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161.
A chiral crown ether column with a pH 1.9 perchloric acid buffered aqueous mobile phase is used to separate amino acid enantiomers by high performance liquid chromatography. An inductively coupled plasma atomic emission spectrometer coupled to the chromatographic system is used as a detector by monitoring the carbon atomic emission line at 193.09 nm. Seven underivatized amino acids are separated and detected resulting in an average mass detection limit of 5 ng (2.5 ng carbon). The chiral crown ether column resolves compounds with a primary amino group near the chiral center by forming a complex between the crown ether and an ammonium ion moiety from the sample. The
-form amino acid always elutes faster than its antipode. The carbon emission detector provides nearly identical sensitivities and similar detection limits for any compounds with comparable mass percents of carbon. Quantification is performed on unknown ratios of amino acids using an internal standard without the need for a calibration curve. Summing the calculated amounts of
and
amino acid and comparing to the known mixture quantity results in an average error of 1.0% for the seven amino acids separated. 相似文献
162.
《Tetrahedron: Asymmetry》2005,16(21):3606-3613
Several arenes possessing a cyclopropyl substituent were subjected to enzymatic oxidation with toluene dioxygenase. The absolute configuration of metabolites was established by chemical means. 相似文献
163.
In response to a growing interest in the use of smaller, faster microchip (mu-chip) methods for the separation of proteins, advancements are proposed that employ the asymmetric squarylium dye Red-1c as a noncovalent label in mu-chip CE separations. This work compares on-column and precolumn labeling methods for the proteins BSA, beta-lactoglobulin B (beta-LB), and alpha-lactalbumin (alpha-LA). Nonequilibrium CE of equilibrium mixtures (NECEEM) represents an efficient method to determine equilibrium parameters associated with the formation of intermolecular complexes, such as those formed between the dye and proteins in this work, and it allows for the use of weak affinity probes in protein quantitation. In particular, nonequilibrium methods employing both mu-chip and conventional CE systems were implemented to determine association constants governing the formation of noncovalent complexes of the red luminescent squarylium dye Red-1c with BSA and beta-LB. By our mu-chip NECEEM method, the association constants K(assoc) for beta-LB and BSA complexes with Red-1c were found to be 3.53 x 10(3) and 1.65 x 10(5) M(-1), respectively, whereas association constants found by our conventional CE-LIF NECEEM method for these same protein-dye systems were some ten times higher. Despite discrepancies between the two methods, both confirmed the preferential interaction of Red-1c with BSA. In addition, the effect of protein concentration on measured association constant was assessed by conventional CE methods. Although a small decrease in K(assoc) was observed with the increase in protein concentration, our studies indicate that absolute protein concentration may affect the equilibrium determination less than the relative concentration of protein-to-dye. 相似文献
164.
165.
R. Douglas Chatham 《Rendiconti del Circolo Matematico di Palermo》2001,50(3):509-542
Résumé D'après D. E. Dobbs, Houston J. Math. 23 (1997), 1–11, nous disons que l'anneau (commutatif)A est un anneau-“going-down” siA/P est un domaine-“going-down” pour chaque idéal premier deA. Etant donné une extension,R⊆T, nous disons que (R, T) est une paire d'anneaux-“going-down” (respectivement, une paire “going-down”) siS est un anneau-“going-down” pour chaque anneau tels queR⊆S⊆T (resp., si “going-down” est satisfait par chaque extension d'anneauxA⊆B tels queR⊆A⊆B⊆T). On montre que siR est un anneau de la dimension 0 (au sens de Krull), alors (R, T) est une paire d'anneaux-“going-down” si et seulement sitr.deg.
R/(P∩R)
T/P≤1 pour chaque idéal premier minimalP deT. Des résultats partiels sont obtenus quandR n'est pas de dimension 0. En outre, si (R, T) est une paire d'anneaux-“going-down” tel queT ait un seul idéal premier minimal, alors (R, T) est une paire “going-down”. Des résultats dans l'esprit ci-dessus sont également obtenus pour quelques autres types de paires.
This paper is taken from the author's doctoral dissertation of May 2000, written under the direction of Professor David E. Dobbs of the University of Tennessee, Knoxville. 相似文献
This paper is taken from the author's doctoral dissertation of May 2000, written under the direction of Professor David E. Dobbs of the University of Tennessee, Knoxville. 相似文献
166.
《Comptes Rendus de l'Academie des Sciences Series IIC Chemistry #》1999,327(9):945-951
We present magnetotransport properties of a multiwalled carbon nanotube. We have developed a technique using electron beam lithography to connect a carbon nanotube with a small contact resistance. The temperature dependence of the nanotube is thermally activated down to 30 mK but with multiple energy gap. At low temperature, the magnetoconductance can be analysed in terms of quantum interferences. Both weak localisation and universal conductance fluctuations have been observed. The phase coherence length extracted from the weak localisation is consistent with the amplitude of the conductance fluctuations. 相似文献
167.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1991,47(11):1595-1605
The IR spectra of ZnF2·4H2O and its deuterated analogues are reported at ambient and liquid-nitrogen temperatures. The OH and OD stretching and bending vibrations of the water molecules are analysed in detail. The two types of water molecules give rise to different absorption peaks in the OH and OD stretching regions in samples that contain isotopically dilute HDO groups. The strongly hydrogen-bonded water molecules H2O(1) and H2O(4) show four broad OH and OD stretching modes at lower frequencies, while the weaker hydrogen-bonded ones H2O(2) and H2O(3) give rise to four narrow bands at higher frequencies. The νOD frequencies of isotopically dilute HDO groups correlate very well with the known R(H---F) and R(H---O) distances in the crystals and the assignment of these modes was done on this basis. It was also found that the ratio νOH/νOD decreases with decreasing values of R(H---O) or R(H---F) in ZnF2·4H2O. 相似文献
168.
Grey Ballard Christian Ikenmeyer J.M. Landsberg Nick Ryder 《Journal of Pure and Applied Algebra》2019,223(8):3205-3224
This is the second in a series of papers on rank decompositions of the matrix multiplication tensor. We present new rank 23 decompositions for the matrix multiplication tensor . All our decompositions have symmetry groups that include the standard cyclic permutation of factors but otherwise exhibit a range of behavior. One of them has 11 cubes as summands and admits an unexpected symmetry group of order 12.We establish basic information regarding symmetry groups of decompositions and outline two approaches for finding new rank decompositions of for larger n. 相似文献
169.
170.
Jennifer A. Rust George L. Donati Mirela T. Afonso Joaquim A. Nóbrega Bradley T. Jones 《Spectrochimica Acta Part B: Atomic Spectroscopy》2009
Electrothermal devices have been employed in analytical atomic spectrometry for more than four decades. Normally these resistively heated devices are used to provide an atomic cloud that is either probed by an external light source (atomic absorption and atomic fluorescence) or swept into a second excitation source such as the inductively coupled plasma (electrothermal vaporization). Less commonly, the electrothermal device both produces the atomic vapor and excites the emission of the atoms in the cloud. This brief review, with 65 selected references, will describe those occasions where electrothermal devices are indeed employed in this manner, with no other source of excitation energy applied. Beginning with a graphite furnace system described in 1975 and ending with a tungsten coil application published in 2008, analytical figures of merit will be reported for methods involving devices fabricated from carbon, molybdenum, and tungsten. The review ends with a discussion of the practical limitations associated with these techniques. 相似文献