Gerard M. O'Connor

Quantitative analysis of cocaine in solid mixtures using Raman spectroscopy and chemometric methods

Near-infrared (785 nm) excitation was used to obtain Raman spectra from a series of 33 solid mixtures containing cocaine, caffeine and glucose (9.8–80.6% by weight cocaine), which were then analysed using chemometric methods. Principal component analysis of the data was employed to ascertain what factors influenced the spectral variation across the concentration range. It was found that 98% of the spectral variation was accounted for by three principal components. Analysis of the score and loadings plots for these components showed that the samples can be clearly classified on the basis of cocaine concentration. Discrimination on the basis of caffeine and glucose concentrations was also possible. Quantitative calibration models were generated using partial least-squares algorithms which predicted the concentration of cocaine in the solid mixtures containing caffeine and glucose from the Raman spectrum with a root mean standard error of prediction (RMSEP) of 4.1%. Caffeine and glucose concentrations were estimated with RMSEPs of 5.2 and 6.6%, respectively. These measurements demonstrate the feasibility of using near-IR Raman spectroscopy for rapid quantitative characterization of illegal narcotics. Copyright

Identifications and quantitative measurements of narcotics in solid mixtures using near-IR Raman spectroscopy and multivariate analysis

Raman spectroscopy offers the potential for the identification of illegal narcotics in seconds by inelastic scattering of light from molecular vibrations. In this study cocaine, heroin, and MDMA were analyzed using near-IR (785 nm excitation) micro-Raman spectroscopy. Narcotics were dispersed in solid dilutants of different concentrations by weight. The dilutants investigated were foodstuffs (flour, baby milk formula), sugars (glucose, lactose, maltose, mannitol), and inorganic materials (Talc powder, NaHCO3, MgSO4·7H2O). In most cases it was possible to detect the presence of drugs at levels down to ∼10% by weight. The detection sensitivity of the Raman technique was found to be dependent on a number of factors such as the scattering cross-sections of drug and dilutant, fluorescence of matrix or drug, complexity of dilutant Raman spectrum, and spectrometer resolution. Raman spectra from a series of 20 mixtures of cocaine and glucose (0–100% by weight cocaine) were collected and analyzed using multivariate analysis methods. An accurate prediction model was generated using a Partial Least Squares (PLS) algorithm that can predict the concentration of cocaine in solid glucose from a single Raman spectrum with a root mean standard error of prediction of 2.3%.

High pressure diamond and diamond-like carbon deposition using a microwave CAP reactor

This paper describes the deposition of diamond and diamond-like carbon coatings using the Circumferencial Antenna Plasma (CAP) reactor. Carbon coatings were deposited at pressures of 8000, 5000 and 3300 Pa onto silicon wafers. The coatings were characterised using electron microscopy and Raman spectroscopy as a function of distance from the centre of the substrate holder. At 8000 Pa, diamond coatings were deposited up to 20 mm from the centre of the silicon wafer, while under the same deposition conditions, diamond-like carbon was observed in an annular region between 62 and 75 mm from the centre. At a deposition pressure of 5500 Pa, homogeneous free-standing diamond films, 120 μm thick and 50 mm in diameter were deposited.

Application of spectral reflectivity to the measurement of thin-film thickness

The aim of this work is to investigate the application of the spectral reflectance technique to thickness measurement of highly localised semi-transparent coatings on miniature geometries, such as those used in the medical devices industry. The paper will describe the application of the technique to coatings on curved or non-uniform surfaces such as narrow-bore metal tubes and thin wires. The paper will describe the equipment used including a spectrometer with micro-focus attachment, and optical modelling software. This work also involved laser-drilling of the polymer films to allow complementary step-height measurements to be made. Special steps were also required to overcome problems in measurement due to the transparency of the thin films. Complementary techniques including white-light interferometry, which were used to benchmark the method, will also be described.

Study of femtosecond laser interaction with wafer-grade silicon

In this paper the interaction of ultra-short pulses (150fs) of laser radiation (wavelength 775nm) over a range of fluences with wafer grade Silicon material in air was analysed using optical and electron microscopy. Optical microscopy was performed by the use of a white light interferometer and a high power optical microscope (magnification 100X). The resolution of both these methods was only sufficient to resolve large dimensions relative to the wavelength of light. For smaller geometries and greater detail, electron microscopy (resolution 1.5nm, 1KV) was used to obtain more information due to its greater resolution and depth of focus. When used in conjunction with surface, cross sectional and transmission imaging, this technique provided the greatest level of detail on the physical processes involved. Using these analysis techniques it was possible to provide a qualitative understanding of the ablation process as a function of laser fluence and to quantitatively describe the depth per pulse over a range of laser fluences, from which a value for the ablation threshold for Silicon (0.17Jcm-2) could be derived.

Langmuir probe investigation of plasma expansion in femto-and picosecond laser ablation of selected metals

A time resolving Langmuir probe was used to study the plasma plumes produced by the ablation of Ag, Ni and Al targets with laser pulses of different pulse durations (0.2 − 10 ps). These metals were chosen because their electron-phonon relaxation times, τe-ph, are of the order of the pulse durations used. The time of flight (TOF) signals have been used to establish the threshold fluences and plume expansion dynamics of the laser produced plasmas for the different pulse durations. The angular dependence of the magnitude of the ion flux was analysed on the basis of Anisimovs self-similar model of the plasma expansion. The amount of charge in the ablation plume is compared for the different pulse durations.

Single-pulse laser ablation threshold of borosilicate, fused silica, sapphire, and soda-lime glass for pulse widths of 500 fs, 10 ps, 20 ns.

In this work, we report a comparative study of the laser ablation threshold of borosilicate, fused silica, sapphire, and soda-lime glass as a function of the pulse width and for IR laser wavelengths. We determine the ablation threshold for three different pulse durations: τ=500  fs, 10 ps, and 20 ns. Experiments have been performed using a single laser pulse per shot in an ambient (air) environment. The results show a significant difference, of two orders of magnitude, between the group of ablation thresholds obtained for femtosecond, picosecond, and nanosecond pulses. This difference is reduced to 1 order of magnitude in the soda-lime substrate with tin impurities, pointing out the importance of the incubation effect. The morphology of the marks generated over the different glass materials by one single pulse of different pulse durations has been analyzed using a scanning electron microscope (FESEM ULTRA Plus). Our results are important for practical purposes, providing the ablation threshold data of four commonly used substrates at three different pulse durations in the infrared regime (1030-1064 nm) and complete data for increasing the understanding of the differences in the mechanisms leading ablation in the nanosecond, picosecond, and femtosecond regimes.

Analysis of thermal damage in bulk silicon with femtosecond laser micromachining

Femtosecond laser micromachining of silicon offers the potential to realize precision components with minimal thermal damage. In this work, an assessment of the damage observed in bulk silicon during femtosecond laser micromachining is presented. The different analysis methods used to determine the structural and chemical changes to wafer grade silicon is first described. The analysis is at or above the ablation threshold - defined as the point where laser induced crystalline- damage is first observed for 1 kHz laser pulses, of 150 fs duration, at a wavelength of 775nm. Structural analysis is based upon electron and optical microscopies, with different sample preparation techniques being used to reveal the micro-machined structure. A key feature of the work presented here is the high-resolution Scanning Transmission Electron Microscope (STEM) images of the laser-machined structures. Below the ablation threshold, electrical experiments were performed with silicon under femtosecond laser excitation to provide a direct method for determining the accumulation of damage to the silicon lattice. Based on this analysis, it will be shown that laser machining of silicon with femtosecond pulses can produce features with minimal thermal damage, although lattice damage created by mechanical stresses and the deposition of ablated material both limit the extent to which this can be achieved, particularly at high aspect ratios.

Luminescence from Fe3+ ions in octahedral sites in LiGa5O8

Luminescence and lifetime measurements are presented for a new emission band in Fe-doped LiGa5O8. The emission is assigned to the 4T1(4G)→6A1(6S) transition in the d5 configuration for Fe3+ ions in octahedral sites. Good-quality single-crystal samples allow fine structure in the zero-phonon line and in the sideband to be resolved.

Investigation of a method for the determination of the focused spot size of industrial laser beams based on the drilling of holes in Mylar film

The focussed spot size of industrial laser beams is a critical processing parameter in most laser machining applications as it determines the machined feature size and the irradiance produced by the laser at the material interface. There are a number of standard methods available for accurately measuring and analysing the focussed spot. These methods often require expensive equipment that can be time consuming and difficult to set up in a production environment. This paper presents an investigation into a cost effective and straightforward method for the measurement of focussed laser spot sizes based on drilling of holes in mylar film. It can be shown that the slope of a plot of the square of the hole diameter versus the natural log of the laser pulse energy is equal to twice the square of the spot radius. A measure of the laser spot size can be calculated by generating laser-drilled holes at number of laser pulse energies. The practicality and accuracy of this method is investigated in this paper for a number of laser types including a diode pumped solid state laser (UV DPSS) operating at the third harmonic (355nm), a femtosecond laser and a flash lamp pumped Nd:YAG laser. A comparison between the measured results and the results generated with other available techniques is also presented.