Mass sensitivity overload

From a practical point of view it is better to use the maximum volume of sample possible, assuming there is no mass overload. This will allow the detector to be operated at the lowest possible sensitivity and in doing so provide the greatest detector stability and, as a consequence, the highest accuracy. 

Ion trapping devices are sensitive to overload because of the detrimental effects of coulombic repulsion on ion trajectories. The maximum number of ions that can be stored in a QTT is about 10 -10, but it reduces to about 10 -10 if unit mass resolution in an RF scan is desired. Axial modulation, a sub-type of resonant ejection, allows to increase the number of ions stored in the QIT by one order of magnitude while maintaining unit mass resolution. [160,161] During the RF scan, the modulation voltage with a fixed amplitude and frequency is applied between the end caps. Its frequency is chosen slightly below V2 of the fundamental RF frequency, because for Pz < 1, e.g., = 0.98, we have z = (0 + 0.98/2) = 0.49 x... 

The piezoelectric microbalance is veiy sensitive, capable of detecting 10 8-10-9 g. The particles collected on the ciystal surface can be chemically analyzed after collection using surface-sensitive techniques. One limitation is possible overloading of the ciystal thus when the collected mass reaches 0.5-1% of the mass per unit of the ciystal, the surface must be cleaned. 

Often a large sample injection (e.g., 200 juJL) will experience only a little additional band broadening over that which would occur for a small injection (e.g., 10 (jlL). Thus, the large injection can result in a higher concentration for detection with no loss in resolution. (This assumes there is no mass overload, which is often a valid assumption in situations where insufficient sensitivity occurs due to very small concentrations of the sought-after substance. However, experimentally this assumption should be verified for each situation.) Also, improved chromatographic precision can result if there is smaller uncertainty of measurement with the larger volume injection. 

The main advantage of OT CEC is that separation efficiency can be doubled using this type of column. The trade-off is that the OT columns can easily be overloaded and therefore require a sensitive detection system. The small diameter of these columns precludes the use of UV detection, and fluorometric detection or mass spectrometry (MS) needs to be used. The use of fused silica capillaries with a bubble cell at the detection window has been reported as an alternative to employ UV detection. This features limit, to a certain extent, the range of practical applications of OT CEC. 

Mass overload occurs when the stationary phase does not have the capacity to retain the amount of sample injected. This can occur even for small injection volumes if the concentration of sample is high enough. This results in a characteristic shark-fin peak shape, where peak tailing starts from the peak s apex. For example, in order to obtain sufficient sensitivity, analytes with weak UV molar absorptivity may require a large enough amount of sample to be injected that the stationary phase becomes overloaded. Injecting less amount of sample, either by a smaller injection volume or by diluting the sample, can solve the problem of mass overload. However, sensitivity will decrease in this case. 

The understanding of the effects of sample concentration (sample mass) in field-flow fractionation (FFF) has being obtained gradually with the improvement of the sensitivity (detection limit) of high-performance liquid chromatography (HPLC) detectors. Overloading, which was used in earlier publications, emphasizes that there is an upper limit of sample amount (or concentration) below which sample retention will not be dependent on sample mass injected into the FFF channels [1]. Recent studies show that such limits may not exist for thermal FFF (may be true for all the FFF techniques in polymer separation), although some of the most sensitive detectors on the market were used [2]. 

Inductively coupled plasma mass spectrometry has several inherent advantages over conventional detection techniques, including its sensitivity and element specificity. However, IC with inductively coupled plasma mass spectrometry was not used for the analysis of iodide and iodate in seawater, except in a very recent report (Chen et al.y 2007). This could be because the high concentration of matrices in seawater led to loss of sensitivity from the build-up of salts on the sampler and skimmer cones of the mass spectrometer. In addition, chloride interfered with eluting very near to iodate because the column was overloaded when the chloride concentration was very high in seawater. Recently, a nonsuppressed IC with inductively coupled plasma mass spectrometry was developed for simultaneous determination of iodate and iodide in seawater (Chen et al., 2007). An anion-exchange column... 

Mass overload. This can be found if one or more constituents of the ple are present in much larger amounts than the analytes of interest An aple of this is the analysis of impurities and degradation products in euticals, which must be quantitatively analyzed if present at levels, to 0.1% of the parent compound. As one is injecting larger and larger ounts to increase the sensitivity for the contaminants, the peak for the Dt compound may become broader as a result of mass overload and start (Interfere with the quantitation of the contaminants. 

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