Pure Absorption Spectra Research Articles

Low-Frequency NARS (LF NARS) by the use of a superheterodyne EPR spectrometer

In this article, the possibilities of recording EPR spectra by the non-adiabatic rapid sweep (NARS) method on a superheterodyne spectrometer are investigated. This method allows recording the pure EPR absorption spectrum of the object under investigation without the need for lineshape–lineheight compromise, while increasing sensitivity by suppressing low-frequency noise. In the NARS method, a low-amplitude sinusoidal modulation of a magnetic field is not used, and an oscillating triangular-shaped bipolar magnetic field is superimposed on the main permanent magnetic field. The triangular field repetition rate should be higher than that of the undesirable noise, and the amplitude is such that the rate of field change satisfies the Bloch non-adiabaticity criterion. The EPR absorption signal is digitized by a fast ADC and accumulated over a large number (n) of triangular field periods. In this case, for low-frequency noise, the spectrum of which is located below the repetition frequency of the triangular field, an accumulation process occurs with an increase in the signal-to-noise ratio (SNR) in proportion to n. A remarkable property of a superheterodyne spectrometer is that the frequency below which low-frequency noise prevails over white noise is significantly lower for it than for a homodyne spectrometer. This allows the use of a low repetition rate (LF NARS), which makes it much easier to obtain a highly linear triangular field of significant amplitude even in microwave resonators with a massive metal case since the harmful effects of eddy currents are reduced. The conditions of non-adiabaticity become easily feasible. At the same time, the noise suppression effect during accumulation turns out to be so significant that the SNR of the LFNARS spectrum exceeds that of the traditional spectrum with magnetic field modulation, with the same recording time, by more than 10 times.

LkHα 225 (V1318 Cyg) South in Outburst

Abstract Magakian et al. called attention to the current bright state of LkHα 225 South, a well-known highly embedded, intermediate-mass young stellar object that over the past two decades has brightened visually from >20 m to <13 m . We present recent optical photometric monitoring showing colorless, nonsinusoidal, periodic brightness oscillations occurring every 43 days with amplitude ∼0.7 mag. We also present new flux-calibrated optical and near-infrared spectroscopy, which we model in terms of a Keplerian accretion disk, and high-dispersion spectra that demonstrate similarity to some categories of “mixed-temperature” accretion-outburst objects. At blue wavelengths, LkHα 225 South has a pure absorption spectrum and is a good spectral match to the FU Ori stars V1515 Cyg and V1057 Cyg. At red optical and infrared wavelengths, however, the spectrum is more similar to Gaia 19ajj, showing emission in TiO, CO, and metals. Sr ii absorption indicates a low-surface-gravity atmosphere. There are also signatures of a strong wind/outflow. LkHα 225 South was moderately bright in the early 1950s as well as in the late 1980s, with evidence for deep fades during intervening epochs. The body of evidence suggests that LkHα 225 South is another case of a source with episodically enhanced accretion that causes brightening by orders of magnitude, and development of a hot absorption spectrum and warm wind. It is similar to Gaia 19ajj, but also reminiscent in its long brightening time and brightness oscillation near peak to the embedded sources L1634 IRS7 and ESO Ha 99.

Harmonic and Anharmonic Oscillator Models for Pure Vibrational Infrared Spectroscopy of Diatomic Molecules

In molecular vibrational infrared spectroscopy, absorption spectra arise from molecular vibration and correspond to transitions between the vibrational energy levels associated with a given electronic state of the molecule. The vibrational transitions, which fall in the near infrared region, are induced through the interaction of the molecular electric dipole with the electric vector of the electromagnetic radiation. The near infrared region extends roughly from 1?m to ?10?^2 ?m. The article explains the pure vibrational absorption spectra of diatomic molecules such as HCl, HBr, HI, CO, … etc. In order to explain the vibrational spectra, diatomic molecules are treated as harmonic oscillator and anharmonic oscillator. In the harmonic oscillator model, we get only one absorption band at the wavenumber value? ?_osc corresponding to frequency of oscillation?_osc while in the actual experimental data, there are many absorption bands corresponding to wave numbers slightly lesser than ? ?_osc, 2? ?_osc, 3? ?_osc, ……..The occurrence of these additional bandsis attributed to the selection rule ?v=±2, ±3, ±4, ……The additional bands are having lesser intensity and are called overtone bands.

Molecular fingerprinting of nanoparticles in complex media with non-contact photoacoustics: beyond the light scattering limit

Optical instruments can probe physical systems even to the level of individual molecules. In particular, every molecule, solution, and structure such as a living cell has a unique absorption spectrum representing a molecular fingerprint. This spectrum can help identify a particular molecule from others or quantify its concentration; however, scattering limits molecular fingerprinting within a complex compound and must be overcome. Here, we present a new, non-contact photoacoustic (PA)-based method that can almost completely remove the influence of background light scattering on absorption measurements in heterogeneous highly scattering solutions and, furthermore, separate the intrinsic absorption of nanoscale objects from their scattering. In particular, we measure pure absorption spectra for solutions of gold nanorods (GNRs) as an example of a plasmonic agent and show that these spectra differ from the extinction measured with conventional UV-VIS spectrophotometry. Finally, we show how the original GNR absorption changes when nanoparticles are internalized by cells.

Synthesis and luminescence properties of CdSe:Eu NPs and their surface polymerization of poly(MMA-co-MQ)

CdSe:Eu nanoparticles (NPs) were synthesized using an oil phase method and the substitution of Cd2+ with Eu3+ was confirmed by XRD, TEM, UV-Vis absorption and fluorescence emission analyses. The CdSe:Eu NPs are monodispersed and uniform spherical particles with a diameter of 3.2 nm, bigger than the pure CdSe NPs (2.3 nm), but with a similar cubic structure as CdSe NPs. Compared with those of pure CdSe NPs, both emission spectrum and absorption spectrum of CdSe:Eu NPs are red-shifted. The CdSe:Eu NPs are incorporated into poly(MMA-co-MQ) to afford poly(MMA-co-MQ)-CdSe:Eu NPs with the cubic structure and particle size (∼3–4 nm) similar to those of CdSe:Eu NPs. The TEM imaging suggests that the CdSe:Eu NPs are uniformly dispersed in poly(MMA-co-MQ) without any obvious aggregation. The fluorescent emission peak and absorption peak of poly(MMA-co-MQ)-CdSe:Eu NPs are between those of CdSe:Eu NPs and poly(MMA-co-MQ), possibly due to the energy transfer caused by the interactions of Cd or Eu atoms on the surfaces of CdSe:Eu NPs with the N and O atoms of poly(MMA-co-MQ). These CdSe:Eu and poly(MMA-co-MQ)-CdSe:Eu NPs with tunable photoluminescence properties can be potentially used for the fabrication of optical and optoelectronic devices.

Transmission versus reflectance spectroscopy for quantitation.

The objective of this work was to compare the accuracy of analyte concentration estimation when using transmission versus diffuse reflectance spectroscopy of a scattering medium. Monte Carlo ray tracing of light through the medium was used in conjunction with pure component absorption spectra and Beer-Lambert absorption along each ray's pathlength to generate matched sets of pseudoabsorbance spectra, containing water and six analytes present in skin. PLS regression models revealed an improvement in accuracy when using transmission compared to reflectance for a range of medium thicknesses and instrument noise levels. An analytical expression revealed the source of the accuracy degradation with reflectance was due both to the reduced collection efficiency for a fixed instrument etendue and to the broad pathlength distribution that detected light travels in the medium before exiting from the incident side.

Voigt deconvolution method and its applications to pure oxygen absorption spectrum at 1270 nm band

Experimental spectral lines of pure oxygen at 1270nm band were analyzed by Voigt deconvolution method. The method gave a total Voigt profile, which arises from two overlapping bands. Deconvolution of total Voigt profile leads to two Voigt profiles, the first as a result of O2 dimol at 1264nm band envelope, and the second from O2 monomer at 1268nm band envelope. In addition, Voigt profile itself is the convolution of Lorentzian and Gaussian distributions. Competition between thermal and collisional effects was clearly observed through competition between Gaussian and Lorentzian width for each band envelope. Voigt full width at half-maximum height (Voigt FWHM) for each line, and the width ratio between Lorentzian and Gaussian width (ΓLΓG−1) have been investigated. The following applied pressures were at 1, 2, 3, 4, 5, and 8bar, while the temperatures were at 298K, 323K, 348K, and 373K range.

A SWIR based algorithm to retrieve total suspended matter in extremely turbid waters

In ocean colour remote sensing, the use of Near Infra Red (NIR) spectral bands for the retrieval of Total Suspended Matter (TSM) concentration in turbid and highly turbid waters has proven to be successful. In extremely turbid waters (TSM>100mgL−1) however, these bands are less sensitive to increases in TSM. Here it is proposed to use Short Wave Infra Red (SWIR) spectral bands between 1000 and 1300nm for these extreme cases. This SWIR spectral region is subdivided into two regions, SWIR-I (1000nm to 1200nm) and SWIR-II (1200nm to 1300nm) which correspond to local minima in the pure water absorption spectrum. For both spectral regions the water reflectance signal was measured in situ with an ASD spectrometer in three different extremely turbid estuarine sites: Scheldt (Belgium), Gironde (France), and Río de la Plata (Argentina), along with the TSM concentration. A measurable water reflectance was observed for all sites in SWIR-I, while in the SWIR-II region the signal was not significant compared to the Signal-to-Noise Ratio (SNR) of current Ocean Colour (OC) sensors. For the spectral band at 1020nm (present in Ocean and Land Colour Instrument — OLCI, onboard Sentinel-3) and at 1071nm, an empirical single band TSM algorithm is defined which is valid for both the Gironde and Scheldt estuarine sites. This means that a single algorithm can be applied for both sites without expensive recalibration. The relationship between TSM and SWIR reflectance at 1020 and 1071nm is linear and did not show any saturation for the concentrations measured here (up to 1400mgL−1), while saturation was observed for the NIR wavelengths, as expected. Hence, for extremely turbid waters it is advised to switch from NIR to SWIR-I wavelengths to estimate TSM concentration. This was demonstrated for an airborne hyperspectral dataset (Airborne Prism Experiment, APEX) from the Gironde estuary having several spectral bands in the SWIR-I. The empirical single band SWIR TSM algorithm was applied to the atmospherically corrected scene providing a TSM concentration map of the Gironde from mouth to more upstream with concentrations expected in this region ranging from a few to several hundreds mgL−1. These results, i.e. the existence of a single relationship for the Scheldt and Gironde, not showing any decrease of sensitivity, highlights the importance of having SWIR bands in future ocean colour sensors for studying extremely turbid rivers, coastal areas and estuaries in the world. A further implication of these results is that there is a TSM limit for application of atmospheric correction algorithms which assume zero SWIR marine reflectance. That limit is defined here as function of wavelength and sensor noise level.

Single-particle absorption spectroscopy by photothermal contrast.

Removing effects of sample heterogeneity through single-molecule and single-particle techniques has advanced many fields. While background free luminescence and scattering spectroscopy is widely used, recording the absorption spectrum only is rather difficult. Here we present an approach capable of recording pure absorption spectra of individual nanostructures. We demonstrate the implementation of single-particle absorption spectroscopy on strongly scattering plasmonic nanoparticles by combining photothermal microscopy with a supercontinuum laser and an innovative calibration procedure that accounts for chromatic aberrations and wavelength-dependent excitation powers. Comparison of the absorption spectra to the scattering spectra of the same individual gold nanoparticles reveals the blueshift of the absorption spectra, as predicted by Mie theory but previously not detectable in extinction measurements that measure the sum of absorption and scattering. By covering a wavelength range of 300 nm, we are furthermore able to record absorption spectra of single gold nanorods with different aspect ratios. We find that the spectral shift between absorption and scattering for the longitudinal plasmon resonance decreases as a function of nanorod aspect ratio, which is in agreement with simulations.

Comparison between Simple and Advanced Data Analysis to Pure Oxygen Absorption Spectrum at the 1270 nm Band

Simple and advanced data analysis was carried out on the pure oxygen absorption spectrum by Voigt deconvolution method between 1200 - 1300 nm, which represented the near-infrared spectrum. Pressures were 1, 5, 10, 15, 20, and 25 bars at 298 K. Experimental spectral lines showed two peaks at 1264 nm belong to O2 dimol and 1268 nm wavelength belong to O2 monomer, but advanced analysis of spectral lines gave many peaks around those two peaks. The total number of energy levels (peaks) decreases when pressure increases.

Optical and structural properties of fullerene films doped with cadmium telluride

Composite thin-film structures based on C60/CdTe were prepared by discrete vacuum evaporation in a quasi-closed volume and by vacuum evaporation from a Knudsen cell. The synthesized fullerene samples contained cadmium telluride with a content ranging from 1 to 50 wt %. The morphology and composition of the films were controlled using scanning electron microscopy and energy-dispersive X-ray analysis. The optimum geometry, the total energy, and the spectrum of excited states of the fullerene-cadmium telluride molecular complex were calculated by quantum-chemical methods. The spectral dependences of the photo-luminescence, Raman scattering, extinction coefficient, and refractive index were measured for different compositions. It was found that, in contrast to the spectra of pure fullerene, the luminescence and absorption spectra of the fullerene doped with cadmium telluride exhibit an additional peak in the wavelength range of 600–620 nm. These data were interpreted as the existence of dipole-allowed transitions in the spectrum of excited singlet states of the fullerene due to the interaction with cadmium telluride. The composite films had an increased resistance to degradation under the action of oxygen and water vapor.

Improvement and Red Shift Toward Visible Spectrum of TiO<sub>2 </sub>Nanofibers through Doping by Fe<sub>2</sub>O<sub>3</sub> and Nitrogen

TiO2has many applications as photocatalyst, photo/electrochromic device, solar cell and sensors. It has been activated only by UV light. In this research we have been investigated on the effect of adding Fe2O3nanoparticles and N on the absorption spectrum in order to increase their activity and shift toward visible light for TiO2nanofibers. We used electrospining method to fabricate nanofibers. Sol gel for spinning pure nanofibers contains Titanium tetraisopropoxide (C12H28O4Ti), acetic acid (CH3COOH), ethanol (C2H5OH) and PVP (C6H9NO-12 wt.% ). Then we used Fe2O3nanoparticle (size: 2040 nm, purity: 98%) and HNO3acid for doping. Among Fe2O3-doped samples, the 2.1 wt% Fe2O3indicated the highest activity for visible light, while with HNO3, excellent photo-catalyst activity obtained at 1.5wt% Fe2O3and 5 wt% HNO3(purity :65%). The final solutions were stacked into the syringe and joined to a high voltage DC supply. A piece of glass stacked on aluminum foilwas used as collector. Finally nanofibers calcined at 500°C. FESEM, XRD and Spectrophotometry techniques were employed to deliberation nanofibers. Diameter of the fibers was approximately 100 nm (anatase: 77%). The Fe2O3-TiO2and Fe2O3-N-TiO2nanofibers indicated respectively 50% and 80% growth in absorption intensities under visible light irradiation compared to pure nanofiber and absorption spectrum peak moved towards larger wavelengths (70 and 100nm) when TiO2doped with Fe2O3and Fe2O3-N. Keywords:Fe2O3nanoparticle; nitrogen; red shift; TiO2nanofiber; UV-visible irradiation.

Moving difference (MDIFF) non-adiabatic rapid sweep (NARS) EPR of copper(II)

Non-adiabatic rapid sweep (NARS) EPR spectroscopy has been introduced for application to nitroxide-labeled biological samples (Kittell et al., 2011) [1]. Displays are pure absorption, and are built up by acquiring data in spectral segments that are concatenated. In this paper we extend the method to frozen solutions of copper-imidazole, a square planar copper complex with four in-plane nitrogen ligands. Pure absorption spectra are created from concatenation of 170 5-gauss segments spanning 850G at 1.9GHz. These spectra, however, are not directly useful since nitrogen superhyperfine couplings are barely visible. Application of the moving difference (MDIFF) algorithm to the digitized NARS pure absorption spectrum is used to produce spectra that are analogous to the first harmonic EPR. The signal intensity is about four times higher than when using conventional 100kHz field modulation, depending on line shape. MDIFF not only filters the spectrum, but also the noise, resulting in further improvement of the SNR for the same signal acquisition time. The MDIFF amplitude can be optimized retrospectively, different spectral regions can be examined at different amplitudes, and an amplitude can be used that is substantially greater than the upper limit of the field modulation amplitude of a conventional EPR spectrometer, which improves the signal-to-noise ratio of broad lines.

Accurate retention time determination of co-eluting proteins in analytical chromatography by means of spectral data

Chromatography is the method of choice for the separation of proteins, at both analytical and preparative scale. Orthogonal purification strategies for industrial use can easily be implemented by combining different modes of adsorption. Nevertheless, with flexibility comes the freedom of choice and optimal conditions for consecutive steps need to be identified in a robust and reproducible fashion. One way to address this issue is the use of mathematical models that allow for an in silico process optimization. Although this has been shown to work, model parameter estimation for complex feedstocks becomes the bottleneck in process development. An integral part of parameter assessment is the accurate measurement of retention times in a series of isocratic or gradient elution experiments. As high-resolution analytics that can differentiate between proteins are often not readily available, pure protein is mandatory for parameter determination. In this work, we present an approach that has the potential to solve this problem. Based on the uniqueness of UV absorption spectra of proteins, we were able to accurately measure retention times in systems of up to four co-eluting compounds. The presented approach is calibration-free, meaning that prior knowledge of pure component absorption spectra is not required. Actually, pure protein spectra can be determined from co-eluting proteins as part of the methodology. The approach was tested for size-exclusion chromatograms of 38 mixtures of co-eluting proteins. Retention times were determined with an average error of 0.6 s (1.6% of average peak width), approximated and measured pure component spectra showed an average coefficient of correlation of 0.992.

Hyperspectral Imaging: A Review of Best Practice, Performance and Pitfalls for in-line and on-line Applications

The objective of this paper is to provide a comprehensive review of best practice in hyperspectral imaging. The paper starts to review the taxonomy of the different spectral imaging techniques together with their advantages and disadvantages. The appropriate selection of cameras and spectrographs and their figures of merit are discussed and a detailed description is given of how to qualify and calibrate a pushbroom imaging system for on-line and in-line control. Special emphasis is given to detection and avoidance of specular reflection which can severely distort quantification of the spectral response. Recommendations for an ideal Lambertian illumination are given and the effects of scatter and absorption are discussed when particulate systems are investigated. Here, first principles are introduced and strategies for how to separate scatter from absorption are developed. A simple method using the Kubelka and Munk approach is examined and separated scatter and pure absorption spectra are shown. The same procedure is applied to show the lateral distribution of the separated scatter and absorption properties of an active pharmaceutical ingredient embedded in an excipient. The terms penetration and information depth are discussed and an example of penetration depth profile over wavelengths is provided. Based on a good quality optical set-up and a validated measurement procedure, a practical procedure is described to analyse the data cube using the chemometrics toolbox for hyperspectral imaging. Finally, a survey on selected applications demonstrates the future potential of hyperspectral imaging.

FTIR microscopy of biological cells and tissue: data analysis using resonant Mie scattering (RMieS) EMSC algorithm

Transmission and transflection infrared microscopy of biological cells and tissue suffer from significant baseline distortions due to scattering effects, predominantly resonant Mie scattering (RMieS). This scattering can also distort peak shapes and apparent peak positions making interpretation difficult and often unreliable. A correction algorithm, the resonant Mie scattering extended multiplicative signal correction (RMieS-EMSC), has been developed that can be used to remove these distortions. The correction algorithm has two key user defined parameters that influence the accuracy of the correction. The first is the number of iterations used to obtain the best outcome. The second is the choice of the initial reference spectrum required for the fitting procedure. The choice of these parameters influences computational time. This is not a major concern when correcting individual spectra or small data sets of a few hundred spectra but becomes much more significant when correcting spectra from infrared images obtained using large focal plane array detectors which may contain tens of thousands of spectra. In this paper we show that, classification of images from tissue can be achieved easily with a few (<10) iterations but a reliable interpretation of the biochemical differences between classes could require more iterations. Regarding the choice of reference spectrum, it is apparent that the more similar it is to the pure absorption spectrum of the sample, the fewer iterations required to obtain an accurate corrected spectrum. Importantly however, we show that using three different non-ideal reference spectra, the same unique correction solution can be obtained.

Effects of intramolecular hydrogen bonding and solvent composition on acidity of some dihydroxy-thioxanthone derivatives in methanol–water binary solvents

► The acidity constant of some new dihydroxy thioxanthone derivatives was determined. ► The effect of inter and intra hydrogen bonding on acidity constants were explained. ► The effect of solvent composition (methanol–water) was also investigated. ► 1,3-Dihydroxy derivative unexpectably showed one ionization step. ► The findings of this article are of especial importance for analytical applications. The acidity constant of four recently synthesized dihydroxy-thioxanthone derivatives (i.e., 1,4-dihydroxy; A1, 3-methyl-1,4-dihydroxy; A2, 2,3-dihydroxy; A3 and 1,3-dihydroxy; A4) in methanol–water binary mixed solvents were determined by multivariate analysis of spectrophotometric-pH-metric data. The solutes A1, A2 and A3 exhibited two ionization steps whereas one ionization step was observed for A4, attributed to the strong intramolecular hydrogen-bonding formed between the hydrogen of 1-hydroxy and its neighbor carbonyl group. The effects of molecular structure on the acid–base behavior of solutes were discussed mainly based on intra and intermolecular hydrogen bonding. The effects of solvent composition on the acidity constant were explained through linear solvation free energy relationship (LSFER) study. It was found that each ionization step of each solute can be modeled by different reduced models of Kamlet–Taft, proposing that they behave differently. Finally, the effect of solvent composition on the calculated pure absorption spectra of the different acid–base species was investigated.

Detection of undistorted continuous wave (CW) electron paramagnetic resonance (EPR) spectra with non-adiabatic rapid sweep (NARS) of the magnetic field

A continuous wave (CW) electron paramagnetic resonance (EPR) spectrum is typically displayed as the first harmonic response to the application of 100 kHz magnetic field modulation, which is used to enhance sensitivity by reducing the level of 1/ f noise. However, magnetic field modulation of any amplitude causes spectral broadening and sacrifices EPR spectral intensity by at least a factor of two. In the work presented here, a CW rapid-scan spectroscopic technique that avoids these compromises and also provides a means of avoiding 1/ f noise is developed. This technique, termed non-adiabatic rapid sweep (NARS) EPR, consists of repetitively sweeping the polarizing magnetic field in a linear manner over a spectral fragment with a small coil at a repetition rate that is sufficiently high that receiver noise, microwave phase noise, and environmental microphonics, each of which has 1/ f characteristics, are overcome. Nevertheless, the rate of sweep is sufficiently slow that adiabatic responses are avoided and the spin system is always close to thermal equilibrium. The repetitively acquired spectra from the spectral fragment are averaged. Under these conditions, undistorted pure absorption spectra are obtained without broadening or loss of signal intensity. A digital filter such as a moving average is applied to remove high frequency noise, which is approximately equivalent in bandwidth to use of an integrating time constant in conventional field modulation with lock-in detection. Nitroxide spectra at L- and X-band are presented.

Atomic absorption background of Ba in EXAFS analysis of BaFe12O19nanoparticles

The approximate barium X-ray atomic absorption in the energy region of L-edges is reconstructed from the absorption spectrum of an aqueous solution of BaCl(2). The result is corroborated by comparison with pure atomic absorption spectra of neighbour elements Xe and Cs. The application of the atomic absorption signal as a proper EXAFS background is demonstrated and discussed in the analysis of Ba hexaferrite nanoparticles with a very weak structural signal. The essential gain is found in the decrease of uncertainty intervals of structural parameters and their correlations. A simple analytical model of the absorption background for the practical EXAFS analysis is demonstrated.

W-band frequency-swept EPR

This paper describes a novel experiment on nitroxide radical spin labels using a multiarm EPR W-band bridge with a loop-gap resonator (LGR). We demonstrate EPR spectroscopy of spin labels by linear sweep of the microwave frequency across the spectrum. The high bandwidth of the LGR, about 1 GHz between 3 dB points of the microwave resonance, makes this new experiment possible. A frequency-tunable yttrium iron garnet (YIG) oscillator provides sweep rates as high as 1.8 × 10 5 GHz/s, which corresponds to 6.3 kT/s in magnetic field-sweep units over a 44 MHz range. Two experimental domains were identified. In the first, linear frequency sweep rates were relatively slow, and pure absorption and pure dispersion spectra were obtained. This appears to be a practical mode of operation at the present level of technological development. The main advantage is the elimination of sinusoidal magnetic field modulation. In the second mode, the frequency is swept rapidly across a portion of the spectrum, and then the frequency sweep is stopped for a readout period; FID signals from a swept line oscillate at a frequency that is the difference between the spectral position of the line in frequency units and the readout position. If there is more than one line, oscillations are superimposed. The sweep rates using the YIG oscillator were too slow, and the portion of the spectrum too narrow to achieve the full EPR equivalent of Fourier transform (FT) NMR. The paper discusses technical advances required to reach this goal. The hypothesis that trapezoidal frequency sweep is an enabling technology for FT EPR is supported by this study.