Mass spectrometry initiation

IATA ICAP ICP ICP-AES ICP-MS ICV ID IDL IDW ISO International Air Transportation Association inductively coupled argon plasma inductively coupled plasma inductively coupled plasma-atomic emission spectrometry inductively coupled plasma-mass spectrometry initial calibration verification identification instrument detection limit investigation-derived waste International Standardization Organization... 

Figeys, D. Aebersold, R. 1997. High sensitivity identification of proteins by electrospray ionization tandem mass spectrometry initial comparison between an ion trap mass spectrometer and a triple quadrupole mass spectrometer. Electrophoresis, 18,360-368. 

Ions are also used to initiate secondary ion mass spectrometry (SIMS) [ ], as described in section BI.25.3. In SIMS, the ions sputtered from the surface are measured with a mass spectrometer. SIMS provides an accurate measure of the surface composition with extremely good sensitivity. SIMS can be collected in the static mode in which the surface is only minimally disrupted, or in the dynamic mode in which material is removed so that the composition can be detemiined as a fiinction of depth below the surface. SIMS has also been used along with a shadow and blocking cone analysis as a probe of surface structure [70]. 

Although there has been some controversy concerning the processes involved in field ionization mass spectrometry, the general principles appear to be understood. Firstly, the ionization process itself produces little excess of vibrational and rotational energy in the ions, and, consequently, fragmentation is limited or nonexistent. This ionization process is one of the mild or soft methods available for producing excellent molecular mass information. The initially formed ions are either simple radical cations or radical anions (M ). 

The nebulization concept has been known for many years and is commonly used in hair and paint spays and similar devices. Greater control is needed to introduce a sample to an ICP instrument. For example, if the highest sensitivities of detection are to be maintained, most of the sample solution should enter the flame and not be lost beforehand. The range of droplet sizes should be as small as possible, preferably on the order of a few micrometers in diameter. Large droplets contain a lot of solvent that, if evaporated inside the plasma itself, leads to instability in the flame, with concomitant variations in instrument sensitivity. Sometimes the flame can even be snuffed out by the amount of solvent present because of interference with the basic mechanism of flame propagation. For these reasons, nebulizers for use in ICP mass spectrometry usually combine a means of desolvating the initial spray of droplets so that they shrink to a smaller, more uniform size or sometimes even into small particles of solid matter (particulates). 

Some of the target molecules gain so much excess internal energy in a short space of time that they lose an electron and become ions. These are the molecular cation-radicals found in mass spectrometry by the direct absorption of radiation. However, these initial ions may react with accompanying neutral molecules, as in chemical ionization, to produce protonated molecules. 

The deterrnination of hydrogen content of an organic compound consists of complete combustion of a known quantity of the material to produce water and carbon dioxide, and deterrnination of the amount of water. The amount of hydrogen present in the initial material is calculated from the amount of water produced. This technique can be performed on macro (0.1—0.2 g), micro (2—10 mg), or submicro (0.02—0.2 mg) scale. Micro deterrninations are the most common. There are many variations of the method of combustion and deterrnination of water (221,222). The oldest and probably most reUable technique for water deterrnination is a gravimetric one where the water is absorbed onto a desiccant, such as magnesium perchlorate. In the macro technique, which is the most accurate, hydrogen content of a compound can be routinely deterrnined to within 0.02%. Instmmental methods, such as gas chromatography (qv) (223) and mass spectrometry (qv) (224), can also be used to determine water of combustion. 

Coupling of analytical techniques (detectors) to high-performance liquid chromatographic (HPLC) systems has increased in the last tree decades. Initially, gas chromatography was coupled to mass spectrometry (MS), then to infrai ed (IR) spectroscopy. Following the main interest was to hyphenate analytical techniques to HPLC. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

Secondly, at an initial stage, the first layer of C2 units diffusing out of the catalyst remains at a Van der Waals distance from the C, layer coordinated to the catalyst surface. Then, if the units of that outer layer bind to one another, this will lead to a half fullerene. Depending on whether the central axis of that half fullerene is a threefold or a fivefold rotation axis, a (9n,0) or a (5n,5 ) tubule will start growing, respectively. The half fullerene can also grow to completion instead of starting a nanotu-bule[ 17]. This assumption is reinforced by the fact that we have detected, by HPLC and mass spectrometry, the presence of fullerenes Qo, C70,. .. 

A number of reviews of mass spectra of carbohydrates have been published from which references to the original papers are available (4, 9, 11, 24, 26). The application of mass spectrometry to this field was initially limited by the relatively low volatility of free carbohydrates and by the complex spectra obtained from some derivatives. These limitations have been partially overcome by new inlet techniques and by pioneering studies on classes and derivatives in order to understand the characteristic fragmentations and rearrangements of the molecular ions of a wide range of carbohydrates. 

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

Cl is an efficient, and relatively mild, method of ionization which takes place at a relatively high pressure, when compared to other methods of ionization used in mass spectrometry. The kinetics of the ion-molecule reactions involved would suggest that ultimate sensitivity should be obtained when ionization takes place at atmospheric pressure. It is not possible, however, to use the conventional source of electrons, a heated metallic filament, to effect the initial ionization of a reagent gas at such pressures, and an alternative, such as Ni, a emitter, or a corona discharge, must be employed. The corona discharge is used in commercially available APCI systems as it gives greater sensitivity and is less hazardous than the alternative. 

The synthetic preparation of 2,8-dichlorodibenzo-p-dioxin was facilitated in that the chemical precursor, 2,4,4 -trichloro-2 -hydroxydiphenyl ether, was available as a pure material. Condensation was induced by heating the potassium salt at 200 °C for 15 hours in bis (2-ethoxyethyl) ether. Product analysis by GLC and mass spectrometry revealed an unexpected dichlorophenol and a monochlorodibenzo-p-dioxin. Further, the product initially isolated by crystallization from the reaction mixture was 2,7-dichlorodibenzo-p-dioxin, rather than the expected 2,8-isomer. Cooling of the mother liquor yielded crystalline plates which were shown to be 2,8-dichlorodibenzo-p-dioxin by x-ray diffraction (Reaction 2). 

Analytical methods for parent chloroacetanilide herbicides in soil typically involve extraction of the soil with solvent, followed by solid-phase extraction (SPE), and analysis by gas chromatography/electron capture detection (GC/ECD) or gas chromatog-raphy/mass spectrometry (GC/MS). Analytical methods for parent chloroacetanilides in water are similarly based on extraction followed by GC with various detection techniques. Many of the water methods, such as the Environmental Protection Agency (EPA) official methods, are multi-residue methods that include other compound classes in addition to chloroacetanilides. While liquid-liquid partitioning was used initially to extract acetanilides from water samples, SPE using... 

Specifically for triazines in water, multi-residue methods incorporating SPE and LC/MS/MS will soon be available that are capable of measuring numerous parent compounds and all their relevant degradates (including the hydroxytriazines) in one analysis. Continued increases in liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/API-MS/MS) sensitivity will lead to methods requiring no aqueous sample preparation at all, and portions of water samples will be injected directly into the LC column. The use of SPE and GC or LC coupled with MS and MS/MS systems will also be applied routinely to the analysis of more complex sample matrices such as soil and crop and animal tissues. However, the analyte(s) must first be removed from the sample matrix, and additional research is needed to develop more efficient extraction procedures. Increased selectivity during extraction also simplifies the sample purification requirements prior to injection. Certainly, miniaturization of all aspects of the analysis (sample extraction, purification, and instrumentation) will continue, and some of this may involve SEE, subcritical and microwave extraction, sonication, others or even combinations of these techniques for the initial isolation of the analyte(s) from the bulk of the sample matrix. 

Figure 8. ° Pb7 Pb vs. Th/Us (derived using Eqn. 5 in the text) diagram for mid-ocean ridge and ocean island basalt based on a recent data set with mostly mass spectrometry measurements (Turner et al. 1997 Bourdon et al. 1996 Dosso et al. 1999 Claude-lvanaj et al. 1998, 2001 Sims et al. 2002). The data show a relatively well defined array that intersect a closed-system hne for the bulk Earth starting with an initial lead isotope composition equal to Canyon Diablo (T = 4.55 Ga). This intersect was used by Allegre et al. (1986) to define the Th/U ratio of the Earth.

In this chapter, we have chosen from the scientific literature accounts of symposia published at intervals during the period 1920 1990. They are personal choices illustrating what we believe reflect significant developments in experimental techniques and concepts during this time. Initially there was a dependence on gas-phase pressure measurements and the construction of adsorption isotherms, followed by the development of mass spectrometry for gas analysis, surface spectroscopies with infrared spectroscopy dominant, but soon to be followed by Auger and photoelectron spectroscopy, field emission, field ionisation and diffraction methods. 

Figure 5.11 Variation in the catalytic activity of an Mg(0001) surface when exposed to a propene-rich propene- oxygen mixture at room temperature. The surface chemistry is followed by XPS (a), the gas phase by mass spectrometry (b) and surface structural changes by STM (c, d). Initially the surface is catalytically active producing a mixture of C4 and C6 products, but as the surface concentrations of carbonate and carbonaceous CxHy species increase, the activity decreases. STM images indicate that activity is high during the nucleation of the surface phase when oxygen transients dominate. (Reproduced from Ref. 39).

---