Mass triangular shaped peaks

The first symptom of mass overload is seen as a broadening of the chromatographic peak as the mass of sample is increased. This is measured as a lowering of the efficiency (reduction in the number of theoretical plates) and increase in peak asymmetry, but as mass load is increased it often results in triangular shaped peaks which show typically a peak maximum at a reduced retention time and a tail which extends to the retention time of a peak resulting from an analytical load. Other, much more bizarre peak shapes can also be found. These represent cases where special interactions between the solute molecules and the stationary phase, the mobile phase or each other occur. 

The spectra obtained for ice Ih, LDA and HDA, using the TFXA spectrometer at 10K [53] is shown in Fig. 11. Ice Ih is the most common and readily obtainable phase of ice which has now been well studied [14,15,48,49]. Its spectrum has a very simple structure, the translational modes below 40 meV are well separated from the librational modes (or hindered rotations) in the energy region between 65-125 meV (very few system shows similar behaviour and this is due to the large mass difference between O and H). The observed acoustic phonon peak is at 7 meV. The two sharp peaks at 28 and 37 meV are the optic-phonon bands and have an unusual triangular-shape. In contrast, only a single feature appears in the IR spectrum, at 27 meV, and the Raman spectrum has an additional shoulder at 36 meV (see Fig. 10). 

In a well-tuned (adjusted) instrument, the shape of a mass spectral peak is approximately triangular (Figure 44.7a), but, in an instrument that is poorly tuned the peak will appear misshapen (Figure 44.7b). Usually, the cause of the skewing of the peak arises from incorrectly adjusted... 

Knox and Piper (13) assumed that the majority of the adsorption isotherms were, indeed, Langmuir in form and then postulated that all the peaks that were mass overloaded would be approximately triangular in shape. As a consequence, Knox and Piper proposed that mass overload could be treated in a similar manner to volume overload. Whether all solute/stationary phase isotherms are Langmuir in type is a moot point and the assumption should be taken with some caution. Knox and Piper then suggested that the best compromise was to utilize about half the maximum sample volume as defined by equation (15), which would then reduce the distance between the peaks by half. They then recommended that the concentration of the solute should be increased until dispersion due to mass overload just caused the two peaks to touch. 

In quadmpole and ion trap spectrometers, it is customary to use the unit resolution definition, which means that each mass can be separated from the next integer mass (500 from 501, 2000 from 2001, etc.). The comparison and conversion between the different resolution definitions is not trivial, considering that they are strongly dependent on peak shape (Gaussian, triangular, trapezoidal, Lorent-zian, etc.).9... 

If the sample size is increased, the shape of the peaks changes to rectangular (in the case of volume overload) or triangular (with mass overload) mixed forms and distorted peak shapes are also observed. Displacement effects can occur where a compound is pushed and concentrated by a following one that has a stronger affinity to the stationary phase. 

Increases in mass resolution are not only accortpanied by a loss in ion transmission efficiency and hence signal intensity (Figure 2.15), but also by a change in peak shape from flat topped to triangular (Figure 2.14(b)).Moreover, the extremely narrow peaks at high mass resolution require an absolutely stable mass calibration (especially for isotope ratio applications). 

This form of the isotherm results in a linear rate of change of the capacity factor with mobile phase concentration and gives rise to a truly triangular peak shape. Knox also concluded that the efficiency of a peak under mass overloaded conditions... 

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