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The @E= constant linked scan is shown to be an excellent tool for quantifications with reversed geometry mass spectrometers. Deuterium-labelled analogues may be used as internal standards, thus providing very simple clean-up procedures. The principle is demonstrated with the example of chloro-substituted benzoic acid methyl esters. Possible interferences arising from isotope peaks are discussed. The method is applied to the quantification of caffeine in beverages.
The @E= constant linked scan is shown to be an excellent tool for quantifications with reversed geometry mass spectrometers. Deuterium-labelled analogues may be used as internal standards, thus providing very simple clean-up procedures. The principle is demonstrated with the example of chloro-substituted benzoic acid methyl esters. Possible interferences arising from isotope peaks are discussed. The method is applied to the quantification of caffeine in beverages.
Witness this army, of such mass and charge" Shakespeare, W.; Hamlet, 1601, Act IV, Scene 4, line 47. CCC 0277-7037/95/05/60359-52 LINKED-SCAN TECHNIQUES FOR MS/MS FIGURE 2. One-dimensional spectra for ions formed from decane, cxtracted from the complete two-dimensional MSiMS domain shown in Fig. I . Such one-dimensional spectra could be obtained by appropriate single scans, without obtaining all the information exhibited in thc full MS/MS domain. Full circles denote ions whose m i z values are fixed by the conditions of the experiment. and open circles denote ions whose m l i values are to be obtained from the appropriate one-dimensional scan. Reproduced from Ref. (2), by permission.appropriate kinds, without generating the complete MS/MS domain. This review is concerned with the experimental methods for generating such scans, using some common mass spectrometer designs. A more fundamental comment on maps like Fig. 1 concerns the significance of the observation. or not, of any particular reaction. Any such observation strictly refers only to the reaction timescale and other conditions pertinent to the particular apparatus in use. For example, the fact that the ion at m/z 43 was observed ( Fig. 2) to arise from the ion at m / z 85, as well as from the ion at m/z 71, does not necessarily imply that the direct (single-step) reaction (m/z 85+ 43) does occur. The only valid conclusion is that some ions of m/z 85, extracted from the ion source, can fragment to yield m/z 43, but whether or not all such events involve an intermediate at m/z 7 1 cannot be determined from these experiments alone. It is also important to note that the great majority of MS/MS experiments investigate fragmentation reactions (m, < m, , where m, is the molecular mass of species X ) . However, at the lower collision energies characteristic of quadrupole instruments, it is also possible to induce chemical reactions between the reactant ion and a suitable collision gas, such that m, > m,. LINKED-SCAN TECHNIOUES FOR MS/MSFIGURE 3. Schematic representations of the action of (a) magnetic and (b) electric sector fields. In each case. 0 is the subject slit (length dimension perpcndicular 10 the plane of the paper. closeable in the plane). The angular spreads of the ion beam in the plane (which is perpendicular to the magnetic field direction, but contains the electric field lines) are a,,, and a,, respectively. (a) F,,? F , , F2, and F3 are the respective direction focus points for ions of mass m, and velocity u , [Eq. (28)]. mass (m, + 8m) and velocity u , , mass m R , and velocity [Eq. (28)] ( u , + Su), and mass (m,< + Sm) and velocity ( u , + 6u). Thus. the differences between F,, and F,. and between Fz and F2, represent the effective mass dispersion of the magnet. The differences between F,, and F2, and between F, and F; reflect the velocity dispersion. (b) F,, is the direction focus point for ions with any combination of m and u consistent with a kinetic energy ke) [Eq. (30)]. and F, is thc direction focus point [Eq. (30)] for ions with a kinetic...
A new, commercially available digital linked scan unit mass display/macker is presented operating in real time in a mass range of 0.0 to 1638.3 u without the need for a data system. The prerequkites and limitations of successful linked scans with their consequences for the new design are discussed. Examples give evidence of the need for increased accuracy. INTRODUCI'ION ~~Linked scans and mapping techniques are well-known means of establishing the fragmentation pathway of ions outside the ion source. Parent ion search experiments at a given daughter ion mass m2 and daughter ion search experiments at a given parent ion mass ml have both been successfully used in mixture analysis and structure elucidation.' More recently, constant neutral loss scans have been introduced. They allow the search for ions fragmentating by loss of a given uncharged mass Am.2*3 These scans are especially useful in mixture analysis when looking for ions with identical functional group^.^ Thus, linked scans on double focusing sector type mass spectrometers now offer possibilities of using MS/MS techniques, which are typically performed by the more complex triple stage instruments. Limitations exist however, arising mainly from the parameters varied by the scans, the geometry of the mass spectrometer and the possible occurrence of artefact peaks.The mass number of singly charged ions rn* transmitted in standard in-focus spectra ( E = E,, V = V,) defines a convenient measure for the magnetic field B.If exclusively singly charged ions are assumed, the daughter ions of dissociations outside the ion source will only be transmitted by the magnetic sector at m* = m22/m1. Examples are the well-known metastable peaks arising from dissociations within the field free region (FFR) preceding the magnetic field. They are observed in single focusing and conventional geometry double focusing mass spectroscopy. In general m* is not sufficient to deduce m,, m2 and Am. The observation of better defined diffuse peaks is, however, possible with double focusing instruments, preferably by linked scans varying B and the electric sector voltage E simultaneously, according to suitable scan laws. The acceleration voltage V, thereby remains constant in order to avoid detuning of the ion source and unnecessary limitation of the range of observable m a~s e s .~ Daughter ions m2 arising from dissociations in the first field free region (FFR1) in between the accelerating field (ACCF) and the first sector field and, on instruments of reversed geometry, from dissociations within WR2, the region in between the two sector fields, are thereby detected. These ions are transmitted by the electrostatic analyser (ESA) with reduced E at E/E, = mJm, due to the loss of mass and hence of energy. The two instrument-independent quantities m" and E/E, characterize the location of diffuse peaks and allow the determination of m,, m2 and Am. The preselection of any of these three masses restricts the location of expected diffuse peaks to a corresponding line on the m*-E/Eo plane (Fig. 1). ...
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