A Raman microprobe study of electrical treeing in polyethylene

The chemical nature of electrical tree growth in a blend of high and low density polyethylene has been studied by confocal Raman microprobe spectroscopy. The observed spectra, which are easily perturbed by the Raman probe beam, can be described in terms of three components, the relative intensities of which vary from place-to-place on the sample. Throughout the body of the tree, the usual Raman bands of polyethylene are seen, but superimposed upon a pronounced fluorescent background. This suggests a degree of material degradation throughout the structure, even where there is no visible evidence of electrical discharge damage or ageing. The individual channels that make up the fractal structure of the tree can then be divided into two distinct categories. Within the core of the tree, their Raman spectra are made up of two elements; fluorescence, plus the G and D bands of sp² hybridized carbon. Here, the tree channels are best thought of as hollow tubules surrounded by a carbonaceous shell. At the tips, the channels are characterized simply by the Raman bands of polyethylene superimposed upon a reduced fluorescent background, irrespective of their size. That is, in this region, the tree channels are simply hollow tubules within the dielectric. The transition between these two structures occurs relatively abruptly. These observations are related to the treeing process.     

A Raman microprobe study of a magnesia partially stabilized zirconia fracture surface

Raman microprobe analysis has been used to determine the extent of phase transformation on the fracture surface of a magnesia partially stabilized zirconia test bar. This has been found to vary across the fracture face and is considered to be related to the amount of microcracking which occurs in the sample during fracture.     

Capillary gel affinity electrophoresis of DNA fragments.

The incorporation of an affinity ligand within a polyacrylamide gel provides a general means of manipulating the selectivity of capillary gel electrophoresis separations. As an example of this approach, high resolution of DNA restriction fragments by capillary gel affinity electrophoresis has been achieved by adding a soluble intercalating agent, ethidium bromide, to the gel-buffer system. A migration model has been developed that can be used for selectivity optimization. Various parameters, such as ligand concentration and applied electric field, have been examined in terms of their influence on retention and selectivity of different-size DNA molecules. From this study, high-resolution separations have been developed with efficiencies as high as 10(7) theoretical plates per meter.     

Raman microprobe: a diagnostic tool for processed silicon. Analysis of microindented silicon

Laser-assisted processes are currently used in silicon technology. The response of the material to the laser beam depends strongly on its own physical properties and on the laser power density. The use of a microRaman system, allows the structural characteristics of the material to be analysed by varying the excitation laser power density on the sample over a large power range with a submicrometre lateral resolution. Results are reported on microindented crystalline silicon, showing that changes in the physical properties of the material, introducing grain boundaries, dislocations and cracking, result in a strong modification of the Raman spectrum. These spectral changes are enhanced for increasing laser power densities. Several mechanisms are pointed out as possible sources of the observed spectral modifications. These results show that Raman microprobe is a very promising technique for the diagnosis of technologically processed semiconductors and devices.     

Separation of DNA restriction fragments by capillary electrophoresis using coated fused silica capillaries.

The abilities of several different capillary electrophoresis techniques to separate DNA restriction fragments up to 23,000 bp were investigated. Methods employing electroosmotic flow in an untreated silica capillary were found to provide, at best, only partial resolution of the 23 fragments in a 1-kbp DNA ladder. By coating the inner walls of a silica capillary with poly(acrylamide) and filling these capillaries with buffers containing methylcellulose as a sleving medium, all fragments in the 1-kbp DNA ladder were separated. In addition, this technique facilitated the separation of the very large fragments in a lambda DNA-HindIII digest. Optimum resolution was obtained at low separation potentials using buffers containing at least 0.5% methylcellulose. The performance of this technique, i.e., resolution and quantitation, make capillary electrophoresis a powerful complement to slab gel electrophoresis and may make it a preferred alternative to both agarose gel electrophoresis and HPLC for applications such as the confirmation of plasmid integrity.     

Lamellar structure in heat-treated polyethylene. A Raman spectroscopic study

Measurement of the low frequency Raman spectrum of polyethylene affords a method of following the effects of sample deformation on lamellar structure and of the process of annealing on lamellar thickness.     

Raman spectroscopy: A technique for estimating extent of polymerization in PMMA

This work shows how laser-Raman spectroscopy can be applied in cases where a combination of an efficient, relatively inexpensive, simple, and non- destructive technique is desirable for the determination of the monomer content in poly(methylmethacrylate), PMMA. The method has been initially applied in order to investigate the optimum conditions for the in situ polymerization of PMMA in the pores of silica sol-gel for the purpose building a solid state dye laser. The calibration line which converts the ratio of two Raman intensities in the PMMA spectrum (at 1640 and 1725 cm^–1) into percentage of monomer content, under the current experimental conditions, has been achieved on the basis of NMR spectroscopy. In addition, it is shown in this work how Raman spectroscopy can allow for the estimation of the reaction enthalpies of partial monomer to polymer conversion after thermal treatment at different temperatures and periods of treatment. Received: 25 August 2000 / Reviewed and accepted: 28 August 2000     

Electrically assisted sampling across membranes with electrophoresis in nanometer inner diameter capillaries

A nondestructive method for sampling from ultrasmall environments has been developed utilizing electrophoresis in nanometer inner diameter capillaries and etched electrochemical detection. The desire to study increasingly smaller biological environments such as mammalian cells has led to the need for capillary electrophoresis techniques with subpicoliter volume sampling capabilities. This sampling technique involves the fabrication of a microinjector at the tip of a 770-nm-inner diameter capillary and the use of electroporation for insertion through the membrane. Separations of catecholamines sampled from the interior of intact liposomes have been achieved. A separation of a cytoplasmic sample taken from an intact mammalian cell has also been obtained.     

Nonisothermal polymerization in a spherical reactor. Temperature distribution and reaction kinetics

The possibility of solving the converse thermal conductivity problem beginning with temperature measurements during the process of nonisothermal polymerization is demonstrated.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 4, pp. 636–641, April, 1978.     

Raman study of the sol to gel transformation under normal and high pressure

The changes in the Raman spectra of the silica sol-gels have been examined as a function of time. The increase of pressure from 1 bar to 3.5 kbar decreased the gelation time from 168 h down to 6 h. The bands associated with the SiO₂ vibrations grow continuously with time indicating the continuous growth of network particles even after the gel had been formed.     

Raman microprobe analysis of elastic strain and fracture in electrophoretically deposited CdSe nanocrystal films

The mechanical stability of nanocrystal films is critical for applications, yet largely unexplored. Raman microprobe analysis used here to probe the nanocrystal cores of thick, fractured electrophoretically deposited films of 3.2 nm diameter CdSe nanocrystals measures approximately 2.5% in-plane tensile strain in cores of unfractured films. The crack dimensions determine the overall in-plane film strain, approximately 11.7%, and the film biaxial modulus, approximately 13.8 GPa, from which the biaxial modulus of the trioctylphosphine oxide ligand matrix is inferred, approximately 5.1 GPa.     

Interactive control of pulsed field gel electrophoresis via real time monitoring.

The migration of DNA fragment bands through a slab gel can be monitored by UV absorption at 254 nm and imaged by a charge-coupled device (CCD) camera. Background correction and immediate viewing of band positions to interactively change the field program in pulsed-field gel electrophoresis is possible throughout the run via this detection scheme. The use of absorption removes the need for staining or radioisotope labelling, thereby simplifying sample preparation and reducing hazardous waste generation. This leaves the DNA in its native state and further analysis can be performed without destaining. The optimization of buffer concentration, electric field strength, temperature, agarose concentration, as well as pulse duration can considerably reduce total run time. For example, DNA from 2 to 850 kb can be separated in 3 h on a 7-cm gel with interactive control of the pulse time, which is 10 times faster than using a constant field program.     

Surface-confined living radical polymerization for coatings in capillary electrophoresis

Surface-confined living radical polymerization is shown to be a controlled means of covalently bonding both linear and cross-linked polymer films on silica. CuCl/bipyridine initiates radical formation through atom transfer with a self-assembled monolayer of benzyl chloride, onto which polymer then grows. The polymerization is intrinsically confined to the surface, avoiding problems associated with polymer formed in the solution. The surface-confined polymerization scheme is generally applicable to radical polymerization of vinyl monomers and was studied here for the case of acrylamide. Infrared spectroscopy shows that the film growth is controllable, and atomic force microscopy reveals that smooth films are prepared. The surface-confinement polymerization scheme was tested for both linear and cross-linked polyacrylamide. Capillary electrophoresis of strongly basic proteins confirms that the coated capillaries provide the high efficiency expected for polyacrylamide. The cross-linked coating exhibits higher reproducibility with respect to migration time than does the linear coating. Surface-confined living radical polymerization prepares linear and cross-linked polymer films without danger of clogging narrow capillaries and will ultimately facilitate cross-laboratory comparisons by enabling control of film thickness.     

Capillary electrophoresis and contactless conductivity detection of ions in narrow inner diameter capillaries

Capillary electrophoresis and conductometry represent a combination of a high-resolution separation method with a sensitive detection principle for the analysis of ionic species. In this paper, results are reported that are obtained with a contactless conductivity detector. This device works without a galvanic contact of the electrolyte and the electrodes. The conductivity sensor is based on two metal tubes that act as cylindrical capacitors. These electrodes are both placed around a fused-silica capillary with a detection gap of 1 mm left in between. When a high audio or low ultrasonic oscillation frequency between 40 and 100 kHz is applied to one of the electrodes, a signal is produced as soon as an analyte zone with a different conductivity compared to the background electrolyte passes the detection gap. An amplifier and rectifier is connected to the other electrode where the signal is further processed. Limits of detection for lithium and fluoride are 4 and 13 ppb, respectively, with a linear range over 4 orders of magnitude from 90 ppb up to more than 1000 ppm for both anions and cations. Furthermore, it is demonstrated that for species with lower equivalent conductivities, such as organic ions, indirect conductivity detection is a sensitive alternative to indirect optical detection methods. Limits of detection of 50 ppb and below are obtained for organic acids.     

Three DNA sequencing methods using capillary gel electrophoresis and laser-induced fluorescence.

Capillary gel electrophoresis is demonstrated for the four-spectral-channel sequencing technique of Smith, the two-spectral-channel sequencing technique of Prober, and the one-spectral-channel sequencing technique of Richardson and Tabor. Sequencing rates up to 1000 bases/h are obtained at electric field strengths of 465 V/cm. At lower electric field strengths, capillary electrophoresis produces useful data for fragments greater than 550 nucleotides in length with 2 times better resolution than slab gel electrophoresis. An on-column detector produces detection limits of 200 zmol (1 zmol = 10(-21) mol = 600 molecules) for the four-spectral-channel technique. A postcolumn detector, based on the sheath flow cuvette, produces detection limits of 20 and 2 zmol for the two- and one-spectral-channel techniques, respectively.     

Capillary electrophoresis in 2 and 5 microns diameter capillaries: application to cytoplasmic analysis

Capillary electrophoresis with electrochemical detection in 2- and 5-micron capillaries has been developed to study ultrasmall biological environments. Sample volumes as low as 270 fL have been injected into the electrophoresis capillary with subattomole detection limits for easily oxidized species. We have applied this method to the analysis of single cell cytoplasm. Sampling of the cytoplasm is accomplished by inserting one end of the electrophoresis capillary directly into a single nerve cell. The high-voltage end of the electrophoresis capillary has been etched with hydrofluoric acid to form a microinjector. This injection scheme represents an improvement over those previously used for similar applications. The excellent selectivity of this method is demonstrated for catecholamine and indolamine neurotransmitters and their metabolites found in the invertebrate system, the pond snail Planorbis corneus.     

Surface-enhanced Raman spectroscopy of chernozem humic acid and their fractions obtained by coupled size exclusion chromatography-polyacrylamide gel electrophoresis (SEC-PAGE)

A humic acid extracted from a chernozem soil was fractionated combining size exclusion chromatography and polyacrylamide electrophoresis (SEC-PAGE). Three fractions named A, B, and C+D, with different electrophoretic mobilities and molecular sizes (MS), were obtained and subsequently characterized by thermochemolysis and surface-enhanced Raman spectroscopy (SERS). The data confirmed that fraction A, with the higher MS, was more aliphatic than fractions B and C+D and, in turn, fractions with lower MS (B and C+D) denoted an enrichment in lignin residues. These structural features explain conformational changes when varying the pH in the humic fraction A and indicated that combination of the two techniques is a good approach for characterizing humic substances.     

Specific base recognition of oligodeoxynucleotides by capillary affinity gel electrophoresis using polyacrylamide-poly(9-vinyladenine) conjugated gel.

Poly(9-vinyladenine) was synthesized and utilized as an affinity macroligand entrapped within the gel matrix. Base-specific separation of oligodeoxynucleotides was achieved with high resolution and high speed by electrophoresis, using capillaries filled with conjugated polyacrylamide-poly(9-vinyladenine) gel. Oligothymidylic acids were selectively separated from the mixture of oligothymidylic and oligodeoxyadenylic acids by utilizing a specific hydrogen bonding between poly(9-vinyladenine) and oligothymidylic acids. Migration time and resolution of oligodeoxynucleotides were influenced by several parameters, such as the size of poly(9-vinyladenine), capillary temperature, and concentrations of poly(9-vinyladenine) and urea. Some guidelines are presented, based on the theoretical formulation of the effect of these parameters, in order to find optimum electrophoretic conditions. Analytical capillary affinity gel electrophoresis was developed for the selective and sensitive base recognition of oligodeoxynucleotides with efficiencies as high as several 10(6) plates/m by using a urea-gel capillary with poly(9-vinyladenine) and temperature-programming.