Hyphenation of gas chromatography to microcoil 1H nuclear magnetic resonance spectroscopy

Whereas the hyphenation of gas chromatography (GC) with mass spectrometry is of great importance, little is known about the coupling to nuclear magnetic resonance spectroscopy (NMR). The investigation of this technique is an attractive proposition because of the valuable information given by NMR on molecular structure. The experiments shown here are to our knowledge the first hyphenating capillary GC to microcoil NMR. In contrast to liquids, gases have rarely been investigated by NMR, mainly due to the experimental difficulties in handling gases and the low signal-to-noise-ratio (SNR) of the NMR signal obtained at atmospheric pressure. With advances in NMR sensitivity (higher magnetic fields and solenoidal microprobes), this limitation can be largely overcome. In this paper, we describe the use of a custom-built solenoidal NMR microprobe with an active volume of 2 microL for the NMR detection of several compounds at 400 MHz, first in a mixture, and then with full coupling to capillary GC to identify them separately. The injected amounts of each analyte in the hyphenated experiments are in the range of 15-50 micromol, resulting in reasonable SNR for sample masses of 1-2 microg.     

Contribution of 1H combined rotation and multipulse spectroscopy nuclear magnetic resonance to the study of tricalcium silicate hydration

Hydration products of tricalcium silicate (C₃S) are the calcium silicate hydrates (C–S–H) and portlandite. Silica fume, added to anhydrous cement in industrial formulations, reacts with portlandite and leads to C–S–H slightly different from the previous one (this reaction is called the pozzolanic reaction). C₃S hydration at 120 °C with and without silica fume has been studied by two ¹H nuclear magnetic resonance techniques: combined rotation and multi-pulse spectroscopy (CRAMPS) and magic angle spinning (MAS). The static spectra are broadened by the proton–proton dipolar interaction. The ¹H MAS technique does not allow us to remove the effects of this interaction completely and cannot be in this case a quantitative method. Therefore the CRAMPS technique, which can remove the broadening of the interaction because of the use of a multipulse sequence associated with the sample rotation, was used. It is shown that the CRAMPS NMR spectra allow us to describe the action of silica fume on the hydrates and to reveal the competition between portlandite formation around the C₃S grains and portlandite dissolution near the silica grains until the complete dissolution of portlandite. This revised version was published online in November 2006 with corrections to the Cover Date.     

Selective ion filtering by digital thresholding: a method to unwind complex ESI-mass spectra and eliminate signals from low molecular weight chemical noise

In this work, we present a simple method by which to preferentially detect either high molecular weight or low molecular weight ions generated by electrospray ionization. This approach, termed selective ion filtering by digital thresholding (SIFdT) is demonstrated on a commercial ESI-TOF instrument that employs a fast digitizer coupled to a microchannel plate detector. The digital representation of each individual scan is digitally filtered prior to spectral coaddition. As larger, more highly charged ions induce a more intense response than low molecular weight singly charged species, a digital threshold can be set that precludes the detection of singly charged species yet permits the efficient detection of larger, more highly charged species. In this work, we demonstrate the applicability of this approach to eliminate low molecular weight chemical noise in ESI-TOF spectra of oligonucleotide and protein ions, demonstrate improved dynamic range for analyte solutions containing high levels of low MW constituents, and show that spectra acquired at different digital thresholds can be subtracted to yield spectra of low molecular weight constituents with improved mass measurement accuracies. A notional scheme is presented in which an alternative digitization approach is employed using multiple differentially thresholded data streams to allow improved internal mass calibration and higher resolution ion partitioning.     

Online coupling of gas chromatography to nuclear magnetic resonance spectroscopy: method for the analysis of volatile stereoisomers

The identification of volatile cis/trans-stereoisomers was accomplished by employing a hyphenated GC-NMR system. The chromatographic and spectroscopic conditions were optimized with respect to the (1)H NMR detection. A special processing technique was developed to handle the recorded NMR spectra in the gas phase with very low sample amounts. The processed stopped-flow (1)H NMR spectra of the investigated chromatographic peaks unequivocally revealed the structure of the corresponding compounds.     

Development of 2D band-target entropy minimization and application to the deconvolution of multicomponent 2D nuclear magnetic resonance spectra

Spectral reconstruction from multicomponent spectroscopic data is the frequent primary goal in chemical system identification and exploratory chemometric studies. Various methods and techniques have been reported in the literature. However, few algorithms/methods have been devised for spectral recovery without the use of any a priori information. In the present studies, a higher dimensional entropy minimization method based on the BTEM algorithm (Widjaja, E.; Li, C.; Garland, M. Organometallics 2002, 21, 1991-1997.) and related techniques were extended to large-scale arrays, namely, 2D NMR spectroscopy. The performance of this novel method had been successfully verified on various real experimental mixture spectra from a series of randomized 2D NMR mixtures (COSY NMR and HSQC NMR). With the new algorithm and raw multicomponent NMR alone, it was possible to reconstruct the pure spectroscopic patterns and calculate the relative concentration of each species without recourse to any libraries or any other a priori information. The potential advantages of this novel algorithm and its implications for general chemical system identification of unknown mixtures are discussed.     

Probabilistic quotient normalization as robust method to account for dilution of complex biological mixtures. Application in 1H NMR metabonomics

For the analysis of the spectra of complex biofluids, preprocessing methods play a crucial role in rendering the subsequent data analyses more robust and accurate. Normalization is a preprocessing method, which accounts for different dilutions of samples by scaling the spectra to the same virtual overall concentration. In the field of 1H NMR metabonomics integral normalization, which scales spectra to the same total integral, is the de facto standard. In this work, it is shown that integral normalization is a suboptimal method for normalizing spectra from metabonomic studies. Especially strong metabonomic changes, evident as massive amounts of single metabolites in samples, significantly hamper the integral normalization resulting in incorrectly scaled spectra. The probabilistic quotient normalization is introduced in this work. This method is based on the calculation of a most probable dilution factor by looking at the distribution of the quotients of the amplitudes of a test spectrum by those of a reference spectrum. Simulated spectra, spectra of urine samples from a metabonomic study with cyclosporin-A as the active compound, and spectra of more than 4000 samples of control animals demonstrate that the probabilistic quotient normalization is by far more robust and more accurate than the widespread integral normalization and vector length normalization.     

1H nuclear magnetic resonance characterization of Portland cement: molecular diffusion of water studied by spin relaxation and relaxation time-weighted imaging

Water molecular dynamics in a hardened Portland cement were characterized by proton Fourier transform nuclear magnetic resonance (NMR) at 400 MHz. Three different types of water molecule (physically bound, chemically bound and porous trapped) were observed. When the hardened cement sample was heated at 105 °C, the physically bound water diffused out of the sample as a function of the heating time while the chemically bound water remained in a stable form. A trace amount of the porously trapped water was also detected to remain in the cavities of the hardened cement even after heating for up to 20 h at this temperature. The loss of the physically bound water proved to be a diffusion-controlled process as evidenced from the NMR data and from a gravimetric technique. A Pake doublet was observed in the NMR spectra. This is a result of the oscillation of the water molecules with hindered molecular motions due to their entrapment in the cement pores. Soaking the dried samples in water resulted in the diffusion of water back into the hardened cement as physically bound water. Nuclear spin–spin relaxation time, T2-weighted imaging showed that the distribution of the physically bound water inside the cylindrical sample formed a doughnut shape after overnight soaking. The residual air in the cement pores may have slowed down the diffusion rate of the water molecules back into the dried cement. © 1998 Kluwer Academic Publishers     

Solid state spatially resolved 1H and 19F nuclear magnetic resonance spectroscopy of dental materials by stray-field imaging

As part of a program to evaluate the use of stray-field magnetic resonance microimaging (STRAFI) in dental materials research spatially resolved nuclear magnetic resonance (NMR) for solid dental cements has been investigated. By applying a quadrature echo pulse sequence to a specimen positioned in the stray-field of a NMR spectrometer superconducting magnet the magnetic resonance within a thin slice was obtained. The specimen was stepped through the field in 500 m increments to record ¹ and ¹⁹F profiles and T₂ values at each point. The specimens were fully cured cylinders made from four types of restorative material (glass ionomer, resin modified glass ionomer, compomer, composite). The values for F¹⁹T₂ varied with material type and reflected the nature of the matrix structure. For all materials containing ¹⁹F in the glass two values were calculated for ¹⁹F T₂, one short and one long. These were relatively invariant. Solid state magic angle spinning (MAS)-NMR showed that they came from the glass. This suggests that a proportion of the element is relatively mobile (in a glass phase) and the remainder is more tightly bound (in a compound dispersed in the glass). This demonstration, that NMR microimaging of both ¹H and ¹⁹F in solid dental cements is possible, opens up exciting new possibilities for investigating the distribution of these elements (in particular fluorine) in solid dental materials. ©©1999©Kluwer Academic Publishers     

A rapid signal processing technique to remove the effect of dispersion from guided wave signals

Guided acoustic and ultrasonic waves have been utilized in various manners for non-destructive evaluation and testing. If a guided wave mode is dispersive, a pulse of energy will spread out in space and time as it propagates. For a long-range guided wave inspection application, this constrains the choice of operating point to regions on the dispersion curves where dispersion effects are small. A signal processing technique is presented that enables this constraint on operating point to be relaxed. The technique makes use of a priori knowledge of the dispersion characteristics of a guided wave mode to map signals from the time to distance domains. In the mapping process, dispersed signals are compressed to their original shape. The theoretical basis of the technique is described and an efficient numerical implementation is presented. The robustness of the technique to inaccuracies in the dispersion data is also addressed. The application of the technique to experimental data is shown and the resulting improvement in spatial resolution is demonstrated. The implications of using dispersion compensation in practical systems are briefly discussed.     

Curve-fitting method for direct quantitation of compounds in complex biological mixtures using 1H NMR: application in metabonomic toxicology studies

A new software tool has been developed that provides automated measurement of signal intensities in NMR spectra of complex mixtures without using data reduction procedures. The algorithm finds best-fit transformations between signals in reference compound spectra and the corresponding signals in analyte spectra. Unlike other algorithms, it is insensitive to variation in chemical shift and can even be used for relative quantitation of compounds whose identities have not yet been established. Additionally, the parameters of the transformation provide information and error metrics that may assist in the streamlining of quality control. The approach presented is general in scope but has been tested by application to peak quantitation in NMR spectra of biofluids. Replicate NMR measurements of solutions of biologically important compounds at various concentrations were made. Further NMR data were collected on urine samples from human, rat, and mouse, which were "spiked" with reference compound solutions at known concentrations. Finally, existing data from an independent toxicology project involving several hundred samples were analyzed, and the consistency of the measurements for metabolites that give multiple NMR signals was assessed. The results of all these tests give confidence that the technique can be used in automated quantitation of compounds in large NMR data sets with minimal operator intervention.     

Wavelength shift analysis: a simple method to determine the contribution of hemoglobin and myoglobin to in vivo optical spectra

The ability to quantify the contributions of hemoglobin (Hb) and myoglobin (Mb) to in vivo optical spectra has many applications for clinical and research use such as noninvasive measurement of local tissue O(2) uptake rates and regional blood content. Recent work has demonstrated an approach to independently measure oxygen saturations of Hb and Mb in optical spectra collected in vivo. However, the utility of this approach is limited without information on tissue concentrations of these species. Here we describe a strategy to quantify the contributions of Hb and Mb to in vivo optical spectra. We have found that the peak position of the deoxy-heme peak around 760 nm in the optical spectra of the deoxygenated tissue is a linear function of the relative contributions of Hb and Mb to the optical spectra. Therefore, analysis of this peak position, hereafter referred to as wavelength shift analysis, reveals the relative concentration of Hb to Mb in solutions and intact tissue. Biochemical analysis of muscle homogenates confirmed that the wavelength shift of the combined Hb/Mb peak in in vivo spectra reflects the ratio of concentrations (Hb/Mb) in muscle. The importance of quantifying the Hb contribution is illustrated by our data demonstrating that Hb accounts for approximately 80% of the optical signal in mouse skeletal muscle but only approximately 20% in human skeletal muscle. This advance will facilitate comparison of the metabolic properties between individual muscles and provides a fully noninvasive approach to measuring local respiration that can be adapted for clinical use.     

1H MAS NMR (magic-angle spinning nuclear magnetic resonance) techniques for the quantitative determination of hydrogen types in solid catalysts and supports

Distinct hydrogen species are present in important inorganic solids such as zeolites, silicoaluminophosphates (SAPOs), mesoporous materials, amorphous silicas, and aluminas. These H species include hydrogens associated with acidic sites such as Al(OH)Si, non-framework aluminum sites, silanols, and surface functionalities. Direct and quantitative methodology to identify, measure, and monitor these hydrogen species are key to monitoring catalyst activity, optimizing synthesis conditions, tracking post-synthesis structural modifications, and in the preparation of novel catalytic materials. Many workers have developed several techniques to address these issues, including 1H MAS NMR (magic-angle spinning nuclear magnetic resonance). 1H MAS NMR offers many potential advantages over other techniques, but care is needed in recognizing experimental limitations and developing sample handling and NMR methodology to obtain quantitatively reliable data. A simplified approach is described that permits vacuum dehydration of multiple samples simultaneously and directly in the MAS rotor without the need for epoxy, flame sealing, or extensive glovebox use. We have found that careful optimization of important NMR conditions, such as magnetic field homogeneity and magic angle setting are necessary to acquire quantitative, high-resolution spectra that accurately measure the concentrations of the different hydrogen species present. Details of this 1H MAS NMR methodology with representative applications to zeolites, SAPOs, M41S, and silicas as a function of synthesis conditions and post-synthesis treatments (i.e., steaming, thermal dehydroxylation, and functionalization) are presented.     

Application of a nuclear quadrupole resonance method to measure stress in a matrix-tracer system in composites

A quantitative correlation has been established between the shift of the nuclear quadrupole resonance frequency of the tracer and the stress in a matrix-tracer system in composites. Measurements are made of the ⁶³Cu resonance frequencies in samples of epoxy resin containing cuprous oxide powder. The stresses in the resin and their dependences on temperature and external pressure are determined. Pis’ma Zh. Tekh. Fiz. 23, 14–18 (April 12, 1997)     

Comment on "Scheimpflug and high-resolution magnetic resonance imaging of the anterior segment: a comparative study"

In a recent paper, Koretz et al. [J. Opt. Soc. Am. A 21, 346 (2004)] conclude that the results of the "corrected Scheimpflug technique" to obtain the shape of the human lens differ significantly from their magnetic resonance imaging (MRI) data. We demonstrate that the conclusions are based on incorrect statistical methods. A straightforward statistical comparison shows that there is no significant difference between the corrected Scheimpflug results and the MRI data.     

Morphology of the poly(vinyl alcohol)–poly(vinyl acetate) copolymer in macrodefect-free composites: a 13C magic-angle-spinning nuclear magnetic resonance and 1H spin-diffusion study

A study was made of the macrodefect-free (MDF) composite based on aluminate cement and a poly(vinyl alcohol)–poly(vinyl acetate) (PVAc) copolymer by ¹³C cross-polarization magic-angle-spinning nuclear magnetic resonance. The spectra were run on both the copolymer and the MDF composite in order to observe the atomic environments of the carbon nuclei. An analysis of the intramolecular hydrogen bonds showed a stronger modification in the sample containing CaAl₂O₄ (CA) than in that containing Ca₃Al₂O₆ (C₃A). From the spectra, it was seen that deacetylation had occurred, and that there was interaction between acetate groups and the calcium and aluminium ions. Proton relaxation times both in the laboratory frame, T ₁(H), and in the rotating frame, T1ρ(H), were exploited for the study of the dimension of the polymer matrix in MDF composites. An extended interphase was identified in the composite containing CA. A comparison with the behaviour of the composite based on silicates Ca₃SiO₅–PVAc highlights the better mixing of the phases in the MDF composites containing CA. This revised version was published online in November 2006 with corrections to the Cover Date.     

Shift-excitation Raman difference spectroscopy-difference deconvolution method for the luminescence background rejection from Raman spectra of solid samples

The feasibility of the shift-excitation Raman difference spectroscopy-difference deconvolution (SERDS-DDM) method for fluorescence suppression from Raman spectra of solid samples is discussed. For SERDS measurements a tunable diode laser source with an emission band centered at 684 nm is coupled to a conventional micro-Raman apparatus and a monochromator device is used for checking the excitation frequency stability. The shifted Raman spectra are then mathematically treated and a deconvolution procedure is used to reconstruct the Raman spectrum devoid of fluorescence. Two different cases are presented. In the first one, fluorescence is intrinsic to the sample and the Raman spectrum of cinnabar pigment is finally reconstructed. In the second, the presence of an external luminescence background in the spectrum of a pure sulfur crystal is considered. The SERDS-DDM reconstructed spectra are compared with spectra obtained via multi-point baseline subtraction and a significant improvement in the detection of weak bands is demonstrated. Practical insights for the application of this method are presented as well.