Detector, atomic spectrometer differential

The detector, which usually can be rotated to various scattering angles, must be able to differentiate among the species present. Surface-ionization detectors with efficiencies of approximately 10% are employed for reactions of metal atoms yielding metal halide products. Laser-induced fluorescence has proved to be an excellent detector for group Ilb halides and a few other systems. In general, products can be detected by electron-bombardment ionization with subsequent analysis by a mass spectrometer. Unfortunately, these mass spectrometer or universal detectors have a low detection efficiency ( 0.1%,) thus, beam machines with such detectors require several differentially pumped chambers to reduce the background pressure (of the mass of interest) in the detector to torr. Modulation tech-... 

The simplest analytical method is direct measurement of arsenic in volatile methylated arsenicals by atomic absorption [ 11 ]. A slightly more complicated system, but one that permits differentiation of the various forms of arsenic, uses reduction of the arsenic compounds to their respective arsines by treatment with sodium borohydride. The arsines are collected in a cold trap (liquid nitrogen), then vaporised separately by slow warming, and the arsenic is measured by monitoring the intensity of an arsenic spectral line, as produced by a direct current electrical discharge [1,12,13]. Essentially the same method was proposed by Talmi and Bostick [10] except that they collected the arsines in cold toluene (-5 °C), separated them on a gas chromatography column, and used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 xg/l for water samples. 

In a typical CMB experiment, beams of atoms and molecules with narrow angular and velocity spread are crossed in a vacuum chamber and the angular and time-of-flight (TOF) distributions of the products are recorded after well defined collisional events take place. The detector is an electron-impact ionizer followed by a quadrupole mass spectrometer (QMS) filter the whole detector unit can be rotated in the collision plane around the axis passing through the collision center (Figure 14.1). The crossed beam machine used in the present experiments has been described in detail elsewhere [67, 79,80]. Briefly, it consists of two source chambers (10 mbar), a stainless-steel scattering chamber (10 mbar), and a rotatable, differentially pumped quadrupole mass spectrometric detector ( <8 X 10" mbar). 

Figure B2.3.3. Crossed-molecular beam apparatus employed for the study of the F + D2 -> DF + D reaction. Indicated in the figure are (1) the effusive F atom source (2) slotted-disk velocity selector (3) liquid-nitrogen-cooled trap (4) D2 beam source (7) skimmer (8) chopper (9) cross-correlation chopper for product velocity analysis and (11) rotatable, ultrahigh-vacuum, triply differentially pumped, mass spectrometer detector chamber. Reprinted with permission from Lee [29]. Copyright 1987 American Association for the Advancement of Scienee. 

Chromatography has as its basis the transformation of a complex multi-component sample into a time-resolved, separated analyte stream, usually observed in the analog differential signal mode. The chromatographic sample is thus distinctive in that analytes are changing in nature and with time. An essential feature of chromatographic instrumentation is a detector for qualitative and quantitative determination of the components resolved by the column this should respond immediately and predictably to the presence of solute in the mobile phase. An important class of solute properly detectors are those giving Selective , or Specific information on the eluates. Spectral property detectors such as the mass spectrometer, the infrared spectrophotometer and the atomic emission spectrometer fall into this class. Such detectors may be element selective , structure or functionality selective or property selective . 

Fig. 2.3. Chromatogram of a solution containing aisenite, arsenate, methylarsonic acid and dimethylarsinic acid (each 100 mg As 1 ) recorded with a differential refractive index detector and an ARL 34000 simultaneous inductively coupled argon plasma atomic emission spectrometer as the arsenic-specifier detector [Hamilton PRP-1 column, hexadecyltrimethylammonium bromide (HTAB) as ion-pairing reagent flow rate 1.5 ml min 0.1 ml injected Waters Associates high pressure liquid chromatograph integration time on ICP 5 sec. As 189.0 nm]. Redrawn from Marine Chemistry [10] by permission of Elsevier Science Publishers and the authors.

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