A discussion about the significance of Absorbance and sample optical thickness in conventional absorption spectrometry and wavelength-modulated laser absorption spectrometry

One of the most frequently used concepts in atomic absorption spectrometry (AAS) is that of Absorbance, an easily measurable quantity that is linear with the concentration of the analyte over a large range of concentrations whenever Beer's law is valid. Laser-based modulation techniques, however, in particular wavelength modulated diode laser absorption spectrometry (WMAS), do not measure exactly the same physical quantities as conventional AAS techniques do, since they do not rely upon separate measurements of the intensities in the presence and in the absence of the sample, but rather (a certain fraction of) the difference between the signals ‘on’ and ‘off’ resonance. Hence, Absorbance is not as natural for the modulated laser-based absorption techniques as it is for the conventional AAS techniques. The entity called sample optical thickness (SOT) appears to be a natural entity as well as a natural unit (denoted SOT units) for the quantification of signals in WMAS. The present paper discusses the concept of measurable quantities and their units in WMAS in some detail and compares them (theoretical and practical considerations) with those of conventional AAS. In particular, it makes a distinction between the ‘observed’ sample optical thickness and the ‘true’ sample optical thickness and shows how these two entities are related to the Absorbance entity. Finally, this work also introduces a dimensionless sensitivity of the WMAS technique, and shows this quantity to be equal to the nth Fourier coefficient of the wavelength modulated, peak-normalized line shape function.     

分子荧光光谱法测定富硒大米中的痕量硒

硒(Ⅳ)与DAN(2,3-二氨基萘)生成具有荧光特性的4,5-苯并苤硒脑,可用分子荧光光谱法测定,优化测定的实验条件,建立了快速准确测定大米中痕量硒的方法.结果表明,其最佳测定条件为:pH=2.0,反应温度为80℃,1.0×10-5 mol/L十六烷基溴化吡啶用作表面活性剂,环己烷萃取条件下,增敏效果明显,在0～0.0...     

The influence of molecular geometry on the fluorescence spectra of biphenyl and the polyphenyls

The shapes of the fluorescence spectra of rigid solutions (in ethanol at 77 K) of biphenyl,p-terphenyl, andp-quaterphenyl are shown to depend on the wavelength of the exciting light: the spectral resolution enhances with increasing wavelength of excitation. These spectra are interpreted in terms of a model wherein the equilibrium configuration of the fluorescent state is planar, but that of the ground state deviates from planarity.Some other results reported in the literature are also discussed and it is concluded that the ground states of the singly and doubly charged ions of these molecules, as well as the lowest triplet states of the neutral molecules, are nearly planar.     

A discussion on mass resolution in secondary ion mass spectrometry

Mass resolution is a very important parameter for mass spectrometry. It is necessary to compare the mass resolution between the newly developed TOF-SIMS and the conventionally high-performance magnetic SIMS. However, the definitions of mass resolution for these two types of instruments are quite different. Whether it is possible to compare mass resolution and how to do such comparison is a challenge. This problem was raised officially during the 2012 ISO/TC 201 meeting at Tampa, Florida, the United States and the long-term cooperation with ISO started afterwards. The definition of mass resolution is one of the most important and fundamental problems for mass spectrometry and should attract significant attention. Here, some detail discussions on mass resolution as well as the related experimental studies in the past few years, including the collaborations with ISO/TC 201/SC6 and SC1 are summarized. This summary covers the common problem for almost all the current existing and still used definitions of mass resolution. A reasonable new definition for mass resolution considering the peak shape or resolution function has been proposed, which has also been confirmed by using experimental studies of the mass resolution comparison between TOF and magnetic SIMS. This study lays a foundation for the future mass resolution comparisons between different mass spectrometry.     

Progress in the determination of metalloids and non-metals by means of high-resolution continuum source atomic or molecular absorption spectrometry. A critical review

This work examines the new possibilities introduced with the arrival of commercially available high-resolution continuum source atomic absorption spectrometers for the determination of metalloids (B, Si, Ge, As, Se, Sb and Te) and non-metals (P, S, F, Cl, Br, I and N-based species), such as the improved potential to detect and correct for spectral overlaps and the strategies available to correct for matrix effects. In particular, and considering the increasing number of papers reporting on the use of molecular absorption spectrometry using graphite furnaces and flames as vaporizers, the work discusses in detail the advantages and limitations derived from the monitoring of molecular spectra from a practical point of view, in an attempt to guide future users of the technique.

Interelement effects in x-ray fluorescence spectrometry. Analysis of the iron-copper-sulfur system

The Rasberry-Heinrich and Claisse-Quintin equations give a good interpretation of the theoretical intensity-concentration relationship in x-ray fluorescence spectrometry. The influence constants (A_x, A_x) of these equations can be calculated. Trace element and structural effects require an empirical correction, which is important in iron-copper-sulfur systems (reverberatory mattes and copper concentrates). The empirical correction in combination with either of the above equations can furnish very good quantitative results from fluorescence intensity measurements.

t005. Analysis of some copper intermediates^a

Specimen	RFe	RCu	RS	%Fe			%Cu			%S
				W	RH	CQ	W	RH	Cl	W	RH	Cl
Reverberatory matte
5-268	0.4717	0.2710	0.1093	35.6	35.52	35.52	32.83	32.82	31.79	26.64	26.62	26.66
5-331	0.4394	0.3042	0.1059	32.0	32.11	32.12	37.17	37.33	37.29	26.42	26.35	26.43
5-336	0.4210	0.3131	0.1126	30.6	30.48	30.45	37.52	37.57	37.65	27.02	26.97	27.03
5-339	0.4295	0.3062	0.1042	31.4	31.29	31.34	37.23	37.13	37.26	26.16	26.04	26.19
5-344	0.4395	0.3094	0.1052	31.8	32.01	31.97	38.42	38.33	38.17	26.09	26.35	26.43
Average deviation (±)		0.13	0.12		0.08	0.11		0.10	0.03
k1					0.899	0.953		1.175	1.133		0.274	0.245
k2					-1.137	-4.146		-20.73	-20.16		14.09	15.26
Concentrate I
2-18	0.3111	0.2809	0.1410	27.7	27.60	27.60	25.79	25.50	25.51	38.83	38.83	38.82
2-20	0.3226	0.2598	0.1445	28.2	28.42	28.42	23.23	23.58	23.57	38.62	38.88	38.88
2-22	0.3335	0.2303	0.1495	29.4	29 31	29.31	20.53	20.62	20.61	39.14	38.90	38.90
3-4	0.3376	0.2007	0.1498	30.2	29.87	29.87	17.82	17.46	17.46	38.59	38.59	38.58
3-5	0.3396	0.1976	0.1515	29.7	30.01	30.00	16.92	17.14	17.14	38.65	38.65	38.65
Average deviation (±)					0.21	0.21		0.26	0.26		0.10	0.11
k1					0.590	0.616		0.783	0.776		0.158	0.162
k2					12.11	10.34		-4.82	-6.60		31.18	30.60
Concentrate II
7-1	0.3821	0.2403	0.1409	28.4	28.29	28.34	27.13	27.38	27.37	32.48	32.54	32.53
7-2	0.3724	0.2406	0.1414	27.9	27.82	27.80	27.33	27.10	27.13	32.32	32.44	32.45
7-3	0.3804	0.2368	0.1414	28.2	28.24	28.29	26.88	26.91	26.87	32.78	32.49	32.49
7-4	0.3825	0.2502	0.1459	28.0	28.20	28.17	28.50	28.45	28.49	32.84	33.08	33.09
7-5	0.3932	0.2559	0.1496	28.7	28.65	28.62	29.41	29.41	29.39	33.74	33.62	33.61
Average deviation ( ± )					0.10	0.10		0.11	0.10		0.17	0.17
k1					0.525	0.649		0.778	0.877		0.216	0.224
k2					10.74	5.740		-0.66	-6.19		21.76	20.87

a

Most of the iron and copper was present as FeS and Cu₂S, respectively; W is the “wet” method. Concentrates I and II contained 6.0% SiO₂-2.2% Al₂O₃ and 4.6% SiO₂-1.8% Al₂O₃, respectively (average values).

     

Liquid chromatography at the critical condition: Thermodynamic significance and influence of pore size

Liquid chromatography at the critical condition (LCCC) is a high performance liquid chromatography (HPLC) technique that lies between size exclusion chromatography and adsorption-based interaction chromatography, where the elution of polymers becomes independent of polymer molecular weight. At LCCC, the balance between the entropic exclusion and the enthalpic adsorption interactions between polymers and stationary phases results in the simultaneous HPLC elution of polymers regardless of molecular weight. Using C18-bonded silica chromatographic columns with 5 μm particle size and different average pore size (diameter = 300 Å, 120 Å, 100 Å, and 50 Å), we report (1) the thermodynamic significance of LCCC conditions and (2) the influence of column pore size on the determination of critical conditions for linear polymer chains. Specifically, we used mixtures of monodisperse polystyrene samples ranging in molecular weight from 162 to 371,100 g/mol and controlled the temperature of the HPLC columns at a fixed composition of a mobile phase consisting of 57(v/v)% methylene chloride and 43(v/v)% acetonitrile. It was found that, at the fixed mobile phase composition, the temperature of LCCC (T_LCCC) is higher for C18-bonded chromatographic columns with larger average pore size. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2533–2540, 2009     

Determination of atomic hydrogen in non-thermal hydrogen plasmas by means of molecular beam threshold ionization mass spectrometry

Atomic hydrogen plays important roles in chemical vapor deposition of functional materials, plasma etching and new approaches to chemical synthesis of hydrogen-containing compounds. The present work reports experimental determinations of atomic hydrogen near the grounded electrode in medium-pressure dielectric barrier discharge hydrogen plasmas by means of molecular beam threshold ionization mass spectrometry (MB-TIMS). At certain discharge conditions (a.c. frequency of 24 kHz, 28 kV of peak-to-peak voltage), the measured hydrogen dissociation fraction is decreased from approximately 0.83% to approximately 0.14% as the hydrogen pressure increases from 2.0 to 14.0 Torr. A simulation method for extraction of the approximate electron beam energy distribution function in the mass spectrometer ionizer and a semi-quantitative approach to calibrate the mass discrimination effect caused by the supersonic beam formation and the mass spectrometer measurement are reported.     

A critical evaluation of the current state-of-the-art in quantitative imaging mass spectrometry

Mass spectrometry imaging (MSI) has evolved into a valuable tool across many fields of chemistry, biology, and medicine. However, arguably its greatest disadvantage is the difficulty in acquiring quantitative data regarding the surface concentration of the analyte(s) of interest. These difficulties largely arise from the high dependence of the ion signal on the localized chemical and morphological environment and the difficulties associated with calibrating such signals. The development of quantitative MSI approaches would correspond to a giant leap forward for the field, particularly for the biomedical and pharmaceutical fields, and is thus a highly active area of current research. In this review, we outline the current progress being made in the development and application of quantitative MSI workflows with a focus on biomedical applications. Particular emphasis is placed on the various strategies used for both signal calibration and correcting for various ion suppression effects that are invariably present in any MSI study. In addition, the difficulties in validating quantitative-MSI data on a pixel-by-pixel basis are highlighted.

Irreversibility,time, and fluctuations in quantum statistical theory of dissipative molecular systems. II. Application to fluorescence spectroscopy

Some basic concepts of the Prigogine quantum statistical theory of irreversible processes are discussed in connection with a previous paper by Chatzidimitriou-Dreismann and Lippert. The removal of the “dual role” that the Hamiltonian plays within conventional quantum theory corresponds to the introduction of a star-unitary operator ?? called “intrinsic time.” The formalism of the previous paper is extended to systems that interact with external fields. A formal specification of ??, furthermore, follows from the Lt invariance of the basic equations of motion and its connection with the intrinsic time. As an application to fluorescence spectroscopy, a photon counting experiment is presented. It allows us to detect certain predicted “dynamical” fluctuations in emission spectra. Some current experimental results are reported concerning the resolution of the broad fluorescence band of fluoranthene (dissolved in ethanol) through detection of the underlying finer vibrational structure.     

The influence of molecular structure and crystallization time on the efficiency of diastereoisomeric salt forming resolutions

Reciprocal resolutions between compounds (racemates and enantiomers) with similar structures have been examined. Amongst structurally similar compounds (so called relative structures) several N-acyl amino acids and amino acid esters were investigated. A part of the resolving agent or the racemic compound could be replaced by an achiral compound with a relative structure and an additive could occasionally improve significantly the efficiency of the resolution. Both the kinetic and the thermodynamic controls were observed as governing factors of the reciprocal resolutions.     

Dependence of the critical temperature on molecular parameters

At the critical temperature the surface tension between coexisting liquid and vapor phases must be zero, and the repulsive contributions associated with cavity formation must exactly counterbalance those from interactions of a molecule in the cavity and the bulk. An expression for the critical temperature of pure fluids in terms of the parameters of scaled particle theory (SPT) has been obtained, and the calculated critical temperatures are compared with experimental data for a range of pure fluids. These include noble and diatomic gases, short and medium length hydrocarbons, aromatic compounds, halogenated compounds, oxygen-containing compounds, and water. Considering the simplicity of this approach, a remarkably good correlation between calculated and experimental values is found for most of these fluids.     

A critical evaluation of high performance liquid chromatography-electrospray ionisation-mass spectrometry and capillary electrophoresis-electrospray-mass spectrometry for the detection and determination of small molecules of significance in clinical and forensic science

A critical review of applications for the period 2000-2003, taken from the Web of Knowledge database, of the techniques high performance liquid chromatography-electrospray ionisation-mass spectrometry (HPLC-ESI-MS) and capillary electrophoresis-electrospray ionisation-mass spectrometry (CE-ESI-MS) to the detection and determination of small molecules of significance in clinical and forensic science is presented. The molecules of mass less than 500 Da are chosen according to selected structural classes in which they give ESI signals primarily as [M+H](+) ions although other ions such as [M-H](-), [M+Na](+) and [M+NH(4)](+) are also reported. The structural classes are drugs with amine-containing side chains, drugs with N-containing saturated ring structures, 1,4-benzodiazepines, carbohydrates, benzimidazoles, other heterocycles, sulphonylureas, anthracyclines, sulphonamides, penicillins, cephalosporins, tetracyclines, nitrocatechols, steroids, flavonoids, oxazaphosphorines, cannabinols, and miscellaneous molecules. Details are given on the fragmentations, where available, that these ionic species exhibit in-source and in ion-trap, triple quadrupole and time-of-flight mass spectrometers. The review then gives a critical evaluation of these recent HPLC-ESI-MS and CE-ESI-MS analytical methods for the detection and determination of small molecules of clinical and forensic significance. Analytical information on, for example, sample concentration techniques, HPLC and CE separation conditions, recoveries from biological media and limits of detection (LODs) are provided.     

A discussion of the robustness of methods for assessing the evidential value of DNA single locus profiles in crime investigations

We discuss three issues relating to the evaluation of DNA single locus probe comparisons in forensic casework: band shift, database size, and population stratification. Illustrations are from experiments carried out in the Forensic Science Service as part of our preparations to implement the technique.     

A fluorescence study on critical exponents during sol-gel phase transition in complex monomeric systems

Methyl methacrylate (MMA), ethyl methacrylate (EMA) and various combinations of MMA with EMA were used during FCC experiments. Pyrene (P_y) was introduced as a fluorescence probe and fluorescence lifetimes from its decay traces were measured during sol-gel phase transitions. The fast transient fluorescence (FTRF) technique was used to study the critical exponents during sol-gel phase transition in free-radical crosslinking copolymerization (FCC). The results were interpreted in the view of percolation theory. The critical exponents of gel fraction, β and weight average degree of polymerization, γ were measured near the point of gel effect and found to be around 0.37 ± 0.015 and 1.69 ± 0.05 in all systems studied respectively.     

Sample preparation for evaluation of detection limits in X-ray fluorescence spectrometry.

The influence of analyte mass concentration on determination of detection limits in X-ray fluorescence spectrometry has been investigated experimentally. Both the total reflection X-ray fluorescence (TXRF) and the conventional energy-dispersive X-ray fluorescence techniques have been used to derive the dependence of analyte mass concentration on the values of detection limits. Results obtained indicate that values of detection limits are optimum, or in other words, they are closer to the true detection limit of the technique, when analyte concentrations are in the range of 10 times of the detection limit.     

The state of the art in thin-layer chromatography-mass spectrometry: a critical appraisal.

Thin-layer chromatography-mass spectrometry (TLC-MS) is a readily implemented technique that, in its simplest form, puts few demands on either chromatography or spectrometry. Nevertheless, compared to the situation with high performance liquid chromatography, it is much less highly developed. Currently, the bulk of the practical applications of TLC-MS are directed towards the use of fast atom, or ion bombardment. Recent developments, however, include the use of matrix assisted laser desorption ionisation (MALDI), surface assisted laser desorption (SALDI) and the development of a TLC-electrospray interface. Here, the state of the art of TLC-MS is described and future trends identified.     

A fluorescence study of the excited-state dynamics of boron dipyrrin molecular rotors

Noticeable viscosity dependence has been revealed for fluorescence spectra of three phenyl derivatives of boron dipyrrin in aqueous glycerol solutions. This dependence is less pronounced for the fluorescence lifetimes. Such behavior is characteristic of molecular rotors used as media microviscosity sensors. A significant growth of the radiative deactivation constant is observed in the range 298-150 K. Quantumchemical calculation of the model fluorophore support the assumption on barrier-free relaxation of the excited state from the pretwisted to the flattened conformation. The spectral-kinetic parameters of fluorophores have been determined, and viscosity graduation curves are presented.