System Upgrade on Feb 12th During this period, E-commerce and registration of new users may not be available for up to 12 hours. For online purchase, please visit us again. Description Chapters Supplementary Using lasers to induce and probe surface processes has the advantages of quantum state specificity, species selectivity, surface sensitivity, fast time-resolution, high frequency resolution, and accessibility to full pressure ranges.
Readership: Graduate students and researchers in surface science, physical chemistry, condensed matter physics, and materials science.
Modern Electronic Structure Theory. Symmetry and Topology in Chemical Reactivity. Synthetic Coordination Chemistry: Principles and Practice. NMR in Structural Biology. Quantum Chemistry. Recent Trends in Surface and Colloid Science. In contrast to standard analytical techniques e. Simultaneous multispecies detection in breath with optical methods has only been demonstrated for few species at a time, at trace levels ppb and below. In many cases it would be useful to have an instrument that can analyze many volatile compounds.
And this is where mass spectrometry still holds an advantage over optical methods. The gold standard in breath research is gas chromatography combined with mass spectrometry GC-MS [ ]. GC-MS allows the unambiguous identification of hundreds of volatile breath compounds and the method has been used extensively in exhaled breath studies. However, it is not suitable for online sampling and the analysis is not real time.
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Some form of pre-concentration adsorbent traps, solid-phase micro-extraction is required and standard analysis time is up to tens of minutes. These limitations can be circumvented using soft chemical ionization instead of electron impact ionization. Instruments based on selected ion flow-tube mass spectrometry SIFT-MS and proton-transfer reaction mass spectrometry PTR-MS allow online sampling and real-time analysis while making it potentially possible to analyses dozens of VOCs simultaneously [ ].
Few studies have carefully addressed the untargeted analysis of medium-to-high molecular weight species that absorb over a broad wavelength range [ ].
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As outlined in Sect. When broadband emission sources are used, such as EC-QCLs or OFCs, the main challenge is to develop robust algorithms that can discriminate between multiple absorbing molecules using their specific narrowband or broadband spectroscopic features. Even so, the existing possibility of scanning very fast over a broad spectral range to obtain a molecular fingerprint of an unprecedented large number of gases with optical techniques is well worth exploring.
The technology of lasers has advanced considerably in the past few years and more wavelengths are available for gas sensing and applications outside the laboratory. Furthermore, new avenues have been opened for the design of compact instruments. This has been demonstrated by examples presented in this article. The MFS approach described in Sect. Few spectroscopic techniques are able to fulfill these demands and a new promising candidate is the intracavity absorption spectroscopy ICAS.
Furthermore, also broadband absorbing molecules, such as acetone [ ], can be detected in breath samples with a portable and compact system. For example, Hancock et al. With an optical cavity mirror reflectivity of The method was validated with measurements made using a chemical ionization mass spectrometer and measurements on individuals under fasting, exercise, and normal conditions have been presented, indicating the utility of the device across a wide dynamic range.
The device has a sufficiently small power requirement that battery operation is possible for a reasonable number of measurements. This device has ready applicability for using breath acetone analysis to provide an alternative to blood testing for ketone measurement, to assist with the management of type 1 diabetes [ ]. The fast development of the QCL combs, e. It is well known that the time to market a device for medical purposes is longer than for other fields of applications.
This might explain why some optical methods are not yet on the market, even though they have shown large potentials, as demonstrated by the examples presented here. For detection of some compounds, such as C 2 H 4 or NH 3 , optical methods are preferred over the MS-based ones, due to a better sensitivity, simplicity in operation and instrument price.
Also, the ability of soft-ionization MS to detect light molecules is limited to those species with sufficient proton affinity. Exhaled NO or fractional exhaled NO in the gas phase, F E NO is a well-known biomarker for airway inflammation [ ], playing an important role in asthma phenotyping and management [ ].
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Moreover, its concentration is dependent on the exhalation flow higher at low flow and vice versa. The use of NO modeling approach linear or nonlinear can solve this issue and provide useful information about the source of NO [ 2 , ]. There are still many unexplored areas for F E NO analysis and examining these will require the implementation of a sampling system allowing different flow rates into the measuring instrument.
This will provide a great value in diagnostic procedures of respiratory diseases and in treatment with anti-inflammatory drugs by predicting the inhaled corticosteroid ICS response. The sampling of exhaled breath is as important as its analysis [ , ].
A careful selection of the laser source for spectroscopic measurements is usually required to limit spectral overlap with water and CO 2 which are quite abundant in breath. However, whenever possible, it would actually be beneficial to monitor CO 2 simultaneously with the molecule of interest within the same scan [ , , ]. A CO 2 -controlled sampling of expiratory air allows a proper determination of the end-tidal metabolite concentration. When broad band emission sources are used, such as EC-QCL or OFC, the main challenge is to develop a robust algorithm that can discriminate between multiple absorbing molecules using their specific spectroscopic features.
The latest incremental developments in MS-based technology seem to strengthen their market position. A representative example is PTR-MS; although the instruments are not becoming cheaper, on the contrary, their features look potentially attractive for multispecies real-time monitoring.
Over time, the PTR-MS instruments have been equipped with a time-of-flight TOF analyzer to improve the mass resolution and enable the analysis of compounds with almost the same molecular mass. Furthermore, commercial PTR-MS instruments now include software that allows convenient data analysis of real-time measurements of breath profiles.
This is of great importance since it aids in the identification of the different respiratory phases dead space, end-tidal by measuring the time-profiles of specific breath volatiles e. The need for an elaborate breath sampler incorporating a CO 2 analyzer, for example can thus be circumvented by multispecies real-time analysis. The question is whether this could change the market beyond the ability of the optical techniques to adapt and secure their position.
For detection of exhaled compounds in high concentration ppm and above , such as methane, the electrochemical sensors are highly competitive with the laser-based techniques, especially that they are already commercially available from various vendors [ ]. Furthermore, an ingestible electronic capsule based on semiconducting metal oxide-based sensor has demonstrated its capability of sensing O 2 , H 2 and CO 2 in the gut during a human pilot trial [ ].
With the technological developments been stimulated by the growing evidence of clinical worth, advanced research moved from measuring relevant molecules in laboratory to pilot studies in hospitals. The transition to clinical practice is in continuous progress; more commercial instruments found their way into clinics and reveal unprecedented information, with the few examples presented here. Like any other techniques, to get into routine practice, the optical techniques need to overcome still several barriers at various levels from instrumental and scientific hurdles to regulatory approvals and financials [ 23 , 27 ].
Breath sampling and measurement. At this stage, a lot of the practical problems regarding breath sampling and measurement have been identified and solutions are in place. This is still a crucial aspect for further implementation, not only for the optical techniques, but also for MS-based ones.
Researchers became more aware of the importance of controlling the exhalation rate and monitoring the exhaled CO 2 in any accessible manner; most measurements are designed for acquisition of the alveolar air and elimination of the dead air space. No matter whether the measurements are online or offline, these set of parameters are the first steps towards standardization of breath sampling.
Lessons have been learned by experience; therefore, rather than still being named a challenge, standardization includes a series of protocols that have been proposed and are currently tested for validation. For example, off-line collection usually in Tedlar bags requires a proper choice for the material and additional tests to check the cleanliness of the bags and the stability over time of the analyte of interest.
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For single or multiple exhalations, reproducibility of the breath samples needs to be also checked. Instrumental challenge. Compact and user-friendly instruments are highly desired, providing accurate measurements, preferably single time-point breath collection with high sensitivity and specificity. They should monitor changes in biomarker concentrations or parameters compared to control or baseline values, etc.
Therefore, calibration of the measuring system for determination of the gas concentration needs to be done taking into account the main compounds present in the matrix of breath, namely H 2 O and CO 2 in air. Scientific challenge and outcomes. Most of the compounds that can be measured with the laser-based techniques have known origin and biochemical pathways. Furthermore, understanding the connection biomarker-disease-health conditions, and response of biomarker to interventions exposure, isotope labeling, therapy, etc. The high precision and absolute accuracy offered by the laser-based techniques allows collection of more data and in consequence, very detailed biomarker studies.
Small differences can be thus easier resolved, gas exchange in the respiratory tract can be characterized so that the origin and chemical pathways can be better understood. In turn, it will become more clear which biomarker reliably correlates with diseases. Another aspect to be considered is that variability of the results may be strongly influenced by diet and life style and exposure to various environmental conditions.
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