A review and tutorial discussion of noise and signal-to-noise ratios in analytical spectrometry—I. Fundamental principles of signal-to-noise ratios

This review consists of two parts which discuss signal-to-noise in a tutorial manner. The sources of noise, the mathematical representation of noise, and the major types of noises in emission and luminescence spectrometry are discussed. An extensive treatment of noise and signal-to-noise ratios of paired readings is given using the relation between the auto-correlation function and the spectral noise power. These signal-to-noise expressions under optimized measurement conditions are given in terms of currents and count rates as well as in terms of charge and counts for the cases of d.c. and a.c. measurements; the present treatment is limited to the cases when the background shows only either shot noise or flicker noise. Finally, the consequences of the combination of these noise sources is considered. Signal expressions for optical spectrometry are also given. Tables give the expressions for signal-to-noise ratios in the various cases.     

3D Printing of Silk Protein Structures by Aqueous Solvent‐Directed Molecular Assembly

Hierarchical molecular assembly is a fundamental strategy for manufacturing protein structures in nature. However, to translate this natural strategy into advanced digital manufacturing like three‐dimensional (3D) printing remains a technical challenge. This work presents a 3D printing technique with silk fibroin to address this challenge, by rationally designing an aqueous salt bath capable of directing the hierarchical assembly of the protein molecules. This technique, conducted under aqueous and ambient conditions, results in 3D proteinaceous architectures characterized by intrinsic biocompatibility/biodegradability and robust mechanical features. The versatility of this method is shown in a diversity of 3D shapes and a range of functional components integrated into the 3D prints. The manufacturing capability is exemplified by the single‐step construction of perfusable microfluidic chips which eliminates the use of supporting or sacrificial materials. The 3D shaping capability of the protein material can benefit a multitude of biomedical devices, from drug delivery to surgical implants to tissue scaffolds. This work also provides insights into the recapitulation of solvent‐directed hierarchical molecular assembly for artificial manufacturing.     

Laser induced double-resonance ionic fluorescence in an inductively coupled plasma

Two pulsed dye lasers pumped by an excimer laser are simultaneously directed into the analytical zone of an inductively coupled argon plasma. When the two beams are tuned to the appropriate ionic transitions, highly excited ionic levels can be efficiently populated in saturated conditions, the resulting fluorescence being then spectrally isolated with a monochromator and measured. A theoretical outline of this technique, variously called double-resonance fluorescence or two-step fluorescence, is given. The experimental results obtained with the alkaline-earth metals Ca, Sr, Ba and Mg show that the technique does provide excellent sensitivity, freedom from scattering problems and unprecedented spectral selectivity. The laser characteristics, the time overlap between the pulses and the spectral characteristics of the transitions used are discussed. Finally, ionic fluorescence in the plasma is the most suitable analytical application of such double-resonance technique since its use in flame atomic fluorescence suffers from the strong depletion of the excited levels due to collisionally assisted ionization.     

Non-linear optical behavior in atomic fluorescence flame spectrometry

The non-linear dependence of the atomic fluorescence radiance upon the irradiance of the excitation source is discussed. Theoretical equations based on the assumption of steady state conditions are derived for a two-level atomic system and for a continuum source of excitation both on a relative as well as on an absolute basis. The theoretical results show that the approach of saturation sets a limit to the fluorescence radiance. The experimental results obtained with the use of a pulsed, tunable dye laser by nebulizing different elements in analytical flames are shown to be in satisfactory agreement with these theoretical results. The theory also predicts that, for a two-level system, the proportional dependence of the fluorescence signal upon the quantum efficiency observed at low irradiances is removed under saturation conditions.     

Relative spatial profiles of barium ion and atom in the argon inductively coupled plasma as obtained by laser excited fluorescence

Relative ionic and atomic fluorescence profiles for barium have been obtained in an argon inductively coupled plasma by exciting different transitions with a nitrogen-laser pumped tunable dye laser and measuring the resulting fluorescence pulses with a boxcar averager. Spatially resolved profiles are directly obtained without the need of an Abel inversion procedure, with a volume resolution of approximately 0.2 mm³. The profiles are given along the excitation axis as well as along the observation axis, for different heights above the coil and different input powers. At low heights, the ion profile resembles a hollow pencil with a typical double-peaked, asymmetric distribution, while the atom profile seems to be complementary to the ion profile. Some scatter from water is also evident at low heights.     

Homogeneous decomposition mechanisms of diethylzinc by Raman spectroscopy and quantum chemical calculations

The gas-phase decomposition pathways of diethylzinc (DEZn), a common precursor for deposition of Zn-VI compounds, were investigated in detail. The homogeneous thermal decomposition of DEZn in N2 carrier was followed in an impinging-jet, up-flow reactor by Raman scattering. Density Functional Theory calculations were performed to describe the bond dissociation behavior using the model chemistry B3LYP/6-311G(d) to estimate optimal geometries and Raman active vibrational frequencies of DEZn, as well as anticipated intermediates and products. Comparison of the measured DEZn decomposition profile to that predicted by a 2-D hydrodynamic simulation revealed that simple bond dissociation between zinc and carbon atoms is the dominant homogeneous thermal decomposition pathway. The calculations suggest several reactions involving intermediates and Raman scattering experiments confirming the formation of the dimer (ZnC2H5)2. In a different set of experiments, photolysis of DEZn gave evidence for decomposition by beta-hydride elimination. The results suggest that beta-hydride elimination is a minor pathway for the gas-phase homogeneous pyrolysis of diethylzinc. A reasonable transition state during beta-hydride elimination was identified, and the calculated energies and thermodynamic properties support the likelihood of these reaction steps.     

Laser-induced fluorescence in a furnace: A viable approach to absolute analysis?

The possibility of performing absolute analysis by laser-induced atomic fluorescence in a graphite tube atomizer is discussed. At the sensitivities which are typical of these measurements, i.e., at absolute levels of the order of femtograms, it is argued that the lack of certified reference materials and the practical limitations in preparing standard solutions constitute the major incentives to the development of absolute analysis. The various theoretical and experimental parameters needed in the procedure of converting a measured fluorescence signal, e.g., a voltage, into the corresponding number of emitting atoms are discussed and evaluated with respect to a typical determination of a toxic element like thallium.     

Theoretical considerations of laser induced fluorescence and ionization spectrometry: how close to single atom detection

The potential capability of detecting single atoms and/or of approaching the intrinsic detection limit, i.e. the limit where all external causes of noise in the system are eliminated, is discussed with respect to laser induced fluorescence and ionization spectrometry. These approaches have been chosen because of their widespread use in laboratories involved in analytical laser spectroscopy. Among the lasers, tunable dye lasers pumped by N₂, Nd-YAG, Excimer, Cu vapor lasers and flashlamps are considered, and flames, graphite furnaces, plasmas and low pressure glow discharges are considered as atom reservoirs. From practical considerations, it is shown that none of the conventional analytical approaches used can satisfy the requirement of a true single atom detection technique, the only exception being provided by the ionization method coupled with atomization under vacuum.     

The impact of several atomic and molecular laser spectroscopic techniques for chemical analysis

Several laser-based methods, namely laser induced fluorescence, laser enhanced ionisation and thermal lensing spectrophotometry are discussed with respect to their capabilities of approaching the extremely high detection sensitivity which is nowadays required in many fields of application, notably in high purity materials, in biomedicine and in the nuclear industry. The discussion is restricted to atomisers operated at atmospheric pressure, i.e., combustion flames, plasmas and graphite furnaces. It is shown that the analytical limit of detection can be in the range of femtograms and that double-resonance excitation possesses significant advantages over single-resonance excitation, both in terms of signal-to-noise ratio and spectral selectivity. In addition, the combination of the fluorescence and ionisation techniques represents a remarkable diagnostic tool. In the nuclear field, the suitability of the technique of thermal lensing for the direct determination and chemical speciation of very low levels of uranium in water is discussed.