WO2014005986A1

WO2014005986A1 - An add-on system including a micro-reactor for an atr-ir spectrometer

Info

Publication number: WO2014005986A1
Application number: PCT/EP2013/063815
Authority: WO
Inventors: Jens Spanggaard PILTOFT; Christian Nybo PETERSEN; Andreas J. KUNOV-KRUSE
Original assignee: Danmarks Tekniske Universitet
Priority date: 2012-07-03
Filing date: 2013-07-01
Publication date: 2014-01-09
Also published as: EP2870460A1

Abstract

The invention relates to an add-on system for an attenuated total reflectance infrared (ATR-IR) spectrometer, the add-on system allowing for time-resolved in situ IR measurements of heterogeneous mixtures. The add-on device comprises a micro-reactor (300A) forming a sample cavity (305) when the micro-reactor (300A) is placed on a sample surface side of an ATR-IR plate (200), a sample magnet (312) being adapted for being placed in the sample cavity (305), and a magnet system (400) comprising an outer magnet (402), the magnet system (400) being adapted for rotating the outer magnet (402) in a plane being substantially parallel to the plane in which the ATR-IR plate (200) lies, wherein, when the micro-reactor (300A) is placed on the ATR-IR plate (200), the outer magnet (402) is adapted for rotating the sample magnet (312) inside the sample cavity (305).

Description

AN ADD-ON SYSTEM INCLUDING A MICRO-REACTOR FOR AN ATR-IR SPECTROMETER

The invention relates to an add-on system for a unit for a spectrometer, the add-on system allowing for time-resolved in -situ IR measurements of heterogeneous mixtures.

Background

When characterizing chemical samples, e.g. gasses, liquids or solid samples, infrared (IR) spectroscopy is a highly useful tool, as it gives a unique finger print signature specific for the vibrational levels in the molecules contained in the sample, thus giving a unique IR spectrum for each molecule.

IR spectrometers are available commercially in multiple designs today designed for different types of IR spectroscopy. For measuring different types of IR spectroscopy, add-on cell / equipment is normally bought separately and installed in a standard IR spectrometer. Such an add-on cell / equipment can e.g. provide for Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), or Attenuated Total Reflectance Infrared (ATR-IR) spectroscopy, all of these methods measuring the IR light scattered from the sample, or transmittance IR spectroscopy, where IR light transmitted through a thin sample is measured.

Transmittance spectroscopy requires a delicate preparation of the sample before an IR spectrum can be obtained, whereas DRIFTS requires a less complicated and delicate sample preparation. For both transmittance spectroscopy and DRIFTS, measurement time of 5-30 minutes for obtaining good IR spectra with a low noise level is required. This is a reasonable long time if one wants to measure kinetics in a sample with these IR spectroscopy techniques. The above described add-on cells / equipments are rather expensive experimental equipment to buy as an accessory to an IR spectrometer. Further, in order to obtain a good time-resolution and/or low noise IR spectra, one would often buy a high- quality IR spectrometer, thereby adding further to the cost of equipment. ATR-IR spectroscopy is often used for analysis of the surface of materials and can be particularly interesting in connection with thick or strongly absorbing materials, where transmission spectroscopy cannot be used. ATR-I is advantageous as it does not require the usual sample preparation of making a powder sample and potassium bromide (KBr) tablet and further provides IR spectra with a low noise level within a significantly shorter time of down to 2-6 seconds.

By using ATR-IR spectroscopy, IR spectra of films having a thickness of only a few micrometers can be obtained. This is possible, since an ATR cell/unit contains sample securing means, which secures solid and/or powder samples to the ATR- plate in such a way, that there is intimate optical contact between the sample and the ATR-IR plate in the ATR cell.

ATR-IR spectroscopy is thus a valuable tool for characterizing in particular solid and/or powder samples at ambient conditions, i.e. room temperature and atmospherically pressure. For spectrometers equipped with an ATR-IR cell, a heating source can further be incorporated in the ATR-IR cell allowing for measurements of the IR spectra at elevated temperatures.

When characterizing chemical reactions, it is desirable to control the atmosphere around the sample or to create a different atmosphere than air in order to get a clear picture of what is really happening during a chemical reaction. Thus, measuring under in-situ conditions is desirable in many cases. Using conventional ATR-IR cells does not allow for direct (possibly time-resolved) in-situ measurements.

It is further quite a challenge to measure IR spectra in liquid samples using ATR-IR cells, as it is difficult to keep large enough the sample volumes in place when the sample is liquid. Further, the sample or part of the sample may evaporate during the IR measurements time thereby giving inaccurate IR spectra. Even further, when measuring heterogenous sample mixtures, deposition in the sample is an additional problem, which needs to be addressed if ATR-IR spectroscopy is to give a meaningful result. The problem with containing a liquid sample on the ATR-IR plate can be solved by using a cell containing the sample. To avoid deposition, the sample can be continuously shaken or rotated. However, this is not an optimum solution to combine with the use of delicate optical equipment, where high stability is required.

As an alternative to shaking and/or rotating the sample, companies such as Specac have developed an ATR reaction cell, which mechanically stirs the sample. This equipment, however, has the disadvantage that it is extremely expensive and requires a large sample volume. The large sample volume further means that it requires a long time to obtain an evenly distributed temperature in the sample after the temperature has been increased or decreased.

An alternative to the above described ATR reaction cell is to use fibre-ATR, where the IR light from the ATR spectrometer and the IR reflected of the sample is guided to and from a separate sample cell, which in turn can be placed on a stirrer for magnet-controlled stirring of the sample. This option is flexible, but far from ideal, as it requires an external reaction cell investment on the required fibre-equipped addon device. On top of that there is a high loss of the IR light in the fibres, which induces a large amount of noise in the IR spectra. As noise scales quadratically, reducing the noise level in fibre- ATR-IR measurements down to the noise level for regular ATR-IR measurements will increase the measurement time by a factor of 4, which significantly reduces the time-resolution obtainable with fibre-ATR equipment. Description of the invention

Disclosed herein is an add-on system for an attenuated total reflectance infrared (ATR-IR) spectrometer, where the ATR-IR spectrometer comprises an ATR-IR plate. Alternatively to the ATR-IR spectrometer, a spectrometer for measuring Raman, ultra violet (UV), visible spectra or other types of spectra may alternatively be used.

The ATR-IR plate comprises a sample surface side whereon a sample, e.g. a homogenous or heterogeneous liquid sample mixture, can be placed; and a light- illuminating surface side situated on the opposite side of the sample surface side, wherein IR light from the ATR-IR spectrometer illuminating the light-illuminating surface side passes through the ATR-IR plate, the IR light thereby interacting with the sample, and wherein light is reflected and/or back scattered from the sample passes through the ATR-IR plate and propagates from the light-illuminating surface side thereby being collected by the ATR-IR spectrometer.

In other words, a sample is placed on one side of the ATR-IR plate, i.e. the sample surface side, and IR light from the IR spectrometer illuminates the sample after having traveled through the ATR-IR plate as the IR light hits the ATR-plate from below, i.e. on the light-illuminating surface side. Thereafter, the IR light interacts with the sample and the reflected and/or back scattered light from the sample travels through the ATR-plate again and into the IR spectrometer as it illuminates from the lower side of the ATR-plate, i.e. the light-illuminating surface side.

The add-on system further comprises a micro-reactor in the shape of an open structure with an opening defined by an ATR-IR plate facing surface, wherein when the micro-reactor is placed on the sample surface side of the ATR-IR plate, a sample cavity enclosing the sample is formed between the sample surface side of the ATR-IR plate and the ATR-IR plate facing surface of the micro-reactor, the sample surface side of the ATR-IR plate thereby covering the opening in the micro- reactor.

The add-on system further comprises a sample magnet adapted for being placed in the sample cavity; and outer magnet system encircling the micro-reactor in a plane substantially parallel to the plane in which the ATR-IR plate lies, wherein, when the micro-reactor is placed on the ATR-IR plate, the outer magnet system is adapted for rotating the sample magnet inside the sample cavity.

By the add-on system described above is obtained a system which allows for time- resolved measurements of IR spectra in liquid solutions using a very small amount of sample. The time-resolution obtainable is in the order of a few seconds. Further it is possible to measure the IR spectra of heterogeneous sample mixtures, where deposition normally would be a large problem, since the outer magnet system provides for efficient stirring of the sample. Further, as the outer magnet system may be constructed such that it is not in physically contact with the micro-reactor or the spectrometer the delicate optical equipment is not affected by stirring of the sample. Thus high optical stability is ensured. At the same time there is not loss of the IR light in fibres or similar (as such is not used) giving the IR spectra a very low amount of noise compared to using conventional fibre-ATR cells combined with traditional stirring of the sample.

By having a small sample volume, it is further possible to obtain an evenly distributed temperature in the sample almost immediately after the temperature has been increased or decreased.

The add-on system described above has a very low production price compared to the equipment commercially available today. In one or more embodiments the micro-reactor further comprises a first opening for introducing the sample and/or reactants into the sample cavity. Thereby the micro- reactor can be fixed to the unit first and the sample applied afterwards, which is highly advantageous when working with liquid samples. In one or more embodiments the micro-reactor further comprises first opening sealing means for sealing the first opening. It is thereby possible to avoid evaporation of the sample over time.

In one or more embodiments the first opening sealing means is a membrane which can be penetrated by a needle, whereby additional substance easily can be applied.

In one or more embodiments the micro-reactor further comprises a second opening for allowing gas and/or liquid to enter the sample cavity and a third opening allowing gas and/or liquid to exit the sample cavity, the second opening and the third opening being independently sealable. When characterizing chemical reactions, it is desirable to control the atmosphere around the sample or to create a different atmosphere than air in order to get a clear picture of what is really happening during a chemical reaction. Thus, measuring under in-situ conditions is desirable in many cases. By having a second and a third opening, it is possible to measure the IR spectra at in-situ conditions, e.g. an atmosphere of nitrogen.

In one or more embodiments the micro-reactor further comprises a fourth opening adapted for changing the pressure inside the sample cavity. In-situ measurements under reduced or increased pressure conditions can thus be performed.

In one or more embodiments the add-on system further comprises an O-ring being positioned between the sample surface side and the ATR-IR plate facing surface of the micro-reactor. Thereby an airtight sealing between the sample surface side and the ATR-IR plate facing surface of the micro-reactor is obtained. The O-ring, e.g. in the form of a membrane can be in a high temperature-stable material, such as e.g. Teflon or an elastomer. In one or more embodiments the system further comprises a source of light, e.g. a diode or an optical fibre connectable to a laser or similar, and a light source containing cavity comprising the source of light, the light source containing cavity being positioned on top of the top surface of the micro-reactor such that light from the source of light can illuminate and/or interact with the sample inside the sample cavity. This allows for measurements of photochemical reactions.

In one or more embodiments the add-on system further comprises light collecting means for collecting light transmitted, reflected and/or back scattered from the sample after light from the additional light source has interacted with the sample.

One advantage obtained thereby is that it becomes possible to measure two types of optical spectra from the same sample simultaneously under the exact same in- situ conditions by illuminating the sample from below with the IR light for obtaining ATR-IR spectra of the sample at the same time as illumination the sample from above with the additional light source and collecting the reflected and/or back scattered light from the sample again for obtaining e.g. UV and/or visible spectra. It is also possible to measure the Raman spectra of the sample 'from above' e.g. by using an optical fibre, which - in addition to providing the additional light source to the sample - also collects the back scattered light from the sample. In one or more embodiments the add-on system further comprises focusing means, e.g. an optical lens, for focusing the light from the source of light onto a specific point inside the sample. A small microscope objective could also be used as the object from which the additional light source illuminates and which collects the backscattered and/or reflected light from the sample.

To further obtain control of the photochemical reaction or the measurements of Raman, UV and/or visible spectra, in one or more embodiments the light source containing cavity comprising the source of light can be adjusted in height.

In one or more embodiments the micro-reactor is coated on the inside, whereby back-ground light from the surroundings doe not influence the measurements. In one or more embodiments the sample magnet is a Teflon magnet.

In one or more embodiments the sample magnet comprises a thermometer integrated in the sample magnet for measuring the temperature of the sample, the thermometer preferably being in remote contact with a read-out system. Thereby a very precise measurement of the temperature in the sample can be measured.

In one or more embodiments the outer magnet system comprises a magnet cavity with a multiple of electromagnets evenly distributed such that they encircle the micro-reactor. In this way, the sample magnet can be rotated without rotating the outer magnet system by alternating the magnetic field of the electromagnets. In one or more embodiments there are six electromagnets encircling the micro- reactor.

In one or more embodiments the magnetic cavity is in a non-magnetic material.

In one or more embodiments the outer magnet system comprises an outer magnet and means for rotating the outer magnet around the micro-reactor. Normally, the outer magnet will be attached to an outer magnet support, e.g. in the form of a rotating ring in a non-magnetic material, thereby making the rotation of the outer magnet easy to do.

In one or more embodiments the add-on system further comprises a temperature probe positioned between the outer magnet system and the micro-reactor. This allows for an optimum control of the temperature inside of the sample, as the temperature probed may heat or cool the sample. The temperature probe may be instructed such that is can heat the sample up to temperatures of approximately 60°C or 80°C such that the existing ATR-IR plate, which cannot withstand high temperatures, are not damaged by use of the add-on system.

In one or more embodiments the temperature probe is attached to or integrated into the outer surface of the micro-reactor. In one or more embodiments the temperature probe is attached or integrated into the inner surface the outer magnet system. Alternatively, the temperature probe can be attached or integrated into the inner surface of the micro-reactor.

In one or more embodiments the temperature probe will be able to measure the temperature of the sample.

In one or more embodiments a temperature probe will be integrated into the sample magnet. Disclosed herein is also the use of the add-on system according to the above for measuring of ATR-IR spectre in an ATR-IR cell.

Disclosed herein is further a method for modifying an attenuated total reflectance infrared (ATR-IR) spectrometer having a ATR-IR plate. The ATR-IR plate comprises a sample surface side whereon a sample, e.g. a gas, a liquid, a powder, a solid sample or sample mixture, can be placed; and a a light-illuminating surface side situated on the opposite side of the sample surface side, wherein IR light from the ATR-IR spectrometer illuminates the light-illuminating surface side, the IR light thereby interacting with the sample, and wherein light is reflected and/or back scattered from the sample propagates from the light-illuminating surface side thereby being collected by the ATR-IR spectrometer. The method comprises the action of securing an add-on system according to the above onto the first ATR-IR plate side. The advantages obtained hereby are described above.

In one or more embodiments, the method further comprises the action of supplying a sample to the sample cavity and subsequently measuring at least one ATR-IR spectrum of the sample in the ATR-IR spectrometer. Brief description of the drawings

Figure 1 shows a schematic overview of a commercially available ATR-IR cell.

Figure 2 shows an add-on system of this invention inserted in the ATR-IR cell of figure 1.

Figures 3a-b show a first embodiment of the add-on system according to the invention in a side-view (fig. 3a) with the add-on system installed on top of the ATR- IR plate, and in a perspective view (fig. 3b), whereas the micro-reactor being part of the add-on system shown in figs. 3a-b is shown in a bottom-up view in fig. 3c.

Figure 4 shows a second embodiment of the add-on system according to the invention in a side-view with the add-on system installed on top of the ATR-IR plate.

Figures 5a-b show a third embodiment of the add-on system according to the invention in a side-view (fig. 5a) with the add-on system installed on top of the ATR- IR plate, and in a perspective view (fig. 5b).

Figure 6 shows a fourth embodiment of the add-on system according to the invention in a side-view with the add-on system installed on top of the ATR-IR plate.

Figures 7a-b show an alternative embodiment of the outer magnet system.

Figure 8 shows the IR spectra following the reaction kinetics during cellulose hydrolysis in ionic liquids by in-situ ATR-FTIR spectroscopy. Figures 9a-b show the stretching modes in the cellulose hydrolysis reaction of figure 8. Figure 10 shows the Arrhenius plot of hydrolysis reactions using different catalysts.

Figure 1 1 shows the dehydration reaction of glucose to form hydroxymethylfurfual (HMF). Figure 12 shows the IR spectra following the dehydration reaction of glucose to form hydroxymethylfurfual (HMF).

Figures 13a-c show the integrated absorbance of the IR band at 1042 cm^"1. Figure 14 shows a specially designed ATR-IR arm. Description of preferred embodiments

In ATR-IR spectroscopy, the IR radiation is passed through an IR transmitting crystal with a high refractive index allowing the radiation to reflect within the ATR element either once (standard ATR) or several times (multi-bounce ATR). The IR radiation from the spectrometer enters the crystal, onto which the sample is pressed, such that the sample is in intimate optical contact with the top surface of the crystal. The IR radiation subsequently reflects through the crystal into the sample, where it interacts with the molecules in the sample. The backscattered IR light is directed out of the crystal and back into the normal beam path of the spectrometer.

By intimate optical contact is meant that the distance between the sample and the crystal is on the order of 0-5 micrometers, in most cases 0-2 micrometers.

Pressing the sample onto the ATR element is essential for measuring IR spectra in solid and/or powder samples, as, if there is not intimate optical contact, IR spectra of solid and/ powder samples cannot be measured. Figure 1 shows a conventional ATR-IR cell 100 having a box 102 containing different optical elements, an opening for IR light propagating towards the sample and an opening for IR light reflected off and/or backscattered from the sample referred to as 104 and 106, respectively. The ATR-cell 100 further comprises a pressure clamp 108 attached to an arm 1 10 adjustable in height and an ATR-IR plate 200 containing an ATR-IR medium 202 such as diamond, ZnSe, ZnS, Ge, Si, sapphire, KRS-5, silver halides (AgX) crystals, or crystals in similar materials which are transparent in the spectral range of interest and which have a suitable index of refraction.

The ATR-IR plate 200 has two surfaces; a sample surface side 201 on which the sample is positioned, and a light-illuminating surface side 203 situated on the opposite side of the sample surface side 201. IR light from the ATR-IR cell / spectrometer 100 illuminates the light-illuminating surface side 203 and passed through the ATR-IR medium 202 thereby interacting with the sample placed on the ATR-IR plate 200. The IR light 106 reflected off and/or backscattered from the sample 202 is directed out of the ATR-IR medium 202 and propagates from the light-illuminating surface side 203 thereby being collected by the ATR-IR spectrometer/cell 100.

When measuring IR spectra, the sample 204, e.g. a homogenous or heterogeneous liquid sample mixture, is placed on the ATR-IR medium 202 and (if it is solid) pressed by the pressure clamp 108 onto it, such that the sample 204 and the ATR- IR medium 202 are in intimate optical contact, the latter being essential when measuring IR spectra in solid and/or powder samples.

Figure 2 shows a conventional ATR-IR cell 100 where an add-on system according to the invention is attached. The add-on system comprises a micro-reactor 300 which is shown in detail in different embodiments in figures 3-6. The micro-reactor 300 is in the shape of an open structure with an opening 301 defined by an ATR-IR plate facing surface 303 as most clearly seen in figure 3c. The micro-reactor further has a top surface 302 and a side surface 304. When the micro-reactor 300 is placed on the sample surface side 201 of the ATR-IR plate 200, a sample cavity 305 for enclosing a liquid sample 204 is formed between the sample surface side 201 of the ATR-IR plate 200 and the ATR-IR plate facing surface 303 of the micro-reactor 300 the sample surface side 201 of the ATR-IR plate 200 thereby covering the opening 301 in the micro-reactor. The micro-reactor 300 is secured in the spectrometer 100 by the pressure clamp 108 attached to the adjustable arm 1 10.

The micro-reactor 300 may comprise a first opening 306, which allows sample 204 to be added to the micro-reactor 300 after the micro-reactor 300 has been secured to the ATR-IR plate 200. The first opening 306 can further be provided with a sealing means 308, e.g. in form of a membrane which can be penetrated with a needle. The sealing means 308 will ensure that the sample does not evaporate out of sample cavity 305. The first opening 306 and sealing means 308 can be omitted in the micro-reactor 300.

The add-on system may further comprises a membrane or a Teflon ring 310, which is positioned between the micro-reactor 300 and the ATR-IR plate 200 for obtaining a tight sealing between the micro-reactor 300 and the ATR-IR plate 200.

The add-on system also comprises a sample magnet 312 adapted for being placed in the sample cavity 305 and an outer magnet system 400 which encircles the micro-reactor 300 in a plane substantially parallel to the plane in which the ATR-IR plate 200 lies. The outer magnet system 400 is adapted for rotating the sample magnet 312 inside the sample cavity 312.

The first embodiment of the outer magnet system 400A shown in detail in figures 3-6 comprises an outer magnet 402 attached to an outer magnet support 401 . The outer magnet support 401 can be made in a variety of non-magnetic materials, such as e.g. non-magnetic steel or plastic if the production costs are to be kept at a minimum. The outer magnet support 401 is further connected to motor 404 by connecting means 406. When the motor 404 is turned on, the outer magnet support 401 rotates in a plane being substantially parallel to the plane in which the ATR-IR plate 200 lies. When the outer magnet support 401 with the outer magnet 402 rotates, it forces the sample magnet 312 placed inside the sample cavity 305 to follow suit, whereby the sample 204 is stirred. The placement of the outer magnet support 401 is highly advantageous in connection with spectroscopic measurements, since it does not affect or influence the path of the essential measuring light from the spectrometer along with the light backscattered, reflected and/or emitted from the sample. Further, by using the outer magnet support 401 of the invention it is not necessary to subject the sample 204 to any kind of shaking or similar in order to avoid segmentation in the sample 204.

Alternatively, the outer magnet 402 can be made to encircle the micro-reactor 300 by means of compressed air, whereby there is no need for the outer magnet support 401 .

Independently of whether an outer magnet support 401 is used to support the outer magnet 402, there is no physical contact between the micro reactor 300 and the outer magnet system 400A. This in turn means that the mechanical parts responsible for the stirring of the sample do not induce vibrations of the ATR-cell, which would disturb the IR-beam and the delicate optics in the spectrometer and/or ART-cell. This significantly reduces the noise level in the spectra as compared to the spectra obtained just by shaking the sample. By using a very small sample magnet 312, in the order of e.g. a few millimeters in length, it is possible to maintain a small sample volume. This ensures that whatever additional reactants added to the sample 204 will be distributed evenly in the sample more or less instantly. Having a small sample volume combined with an effective stirring in the sample further gives optimum conditions for minimizing the temperature variation in the sample 204. This is advantageous when the spectra need to be measured at elevated temperatures.

Figures 3a-b show a first embodiment of the add-on system according to the invention in a side-view (fig. 3a) with the add-on system installed on top of the ATR- IR plate 200, and in a perspective view (fig. 3b), the latter only showing the micro- reactor 300A and the first embodiment of the outer magnet system 400. The first embodiment of the add-on system comprises a first embodiment of the micro- reactor 300A having a first opening 306, which allows sample 204 to be added to the micro-reactor 300 after it has been secured to the ATR-IR plate 200. The first opening is provided with a sealing means 308, e.g. in form of a membrane which can be penetrated with a needle. The sealing means 308 will ensure that the sample does not evaporate out of sample cavity 305. Figure 4 shows a second embodiment of the add-on system according to the invention in a side-view with the add-on system installed on top of the ATR-IR plate 200. The add-on system comprises the first embodiment of the micro-reactor 300A described in figures 3a-b. On top of the micro-reactor 300A is added a photochemical add-on device 500 comprising a cavity 506 containing a light source 502, e.g. an optical fibre, for initiating and/or making a reaction progress continuously a photochemical reaction, which can subsequently be followed by IR spectroscopy. The light 510 out of the light source can be e.g. UV light or visible light. The photochemical add-on device 500 can be integrated with the micro-reactor 300A, the two parts being one item, or the photochemical add-on device 500 may be separately connected to the micro-reactor 300A.

The photochemical add-on device 500 comprises a window 504 for allowing the light from the light source 502 to enter the sample cavity 305. The window can be an integral part of the photochemical add-on device 500 and be made in e.g. quarts, calcium fluoride or a similar material, which allows e.g. UV and visible light to pass through. Integrated into the add-on system - most likely as part of the additional light source - light collecting means for collecting light transmitted, reflected and/or back scattered from the sample after light from the additional light source has interacted with the sample can also be found. One advantage obtained thereby is that it becomes possible to measure two types of optical spectra from the same sample simultaneously under the exact same in- situ conditions by illuminating the sample from below with the IR light for obtaining ATR-IR spectra of the sample at the same time as illumination the sample from above with the additional light source and collecting the reflected and/or back scattered light from the sample again for obtaining e.g. UV and/or visible spectra. It is also possible to measure the Raman spectra of the sample 'from above' e.g. by using an optical fibre, which - in addition to providing the additional light source to the sample - also collects the back scattered light from the sample. A small microscope objective could also be used as the object from which the additional light source illuminates and which collects the backscattered and/or reflected light from the sample.

The photochemical add-on device 500 may further comprise focusing means 508 for focusing the light 510 out of the light source onto the sample 204. Focusing the light 510 from the light source 502 may be necessary, if the photochemical reaction needs high intensity light in order to be initiated.

Between the photochemical add-on device 500 and the pressure clamp 108 on the spectrometer 100 may be a membrane (not shown in the figure) if the photochemical add-on device 500 is in a fragile material, which could be damaged by pressing the pressure clamp 108 too hard on the device.

Instead of an optical fibre as the light source 502, a photodiode could also be used.

Figures 5a-b show a third embodiment of the add-on system according to the invention in a side-view (fig. 5a) with the add-on system installed on top of the ATR- IR plate 200, and in a perspective view (fig. 5b). The add-on system comprises the first embodiment of the outer magnet system 400 as described above and a second embodiment of the micro-reactor 300B. The second embodiment of the micro- reactor 300B comprises the features as the first embodiment 300A described in figures 3a-b and further has a second opening 314, a third opening 316, and a fourth opening 318, which are independently sealable by means of second sealing means 324, third sealing means 326, and fourth sealing means 328, respectively.

The second opening 314 and the third opening 316 are for allowing gas in and out of the sample cavity 305, thereby providing for in-situ measurements of the IR spectra, e.g. in an atmosphere of nitrogen. The second and third openings 314, 316 can also be used for supplying a gaseous reactant continuously or in pulses, extract a product with a second liquid phase from the top, or to pressurize the micro-reactor cell 300B during the reaction. The fourth opening 318 is for reducing the pressure in the sample cavity 305. The fourth opening 318 or alternatively, the second and third openings 314, 316 can be omitted in the design of the micro-reactor 300.

Figure 6 shows a fourth embodiment of the add-on system according to the invention in a side-view with the add-on system installed on top of the ATR-IR plate. The fourth embodiment on the add-on device comprises first embodiment of the outer magnet system 400 as described above, a micro-reactor 300B as described in figures 5a-b and a photochemical add-on device 500 as shown and described in figure 4. Like in the second embodiment of the add-on device shown in figure 4, the photochemical add-on device 500 can be an integrated part of the micro-reactor 300B or a detachable part. Figure 7a shows a second embodiment of the outer magnet system 400B seen inside in a view from above or below. Figure 7b shows the outer magnet system 400B and a micro-reactor 300, where any of the micro-reactors described in the preceding figures could be used. The outer magnet system 400B may be used in connection with all of the add-on systems described in the preceding figures. The outer magnet system 400B comprises a multiple of outer electromagnets 402' evenly distributed such that they encircle the micro-reactor 300. The outer electromagnets 402' are stationary in position. By changing the magnetic field generated by the individual electromagnets 402', the total magnetic field of the outer magnet system rotates around the micro- reactor 300 and thereby forces the sample magnet 312 placed inside the sample cavity 305 to follow suit, whereby the sample 204 is stirred. In the outer magnet system 300B shown in figure 7a, there are six electromagnets 402' encircling the micro-reactor 300. Alternatively, two, three, four, five, seven, eight or more than eight electromagnets 42' could be used.

The outer magnet system 400B normally comprises a magnet cavity 403 wherein the electromagnets 402' are placed. The magnetic cavity 403 is normally made in a non-magnetic material, such as e.g. aluminum. In all the above described add-on systems, the sample magnet 312 may comprise a thermometer integrated in the sample magnet 312 for measuring the temperature of the sample 204. The thermometer will preferably be in remote contact with a read- out system. In this manner, the precise temperature of a liquid sample 204 can be measured during the measurements period.

The micro-reactor 300 and the outer magnet system 400B can be integrated into one unit possibly be a glass-metal thread. This may assist in stabilizing the system.

The add-on system may further comprise a temperature probe 408 positioned between the outer magnet system 400B and the micro-reactor 300 as shown in figure 7b. The temperature probe 408 is adapted for heating and/or cooling the sample such that a constant temperature or an increasing/decreasing temperature is obtainable. Normally the temperature probe 408 can heat up the sample to temperatures of approximately 60°C or 80' C. By keeping the temperature in this rage, the add-on system can be used with existing ATR-IR plates 200, which may not be able to withstand temperatures above 100' C.

The temperature probe 408 can be a separate item or could be attached to or integrated into the outer surface of the micro-reactor 300. This applies to all the micro-reactors 300 shown and described in the preceding figures.

Alternatively, the temperature probe 408 can be attached or integrated into the inner surface the outer magnet system 400B shown in figure 7a or the outer magnet system 400A shown in the preceding figures - the latter with the temperature probe 408 integrated in the outer magnet support 401.

Alternatively, the temperature probe can be attached to or integrated into the inner surface of the micro-reactor 300. The temperature probe can be able to measure the temperature of the sample. Also, the temperature probe can be integrated into the sample magnet. Figure 8 shows an example of in-situ difference ATR-FTIR spectra obtained using the add-on device of the present invention. In figure 8 is shown the difference IR spectra 600 following the reaction kinetics during cellulose hydrolysis in ionic liquids. The acid catalyzed hydrolysis of cellulose in ionic liquids has been subject to huge interest in the last decade, since it discovered that some ionic liquids are able to dissolve cellulose. The dissolution of cellulose makes the glycoside bond more available for attack by the catalyst than the crystalline fibres in aqueous slurry. Several studies describing hydrolysis in ionic liquids in the presence of both acidic catalysts seem to focus on the product distribution after hydrolysis and are concerned with the kinetics of cellulose in ionic liquids.

One problem when describing kinetics of cellulose in ionic liquids is that the cellulose is insoluble in conventional solvents, thereby making it very challenging to estimate the initial reaction rate by conventional analytical methods. Further, the analysis of hydrolysis rates is complicated as the acidic catalyst is active for glucose dehydration to e.g. Hydroxymethyl- furfural (HMF) and livulinic acid occurs simultaneously at considerable rates which makes the produced glucose an unsuitable measure for the hydrolysis rate. Instead of monitoring the formation of hydrolysis products figure 8 follows the disappearance of glycoside bonds during hydrolysis by in-situ ATR-FTIR in a magnetic stirred micro-reactor using the add-on device of the present invention. The spectra obtained at early time are marked 608 and the pseudo steady state spectra for the first 30 minutes at 120 °C are marked 610.

The two group vibrations comprising C-O-C stretching vibrations are located around 1 165-1 155 cm^"1 and 965 cm^"1 marked 602 and 604 in figure 8, respectively. The C- O-C stretching vibrations located around 1 165-1 155 cm^"1 are anti-symmetric stretching modes as illustrated in figure 9a and the C-O-C stretching vibrations around 965 cm^"1 are symmetric stretching modes as shown in figure 9b.

When using the popular ionic liquids 1-butyl-3-methylimidazolium chloride ([BMIM]CI) and 1 -ethyl-3-methylimidazolium chloride ([EMIMjCI) it was only possible to follow the very weak symmetric stretching mode at 965 cm^"1, that turned out to be unsuited for accurate quantitative interpretation, due to overlap with a very strong mode comprising bending of the C2-H proton on the [BMIM] cation. However this C- H bending mode disappears when substituting the hydrogen in the C2 position with deuterium, thereby obtaining the ionic liquid 1 -butyl-2,3-dimethylimidazolium chloride [BDMIM]CI. This allows for a quantitative analysis of the intense band due to anti-symmetric stretching mode of the glycoside bond.

The difference spectra 600 of H₂S0₄ catalyzed hydrolysis of Avicel cellulose in [BDMIMJCI seen in figure 8 show the total change in the composition during the hydrolysis reaction. The difference spectra 600 show a mixture of bands due to both cellulose and glucose disappearing as the reaction proceeds and care must be taken with a quantitative interpretation. As the pyranose backbone of the cellulose chains shows significant similarities with glucose, several of the types of vibrational modes are found in spectra of both structures.

In the polymeric species, the introduction of the acetal group with the glycoside bond, however, changes several modes around the oxygen atom inside the pyranose ring. Especially the intense C-0 stretching modes inside the puranose ring are blue-shifting the C-0 stretching modes significantly up to around 1070-1060 cm^"1 (marked 606 in figure 8). The intensity of these bands undergoes a significant decrease under hydrolysis and thereby indicates the hydrolysis of cellulose. Bands due to these intense C-0 stretching modes did show some linear decrease as a function of time. However, glucose also has bands in the same area, therefore these bands must be considered unsuitable for quantitative analysis of the hydrolysis rate, as they would express an unknown fraction of both the rate of cellulose and the glucose conversion.

The anti-symmetric stretching of the glycoside bond has a relatively strong absorption band around 1 155-1 160 cm^"1, which is found in cellulose and in celluoligiomers including cellobiose. The spectra of glucose show an intense band around 1 140 cm^"1 due to stretching of the C1 -01 bond and bending of the C1-H and 01 -H bonds, whereas cellulose and the celluoligiomers only show a weak absorption band. However, these weaker absorption bands can easily be distinguished and quantified by mathematical deconvolution of the ATR-corrected difference spectra.

No general linear time dependency of the area of this 1 140 cm^"1 was observed. The kinetics of cellulose hydrolysis in ionic liquids are - due to the analytical difficulties mentioned earlier - quite limited, but activation energies of cellulose hydrolysis catalyzed by strong mineral acids or similar systems are reported. Therefore, the hydrolysis was initially investigated using sulfuric acid to compare the in-situ FTIR method with kinetics derived from studies using more conventional analytical methods.

Cellulose hydrolysis was investigated in-situ in [BDMIMJCI using 1.7 wt % sulfuric acid as catalyst from 90 to 140 °C in steps of 10 °C. The small volume of the stirred sample in the detachable stirred micro-reactor 300 ensured fast heating of the cooled premixed reaction mixture. Initially a slight increase of the intensity of the 1 157 cm^"1 band was observed probably due to cellulose being redissolved. However, within short time depending of the temperature, a steady decrease of the band was observed. The initial reaction rates can be determined from the integration of the 1 157 cm^"1 band of the deconvoluted IR spectra. The determination of first order rates k expressed as absorbance s^"1 can be established with high accuracy from the in-situ experiments. Figure 10 shows the Arrhenius plot 700 for the sulphated nano titania (SO -Ti0₂) catalyzed hydrolysis of cellolose in [BDMIMJCI marked 702, the H₂S0₄ catalyzed hydrolysis of cellolose in [BDMIMJCI marked 704, and the H₂S0₄ catalyzed hydrolysis of cellobiose in [BDMIMJCI marked 706. The data are obtained on the basis of IR spectra measured at in-situ conditions at different elevated temperatures using the add-on device of the present invention comprising the micro- reactor 300 and the outer magnet system 400.

Cellobiose is often used as a model to describe cellulose hydrolysis as it is much easier to handle the analysis due to the high solubility of cellobiose in a lot of solvents. Cellobiose hydrolysis was investigated under the same conditions as described above. As can be seen in figure 10, the apparent activation energy for cellobiose hydrolysis is significantly lower than for cellulose when comparing the slopes in the Arrhenius plot of cellobiose 706 with that of cellulose 704.

The cellulose hydrolysis in S0₄ ² -Ti0₂ (706 data) was investigated in the temperature range from 130-160 °C in steps of 10 °C in cellulose solution containing 20 wt.% sulphated nano titania. Due to the strong broad band of Ti-0 vibrations the homogenosity of the sample could be monitored, and the catalyst density was found to be constant during hydrolysis experiments. The catalyst was found to be active both for cellulose hydrolysis and glucose conversion and the band due to glucosidal C-0 stretching decreased in similar way that was observed when sulfuric acid.

Figure 12 shows the IR spectra 800 following the dehydration reaction of glucose to form hydroxymethylfurfual (HMF) shown in figure 1 1. The reaction occurs in the ionic liquid 1 -butyl-3methyl-imidazolium chloride ([BMIMJCI) in an ATR-FTIR spectrometer using a micro-reactor 300 according to the invention, equivalent to a batch reactor, containing both reactants and products throughout the reaction. The experiments were performed using a Nicolet iS5 spectrometer using a Specac Golden Gate ATR unit with a High temperature diamond ATR-IR cell. The IR spectra in figure 12 are measured at 1 10 °C over a time interval of 70 minutes, with the spectrum measured at 0 seconds marked as 802 and the spectrum measured at 70 minutes marked 804. The arrows mark the trend in increase/decrease of the different IR bands as time increases. The time resolution in the experiment shown in figure 12 is 60-15 seconds. This is significantly faster than if the spectra were to be measured with e.g. conventional DRIFTS FT-IR spectroscopy.

The IR band at 1042 cm^'1 marked 806 in the figure is characteristic of glucose. As is apparent from figure 12, the glucose characteristic band at 1042 cm^"1 decreases as a function of time while at the same time, new bands around 1510 cm^"1 (marked 808) and 1680 cm^"1 (marked 810) increase in intensity. The 1510 cm^"1 and 1680 cm^" ¹ bands are characteristic of HMF and is an indication of formation of HMF. Figures 13a-c show the integrated absorbance of the IR band at 1042 cm^"1 assigned to glucose as a function of time obtained from IR spectra measured at 80 °C (902 in figure 13a), 1 10 °C (904 in figure 13b) and 130 °C (906 in figure 13c). The decrease in the absorbance mimics the decrease in glucose concentration as glucose is dehydrated thus forming HMF. Note the difference in time scale, mimicking difference in reaction rate dependent on the temperature. In this example, the reaction rate is exponentially dependent on the temperature.

From figures 12-13 it is clear that the dehydration reaction of glucose to form HMF is significantly faster at high temperatures compared to lower temperatures, thus following an Arrhenius trend.

By using an add-on system according to the invention, it is thus possible to measure dehydration reactions at high temperatures with an excellent time resolution in a batch reactor mode. The latter option provides a fast way of obtaining Arrhenius plots, which otherwise is a very time-consuming process using conventional IR spectroscopic methods.

The outer magnet system 400B and the micro-reactor 300 may be connectable to a specially designed ATR-IR arm 1000 shown in figure 14, which is designed such that it either substitutes the existing adjustable arm and the pressure clamp on the conventional ATR-IR cell/unit in the ATR-IR spectrometer or is directly connectable to the existing arm. The ATR-IR arm 1000 is constructed such that when it is connected to the conventional ATR-IR cell/unit in the ATR-IR spectrometer and to any of the outer magnet system 400B and/or the micro-reactors 300 described herein, such that the user can move the arm 1000 and the outer magnet system 400B and/or the micro-reactor 300 together.

The ATR-IR arm 1000 comprises one end 1002, which is connectable to the conventional ATR-IR cell/unit and an opposite end 1004, which the user fixates to the outer magnet system 400B and/or the micro-reactors 300. The first end 1002 is normally connected to a base part (not shown in figure) which in turn is connectable directly to the conventional available ATR-IR cells. The interface between the ATR-IR arm 1000 and the base part functions like a pickup on an old record player, where the ATR-IR arm 1000 can be turned to one side and be lifted up and down. When the ATR-IR arm 1000 is positioned in the measuring position, a magnet mounted on the base can function such that it is ensured that outer magnet system 400B and/or the micro-reactors 300 is positioned directly over the ATR-IR plate. In this way, the ATR-IR arm 1000 can be moved to the side and back again several times and still be positioned on top of the ATR-IR plate at the same position each time.

On the second end of the ATR-IR arm 1000, the two 'claws' may have a springs which ensures a quick and easy mounting of the outer magnet system 400B and/or the micro-reactors 300 in the ATR-IR arm 1000. Alternative means for fastening the outer magnet system 400B and/or the micro-reactors 300 in the ATR-IR arm 1000 could also be imagined.

The ATR-IR arm 1000 comprises a mount 1006, which is adapted for inserting different equipment to enhance the functionality of the ATR-IR arm 1000. Examples of such additional equipment could be a clamp which fixates the micro-reactor 300 to the ATR-IR plate while the ATR-IR arm 1000 itself fixates/holds the outer magnet system 400B.

In figure 14, a slightly different design version of the outer magnet system 400B and the micro-reactors 300 is shown. However, it should be understood that the principle of the ATR-IR arm 1000 described above applies to different designs of both the outer magnet system 400B and/or the micro-reactors 300.

A motor incorporated in the mount 1006 could also be imagined. The use of the motor could be control the rotation of outer magnets or activate the magnet system in the case where stationary magnets are used. If the outer magnet system 400B and the micro-reactor 300 are in a design where the ATR-IR arm 1000 engages with a toothed wheel as shown in figure 14, the motor in the mount 1006 may have a corresponding toothed wheel which interactions with that on the micro-reactor 300 or the outer magnet system 400B. Yet alternatively, the motor could be integrated as a part of the outer magnet system 400B and/or the micro-reactor 300 itself.

To further automate the ATR-IR cap arm 1000, a sensor 1008 can be placed on the ATR-IR cap arm 1000 - either at the first end 1002 of the arm 1000 as shown in figure 14 or at the second end 1004 of the arm 1000, where it connects to the outer magnet system 400B and/or the micro-reactor 300.

When a outer magnet system 400B and/or the micro-reactor 300 is placed over the ATR-IR plate 200, the sensor 1008 will then activate the motor and thereby initiate rotation or activate of the magnet system such that the sample magnet 312 rotates.

Furthermore, the sensor 1008 may also deactivate the motor or stop the magnet system from rotating the sample magnet 312 when the ATR-IR cap arm 1000 is lifted. In this way the sensor 1008 functions to give the system information on whether the ATR-IR cap arm 1000 is in an active state/inactive state or moved between these states. The sensor 1008 can on this account activate the motor (or motors if a multiple is used) accordingly.

Integrated into the ATR-IR arm 1000 can also be a complete 'Plug'n'Play' system, where high temperature quick-connect gas connections, e.g. produced by Swagelok can be placed on the ATR-IR arm 1000 to automatically interact with the openings 314, 316, 318 in the micro-reactor 300 when the outer magnet system 400B and/or the micro-reactor 300 is inserted into the ATR-IR arm 1000. The ATR-IR arm 1000 will in this scenario also act as a heat sink for both the gas connections and the motor (if present) to lead heat away and avoid overheating the normally plastic based quick-connect gas connectors.

References

100 conventional ATR-IR cell

102 ATR-IR cell box containing optical elements

104 IR light propagating towards the sample

106 IR light emitted/reflected off the sample

108 pressure clamp

1 10 adjustable arm holding the pressure clamp

200 ATR-IR plate

201 sample surface side of the ATR-IR plate

202 ATR-IR medium

203 light-illuminating surface side of the ATR-IR plate

204 sample

300 micro-reactor

300A first embodiment of the micro-reactor

300B second embodiment of the micro-reactor

301 opening in the micro-reactor

302 top surface of micro-reactor

303 ATR-IR plate facing surface

304 side surface of micro-reactor

305 sample cavity

306 first opening

308 first sealing means

310 membrane, e.g. a Teflon ring

312 sample magnet

314 second opening

316 third opening

318 fourth opening

324 second sealing means

326 third sealing means

328 fourth sealing means

400 outer magnet system

400A first embodiment of the outer magnet system

400B second embodiment of the outer magnet system

401 outer magnet support 402 outer magnet

402' outer electromagnet

403 magnetic cavity containing the outer electromagnets

404 motor

406 connection between the motor and the outer magnet support

408 temperature probe

500 photochemical add-on device

502 light source, e.g. an optical fibre

504 window

506 cavity in the photochemical add-on device

508 focusing means

510 light from the light source

600 in-situ difference ATR-FTIR spectra of cellulose hydrolysis in IL

602 IR band around 965 cm^"1

604 IR band around 1 165-1 155 cm^"1

606 IR band around 1070-1060 cm^"1

608 IR different spectrum at early time (t = 0 seconds)

610 pseudo steady state IR different spectrum after 30 minutes

700 Arrhenius plot (AP)

702 AP of S0₄ ² -Ti0₂ catalyzed hydrolysis of cellolose in [BDMIMJCI

704 AP of H₂S0₄ catalyzed hydrolysis of cellolose in [BDMIMJCI

706 AP of H2SO4 catalyzed hydrolysis of cellobiose in [BDMIMJCI

800 IR spectra of the dehydration reaction of glucose to form HMF

802 IR spectrum at t = 0 seconds

804 IR spectrum at t = 70min

806 IR band around 1042 cm^"1

808 IR band around 1510 cm^"1

810 IR band around 1680 cm^"1

902 IR spectra measured at 80 °C

904 IR spectra measured at 1 10 °C

906 IR spectra measured at 130 °C

1000 ATR-IR cap arm

1002 first end of the ATR-IR arm

1004 second end of the ATR-IR arm 1006 mount in the ATR-IR arm

Claims

An add-on system for an attenuated total reflectance infrared (ATR-IR) spectrometer, the ATR-IR spectrometer comprising an ATR-IR plate with:

- a sample surface side whereon a sample, e.g. a homogenous or heterogeneous liquid sample mixture, can be placed; and

- a light-illuminating surface side situated on the opposite side of the sample surface side, wherein IR light from the ATR-IR spectrometer illuminating the light-illuminating surface side passes through the ATR-IR plate, the IR light thereby interacting with the sample, and wherein light is reflected and/or back scattered from the sample passes through the ATR-IR plate and propagates from the light-illuminating surface side thereby being collected by the ATR-IR spectrometer,

the add-on system comprising:

- a micro-reactor in the shape of an open structure with an opening defined by an ATR-IR plate facing surface, wherein, when the micro-reactor is placed on the sample surface side of the ATR-IR plate, a sample cavity enclosing the sample is formed between the sample surface side of the ATR-IR plate and the ATR-IR plate facing surface of the micro-reactor, the sample surface side of the ATR-IR plate thereby covering the opening in the micro-reactor;

- a sample magnet adapted for being placed in the sample cavity; and

- an outer magnet system encircling the micro-reactor in a plane substantially parallel to the plane in which the ATR-IR plate lies, wherein, when the micro-reactor is placed on the ATR-IR plate, the outer magnet system is adapted for rotating the sample magnet inside the sample cavity.

An add-on system according to claim 1 , wherein the micro-reactor further comprises a first opening for introducing the sample and/or reactants into the sample cavity.

An add-on system according to claim 2, wherein the micro-reactor further comprises first opening sealing means for sealing the first opening.

4. An add-on system according to claim 3, wherein the first opening sealing means is a membrane which can be penetrated by a needle.

5. An add-on system according to any of the preceding claims, wherein the micro-reactor further comprises a second opening for allowing gas and/or liquid to enter the sample cavity and a third opening allowing gas and/or liquid to exit the sample cavity, the second opening and the third opening being independently sealable.

6. An add-on system according to any of the preceding claims, wherein the micro-reactor further comprises a fourth opening adapted for changing the pressure inside the sample cavity.

7. An add-on system according to any of the preceding claims, wherein the add- on system further comprises an O-ring being positioned between the sample surface side of the ATR-IR plate and the ATR-IR plate facing surface of the micro-reactor.

8. An add-on system according to any of the preceding claims, wherein the system further comprises a source of light, e.g. a diode or an optical fibre connectable to a laser or similar, and a light source containing cavity comprising the source of light, the light source containing cavity being positioned on top of the micro-reactor such that light from the source of light can illuminate and/or interact with the sample inside the sample cavity.

9. An add-on system according to claim 8, wherein the add-on system further comprises focusing means, e.g. an optical lens, for focusing the light from the source of light onto a specific point inside the sample.

10. An add-on system according to claims 8 or 9, wherein the light source containing cavity comprising the source of light can be adjusted in height.

1 1 . An add-on system according to any of the preceding claims, wherein the sample magnet is a Teflon magnet.

12. An add-on system according to any of the preceding claims, wherein the sample magnet comprises a thermometer integrated in the sample magnet for measuring the temperature of the sample, the thermometer preferably being in remote contact with a read-out system.

13. An add-on system according to any of claims 1-12, wherein the outer magnet system comprises a magnet cavity with a multiple of electromagnets evenly distributed such that they encircle the micro-reactor. 14. An add-on system according to claim 13 with six electromagnets encircling the micro-reactor.

15. An add-on system according to claims 13 or 14, wherein the magnetic cavity is in a non-magnetic material.

16. An add-on system according to any of claims 1 -12, wherein the outer magnet system comprises an outer magnet and means for rotating the outer magnet around the micro-reactor.

17. An add-on system according to claim 16, wherein the outer magnet is attached to an outer magnet support, e.g. in the form of a rotating ring in a non-magnetic material. 18. An add-on system according to any of the preceding claims further comprising a temperature probe positioned between the outer magnet system and the micro-reactor.

19. An add-on system according to claim 18, wherein the temperature probe is attached to or integrated into the outer surface of the micro-reactor. 20. An add-on system according claim 18 or 19, wherein the temperature probe is attached or integrated into the inner surface the outer magnet system.

21 . An add-on system according to any of claims 18-20, wherein the temperature probe can heat the sample up to temperatures of approximately 60°C or 80°C.

22. A method for modifying an attenuated total reflectance infrared (ATR-IR) spectrometer, the ATR-IR spectrometer comprising an ATR-IR plate with:

- a sample surface side whereon a sample, e.g. a homogenous or heterogeneous liquid sample mixture, can be placed; and - a light-illuminating surface side situated on the opposite side of the sample surface side, wherein IR light from the ATR-IR spectrometer illuminates the light-illuminating surface side, the IR light thereby interacting with the sample, and wherein light is reflected and/or back scattered from the sample propagates from the light-illuminating surface side thereby being collected by the ATR-IR spectrometer,

the method comprising the action of securing an add-on system according to any of claims 1 -21 onto the sample surface side of the ATR-IR plate .

23. A method according to claim 22 further comprising the action of supplying a sample to the sample cavity and subsequently measuring at least one ATR-IR spectrum of the sample in the ATR-IR spectrometer.