Time Basis of Mass Spectral Data

Column-based chromatographic methods continuously elute sample into the ionization source of the mass spectrometer. For the most part, ionization sources also operate in a continuous fashion, generating an uninterrupted stream of ions for mass analysis. However, most mass analysers do not provide continuously measured mass dispersion. The mass range selected by the user is scanned. Usually, the mass analyser scans from a high mass to a low mass. At any given instant, ions of only one mass-to-charge ratio are passed through the mass analyser to the detector, which registers the arrival of the mass-selected ion as an electrical signal. Ions that do not possess the 'correct' mass-to-charge ratio at that particular instant follow a flight path that brings them into collision with some part of the instrument. The ions that are not selected are neutralized at the surface, and the deposited material eventually desorbs from the surface, diffuses through the vacuum, and is removed from the instrument by the vacuum pumps. The consequence of analyser scanning is that only a small fraction of the molecules that are ionized in the source proceed successfully through the mass analyser to the detector to produce a measured signal. It is the extraordinarily high gain (106-108) available with modern electron multiplier detectors that compensates for the inherent 'transmission inefficiency' associated with a scanning mass analyser.

Therefore, in a hyphenated chromatography-mass spectrometry method, the time behaviour of the data as established by the chromatography is convoluted with the time behaviour of the data as established by the operation of the mass spectrometer. The latter is usually determined by scanning, but in the past was sometimes delineated by how fast the data could be captured and stored. In the ideal analytical design, the chromatography time base would predominate, and the time evolution of the data would be biased at least 100 : 1 in favour of the chromatographic determinant. In practice, the ratio is much closer to 10 : 1, and the time bases for operation of the mass spectrometer should be of constant concern in analysis of the data.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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