Investigation of photometric errors in FTIR-spectra obtained inopen-path monitoring

U. Muller, R. Kurte, H.M. Heise*

Institut fur Spektrochemie und Angewandte Spektroskopie, Bunsen-Kirchhoff-Str. 11, D-44139 Dortmund, Germany

Received 25 August 1998; accepted 22 September 1998

Abstract

Open path monitoring of the atmosphere can be carried out using FTIR-spectrometers, for which high sensitive, nitrogencooled mercury–cadmium–telluride (MCT) detectors are routinely employed. Because of non-linearities in the MCT-detectorresponse during the interferogram measurement and not correctly compensated ambient blackbody emission, photometricinaccuracies arise for mainly employed bistatic instruments with the unmodulated source radiation being attenuated by atmo-spheric infrared absorbing species. Especially, the mid-infrared fingerprint-range is affected where many pollutants have theirstrongest spectral signatures. An important example is the determination of benzene in atmospheric air using the strong Q-branch absorption at 673.9 cm21, which is severely overlapped by then2 band of carbondioxide.q 1999 Elsevier Science B.V.All rights reserved.

Keywords:Fourier transform infrared spectroscopy; Bistatic instruments; Open-path monitoring; Detector non-linearity correction; Ambientblackbody emission; Photometric accuracy

1. Introduction

In recent years FTIR-spectroscopy has become animportant tool in gas analysis of the atmosphere,where emissions from a large number of sources ina wide range of concentrations can be monitored.Whilst robust spectrometers with different maximumspectral resolution are nowadays commercially avail-able for routine measurements, further emphasis forimproving the acceptance of this methodology has tobe laid on photometric and chemometric aspectsimportant in the quantitative evaluation of the spectra.In spite of the importance of such instrumentation, theexamination of the photometric accuracy especially at

the high absorbance end is still not without problems,as in particular, transmittance standards for calibra-tion have not been available hitherto. For laboratorymeasurements the use of secondary standards oforganic solvents measured in liquid cells [1], and ofmatt finished polystyrene films have been proposedfor a performance check of FTIR-spectrometers [2].

There is a large list of possible errors of differentmagnitude and of specific spectral influence in FTIR-spectroscopy [3]. Typical detectors for FTIR-spectro-meters are thermal pyroelectric DTGS detectors,which show linearity undisputed also for high photonfluxes [4], but miss the sensitivity needed for trace gasanalysis. This goal can be achieved by liquid nitrogencooled mercury–cadmium–telluride (MCT) detec-tors. However, these suffer from non-linearityproblems at high photon fluxes, which is critical inFT-spectroscopy when recording the interferogram.

Journal of Molecular Structure 482–483 (1999) 539–544

0022-2860/99/$ - see front matterq 1999 Elsevier Science B.V. All rights reserved.PII: S0022-2860(98)00673-5

* Corresponding author. Tel.: 0049 2311392215; fax: 00492311392120.

E-mail address:heise@isas-dortmund.de (H.M. Heise)

Although the problems caused by this for the photo-metric accuracy are described in the literature, it hasjust recently been addressed by analytical spectrosco-pists [5–7].

For open-path monitoring using FT-spectrometers,two primary configurations exist [8]. In the commonlyused bistatic configuration, the source is at one end ofthe path and the interferometer and detector at theother. There is the need to subtract the ‘detector portradiation’ seen within the field of view of the cooleddetector, which is emitted from the detector window,casing or post sample apertures [9]. This arises only inbistatic systems with an unmodulated infrared sourceat one end of the monitored atmospheric column. Theother setup is a so-called monostatic system withinfrared source and detector at the same end of thepath with the typical setup of a retroreflector giving an

optical path of twice the distance of the spectrometerand the latter element. This is similar to laboratoryinstruments, where the source radiation is modulatedby the interferometer before it is attenuated by thesample medium. For most monostatic systems thereis an additional beam-splitter at the interferometer exitport which can cause the occurance of stray light witha similar effect on the spectral measurement as theblackbody detector port radiation [10]. Owing to thesecond beam-splitter the radiation power available foropen-path monitoring is reduced compared to bistaticsystems.

For such a bistatic instrument we investigated theinfluence from the non-linear detector response andthe change in detector responsitivity when recordingsingle beam sample spectra and the ambient black-body emission as a result of a slight temperature

U. Muller et al. / Journal of Molecular Structure 482–483 (1999) 539–544540

Fig. 1. Raw (A) and corrected (B) single beam spectra of an SF6 sample and the ambient blackbody emission (respective difference spectra ofboth are shown in the insets).

change because of the different radiation powershitting the MCT-detector. A software correction ofthe non-linear detector response is applied to the inter-ferograms as described by Keens and Simon [11]. Anexample is provided for the evaluation of spectra fromair containing benzene concentrations in the ppb-range.

2. Experimental

Our spectral measurements at 0.2 cm21 resolutionwere carried out using a K300 FTIR-spectrometerfrom Kayser-Threde GmbH (Munich, Germany),which was equipped with an MCT-detector cooledby a Joule-Thompson cryostat and a White cell fromBastian Feinmechanik (Wuppertal, Germany)offering a variable optical pathlength (maximum110 m). The spectra were calculated by means of aPC using programs for interferogram processing andFourier transformation written in MATLAB (TheMathworks Inc., South Natick, MA). The K300 spec-trometer gives two sided interferograms, so thatpower spectra were calculated avoiding phase errorcorrection.

Test gases of different SF6 concentrations weregenerated from a commercial test gas (104 vppm)from Messer Griesheim (Krefeld, Germany) and dilu-tion by N2 using critical orifices of different holediameters and accurate backpressure measurements[12]. For providing spectra with zero transmissionintervals, gas samples were produced manometricallyfrom SF6 of purity 99.9% and CO2 (4% in N2), bothfrom Messer Griesheim. Further N2 was added toyield a total pressure of 1000 hPa.

3. Results and discussion

The approach using a post-processing software toaccount for photometric errors in single beam spectraowing to a non-linear detector response has someremarkable advantages, because no elaborate hard-ware correction is necessary which requires substan-tial fore-knowledge on the detector photoelectricalcharacteristics. Reducing the photon flux by anattenuator or an optical filter can reduce non-linearityeffects, on the other side the signal-to-noise ratio islowered to an undesirable fraction. Within the

patented algorithm two factors are calculated fromthe original single beam spectrum, so that a secondorder approximation to the true interferogram can becalculated which undergoes in the second step aconventional Fourier transformation [11].

For demonstrating the effects from stronglyabsorbing samples we used gas samples of CO2 andSF6 and mixtures of both at different optical path-lengths. In Fig. 1A the single beam spectra of anSF6 sample are shown along with the ambient detectorport blackbody emission spectrum calculated from themeasured original interferograms. The effect from thenon-linear detector response is evident from the posi-tive signal below the MCT-detector cutoff at about550 cm21. In the inset the subtraction result isshown elucidating serious photometric deviations forthe opaque spectral regions. In Fig. 1B the spectracorrected for the non-linear detector response arepresented where the effects for the ambient blackbodyemission spectrum are negligible because of the factthat its integral radiation power is only a fraction of0.03 compared to the SF6 single beam spectrum.

We noticed that even after correction the opaqueregions in the sample single beam spectrum did notmatch the amplitudes of the blackbody emission spec-trum, but were lower. This would result in unaccep-table negative intensity values when compensating forthe latter by subtraction. A remedy is to apply anadapted factor slightly above 1 for scaling the samplesingle beam spectrum up to reach a better agreementfor the intensity values in the spectral intervals withzero transmission. For the spectra recorded at 6.90 mthe matching was done taking into account theinterval below 1000 cm21 (scaling factor 1.031).The final result of the subtraction based on a scaledspectrum from a corrected interferogram is shown inthe inset of Fig. 1B.

The same procedures were applied for spectrarecorded, for example, at an optical pathlength of42 m. Owing to the multiple reflections in the cell,by a factor of 0.58 lower integral intensity is measuredcompared to the spectrum at the shortest optical pathavailable with the White cell. As expected, the effectsfrom the non-linear detector response are reducedwhich is obvious again below the detector cuttoffwavenumbers. Another positive result is that thefactor for scaling the single beam sample spectrumis smaller than for the previous case (1.017). The

U. Muller et al. / Journal of Molecular Structure 482–483 (1999) 539–544 541

U. Muller et al. / Journal of Molecular Structure 482–483 (1999) 539–544542

Fig. 2. Calibration curve for SF6 absorbance at 948 cm21 for various gas mixing ratios using different evaluation procedures; abbreviations inthe legend: ambient blackbody emission (abbe), detector non-linearity (dnl).

Fig. 3. A Single beam spectra of resolution 1 cm21, necessary for the determination of benzene in air;B logarithmized, single beam sample andbenzene free atmospheric spectra, adjusted by least-squares for the same CO2 absorbance andC difference spectrum showing the unscaledbenzene Q-branch (see also text).

correction using linear and quadratic terms for themeasured interferogram produces an even better resultthan shown in Fig. 1B within the opaque intervals.

In Fig. 2 the results from the different spectrumevaluation procedures are shown, where themaximum absorbance values of SF6 at 947.9 cm21

are plotted versus different concentrations. The largestphotometric errors are certainly introduced byneglecting the ambient blackbody emission, the corre-sponding calibration curve is the lowest in Fig. 2.Subtracting the latter spectrum, but without correctionfor detector nonlinearity, the second curve from belowis obtained, whereas the most upper line is calculatedafter application of that correction. Under these condi-tions, the single beam spectrum at sample transmis-sion zero not yet matches the same intensity of theambient blackbody emission spectrum, which hadbeen treated in the same way as the sample spectrum.After application of a correction factor to the samplespectrum to cope with slightly reduced detectorresponsivity, the spectra are prepared for the calcula-tion of unperturbed absorbance values.

These considerations have an impact on the quan-titative determination of benzene in the atmosphere.The strongest benzene absorption band is found at673.9 cm21 which is convenient to detect low concen-trations; on the other side this Q-branch is heavilysuperimposed by the strongn2 absorption band fromCO2. For the evaluation the single beam spectra of thebenzene contaminated air and that of an atmosphericbackground spectrum measured at a comparable CO2

absorption are necessary. In addition, another back-ground spectrum, preferably a short-path spectrum,for which the source radiation has to be attenuatedto avoid detector saturation, and the ambient black-body emission spectrum are required. An example ofthe scaled spectra is shown in Fig. 3A. The ordinaryprocedure as described before, did not lead to satis-factory difference spectra after CO2 absorbancecompensation because of large absorbance noise.Therefore we chose a least squares fit of the CO2 R-branch lines in the logarithmized single beam spectra(671.8–673.3 cm21 and 674.2–676.5 cm21) with nocorrection of the ambient blackbody emission and apostscaling of the absorbance values according to theadjusted single beam intensities. For this, the Q-branch absorption at 667.5 cm21 provides the photo-metric anchor-point, whereas the short-path back-

ground was matched using spectral data below600 cm21 and above 750 cm21. In Fig. 3B the loga-rithmized atmospheric single beam spectra adjusted inintensities by least-squares are shown, whereas in Fig.3C the subtraction result is plotted. A factor of 9.46was necessary to yield the unperturbed absorbancevalue taking the ambient blackbody emission intoaccount. Using spectral benzene reference data [13],a concentration of 104 ppb was derived compared tothe test gas preparation of 100 ppb. Atmosphericspectra up to a pathlength of 150 m can be utilized.

4. Conclusions

Improvements for the accuracy in the absorbancescale using the procedures above are significant. Thishas its special consequences when strong absorptionbands arising from atmospheric water and carbondi-oxide have to be compensated without the availabilityof exactly matched atmospheric background spectra,ideally recorded under the same atmospheric andpathlength conditions.

Acknowledgements

The authors acknowledge gratefully the financialsupport by the Ministerium fu¨r Schule und Weiterbil-dung, Wissenschaft und Forschung des Landes Nord-rhein-Westfalen and the Bundesministerium fu¨rBildung Wissenschaft, Forschung und Technologie.Drs. Andrea Ga¨rtner and T. Ha¨usler from the Land-esumweltamt (Essen, Germany) are thanked for thesupply of spectrum of benzene in air.

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