Experimental spectrometry

The discussion of Rutherford backscattering spectrometry starts with an overview of the experimental target chamber, proceeds to the particle kinematics that detennine mass identification and depth resolution, and then provides an example of the analysis of a silicide. 

Following the movement of airborne pollutants requires a natural or artificial tracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source. Limitations placed on the tracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emitted from high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable. On the basis of these criteria, rare earth isotopes, such as those of Nd, were selected as tracers. The choice of tracer, in turn, dictated the analytical method (thermal ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of... 

Stolzberg, R. J. Screening and Sequential Experimentation Simulations and Elame Atomic Absorption Spectrometry Experiments, /. Chem. Educ. 1997, 74, 216-220. 

Desorption ionization (DI). General term to encompass the various procedures (e.g., secondary ion mass spectrometry, fast-atom bombardment, californium fission fragment desorption, thermal desorption) in which ions are generated directly from a solid or liquid sample by energy input. Experimental conditions must be clearly stated. 

During gc/ms or liquid chromatography/mass spectrometry (Ic/ms) acquisitions, it is possible to perform a mixture of the experiments described in Table 2 for different time windows, with the experimental parameters, such as the coUision energy, optimized for each analyte. 

In the context of chemometrics, optimization refers to the use of estimated parameters to control and optimize the outcome of experiments. Given a model that relates input variables to the output of a system, it is possible to find the set of inputs that optimizes the output. The system to be optimized may pertain to any type of analytical process, such as increasing resolution in hplc separations, increasing sensitivity in atomic emission spectrometry by controlling fuel and oxidant flow rates (14), or even in industrial processes, to optimize yield of a reaction as a function of input variables, temperature, pressure, and reactant concentration. The outputs ate the dependent variables, usually quantities such as instmment response, yield of a reaction, and resolution, and the input, or independent, variables are typically quantities like instmment settings, reaction conditions, or experimental media. 

A predictive macromolecular network decomposition model for coal conversion based on results of analytical measurements has been developed called the functional group, depolymerization, vaporization, cross-linking (EG-DVC) model (77). Data are obtained on weight loss on heating (thermogravimetry) and analysis of the evolved species by Eourier transform infrared spectrometry. Separate experimental data on solvent sweUing, solvent extraction, and Gieseler plastometry are also used in the model. 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

Busch, K. L. Getting a charge out of mass spectrometry. Spectroscopy, 9, 12,1994. Russell, D. H., Ed. Experimental Mass Spectrometry. New York Plenum Press 1994. 

The substituted radical cations [Me2S.. SMe2], [Et2S. .SEt2] and [Et2S.. SMe2] (Fig. 4) have been studied by lilies, McKee and co-workers using a combined experimental/theoretical approach [127-129]. Mass spectrometry experiments on the gas-phase association reactions... 

Table 5.3 Effect of experimental parameters on the LC-MS analysis of recombinant antibodies. Reprinted from J. Chromatogr., A, 913, Wan, H. Z., Kaneshiro, S., Frenz, J. and Cacia, J., Rapid method for monitoring galactosylation levels during recombinant antibody production by electrospray mass spectrometry with selective-ion monitoring , 437-466, Copyright (2001), with permission from Elsevier Science...

Method development is important. LC-MS performance, probably more than any other technique involving organic mass spectrometry, is dependent upon a range of experimental parameters, the relationship between which is often complex. While it is possible (but not always so) that conditions may be chosen fairly readily to allow the analysis of simple mixtures to be carried out successfully, the widely variable ionization efficiency of compounds with differing structures often makes obtaining optimum performance for the study of all components of a complex mixture difficult. In such cases, the use of experimental design should be seriously considered. 

When water undergoes self-ionization, a range of cationic species are formed, the simplest of which is the hydronium ion, HjO (Clever, 1963). This ion has been detected experimentally by a range of techniques including mass spectrometry (Cunningham, Payzant Kebarle, 1972), as have ions of the type H+ (HaO) with values of n up to 8. Monte-Carlo calculations show that HjO ions exist in hydrated clusters surrounded by three or four water molecules in the hydration shell (Kochanski, 1985). These ions have only a short lifetime, since the proton is highly mobile and may be readily transferred from one water molecule to another. The time taken for such a transfer is typically of the order of 10 s provided that the receiving molecule of water is correctly oriented. 

Using the same threshold ionization mass spectrometry setup, Perrin et al. [317] have measured the temporal decay of radical densities in a discharge afterglow. From these experiments the coefficient p for the radical SiH. has been determined to be 0.28, which is in agreement with already known results from other (indirect) experimental approaches [136,137,318]. For the Si2H5 radical is determined to be between 0.1 and 0.3. The coefficient p for atomic hydrogen on a-Si H lies between 0.4 and 1, and is thought to represent mainly surface recombination to H. ... 

From the beginning of the 1980s, some effective experimental approaches based on new principles have been invented for the study of interfacial reactions in solvent extraction chemistry. Recently, some methods were developed from our laboratory, the highspeed stirring (HSS) method [4,5], the two-phase stopped flow method [6], the capillary plate method [7], the reflection spectrometry [8], and the centrifugal liquid membrane (CLM) method [9]. 

Jenner GA, Foley SF, Jackson SE, Green TH, Fryer BJ, Longerich HP (1994) Determination of partition coefficients for trace elements in high pressnre-temperature experimental ran products by laser ablation microprobe-inductively conpled plasma-mass spectrometry (LAM-ICP-MS). Geochim Cosmochim Acta 58 5099-5103... 

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 6.8 Experimental set-up for FAB. After Claeys and Claereboudt [76], Reprinted from M. Claeys and J. Claere-boudt, in Encyclopedia of Spectroscopy and Spectrometry (J.C. Lindon, ed.), Academic Press, pp. 505-512, Copyright (2000), with permission from Elsevier...

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