Trends in Analytical Chemistry, Vol. 25, No. 1, 2006 Trends

Resolution and suppression ofmechanical noise in FT-NIRspectroscopyThomas Meyer, Joachim Oelichmann, Hanspeter Kellerhals

We summarize the various techniques for suppressing the influence of me-

chanical disturbances in Michelson interferometers and describe the polar-

ization interferometer in detail as an alternative. Since the best achievable

resolution has to be pre-selected for polarization interferometers, we discuss

the relationship between resolution and accuracy obtained for quantitative

analyses in Fourier transform near-infrared spectroscopy.

ª 2005 Elsevier Ltd. All rights reserved.

Keywords: Fourier transform; Interferometer; Near-infrared; Polarization; Resolution;

Spectroscopy

Thomas Meyer,

Joachim Oelichmann

Buchi Labortechnik AG,

CH-9230 Flawil, Switzerland

Hanspeter Kellerhals*

Kellerhals Analytik und

Informatik, Hegetsbergstrasse 6,

CH-8610 Uster, Switzerland

*Corresponding author.

Tel.: +41 1 994 11 31;

Fax: +41 1 994 11 39;

E-mail: kellerhals@active.ch

0165-9936/$ - see front matter ª 20050165-9936/$ - see front matter ª 2005

1. Introduction

Interferometry has become the favoritetechnical approach for recording near-infrared (NIR) spectra due to its perfectreproducibility of spectra and its instru-ment-independent portability of methods.The basic set-up is a laser-controlledMichelson interferometer, as shown inFig. 1.Incoming light is split by the beam

splitter, B. One part of the beam is reflectedby a fixed mirror, F, the other part by amoving mirror, M. The recombined outputbeam is an interferogram, which can besampled and Fourier-transformed into thespectrum. By means of an HeNe laser, L, itis possible to sample the signal at veryprecise, reproducible points. The laser lightis directed through the same opticalarrangement. It produces a sinusoidalsignal at the detector, S, which is used totrigger the sampling of the NIR interfero-gram. The technique has been thoroughlydiscussed in a large number of textbooks(e.g. [1,2]).The most critical point in the design of

such an interferometer, if it is meant to beused for precise NIR measurements inrough environment, is the opticalalignment. For example, tilt caused by

Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2005.06.007Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2005.06.007

vibrations or other mechanical noise leadsto spectrum instabilities even for relativelysmall tilt angles, d. A common criterion forthe maximum permissible difference inretardation is that it should be less thanone-tenth of the shortest wavelengthacross the width of the beam. For NIRspectrometers (assuming a beam diameterof 5 cm and a maximum wavenumber of10,000 cm�1), this means a permissibletilt of only 1 lrad. The basic Michelsoninterferometer therefore has to be im-proved for NIR spectroscopy.In Section 2, we summarize several

concepts for improved designs of the basicMichelson interferometer. They have beendiscussed previously in the literature [2].In Section 3, we describe an alternative

approach, which makes use of the opticalretardation in birefringent crystals that isless affected by mechanical distortions.Since the nominal resolution for crystalinterferometers heavily affects the costs ofthe instrument, resolution has to bechosen carefully. This is why we discuss inSection 4 the interdependence of perfor-mance and resolution for quantitative NIRapplications.

2. Improved interferometers of theMichelson type

2.1. Dynamic alignmentLaser beams can be used to trigger sam-pling of the interferogram as well as fordynamic adjustment of the optical align-ment (see Fig. 2).For this purpose, the laser beam is either

expanded to a few millimeters or split intothree beams and then detected by three

1919

Figure 3. Double-pendulum interferometer (details explained inthe text; the laser control is not shown).

Figure 4. Fully-compensated Michelson interferometer (detailsexplained in the text; the laser control is not shown).

Figure 1. Standard Michelson Interferometer (details explained inthe text).

Figure 2. Michelson interferometer expanded for dynamic align-ment (details explained in the text).

Trends Trends in Analytical Chemistry, Vol. 25, No. 1, 2006

segments of a quadrant detector or three separatedetectors. These three detectors receive sinusoidal signalswith phase differences representing the actual differencein retardation caused by misalignments. They are usedfor controlling a servo system correcting the position ofthe mirror, F, for example, mounted on piezoelectricelements, P [3].In other designs, the tilt of the moving mirror, M, is

controlled magnetically [4].The commercial NIR instruments of Thermo Nicolet

Corporation currently apply dynamic alignment. It is avery suitable extension of the Michelson interferometer,which is relatively inexpensive because no additionaloptical components are required. However, the supple-mentary mechanics, electronics and software are quitecomplex.

2.2. Double-pendulum interferometersIn the double-pendulum interferometer (Fig. 3), two cubecorner retroreflectors, R, are mounted on a support,which is movable for small angles around a pivot point, P.

20 http://www.elsevier.com/locate/trac

The optical retardation changes in both arms of theinterferometer during the scan. Because both mirrors, R,move, the retardation corresponds to four times themechanical displacement. Retroreflectors ensure thatany tilt is compensated for automatically. However,shearing normally leads to differences in retardation.Because of symmetry, the double-pendulum interferom-eter is optically compensated for shear of the mirrorassembly and for rotation about an axis parallel to theline connecting the two mirrors. Rotation about an axisparallel to the line connecting the pivot and the beamsplitter will cause misalignment, so a suitable rigiddesign of the pivot is crucial.Double-pendulum interferometers are used commer-

cially in the NIR spectrometers of ABB (Bomem) [5] andin a modified version in those of Bruker [6]. They sup-press mechanical distortions in an elegant way, the onlydisadvantage being the costliness of the extra opticalcomponents.

2.3. Fully-compensated Michelson interferometerAn example of a fully-compensated Michelson interfero-meter is shown in Fig. 4. It has been introduced into

Trends in Analytical Chemistry, Vol. 25, No. 1, 2006 Trends

commercial instruments by Perkin–Elmer [7] and formsthe base of its current NIR instrument line.The retardation is obtained by rotation of a pair of

parallel mirrors, M, about a symmetric pivot, P, in one ofthe interferometer arms.. The fixed mirrors, F, and thebeam splitter, B, outside the moving device, complete thearrangement. This design is inherently free of dynamicalignment errors. Again, this is an elegant solution forsuppressing mechanical disturbances in a Michelson-type interferometer.

Figure 5. Two orthogonal components of a linearly polarized lightwave undergo different phase shifts when passing through a bi-refringent crystal plate. A phase shift of 180� results in linear polar-ization (a). The state of polarization is unchanged by a phase shiftof 360� (b). In general, the outgoing light is elliptically polarized(a phase shift of 54� is shown in (c)).

3. Polarization interferometers

While a Michelson interferometer uses a moveablemirror to alter the optical path between two separatebeams, the polarization interferometer [8] uses a bire-fringent crystal plate to introduce a phase shift betweentwo orthogonally-polarized light waves. Upon passingthrough a medium (index of refraction n, and thicknessx), a beam of light (wavelength k) experiences a phaseshift:

Du ¼ 2pknx.

In a birefringent crystal, two orthogonally-polarizedwaves are affected by different indices of refraction. Theordinary beam ‘‘sees’’ the ordinary index of refraction,no, the extraordinary beam propagates according to theextraordinary index, ne, so a phase shift is introducedbetween the two beams:

Du ¼ 2pkðne � noÞx.

For a plate of thickness x0.5 = k/2(ne � no), the wavesare shifted by 180�. This situation is depicted in Fig. 5(a).The incident light is linearly polarized at 45�, creatingequal amounts of light along and perpendicular to theextraordinary axis of the crystal. The amplitudes of theelectrical field before and after the transition throughthe crystal are drawn as waves. After the crystal, therelative change in phase of 180� results in a linearpolarization of �45� of the outgoing light.A crystal plate twice as thick [i.e., x1 = k/(ne � no)]

creates a shift of 360� between the two polarizationmodes, which leaves the polarization unaffected[Fig. 5(b)]. The same obviously happens if no crystalplate at all is placed in the beam, and if the thickness ischosen as a multiple of x1. For arbitrary values of x,where the phase shift between the two polarizationmodes is not 180� or 360�, the outgoing light is ellipti-cally polarized. An example [x0.3 = 0.3k/(ne � no)] isshown in Fig. 5(c).The changes in the state of polarization are trans-

formed into an intensity modulation, if a polarizer isbrought into the outgoing beam. We chose the orienta-

tion of the polarizer as �45�, which means that all thelight is blocked when no crystal plate is present. For lightof intensity, I0, and wavenumber, �m ¼ 1=k, passedthrough a crystal plate of thickness, x, and birefringence,Dn = (ne � no), the intensity at the output is then vary-ing according to

I ¼ I02

1� cos 2p�mDnxð Þ½ �.

This formula is almost equivalent to that we obtain fora Michelson interferometer. By changing the platethickness, we can modulate monochromatic light with afrequency that depends on its wavenumber.At first sight, the technical task of building a crystal

plate with variable thickness (and even ‘‘negative’’thickness!) does seem a little demanding, but it is solvedin an elegant way, as shown in Fig. 6. The plate is cutalong a diagonal line into two wedges, and the wedgesare diagonally moved relative to each other to form aplate of varying thickness, x2.‘‘Negative’’ values of the plate thickness are obtained

with the help of a compensator, C, a crystal plate cut sothat the optical axis is rotated around the beam axis by90�. Within the compensator, the indices of refractionare interchanged and the sign of Dn is reversed. Theeffective thickness, x, is thus reduced by x1, the thicknessof the compensator, such that x = (x2 � x1). This num-ber can also have negative values. This is importantbecause, in a measurement, data needs to be recorded onboth sides of the interferogram peak (occurring at x = 0).This optical arrangement is also known as a Soleilcompensator [9].

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Trends Trends in Analytical Chemistry, Vol. 25, No. 1, 2006

The input beam is prepared by passing unpolarized,polychromatic light from a collimated tungsten halogenlamp through a polarizer oriented at 45� (Pol 1). Thepolarization of the passing light is altered in the wedgeassembly, depending on the position of the larger wedge.After the second polarizer (Pol 2, �45�), the intensity oflight is recorded using a detector suited to the NIRspectral range. A laser beam (e.g., from an HeNe laser) iscoupled into the beam path by means of a beam splitter(BS2) and passes through the interferometer in thedirection opposite to the polychromatic light. The laserlight after the interferometer is directed onto a detectorusing a second beam splitter (BS1). The laser signal ispassed through a bandpass filter. At the zero points ofthe laser signal, the intensity at the interferogramdetector is measured. In this way, an interferogram isobtained.Compared to a Michelson interferometer, there is a

new factor of Dn in the argument of the cosine function.This is one of the major advantages of the polarizationinterferometer, because Dn is usually a small number.When the wedges are mechanically moved by a distanceof one wavelength, for example, the distance in thick-ness, x, is smaller because of the cutting angle of thewedge. Because of this and birefringence, the resultingphase difference will amount to only a fraction of 2p. Inthe case of TeO2 wedges (Dn � 0.15) cut at 20�, themechanical movements are optically reduced by about afactor of 40. The interferometer is therefore more ruggedand insensitive towards external mechanical distur-bances. In addition, there is no spatial separation intotwo partial beams, which have to be joined to interfere,but the phase shift is introduced between two orthogonalmodes of polarization, which propagate along the samedirection. This makes it easier to adjust the optical pathand to keep the optics aligned.The spectrum resulting from the Fourier transform of

the interferogram does not have an equidistant wave-number axis. The data points are equidistant inu ¼ �mDnð�m; T Þ and must be interpolated to an equidistant

Figure 6. Polarization interferometer (details explained in the text).

22 http://www.elsevier.com/locate/trac

wavenumber axis. For this dispersion correction, thebirefringence must be known as a function of wave-length and temperature.For Michelson interferometers, the nominal resolu-

tion automatically exceeds the requirements by at leastan order of magnitude, if we restrict our discussion toqualitative and quantitative NIR measurements forliquid and solid samples. This is not the case forpolarization interferometers, if the extra advantages ofcompact, affordable design are maintained. Themaximum retardation is given by the dimensions ofthe birefringent single crystal, which is cost effective. Itis therefore important to explore the effect of resolutionon the quality of NIR results, as discussed in thenext section. Polarization interferometers are used inthe current NIR instrument line of Buchi Labor-technik.

4. Optimum resolution for NIR spectroscopy

To evaluate the effect of resolution to NIR measure-ments, we selected a demanding quantitative applica-tion, the so-called BTX experiment. The objective was toquantify the components of mixtures of benzene, tolu-ene, o-xylene, m-xylene and p-xylene. The study used 50samples, carefully mixed from pure components in thefollowing concentration ranges: benzene 74–94%; tolu-ene 0–20%; and, xylene isomers 0–5%.With these samples, five sets of 150 spectra each were

recorded at resolutions of 2, 4, 8 and 16 cm�1 with aBruker Matrix FT-NIR spectrometer. The number ofscans was adapted to the resolution selected in such away that the same measuring time was available foreach record. Spectra were measured using a trans-mission probe with a 2-mm path-length.Of main interest were the standard errors of predic-

tions for the xylene isomers, which represented solidestimates for the limits of precision for quantifying liquidmixtures. The values were evaluated with Buchi NIRCal5.0 Chemometric Software and summarized in Table 1.Partial least squares (PLS) was applied to evaluate opti-mum calibrations. They were based on the spectra in therange 4150–6300 cm�1.The results show almost no relationship between res-

olution and the predicted quality of calibrations withinthe selected resolution range. This is due to the fact thatthe theoretical gain of information at higher resolution isbalanced by the lower signal-to-noise ratio. 16 cm�1

could therefore be chosen without significant losses. Inits commercial spectrometer with a polarization inter-ferometer, Buchi Labortechnik uses a nominal resolutionof 8 cm�1.For powder analyses too, resolution is not critical. It

was shown in one study that, for powder blends, reso-lutions better than 32 cm�1 did not improve the results

Table 1. Standard errors of predictions obtained for the quantifica-tion of xylene isomers with NIR

Resolution(cm�1)

SEP foro-xylene (%)

SEP form-xylene (%)

SEP forp-xylene (%)

16 0.023 0.020 0.0178 0.021 0.019 0.0134 0.038 0.035 0.0362 0.027 0.029 0.028

Calibration spectra were recorded with the same spectrometer atvarious resolutions. Results show that resolutions below 16 cm�1

do not significantly affect the precision attainable.

Trends in Analytical Chemistry, Vol. 25, No. 1, 2006 Trends

[10]. This limit was reported as 8 cm�1 for a powderblend with different ingredients [11].

5. Conclusions

Michelson interferometers are well established for FT-IRand FT-NIR spectroscopy. Their inherent weakness,sensitivity to mechanical disturbances, can be sup-pressed by various mechanical, optical or electronicmethods. The polarization interferometer provides a truealternative for NIR interferometry. First, it is insensitiveto mechanical distortions, and, second, compact designsfor optimal resolution are possible due to the availabilityof strongly birefringent material. Polarization interfer-ometers are therefore an excellent choice for spectro-meters to be used in quality control at site.Michelson interferometers have the advantage that

they can be switched to ultrahigh resolution, whichallows for the separation of water-vapor absorptionbands. Selected bands are ideal signals for the absolutecalibration of the wavenumber scale.For the calibration of polarization interferometers,

special gas samples with well-separated NIR signals haveto be used. This calibration is required only once duringfinal tests of the interferometer at the factory so it doesnot concern the user directly. For both solid and liquidsamples, the resolution of polarization spectrometers can

be selected so that optimum results are obtained even forthe most demanding measurements.

References

[1] A.G. Marshall, F.R. Verdun, Fourier Transforms in NMR, Optical,

and Mass Spectrometry, Elsevier, Amsterdam, The Netherlands,

1990.

[2] R.S. Jackson, Handbook of Vibrational Spectroscopy, John Wiley &

Sons, Chichester, West Sussex, UK, 2002.

[3] H.L. Buijs, G.L. Vail, J-N. Berube, US Patent No. 4,345,838, 1982

or L. Gerald, US Patent No. 4,693,603, 1985.

[4] J.M. Coffin, US Patent No. 5,883,712, 1999.

[5] P. Burkert, US Patent No. 4,383,762, 1983.

[6] A. Simon, J. Gast, A. Keens, US Patent No. 5,309,217, 1994.

[7] R.A. Hoult, US Patent No. 4,881,814, 1989.

[8] E. Wagner, M. Labhart, U. Glaus, US Patent No. 5,157,458, 1992.

[9] H. Soleil, C. R. Acad. Sci. 21 (1845) 426.

[10] O. Kolomiets, H.W. Siesler, J. NIR Spectrosc. 12 (2004) 271.

[11] H.S. Wang, K. Osaki, Y. Ozaki, Presentation, 12th International

Conference on NIR Spectroscopy, Auckland, New Zealand, 2005.

Thomas Meyer is working for Buchi Labortechnik AG as a project

manager in R&D. He is primarily engaged with the development of the

Buchi NIR spectrometer family. He received his Dr. Sc. Nat. in Physics

at the ETH Zurich in 1990.

Joachim Oelichmann is Marketing Manager for the Business Area

NIRSolutions at Buchi Labortechnik AG. Before joining Buchi, he had

been first Applications Manager and then Product Manager for IR/NIR

spectroscopy at Perkin Elmer in Germany for 16 years. He worked for

more than four years in research at Carl Zeiss in Oberkochen,

Germany. His main interests are industrial applications of vibrational

spectroscopy and software applications to spectroscopy. He studied

chemistry at the University of Dortmund, Germany, and did his

diploma and doctoral theses with Prof. Bernhard Schrader on infrared

and Raman spectroscopy.

Hanspeter Kellerhals is a consultant to Buchi Labortechnik AG and

other manufacturers and users of analytical instruments. Before

starting activities on his own, he was managing NMR and MS devel-

opment teams at Spectrospin AG, the Swiss production and research

center of the Bruker group of companies, for 18 years. His main interest

is the combination of modern computer techniques, such as multi-

variate analysis, with spectroscopic methods. He holds a PhD in physics

from the University of Basel and is the author of several patents and

publications in the fields of NMR, MS and computer applications.

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