Lead, analytical contamination sources

The prerequisite for implementing MS in biochemical analysis is that the analytes are amenable to ionization using one of the available ion sources (cf. Chapter 2). A vast majority of biochemical processes occur in the condensed phase. Most enzymes are active in aqueous solutions while only a few of them can catalyze reactions in organic solvents (e.g., transesterification catalyzed by lipase [5]). Water-based buffers are not always the optimum solvents for the operation of ion sources. In fact, concentrated buffer solutions - which are often used in biochemical assays - can suppress ionization, and lead to contamination of... 

The ability to provide data on the isotopic composition of analyte elements, is a further special feature of the technique. This has already been exploited in a number of ways, including geological dating and the identification of sources of environmental lead contamination and poisoning. The radiogeneric source of lead means that different deposits may have different isotopic composition. In the UK this feature has been used to... 

The most straightforward tool for the introduction of a sample into a mass spectrometer is called the direct inlet system. It consists of a metal probe (sample rod) with a heater on its tip. The sample is inserted into a cmcible made of glass, metal, or silica, which is secured at the heated tip. The probe is introduced into the ion source through a vacuum lock. Since the pressure in the ion source is 10-5 to 10-6 torr, while the sample may be heated up to 400°C, quite a lot of organic compounds may be vaporized and analyzed. Very often there is no need to heat the sample, as the vapor pressure of an analyte in a vacuum is sufficient to record a reasonable mass spectrum. If an analyte is too volatile the cmcible may be cooled rather than heated. There are two main disadvantages of this system. If a sample contains more than one compound with close volatilities, the recorded spectrum will be a superposition of spectra of individual compounds. This phenomenon may significantly complicate the identification (both manual and computerized). Another drawback deals with the possibility of introducing too much sample. This may lead to a drop in pressure, ion-molecule reactions, poor quality of spectra, and source contamination. 

Trace analysis has its special hazards for the unwary. The most important of these are loss of material in the analytical process and contamination by outside sources. Everyone realizes that trace constituents can be lost from samples, but few are aware of the many ways in which this can occur. For example, phosphate has been observed to disappear mysteriously from water samples in polyethylene bottles (10). Nitric acid, used to clean plastic vials, has been observed to convert these surfaces to ion exchangers, which readily take up as much as 10 12 moles per sq. cm. of trace metals (16). Lead nitrate solutions unless made distinctly acidic, plate out much of the lead on the walls of glass bottles. While everyone realizes that formation of a precipitate is liable to carry out trace constituents either by adsorption or occlusion, it is not as well-known that vanishingly small amounts of precipitates—amounts likely to be overlooked on casual observation—may also do this. The fly-ash and soot, which seem to be inescapable components of city air,... 

Matrix components are not efficiently removed with protein precipitation and will be contained in the isolated supernatant or filtrate. In MS/MS detection systems, matrix contaminants have been shown to reduce the efficiency of the ionization process with atmospheric-pressure ionization (API) techniques [3-12]. The observation seen is a loss in response, and this phenomenon is referred to as ionization suppression. This effect can lead to decreased reproducibility and accuracy for an assay and failure to reach the desired limit of quantitation. Additionally, the efficiency of protein removal witli organic solvents is not complete and typically ranges from 98.7% to 99.8% [2] to leave residual amounts of protein that carry over into the analytical system and foul the ionization source of a mass spectrometer after repeated injections. 

Lead isotope ratios can provide analytical information about sources of lead contamination whereas lead concentration measurements cannot. Due to this fact, studies of the isotopic composition of lead are commonly used in environmental science as well as in geological and anthropological studies. Among all of the naturally occurring lead isotopes only °" Pb is nonradiogenic, whereas ° Pb, ° Pb, and ° Pb are daughter products from the radioactive decay of and Th, respectively. This fact... 

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