henry buijs ir spectroscopy and ftir - abb group · ftir observes all spectral elements at the same...
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IR Spectroscopyand FTIR
Henry Buijs
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Molecular Vibrations
Material is made up of atomsA group of atoms linked together by chemical bonds is called a molecule
Atoms of hydrogenand oxygen
Molecule of water
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Molecular Vibrations
The atoms of a molecule are always in motionThese motions are called vibrations
symmetrical vibrationH-O-H
asymmetrical vibrationH-O-H
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Properties of Molecular Vibrations
The rate of vibration is millions of cycles per second (it never stops)Each vibration has a unique frequencyThe frequency of a vibration depends on:
The strength of the chemical bond between atomsThe mass of each atom
So we can learn a lot about a molecule when we study its vibrations!
Infrared spectroscopy is a method for the analysis of molecular vibrations
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IR Spectroscopy
Analysis based on the absorption at different wavelengths of an infrared beam by a sample
Sample
Source radiation Transmitted radiation
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transmittance spectrum of the sample
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Relative intensity of IR beam
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Principles of Infrared Spectroscopy
When a frequency of light corresponds to a molecular vibration it is absorbed by the sample
The fraction of light transmitted by the sample compared with the light incident as a function of frequency gives the infrared spectrum of the sample
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Infrared Spectrum of a Polystyrene Film Wavelength is expressed in number of waves per cm.
(e.g. 1000 /cm is the same as wavelength λ=1/1000th cm)Vertical scale is percent remaining intensity
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transmittance spectrum of the sample
Wavenumbers (cm-1)
%
InfraredUV
Visible
Near IR Mid IR
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Properties of an IR Spectrum There is much detail in a spectrum.Much of it is unique to a sample
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A band as narrow as 6 cm-1 at3100 cm-1 (0.2% of wavelength)
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But What Is Really Happening Here?
In fact recording a spectrum is a little like tuning in to radio stations...We turn the button (change the frequency)Sometimes we catch a signal, and we hear music (we “absorb” the signal at this frequency)We know which stations to find, because we know the frequency where they are (we know which vibrations to pickup)Sometimes the signal is strong, sometimes weaker (the peak is higher or lower)Sometimes we hear nothing at all!
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Absorbing and Non-Absorbing Species
Absorbing:Any molecular vibration which displaces an electric charge will absorb infrared radiation.Diatomic molecules with dissimilar atoms HCl, COAsymmetric vibrations in polyatomic moleculesFunctional groups
Non-absorbing:Atomic Species: He, Ne, Ar, KrAll Atomic SpeciesMolecular Species: H2, N2, O2, F2, Cl2, Br2These cannot be analyzed by infrared
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Mid IR SpectroscopyWavelength range 2500 nm to 25000 nmFrequency range 4000 to 400 cm-1Mid IR spectra provide distinctive patterns for many compoundsPermits distinction of several hundred thousand different compoundsMost compounds have high absorptivity in mid IR
Condensed phase compounds>50% transmittance for sample thickness in 10 to 100 micrometer rangeNeed sample accessories that assure short light path through sample
Capillary liquid transmission cell, ATR and diffuse reflectance
Gas phase compounds>50% transmittance for sample thickness in 10 to 100 cm rangeMid IR is suitable for quantitative gas phase analysis
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IR Spectrum of a Liquid Vegetable Oil (path: 12 microns)
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Abso
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IR Spectra of Hand Cream and Water (path: 12 microns)
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Frequency (cm )
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water
hand cream
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-Infrared Spectrum of Gaseous HCl (10-cm path cell, resolution 1 cm-1)
3200 3000 2800 2600 Frequency (cm )
Abso
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2970 2960 2950 2940
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Near IR SpectroscopyWavelength range 700 nm to 2500 nmFrequency range 14000 to 4000 cm-1Near IR spectra have less distinctive patterns than Mid IR spectra
Harmonics and combination frequencies of mid IR bandsPrincipally harmonics of H-C, H-N, H-O vibrations
Absorptivity is lower than in mid IRLow absorptivity is offset with longer path sampling
Sampling with pathlength of several millimeters is much easier and permits greater reproducibility
Disposable glass vial samplingSampling of flowing sample in process pipe
Quantitative analysis using chemometrics
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Overtones (Harmonics) and Combinations
Infrared spectrum of H2O:OH stretching: 3400 cm-1
OH bending: 1630 cm-1
First overtone: 2 x 3400 = 6800 cm-1
Second overtone: 3 x 3400 = 10200 cm-1
Combination: 3400 + 1630 = 5030 cm-1
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The pathlength is increased by a factor of 150 in the NIR to get intensity similar to the mid-IR
Mid and Near Infrared Spectra of Isopropanol
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Isopropanol, mid-IR
Isopropanol, NIR2 mm cuvette
Horizontal ATR
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NIR Spectra of Surfactants
9000 8000 7000 6000 5000 Frequency (cm )
Nor
mal
ized
Abs
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From the bottom to the top:
- OH value = 217.6, sap value = 145.7
- OH value = 185.6, sap value = 153.2- OH value = 107.4, sap value = 159.5
Hydrocarbon region
OH region
Hydrocarbon region
moisture
OH region
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Fundamentals and Overtones
The C-H Stretching Vibration for CHCl3 Liquid
Anharmonic Observed oscillator Absorbance Pathlength
Transition wavenumber calculation for a forυ = 0 to υ= (cm-1) (cm-1) 1 cm cell measurement
1 3019 3019 170 5 µm2 5912 5912 17 0.5 mm3 8677 8679 0.55 1.0 cm4 11318 11320 0.02 5.0 cm5 13850 13835 0.001 N/A6 16270 16224 0.0001 N/A
Note the reduction in absorbance. This requires very short path at 3019cm-1, convenient 0.5 mm and 10 mm path at 5912 and 8677 respectively.
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FTIR principlesHenry Buijs
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Dispersive Infrared Spectroscopy
Geometric wavelength separation
Prism dispersionDiffraction grating
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The Michelson Interferometer Optical part of FT-IR2 flat mirrors and a beamsplitter
One mirror movesChanges optical path of split light beamsRecombine light and get variable interference
Wavelength separation by modulation
Each wavelength hasa unique modulation frequency
Moving mirror
Fixed mirror
Beamsplitter
Compensator
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FTIR technologyMany Interferometer types are used by different FT-IR manufacturers. Why?
To achieve more stable and reproducible operation
The original Michelson interferometer is sensitive to misalignment
Tilt of 1 micrometer across the IR beam (25 micro radians) will changeinterferogram by >10%
The scan mechanism can wear out or become irratic
1 micro meter1 micro meter
Scan MechanismScan Mechanism
fixed mirrorfixed mirror
moving mirrormoving mirror
compensatorcompensator
beamsplitterbeamsplitter
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The ABB Michelson FT-IR
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FT-IR with Laser Controlled Measurement
Modulation frequencies of all wavelengths are locked to modulation frequency of an internal He-Ne laser
Provides ultra precise measurement of spectral frequenciesProvides reproducible measurement of spectral frequencies
fixed mirrorfixed mirror
Scan MechanismScan Mechanism
moving mirrormoving mirror
compensatorcompensator
beamsplitterbeamsplitter
IR beam
Laser beam
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Vol
ts si
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Vol
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Vol
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Vol
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Interferogram / Arbitrary X
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TypicalTypical interferograminterferogram4 volts at4 volts at centerburstcenterburst
0.004 volts at end of scan0.004 volts at end of scan
Measurement with FTMeasurement with FT--IRIR
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Vol
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Vol
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.015
31200 31100 31000
Interferogram / Arbitrary X
Two interferograms (near end of scan)Two interferograms (near end of scan)difference difference
Measurement with FTMeasurement with FT--IRIR
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Vol
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Vol
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-10E-05
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31400 31300 31200
Interferogram / Arbitrary X
Difference between 2 interferograms (16Difference between 2 interferograms (16 coaddedcoadded scans )scans )(peak signal (4volts)/p. to p. noise =40000:1)(peak signal (4volts)/p. to p. noise =40000:1)
Measurement with FTMeasurement with FT--IRIR
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Vol
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Vol
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-.001
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Interferogram / Wavenumber (cm-1)
Interferogram of blocked beam Interferogram of blocked beam (3 levels of ADC)(3 levels of ADC)
Measurement with FTMeasurement with FT--IRIR
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S/N achievableTable 1figure RMS noise in
Micro AAt 4000 cm-12500nm
At 5000 cm-12000nm
At 5800 cm-11725nm
At 6600cm-11515nm
A 0A 4.3 2.6 3.25 4.6B 1.634A 168 160 148 177C 1.634A +gain 11 6 8.5 14D 2.683A +gain 125 83 105 145
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Absorbance / Wavenumber (cm-1)
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-50E-060
50E-06
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2.6832.6835
2.6842.6845
5400 5200 5000 4800 4600 1.633
1.63351.634
1.63451.635
5400 5200 5000 4800 4600
1.6341.6345
1.6351.6355
A
CB
D
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Interferometer Features
Patented Michelson type interferometerCube corner retroreflectors mounted on a "wishbone" swing arm.
Factory prealigned interferometerand input/output optics. Does not require any adjustment by user.
Scanning by rotating the swing arm on a flex pivot, driven by an induction motor
This provides smooth, constant-velocity, perturbation-free scanning without wear.
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The ABB Michelson FT-IR
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Acquisition of an Infrared Spectrum
The light coming from the source goes through the interferometer; the movement of the mirrors causes an interference pattern which is called the interferogramNext the light passes through the sample; theinterferogram of the source is modified as the light is absorbed by the sampleThe interferogram is measured by the detectorThe interferogram is transformed into a spectrum by a mathematical operation called the Fourier Transformalgorithm
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Acquisition of an Infrared Spectrum
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Interferogram of the source & sample
Interferogram of the source Spectrum of the source
Spectrum of the source & sample
FT
FT
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Acquisition of an Infrared Spectrum
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transmittance spectrum of the sample( polystyrene film )
Spectrum of source only(no sample present)
Spectrum of source & sample
divided by
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Advantages of FT-IR Over DispersiveOptical efficiencyMeasurement efficiencyPrecision of measurement
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Optical Efficiency of FT-IRA dispersive spectrometer has much greater light restriction than FT-IR for the same size and resolution spectrometer
A Dispersive spectrometer has a slit width of 0.25mm by 5mm highFT-IR has a round aperture of 7.5mm diameter
Ratio of areas for FT-IR/dispersive is 35
Dispersive spectrometerDispersive spectrometerSlit 0.25x 5mmSlit 0.25x 5mm
1.25mm21.25mm2
FTFT--IRIRJacquinotJacquinot stop 7.5 mm stop 7.5 mm diamdiam
44 mm244 mm2
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Optical efficiency of FT-IR
FT-IR has sufficient throughput to simultaneously illuminate 8 sampling accessories via fiber optic cables
The ABB FTPA2000-200 (Networkir)
modulated outputs8 channels
Direct FO coupled Detectors8 channels
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Measurement Efficiency of FT-IRMultiplexing
Grating observes 1 spectral element at a time1000 to 2500 nm with 4 nm resolution = 375 elements
FTIR observes all spectral elements at the same time4000 to 10000 cm-1 at 16 cm-1 =375 elementsTypical advantage 20x
All wavelengths are modulated simultaneouslyA single detector measures all wavelengths continuously
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Precision Requirements in IR Spectroscopy
For quantitative analysis via IR spectroscopy, the spectra of a sample should be reproducible
For a reproducibility of 1%,The absorbance scale must be reproducible to <1%
Requires <1% error in transmittance when transmittance is >50%Requires <0.1% error in transmittance when transmittance is near10%
Frequency scale must be reproducible to <1% of resolutionAt 4 cm-1 resolution error must be <0.04 cm-1
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Precision: Dispersive Spectrometry vs. FT-IRDispersive Spectrometers
Wavelength is established by mechanical tolerance on slit position
Best mechanical tolerance is
1 micrometer1 part in 25000
for 2.5 cm structure
+/-0.3 cm-1 at 7500 cm-1
Any detected light not passing through spectrometer adds offset to transmittance of sample
Absorbance scale reproducibility limited by stray light
Reproducibility limited to low resolution and low absorbance applications only
FT-IRWavelength is established by internal He-Ne Laser
1 part in 1 000 000repeatability <+/-0.008 cm-1 at 7500 cm-1
For ABB FT-IRsGuaranteed Absolute Accuracy of frequency scale
+/-0.04 cm-1 at 7300 cm-1 Any detected light not modulated by FT-IR does not cause offset to transmittance of sample
There is no stray light in FT-IR
Better Reproducibility over a wider range of resolution and absorbance
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Repeatability and Stability of ABB’s FT-IRs
Internal laser controlled mirror scanLaser coaxial with IR beam provides frequency scale repeatability of +/-0.001 cm-1 (2 σ )Permanent factory alignment of interferometer provides
100% line repeatability < 0.1% short term100% line repeatability < 2% over 16 hour intervalLong-term 100% line repeatability of spectral structure <0.1%
Linear baseline shift and tilt < 2%
It is common to use the same spectral response reference for as long as a year.
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Identical Spectral Response
All units have identical spectral responseReproducibility of spectral response approaches repeatabilityCalibration transfer without chemometric correction
From analyzer to analyzerAfter repair of an analyzer
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Reproducibility of ABB’s FT-IRs
Spectrometer validation using standard samples provides
Frequency scale reproducibility within +/- 0.04 cm-1 (2σ )For FT-NIR, narrow water vapor line at 7299.85 cm-1For FT-IR, narrow water vapor line at 1917.65 cm-1
Absorbance reproducibility within +/-0.002A (2σ )High purity Toluene in temperature controlled liquid cell or sealed vial, 0.5 mm path at 28ºC
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Frequency Validation Water Spectrum
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7360 7340 7320 7300 7280 7260 7240 Wavenumber (cm-1)
Validation bandValidation bandH2OH2O
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Frequency Reproducibility
Result of VALID test
7299.4
7299.44
7299.48
7299.52
7299.56
7299.6
7299.64
7299.68
7299.72
7299.76
7299.8
7299.84
7299.88
7299.92
7299.96
7300
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frequ
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Band Position
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Absorbance Reproducibility: Toluene
Toluene SpectrumPeak absorbance = 1.2 at 4050 cm-1Average peak heights = 0.5 at 4200 to 4700 and near 6000 cm-1Maximum absorbance deviation <0.002 A from 4200 to 7500 cm-1 ( 2 cm-1 resolution, 50 scans)
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Typical Toluene Reproducibility
-.002
-.001
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Absorbance / Wavenumber (cm-1)
Toluene Difference Spectrum
MB160 (s/n SZM480CW)
MB160 (s/n SZM480CM)
16 cm-1 resolution
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Toluene Reproducibility Statistic
Toluene Validation
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An Example: OH Value Calibration
OHvalue (Actual v.s. Predicted)
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actual
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RMSD=0.7 (2 Factors)
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OH Value Repeatability
OH value
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cal samplesValidationLinear (cal samples)
Repeatability RMSD 0.05
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OH Value Repeatabilitycontrol chart 10 weeks
y = 0.0002x + 57.202
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OH
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OH Value Reproducibility
System to System Reproducibility
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MB160 unit
OH
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Advantages of No Drift and Identical Spectral Response
Less effort is required to develop a calibrationThere are no analyzer variations to model
A robust calibration for OH value for a polyol requires 10 to 15 reference samples.
Less calibration maintenanceOnce a calibration is developed it can be used indefinitely as long as the product does not change
Some customers are working more than 5 years with the same calibration without any need for adjustment
Can do applications that are difficult or costly to calibrateCostly reference methodsProcesses that are difficult to vary.
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The ABB Spectrometer Line
The infrared band can be divided into three regions: The Far infrared, the Mid infrared and the Near infrared.In the Far Infrared (< 400 cm-1), we particularly study the structure of complex solids. This region is covered by the FTLA2000-102In the Mid Infrared (4000 - 400 cm-1), we study the structure of many natural products and gas phase samples. This region is covered by the FTLA2000-100, FTLA2000-104, FTLA2000-154The Near Infrared (12000 - 4000 cm-1) region is used particularly for quantitative analysis. It is covered by the FTLA2000-154, and FTLA2000-160
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The ABB Spectrometer Line
On-line analyzers:FTPA2000-300 (Workir)
Single-point analyzer existing in Mid IR and Near IR versions.
FTPA2000-200 (Networkir)Near IR fiber-optic coupled multipoint analyzer. Can monitor up to eight sampling points.
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Summary
With FT-NIRs from ABB expectAnalyzers that do not drift over timeAnalyzers that have identical spectral response
Calibrations give the same results on any analyzerCalibrations require no correction after repair of analyzer
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