S. Sreedhar

Laser-induced breakdown spectroscopy-based investigation and classification of pharmaceutical tablets using multivariate chemometric analysis

We report the effectiveness of laser-induced breakdown spectroscopy (LIBS) in probing the content of pharmaceutical tablets and also investigate its feasibility for routine classification. This method is particularly beneficial in applications where its exquisite chemical specificity and suitability for remote and on site characterization significantly improves the speed and accuracy of quality control and assurance process. Our experiments reveal that in addition to the presence of carbon, hydrogen, nitrogen and oxygen, which can be primarily attributed to the active pharmaceutical ingredients, specific inorganic atoms were also present in all the tablets. Initial attempts at classification by a ratiometric approach using oxygen (∼777 nm) to nitrogen (742.36 nm, 744.23 nm and 746.83 nm) compositional values yielded an optimal value at 746.83 nm with the least relative standard deviation but nevertheless failed to provide an acceptable classification. To overcome this bottleneck in the detection process, two chemometric algorithms, i.e. principal component analysis (PCA) and soft independent modeling of class analogy (SIMCA), were implemented to exploit the multivariate nature of the LIBS data demonstrating that LIBS has the potential to differentiate and discriminate among pharmaceutical tablets. We report excellent prospective classification accuracy using supervised classification via the SIMCA algorithm, demonstrating its potential for future applications in process analytical technology, especially for fast on-line process control monitoring applications in the pharmaceutical industry.

Silver nano-entities through ultrafast double ablation in aqueous media for surface enhanced Raman scattering and photonics applications

We have fabricated stable silver nanoparticles (NPs) and nanostructures (NSs) effectively through double ablation of bulk silver substrate immersed in double distilled water using ∼2 ps laser pulses. The effects of multiple/double/single ablation on silver substrates via surface morphology studies along with average size distribution of Ag NPs were investigated. Prepared Ag NPs in solution exhibited yellow color with an absorption peak near 410 nm, assigned to localized surface Plasmon resonance of nano-sized silver. Depending on the ablation parameters average sizes observed were ∼13 nm/∼17 nm in multiple/double ablation case and ∼7 nm in single ablation case. High resolution transmission electron microscope studies highlighted that most of the Ag NPs were spherical and polycrystalline in nature. Surface morphology of the substrates was characterized by field emission scanning electron microscope and atomic force microscope. A different scenario was observed in the double ablation case compared to single...

Stoichiometric analysis of ammonium nitrate and ammonium perchlorate with nanosecond laser induced breakdown spectroscopy

We present our results on the stoichiometric analysis of ammonium nitrate (AN) and ammonium Perchlorate (AP) studied using laser induced breakdown spectroscopy (LIBS) with nanosecond pulses. The LIBS spectra collected for AP and AN, without any gating and using a high resolution spectrometer, exhibited characteristic lines corresponding to O, N, H, C, and K. The Oxygen line at 777.38 nm and three Nitrogen lines (N1, N2, N3) at 742.54 nm, 744.64 nm, 747.12 nm were used for evaluating the Oxygen/Nitrogen ratios. The intensities were calculated using area under the peaks and normalized to their respective transition probabilities and statistical weights. The O/N1 ratios estimated from the LIBS spectra were ~4.94 and ~5.11 for AP and O/N3 ratios were ~1.64 and ~1.47 for AN obtained from two independent measurements. The intensity ratios show good agreement with the actual stoichiometric ratios - four for AP and one for AN.

Femtosecond and nanosecond LIBS studies of nitroimidazoles: correlation between molecular structure and LIBS data

In the present study, seven novel explosive molecules (nitroimidazoles) have been investigated for laser induced breakdown (LIB) spectral signatures of molecular and atomic species in air and argon atmospheres utilizing both femtosecond (fs) and nanosecond (ns) laser pulse excitation. The molecular emissions were observed to be stronger in the fs spectra whereas atomic emissions were prominent in the ns spectra recorded in both air and argon atmospheres. The C2 Swan band was strongest in an argon atmosphere while the CN violet band was strongest in ambient environment. The LIB spectra were analysed for understanding (a) the influence of molecular structure i.e. type of bonds (C–C, CC, C–N and CN) on atomic (C, H, N and O) and molecular (CN, C2 and NH) emissions, (b) effect of surrounding atmosphere on the fs and ns LIB spectra, (c) correlation between stoichiometric and intensity ratios of molecular as well as atomic species, (d) effect of the number of substituents and their position in the ring on the fragmentation pathways and (e) correlation between oxygen balance and LIB spectra. Furthermore, time resolved spectroscopic studies of the plasma induced by fs and ns laser pulses have been carried out to understand the temporal evolution and possible reaction mechanisms of various molecular species. An attempt has been made to correlate the spectral emission with the chemical structure for this series of energetic materials. Our detailed studies and analyses clearly suggest that the atomization/fragmentation ratio could serve as a performance metric for high energy materials.

Femtosecond and picosecond ablation of aluminum for synthesis of nanoparticles and nanostructures and their optical characterization

In this paper we report the fabrication of nanoparticles and nanostructures through the interaction of ultrashort (~40 fs) and short (~2 ps) laser pulses with bulk Aluminum immersed in various liquid media of different polarity [chloroform which is polar, carbon tetrachloride which is non-polar, water which is polar, dichloromethane (DCM) which is polar, and Cyclohexane which is non-polar] using the laser ablation technique. Except water and Cyclohexane, other media showed yellow coloration after ablation took place indicating formation of nanoparticles in the solution in both fs and ps domains. The coloration of the laser exposed portion in the Al substrate was golden yellow and its closer view depicted micro-grating (~1-2 μm) and nano-ripple (period 330 nm) formation depending on the focal conditions. The investigation of polarization dependence on the ablation was performed for water media. Depending on the ablation threshold, we observed micron sized structures and nano-ripples on the surface. As the rate of ablation depends on the position of the focus on the Al substrate and beam waist parameters, we have studied the liquid level dependence of ablation with different water levels on the Al substrate and we compared these patterns obtained below, near, and above the ablation thresholds of the sample. Field Emission-Scanning Electron Microscope (FE-SEM), UV-Vis absorption spectra, Electron Diffraction Pattern and Transmission Electron Microscope (TEM) were used for the characterization and comparison of products in both domains.

Laser-induced breakdown spectroscopy of RDX and HMX with nanosecond, picosecond, and femtosecond pulses

Herein we present some of our initial experimental results obtained from the laser induced breakdown spectroscopic (LIBS) measurements of RDX and HMX using nanosecond (ns), picosecond (ps), and femtosecond (fs) laser pulses acquired without gating and delay. RDX and HMX were mixed with KBr and pellets were prepared for the spectroscopic studies. Nanosecond pulses at 532 nm, ps/fs pulses at 800 nm were used for the experiments. The spectra were collected using Ocean Optics 4000/Maya spectrometer using a UV transmitting, 400 μm core diameter fiber in one case and a combination of lenses to collect the light from plasma in the second case. Several features were observed in the spectra exclusive for each pulse domain. The differences/similarities in the spectra collected using different pulses are presented.

Laser induced breakdown spectroscopy of high energy materials using nanosecond, picosecond, and femtosecond pulses: challenges and opportunities

We present some of our initial experimental results from laser induced breakdown spectroscopy (LIBS) studies of few high energy materials such as a simple match stick (MS) and BKNO3 (BPN), and ammonium perchlorate (AP) using nanosecond (ns), picosecond (ps), and femtosecond (fs) pulses. The characteristic peaks of each sample in different time domains are analyzed. The merits and de-merits of ultrashort pulses in LIBS experiments for discrimination of high energy materials are highlighted.

On-axis time-resolved spatial characterization of shock-induced refractive fringes in liquid water

We report here the on-axis measurement of time-resolved spatial characterization of refractive fringes due to nanosecond-laser-induced shock waves in liquid water. The complex shadowgraphic fringes due to interference of multiple shock waves observed in the transverse measurements are completely avoided in the on-axis measurements due to the fact that the outermost region of the shock front acts as a radially symmetric phase object to the probe beam, refraction from which results in clean and continuous fringes observed by the intensified charge coupled detector (ICCD) detector. A detailed analysis of different types of time-resolved fringes obtained in the on-axis measurement for fixed laser pulse energy leads us to an alternate and better way to analyze the fringes to obtain the shock wave velocity and the density profile in the entire region surrounding the shock origin, which will enable 3D imaging of shock wave dynamics.

Fabrication and Characterization of Aluminum Nanostructures Using Femtosecond Ablation Technique

In this paper we report the fabrication of nanostructures through the interaction of ultrashort laser pulses (~40 fs) with bulk Aluminum immersed in various liquid media of different polarity (chloroform which is polar, carbon tetrachloride which is non-polar, and water which is polar) using femtosecond laser ablation. Except water medium other two media showed yellow coloration after ablation indicating the formation of nanoparticles in the solution. The coloration of the laser exposed portion in the Al substrate was golden yellow and its closer view depicted micro-grating and nano-ripple (period 330 nm) formation. The investigation of polarization dependence on the ablation was performed for the liquid media. Depending on the focal position we observed sub-micron structures and nano-ripples on the surface. As the rate of ablation depends on the position of the focus on the Al substrate and beam waist and other laser parameters, we have studied the liquid level dependence of ablation with different water levels on the Al substrate and we compared these patterns when focus was beyond the sample and on the sample. Size distribution of the generated nanoparticles in the liquid in which ablation took place and surface morphology of the substrates were investigated using SEM technique.

Supercontinuum Emission from Water using 40 fs Pulses in the External Tight Focusing Limit

We present our results from the measurements of Supereonlinuum emission (SCE) resulting from the propagation ol” tightly foe used 40 femtosecond laser pulses through distilled water. The e fleet of linearly polarized (LP) and circularly polarized (CP) light pulses on the SCE: in different external focal geometries (f/6 & f/12) is studied in detail. A considerable shift in the minimum wavelength of SCF under tighter focusing limit is observed.