DE102014113354B4 - Method for spectrometry and spectrometer - Google Patents

Abstract

und
Einstellen des Einfallswinkels α, so dass gilt α = Θ_B,Mittel.

Method for spectrometry on a sample (12) comprising the steps of providing the sample (12) in the form of a chemical substance (14) between at least two planar sections of a packaging material (15, 16, 17), irradiating the sample (12) with electromagnetic Radiation (2) having a plurality of frequencies or a frequency band from a frequency range of 1 GHz to 30 THz, wherein the electromagnetic radiation (2) at an incident angle α on one of the flat sections of packaging material (15, 16, 17) impinges, and frequency-resolved acquisition of a measure of the intensity of the electromagnetic radiation (2) emitted at the angle of incidence on the sample (12) and transmitted through the sample (12) or reflected by the sample (12) as a spectrum,
characterized in that the method additionally comprises the steps of:
Determining a type of packaging material (15, 16, 17) of the sample (12), determining a mean Brewster angle Θ _{B, means} for packaging materials (15, 16, 17) of this type at a frequency in the frequency domain, with the steps of determining Maximum value Θ _{B, max} for the Brewster angle of packaging materials (15, 16, 17) of this type,
Determining a minimum value Θ _{B, min} for the Brewster angle of packaging materials (15, 16, 17) of this type and
Calculating a mean Brewster angle Θ _{B, means} for packaging materials (15, 16, 17) of this kind as $${\mathrm{\Theta }}_{B.Mittel}=\frac{{\mathrm{\Theta }}_{B.min}+{\mathrm{\Theta }}_{B.max}}{2}$$

and
Adjusting the angle of incidence α such that α = Θ _{B, mean} .

Description

The present invention relates to a method for spectrometry on a sample comprising the steps of providing the sample in the form of a chemical substance between at least two planar sections of a packaging material, irradiating the sample with electromagnetic radiation and a plurality of frequencies or a frequency band from a frequency range of 1 GHz to 30 THz, wherein the electromagnetic radiation impinges on one of the flat sections of packaging material at an angle of incidence α, and frequency-resolved detection of a measure of the intensity of the radiated at the angle of incidence on the sample and transmitted through the sample or reflected from the sample electromagnetic Radiation as a spectrum.

In a frequency range of electromagnetic radiation between 1 GHz and 30 THz, also referred to as terahertz frequency range or far-infrared frequency or spectral range, many materials, such as paper, paperboard, ceramics and a variety of plastics, are transparent and can be screened. This results in potential applications in particular for imaging systems in quality control, packaging control, security control and for the monitoring of chemical reactions.

The frequency of 1 THz corresponds to a wavelength of the electromagnetic wave of 300 μm and a photon energy of 4.14 meV or 33 wave numbers. The radiation is therefore non-ionizing and unlike, for example, X-radiation at the other end of the electromagnetic spectrum, it does not encounter health concerns.

However, systems operating in said frequency range of the electromagnetic spectrum are used not only for the purpose of imaging but also for the identification of chemical substances, i. for spectrometry. It has in fact been found that certain chemical substances, for example those which are safety-relevant, such as anthrax or explosives, have characteristic absorption bands in the aforementioned frequency range. If it succeeds, a spectrum, i. To record a frequency-resolved measure of the intensity of the reflected or transmitted by a chemical substance electromagnetic radiation in said frequency range, this spectrum can be compared with known reference spectra, which are stored for example in a database, and the substance can be identified.

Of particular interest is the identification of chemical substances in multi-layered structures, such as those found in packaging such as cartons or envelopes. However, it has been found that even if the substance contained in a packaging has a characteristic spectrum in the aforementioned frequency range, the substances can not always be clearly identified, since the packaging itself leads to features, in particular extinction features, in the spectrum. which superimpose the characteristic absorptions of the actual chemical substance.

The publication of Egert et al. THz system, IEEE Sensors Journal, Vol. 10, No. 3, pp. 379-383, March 2010 discloses a THz spectrometer in which a sample is located at the Brewster angle in the THz radiation.

The publication by R. Beigang et al. "Comparison ofterahertz technologies fordetection and identification of explosives" Proceedings of SPIE, vol. 9102 discloses a comparison of various THz technologies for detecting and identifying a variety of explosives under identical experimental conditions.

In contrast, it is an object of the present invention to provide a method for spectrometry on a chemical substance in a multilayer structure and a spectrometer, which improve the uniqueness of the identification of a chemical substance in a multilayer structure on the basis of their spectrum in said frequency range or a section thereof ,

und Einstellen des Einfallswinkels α, so dass gilt α = Θ_B,Mittel.At least one of the aforementioned objects is achieved by a method for spectrometry on a sample comprising the steps of providing the sample in the form of a chemical substance between at least two flat sections of a packaging material, irradiating the sample with electromagnetic radiation having a plurality of frequencies or one Frequency band from a frequency range of 1 GHz to 30 THz, wherein the electromagnetic radiation impinges on one of the flat portions of packaging material at an angle of incidence α, and frequency-resolved detecting a measure of the intensity of the irradiated at the angle of incidence on the sample and transmitted through the sample or electromagnetic radiation reflected from the sample as a spectrum,
the method additionally comprising the steps of: determining a type of packaging material ( 15 . 16 . 17 ) of the sample ( 12 ), Determining a mean Brewster angle Θ _{B, means} for packaging materials ( 15 . 16 . 17 ) of this kind at a frequency from the Frequency range with the steps Determining a maximum value Θ _{B, max} for the Brewster angle of packaging materials ( 15 . 16 . 17 ) of this kind, determining a minimum value Θ _{B, min} for the Brewster angle of packaging materials ( 15 . 16 . 17 ) of this type and calculating a mean Brewster angle Θ _{B, means} for packaging materials ( 15 . 16 . 17 ) of this kind as ${\mathrm{\Theta }}_{B.Mittel}=\frac{{\mathrm{\Theta }}_{B.min}+{\mathrm{\Theta }}_{B.max}}{2}$

and adjusting the angle of incidence α such that α = Θ _{B, mean} .

It is the basic idea of the present invention that in the spectrometry of a chemical substance between two flat sections of packaging material, the flat sections of packaging material due to multiple reflections and the associated interference cause extinction effects in the frequency-resolved measurement of transmission or reflection, which is the actual superimpose characteristic features of the spectrum of the chemical substance.

However, these effects are dependent on the angle due to the multilayer nature of the flat packaging material. This angular dependence is due to the fact that the reflection on the flat sections of packaging material is also dependent on the angle. If no reflection occurs on the sections of packaging material, it follows that no interference effects due to multiple reflection occur on the sections of packaging material.

If there are no interference effects due to multiple reflections on the two flat sections of packaging material, the characteristic features of the chemical substance between the two flat sections of packaging material become more apparent in the detected spectrum.

The reflections on the planar sections of the packaging material are then minimized if the angle of incidence α of the electromagnetic radiation is equal to the Brewster angle Θ _{B of} the packaging material at a frequency from the frequency range of the high-frequency radiation.

This applies in particular when the electromagnetic radiation which is radiated onto the sample is linearly polarized, the oscillation plane of the electromagnetic radiation being parallel to the plane of incidence of the electromagnetic radiation (so-called p-polarization).

Chemical substances which can be detected with one embodiment of the method according to the invention and / or the spectrometer according to the invention are, for example, drugs such as heroin, methamphetamine or extasy, explosives such as PETN, RDX or SEMTEX-H, fillers and extenders such as α-lactose, paracetamol or Caffeine, or so-called simulants such as β-lactose, p-aminobenzoic acid (PABA), tartaric acid, maltose.

In one embodiment, the chemical substance may be present in the form of a geometrically defined body, for example a tablet, but alternatively also as a powder.

The inventive method is used in particular for the detection of hazardous substances in envelopes and parcels. In this case, it will be the normal case that the samples which are subjected to the spectroscopic method according to the invention contain no safety-relevant substances, but that uncritical materials, for example paper or air, are arranged between the flat sections of packaging material. These also fall within the meaning of the present application under the concept of chemical substance.

A sample in the sense of the present application is a structure of at least three layers or layers, namely the two flat sections of packaging material and the chemical substance in between.

In one embodiment, the sample has at least three, but preferably exactly three, layers or layers, for example a layer of the chemical substance in a paper envelope as the packaging material of the flat sections.

An embodiment of the method according to the invention is particularly useful if the sample in addition to the chemical substance comprises an outer packaging of a dielectric material, preferably of paper, cardboard, plastic or plastic film, or from a combination of these materials.

Particularly pronounced, the advantages of the present invention can be realized if the sample consists of the chemical substance and an outer packaging made of paper or cardboard. Particularly suitable is an embodiment of the invention for spectrometry on a chemical substance in an envelope made of paper or paperboard, an envelope made of paper or cardboard with a viewing window, a padded envelope made of paper or cardboard or an envelope comprising paper or paperboard and a bubble wrap ,

A planar section in the sense of the present application is any section of material which is suitable for causing a reflection of the electromagnetic radiation, so that together with a corresponding reflection from a second such flat portion of packaging material an interference effect occurs. This is especially given for flat planar sections. In one embodiment of the invention, the sample has at least two flat planar sections, which are arranged substantially parallel to one another. This condition is met in the context of the present application, for example, in an envelope made of paper or cardboard.

While embodiments are conceivable in which the sample has two flat sections each of a packaging material, wherein the packaging materials of the two flat sections differ from each other, in a particularly preferred embodiment of the invention, the two flat sections of the same packaging material.

In one embodiment of the invention, the sample is formed from a chemical substance and exactly two flat sections from a packaging material. The sample is then provided by, for example, bagging the chemical substance into an envelope. However, embodiments are conceivable in which the outer packaging comprises more than two flat sections of a packaging material.

For the purposes of the present application, the terahertz frequency range (THz frequency range) is understood as meaning a frequency range from 1 GHz to 30 THz, but preferably a frequency range from 100 GHz to 5 THz.

In order to be able to detect the characteristic (absorption) spectrum of a chemical substance, it is necessary to irradiate the substance either with a plurality of mutually different frequencies or with an entire frequency band from the said frequency range, wherein the frequency band can also cover the entire frequency range.

In order to record the spectra, it is necessary to measure in a frequency-resolved manner a measure of the intensity of the electromagnetic radiation guided at the angle of incidence onto the multilayer structure and either transmitted through the multilayered structure or reflected by the multilayered structure.

While it is initially irrelevant to the realization of the basic idea of the present invention whether the spectrum of the sample is detected in a reflection geometry or in a transmission geometry, in one embodiment a transmission geometry in which the sample is located between a source for the electromagnetic radiation and a detector for the transmitted radiation is preferred. In transmission geometry, the absorption lines of the chemical substance to be identified are more pronounced.

In particular, when the measurement is made at the Brewster angle of the packaging material or with very little deviation therefrom, i. essentially without reflections on the packaging material, the method according to the invention is also suitable for spectroscopy in reflection geometry.

For the purposes of the present application, the angle of incidence of the electromagnetic radiation on the sample is always measured as a deviation from the solder on the surface of one of the flat sections of packaging material.

Assuming collimated electromagnetic radiation impinging on the sample, the angle of incidence for the purposes of the present application is the angle between the solder on the planar section of packaging material in the surface region of the sample irradiated by the electromagnetic radiation and a straight line parallel to the propagation direction of the sample collimated radiation.

In the case of electromagnetic radiation focused on the sample, the angle of incidence in the context of the present application is understood to mean the angle between the solder on the surface of the sample or its packaging material and the axis of symmetry of the focused beam of the electromagnetic radiation which can be roughly described as a cone.

For the purposes of the present invention, a measure of the intensity of the electromagnetic radiation is understood in particular to mean the intensity of the radiation itself, but also the electric field of the radiation whose square is proportional to the intensity.

In one embodiment of the invention, the detected spectrum is an absorbance spectrum in which the absorbance of the electromagnetic radiation through the chemical substance sample is plotted against the frequency.

In one embodiment of the invention it may be expedient if the angle of incidence α is set individually for each sample whose spectrum is to be detected by means of the method according to the invention. This assumes that one first analyzes the packaging material of the flat sections of the sample in order to determine its Brewster angle. In such an embodiment, it is useful if the angle of incidence is equal to the Brewster angle of the Packaging material is at a frequency in the frequency range.

However, according to the invention, one and the same angle of incidence should be used for a plurality of samples. This is provided that all samples to be detected have two flat sections of packaging material, all packaging materials of the samples to be spectroscopically having a similar refractive index in the selected frequency range and thus a similar Brewster angle. Two typical types of packaging material in the context of the present application are, for example, paper and cardboard on one side and plastics on the other side.

First, the type of packaging material of the sample is determined. In one embodiment of the invention, this determination of the type of packaging material for the specimen to be spectroscopically can then take place when it is determined to which samples with which type of areal sections of a packaging material the method is to be applied in principle. Alternatively, the type of packaging material for each individual sample pending for spectroscopy may be determined and then the angle of incidence may be adjusted. If the type of packaging material of the sample to be analyzed is determined, then a mean Brewster angle for packaging materials of this type is determined and the angle of incidence α is set so that the angle of incidence α is equal to the mean Brewster angle Θ _{B, average} .

It is understood that the angle of incidence α set in this manner can deviate from the actual Brewster angle Θ _{B of} the specific sample to be spectroscopically by ± 15 °. However, this deviation of the average Brewster angle Θ _{B, average} , from the actual Brewster angle Θ _{B of} the packaging material of the concrete sample is low enough that the advantages of approximating the angle of incidence to the actual Brewster angle Θ _{B are still} realized.

The determination of a mean Brewster angle Θ _{B, means} for packaging materials of one kind, further comprises the following steps: determining a maximum value Θ _{B, max} for the Brewster angle of the packaging materials of this kind, determining a minimum value Θ _{B, min} for the Brewster angle the packaging materials of this type and calculating a mean Brewster angle Θ _{B, means} for packaging materials of this kind as ${\mathrm{\Theta }}_{B.Mittel}=\frac{{\mathrm{\Theta }}_{B.min}+{\mathrm{\Theta }}_{B.max}}{2}\text{,}$

If the type of packaging material is, for example, "paper and board", then the mean Brewster angle Θ _{B, means} in one embodiment about 56 °.

In an embodiment of the invention, the method additionally comprises the steps of: comparing the detected spectrum with one or a plurality of predetermined and stored comparison spectra and identifying the chemical substance.

wobei Θ_B,max ein Maximalwert für den Brewster-Winkel von Verpackungsmaterialien (15, 16, 17) dieser Art ist und Θ_B,min ein Minimalwert für den Brewster-Winkel von Verpackungsmaterialien (15, 16, 17) dieser Art ist, und wobei die Steuereinrichtung (11) so eingerichtet ist, dass sie im Betrieb des Spektrometers (1) den Einfallswinkel α so einstellt, dass gilt α = Θ_B,Mittel.At least one of the aforementioned objects is also achieved by a spectrometer for spectrometry on a chemical substance in a sample, the spectrometer being equipped with a source of electromagnetic radiation having a plurality of frequencies or a frequency band from a frequency range of 1 GHz to 30 THz, a detector for frequency-resolved detection of a measure of the intensity of the guided at an angle of incidence on the sample and transmitted through the sample or reflected by the sample electromagnetic radiation as a spectrum and the sample in the form of the chemical substance between at least two planar sections of a Packaging material, the spectrometer ( 1 ) a control device ( 11 ) and the spectrometer ( 1 ) is arranged so that the angle of incidence α, under which the electromagnetic radiation ( 2 ) to the test ( 12 ), by the control device ( 11 ) is adjustable, wherein the control device ( 11 ) is set up to operate in the operation of the spectrometer ( 1 ) after inputting one type of packaging material ( 15 . 16 . 17 ) into the control device ( 11 ) a mean Brewster angle Θ _{B, means} for packaging materials ( 15 . 16 . 17 ) of this type is determined at a frequency from the frequency range, wherein the average Brewster angle Θ _{B, means} for packaging materials ( 15 . 16 . 17 ) of this kind is calculated as ${\mathrm{\Theta }}_{B.Mittel}=\frac{{\mathrm{\Theta }}_{B.min}+{\mathrm{\Theta }}_{B.ma\mathrm{<figure-callout\; id="2"\; label="x"\; filenames="DE102014113354B4\_0007.png,DE102014113354B4\_0008.png"\; state="\{\{state\}\}">x</figure-callout>}}}{2}\text{.}$

where Θ _{B, max is} a maximum value for the Brewster angle of packaging materials ( 15 . 16 . 17 ) of this kind and Θ _{B, min is} a minimum value for the Brewster angle of packaging materials ( 15 . 16 . 17 ) of this type, and wherein the control device ( 11 ) is set up to operate in the operation of the spectrometer ( 1 ) sets the angle of incidence α such that α = Θ _{B, mean} .

While the spectrometer according to the invention can be realized with any type of source and detector, which make it possible to simultaneously and successively different frequencies from the frequency range of 1 GHz to 30 THz to the sample to conduct and detect, in a special way optoelectronic approaches to coherent generation and detection of the terahertz radiation for the realization of the spectrometer according to the invention.

To such an optoelectronic spectrometer with a source and a detector for the frequency range of 1 GHz to 30 THz too this has a laser, a first optoelectronic high-frequency component as a source and a second optoelectronic high-frequency component as a detector for the electromagnetic radiation and a beam splitter, which is arranged and arranged so that it is part of the electromagnetic radiation emitted by the laser during operation of the spectrometer leads to the first high-frequency component and directs a portion of the electromagnetic radiation emitted by the laser to the second high-frequency component, on.

In one embodiment of such a spectrometer, the laser is a laser for generating ultrashort electromagnetic pulses, preferably with a pulse duration of 150 fs or less.

The first and second high-frequency optoelectronic components as electromagnetic radiation source and detector in a frequency range of 1 GHz to 30 THz are each alternatively a photoconductive switch or an electro-optic crystal in one embodiment. When using an electro-optic crystal, the non-linear effects in the crystal are exploited to generate from the electromagnetic radiation of the laser, the electromagnetic radiation in the frequency range of 1 GHz to 30 THz.

When using a photoconductive or photoconductive switch, optionally in combination with an antenna connected thereto, the impact of a short electromagnetic pulse on the photoconductive switch at a corresponding electrical bias of the high-frequency component causes a short pulse of current in the component and thus the emission of a broadband pulse in the terahertz frequency range. By contrast, the short electromagnetic pulse of the laser on the detector side serves to briefly gate the detector with the aid of the photoconductive switch, thus making it possible to measure the electric field of the electromagnetic radiation impinging on the high-frequency component of the detector in the THz frequency range. If a current is measured at the supply lines of the photoconductive switch of the high-frequency component used as the detector, then the field of the electromagnetic terahertz radiation which impinges on the high-frequency component can be detected in a time-resolved manner.

The electric field of the electromagnetic THz radiation impinging on the detector drives charge carriers in the longitudinal direction over the switch. In this case, a current flow is only possible if at the same time the photoconductive switch is closed, that is, the switch is irradiated with electromagnetic radiation of the laser. Since the electromagnetic pulse used to switch the photoconductive switch is short of the time course of the electric field of the pulse received by the detector in the THz frequency range, the electric field of the THz signal can be time-resolved sampled by taking a time offset between the THz pulse striking the switch and the electromagnetic pulse used to switch the photoconductive switch are introduced and varied during the measurement.

The time-resolved electromagnetic field thus detected can be converted by Fourier transformation into a corresponding frequency-resolved spectrum.

It should be understood that in one embodiment, the spectrometer includes a photoconductive switch for detecting the THz radiation, a suitable current or voltage amplifier connected to it for detecting the currents through the switch of the detector.

However, the optoelectronic spectrometer with the high-frequency components according to the invention can also be operated with monochromatic laser radiation as an alternative to a laser with short electromagnetic pulses. For this purpose, a laser or even two lasers coupled together then provide two laser frequencies with a frequency spacing equal to the electromagnetic radiation to be used for the spectrometry in the frequency range from 1 GHz to 30 GHz. These are spatially superimposed. The resulting electromagnetic beating signal will be applied to the optoelectronic high frequency components as source and detector. The beat signal generates monochromatic electromagnetic radiation in the THz range.

In such an embodiment, the frequency of the electromagnetic radiation in the THz range is set or tuned by tuning the difference frequency between the two electromagnetic radiation components generated by the one or more lasers.

The spectrometer has a control device and the spectrometer is set up such that the angle of incidence at which the electromagnetic radiation impinges on the sample can be set by the control device, wherein the control device is set up in such a way that, after the input of a Type of packaging material in the control means a mean Brewster angle Θ _{B, means} for packaging materials of this type determined and the angle of incidence α adjusted so that α = Θ _{B, means} .

In this case, the input of the type of packaging material of the sample can be done in particular by a user. For the determination of the average Brewster angle Θ _{B, means} for the type of packaging material, it is recommended that the control means have a memory in which the average Brewster angles are stored for different types of packaging materials.

It is basically irrelevant to the operation of the spectrometer according to the invention, whether to adjust the angle of incidence, the sample is pivoted relative to the source and the detector, so that the angle of incidence changes or if the arrangement of source and detector with respect to the fixed sample is rotated.

In one embodiment, relative rotation or pivoting between the source and the detector on the one hand and the sample on the other hand can be effected mechanically on the one hand by rotating the sample or the arrangement of source and detector with respect to one another. However, purely electronic embodiments are also possible in which the angle of incidence is achieved via an electronic beam control, for example with the aid of a phased array.

As far as aspects of the invention have been described above with regard to the spectrometer according to the invention, these also apply to the corresponding method for spectrometry on a chemical substance in a multilayer structure and vice versa. As far as the method is carried out with a spectrometer according to this invention, this has the corresponding facilities for this purpose. In particular, embodiments of the spectrometer are suitable for carrying out the previously described embodiments of the method.

• 1 zeigt eine schematische Draufsicht auf ein Spektrometer gemäß der vorliegenden Erfindung.
• 2 zeigt eine graphische Darstellung der erfassten Extinktion der Probe aus 1 in Abhängigkeit von der Frequenz bei einem Einfallswinkel, der gleich dem mittleren Brewster-Winkel für die Art des Verpackungsmaterials der Probe ist.
• 3 zeigt zum Vergleich eine grafische Darstellung der erfassten Extinktion der Probe aus 1 in Abhängigkeit von der Frequenz bei einem Einfallswinkel von 0°.

Further advantages, features and applications of the present invention will become apparent from the following description of an embodiment and the associated figures.

• 1 shows a schematic plan view of a spectrometer according to the present invention.
• 2 shows a graphical representation of the detected absorbance of the sample 1 depending on the frequency at an angle of incidence equal to the average Brewster angle for the type of packaging material of the sample.
• 3 shows for comparison a graphical representation of the detected absorbance of the sample 1 as a function of the frequency at an angle of incidence of 0 °.

1 schematically shows the structure of a spectrometer according to the invention 1 in which the electromagnetic high-frequency radiation 2 in a frequency range of 1 GHz to 30 THz with photoconductive switches as a source 3 and detector 4 for the high-frequency radiation 2 is generated and recorded.

At the heart of the spectrometer shown 1 is a laser 5 for generating short electromagnetic pulses 6 in the infrared spectral range. The pulse duration of the laser 5 generated pulses is about 100 fs. Everyone from the laser 5 generated pulse 6 is at a beam splitter 7 into two spatially separated impulses 8th . 9 split about the same power. The path lengths of the optical and high frequency signals are on the path from the beam splitter 7 and the source 3 to the detector 4 on the one hand and from the beam splitter 7 over a delay line 10 to the detector 4 on the other hand set up so that the high-frequency signal 2 simultaneously with the second part 8th of the impulse 6 , its first part 9 the generation of the high-frequency pulse 2 in the source 3 has caused the detector 4 incident.

In the illustrated embodiment, the source is 3 and the detector 4 each of a high-frequency component, which as a photoconductive switch, each with an antenna for the high-frequency radiation 2 is designed.

Each of the high frequency components 3 . 4 has a dipole structure as an antenna. Each of the dipole antennas has an interruption at its center which, together with a photoconductive semiconductor substrate underlying the antenna structure, forms a photoconductive switch. The one from the laser 5 generated electromagnetic radiation in the form of short pulses 6 is focused on these photoconductive switches. It is alternatively possible that of the laser 5 generate generated electromagnetic radiation by means of optical fibers on the photoconductive switch. In such an embodiment, it may be necessary to pre-compensate the dispersion experienced by the individual pulses in the fiber.

Both the source 3 as well as the detector 4 each have two leads for the respective dipole antenna.

In the illustrated embodiment, the semiconductor substrate for the photoconductive switch is a low-lived gallium arsenide with short carrier lifetime or charge carrier trapping times.

In operation of the spectrometer 1 gets to that as a source 3 used high-frequency component a rectangular-modulated bias voltage applied via the leads of the antenna. The rectangular modulation is used to detect the electromagnetic radiation detector side with the aid of a lock-in amplifier, whose reference signal to the modulation of the bias of the source 3 is phase locked. The short-term closure of the photoconductive switch causes a brief current pulse through the antenna, which causes the emission of a broadband high-frequency pulse 2 from the antenna leads.

Considering the high-frequency component used on the detector side 4 , the current flowing through the antenna and the photoconductive switch formed by it is detected or measured there. For this purpose, the high-frequency component 4 with a control device and evaluation device 11 connected. This is formed in the present case of a conventional industrial PC.

This leads to the antenna of the detector 4 impinging electromagnetic field of high frequency pulse 2 to a driving of charge carriers via the antenna or the switch and thus to a stream. However, the current through the antenna can flow only at the moment when the photoconductive switch by the arrival of an electromagnetic pulse 8th the laser 5 closed is. Since the electromagnetic pulse used to close the photoconductive switch on the detector side is short in time to the high frequency pulse impinging on the antenna, by shifting the arrival times of the two relative to one another by means of the delay line 10 the electric field of the high frequency pulse 2 be scanned temporally resolved. This time-resolved electric field of the electromagnetic radio-frequency pulse 2 can be transmitted by means of a Fourier transformation into an intensity spectrum in the frequency domain.

In the illustrated embodiment of the spectrometer 1 out 1 is the sample 12 on a sample holder 13 arranged.

In the sample 12 this is a tablet in this example 14 from p-aminobenzoic acid (PABA) with a surface density of 0.1 mg / mm ² . This PABA tablet is in two paper bags 15 . 16 as well as an envelope 17 embedded. PABA serves as a model substance for other, safety-relevant substances in the testing of spectrometers for the detection of safety-relevant chemical substances. PABA 14 forms the chemical substance to be detected within the meaning of the present application. The paper bags 15 . 16 as well as the envelope form a total of six flat sections of packaging material in the context of the present application. This is PABA 14 between three flat sections on the one and three flat sections of the packaging material 15 . 16 . 17 embedded on the other side.

The angle of incidence α is in the illustrated embodiment of the spectrometer 1 out 1 adjusted so that it corresponds to the middle Brewster angle Θ _{B, means} for paper. The angle of incidence α is 56 ° here.

At this time, the average Brewster angle Θ _{B became mean} for paper as a kind of packaging material of the sample 12 determined by adding the values for the minimum and maximum Brewster angles Θ _{B, min} , Θ _{B, max} and dividing by two. True, the actual Brewster angle for the material of the envelope 17 or the paper bags 15 . 16 the sample 12 from this mean Brewster angle Θ _{B, mean} , however, the advantages of minimizing the reflections in the surfaces of the packaging material 15 . 16 . 17 nevertheless realized.

It is understood that the reflections on the packaging material 15 . 16 . 17 can only be minimized when the on the sample 12 incident electromagnetic radiation 2 p-polarized. That is, the linear polarization is parallel to the plane of incidence (the plane defined by the incident slot and the central axis of the electromagnetic radiation 2 spanned). For this purpose, the radiating dipoles of the antennas of the source 3 and the detector 4 aligned so that they are also parallel to the plane of incidence and the radiation 2 lie.

In 2 is now the extinction spectrum for a measurement at the angle of incidence α of 56 °, ie the average Brewster angle Θ _{B, means} for the type "paper and cardboard" of the packaging material using the spectrometer 1 shown.

Plotted is the extinction on the y-axis in arbitrary units as well as the frequency in units of THz on the x-axis. Clearly visible are the absorptions characteristic of PABA at about 0.6 THz and 0.8 THz.

The spectrum 2 can be easily compared with reference spectra, for example by correlation, and thus identify the detected chemical substance within the packaging material, in this case as PABA.

3 shows in comparison to an extinction spectrum, which is different than in 1 represented (and thus not according to the invention), at an angle of incidence α of 0 °, ie at a vertical incidence, was measured. Due to the multiple reflections on the packaging material 15 . 16 . 17 in particular are the characteristic absorptions of the PABA tablet 14 are not identifiable at 0.6 and 0.8 THz because they are superimposed by the interference effects due to multiple reflection.

In an alternative embodiment of the spectrometer 1 is the sample holder 13 a turntable, which makes it possible, the angle of incidence α of the electromagnetic high frequency radiation 2 to the test 12 adjust. This setting is made in one embodiment by means of control and evaluation 11 , In this case, this adjusts the angle of incidence α so that it is equal to the average Brewster angle for the type of packaging material which comprise the samples to be spectroscoped.

For purposes of the original disclosure, it is to be understood that all such features as will become apparent to those skilled in the art from the present description, drawings, and claims, even if concretely described only in connection with certain other features, both individually and separately any combination with other of the features or feature groups disclosed herein are combinable, unless this has been expressly excluded or technical conditions make such combinations impossible or pointless. On the comprehensive, explicit representation of all conceivable combinations of features is omitted here only for the sake of brevity and readability of the description.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description is exemplary only and is not intended to limit the scope of the protection as defined by the claims. The invention is not limited to the disclosed embodiments.

Variations of the disclosed embodiments will be apparent to those skilled in the art from the drawings, the description and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain features are claimed in different claims does not exclude their combination. Reference signs in the claims are not intended to limit the scope of protection.

LIST OF REFERENCE NUMBERS

1

spectrometer

2

RF radiation

3

source

4

detector

5

laser

6

electromagnetic pulse in the infrared spectral range

7

beamsplitter

8th

second part of the pulse 6

9

first part of the impulse 6

10

delay path

11

Control and evaluation device

12

sample

13

sample holder

14

PABA tablet

15, 16

paper bag

17

envelope

Claims (8)

Method for spectrometry on a sample (12) comprising the steps of providing the sample (12) in the form of a chemical substance (14) between at least two planar sections of a packaging material (15, 16, 17), irradiating the sample (12) with electromagnetic Radiation (2) having a plurality of frequencies or a frequency band from a frequency range of 1 GHz to 30 THz, wherein the electromagnetic radiation (2) at an incident angle α on one of the flat sections of packaging material (15, 16, 17) impinges, and frequency-resolved detection of a measure of the intensity of the electromagnetic radiation (2) emitted at the angle of incidence on the sample (12) and transmitted through the sample (12) or reflected by the sample (12) as a spectrum, characterized in that the method additionally the steps comprise: determining a type of packaging material (15, 16, 17) of the sample (12), determining a mean Brewster-W els _{B, means} for packaging materials (15, 16, 17) of this type at a frequency from the frequency range with the steps of determining a maximum value Θ _{B, max} for the Brewster angle of packaging materials (15, 16, 17) of this type, determining a minimum value Θ _{B, min} for the Brewster angle of packaging materials (15, 16, 17) of this type and calculating a mean Brewster angle Θ _{B, means} for packaging materials (15, 16, 17) of this type $${\mathrm{\Theta }}_{B.Mittel}=\frac{{\mathrm{\Theta }}_{B.min}+{\mathrm{\Theta }}_{B.max}}{2}$$

and adjusting the angle of incidence α such that α = Θ _{B, mean} . Method according to the preceding claim, characterized in that the electromagnetic radiation (2) is linear p-polarized. Method according to one of the preceding claims, characterized in that the two flat sections are made of paper or cardboard. Method according to one of Claims 1 to 3 , characterized in that the two sheet-like sections are made of plastic, preferably of a plastic film. Method according to one of the preceding claims, characterized in that the flat sections form an envelope. Method according to Claim 1 , characterized in that the type of packaging material (15, 16, 17) is paper and paperboard and the average Brewster angle Θ _{B, means} of paper and cardboard is about 56 °. Method according to one of the preceding claims, characterized in that the method additionally comprises the steps of: comparing the detected spectrum with one or a plurality of predetermined comparison spectra and identifying the chemical substance (14). Spectrometer (1) for spectrometry on a chemical substance (14) in a sample (12) having a source (3) designed to generate electromagnetic radiation (2) having a plurality of frequencies or a frequency band from a frequency range of 1 GHz to 30 THz, a detector (4) designed for frequency-resolved detection of a measure of the intensity of the guided at an angle of incidence α on the sample (12) and transmitted through the sample (12) or from the sample (12) electromagnetic radiation (2) as a spectrum and the sample (12) in the form of the chemical substance (14) between at least two flat sections of a packaging material (15, 16, 17), characterized in that the spectrometer (1) comprises a control device (11) and the spectrometer (1) is arranged so that the angle of incidence α at which the electromagnetic radiation (2) impinges on the sample (12) by the control device (11) e is adjustable, wherein the control device (11) is set up such that, during operation of the spectrometer (1), after an input of one type of packaging material (15, 16, 17) into the control device (11), a mean Brewster angle Θ _{B, Means} for packaging materials (15, 16, 17) of this type determined at a frequency from the frequency range, wherein the average Brewster angle Θ _{B, means} for packaging materials (15, 16, 17) of this type is calculated as $${\mathrm{\Theta }}_{B.Mittel}=\frac{{\mathrm{\Theta }}_{B.min}+{\mathrm{\Theta }}_{B.max}}{2}$$

where Θ _{B, max is} a maximum value for the Brewster angle of packaging materials (15, 16, 17) of this type and Θ _{B, min is} a minimum value for the Brewster angle of packaging materials (15, 16, 17) of this type, and wherein the control device (11) is set up so that it adjusts the angle of incidence α in the operation of the spectrometer (1) such that α = Θ _{B, means} .

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