitute the nearest measurable wavelength(frequency)

47、.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 sinusoids)sometimes indicate digital problems. Isolated noise spikes

48、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 spectromete

49、r.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 Spectrum 2(or 1) to produce a check sample transmittance spectrum (orreflectance spectrum, when applicable). Convert all spectra toabsorbance spectra. Subtract the stored absorbance checksample spectrum from this new absorbance check samplespectrum. Note any changes.7.3.1 ReportagePlot the check sample absorbance spec-trum over the reported dynamic range of the