The Difference Between VFA and FT-IR Spectra - Wilks Enterprise, Inc.

Paul WilksTech NoteThe Difference Between VFAand FT-IR SpectraThe basic characteristics of variable filter array (VFA) spectrometers, both nearandmid-infrared, make them well suited for routine infrared analysis applicationsoutside and inside the laboratory. The performance of this spectrometer isdiscussed here.The basic characteristics of variable filter array (VFA)spectrometers, both near- and mid-infrared, availablefrom several manufacturers, make them well suited forroutine infrared analysis applications outside and inside thelaboratory. Although the infrared data gathered by a VFAspectrometer is similar to that obtained from an FT-IR instrument,it differs in one key respect — resolution. In themid-IR region, FT-IR spectra usually are recorded at resolutionsbetween 0.5 cm -1 and 8 cm -1 . A VFA spectrometer with64 pixels has a theoretical resolution of 16 cm -1 . Because ofcrosstalk from pixel to pixel, both thermal and optical, theresolution is reduced further to the neighborhood of 50 cm -1 .What effect does low resolution have upon the performanceof a VFA spectrometer? To answer this question, wehave used a VFA spectrometer in several different applicationsto determine the effect, if any, of low resolution.Raw material verification. A company uses a solvent thatcomes in three isomers, each specific to a different process.Sending a sample to the laboratory can take over an hour forverification, delaying the offloading of the material. Figure 1shows VFA spectra of the three isomers. Even though thespectral information in the VFA spectra is considerably lessthan that of FT-IR traces, there is enough data from the VFAspectrometer to verify the isomer in the shipment. Because aVFA spectrometer has no moving parts and no free air path,it is unaffected by atmospheric absorptions and can be situatedat the receiving dock. In less than 5 min, the incomingraw material can be verified and off-loaded to the correctlocation.Process monitoring. In a process, a material that has anabsorption band “A” between 8.5 and 9 µm (Figure 2) convertsto a material that has band “B” at 7.0 µm. The reactioncan take several hours. The figure shows spectra periodicallytaken by the VFA instrument situated close to the productionline, which follow the switch from “A” to “B”. Again, in this(a)% Reflectance(b)% Reflectance100806040130120110100901800 1600 1400 1200 1000Wavenumber1800 1600 1400 1200 1000WavenumberFigure 1. (a) Infrared spectra of three isomers. (b) UsingIsomer 1 as a reference, a spectrum of Isomer 1 produces astraight line while the spectra from the other two isomersshow deviations from a straight line.situation, the resolution is sufficient to tell when a reaction iscomplete.Flexible film manufacturing. There are many areas in flexiblefilm manufacturing in which an onsite test is invaluable.2 Spectroscopy 20(12) December 2005www.spectroscopyonline.com

% Reflectance% Transmission908580757065606.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0Wavelength (m)Figure 2. Following a reaction. As the reaction progresses, the band between 8.5 and 9 µmdecreases to be replaced by the band at 7.0 µm.80604020T = 0T = 1T = 2T = 3T = 41000 1200 1400 1600Wavenumber (cm -1 )Figure 3. Spectra of common materials used in flexible film production.With multilayer films, rolls of film arelaminated together with individualrolls, often having different top andbottom materials. Figure 3 shows spectraof common materials used in flexiblefilm production. The spectra differenough so that an operator in the productionarea can verify the film and orientit in the right direction forlamination. In the warehouse,rolls of finished films can be verifiedon-site before shipment toensure that the customer isreceiving the desired product.Another application involvesmeasuring an inside layer thickness,such as nylon, in a multilayerfilm. Nylon has anabsorbance different from thespectra of the outside polyethylenelayers, which can be distinguishedwith a lower resolutionVFA spectrometer.FT-IR Spectra“White powder” verification. Partlyas a result of 9/11, and partly becausematerials are being examinedfrequently for identificationpurposes, mid-IR FT-IR spectrometershave become widelyused for this purpose to the extentthat libraries containingthousands of spectra of knownmaterials have been constructed.Is it feasible to use a VFA spectrometerto search the librariesfor identification purposes?The spectra of common pharmaceuticalproducts shown inFigure 4 are a good example ofthe differences between VFAspectra and FT-IR spectra. Whileeach VFA spectrum is distinctlydifferent from the others, eachalso is quite different from itsFT-IR counterpart. Because ofthese substantial differences, itappears to be impractical tocompare spectra from FT-IRlibraries directly.On the other hand, if oneexamines the VFA spectra, itbecomes apparent that eachproduct has a characteristic VFAspectrum, sometimes more readilyidentifiable than its FT-IRspectrum, so libraries of VFAspectra could serve to identifyvarious groups of materials. Forexample, a pharmaceutical companymight use one of the VFAspectral libraries VFA spectracould be created. [Author:www.spectroscopyonline.comDecember 2005 Spectroscopy 20(12) 3

VFA Spectra(a)% Transmittance % Reflectance(c)% Reflectance% Transmittance9590858075908070609085807590858075701600 1400 1200 1000Wavenumber1600 1400 1200 1000Wavenumber1600 1400 1200 10001600 1400 1200 1000Wavenumber(b)% Reflectance % Transmittance% Reflectance100959085959085807570(d)% Transmittance100959085809590851600 1400 1200 1000Wavenumber1600 1400 1200 1000Wavenumber1600 1400 1200 10001600 1400 1200 1000Wavenumber% Transmission % Transmission% Transmission10095908595908580959085807570Bufferin, VFA1600 1400 1200 1000WavenumberBufferin, FT-IR 32-cm -1 resolution1600 1400 1200 1000WavenumberBufferin, FT-IR 2-cm -1 resolution1600 1400 1200 1000WavenumberFigure 4. VFA (upper) and FT-IR (lower) spectra of commonpharmaceuticals (a) aspirin, (b) Bufferin, (c) Advil, and (d)Tums.Figure 5. Changing resolution on spectral libraries to matchVFA resolution.Please clarify this sentence]While it does not appear that the VFA spectrometer willreplace the FT-IR instrument as a general purpose unknownmaterial identifier, it can be used for this purpose when a relativelysmall number of materials is involved. Because VFAspectrometers are small, rugged, and usually contain no movingparts or exposed optical paths, they can be practical alternativesto FT-IR instruments when applicable.It is possible to degrade FT-IR spectra from spectrallibraries to lower resolution, as in Figure 5, so that they tendto match VFA spectra. However, in so doing, search programswould be far less specific and generally would produce anumber of possible hits rather than one or two.spectra and low-resolution VFA spectra are useful, dependingupon the application. ■Paul Wilks is president of Wilks Enterprise, Inc. (South Norwalk,CT).Low Resolution Effects Can Be PositiveFrom the examples presented here, it is apparent that low-resolutionmid-IR data is not necessarily a deterrent and oftencan have a positive effect. As the examples show, low-resolutionspectra frequently contain gross differences betweenspectra of materials, so that visual verification of materials isfeasible. Automatic identification from a VFA spectra libraryalso is feasible. Although FT-IR spectra can contain muchmore detail, visual differences tend to be less apparent, andtherefore, require a more detailed systematic evaluation forpositive identification. Obviously, both high-resolution FT-IR4 Spectroscopy 19(11) 20(12) November December 2004 2005www.spectroscopyonline.com