DEVELOPMENT AND OPTIMIZATION OF THE UV REDUCER FOR THE DETERMINATION OF EXPLOSIVES

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1 DEVELOPMENT AND OPTIMIZATION OF THE UV REDUCER FOR THE DETERMINATION OF EXPLOSIVES Alena Bumbová, Zbyněk Večeřa, Pavel Mikuška, Josef Kellner and Alena Langerová Abstract: In the study the procedure of developing and optimizing the UV reductor and the chemiluminescence detector for the determination of explosives has been described. The final miniaturized version of the photolytic reductor uses a 15 W Xe flashing lamp. Thanks to the optimization of flow rates and composition of reagents very low detection limits have been achieved. The use of this miniaturized optimized reductor and chemiluminescence detector in connection with an original microcolony liquid chromatograph is a prerequisite for the construction of an analyzer which will be able to determine energetic compounds in the air. Keywords: chemiluminescence detector, nitramines, nitroesters, UV reductor 1. Introduction It is known that the photolytic reduction of nitrate induced by UV radiation absorption provides a simple method for its conversion into nitrite, for the detection of which we developed in the past an extremely sensitive detector which works on the chemiluminescence principle [1, 2]. We have found out that a similar reaction also happens in photolysis of some organic compounds which have nitrogen in their molecule. Energetic compounds [3], e.g. trinitrotoluene (TNT), pentaerythrite tetranitrate (PETN), hexogene (RDX), octogene (HMX), ethylenglycol dinitrate (EGDN), trinitrobenzoic acid (TNBK), glycerol trinitrate (NG) and picric acid (PA), also fall into the class of these compounds. Dimethyldinitrobutane (DMDNB) also undergoes photolysis; it is used as a marker of energetic compounds. It can be expected that also other energetic compounds which include C-NO 2, C-ONO, N-NO 2 groups or, if need be, NH 2 group in their molecule will undergo this type of photolysis as well. Although the photolysis of the abovementioned nitrogen compounds is a complicated process comprehensive of many reactions and reaction intermediates, the resulting reaction, e.g. in case of nitrate, can be described using a simple stoichiometric equation (R1 reaction) as follows: NO 3 + h NO 2 + ½ O 2 (R1) The yield of R1 reaction depends on UV energy, UV radiation wavelength, composition of the medium in which the reaction proceeds and, last but not least, also on the time, during which UV radiation affects a given compound. 2. Results 2.1 UV reductor Optimization For simplification nitrate was used as a model compound during the optimization of the UV reductor. The aim was to develop a reductor which will support the on-line 15

2 conversion of selected nitrogen oxo compounds into nitrite with the subsequent rapid and sensitive detection of the produced nitrite. At the beginning the chemiluminescence (CL) method for the detection of nitrite produced due to photolytic conversion of nitrate was used. This method is based on the oxidation of nitrite by hydrogen peroxide in an acid medium into peroxonitrous acid (R2 reaction) which is determined as peroxonitrite through chemiluminescence reaction with luminol. H + HONO + H 2 O 2 > HOONO + H 2 O (R2) For the detection itself the continual system was used when a sample solution (i.e. the nitrite after the output from the UV reductor) was being mixed continuously with the reagent solution (4 mm H 2 O 2 and 3 mm EDTA in 0.3 M H 2 SO 4 ). During the flow through the reaction capillary tube (PTFE, 0.5 mm I.D., length 30 cm) H 2 O 2 in an acid medium oxidizes the nitrite into the peroxonitrous acid which is determined directly in the chemiluminescence detector using the chemiluminescence reaction with luminol after mixing with the CL solution (2 mm luminol and 3 mm EDTA in 0.6 M KOH). The emitted CL radiation is detected using the photomultiplier. The area of peak is measured as a CL signal. The detection limit of nitrite is M (for the signal/noise ratio = 3). In optimizing the UV reduction conditions a few parameters which affect the reduction efficiency of nitrogen oxo compounds into nitrate fundamentally have been tested experimentally: Composition of the carrier liquid; Flow rate of the carrier liquid and the exposure time; Material (PTFE, quartz) and dimensions (length and bore) of the reaction capillary tube; Parameters of UV light sources (i.e. wavelength of emitted radiation, performance of a UV source); Optimization of the UV reductor geometric arrangement; Temperature. In the course of the UV reductor development a few UV reductor versions varying in a used lamp and material of the capillary tube for the flow of liquids or, if need be, geometry of their mutual configuration, were tested. The result of the first phase of optimizing both the UV reductor and the new detection method was the simplification of the whole system. The optimized final version of the reductor included a 125 W Hg lamp (RVK 125 W, Teslamp Praha). The capillary tube is fixed parallel to the lamp longitudinal axis at the distance of 35 mm from the lamp body. The optimum flow rate of a sample through the reductor is 300 µl.min -1 ; the temperature inside the reductor is 190 C. The detection limit for the determination of nitrate under the abovementioned optimum conditions is M (S/N = 3). The total analysis time comprehensive of both the sample flow rate through the UV reductor, and the subsequent continuous detection is 12 s. In accordance with the preliminary experiments it is possible to detect many energetic compounds in aqueous solutions, the concentration of which is M (TNT) and M (Pentaerythrite tetranitrate, HEXOGENE, OCTOGENE, DMDNB). The final UV reductor version was tested successfully for the photolytic conversion of organic nitrogen oxo compounds (TNT, Pentaerythrite tetranitrate, HEXOGENE OCTOGENE) with the subsequent continuous CL detection of products. Further, the 16

3 possibility for the determination of the abovementioned compounds in the air was tested. It has been confirmed that the developed UV reductor can be used with success within the rapid and sensitive detection of selected energetic compounds. The photolytic reductor developed in 2010 (FR 2010) is not too suitable for the terrain and mobile use due to its large size. When using the FR2010 for the direct detection of real samples of nitrogen oxo compounds in the air the interference of nitrous acid and sulphur dioxide occurring in the air together with the studied nitrogen compounds was detected. Therefore, at first it was necessary to approach the elimination of these interferences before the reductor miniaturization. On the basis of the literature search it was decided to remove HNO 2 from the sample solution through its reaction with sulfamidic acid (SMA) to form nitrogen. The reaction can be described using the following equation (R3): HNO 2 + H 2 NSO 3 H H 2 SO 4 + N 2 + H 2 O (R3) The sulfamidic acid solution was being mixed with the sample solution before its entering the UV reductor. The optimization of sulfamidic acid (SMA) concentration and the reaction time was carried out with the aim of removing HNO 2 quantitatively from the sample solution before its entering the reductor. The optimum SMA concentration is 2 g/l and the reaction time is dozens of seconds. The SO 2 interference can be eliminated with the addition of formaldehyde which reacts with SO 2 to form hydroxymethansulfonate (HCHO). HCHO was added to the SMA solution; the optimum formaldehyde concentration was 0.02 M. The interferences of other pollutants were not detected Miniaturization of the reductor and optimization of detection Arrangement of the measuring apparatus The measuring apparatus includes a photolytic reductor, chemiluminescence (CL) detector and continuous liquid-flow system which enable the continuous connection of photolytic conversion and CL detection in figure 1. At first, the sample solution is being mixed with the SMA solution continuously. Then the resulting mixture passes through the reductor where the sample is exposed to radiation from the UV lamp during the flow rate through the reaction quartz capillary tube. The absorption of UV radiation induces the photolytic conversion of nitrate and nitrogen oxo compounds occurring in the sample into unidentified reaction products. The solution coming out of the reductor is being mixed with the luminol solution in the CL detector; reaction products react with luminol and the emitted CL radiation was detected using a photomultiplier. The area of peak is measured as a CL signal. 17

4 Figure 1: Apparatus scheme PC computer, PP peristaltic pump, FR photolytic reductor, FL flashing lamp, PMT photomultiplier, CLD chemiluminescence detector When developing the miniaturized photolytic reductor (MFR) we compared its characteristics with the reductor developed in 2010 (FR2010) Optimization of determination During the development of the MFR a few parameters were tested experimentally. These parameters affect fundamentally the reduction efficiency of nitrate and energetic compounds as well as the detection limit for the determination of these compounds as follows: - Influence of radiation wavelength upon the conversion of nitrogen compounds, - UV reductor geometric arrangement, - Flow rate of the luminol and SMA solution through the redactor, - Flow rate of the sample through the redactor, - Composition of the SMA solution, - Composition of the luminol solution. When developing and optimizing we used nitrate, TNT, PETN, RDX, HMX, EGDN, TNBK, NG, PA and DMDNB. We tested another UV radiation source than the Hg lamp in the miniaturized photolytic reductor (MFR) for the reason of miniaturization. On the basis of the literary search a flashing Xe lamp was selected as a UV radiation source, the performance of which is 15 W (Hamamatsu, L type, lamp diameter 26 mm). Its advantage is the following: small size, lower power and no heating of the sample when passing through the reaction capillary tube inside the redactor. Sample heating results in sample degassing; the escaped gases (O 2 and N 2 ) cause that the detector shows increased signal fluctuations, which results in the decrease in reliability of the determination as well as lowering of the method detection limit. On the other hand, due to the smaller diameter of the flashing lamp, only a shorter part of the reaction capillary tube is irradiated in comparison with the arrangement in the FR2010. Hypothetically, it was expected that the lower lamp performance when compared with the Hg lamp in the FR 2010 could probably lead to the lower photolysis efficiency of the studied compounds even if, on the basis of the literature, it was evident that it wouldn t have to be like that. In spite of that, the MFR was constructed so that the Xe lamp with a parabolic collimator was used for the final version. The parabolic collimator concentrated UV radiation rays on a cylindrical parabolic mirror which was made of aluminium and positioned vertically to the lamp axis Final configuration of the miniaturized photolytic redactor The final configuration of the miniaturized photolytic reductor includes a 15 W Xe flashing lamp which irradiates a quartz capillary tube (ID 530 µm, the length of irradiated part is 25 mm) fixed vertically to the longitudinal lamp axis at the distance of 30 mm from the lamp. The parabolic aluminium mirror is installed beyond the capillary tube and focuses the light on the capillary tube. The advantage of the Xe lamp is that it does not cause the heating of the sample due to its lower performance. However the necessity of cooling does not take place and the apparatus is simplified. The optimum composition and flow rates of reagent and sample solutions are summarized in Table Limit of detection The final optimized version of the miniaturized reductor (flashing 15 W Xe lamp) in combination with optimized flow rates and reagent composition has been used for determination of the limit of detection (LOD) of the studied compounds. The values of the obtained limits of detection are given in Table 2. For the comparison the values of 18

5 limits of detection obtained using the photolytic reductor from 2010 (150 W Hg lamp) are also mentioned. Table 1 Optimized compositions and flow rates of solutions for the MFR Flow velocity Composition (µl.min -1 ) Sample SMA solution M SMA (2 g/1 L), HCHO (1.6 ml 37% HCHO/1 L), acetone (0.5 ml/1 L) Luminol solution M luminol, 0.01 M sodium pyrophosphate, 0.1 M KOH Table 2 Comparison of detection limits (mol/l) of studied compounds (3 S/N) Compound LOD (mol/l) LOD (mol/l) LOD ratio 150 W Hg lamp 15 W Xe lamp for Xe / Hg lamp Trinitrotoluene Pentaerythrite tetranitrate Hexogene Octogene Ethylenglycol dinitrate Picric acid ,3-dimethyl-2,3-dinitrobutane G glycerol trinitrate Trinitrobenzoic acid HNO Conclusion It results from the comparison of LODs values that the photolytic reductor with the 150 W Hg lamp (i.e. FR2010) has minimum 5x lower LOD value for all the studied compounds than the newly developed miniaturized version of the photolytic reductor with the 15 W flashing Xe lamp (i.e. MFR). Nevertheless, despite the lower efficiency of the UV lamp in the MFR it was a success to achieve such limits of detection through the optimization of flow rates and the composition of reagents. The limits of detection in connection with an original microcolony liquid chromatograph are a prerequisite for the construction of an analyzer which will be able to determine energetic compounds in the air. The financial support of the Ministry of Defence of the Czech Republic by the grant No R is also gratefully acknowledged. References [1] SMITH, J.P., HINSON-SMITH, V. The New Era of SAW Device. Anal. Chem. 78(11), pp ,

6 [2] GOODPASTER, J.V., McGUFFIN, V.L. Fluorescence quenching as an indirect detection method for nitrated explosives. Anal. Chem. 73(9), pp , [3] MEANEY, M.S., McGUFFIN, V.L. Luminescence-based methods for sensing and detection of explosives. Anal. Bioanal. Chem., 391, pp ,