Vapour phase Fourier transform infrared spectrometric determination of thiourea

A novel and precise procedure for the determination of thiourea based on vapour-generation Fourier transform infrared spectrometry is described. A 4 ml volume of 1.4% m/v iodine solution was injected into a glass vessel containing 5 ml of thiourea and 0.2 ml of 1 M sodium hydrogencarbonate solution. The CO2 evolved under these conditions was swept by a stream of nitrogen to an infrared gas cell. At 200 s after injection of the iodine solution, the vapour phase FTIR spectra were continously recorded, as a function of time, between 2500 and 2200 cm(-1), which includes the CO2 absorption band at 2350 cm(-1). From the absorbance data in the 2399-2284 cm(-1) range, integrated absorbance measurements were obtained providing transient recordings which correspond to the absorbance of CO2 in the selected wavenumber range. The method provided a limit of detection of 10 ppm of thiourea, a throughput of 14 samples h(-1) and an RSD of 1.1% for three independent analyses of a 500 ppm thiourea solution. Results obtained for a series of real samples compared well with those obtained using potentiometric titration as a reference method.     

Fourier transform infrared spectrometric determination of Ziram

A procedure has been developed for vapour-phase Fourier transform infrared determination of Ziram, a dithiocarbamate pesticide. The method is based on the evolution of CS₂, after decomposition of the dithiocarbamate with diluted H₂SO₄ at 50°C. The CS₂ evolved was swept by a carrier flow of nitrogen to a laboratory-made infrared gas cell of 39 mm pathlength and 490 μl volume. The signals were registered as a function of time. The area of peaks obtained from absorbance measurement in the wavenumber range between 1600 and 1450 cm⁻¹ were interpolated in a calibration line established from Ziram standards treated in the same way as samples. The method provided an absolute limit of detection of 0.055 mg, a variation coefficient of the order of 6% for an analyte mass of 50 mg, and an analysis time of 3.5 min.     

Fourier transform infrared spectrometric determination of Malathion in pesticide formulations

An environmentally friendly methodology has been developed for quality control analysis of emulsifiable concentrate pesticide formulations containing Malathion as active ingredient, using flow injection analysis (FIA)-Fourier transform infrared (FTIR) spectrometry. Five microlitres samples were directly injected into a 3 ml closed FIA manifold, in which 2 ml of CHCl₃ was re-circulated at 1.96 ml min⁻¹. After homogenisation and sample measurement, 2 μl volumes of a Malathion standard were injected, taking absorbance measurements after each injection. Peak height of the chemigrams, established from peak area values between 1027 and 1017 cm⁻¹, corrected with a baseline fixed from 1087 to 993 cm⁻¹ were employed for Malathion quantification using the standard addition approach, after reaching the steady state for every injection. A limit of detection of 12 μg ml⁻¹ was achieved. Results found by standard addition-FTIR in commercially available samples showed a good correlation with those obtained by the reference gas chromatography-flame ionisation detection procedure.     

Seafood freshness determination through vapour phase Fourier transform infrared spectroscopy

A new vapour-phase manifold has been developed to determine trimethylamine (TMA) in fish and cephalopod samples by Fourier transform infrared (FT-IR) spectroscopy. Samples were treated off-line for 1 h with trichloroacetic acid (TCA), filtered and washed. The obtained extracts were aspirated and alkalinized with NaOH 2.0 M, in an on-line system. TMA was separated from the solution in a gas phase separator and then transported by means of a nitrogen carrier into a home made 10 cm pathlength IR gas cell, where the corresponding FT-IR spectra were acquired by accumulating 30 scans per spectrum with 2 cm⁻¹ nominal resolution. The method was applied to the determination of TMA in natural samples providing concentration values statistically comparables with those obtained by a head space gas chromatography (HS-GC) reference procedure. The sample throughput by FT-IR is increased by a factor of 6 as compared with HS-GC.     

Fourier transform infrared spectrometric strategies for the determination of Buprofezin in pesticide formulations

Two different strategies for Buprofezin determination, an off-line extraction and stopped-flow determination and an automated procedure, based on the on-line extraction of Buprofezin samples with chloroform and flowing action analysis-fourier transform infrared (FIA-FT-IR) spectrometric measurement of the extracts, have been developed. For the treatment of the off-line extraction mode, data a three-factor partial least squares (PLSs) calibration was developed, using the region from 1465.7 to 1342.3 cm⁻¹ with a single point baseline defined at 2051.9 cm⁻¹ and based on the use of chloroform solutions of Buprofezin. The method provides a R.S.D. <0.1%, recoveries of the order of 100% and generates 25 ml of CHCl₃ waste for each sample.On the other hand, the recommended FIA method provided a 3 s limit of detection of 20 μg ml⁻¹, which corresponds to 0.12% (w/w) in the solid sample, a repeatability of 0.8% as R.S.D., and a maximum sampling frequency for the whole procedure of 6 h⁻¹.The waste generation, being lower than the off-line strategy, is only 3 ml of CHCl₃ per sample.     

Flow analysis-vapor phase generation-Fourier transform infrared (FA-VPG-FTIR) spectrometric determination of nitrite

In this work, the coupling between flow analysis (FA)–vapor phase generation (VPG) and Fourier transform infrared spectrometry (FTIR) has been proposed as a novel and alternative strategy for the determination of nitrite. The analyte was transformed into the gaseous nitric oxide (NO) by on-line reaction with potassium iodide (KI) or ascorbic acid in acidic medium. The gaseous NO generated was transported by means of a N₂ gas carrier stream inside the IR gas cell and the corresponding FTIR spectrum was acquired in a continuous mode. The absorbance at 1876 cm⁻¹, corrected by a baseline established between 1879 and 1872 cm⁻¹ at a nominal resolution of 2 cm⁻¹, was selected as a measurement criterion. The effect of different spectroscopic and flow analysis experimental parameters, such as nominal resolution, number of scans, reducing agent and its concentration, acidic medium, reagents and sample flow rates, and the carrier gas flow rate on the analytical signal, and then in the figures of merit were initially evaluated by using a standard short path length (10 cm) IR gas cell. The optimization of the system was carried out by the univariate method. The main aims of this study were: (i) to investigate the on-line generation of gaseous nitric oxide in a continuous flow system, and (ii) the use of Fourier transform infrared spectrometry as an alternative and selective detector for the determination of nitrite. The proposed method was initially tested and applied for the determination of nitrite in samples with very high concentration of nitrite, such as frankfurters.     

Flow injection Fourier transform infrared determination of nicotine in tobacco

A fully automated procedure is proposed for the Fourier transform infrared (FTIR) determination of nicotine in tobacco. The method is based on the on-line extraction of nicotine with CHCl3. Samples, weighed inside empty extraction cartridges, were humidified with NH3 and the cartridges were installed in a flow manifold in which they were extracted with 2 ml CHCl3 for 2 min, then 400 microliters of the extract were introduced into a micro-flow cell using a carrier of CHCl3 and the IR spectrum was registered continuously. The absorbance, in the wavenumber range 1334-1300 cm-1, was measured, obtaining a peak as a function of time. The area of this peak was interpolated on a calibration line established from standard solutions of nicotine in chloroform treated in the same way as samples. The method provided a limit of detection of 0.1 mg ml-1 nicotine, an RSD lower than 2% and a sampling frequency of the whole procedure of 6 h-1. Results obtained for natural samples of cut tobacco and cigar compared well with those obtained by a batch FTIR procedure, involving an off-line extraction with a total time of 16 min. However, for yellow tobacco cigarette, an on-line extraction time of 10 min was required to obtain a good recovery of nicotine.     

Fourier transform infrared determination of caffeine in roasted coffee samples

A new procedure has been developed for the FT-IR determination of caffeine in roasted coffee samples. The method involves wetting the coffee samples with a 0.25 M aqueous NH3 solution, extracting the caffeine with CHCl3, and measuring absorbance at 1,659 cm(-1) using a baseline established between 1,900 and 830 cm(-1). The procedure proposed is fast, only requiring a total extraction time of 16 min for each sample, and provides a drastic reduction of the organic solvent consumed, from the 200 mL diethyl ether and 50 mL CHCl3, required for each sample by the reference chromatographic UV-spectrometric determination to only 5 mL CHCl3. The method provides a limit of detection of the order of 3 mg L(-1) caffeine and a relative standard deviation of 0.4% for 3 independent analyses of a sample containing 18.6%mg/g caffeine. The accuracy of the FT-IR procedure was evaluated from recovery experiments on spiked samples providing values from 94.4 to 100.1% and from the comparison of results found for a series of commercial samples, by both FT-IR and the official reference procedure.     

Fourier transform infrared determination of caffeine in roasted coffee samples

A new procedure has been developed for the FT-IR determination of caffeine in roasted coffee samples. The method involves wetting the coffee samples with a 0.25 M aqueous NH₃ solution, extracting the caffeine with CHCl₃, and measuring absorbance at 1659 cm^–1 using a baseline established between 1900 and 830 cm^–1. The procedure proposed is fast, only requiring a total extraction time of 16 min for each sample, and provides a drastic reduction of the organic solvent consumed, from the 200 mL diethyl ether and 50 mL CHCl₃ required for each sample by the reference chromatographic UV-spectrometric determination to only 5 mL CHCl₃. The method provides a limit of detection of the order of 3 mg L^–1 caffeine and a relative standard deviation of 0.4% for 3 independent analyses of a sample containing 18.6%mg/g caffeine. The accuracy of the FT-IR procedure was evaluated from recovery experiments on spiked samples providing values from 94.4 to 100.1% and from the comparison of results found for a series of commercial samples, by both FT-IR and the official reference procedure. Received: 9 July 1999 / Revised: 23 September 1999 / /Accepted: 24 September 1999     

Flow analysis-hydride generation-Fourier transform infrared spectrometric determination of antimony in pharmaceuticals

In this work, a flow analysis system with hydride generation and Fourier transform infrared (FTIR) spectrometric detection has been developed for the determination of antimony in pharmaceuticals. The method is based on the on-line mineralization/oxidation of the organic antimonials present in the sample and pre-reduction of Sb(V) to Sb(III) with K₂S₂O₈ and KI, respectively; prior to the stibine generation. The gaseous SbH₃ is separated from the solution in a gas phase separator, and transported by means of a nitrogen carrier into a short pathway (10 cm) IR gas cell, where the corresponding FTIR spectrum is acquired by accumulating 3 scans in a continuous mode. The 1893 cm⁻¹ band was used for the quantification of the antimony. The procedure is carried out in a closed system, which reduces sample handling and makes possible the complete automation of the antimony determination. The figures of merit of the proposed method (linear range: 0-600 mg l⁻¹, limit of detection (3σ)=0.9 mg l⁻¹, limit of quantification (10σ)=3 mg Sb l⁻¹, precision (R.S.D.) less than 1% and sample frequency=28 h⁻¹), are appropriate for the designed application. Furthermore, precise and accurate results were found for the analysis of different antimonial pharmaceutical samples, indicating that the methodology developed represents a valid alternative for the determination of antimony in pharmaceuticals, which could be suitable for the routine control analysis.     

Fourier - transform infrared - spectrometric investigation of blends of terephthalic polyesters with polycarbonate

Blends of polycarbonate (PC) and poly(ethyleneterephthalate) (PET) or poly(butyleneterephthalate) (PBT) heated above the melting temperature of the crystalline component show transesterification reactions. The crystallizability of the resulting system (mixture of copolyesters with homopolymers) decreases with time. Blends of PET and PC loose their crystallizability after 10 - 20 min at 543 K. After that treatment samples with low content of PC have lost the characteristic IR absorptions of PC. Transesterification is faster in blends of PBT and PC: after 3–15 min at 543 K the crystallizability has vanished. After transesterification has reached equilibrium a homogeneous statistical copolymer has developed from the homopolymers.     

Solid sampling Fourier transform infrared determination of Mancozeb in pesticide formulations

A series of approaches have been assayed for FTIR determination of Mancozeb in several solid commercial fungicides using different calibration strategies. The simplest procedure was based on the use of the ratio between the absorbance of a characteristic band of Mancozeb and that of a KSCN internal standard measured in the FTIR spectra obtained from KBr pellets. It was employed the quotient between peak height absorbance values at 1525 cm⁻¹ for Mancozeb and 2070 cm⁻¹ for KSCN. In these conditions a precision as relative standard deviation (RSD) of 0.6% and a relative accuracy error of 0.8% (w/w) were found. For complex formulations, containing other compounds with characteristic absorption bands at different wavenumbers than Mancozeb, one of them was used as internal reference being employed the standard addition approach. In this case, the Mancozeb bands at 1525 cm⁻¹ or at 1289 cm⁻¹ were employed, being used the ferrocyanide band at 2075 cm⁻¹ as internal reference. RSD values between 0.7-1.4% and a relative accuracy error of 3% (w/w) were found. A third strategy was based on the use of partial least squares (PLS) calibration. A reference set was prepared mixing Mancozeb, Kaolin, Cymoxanil and KBr, being predicted the Mancozeb concentration in pesticide formulations by using the quotient between absorbance bands of Mancozeb and those of Cymoxanil. In these conditions a relative accuracy error of 0.6% (w/w) and a relative standard deviation of 1.3% were found.     

Ultra-rapid-scanning Fourier transform infrared spectrometry

Several applications of Fourier transform infrared (FT-IR) spectrometry are discussed where both conventional rapid-scanning interferometers and step-scan interferometers are unable to provide the needed information. A new interferometer is described by which it is possible to acquire interferograms with 0.25 cm optical path difference (4 cm⁻¹ spectral resolution) in less than 1 ms. The moving element of this interferometer is a tilted mirror that continuously rotates at a speed of up to 500 Hz. The performance of this spectrometer is demonstrated by examining the spark-initiated combustion of ethane and methane in air. The formation of vibrationally excited CO₂ and H₂O, with lifetimes approaching 50 ms, was observed.     

Fourier transform infrared spectroscopic determination of cypermethrin and deltamethrin in emulsifiable concentrate formulations

Fourier tranform infrared (FT-IR) spectroscopic method has been developed for determination of cypermethrin and deltamethrin in emulsifiable concentrate formulations. The known concentration of formulation was subjected to preparative thin layer chromatography and the active ingradient zone was scrapped from the plate. Pyrethroids were eluted from the adsorbent with chloroform and estimated by measuring the ester carbonyl absorption band at 1749 cm(-1) in cypermethrin and at 1743 cm(-1) in deltamethrin using base line technique. Recoveries of cypermethrin and deltamethrin from commercial and laboratory prepared formulations were 90 to 97% in both the cases. The validity of FT-IR method was confirmed by comparing the results with standard HPLC method.     

Polarization modulation Fourier transform infrared spectroscopy

Polarization modulation Fourier transform infrared spectroscopy can be measured by either double modulation FTIR or interferometric modulation FTIR techniques. The principles of both techniques are presented with examples of representative measurements. The relative advantages and disadvantages of each method are compared.     

Fourier transform infrared spectroscopy and theoretical study of dimethylamine dimer in the gas phase

Dimethylamine (DMA) has been studied by gas-phase Fourier transform infrared (FTIR) spectroscopy. We have identified a spectral transition that is assigned to the DMA dimer. The IR spectra of the dimer in the gas phase are obtained by spectral subtraction of spectra recorded at different pressures. The enthalpy of hydrogen bond formation was obtained for the DMA dimer by temperature-dependence measurements. We complement the experimental results with ab initio and anharmonic local mode model calculations of monomer and dimer. Compared to the monomer, our calculations show that in the dimer the N-H bond is elongated, and the NH-stretching fundamental shifts to a lower wavenumber. More importantly, the weak NH-stretching fundamental transition has a pronounced intensity increase upon complexation. However, the first NH-stretching overtone transition is not favored by the same intensity enhancement, and we do not observe the first NH-stretching overtone of the dimer. On the basis of the measured and calculated intensity of the NH-stretching transition of the dimer, the equilibrium constant for dimerization at room temperature was determined.     

Fourier transform infrared determination of CO2 evolved from carbonate in carbonated apatites

A quantitative method based on FTIR has been developed to determine carbonate in synthetic apatites. The method measures the evolved CO₂ after reaction of 50 mg apatite with 2 mL of hydrochloric acid (0.5 M) in a reaction vessel, heated to 40?°C. The CO₂ evolved was swept by a carrier of nitrogen to a laboratory-made infrared gas cell of 39 mm pathlength and 490 μL volume. The signals were recorded as a function of time and the areas of the chemigram peaks obtained from the measurements in the wavenumber range of 2500–2150 cm^–1, were interpolated using a calibration curve. The method can be used to study apatites with carbonate contents below 0.2% with a sampling frequency of 8 h^–1.     

Fourier transform infrared determination of CO2 evolved from carbonate in carbonated apatites

A quantitative method based on FTIR has been developed to determine carbonate in synthetic apatites. The method measures the evolved CO2 after reaction of 50 mg apatite with 2 mL of hydrochloric acid (0.5 M) in a reaction vessel, heated to 40 degrees C. The CO2 evolved was swept by a carrier of nitrogen to a laboratory-made infrared gas cell of 39 mm pathlength and 490 microL volume. The signals were recorded as a function of time and the areas of the chemigram peaks obtained from the measurements in the wavenumber range of 2,500-2,150 cm(-1), were interpolated using a calibration curve. The method can be used to study apatites with carbonate contents below 0.2% with a sampling frequency of 8 h(-1).     

Fourier transform infrared analysis of paint solvents

The composition of solvent mixtures employed in the paint industry was determined by Fourier transform infrared spectrometry. A procedure based on the use of the ratio between the absorbance of characteristic bands of each component permits the ratio between two solvents in a binary mixture to be determined, independently of the thickness of the sample film. Typical mixtures of xylol with butan-1-ol and with butyl acetate (xylol=mixture of o-, m- and p-xylenes) were used as model systems to develop the proposed procedure and the accuracy was determined using synthetic formulations. Another possible application of the procedure is the characterization of azeotropic mixtures.     

Trends in Fourier transform infrared spectroscopic imaging

Fourier transform infrared (FTIR) spectroscopic imaging is a relatively new method that has received great attention as a new field of analytical chemistry. The greatest benefit of this technique lies in the high molecular sensitivity combined with a spatial resolution down to a few micrometers. Another advantage is the ability to probe samples under native conditions, which allows new insights into samples without the need for fixation, stains, or an additional marker. Advances in instrumentation have made FTIR spectroscopic imaging the tool of choice for an increasing number of applications. The main applications are in the bioanalytical chemistry of cells and tissue, polymers, and recently as well as in homeland security. This report gives a short overview of current developments and recent applications. Figure FTIR image of a polymer blend reveals the chemical composition. Online Abstract Figure (365 KB).