Scanning Of Forensic Samples On Ftir - Sample Preparation

摘要

A variety of methods are used for sample preparation to study the infrared spectrum of solids. One of them is preparation of KBr disc and its scanning under transmission mode. An attempt has been made to calibrate the method for preparation of KBr disc with the solid samples. All possible variables, which affect the quality of KBr disc vis-a-vis their effect on quality of IR spectra have been studied with special references to one of the drug i.e. Atropine sulphate. The variables have been suitably controlled to produce the best quality of IR spectra for the sample under study. The advantages of having prepared a quality disc have been discussed for FTIR in the paper. Introduction For studying solid samples on FTIR formation of KBr disc, is one of the most accepted methods of routine. Drugs, Fibers, Poisons, Polymers and Explosive of forensic interest are mostly analyzed in their solid form using FTIR for the purpose of their identification and characterization [1-5]. The method is not only nondestructive but is useful when samples are available in very small quantity. Scanning of the samples using FTIR is preferred in transmission mode. KBr is an important sample matrix for FTIR scanning. The KBr used should be of IR grade and must be pure. Before preparing the disc, KBr should be vacuum dried as per U.S. pharmacopoeia. To avoid water uptake, the KBr is stored in a sealed bottle, which is placed in a drying cabinet heated to 40 oC. Beside the quality of KBr, the number of sample molecules in the beam and path length are two important factors to be monitored in preparing disc for the sample. Materials that are not self-supporting need to be dispersed in potassium bromide (KBr) to study their spectra. The other factors, which affect the IR spectra, are error in measuring which may be sometimes due to errors in preparation of the disc such as it may have poor distribution of sample, larger amount of sample with KBr or holes in the disc these may be minute, which could not be noticed while its preparation. The greatest obstacle for the analysis of powdered solid is making the particle size small, uniform and reproducible. Reproducibility in particle size can be improved by grinding the same sample for same time period. The concentration of the sample and sample size are critical when preparing the KBr disc. Organic sample may be mixed with KBr powder at a sample concentration of 0.5%, while inorganic sample can be prepared up to same concentration provided that path length/thickness are correctly adjusted. Scratches and dents in the cylindrical anvil harpers the performance and can cause the anvil to get stuck inside the collar. The die set should never be compressed when empty. Considering various aspect of disc preparation the method has been standardized to study the FTIR spectra. Materials 35 mm mortar and pestle, Spatula, Auto press comprising of SS Die (13mm), pellet holder, plunger, envil and EXCALIBER BIO-RAD FTIR Instrument. Potassium bromide (KBr) IR grade and Sample (Atropine sulphate) procured from M/s Pike Technologies and M/s Loba Chemie respectively. Recommended Method: To prepare a good quality disc, a standard method should be followed. The following points should be carefully followed in preparing the disc:Place about 500 mg of KBr into a mortar and grind it until there is no evidence of crystalinity. Transfer this KBr powder into the drying box at a temperature of 40 oC. Place 10 mg of solid sample into the mortar and again grind it until a fine powder is form. Weigh 1-2 mg of solid fine powder of sample (as per requirement of the die) and 200-300 mg of dry fine powder of KBr. Transfer these weighed quantities into a mortar and mix well with the help of a spatula. Do not grind during the mixing procedure, since reduction in particle size is not required and will lead to absorption of moisture by KBr. Assemble bottom and top portion of KBr press assembly and place one of the 13 mm die with the polished surface u