More advanced mass-spectrometric techniques

In some applications, determination of the mass of the ions leaving the source is not sufficient to provide the required confidence in the assignment of the spectrum. Extra information or specificity can be obtained by making use of ions that decompose after leaving the spectrometer source. In a double focusing instrument of the type 

A mass spectrometer containing a single quadrupole mass filter cannot be used to perform this type of experiment. However spectrometers consisting of three quadrupoles in series (triple quadrupole mass spectrometers) are available for this purpose. The first quadrupole is used to select the parent ion, fragmentation takes place in the second quadrupole and daughter ions are selected by the third quadrupole. 

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Innovations in the held of mass spectrometry are constantly being developed. Often they are initially investigated for basic research and then eventually transferred to more applied areas such as the analyses of contaminants in food. Several advanced mass spectrometric techniques are described in this section, along with possible applications to residue analysis. 

The availability of mass spectrometric techniques (such as electrospray) for observing molecular ions from intact enzymes is an important advance in the study of the covalent chemistry of proteins. Although the use of radioactively labelled molecules has previously allowed the average level of protein modification to be deduced, mass spectrometry is much quicker, more convenient and safer it also shows the distribution between the various multiply modified species and, most importantly, gives the molecular weight of the modified species, allowing deductions to be drawn as to the nature of the reaction occurring. 

There have recently been significant advances in the technique of carbon 14 measurements, which have permitted the determination of the concentration ratio C14/C12 in samples of small size. Two developments have occurred the first is an entirely new mass-spectrometric separation of Cl4 and Cl2 ions and their subsequent estimation by counting [1-8]1, while the second is simply the extension of conventional proportional counter operation to very small size carbon samples [9]. The first method is very fast, precise, and capable of treating samples of even submilligram size, but requires an expensive installation. The second method is slow (counting times of two months or more are necessary), can probably be made sufficiently precise to handle most problems, works down to sample sizes of 10 mg carbon, and is relatively inexpensive, especially to install in already-existing radiocarbon laboratories. 

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