DE102018000307B4 - Static Fourier transform spectrometer and a method for operating the static Fourier transform spectrometer - Google Patents

Abstract

Static Fourier transformation spectrometer with a radiation source (4), which is set up to emit infrared radiation during operation of the static Fourier transformation spectrometer (1), which during operation along an optical path (5) of the static Fourier transformation Spreads spectrometer (1), which has a sample path (14) and a reference path (15), a sample (7), which is only arranged in the sample path (14) and thus interacts with the infrared radiation of the sample path (14) during operation and an interferometer section (3), which is set up to generate a sample interferogram (27) of the sample path (14) after the interaction with the sample (7) and a reference interferogram (28) of the reference path (15), and a detector ( 11), which is set up to record the sample interferogram (27) and the reference interferogram (28) at the same time, the static Fourier transformation spectrometer (1) ei It is directed to determine a sample spectrum (20) from the sample interferogram (27) and a reference spectrum (21) from the reference interferogram (28) by means of a Fourier transformation and to correct the sample spectrum (20) with the reference spectrum (21).

Description

The invention relates to a static Fourier transformation spectrometer and a method for operating the static Fourier transformation spectrometer.

In a Fourier transform spectrometer, a sample is irradiated with infrared radiation. After the infrared radiation has passed through the sample, the infrared radiation is divided into two partial beams and the two partial beams are brought together again, so that the infrared radiation forms an interferogram. The Fourier transformation spectrometer conventionally has a Michelson interferometer, in which a mirror is moved and a variation of the optical path length of one of the two partial beams is thus generated.

In addition to the Fourier transform spectrometers with the Michelson interferometer, there is a so-called static Fourier transform spectrometer, which is designed without a movable mirror. In the static Fourier transformation spectrometer, the two partial beams are combined on a detector in such a way that the difference in the optical path length of the two partial beams is varied along one dimension of the detector. Such a static Fourier transform spectrometer is, for example, in WO 2016/180551 A1 described. With the static Fourier transform spectrometer, spectra can be recorded in a shorter time than with the Fourier transform spectrometer with the Michelson interferometer, but the spectra with the static Fourier transform spectrometer are less accurate.

The object of the invention is therefore to create a static Fourier transformation spectrometer and a method for operating the static Fourier transformation spectrometer, with which spectra can be measured with a high degree of accuracy.

The static Fourier transformation spectrometer according to the invention has a radiation source which is set up to emit infrared radiation in an operation of the static Fourier transformation spectrometer, which in operation propagates along a beam path of the static Fourier transformation spectrometer which has a sample path and has a reference path, a sample which is only arranged in the sample path and thus enters into an interaction with the infrared radiation of the sample path during operation, and an interferometer section which is set up, a sample interferogram of the sample path after the interaction with the sample and a reference interferogram of the Generate reference path, and has a detector which is set up to record the sample interferogram and the reference interferogram simultaneously, the static Fourier transformation spectrometer being set up by means of a Fourier transformation determine a sample spectrum from the sample interferogram and a reference spectrum from the reference interferogram, and correct the sample spectrum with the reference spectrum.

The method according to the invention for operating the static Fourier transformation spectrometer has the steps: a2) introducing a sample only into the sample path; b) spreading infrared radiation along the beam path so that the sample interacts with the infrared radiation of the sample path; c) generating a sample interferogram of the sample path after the interaction with the sample and a reference interferogram of the reference path and simultaneous recording of the sample interferogram and the reference interferogram; d) determining a sample spectrum from the sample interferogram and a reference spectrum from the reference interferogram by means of a Fourier transformation in each case; e) Correcting the sample spectrum with the reference spectrum.

Surprisingly, it was found that the corrected sample spectra can be measured with a high degree of accuracy using the static Fourier transformation spectrometer according to the invention and the method according to the invention for operating the static Fourier transformation spectrometer. The accuracy achieved was almost as high as that of a conventional Fourier transform spectrometer with a Michelson interferometer.

The interaction of the sample with the infrared radiation can be, for example, transmission, reflection, scattering, remission or attenuated total reflection (English: attenuated total reflection, ATR).

It is preferred that the detector has a sample area that is set up to record the sample interferogram and a reference area that is set up to record the reference interferogram, the sample area being three to ten times as large as the reference area. It was surprisingly found that the signal to noise ratio is particularly high with this area ratio, as a result of which the corrected sample spectra have a particularly high accuracy.

The sample path and the reference path are preferably arranged spatially separated from one another in the region of the sample. As a result, the sample can advantageously be arranged in the sample path such that no end of the sample is arranged in the beam path, which would disadvantageously lead to the formation of interference. The interference would reduce the accuracy of the corrected spectra.

Alternatively, it is preferred that the entire beam path from the radiation source up to and including the sample is arranged in a non-separated manner. This is an advantageously simple structure of the static Fourier transformation spectrometer, which can also be adjusted particularly easily.

It is preferred that the static Fourier transformation spectrometer has an ATR crystal which is arranged in the sample path, the sample contacting the surface of the ATR crystal, so that in operation the infrared radiation is reflected at the interface between the and total reflection ATR crystal and the sample spreads. Because the evanescent waves generated by the total reflection have only a small depth of penetration into the sample, water produces only a low absorption. This makes the ATR crystal particularly suitable for aqueous samples, so that the aqueous samples can be measured with a particularly high level of accuracy.

The optical path lengths of the sample path and the reference path are preferably of the same length. As a result, the accuracy of the corrected spectra is particularly high. It is preferred that the beam path has several of the sample paths in which different samples are arranged. As a result, several of the samples can advantageously be measured simultaneously. The detector preferably has a two-dimensional matrix of infrared-sensitive elements. The size of the sample area and the reference area can be selected here via the number of infrared-sensitive elements, because the number of infrared-sensitive elements is proportional to the size of the respective area. Alternatively, the detector preferably has a line detector for the reference path and a line detector for each of the sample paths.

The method preferably has the step: a0) selecting a sample area of a detector which is set up to record the sample interferogram is three to ten times larger than a reference area of the detector which is set up to record the reference interferogram. In the event that the detector has the two-dimensional matrix on the infrared-sensitive elements, the size of the sample area and the size of the reference area can be selected in accordance with the number of infrared-sensitive elements.

It is preferred that the method has the steps: f) performing steps b) to d) without the sample being arranged in the sample path; g) calculating at least one correction factor in order to scale the sample spectrum determined in step f) and the reference spectrum determined in step f) such that a difference between the intensity of the sample spectrum and the intensity of the reference spectrum is minimized; and wherein in step e) the correction factor is used. With the correction factor, an uneven illumination of the detector by the radiation source can advantageously be corrected, which leads to an increased accuracy for the corrected sample spectra.

The method preferably has the steps: a1) arranging an ATR crystal in the sample path and performing steps b) to e) without the sample contacting the surface of the ATR crystal; h) repeating step a1) and comparing the sample spectra corrected in steps a1) and h). This advantageously shows aging of the ATR crystal or material deposition on the ATR crystal.

It is preferred that the method has the step: a0) arranging a reference sample in the reference path. In this way, differences between the sample and the reference sample can be measured with particularly high accuracy. If, for example, a particularly pure reference sample is arranged in the reference path, contamination of the sample can be detected particularly precisely and moreover quickly.

• 1 zeigt eine erste Ausführungsform des statischen Fourier-Transformations-Spektrometers in einer ersten Draufsicht.
• 2 zeigt die erste Ausführungsform in einer zweiten Draufsicht.
• 3 zeigt eine Draufsicht auf eine zweite Ausführungsform des statischen Fourier-Transformations-Spektrometers.
• 4 zeigt eine Draufsicht auf eine dritte Ausführungsform des statischen Fourier-Transformations-Spektrometers.
• 5 zeigt eine Draufsicht auf eine vierte Ausführungsform des statischen Fourier-Transformations-Spektrometers.
• 6 zeigt von einem Detektor des statischen Fourier-Transformations-Spektrometers aufgenommene Messdaten.
• 7 bis 10 zeigen von dem statischen Fourier-Transformations-Spektrometer ermittelte Spektren sowie deren Korrekturen.
• 11 zeigt einen Versuch, bei dem die Probenfläche und die Referenzfläche des Detektors variiert wurden.

The invention is explained in more detail below with the aid of the attached schematic drawings.

• 1 shows a first embodiment of the static Fourier transform spectrometer in a first plan view.
• 2 shows the first embodiment in a second plan view.
• 3 shows a top view of a second embodiment of the static Fourier transform spectrometer.
• 4 shows a top view of a third embodiment of the static Fourier transform spectrometer.
• 5 shows a top view of a fourth embodiment of the static Fourier transform spectrometer.
• 6 shows measurement data recorded by a detector of the static Fourier transformation spectrometer.
• 7 to 10 show spectra determined by the static Fourier transformation spectrometer and their corrections.
• 11 shows an experiment in which the sample area and the reference area of the detector were varied.

Like it out 1 to 5 can be seen, has a static Fourier transform spectrometer 1 a radiation source 4 which is set up in an operation of the static Fourier transformation spectrometer 1 To emit infrared radiation. The static Fourier transform spectrometer 1 points in the direction of propagation of the infrared radiation towards the radiation source 4 a sample section 2 and after the sample section, an interferometer section 3 on. The infrared radiation can have a wavelength of 2 µm to 20 µm, for example. The radiation source 4 can be set up to emit the infrared radiation in a continuous wave mode. Alternatively, the radiation source 4 be set up to emit the infrared radiation and emit it with a duty cycle of more than 50%. The infrared radiation spreads along a beam path during operation 5 of the static Fourier transform spectrometer 1 out. The ray path 5 has a sample path 14 and a reference path 15 on. The static Fourier transform spectrometer 1 has a collection optics 6 on that in the beam path 5 is arranged, which comes from the radiation source 4 parallelize emerging infrared radiation.

The static Fourier transform spectrometer 1 assigns a sample 7 on that in the sample section 2 and only there in the sample path 14 is arranged and thus an interaction with the infrared radiation of the sample path during operation 14 received. The interferometer section 3 a sample interferogram is set up 27 of the sample path 14 after interacting with the sample 7 and a reference interferogram 28 of the reference path 15 to create. The interferometer section 3 has a detector 11 that is set up, the sample interferogram 27 and the reference interferogram 28 to record at the same time. The static Fourier transformation spectrometer is set up by means of a Fourier transformation from the sample interferogram 27 a spectrum of samples 20 and from the reference interferogram 28 a reference spectrum 21 to determine as well as the sample spectrum 20 with the reference spectrum 21 to correct so that one with the reference spectrum 21 corrected sample spectrum 22 arises.

In particular 1 and 2 illustrate the operation of the interferometer section 3 , 1 shows a top view of the static Fourier transform spectrometer 1 where the sample path 14 and the reference path 15 are arranged side by side. 2 shows a top view of the static Fourier transform spectrometer 1 , in which the arrangement 1 90 ° around the optical axis of the static Fourier transform spectrometer 1 is rotated. This is in 1 by an arrow marked x, the one x Direction, and in 2 represented by an arrow labeled y, which denotes a y direction. The x Direction and the y Direction include an angle of 90 °. The interferometer section 3 has a split plane 8th , in which a gap can be arranged and via which the infrared radiation during operation into the interferometer section 3 entry. The gap has in the x Direction a gap height and in the y Direction a gap width. The gap height can be chosen so that the detector 11 in the x Direction is fully illuminated by the infrared radiation. The gap can be such that the gap width is variably adjustable. This allows the spectral resolution to be changed.

In addition, the interferometer section points 3 a first lens 9 and a second lens 10 on that in the beam path 5 are arranged, the split level 8th on the detector 11 map. Furthermore, the interferometer section 3 a beam splitter 12 and a mirror 13 on that in the beam path 5 between the first lens 9 and the second lens 10 are arranged. 2 shows, in operation, part of the infrared radiation the beam splitter 13 happens and then via the second lens 10 on the detector 11 arrives. Another part of the infrared radiation is from the beam splitter 12 and then from the mirror 13 reflects and then passes through the second lens 10 on the detector 11 , One part and the other part of the infrared radiation together form the sample interferogram on the detector 27 and the reference interferogram 28 that in the x Direction are arranged side by side. One part and the other part of the infrared radiation, because they have traveled a different path in the interferometer section, have an optical path length difference (which can also be zero) when they are on the detector 11 arrived. This optical path length difference is experienced in the y -Direction a variation 16 and this variation 16 leads to the sample spectrum using the Fourier transformations 20 and the reference spectrum 21 can be determined. This and other details about the interferometer section 3 are in WO 2016/180551 A1 described.

1 and 2 show a first embodiment of the static Fourier transform spectrometer 1 , in which the entire beam path 5 from the radiation source 4 up to and including the sample 7 is arranged separately, which means that the sample path 14 and the reference path 15 are arranged immediately adjacent to each other. In addition, the entire beam path is from the sample 7 up to the detector 11 arranged separately. 3 shows a second embodiment of the static Fourier transform spectrometer 1 , at the sample path 14 and the reference path 15 in the area of the sample 7 are spatially separated from each other, which means that here there is a space between the sample path 14 and the reference path 15 is arranged. The full sample path can be advantageous here 14 through the sample 7 can be passed through without diffraction of the infrared radiation of the sample path 14 at one end of the sample 7 takes place. In the first embodiment and the second embodiment, the transmission of the sample 7 measurable. At the rehearsal 7 For example, it can be a liquid sample that is arranged in a cuvette. It is also conceivable that the sample 7 is a solid sample.

4 shows a third embodiment of the static Fourier transform spectrometer 1 where the static Fourier transform spectrometer is a gas measuring cell 17 having. The sample 7 is gaseous and inside the gas measuring cell 17 arranged and the sample path 14 runs inside the gas measuring cell 17 , The gas measuring cell 17 can, as in 4 shown a first curved mirror 18a and a second curved mirror 18b have, which are arranged such that the infrared radiation multiple times within the gas measuring cell 17 is reflected. It is also conceivable that the static Fourier transformation spectrometer 1 has a further gas measuring cell which is structurally identical to the gas measuring cell 17 is and through which the reference path 15 runs.

5 shows a fourth embodiment of the static Fourier transform spectrometer 1 , where the static Fourier transform spectrometer 1 an ATR crystal 19 having in the sample path 14 is arranged, the sample 7 the surface of the ATR crystal 19 contacted, so that during operation the infrared radiation under total reflection at the interface between the ATR crystal 19 and the sample 7 spreads. It is conceivable that the static Fourier transform spectrometer 1 has another ATR crystal that is structurally identical to the ATR crystal 19 and that is in the reference path 15 is arranged.

The detector 11 can have a two-dimensional matrix of infrared-sensitive elements. Alternatively, the detector 11 can use a line detector for the reference path 15 and a line detector for the sample path 14 exhibit. In the event that the static Fourier transform spectrometer 1 a plurality of the sample paths 14 in which different samples 7 are arranged, the static Fourier transform spectrometer 1 a respective line detector for each sample path 14 on.

6 shows a measurement of the intensity of infrared radiation, the measurement with the detector 11 was recorded with the two-dimensional matrix on the infrared-sensitive elements. For this purpose, the sample was in step a2) 7 only in the sample path 14 brought in. As the sample 7 a plastic film was used. In step b) the infrared radiation is along the beam path 5 spread out so that the sample 7 an interaction with the infrared radiation of the sample path 14 received. In step c) the sample interferogram is 27 of the sample path 14 after interacting with the sample 7 and the reference interferogram 28 of the reference path 15 generated as well as the sample interferogram 27 and the reference interferogram 28 recorded at the same time. The detector points to this 11 how it out 6 one can see a sample area 29 that is set up, the sample interferogram 27 record, and a reference surface 30 that is set up, the reference interferogram 28 take. The detector 11 can also be an interface 31 have between the sample surface 29 and the reference surface 30 is arranged and on which no usable interferogram is produced. This may be due to an insufficiently precise adjustment of the sample, for example 7 in the beam path 5 to be attributed. It is also conceivable that this is due to the fact that the radiation source 4 has a non-punctiform extension.

Like it out 6 can be seen, the detector 11 a plurality of lines in the x Direction are arranged side by side, and a plurality of columns in the y Direction are arranged side by side. To generate the sample interferogram 27 in step c) all of the sample area 29 measured intensities averaged in columns, so that a vector I _sample (column) is formed. To generate the reference interferogram 28 can all from the reference surface 30 measured intensities are column-averaged so that a vector I _Ref (column) is formed. You can then use the variation 16 the optical path length difference and the speed of light of the vector I _sample (column) into a vector I _sample (t) and the vector I _ref (column) into a vector I _ref (t), where t is the time. The variation 16 The optical path length difference can be determined, for example, by using a standard sample with a known spectrum in the sample path 14 is inserted and measured.

In a step d) the sample spectrum 20 from the sample interferogram 27 and the reference spectrum 21 from the reference interferogram 28 determined by means of a Fourier transformation. In step d) the sample spectrum can 20 from the vector I _sample (t) using the Fourier transform and the reference spectrum 21 can be determined from the vector I _Ref (t) by means of the Fourier transformation. 7 shows the sample spectrum 20 and the reference spectrum 21 in a plot in which the frequency in cm ^{-1 is} plotted on the abscissa and the intensity in arbitrary units is plotted on the ordinate.

In a step e) the sample spectrum 20 with the reference spectrum 21 corrected. For this purpose, the intensity of the sample spectrum is used for each frequency 20 by the intensity of the reference spectrum 20 divided. This with the reference spectrum 21 corrected sample spectrum 22 is in 8th presented in a plot in which the frequency in cm ^{-1 is} plotted on the abscissa and a transmission in percent is plotted on the ordinate. An FTIR spectrum was used for comparison 23 of the plastic film by means of a Fourier transformation spectrometer, which has a Michelson interferometer, and also in the application in 8th entered. It can be seen that this is with the reference spectrum 21 corrected sample spectrum 22 still significant from the FTIR spectrum 23 differs. This may be due to the fact that in the sample path 14 and in the reference path 15 there is a different intensity of the infrared radiation, which indicates an incorrect adjustment, in particular of the radiation source 4 and the collection optics 6 is due.

To the different intensity in the sample path 14 and in the reference path 15 To correct, steps b) to d) can be carried out in a step f) without the sample 7 in the sample path 14 is arranged. 9 shows a sample spectrum determined in step f) 24 and a reference spectrum determined in step f) 25 which are entered in a plot in which the frequency in wave numbers is plotted on the abscissa and the intensity in arbitrary units is plotted on the ordinate. A correction factor is then calculated in a step g) by the sample spectrum determined in step f) 24 and the reference spectrum determined in step f) 25 to scale each other such that there is a difference between the intensity of the sample spectrum 20 and the intensity of the reference spectrum 21 is minimized. In step e) the sample spectrum is corrected 20 the correction factor is also used. The correction factor can be determined, for example, by the areas below the sample spectrum determined in step f) 24 and below the reference spectrum determined in step f) 25 be related. Alternatively, it is also possible to calculate a correction factor for each frequency. 10 shows one with the reference spectrum 21 and sample spectrum corrected with the correction factor 26 in a plot in which the frequency in wave numbers is plotted on the abscissa and the transmission in percent is plotted on the ordinate. The FTIR spectrum is also for comparison 23 entered. The remaining differences can be achieved by lower spectral resolution of the static Fourier transform spectrometer 1 compared to the Fourier transform spectrometer with the Michelson interferometer.

An experiment was subsequently carried out in which the ratio of the sizes of the sample area 29 and the reference surface 30 was varied. This was done by using the first embodiment of the static Fourier transform spectrometer 1 the plastic film in the x -Direction at different depths in the beam path 5 was pushed in. This changed the sizes of the sample path 14 and the reference path 15 and thus the sizes of the sample area 29 and the reference surface 30 , Steps b) to e) were carried out for each position of the plastic film, and each one with a reference spectrum 21 corrected sample spectrum 20 added. Then each of the corrected sample spectra 20 with the FTIR spectrum 23 compared by _minimizing the expression χ ² = Σ _{column = 1..n} (I _sample (column) - α * I _FTIR (column)) ² by varying a factor a, where I _sample (column) is the corrected sample spectrum 20 , I _FTIR (column) the FTIR spectrum 23 and n is the number of columns. In addition, a ratio A (sample area) / A (reference area) was formed for each position of the plastic film, where A (sample area) is the size of the sample area 29 and A (reference area) the size of the reference area 30 is. 11 shows a plot in which the ratio is plotted on the abscissa and the respective χ ^{2 is} plotted on the ordinate. It applies that the lower the χ ² , the smaller the deviations of the corrected sample spectrum 22 to the FTIR spectrum 23 are. Surprisingly, it was found from a ratio of 3 to a ratio of 10 that χ ^{2 is} significantly lower than in the remaining range. This is due to the fact that in this area the signal-to-noise ratio of the corrected sample spectra 22 is significantly higher than in the remaining area.

LIST OF REFERENCE NUMBERS

1

Static Fourier transform spectrometer

2

sample section

3

interferometer

4

radiation source

5

beam path

6

collection optics

7

sample

8th

cleavage plane

9

first lens

10

second lens

11

detector

12

beamsplitter

13

mirror

14

sample path

15

reference path

16

Variation of the optical path length difference

17

Gas cell

18a

curved mirror

18b

curved mirror

19

ATR crystal

20

sample spectrum

21

reference spectrum

22

Sample spectrum corrected with reference spectrum 21

23

FTIR spectrum

24

Sample spectrum determined in step f)

25

Reference spectrum determined in step f)

26

Sample spectrum corrected with reference spectrum 21 and correction factor

27

Probeninterferogramm

28

reference interferogram

29

sample area

30

reference surface

31

interface

x

-Direction

y

-Direction

Claims (15)

Static Fourier transformation spectrometer with a radiation source (4), which is set up to emit infrared radiation during operation of the static Fourier transformation spectrometer (1), which during operation along an optical path (5) of the static Fourier transformation Spreads spectrometer (1), which has a sample path (14) and a reference path (15), a sample (7), which is only arranged in the sample path (14) and thus interacts during operation with the infrared radiation of the sample path (14) and an interferometer section (3), which is set up to generate a sample interferogram (27) of the sample path (14) after interaction with the sample (7) and a reference interferogram (28) of the reference path (15), and a detector ( 11), which is set up to record the sample interferogram (27) and the reference interferogram (28) at the same time, the static Fourier transformation spectrometer (1) ei It is directed to determine a sample spectrum (20) from the sample interferogram (27) and a reference spectrum (21) from the reference interferogram (28) by means of a Fourier transformation and to correct the sample spectrum (20) with the reference spectrum (21). Static Fourier transform spectrometer according to Claim 1 , wherein the detector (11) has a sample surface (29) which is set up to record the sample interferogram (27) and a reference surface (30) which is set up to record the reference interferogram (28), the sample surface (29) three times up to ten times the reference surface (30). Static Fourier transform spectrometer according to Claim 1 or 2 The sample path (14) and the reference path (15) are arranged spatially separated from one another in the region of the sample (7). Static Fourier transform spectrometer according to Claim 1 or 2 , wherein the entire beam path (5) from the radiation source (4) up to and including the sample (7) is arranged separately. Static Fourier transform spectrometer according to one of the Claims 1 to 4 , wherein the static Fourier transformation spectrometer (1) has an ATR crystal (19) which is arranged in the sample path (14), the sample (7) contacting the surface of the ATR crystal (19) so that during operation, the infrared radiation propagates under total reflection at the interface between the ATR crystal (19) and the sample (7). Static Fourier transform spectrometer according to one of the Claims 1 to 5 , wherein the optical path lengths of the sample path (14) and the reference path (15) are the same length. Static Fourier transform spectrometer according to one of the Claims 1 to 6 , wherein the beam path (5) has several of the sample paths (14) in which different samples (7) are arranged. Static Fourier transform spectrometer according to one of the Claims 1 to 7 , wherein the detector (11) has a two-dimensional matrix of infrared-sensitive elements, or wherein the detector (11) has a line detector for the reference path (15) and a line detector for each of the sample paths (14). Static Fourier transform spectrometer according to one of the Claims 1 to 8th , wherein the radiation source (4) is set up to emit the infrared radiation in a continuous wave mode. Method for operating a static Fourier transformation spectrometer (1) which has a beam path (5) which has a sample path (14) and a reference path (15), comprising the steps: a2) introducing a sample (7) only into the sample path (14); b) spreading infrared radiation along the beam path (5) so that the sample (7) interacts with the infrared radiation of the sample path (14); c) generating a sample interferogram (27) of the sample path (14) after the interaction with the sample (7) and a reference interferogram (28) of the reference path (15) and simultaneous recording of the sample interferogram (27) and the reference interferogram (28); d) determining a sample spectrum (20) from the sample interferogram (27) and a reference spectrum (21) from the reference interferogram (28) by means of a Fourier transformation in each case; e) correcting the sample spectrum (20) with the reference spectrum (21). Procedure according to Claim 10 , with the step: a0) selecting a sample area (29) of a detector (11) which is set up to record the sample interferogram (27) three to ten times as large as a reference area (30) of the detector (11) which is set up to record the reference interferogram (28). Procedure according to Claim 10 or 11 , with the steps: f) performing steps b) to d) without the sample (7) being arranged in the sample path (14); g) calculating at least one correction factor in order to scale the sample spectrum (24) determined in step f) and the reference spectrum (25) determined in step f) such that there is a difference between the intensity of the sample spectrum (20) and the intensity of the reference spectrum (21) is minimized; and wherein in step e) the correction factor is used. Method according to one of the Claims 10 to 12 , with the steps: a1) arranging an ATR crystal (19) in the sample path (14) and performing steps b) to e) without the sample (7) contacting the surface of the ATR crystal (19); h) repeating step a1) and comparing the sample spectra (22) corrected in steps a1) and h). Method according to one of the Claims 10 to 13 , with the step: a0) arranging a reference sample in the reference path. Method according to one of the Claims 10 to 14 , wherein in step b) the infrared radiation is spread in a continuous wave mode.

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