Spectrometer sources ultraviolet

One of the most direct methods is photoelectron spectroscopy (PES), an adaptation of the photoelectric effect (Section 1.2). A photoelectron spectrometer (see illustration below) contains a source of high-frequency, short-wavelength radiation. Ultraviolet radiation is used most often for molecules, but x-rays are used to explore orbitals buried deeply inside solids. Photons in both frequency ranges have so much energy that they can eject electrons from the molecular orbitals they occupy. 

Visible and ultraviolet absorption spectra are measured in an absorption spectrometer. The source gives out intense visible light or ultraviolet radiation. The wavelengths can be selected with a glass prism for visible light and with a quartz prism or a diffraction grating for ultraviolet radiation (which is absorbed by glass). A typical absorption spectrum, that of... 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

Photolysis Reactors and Ultraviolet Sources. For 253.7-m/ irradiation, a modified irradiation apparatus purchased from Delmar Co. was used. The reactor was a 2-necked, 500-ml., round-bottomed flask. One neck was an O-ring joint, and the other was a 24/40 joint. A 4- X 1-inch coiled low pressure mercury quartz lamp was placed inside the flask through the O-ring neck, and the joint was sealed with removable O-rings. The reactor was connected directly to the mass spectrometer by the 24/40 joint. The samples were placed inside the flask and irradiated internally. The O-ring was shielded from direct radiation so as not to induce degradation. The estimated output of the lamp was 30 watts, and the ambient temperature within the reactor during irradiation was 70°C. 

Major methods for introducing proteins and other macromolecules into mass spectrometers are electrospray and matrix-assisted laser desorption/ionization (MALDI).18-27 Most often, MALDI is used with a time-of-flight mass spectrometer, which can measure mlz up to 106. Typically, 1 p,L of a 10 jxM solution of analyte is mixed with 1 p,L of a 1-100 mM solution of an ultraviolet-absorbing compound such as 2,5-dihydroxybenzoic acid (the matrix) directly on a probe that fits into the source of the spectrometer. Evaporation of the liquid leaves an intimate mixture of fine crystals of matrix plus analyte. 

The photolyses of CH2I2 and CHI3 in molecular beams have been investigated by Kawasaki ct al. (560) using a broad-band polarized light source in conjunction with a mass spectrometer. The primary product of the photolysis in the near ultraviolet is the I atom. Hence, primary processes are... 

Visible and ultraviolet (UV) absorption spectra are measured in an absorption spectrometer. The source... 

Isotopes in interstellar gas With the aid of the Hubble Space Telescope it has been possible for the first time to measure the boron isotopic ratio within diffuse clouds of the Milky Way. The interstellar ultraviolet radiation renders B ionized (B+) in the diffuse clouds therefore its spectrum is similar to that of the element Be, but at shorter wavelengths. The strongest resonance line lies in the ultraviolet, visible to Hubble spectrometers. The smaller mass of the 10B isotope shifts its line by 0.013 A (about 0.001%) toward longer wavelengths. Very detailed analysis of the line pair has shown in several clouds that today s interstellar abundance ratio is UB/ 10B = 3.4 0.7, which is consistent with the solar ratio 4.05. For the first time one can conclude that the solar ratio is not an abnormal one, but is shared by interstellar gas at a value larger than the ratio 2.5 that is produced by cosmic-ray collisions in the interstellar gas. Another source of11B is needed. 

Figure 6. Block diagram of the experimental. RB system, vhich consists of three basic components a laser system capable of producing tunable ultraviolet radiation, a magnetic sector mass spectrometer with a suitably modified thermal atomization source, and a detection and measurement circuit capable of quantifying the pulsed ion currents produced in the experiment.

The photoelectron spectra were taken on a Perkin-Elmer PS-16 instrument with a Hel source. The ultraviolet absorption spectra were ddermined in the vapor phase on a McPherson model 225 vacuum ultraviolet spectrometer mounted with a 1200 lines/mm grating and a hydrogen light source. A Cary-17 spectrometer was also used. 

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