Ftir why use kbr

Generally, it is easy to produce a good quality KBr pellet if an evacuable pellet die is used correctly. When the sample is added to the halide salt the clarity of the disc will depend to a large extent on the quantity and type of sample. For a 13mm diameter die - mg KBr and 1 mg sample at load of 8 tons are a good starting point.

To ensure that a sample pellet is produced which will enable accurate spectra of samples to be obtained, it is essential that the sample is thoroughly blended with the halide salt powder. Blending can be achieved either by using a mortar and pestle or by using a grinding mill. The overall quality of a pellet is largely dependent upon the quality of the KBr or halide salt powder used, which should always be of a spectroscopic grade of purity.

However, some faults in the produced sample pellet may occur due to a variety of reasons. Some of these faults and their remedies are tabulated below.

The faults described are for pure KBr or other halide salts which do not contaminate the sample. Powder not uniformly flat in the die, leaving large particles which do not vitrify when pressed. Good to know about KBr pellets KBr pellet formation is a classic technique for the analysis of solids that retains attractive features over newer techniques such as ATR.

The latter is then pressed into a disk. If information on the physical state, e. Some substances, such as base hydrochloride, may exchange halogen with potassium bromide powder, again mulling is preferable. However most mulling agents contain bands in the spectrum which may mask bands in the sample spectrum. KBr does not contain bands in the mid-IR region of the spectrum, and therefore preparation as halide disks potentially loses less information.

The strength of an IR absorption spectrum is dependant on the number of molecules in the beam. With a KBr disk the strength will be dependant on the amount and homogeneity of the sample dispersed in the KBr powder.

The amounts stated below are for guidance only, the bulk density of the sample or other diluents may require these to be varied. They will also have to be varied according to the diameter of the disk required. The weights quoted are for a 16 mm diameter disk. Approximately half should be used for the 13 mm diameter disks.

The spectrum quality is affected by the quality of the disk. The flatness of the baseline is dependent on the particle size and dispersion of the sample in the KBr powder. After drying the powder, store it in a desiccator. When performing measurements, the background can be measured with an empty pellet holder inserted into the sample chamber.

However, background measurements on a pellet holder with a pellet of KBr only, that contains no sample, can correct for infrared light scattering losses in the pellet and for moisture adsorbed on the KBr. This is a method for measuring powder samples with simple sample pretreatment. The sample is distributed in a liquid of approximately equal refractive index, and the infrared spectrum is measured.

Generally, the powder is dispersed in non-volatile liquid paraffin Nujol that has low absorption in the infrared region. To prepare a sample, pulverize approximately 10 mg sample powder in a mortar and pestle. Add one or two drops of liquid paraffin and mix to distribute the sample powder in the liquid paraffin.

Apply the paste to a liquid cell KBr crystal plate, etc. Since liquid paraffin exhibits absorption near to cm -1 , cm -1 , cm -1 , and cm -1 as shown in Fig. When light is shone onto a powder sample, some of the light undergoes specular reflection at the powder surface and other light enters the sample and reappear from the surface as diffuse reflected light after repeated transmission and reflection.

The diffuse reflection method is used to obtain spectrum using the diffuse reflected light. When measuring an infrared spectrum using the diffuse reflection method, the sample powder is normally not measured directly but diluted in an alkali halide, such as KBr.

As we have gained from former experience in this field, the influence of the viscosity and rheological properties of embedding materials on the potential for contamination of samples is not significant except for samples with extremely high open porosity. Another way to reduce the potential contamination of the sample with resin is the barrier method.

This involves the covering of the sample by an insulating material prior to embedding the sample in synthetic resin. The authors [ 3 ] tested various methods to prevent the infiltration and to minimize contamination with resin using very thin layer of Rhoplex AC acrylic dispersion thickened with fumed silica.

Another successful barrier is cyclododecane [ 2 ],[ 10 ] which has been commonly used as a temporary consolidant and barrier coating in conservation processes. When fixing a sample, synthetic resin may be substituted by halide salts i. These halide salts are inactive in infrared light. A cross-section of a multi-layer sample may be made by pressing the sample into the halide salt pellets [ 8 ]-[ 14 ].

This method eliminates any contamination of the studied sample. However, the pellet is very hydroscopic, and therefore must be kept in a desiccator for long-term storage. Eleven samples of paint layers were taken from the surface of one historical railway car from the collection of the National Technical Museum in Prague, Czech Republic. From these samples, eleven cross-sections were prepared using different methods:.

The sample was cut through the center by two parallel cuts and pressed again into the new pellet, perpendicularly to the direction of the first compression. Before embedding into the Dentacryl samples DC and DCt were immersed into the cyclododecane Sandragon, Czech Republic either in the form of melt sample DCt or as a solution in diethylether p.

Before being embedded into the Polylite, samples PCt and PC were immersed into melted cyclododecane and a solution in diethylether, respectively. Sample PS was first immersed into a potassium liquid glass Water glass Inc. Sample PH was first immersed into an aqueous dispersion of polyvinyl acetate Herkules Druchema, Czech Republic and then embedded into the Polylite resin.

A sample of paint layers of the painting Madonna from the church of St. Thomas in Brno in the Czech Republic was divided in two parts. The preparation of its cross-sections followed the preparation methods of sample KBr sample no.

The preparation of all samples is summarized in Table 1. Cross-sections have been observed using optical fluorescence stereomicroscope Leica M FC Leica Microsystems, Switzerland equipped with various objectives zoom ; magnification 7. Each spectrum has been achieved in the range of — cm —1 at a spectral resolution of 8 cm —1 and with scans on the average using Omnic software Thermo Scientific Inc.

During the analysis of the cross-section sample taken from the paint layers of the historical railway car collection of the National Technical Museum, Prague, Czech Republic different barrier methods to prevent the infiltration of embedding materials in the samples were used.

The two embedding materials used were synthetic resin [polyacrylic resin Dentacryl and polyester resin Polylite ]. First, samples were fixed into the resin without any protective layers to verify the infiltration of embedding resins into the layers of analysed sample. Next, before embedding, samples were covered by a protective layer. The barrier layers studied were: cyclododecane in melted form and as a solution, liquid glass, dispersion of polyvinyl acetate Herkules , a coat of sputtered gold and a coat of carbon.

Infiltration of the sample had been verified comparing the infrared spectra ATR mode with germanium crystal of the peripheral layer of the sample in Figure 1 marked with number 1 which was embedded into synthetic resin and another sample which had been pressed in KBr pellet. This layer was characterized separately by the methods of FTIR micro spectroscopy ATR mode — see Experimental, and transmittance mode — the first layer was scraped off from the surface of the sample, mixed with KBr powder, pressed into a pellet and analysed in the range of — cm —1 at a spectral resolution of 8 cm —1 with scans , Raman micro spectroscopy at the National Gallery in Prague by Ing.

Results of all these methods confirmed that this layer consists of titanium white rutile and phthalocyanine blue bound with linseed oil. The presence of vibration spectral bands characteristic for the synthetic resin, used as the embedding material, appeared in the spectrum of this first layer of the analysed sample confirms the infiltration of embedding material in the sample. Multilayer sample pressed twice in a KBr pellet.

Scheme left : First the multilayer sample is pressed into the flat KBr pellet. The middle part of the pellet is precisely cut by two parallel cuts. This part is pressed again into the new pellet, perpendicularly to the direction of compression of the original pellet; Microscopic image of the sample of paint-layers of historical railway car in KBr pellet right. The contamination of sample with embedding material may also occur as a result of smearing during dry or wet polishing.

When such smearing does occur, all layers of the sample, peripheral layers as well as inner layers, are affected the same way without any distinction.

Nevertheless, the contamination of inner layers of the sample with synthetic resin had been tested as well. On the contrary, the use of liquid glass and Herkules as a barrier method can show a higher smearing effect on the contamination of surface of the sample than the synthetic resin. The pressing of the sample into the KBr pellet was chosen as the comparative method - without any contamination. The preparation of pellets for FTIR microscopy may proceed in several ways, depending on the size and nature of sample:.

In the KBr pellet, layers cut flat at an angle are inserted so that all layers are gradually revealed. Multilayer sample is pressed into the KBr pellet perpendicularly to the plane of the layers and then the pellet is carefully ground down to the reveal the cross-section of the sample material.

First, the multilayer sample is pressed into the flat KBr pellet. The middle part of the pellet the sample in pellet is precisely cut by two parallel cuts. This part is pressed again into the new pellet, perpendicularly to the direction of compression of the original pellet Figure 1. Results of analyses of samples prepared in this way are satisfactory and definitely undistorted by the presence of any auxiliary substances.

FTIR spectrum of the sample fixed in KBr pellet is presented in Figures 2 and 3 the upper red spectrum — sample KBr and was taken as reference for the other barrier methods. FTIR spectra of the cross-section of paint-layers of historical railway car embedded into the acrylic resin.