Simultaneous determination of creatinine, creatine, and UV-absorbing amino acids using dual-mode gradient low-capacity cation-exchange chromatography

A simple and versatile cation-exchange chromatography technique for the simultaneous determination of urinary creatinine (Cre), creatine (Crn), methionine (Met), tyrosine (Tyr), phenylalanine (Phe), histidine (His), and tryptophan (Trp) was developed. A novel low-capacity cation-exchange column packed with a newly developed sulfoacylated hypercross-linked macroreticular polystyrene-divinylbenzene resin, referred to as TMR-A/75 (capacity: 75 microequiv/column), was successfully used with a binary dual-mode gradient eluting system. Two solvents, (A) 25 mM phosphoric acid-methanol (30:70, v/v) and (B) 25 mM disodium hydrogenphosphate-methanol (30:70, v/v) were pumped through the column by programming solvent delivery ratios as 0 to 5 min: A-B (55:45, pH 3.6); 5-21 min: A-B (49:51, pH 5.3); and 21-35 min: A-B (55:45, pH 3.6). The flow rate was simultaneously time-programmed to be 0.6 mL/min from 0 to 19 min and to be 1.0 mL/min from 19 to 35 min. This eluting system could permit the use of the UV detection at 210 nm. The analytes, Crn, Met, Tyr, His, Cre, Phe, and Trp, were well separated in this order in 27 min with minimum resolution of approximately 2, and the cycle time was about 35 min. Retention time of each analyte was very reproducible with relative standard deviations (RSDs) between 0.05 and 0.38% (n = 5). The peak area responses were also reproducible with RSDs between 0.74 and 2.24% (n = 5). Calibration lines based on area data were linear from 1 to 1000 microM with r2 values of 0.9998 (Crn), 0.9998 (Met), 0.9999 (Tyr), 0.9999 (His), 1.0000 (Cre), 1.0000 (Phe), and 0.9999 (Trp). The method was applicable to the screening and/or chemical diagnosis of inherited metabolic disorders such as phenylketonuria (PKU), tyrosinemia, and Lowe syndrome. The creatinine ratios of diagnostic markers (microM/microM Cre) were easily determined. The Phe/Cre ratios for five urines from patients with PKU ranged from 0.162 to 0.521, and the Tyr/Cre ratio for tyrosinemia was 0.147. The ratios of Tyr/Cre, Phe/Cre, and Trp/Cre for Lowe syndrome were 0.497, 0.321, and 0.495, respectively. In contrast, the creatinine ratios for healthy newborns showed one digit lower than those for patients did. The developed method is very practical and can provide useful information and results for the clinical or biomedical researches with low analytical run costs.     

Cascaded z-filters for efficient single-scan suppression of zero-quantum coherence

A simple and robust method to suppress zero-quantum coherence (ZQC) in NMR experiments, in a single scan and with very high suppression ratio, is described. It is an appreciable improvement on a previous technique by Thrippleton and Keeler [Angew. Chem. Int. Ed. 42 (2003) 3938]. The method, called a z-filter cascade, preserves longitudinal, or z-magnetization, with high efficiency. Losses depend mostly on T1 relaxation but not T2 relaxation mechanisms. At the same time, suppression of ZQC can be essentially complete in a single scan. The time duration of the z-filter cascade scales inversely to representative chemical shift differences between the coupled spins, and is typically a few tens of milliseconds. The high efficiency of the zero-quantum suppression and excellent retention of the desired z-magnetization, in a single scan without resort to phase cycling or difference spectroscopy, makes the z-filter cascade a useful new pulse sequence building block for a whole range of NMR experiments. In cases where unwanted residual ZQC may have previously contributed to baseline " t1-noise" in two-dimensional NMR spectra, the z-filter cascade can deliver a noteworthy improvement in spectral quality.     

Analysis of the magic asymmetric gradient stimulated echo sequence with shaped gradients

Recently a 13-interval magic asymmetrical gradient stimulated echo (MAGSTE) sequence has been proposed for accurate displacement measurements in the presence of spatially varying background gradients. In this paper, the commonly used trapezoidal and sine shaped gradients are studied for the MAGSTE sequence, and the magic asymmetrical gradient ratio and b-factor are provided. The derivation enables the MAGSTE sequence to be implemented on systems with non-negligible gradient rise times.     

59Co, 23Na NMR and electric field gradient calculations in the layered cobalt oxides NaCoO2 and HCoO2

⁵⁹Co and ²³Na NMR has been applied to the layered cobalt oxides NaCoO₂ and HCoO₂ at three different magnetic field strengths (4.7, 7.1 and 11.7 T). The ⁵⁹Co and ²³Na quadrupole and anisotropic shift tensors have been determined by iterative fitting of the NMR line shapes at the three magnetic field strengths. Due to the large ⁵⁹Co quadrupole interaction in NaCoO₂, a frequency-swept irradiation procedure was used to alleviate the limited bandwidth of the excitation. While the ⁵⁹Co and ²³Na shift and quadrupole coupling tensors in NaCoO₂ are found to be coincident and axially symmetric in agreement with the crystal symmetry requirements, the fits of the ⁵⁹Co NMR spectra clearly show the presence of structural disorder in HCoO₂. The ²³Na chemical shift anisotropy can be reproduced by shift tensor calculations using a point dipole model and considering that the magnetic susceptibility in NaCoO₂ is due to Van Vleck paramagnetism for Co³⁺. Electric field gradient calculations using either the empirical point charge model or the ab initio full potential-linearized augmented plane wave method are compared with the experimental NMR data.     

A theoretical analysis on efficiency of some Newton-PCG methods

In this paper, we study the efficiency issue of inexact Newton-type methods for smooth unconstrained optimization problems under standard assumptions from theoretical point of view by discussing a concrete Newton-PCG algorithm. In order to compare the algorithm with Newton's method, a ratio between the measures of their approximate efficiencies is investigated. Under mild conditions, it is shown that first, this ratio is larger than 1, which implies that the Newton-PCG algorithm is more efficient than Newton's method, and second, this ratio increases when the dimension n of the problem increases and tends to infinity at least at a rate In n/In 2 when n→∞, which implies that in theory the Newton-PCG algorithm is much more efficient for middle- and large-scale problems. These theoretical results are also supported by our preliminary numerical experiments.     

Alternating oblique projections for coupled linear systems

In this work we propose the use of alternating oblique projections (AOP) for the solution of the saddle points systems resulting from the discretization of domain decomposition problems. These systems are called coupled linear systems. The AOP method is a descent method in which the descent direction is defined by using alternating oblique projections onto the search subspaces. We prove that this method is a preconditioned simple gradient (Uzawa) method with a particular preconditioner. Finally, a preconditioned conjugate gradient based version of AOP is proposed. AMS subject classification 65F10, 65N22, 65Y05     

Linear Systems Arising for Second-Order Mimetic Divergence and Gradient Discretizations

Recent investigations by Castillo and Grone have led to a new method for constructing mimetic discretizations of divergence and gradient operators. Their technique, which employs a matrix formulation to incorporate mimetic constraints, is capable of producing approximations whose order at a grid boundary is equal to that in the grids interior. In this paper, we construct a second-order mimetic discretization using Castillo and Grones approach and compare it to other second-order discretizations by applying them to an elliptic boundary value problem in one dimension. A detailed perturbation analysis is provided to offer some insight into the two discretizations yielding the best numerical results in the study.Mathematics Subject Classifications (2000) 65D25, 65M06, 65G99.This paper is an expanded version of the article [2] presented at the International Conference on Parallel and Distributed Processing Techniques and Applications held in Las Vegas, Nevada in June of 2003.     

Fractional Generalization of Gradient Systems

We consider a fractional generalization of gradient systems. We use differential forms and exterior derivatives of fractional orders. Examples of fractional gradient systems are considered. We describe the stationary states of these systems.Mathematics Subject Classification (2000). 426A33, 70G60     

Gradient-enhanced FAWSETS perfusion measurements

This work describes the use of custom-built gradients to enhance skeletal muscle perfusion measurements acquired with a previously described arterial spin labeling technique known as FAWSETS (flow-driven arterial water stimulation with elimination of tissue signal). Custom-built gradients provide active control of the static magnetic field gradient on which FAWSETS relies for labeling. This allows selective, 180 degrees modulations of the phase of the perfusion component of the signal. Phase cycling can then be implemented to eliminate all extraneous components leaving a signal that exclusively reflects capillary-level perfusion. Gradient-enhancement substantially reduces acquisition time and eliminates the need to acquire an ischemic signal to quantify perfusion. This removes critical obstacles to application of FAWSETS in organs other than skeletal muscle and makes the measurements more desirable for clinical environments. The basic physical principles of gradient-enhancement are demonstrated in flow phantom experiments and in vivo utility is demonstrated in rat hind limb during stimulated exercise.     

A modified PGSE for measuring diffusion in the presence of static magnetic field gradients

With a proper timing of pi pulses, it is possible to reduce the effect of the static internal magnetic field gradient on the measurement of diffusion with the pulsed gradient spin echo (PGSE). A pulse sequence that in the first order eliminates the effect of weak internal static gradients in a standard PGSE experiment is introduced. The method should be applied in the cases, where strong and short magnetic gradient pulses are used to investigate the motion of liquid in heterogeneous samples with large susceptibility differences such as porous media.