Analysts atmosphere

It is necessary to draw attention to the variable pH of water which may be encountered in quantitative analysis. Water in equilibrium with the normal atmosphere which contains 0.03 per cent by volume of carbon dioxide has a pH of about 5.7 very carefully prepared conductivity water has a pH close to 7 water saturated with carbon dioxide under a pressure of one atmosphere has a pH of about 3.7 at 25 °C. The analyst may therefore be dealing, according to the conditions that prevail in the laboratory, with water having a pH between the two extremes pH 3.7 and pH 7. Hence for indicators which show their alkaline colours at pH values above 4.5, the effect of carbon dioxide introduced during a titration, either from the atmosphere or from the titrating solutions, must be seriously considered. This subject is discussed again later (Section 10.12). 

If a high -molecular-weight compound is being studied by LC-MS, the analyst has little choice in the ionization method to use, with atmospheric-pressure chemical ionization (APCl) being wholly inappropriate. However, when low -molecular-weight componnds are involved, both electrospray ionization and APCl are potentially of value. 

A. L. Gray. Mass-Spectrometric Analysis of Solutions Using an Atmospheric Pressure Ion Source. Analyst, 100(1975) 289-299. 

Daniel JM, Ehala S, Eriess SD, Zenobi R. 2004. On-line atmospheric pressure matrix-assisted desorption/ionization mass spectrometry. Analyst 129 574. 

Fenn published work in 1989 [7-9] showing ionisation of large molecules by electrospray ionisation (ESI). Fenn built on the early work of Malcolm Dole [10] but Fenn used a counter current gas to assist with desolvation of the droplets and aid the formation of the ions. In the early 1990s, experiments with atmospheric pressure ionisation (API) showed promise and in a short space of time the first commercial systems utilising the new techniques of ESI [11] and Atmospheric Pressure Chemical Ionisation (APCI) began to appear on analysts benches. The sensitive, reliable and easily operated LC-MS system had arrived. 

Six atmospheric issues are currently being assessed by scientists, modelers, and policy analysts ... 

The optical properties of the components of petroleum have been of major importance in connection with their identification and in the determination of purity. The primary effort has been directed to the study of pure hydrocarbons and only limited work has been concerned with the prediction of the index of refraction and the specific rotation of hydrocarbon mixtures. Table V summarizes the optical properties of a number of the principal components of petroleum. Only a few references to the optical properties of pure hydrocarbons of primary interest to the analyst have been included. Developments (9) in refractometers have materially increased the potentialities of the index of refraction measurements at atmospheric pressure as an analytical method. Consideration of the pertinent data in this field is beyond the scope of the present discussion. Reviews of developments in infrared (24, 26) and mass spectrometry (68) are available. 

A large proportion of atmospheric samples will be obtained in containers similar to those shown in Figure 7.1. It should be recognized that these samples may have been taken under circumstances in which the temperature of the sample was much higher than the laboratory in which analyses are to be made. An immediate result of this would be that the sample contained, as available to the analyst, is present at an unknown pressure less than atmospheric. In addition, some samples are taken by using evacuated containers, then opening them at the site to transfer sample to the container. If the container was not opened for a sufficient period of time, the contents may still be below atmospheric... 

B Delephine, D Hurtaud-Pessel, P Sanders. Simultaneous determination of six quinolones in pig muscle by liquid chromatography-atmospheric pressure chemical ionization mass spectrometry. Analyst 123 2743-2747, 1998. 

The use of liquid chromatography-mass spectrometry (LC-MS) is becoming more popular because of the increasing number of LC-MS interfaces commercially available thermospray (TSP), particle beam (PB), and atmospheric pressure ionization (API). Coupled with mass spectroscopy, HPLC provides the analyst with a powerful tool for residue determination. 

Recent advances in electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), thermospray, and particle beam LC-MS have advanced the analyst toward the universal HPLC detector, but price and complexity are still the primary stumbling blocks. Thus, HPLC-MS remains expensive and the technology has only recently been described. Early commercial LC-MS uses particle beam and thermospray sources, but ESI and APCI interfaces now dominate. Liquid chromatography MS can represent a fast and reliable method for structural analyses of nonvolatile compounds such as phenolic compounds (36,37), especially for low-molecular-weight plant phenolics (38), but the limited resolving power of LC hinders the widespread use of its application for phenolics as compared to GC-MS. 

Contamination problems can arise during the preparation and analysis of soils. Sources of trace elements can be atmospheric dust contamination after the initial sample was taken, laboratory equipment, adventitious contaminants such as cosmetics, and reagents used during the analysis. The analyst should take suitable precautions to reduce these to a minimum. Some of these problems have been reviewed by Mitchell [18]. 

In 1995, Taylor and co-workers also described the use of an open-access LC/MS system for routine structure confirmation, featuring atmospheric pressure chemical ionization (APCI). This system featured dual personal computers (PCs) for automated instrument control and sample log-in. A system-PC is responsible for running the Windows NT for Workgroups operating system and interfaces with the network for instrument control. A separate log-in PC, isolated from the LC/MS system, is used by the synthetic chemist to enter details about the samples. The analyst prepares the sample in an autosampler vial in one of several solvent options. The system specifies where to place the sample vial in the autosampler, and following analysis with a standard method, spectra are automatically processed and printed without any chemist intervention. 

Bailey, J. B. W., Brown, N. E., Phillips, C. V. A method for the determination of carbon monoxide, carbon dioxide, sulphur dioxide, carbonyl sulphide, oxygen and nitrogen in furnace gas atmospheres by gas chromatography. Analyst 96, 447 (1971). 

The reaction of an aryl diazonium halide with an aliphatic unsaturated compound to yield an a-halo-P-phenyl alkene and alkanes. The reaction is performed in the presence of cupric ious. The presence of an electron-withdrawing group is useful in promoting the reactivity of the alkene. See Kochi, J.K., The Meerwein reaction. Catalysis by cuprous chloride, J. Am. Chem. Soc. 11, 5090, 1955 Morales, L.A. and Eberlin, M.N., The gas-phase Meerwein reaction, Chemistry 6, 897-905, 2000 Riter, L.S., Meurer, E.C., Handberg, E.S. et al, lon/molecule reactions performed in a miniature cylindrical ion trap mass spectrometer. Analyst 128, 1112-1118, 2003 Meurer, E.C., Chen, H., Riter, E.S. et al., Meerwein reaction of phosphonium ions with epoxides and thioepoxides in the gas phase, J. Am. Soc. Mass Spectrom. 15, 398 05, 2004 Meurer, E.C. and Eberlin, M.N., The atmospheric pressure Meerwein reaction, J. Mass Spectrom. 41, 470-476, 2006. 

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