Mass analyzer actions

Quadmpoles or hexapoles are used as transmission guides for both slow and fast ions. In both cases, the objective is to ensure that as many ions as possible are guided from the entrance of the device to its exit. The ions are usually in transit in a straight line between an ion source and a mass analyzer. Any ions within the transmission guides that are deflected from the desired trajectory are pushed or pulled back on course by the action of the inhomogeneous RF fields applied to the poles of the guides. 

The heart of the mass spectrometer is the mass analyzer, the function of which is to measure the mass-to-charge ratios of ions and provide a means of their identification. This is achieved by a combination of a dispersive action to separate the ions according to their m/e ratios and a focusing action to maximize the resolved ion intensities... 

The dwelltime of ions within the ion source is defined by the extraction voltages applied to accelerate and focus them into an ion beam and by the dimensions of that ion source. In standard El ion sources the freshly formed ions dwell about 1 ps before they are forced to leave the ionization volume by action of the accelerating potential. [41] As the ions then travel at speeds of some 10 m s they pass the mass analyzer in the order of 10-50 ps (Fig. 2.9). [9] Even though this illustration has been adapted for a double focusing magnetic sector mass spectrometer, an ion of m/z 100, and an acceleration voltage of 8 kV, the effective time scales for other types of instruments (quadrupole, time-of-flight) are very similar under their typical conditions of operation (Table 2.4). 

Secondary ionization mass spectrometry (SIMS) is yet another method to analyze solid samples directly. SIMS uses a focused beam of ions (called the primary ion beam) to sputter atoms from the surface of a sample. A small fraction of the sputtered atoms are ionized by the sputtering action, hence the term secondary ionization. The secondary ions are extracted, accelerated, and analyzed by a mass analyzer. Two main configurations exist conventional SIMS, which uses electric... 

Methods Ambient ionization methods, of which there are now over 20, e.g., desorption electrospray ionization (DESI), desorption atmospheric pressure chemical ionization (DAPC), desorption atmospheric pressme photo-ionization (DAPPI), and direct analysis in real time (DART), are now joined by paper spray, a method where ESI is initiated at the pointed tip of a piece of filter paper. A drop of blood ( 15 pi) is dried on the paper, and then the paper is moistened with 25 pi of a solvent suited to both the extraction of the analytes from the blood and the ESI process (e.g., 90% methanol 10% water with either 100 ppm acetic acid or 200 ppm sodium acetate). When the paper is exposed to high voltage (3-5 kV) while held close ( 5 mm) to the entrance of the mass analyzer, a spray (similar to electrospray) is induced at the tip of the paper as capillary action carries extracted compounds through the paper (Figure 4.5). The spray is maintained for 30-90 s at a flow rate comparable to that used in nano-electrospray. 

The beam of neutral gaseous analyte molecules enters the ionization chamber or ion volume, i.e., the actual region of ionization within the ion source block, in a line vertical to the paper plane and crosses the electron beam in the center. In order to reduce loss of ions by neutralizing collisions with the walls, the ions are pushed out immediately after generation by action of a low voltage applied to the repeller electrode [3,4]. They are then accelerated and focused towards the mass analyzer. Efficient ionization and ion extraction are of key importance for the construction of ion sources producing focusable ion currents in the nanoampere range [5]. 

A recent development in instrumentation that can potentially remove or minimize the prevalence of molecular ions in the mass spectrum is the reaction cell (sometimes called a collision cells).The reaction cell is a device that is mounted in the mass spectrometer, positioned between the ion lenses and the mass analyzer. A diagram of a typical reaction cell is shown in Figure 8.2.The sample ion beam produced by the ICP, which is collimated by the ion lens, is directed into the cell through an aperture. The beam, which is composed of analyte ions, matrix component ions, and polyatomic molecules, is transmitted through the cell by the action of a quadrupole (reaction cell) or hexapole (collision cell) transmission optic element. An externally supplied fill gas in the cell selectively reacts with the polyatomic molecular... 

Effects of Cold Gas Recycle and Approach to Equilibrium. Product gases resulting from various CGR ratios were analyzed (Table XI). For the experiments tabulated, a decrease in the cold recycle ratio resulted consistently in increases in the product gas concentrations of water vapor, hydrogen, and carbon dioxide and a decrease in methane concentration. These trends may be noted in experiment HGR-12 as the CGR ratio decreased from 8.7 1 to 1.2 1, in experiment HGR-13 as it increased from 1.0 1 to 9.1 1, and in experiment HGR-14 as it decreased from 3.0 1 to 1.0 1. These trends indicate that the water-gas shift reaction (CO + H20 —> C02 + H2) was sustained to some degree. Except for the 462-hr period in experiment HGR-14, the apparent mass action constants for the water-gas shift reaction (based on the product gas compositions in Table XI) remained fairly constant at 0.57-1.6. These values are much lower than the value of 11.7 for equilibrium conversion at 400°C. In... 

The emption of Mount St. Helens on May 18, 1980, provided geologists with a unique opportunity to study the action of volcanos. Gas samples from the plume were collected and analyzed for toxic heavy metals. To collect mercury (Hg), gas samples were passed over a piece of gold metal, which binds Hg atoms veiy tightly. The mass of the metal increased as it absorbed Hg from the plume. From a plume-gas sample containing 200 g of ash, 3.60 ft g of Hg was deposited on the gold. How many moles of mercury were present in the gas sample How many atoms is this ... 

The development of chemistry itself has progressed significantly by analytical findings over several centuries. Fundamental knowledge of general chemistry is based on analytical studies, the laws of simple and multiple proportions as well as the law of mass action. Most of the chemical elements have been discovered by the application of analytical chemistry, at first by means of chemical methods, but in the last 150 years mainly by physical methods. Especially spectacular were the spectroscopic discoveries of rubidium and caesium by Bunsen and Kirchhoff, indium by Reich and Richter, helium by Janssen, Lockyer, and Frankland, and rhenium by Noddack and Tacke. Also, nuclear fission became evident as Hahn and Strassmann carefully analyzed the products of neutron-bombarded uranium. 

Analyzing the conductance data for sodium in NH8 solutions led to the original mass action equilibria postulate of Kraus (27). It was originally proposed that metal in NH8 solutions behave as weak electrolytes or ionogens, producing metal ions and solvated electrons in accord with the equation... 

The two mass action equilibria previously indicated have been used in conjunction with a modified form of the Shedlovsky conductance function to analyze the data in each of the cases listed in Table I. Where the data were precise enough, both K2 and K were calculated. As mentioned previously, the K s so evaluated are practically the same as those obtained for ion pairing in solutions of electrolytes in ammonia and amines. This is encouraging since it implies a fairly normal behavior (in the electrolyte sense) for dilute solutions of metals. Further support of the proposed mass action equilibria can be found in the conductance measurements of sodium in NH8 solutions with added salt. Bems, Lepoutre, Bockelman, and Patterson (4) assumed an additional equilibrium between sodium and chloride ions, associated to form NaCl, to compute the concentration of ionic species, monomers, and dimers when the common ion electrolyte is added. Calculated concentrations of conducting species are employed in the Onsager-Kim extension of the conductance theory for low-field conductance of a mixture of ions. Values of [Na]totai ranging from 5 X 10 4 to 6 X 10 2 and of the ratio of NaCl to [Na]totai ranging from zero to 28.5 are included in the calculations. 

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