CEMS 2010 spectrometer

General view of CEMS 2010 spectrometer based on continuous gas flow counter. 

Figure 6 CEMS spectrometer used to operate at 4.2 K. M, mylar window, B, cold finger. The detection assembly is screwed on to the dewar at SS. ...

Pd removal was determined as follows. An aliquot of a representative liquid or solid sample was accurately weighed and subsequently digested by refluxing in nitric and/or hydrochloric acid using a closed vessel microwave procedure (CEM MARS5 Xpress or Milestone Ethos EZ). Cooled, digested samples were diluted, matrix matched to standards, and referenced to a linear calibration curve for quantitation an internal standard was employed to improve quantitation. All samples were analyzed by an Inductively Coupled Plasma Mass Spectrometer or ICP/MS (Perkin Elmer SCIEX Elan DRCII) operated in the standard mode. 

Discrimination of slower ions as compared to faster ions is observed with SEMs, [243] CEMs, and MCPs as well. [239,244,245] This means a reduction in sensitivity upon reduction of the acceleration voltage of a mass spectrometer, and of course, with increasing ion mass (Eq. 4.3). In order to reduce such effects and especially to improve sensitivity for high-mass ions, post-acceleration detectors have been developed. [236,245] In post-acceleration detectors the ions are accelerated immediatly in front of the detector by a voltage of 10-30 kV before they hit the first dynode or the first MCP. 

Fig. 1.32 Straight channel electron multipliers (CEM) are typically used in quadrupole-type mass spectrometers.

CEMS = conversion electron Mossbauer spectroscopy DFT = density functional theory EFG = electric field gradient EPR = electron paramagnetic resonance ESEEM = electron spin echo envelope modulation spectroscopy GTO = Gaussian-type orbitals hTH = human tyrosine hydroxylase MIMOS = miniaturized mossbauer spectrometer NFS = nuclear forward scattering NMR = nuclear magnetic resonance RFQ = rapid freeze quench SAM = S -adenosyl-L-methionine SCC = self-consistent charge STOs = slater-type orbitals TMP = tetramesitylporphyrin XAS = X-ray absorption spectroscopy. 

MCPs operate on the same principle as CDEMs. A microchannel plate is essentially a monolithic array of miniaturized CDEMs fabricated in a single wafer or disk of glass. The disk contains pores extending from the upper surface to the lower surface. These pores are known as channels and perform the same function as the interior of the tube in a CEM, but in contrast to a typical CEM, the entrance ends of the channels are not flared. The channels are typically 3 to 30 micrometers in diameter depending on the design. The length of the channels is set by the thickness of the disk, typically 200 to 1000 micrometers. MicroChannel plates can be fabricated in areas measured in cm, and because the disk-shaped profile forms a nearly flat stopping surface for ions, they are ideal detectors for TOE mass spectrometers. Burle and Hamamatsu are the major suppliers of these devices. 

The representative experimental procedure is as follows 1,4-dibromobutane (1 mmol, 0.215 g) and sodium azide (2.5 mmol, 0.163 g) in water (2 mL) were placed in a 10 mL crimp-sealed thick-walled glass tube equipped with a pressure sensor and a magnetic stirrer. The reaction tube was placed inside the cavity of a CEM Discover focused microwave synthesis system, operated at 120 + 5°C (temperature monitored by a built-in infrared sensor), power 70-100 Watt and pressure 60-100 psi, for 30 minutes. After completion of the reaction, diethyl ether was added to extract the alkyl azide. GC/MS analysis indicated the disappearance of alkyl halides. FT-IR spectrum of crude product was obtained using a FT-IR spectrometer and the formation of alkyl azide was confirmed by the characteristic IR adsorption around 2100 cm Removal of the solvent under reduced pressure (rotary evaporator) afforded the product, 1,4-diazido-butane (0.125 g) in 89% yield. 

After being dried, the samples were reexamined by nitrogen sorption and mercury intrusion, and a portion of the material was analyzed to determine the residual mercury levels. This analysis was achieved by acid digestion (10 mL of 50% aqua regia sample sizes were approximately 0.2 g in all cases) in pressure-sealed poly(tetrafluoroethylene) (PTFE) tubes heated to 140 °C for 10 min in a microwave oven (CEM). The solutions were analyzed after suitable dilution in distilled water with a graphite furnace atomic absorption spectrometer (Perkin Elmer 5100-PC). The detection limit for this method is estimated to be 6 ppm of mercury on the dry solid. 

An online gas chromatograph/mass spectrometer (GC/MS) was thus installed just before the CEMS unit, and we watched, with the anticipation only other PAT geeks can understand, for the next CEMS spike. The GC/MS reading at the next CEMS spiked is shown in Figure 23.7. 

Most of the disadvantages of discrete dynode SEMs are overcome by the development of channel electron multipliers (CEMs) (sometimes also referred to as continuous dynode electron multipliers, CDEMs) that are by far the most widely used ion detectors in analytical mass spectrometers. 

DCEMS together with low-temperature and in-field measurements require much more sophisticated experimental equipment. Gas counters, scintillation detectors, electron multipliers (Channeltron, Ceratron), surface barrier silicon semiconductor detector, and electron energy analyzers belong to the most frequently used detectors applied in CEMS and CXMS, respectively. Differences among individual constructions can be found in Ref. 123. Commercially available version of CEMS/CXMS spectrometer is depicted in Fig. 18.36 [127]. Device is based on 27t proportional continuous gas flow counter for room-temperature zero-magnetic field measurements. 

Conversion electron Mossbauer spectra (CEMS) of the samples were measured at 295 K with the aid of the same Haider Mossbauer spectrometer capable of operating in conventional constant acceleration mode using a backscatter-type gas flow detector. 

Milano et al. [153, 154] and Cook [34] introduced an approach to derivative spectra by substituting electronic wavelength modulation for the mechanical systems used in derivative spectrometers. This effect is achieved by superimposing a low-amplitude, periodic wave form on the horizontal sweep signal. In this way spectra were generated. Warner et al. [155] applied a vidicon detector for fast detection of fluorescence spectra and obtained derivatives of the stored data by digital computation. Cook et al. [156] also made use of a silicon vidicon detector for multichannel operations in rapid UV-VIS spectrophotometers with the possibility of first-order differentiation. For the same purpose Milano et al. [93, 157] used a multichannel linear photodiode array for detection of spectra in polychromator optics and stored data manipulations (d ). Technical explanations of the principles of diode array and vidicon devices cem be found in [158-161]. 

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