Sample transmission efficiency

The energies of the Auger electrons leaving the sample are determined in a manner similar to that employed for photoelectrons already described in chapter 2 Section 4. Modern instruments nearly always incorporate cylindrical mirror analysers (CMA) because their high transmission efficiency leads to better signal-to-noise ratios than the CHA already described. 

Polarization effects The transmission efficiency of a monochromator depends on the polarization of light. This can easily be demonstrated by placing a polarizer between the sample and the emission monochromator it is observed that the position and shape of the fluorescence spectrum may significantly depend on the orientation of the polarizer. Consequently, the observed fluorescence intensity depends on the polarization of the emitted fluorescence, i.e. on the relative contribution of the vertically and horizontally polarized components. This problem can be circumvented in the following way. 

Let Ix, Iy and Iz be the intensity components of the fluorescence, respectively (Figure 6.3). If no polarizer is placed between the sample and the emission monochromator, the light intensity viewed by the monochromator is Iz + Iy, which is not proportional to the total fluorescence intensity (Ix + Iy + Iz). Moreover, the transmission efficiency of the monochromator depends on the polarization of the incident light and is thus not the same for Iz and Iy. To get a response proportional to the total fluorescence intensity, independently of the fluorescence polarization, polarizers must be used under magic angle conditions (see appendix, p. 196) a polarizer is introduced between the excitation monochromator and the sample and... 

Attainable detection limits depend on the amount of analyte that enters the ICP per second, the efficiency of aerosol conversion into analyte ions in the ICP, and the transmission efficiency of ions from the plasma to the MS detector. The detection limits also depend on the variation of the background and the integration time. Typical pneumatic nebulizer/spray chamber systems operated at sample uptake rates from 0.1 to 2.0 mL/min introduce an amount of analyte equivalent to that in 10 to 30 JiL/min of sample solution into the ICP. At a sample uptake rate of 1 mL/min, only 1% to 2% of the analyte enters the plasma most of the sample is lost in the spray chamber and exits through the drain. Concentration based detection limits can be improved by approximately a factor of 10 by using a high-... 

Models [105,177] and experimental measurements [178-180] suggest that the most severe chemical matrix effects are due to space charge induced decreases in the ion transmission efficiency from the plasma to the detector of the mass spectrometer. Unlike the deposition effects, these depend only on the composition of the sample being introduced into the plasma, not on previously run samples. 

Another significant challenge remains in further increasing the efficiency with which sample species are utilized through increased ion throughput and duty factor. Currently many of the ions generated by continuous ionization sources are lost because of the pulsed nature of the instrument. Realization of higher duty factor and the unit transmission efficiency of which TOF-MS is capable could propel the TOF-MS into a sensitivity realm well beyond that of current mass analyzers. 

Improvements in the ionization/transmission efficiency directly result in improved counting statistics and/or smaller sample sizes. Such improvements generally benefit all of the sub-disciplines that utilize isotopic measurements, but hold especially important practical implications for dating slow-growth speleothems. 

Today, MALDI ion mobility MS instruments have longer drift cells and use both ultraviolet (UV) and infrared (IR) laser ionization. In addition, the longer drift cells have nonlinear electric fields, which help to focus the ions into the center of the tube and increase the transmission efficiency of the ion, reducing both resolving power and cross-section accuracy. MALDI ion mobility MS has been used for the analysis of very complex biological samples. Figure 9.5 is a typical MALDI ion mobility MS... 

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