Multiple ionization source

An other approach has been described by Syage et al. [36], who investigated the potential of various ionization sources (ESI, APCI, APPI) either in simultaneous or in switching mode. They suggest that ESI/APPI is the best combination because APPI covers a broad range of analytes while ESI covers the larger molecules. 

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The type of ionization method is determined by the method of sample introduction. The most useful sources with GC are El and Cl, and with LC, ESI (or another API method, e.g., APCI or APPI). MALDI is a stand-alone ionization method that is best suited to peptides and proteins. Because MALDI is an off-line technique, samples can be investigated multiple times as opposed to LC, where peaks cannot be revisited without reinjecting the sample. Although it is highly desirable to have multiple ionization sources, the budgets for dedicated instruments may not allow such luxuries. 

Various analyzers have been used to analyze phenolic compounds. The choice of the MS analyzer is influenced by the main objective of the study. The triple quadrupole (QqQ) has been used to quantify, applying multiple reaction monitoring experiments, whereas the ion trap has been used for both identification and structure elucidation of phenolic compounds. Moreover, time-of-flight (TOF) and Fourier-transform ion cyclotron resonance (FT-ICR) are mainly recommended for studies focused on obtaining accurate mass measurements with errors below 5 ppm and sub-ppm errors, respectively (Werner and others 2008). Nowadays, hybrid equipment also exists, including different ionization sources with different analyzers, for instance electrospray or atmospheric pressure chemical ionization with triple quadrupole and time-of-flight (Waridel and others 2001). 

M. G. Inghram and W. A. Chupka. Surface Ionization Source Using Multiple Filaments. Rev. Sci. Instrum., 24(1953) 518-520. 

Limited sample clean-up could overload the analytical column, and residual matrix components can accumulate on the column after multiple injections. The residual matrix components can also solidify and deposit over a period of time in the LC-MS ionization source or vacuum interface, resulting in a decrease in ion transfer efficiency. The decrease in instrumentation performance (i. e., signal intensity) can be monitored by the signals of system-suitability samples dispersed within an analytical batch. The practice of replacing the pre-column in every run and scrubbing the analytical column periodically with a cleaning mobile phase will help to maintain instrument performance. 

The obvious benefit of the quadrupole ion trap is that it is an ion storage device. Therefore, ions can be both accumulated and stored for extended periods. Accumulation can occur over a continuous ionization event or over multiple pulsed ionization periods. When used with pulsed ionization sources, duty cycle, defined in terms of sample utilization, can be as high as 100%. Because a broad range of atomic ions can be stored simultaneously, the quadrupole ion trap is a promising analyzer for transient peak analysis. 

It is important to note that each of these ionization sources, either a laser in MALDI-TOF or the high voltage ionization of droplets in ESI-MS/MS or LC-MS/ MS, will each produce a different spectrum of detectable ions and intensities because the effectiveness and nature of peptide ionization is quite different for each source. In addition, the presence of multiple peptides that influence each other s ionization potential notably through ion suppression makes most peptide ion measurements only semiquantitative. 

Theoretical treatments predict and sophisticated mass-spectrometric experiments confirm that such recoil and electronic excitation sources may cause fragmentation and multiple ionization only in a small fraction of the primary decay species [Eq. (7a)], whereas the remainder ( 80%) is formed in the... 

Under acidic conditions, an analyte protein of unknown molecular weight is known to possess multiple positive charge, between +5 and +8. A mixture containing this protein and others were subjected to capillary electrophoresis in 10-mM trifluoroacetic acid, and the eluate from the capillary was fed directly into the ionization source of an electrospray mass spectrometer. As the protein eluted from the capillary, the mlz range of the mass spectrometric detector was scanned, and peaks were observed at mlz values of 938, 1071, 1250, and 1500. What is the molecular weight of the protein ... 

The most common methods used for pharmaceutical pK values are based on pH measurements, Eqs. (3-8). Thus, they cannot be interpreted with greater accuracy than 0.02 pKa unit [see the definition in Section 2.1, Eq. (1)]. This level of precision and accmacy should always be the aim in determining pK values for inclusion in the drug sciences literature. Potentiometric titrations [Eqs. (3-4)] are often performed with this level of accmacy, primarily for compounds with either a single ionization step or for multiple ionizations with >4 log units between the pKa values. The careful use of precise pH meters (e.g., the series of Beckman Research models, or the corresponding Radiometer, Orion, or Metrohm instruments) for the determination of pH data means that reproducibility for replicate measurements may be rather better than 0.02. In the author s experience, these instruments may be calibrated with a reproducibility of 0.002 pH imit, which can be maintained (with proper temperature control and exclusion of CO2) for at least 8 h. This does not imply accuracy of 0.002 pH unit, which is not possible according to the current definition of pH. Spectrophotometric [Eq. (5)] and solubility-pH dependence [Eqs. (7-8)] methods are potentially capable of similar accuracy, but often do not give results better than 0.05 pKa unit, due to the inevitable inclusion of additional sources of error from the absorbance or concentration measurements. 

MALDl acts as a soft ionization source and generally produces singly charged molecular ions from even very large polymers and biomolecules, although a few multiple-charge ions and some fragment ions may occur (Fig. 9.11). 

The data shown in Figure 9.5 are from a mixture of sphingomyelin (ML), dyn-orphin (MP), and chlorisondamine (Chi). The primary feature of this 2D spectrum is the presence of two families of ions. Because this spectrum was obtained with a MALDI ionization source, all the ions are singly charged. So, unlike the ion families discussed for Figure 9.3, these trend lines do not arise from multiple charges but from structure similarities among compound classes. 

A wide variety of lasers have been used in combination with trapping mass spectrometers. Two lasers in particular are incorporated already into many commercial mass spectrometers the CO2 (X= 10.6 pm or 0.12 eV photon ) and nitrogen (X=337 nm or 3.68 eV photon ) lasers. The former is used for performing infrared multiple photon dissociation (IRMPD) experiments (mostly in ICR instruments, see Section 9.3) while the latter is used commonly in MALDl ionization sources. Other commonly-used lasers are Q-switched Nd YAG lasers (fundamental X= 1064 nm... 

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