Closed detector , definition

The first part of this book is dedicated to a discussion of mass spectrometry (MS) instrumentation. We start with a list of basic definitions and explanations (Chapter 1). Chapter 2 is devoted to the mass spectrometer and its building blocks. In this chapter we describe in relative detail the most common ion sources, mass analyzers, and detectors. Some of the techniques are not extensively used today, but they are often cited in the MS literature, and are important contributions to the history of MS instrumentation. In Chapter 3 we describe both different fragmentation methods and several typical tandem MS analyzer configurations. Chapter 4 is somewhat of an outsider. Separation methods is certainly too vast a topic to do full justice in less than twenty pages. However, some separation methods are used in such close alliance with MS that the two techniques are always referred to as one combined analytical tool, for example, GC-MS and LC-MS. In effect, it is almost impossible to study the MS literature without coming across at least one separation method. Our main goal with Chapter 4 is, therefore, to facilitate an introduction to the MS literature for the reader by providing a short summary of the basic principles of some of the most common separation methods that have been used in conjunction with mass spectrometry. 

In a traditional PTR-MS, the detector is a quadrupole and its mass resolution is rather limited, the ratios of mass to charge (tn/z) of the product ions cannot be served for a definite indicator of the identity of trace gases because numerous isomeric or isobaric compounds have the same or close molecular weight. In addition, mass overlap from probable fragmentation and cluster ions may be in operation. In particular, when a mixture containing unknown VOC components is investigated, it is inevitable to meet with a question how to identify the compounds. 

Fig. 1.20. Since the introduction of multislice detectors, it is a known issue that at the start and the end of each spiral scan, a region is irradiated for which no images can be reconstructed (red). That portion depends on the width of the detector and becomes more severe the wider the detector becomes and the shorter the scan region is. That problem can be overcome, but introducing a tube side collimator that continuously opens at the start of the scan and closes at the end of the scan. The Siemens SOMATOM Definition AS-i- is the first scanner offering that technology, which saves 10-25 % dose depending on the application. Typical dose savings using this technology are 10% for abdominal, 15% for thorax, 20% for head and 25% for cardiac examinations...

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