A clean work station – integrated solution of the samples contamination problem during the trace elemental analysis
The main advantage of PRIZ adapter designed and manufactured by SIMEX Company is an ability to obtain high quality spectrums of micro samples having been transformed into a thin layer on mirror-like finished metal plates. Visual monitoring system with built-in camera greatly increases information scope when adjusting as well as reliability of the examination results. This adapter can also be useful in registering reflection spectrums from bulk materials and solid objects of random form including optical parts.
Classic method for registering substance spectrum in a mixture with potassium bromide powder (KBr) is widely known. It provides good quality spectrum easy to further processing and identification in spectrum data bases. Disadvantages of pressing with potassium bromide include irrecoverable loss of sample during examination and complicated sample preparation: fine grinding, selection of optimum proportion of the substance to be examined and high purity potassium bromide with subsequent pressing of pills using press and mold.
Many users tend to modify pressing with more convenient and advanced methods, first of all – ATR (Attenuation Total Reflectance) method. In spite of many advantages this method cannot provide good measuring results in all cases. Sample layer thickness exposed to radiation is relatively small (a few microns to several dozens of microns) and depending on physical and chemical properties of the substance the spectrum can be weekly expressed especially in its short wave section. This impairs identification in common data bases and in addition to that the crystals have their own transmission spectrum and reduce spectral measurement range. Absorption bands of impurities in low concentrations can be practically not recognized in ATR spectrums. If a sample is of irregular shape and too hard, there is no good contact with ATR crystal, besides many crystals are quickly worn out during the operation. Samples may be contained in aggressive environments and have refraction index which prevents obtaining noticeable absorption bands in conjunction with available ATR crystal. It’s good that many problems can be solved by selection of ATR element material (diamond, germanium), utilizing multiple reflection (MATR) or sample preheating (ATR thermal cell).
Introduction of complex highly sensitive broad range IR-microscopes (particularly MIKRAN [1, 2] in Russia, manufactured by SIMEX since 2006) has enabled taking spectrums of samples with dimensions starting from 5 µm, performing automated mapping of surfaces, registering spectrums of inclusions and discontinuities with application of diaphragms, reaching high sensitivity and rate of measurements using cooled MST-detectors.
Out of several methods applicable in IR-microscope, examination of samples in form of thin films on mirror plates produced from alloyed steel has been most frequently used. Basically, the sample preparation is about giving a micro object the form of thin layer (several microns thick) with special roller. Spectrum of such layer is taken in so called double pass mode when emission passes through the layer two times reflecting from the mirror backing. The layer after rolling has irregular thickness which allows selection of optimal area via masking and obtain highly detailed standard type spectrum.
There is great number of cases where dimensions of the samples examined are bigger than 200 µm and seeking small areas for the analysis is not required. In such cases the double pass method can be employed without using highly sensitive IR-microscope. This approach is implemented in low-cost PRIZ adapter by SIMEX (Fig.1). According to operating functions this adapter resembles a simplified IR or small-sized microscope. Mini microscope integrated camera is used as binocular and shows an image of the examined sample on a computer monitor. PRIZ is also applicable for powder type samples which will have thin layer form on mirror plate surface after sample preparation. Visualization system helps user in location of an examination object and defining maximum level of IR-signal.
PRIZ adapter benefits:
• sample will not be lost and can be analyzed by other methods as required;
• quenched plates made of alloyed steel are chemically and mechanically stable which allows examination of relatively hard materials as well as aggressive substances including liquid and paste-like ones;
• standard type spectrum with an expressed pattern; By adjusting layer thickness user can achieve prominent registration of week absorption bands either;
• quick sample preparation, visual control in photometric measurements.
In order to facilitate sample preparation users are offered an inexpensive mini-press (Figure 2) designed and manufactured by SIMEX. Samples are crushed in mini-press between two steel plates each of which can be used afterwards. Comparing to rolling method, the mini-press procedure excludes sample loss likelihood, allows better pressure control and prevents risk of micro particle “sticking” to the roll working surface.
Measuring procedure is very simple. A small fraction shall be taken and placed onto one of the mini-press plates at nearly centre location. Then the other plate is placed from above and several twisting movements are made with low torque. Clean mirror plate is installed into the adapter and base spectrum is registered following the surface aiming. After positioning one of the plates with sample, the sample preparation quality can be checked by visualization system while engaging an online sample spectrum tracing. Spectrum registration resolution is 2 to 8 cm–1, one spectrum registration time – about 20 seconds. Fig.3 shows FCM pigment spectrums taken by PRIZ and ATR adaptors.
If the layer is too thick and absorption bands are hardly separated, the plates again are placed into the mini-press for further rolling. Roller can also be used for this purpose. Liquids and paste-like substances are simply deposited with thin layer onto the plate surface. Mirror-diffuse reflection spectrums from powders are taken within a special cell included in supply package. Before placing inside the cell it is recommended to mix the powder with potassium bromide in small proportion.
Solid samples, optical parts, crystals etc. are positioned in special holder. Accuracy and high repeatability of measurements are obtained through precise visual aiming at photometric surface.
LIQUID CELL WITH ADJUSTABLE THICKNESS OF THE TESTED LIQUID LAYER
There are three main methods to obtain liquid spectrums in contemporary spectroscopy: using standard liquid cells, backing windows and ATR (MATR) adaptors.
The standard type liquid cell is made up of an enclosure with two attached windows and a shim between them. By exchanging shims, you can adjust the liquid layer thickness and therefore the absorption intensity in resultant spectrums. This method is optimal for quantitative analysis but is very labor consuming, not good for viscous liquids and requires expensive consumables (crystal windows).
Backing windows of small diameter are applicable when registering spectrums of viscous liquids and suspensions. They are used particularly for examination of highly blurring or chemically aggressive liquids, but the backing windows are impracticable for quantitative analysis since the liquid layer thickness cannot be precisely set up.
The Attenuation Total Reflectance method is the fastest and more convenient for its ability to take premium quality liquid spectrums with high repeatability of the results. The use of MATR prisms will improve sensitivity of the method. Disadvantages may include non-adjustable layer thickness and therefore limited application in quantitative measurements and defining impurities in low concentrations. Limitations can also be caused by chemical activity of the tested substances – for all types of crystals, excluding diamond.
Liquid cell with adjustable test liquid layer thickness designed by SIMEX Research and Production Company is an optical and mechanical adapter installed into the sample compartment of FT-801 spectrometer. This liquid cell contains focusing optics, removable holders of two backing windows 10mm in diameter and two adjusting screws located on top panel of the adapter enclosure for movement of the windows (see Fig.4).
Liquid of any viscosity is firstly deposited in form of small drop onto the lower window then it uniformly fills the gap between the windows while they are gradually closing in. By rotating the adjustment screws and online monitoring the transmission spectrum user can set up the required liquid layer thickness considering general expressiveness of the whole spectrum or intensity of certain absorption bands. If there is a calibration feature which can easily be created by ZAIR 3.5 software using several samples with already known concentrations the liquid cell allows for making quantitative measurements. The cell is indispensable when analyzing mixtures that contain impurities in low proportions.
Figure 5 represents several spectrums of Buksol viscous grease showing the process of sample layer thickness selection between the windows.
The liquid cell with adjustable liquid layer thickness has the following advantages: quick analysis with high repeatability, low-cost replaceable backing windows, the system is easy to setup and clean, very small amount of sample is required to obtain quality spectrum.
ADAPTER WITH COOLED MST-DETECTOR
IR Fourier spectroscopy method is often used as a solution for tasks requiring extra sensitivity in measurements. Relative example is a fiber spectroscopy for registering spectrums of substances contained in remote closed volumes where sample selection for normal laboratory testing cannot be done. This is monitoring of chemical reactions, industry production technological processes, ecological control, express-control of liquid composition in pipelines and solids on conveyor belts. Considering small time available for spectrum registration and significant losses of energy in fiber probes themselves, the sensitivity value of typical non-cooled detectors is not always enough for obtaining high quality spectrums.
Special adapter to FT-801 Fourier-spectrometer which is an optical module with integrated high sensitivity MST-detector cooled by liquid nitrogen is designed to solve this problem. This adapter is installed into the sample compartment of spectrometer beside fiber probe coupling device and connected to the respective external electric port; the detector is activated by software command. Figure 6 shows the fiber probe coupling device with fiber made by Art photonics Company (Berlin, Germany), on the left side in sample compartment, and adapter with MST-detector to the right of the fiber probe coupling device. The method sensitivity is thereby increased five times, yet the spectrum registration time is decreased. Operating time after filling the cryostat capacity (200 ml) with liquid nitrogen is no less than 6 hours.
Improving the spectrometer sensitivity via adapter with MST-detector may be required in many applications such as: registering low-power radiator emission spectrums, route gas analysis with high spectral resolution, detecting impurities in ultra-low concentrations, using multi-way gas cells etc.
ATR THERMAL CELL WITH TEMPERATURE CONTROL
Sample preheating system is designed to expand functionality of typical ATR-MDF and ATR-D adaptors to the FT-801 Fourier spectrometer. It includes replaceable heated frame with ATR diamond crystal  made in Russia and temperature controller unit (Fig.7). Maximum operating temperature – 220°С, adjustment accuracy – 1°С, time to reach the maximum temperature – 5 minutes.
Many hard samples in preheated condition can produce much better expressed ATR spectrum than in cold state. Thus the method sensitivity can be increased by several times. Two spectrums for hard polymer registered at room temperature and temperature of 140°С are shown in Fig.8.
Properties of certain substances and compounds are drastically changed under temperature rise, there may occur chemical interaction processes between components, their decomposition, oxidation in air environment etc. And subject to changes in substance chemical condition during preheating, the ATR adapter with diamond thermal cell and temperature controller is able to take spectrum set corresponding to process kinetics which greatly expands the method functionality in tests.
In conclusion it should be noted that presently the FT-801 IR-Fourier-spectrometer manufactured by SIMEX (Novosibirsk, Russia) includes so many adaptors and fixtures for making molecular composition analysis and express-analysis in different objects that an examiner can solve tasks in the best way with minimum labor and financial investments in the equipment.
Now you can choose not only broad range MIKRAN IR-microscope, but also the inexpensive PRIZE spectral mini-microscope unless your objective does not allow it; acquire the inexpensive ATR-MDF adapter with zinc selenide or germanium crystal which can further be equipped with diamond thermal cell if the need arises. Liquid mini-cell with adjustable thickness and quick sample preparation will be required in high-volume industrial production control measurements, especially when analyzing mixtures, meanwhile the adapter with cooled MST-receiver has great future in using optic fiber probes as well as raising sensitivity of IR-measurements when solving various scientific tasks.
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3. Ezhevskaya T., Bublikov A., Palyanov Yu., Khokhryakov A. Russian diamonds in IR Fourier spectrometry // Analytics. 2015. No. 2. P. 118–123.