Analytes ionization process

Clearly, in view of the complexity of this ion-molecule chemistry in the Cl plasma ( plasma is a gas in which the total concentration of ions is high), the conditions inside the Cl source must be carefully controlled if any level of reproducibility is to be achieved. Fortunately, at the methane pressures in the source, >0.1 torr, the abundance ratios of the methane-derived ions are essentially independent of pressure but do vary with temperature. At 200 C typical relative abundances might be mJz 17 (CH5 ), 100 mtz 29 (C2H5+), 85 m/z 41 (C3H5+), 15 % however, these abundance ratios do vary with temperature and with levels of impurities (oxygen, water, etc.) such that, at even quite low water concentrations, HjO becomes the dominant positive ion in a methane Cl plasma. The relative efficiencies of the three analyte ionization processes depend strongly on the chemical nature of A but in most cases protonation is the major contributor. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

Strack, D. et al., Cyanidin 3-oxalylglncoside in orchids, J. BioscL, 41, 707, 1986. Choung, M.-G. et al.. Isolation and determination of anthocyanins in seed coats of black soybean (Glycine max (L.) Merr.), J. Agric. Food Chem., 49, 5848, 2001. Covey, T., Analytical characteristics of the electrospray ionization process, in Biochemical and Biotechnological Applications of Electrospray Ionization Mass Spectrometry, ACS Symposium Series, Snyder, A.P. and Anaheim, C. A., Eds., Washington, D.C., 1995, chap. 2. 

Penning ionization occurs with the (trace) gas M having an ionization energy lower than the energy of the metastable state of the excited (noble gas) atoms A. The above ionization processes have also been employed to construct mass spectrometer ion sources. [21,24] However, Penning ionization sources never escaped the realm of academic research to find widespread analytical application. 

The physicochemical aspects of the ionization process in general, ion internal energy, and the principles determining the reaction pathways of excited ions have already been addressed (Chap. 2). After a brief repetition of some of these issues we will go more deeply into detail from the analytical point of view. Next, we will discuss technical and practical aspects concerning the construction of El ion sources and sample introduction systems. Finally, this chapter directly leads over to the interpretation of El mass spectra (Chap. 6). 

Strictly speaking, every species has an ionization efficiency curve of its own. The overall efficiency of El depends on the intrinsic properties of the ionization process as well as on the ionization cross section of the analyte (Chap. 2.4). Fortunately, the curves of ionization cross section versus electron energy are all of the same type, exhibiting a maximum at electron energies around 70 eV (Fig. 5.4). This explains why El spectra are almost exclusively acquired at 70 eV. 

Atmospheric pressure ionization (API) was the first technique to directly interface solution phase with a mass analyzer. [26] In API, a solution of the analyte is injected into a stream of hot nitrogen to rapidly evaporate the solvent. The vapor passes through a Ni source where electrons emitted from the radioactive Ni isotope initiate a complex series of ionizing processes. Beginning with the ioniza-... 

Ionization changes can be efficiently corrected with the use of an isotopically labeled IS, which possesses identical ionization response and fragmentation pattem. Therefore, deuterated IS can be used to correct both the overall method variability (e.g., sample preparation, injection, electrophoretic process, etc.) as well as matrix effects since the amount of suppression from interferents is expected to be similar. However, the total concentration of analyte and IS should be below the saturation of the ionization process. Guidelines to obtain a reproducible CE—MS method were published by Ohnesorge et al. and took into account the use of an isotopically labeled IS. 

Atmospheric pressure chemical ionization (APCI) is a gas phase ionization process based on ion-molecule reactions between a neutral molecule and reactant ions [31]. The method is very similar to chemical ionization with the difference that ionization occurs at atmospheric pressure. APCI requires that the liquid sample is completely evaporated (Fig. 1.12). Typical flow rates are in the range 200-1000 xL min , but low flow APCI has also been described. First, an aerosol is formed with the help of a pneumatic nebulizer using nitrogen. The aerosol is directly formed in a heated quartz or ceramic tube (typical temperatures 200-500 °C) where the mobile phase and the analytes are evaporated. The temperature of the nebulized mobile phase itself remains in the range 120-150 °C due to evapo-... 

The CE/MS coupling was first reported by Smith and coworkers (10). In principle, the problem of CE/MS coupling is similar to that of LC/MS. The analyte is dissolved in a liquid mobile phase that is removed in the course of the ionization process. Therefore, the same types of interfaces as known from the historically older LC/MS should be available for CE/MS. The smaller flow rates should be an advantage with respect to the vacuum system of the MS. Unfortunately, it is not that simple. 

Two models can explain the events that take place as the droplets dry. One was proposed by Dole and coworkers and elaborated by Rollgen and coworkers [7] and it is described as the charge residue mechanism (CRM). According to this theory, the ions detected in the MS are the charged species that remain after the complete evaporation of the solvent from the droplet. The ion evaporation model affirms that, as the droplet radius gets lower than approximately 10 nm, the emission of the solvated ions in the gas phase occurs directly from the droplet [8,9]. Neither of the two is fully accepted by the scientific community. It is likely that both mechanisms contribute to the generation of ions in the gas phase. They both take place at atmospheric pressure and room temperature, and this avoids thermal decomposition of the analytes and allows a more efficient desolvation of the droplets, compared to that under vacuum systems. In Figure 8.1, a schematic of the ionization process is described. 

In direct APPI, the ionization process is activated by the photons emitted by the source and takes place if the ionization energies of the molecules are below 10 eV. This is the case of most analytes however, the ionization energy of most HPLC solvents is higher. This implies that the sample must be vaporized prior to detection. The molecular ion is first generated by an impact with photons ... 

But bremsstrahlung must also be considered as a part of the background spectrum.17 The main ionization processes of an analyte (M) in argon inductively coupled plasma are ... 

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