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Measurement of Vapors and Gases
Published in S.M. Rappaport, Thomas J. Smith, Exposure Assessment for Epidemiology and Hazard Control, 2020
Robert F. Herrick, Eugene R. Kennedy, Mary Lynn Woebkenberg
Flame ionization is a technique widely used in both the laboratory and the field. The flame ionization detector (FID) is commonly used on gas chromatographs but has also been successfully adapted into a portable, direct-reading field instrument. Part of the attractiveness of an FID is the low noise level (10-12 amperes), which allows for high sensitivity, and the wide linear range (107). In an FID, hydrogen and the sample gas-stream are mixed with combustion air or oxygen and ignited; the current is then measured across an electrode gap above the flame. This current is proportional to the number of ions generated during the burning of the sample.25 The FID is useful in the analysis of most organic compounds, although it is most sensitive to hydrocarbons.
MR Imaging
Published in George C. Kagadis, Nancy L. Ford, Dimitrios N. Karnabatidis, George K. Loudos, Handbook of Small Animal Imaging, 2018
To improve the quality of MR images, the signal is often measured in the form of a spin or gradient echo rather than FID. A spin echo is generated by applying an inverting 180° pulse at time TE/2 following the excitation pulse. This pulse inverts the magnetization of spins precessing in the transversal plane and completely refocuses all effects of B0 inhomogeneity at time TE/2 after the pulse. The spin-echo is therefore formed at time TE (echo time) after the initial excitation pulse (Figure 9.4a). The gradient echo is formed by switching the polarity of the read gradient so that the echo signal is formed at time TE after the pulse when the integral value of the inverted gradient becomes equal to that of the positive gradient (Figure 9.4b). A gradient echo does not refocus static B0 inhomogeneities and therefore is very sensitive to magnetic susceptibility gradients present in the sample.
Nuclear Magnetic Resonance
Published in Grinberg Nelu, Rodriguez Sonia, Ewing’s Analytical Instrumentation Handbook, Fourth Edition, 2019
A signal is induced from the relaxing nuclei that may be likened to a decaying sine wave. This signal arises as the result of the decay of transverse magnetization and is detected by the spectrometer’s rf receiver (Figure 11.10a). The term describing this signal is the free induction decay or FID. Because NMR samples usually contain nuclei with different resonance frequencies, the decay curves arising from the different magnetization frequencies are superimposed causing the FIDs to interfere with one another. This collection of signals is also described as an interferogram. The FID represents a time domain spectrum, however, because it is not possible to directly interpret the FID signals, a mathematical function is applied known as Fourier transformation (FT) [33]. When FT is applied to the FID, a frequency domain spectrum is produced, which upon phase and baseline correction may be interpreted by the analyst (Figure 11.10b). Because NMR is an inherently insensitive technique, the FID signals are small compared to the noise. This is especially problematic for nuclei with low natural abundance such as l3C. To compensate for these weak signals, the FIDs of many pulses are acquired and added together. However although the signal is additive, the noise is random, and both adds and subtracts. Therefore the signal-to-noise (S/N) does not increase linearly with the number of scans (NS) but instead increases in proportion to the square root of NS:S:N~NS
Groundwater investigation using ground magnetic resonance and resistivity meter
Published in ISH Journal of Hydraulic Engineering, 2021
Uttam Singh, Pramod Kumar Sharma, C. S. P. Ojha
It is also observed that the use of the geophysics for both groundwater resource mapping and water quality evaluation is increased due to the rapid advances in computer software and numerical modeling (Gev et al. 1996; Yaramanci et al. 1999). The technique of nuclear magnetic resonance (NMR) is used in geophysical applications for determining aquifer properties such as porosity, permeability and water content (Yaramanci et al. 1999; Muller-Petke et al. 2011, 2013). The technique of NMR combines the information content accessible via the NMR measurements with the non-destructive approach to derive the subsurface information from the surface-based measurements (Burger et al. 2006). As such, the NMR is the only geophysical exploration method providing direct and non-destructive information on hydraulic subsurface aquifer properties. The NMR is based on the free induction decay (FID) experiment, emitting an excitation pulse and recording the relaxation signal using large surface coils of tens of meters and detecting signals from depths up to 150 m (Walsh 2008).
Metabolomics profiling of valproic acid-induced symptoms resembling autism spectrum disorders using 1H NMR spectral analysis in rat model
Published in Journal of Toxicology and Environmental Health, Part A, 2022
Hyang Yeon Kim, Yong-Jae Lee, Sun Jae Kim, Jung Dae Lee, Suhkmann Kim, Mee Jung Ko, Ji-Woon Kim, Chan Young Shin, Kyu-Bong Kim
NMR data were obtained in the form of FID files and then the spectrum was acquired by Fourier transformation. All acquired spectra were phased, baseline corrected and referenced to the TSP-d4 peak (chemical shift of 0 ppm) using VnmrJ 4.2 software (Aligent Technologies, Santa Clara, CA, USA). The identification and quantification of metabolites were performed using the Chenomx NMR Suite 8.3 Professional software (Chenomx, Inc., Edmonton, Alberta, Canada) with a library database of Chenomx. The internal standard for chemical shift and quantification was TSP-d4 for brain and serum and DSS for urine. For urine samples, the concentration of metabolites was expressed as a relative ratio value normalized by creatinine concentration, assuming a constant % creatinine excretion.
The effect of periodic intermittency on the cyclic behavior of marine sedimentary clay
Published in Marine Georesources & Geotechnology, 2019
Qingqing Zheng, Tangdai Xia, Zhi Ding, Shaoheng He
Magnetic resonance is a unique physical phenomenon based on proton spin. The process by which a magnetized vector returns to a balanced state is called relaxation. There are two types of relaxation: longitudinal relaxation and transverse relaxation. The process by which a horizontally magnetized vector that is vertical to an external magnetic field B0 transforms from an unbalanced state to a balanced state is called the transverse relaxation process (or spin-spin relaxation). The transverse relaxation time T2 represents the speed of the recovery process. The time variation of the nuclear magnetic signal is called a free induction decay (FID) curve. The first point on the FID curve is proportional to the water content of the specimen. The FID curve is converted to a T2 spectrum via a Fourier transform. The T2 distribution measured by nuclear magnetic resonance is the key to calculations relevant to the pores: where is the surface relaxation rate of the material, which is related to the type of soil, and refers to the specific surface area of the pores. The specific surface area and the pore diameter D are reciprocals. In this case, we set T2=CD (Li, Zhu, and Guo 2008). C is the aperture conversion coefficient, which is related to and the pore shape defined in the calculation. For isotropic soil, C is approximately constant.