FESEM imaging at low voltage The S-4500 scanning electron microscope, which uses a cold field emission electron source, is designed for high-resolution imaging at low accelerating voltages. The image resolution of < 4.5 nm at 1 kV allows the microscope to be used for studies of beam-sensitive or nonconducting materials such as glasses, polymers, ceramics, electron beam resists, or thin films without the need to coat specimens. The microscope operates in the accelerating voltage range of 0.530 kV and magnifies objects in the range of 20-500,000 x. At 15 kV, resolution is < 1.5 nm. To enhance resolution at 1 kV, the microscope incorporates dual second-electron (SE) detectors. In addition to the standard SE detector placed in the specimen-scanning chamber, a second "upper" detector sits in the lens system. The ability to image at low voltage without coating specimens means that products such as semiconductors can be studied nondestructively and that coating artifacts can be eliminated. The objective aperture has four openings that can be selected and aligned outside the vacuum system. Scanning modes include normal, split/dual magnification, line, position set, spot, analysis area finder, selected area analysis, and oblique (y-modulation) scanning. Hitachi U 407
lytes, separation media, column types, and applications is included. 28 pp. Phenomenex • 408
water. The second note in the series describes the use of flow injection for rapid sample introduction in ICPMS. Perkin Elmer • 409
FIA with ICPMS "Flow Injection Atomic Spectrometry (FIAS) Application Reports" is a series of application notes describing the use of flow injection with ICPMS. The first note describes optimization and use of the technique. It demonstrates an application of FIA/ICPMS with hydride and cold vapor generation for the determination of heavy metals in drinking, river, and sea
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Real-time DNA sequencing of long strands Model 4000L automated DNA sequencer detects and identifies 800 or more bases in a single electrophoretic gel separation of a DNA digest An IR fluorophore dye is used for cycle-sequencing protocols or Sanger dideoxy protocols to prepare the samples, which are loaded onto the system's gel apparatus. The 4000L is an upgrade of Model 4000, which was offered for sale at the beginning of 1992. The 4000L accommodates 66-cmlong gels in addition to the 33-cm gels used with the Model 4000 for a typical single-base resolution accuracy of 99% at the 800th base using single-strand DNA During the run, a solid-state laser diode excites the dye at 785 nm. Individual bands are detected by a fluorescence microscope with a silicon avalanche photodiode filtered at 820 nm. The microscope, which is integrated with the laser and positioned at the bottom center of the gel, scans at 1.4-17.9 cm/s and can be focused automatically through the system's dedicated sequencing and control software. The first bands reach the bottom of the gel within 1 h, and a typical run lasts 20 h for a sample with 800
or more bases. Because the system records the bands as they run off the gel, the gel doesn't need to be stained, photographed, or handled for preservation. However, it cannot be used as a permanent record as conventional DNA gels can. The software displays a densitometric image of the gel as the bands run down to the bottom. It assigns band positions, makes base identifications, and flags ambiguous bands with an IUPAC code to enable rapid file searching. The assigned DNA sequence is displayed alongside the gel image. The software also allows automatic, semiautomatic, and manual base assignment as well as image manipulations such as scaling, colorization, background subtraction, and band normalization or "smile" correction. Data can be stored in TIFF files and a variety of other image formats, and sequences can be stored in ASCII files for exporting to publishing software. Li-Cor 1 4 1 0
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