Mass spectrometer, detectors

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

Analytical standards are prepared for two purposes for fortifying control matrices to determine the analytical accuracy and for calibrating the response of the analyte in the mass spectrometer detector. The purity of all standards must be verified before preparation of the stock solutions. All standards should be refrigerated (2-10 °C) in clean amber-glass bottles with foil/Tefion-lined screw-caps. The absolute volume of the standard solutions may be varied at the discretion of the analyst, as long as the correct proportions of the solute and solvent are maintained. Calibrate the analytical balance before weighing any analytical standard material for this method. 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

Both the GC-MS and GC-IR instruments obviously require that the column effluent be fed into the spectrometer detection path. For the IR instrument, this means that the IR cell, often referred to as a light pipe, be situated just outside the interferometer (Chapter 8) in the path of the light, of course, but it must also have a connection to the GC column and an exit tube where the sample may possibly be collected. The infrared detector is nondestructive. With the mass spectrometer detector, we have the problem of the low pressure of the mass spectrometry unit coupled with the ambient pressure of the GC column outlet. A special method is used to eliminate carrier gas while retaining sufficient amounts of the mixture components so that they are measurable with the mass spectrometer. 

Match the detector with the items that follow thermal conductivity detector, flame ionization detector, electron capture detector, and mass spectrometer detector. 

Various mass spectrometer configurations have been used for the detection of explosives, such as ion traps, quadrupoles and time-of flight mass analyzers and combinations as MS/MS systems. The ionization method is usually APCI with corona discharge [24, 25]. An example is given in Figure 20, which shows the schematic diagram of an explosive mass spectrometer detector [25]. It is based on an ion trap mass analyzer, an APCI source with corona discharge and a counter-flow introduction (CFI) system. The direction of the sample gas flow introduced into the ion source is opposite to that of the ion flow produced by the ion source. 

A personnel screening portal (Figure 21) was developed using a MS/MS mass spectrometer detector [27]. The MS detector consisted of ion trap and time-of-f-light mass (IT-TOF) analyzers with a discharge ionization source (Figure 22). MS/ MS product ions of the various explosives were used for identification. 

Chamberlain, T., K. Hanold, M. Hanning-Lee, Y. Liu, J. Syage, K. Linker, C. Rhykerd, and R.Bouchier. Multi-threat mass spectrometer detector. Proc. of the ICAO Workshop, DERA, UK, 2000. 

Thus atoms with thermal energy of about 0.02 eV have X = 1 A and can readily diffract from surfaces. A beam of atoms is chopped with a variable frequency chopper before striking the surface. This way, an alternating intensity beam signal is generated at the mass spectrometer detector, that is readily separated from the noise due to helium atoms in the background. 

The mass spectrometer detectors place new demands on the HPLC system. The MS interface requires use of volatile buffers and reagents. Nanospray interfaces especially benefit from low-volume, high-resolution separations. The mass spectrometer is a fast response system and benefits from separation speeds higher than normally supplied by HPLC systems. All of these requirements have provided constraints on new development directions for HPLC systems. 

Li, M. X. Wu, J. T. Parus, S. Lubman, D. M. 1998. Development of a three-dimensional topographic map display for capillary electrophoresis/mass spectrometry with an ion trap/reflectron time-of-flight mass spectrometer detector applications to tryptic digests of isoforms of myelin basic protein. ./. Am. Soc. Mass Spectrom., 9,701-709. 

Of all the methods investigated for organics in FDR it would appear that high-performance liquid chromatography (HPLC) and gas chromatography with a mass spectrometer detector (GC/MS) are currently the most promising. 

Matrix-Induced Changes in the Transmission of Ions from the Inductively Coupled Plasma to the Mass Spectrometer Detector. The most severe chemical matrix effects in ICP-MS are due to changes in the transmission efficiency of ions... 

Fig. 3. A schematic view of a crossed-molecular beam apparatus used for studying the reactions of chlorine atoms with halogen molecules. The mass spectrometer detector is rotatable about the scattering centre for measuring the angular distributions of the reaction products whose recoil velocities are determined by time-of-flight analysis. (Reproduced from ref. 558 by permission of the authors and the American Institute of Physics.)...

Figure 19.3. A Varian 3400 gas chromatograph with a Tekmar 3000 purge-and-trap concentrator and a Varian Saturn 2000 mass spectrometer detector operated by Varian Saturn GC/MS Workstation Software Version 5.41 k.

FIGURE 11.1 Top view of the crossed molecular beams machine. Shown are the main chamber, the primary (laser ablation configuration) and secondary source chambers, and the rotatable differentially pumped mass spectrometer detector. 

Microcapillary (0.200 -s- 0.300 mm diameter) and nanocapiUary (0.075 -t- 0.100 mm diameter) columns limit solvent consumption and interface more easily with mass spectrometer detectors. They can assay only small amounts of sample and are superior for managing Joule heating due to their enhanced surface area to volume ratio and lower volumetric flow rate (uL/min) for a given linear mobile phase velocity (mm/sec). 

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