Atmospheric pressure source

Electron affinitive species present in the atmospheric pressure source can capture such low energy electrons. In particular, oxygen can produce a superoxide ion ... 

Furnace atomisation plasma emission spectrometry (FAPES) this consists of an atmospheric pressure source combining a capacitively coupled radiofrequency helium plasma formed inside a graphite tube which contains an axial powered electrode. This miniplasma has rarely been used in analytical atomic spectrometry, probably because of the small number of users and a lack of information about its applications and capabilities [7]. 

Such sources ionize the sample at atmospheric pressure and then transfer the ions into the mass spectrometer. An atmospheric pressure interface is then used to transfer ions into the high vacuum of the mass analyser. The problem lies in coupling an atmospheric pressure source compartment with an analyser compartment that must be kept at a very low pressure or at a very high vacuum (10 5 Torr). 

The APPI source is one of the last arrivals of atmospheric pressure sources [80,81]. The principle is to use photons to ionize gas-phase molecules. The scheme of an APPI source is shown in Figure 1.34. The sample in solution is vaporized by a heated nebulizer similar to the one used in APCI. After vaporization, the analyte interacts with photons emitted by a discharge lamp. These photons induce a series of gas-phase reactions that lead to the ionization of the sample molecules. The APPI source is thus a modified APCI source. The main difference is the use of a discharge lamp emitting photons rather than the corona discharge needle emitting electrons. Several APPI sources have been developed since 2005 and are commercially available. The interest in the photoionization is that it has the potential to ionize compounds that are not ionizable by APCI and ESI, and in particular, compounds that are non-polar. 

Atmospheric pressure sources need to have a device for the desolvation of the ions, such as a heated capillary, a heated gas curtain, collisions at intermediate pressure in a focusing multipole. The increase in internal energy induces the dissociation of the associated ion-molecules. Furthermore, the ions are at low pressure and thus the probability of the reverse reaction is strongly decreased. For example, if a molecule forms a complex with an... 

The orbitrap is an electrostatic ion trap that uses the Fourier transform to obtain mass spectra. This analyser is based on a completely new concept, proposed by Makarov and described in patents in 1996 [26] and 2004 [27], and in Analytical Chemistry in 2000 [28], A third patent describes a complete instrument including an atmospheric pressure source [29], Another article was also published with Cooks in 2005 [30]. The first commercial instrument was introduced on the market by the Thermo Electron Corporation in June 2005. 

Electrospray ionization involves the introduction of a liquid solution directly into the atmospheric pressure source through an emitter. The liquid forms a droplet at the end of the emitter, where it is exposed to a high electrical field (Fig. 1). This results in a buildup of multiple charges on the surface of the droplet. The coulombic forces from these charges ultimately result in the droplet s expulsion from the surface. The ions produced in the ion source are then extracted into the mass analyzer. ESI is now widely used for identifying small molecules, proteins, studying large non-covalent complexes, structural analysis, and as a detector for separation methods such as HPLC and capillary... 

MS is becoming the detection system of choice for LC by virtue of its flexibility and high selectivity for individual solutesHowever, LC-MS is always less sensitive than GC-MS as a result of the need to transfer the analytes from the liquid phase into a high-vacuum gas phase. Other limitations of LC-MS combination include the inability to use nonvolatile buffers, the narrow optimum range for eluent flow rate influence of the proportion of organic modifier on the sensitivity, and the narrow choice of ionization methods.Nevertheless, LC-MS has been widely accepted as an advantageous choice for the determination of carbamate pesticides in water matrices, which is more robust and flexible in the absence of derivatization. Thermospray and particle-beam interfaces are probably most commonly used for offline and online determination of carbamates in Atmospheric pressure sources such as... 

All the atmospheric pressure sources use ions from HPLC eluent which are already formed in solution. For example, an acidic mobile phase in HPLC will generate protonated sample. Such ions will pass into the mass analyser and will be separated in the usual manner. 

The main issue with atmospheric pressure sources is efficient sampling of the molecular ion species which must be transferred from normal atmospheric pres-siue to a vacuum chamber. This is effected by passing the ion stream through a pin hole and a series of focusing lenses into a two-stage pumped separator which removes the bulk of the neutral molecules. However, the nature of this transfer process makes the inlet prone to contamination, and the use of inorganic and non-volatile buffers in the HPLC separation will rapidly block these systems. Volatile buffers, such as ammonium formate, ammonium acetate and small concentrations of trifluoroacetic acid are best suited for LC/MS procedures. 

A similar effect can be generated by manipulating the cone voltage in the atmospheric pressure source. Using a triple quadrupole instrument fitted with an identical source to the quadmpole-ToF, but scanning a normal spectrum, spectra were obtained for SB-243213 at various cone voltages (Fig. 5.15). 

The ion chemistry and electrical stability of a point-to-plane corona discharge (CD) ion source was described using a tandem mass spectrometer equipped with an atmospheric pressure source. This source was a continuous current-regulated discharge with a direct current (DC) power supply and danonstrated the regions of stability with potential and distance between the needle and plane. A sch atic overview of the mechanism of formation of ions in a CD is shown in Figure 4.1. 

Figure 8.2 An apparatus for growing VGCF at atmospheric pressure. Source Reprinted from Tibbetts GG, Rodda EJ, High temperature limit for the growth of carbon filaments on catalytic iron particles, Mater Res Soc Symp Proc, 111, 49, 1988.

Figure 5.15 (a) Sketch of a gas curtain interface for API-MS coupling (Buckley 1974, 1974a French 1977). The ultra-dry gas (N2) curtain separates the ionization chamber (atmospheric pressure) from the orifice leading to the skimmer cone and thence to the mass spectrometer vacuum, (b) Sketch of an API-MS interface based on a heated glass capillary that connects the atmospheric pressure source to the low vacuum region preceding the sampling cone (Figure 5.17). In both cases an electric field E helps direct the ions into the sampling orifice. Reproduced from Bruins, Mass Spectrom. Revs. 10, 53 (1991), with permission of John Wiley Sons, Ltd.

All the data reported here were acquired on a hybrid quadrupole/ion mobility/ orthogonal acceleration time-of-flight (oa-TOF) instrument (Synapt G2 HDMS, Waters Corp., Milford, MA). A schematic of the instrument is shown in Figure 8.1. Ions generated in the atmospheric pressure source enter the vacuum system and pass through an ion guide and quadrupole mass filter to the IMS section of the instrument, which comprises three ion guides. 

Fine vacuum and atmospheric pressure sources for MALDI... 

Figure 4.14. Area to be cleaned inside the atmospheric pressure source region.

The atmospheric pressure MALDI sources (Pigme 6.16)" " have many of the same advantages as the high pressure MALDI sources. " " The expected trade-off between the two source geometries is likely to be one of sensitivity versus extent of collisional cooling however, neither source geometry has yet been extensively tested with respect to either parameter. Early indications, however, seem to point in the direction of better collisional cooling but lower sensitivity for the atmospheric pressure sources. A prototype intermediate-pressure MALDI source demonstrated 10-attomole sensitivity, " compared... 

A major benefit of the oaTOF technology has been the development of hybrid quadrupole/ TOF mass spectrometers. In these instruments a number of RF quadrupole (or other multipole) devices are used to provide the transition from atmospheric pressure sources. 

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