Detector saturation

If, e.g., during arunning experiment the danger of detector saturation is observed, the experienced user may broadcast a command to the detector (by writing into a pipe) change the exposure but keep the cycle time constant. 

The use of the volatile salt CH3CO2NH4 (1 mM) for controlling adduct formation showed similar improvements to the spectra (data not shown), whilst its relative volatility minimised the negative aspects of salt addition in ESI-MS (i.e. detector saturation and precipitation on the sampling orifice) [29],... 

Linear Range The concentration range where increasing concentrations of an analyte have a proportional increase in LC-MS response. Overall QqQ-type mass spectrometers (triple quadmpoles, Q-TRAPS) are superior in terms of linearity. Most common causes for nonlinear response include MS detector saturation, dimmer/adduct formation, API droplet/vapor saturation at high concentrations, and space charge effects. 

The non-linear calibration models are also acceptable. Figure 4.6 shows an example of a non-linear calibration approximated with a quadratic fit. Non-linear calibration curves are not acceptable if used to compensate for the detector saturation at a high concentration level. 

Very low RNA concentrations (<0.1 O.D.) produce low signal to noise while at high RNA concentrations (>1 O.D.) the detector saturates and does not yield accurate data. At higher concentrations, RNA molecules... 

There are a variety of other factors that influence the accuracy of quantitative analysis. Noise, in the form of baseline disturbances and baseline drift, affects area more than it does height, as it can cause area to be lost at the tailing edges of the peaks where they are widest. Peak asymmetry and detector saturation or nonlinearity, however, have a more detrimental effect on peak height. Figure 7.6 shows a calibration curve comparing peak height measurements with peak area measurements.13... 

Figure 7. EE and PIE accompanying the fracture of sodium trisilicate glass in three point bend. Fracture occurred at time t = 0. Both emissions show evidence of detector saturation at fracture. (Reproduced with permission from Ref. 13. Copyright 1988 American Vacuum Society.)...

An optical consideration specific to the use of the TOF-MS with high-intensity sources is the removal of background ions, plasma gas ions, and matrix ions to prevent detector saturation. To date, this has been accomplished with parallel-plate deflection in the flight path, which is depicted in Fig. 12.6. Removal of specific m/z ions is accomplished by the application of a time-dependent potential to one, or both, of the plates at some time delayed with respect to the repeller pulse. In this way, those ions between the plates at the time of the pulse experience a field transverse to the flight axis and are removed from the flight path. The voltage pulses employed here must have fast rise and fall times (<10-20 nsec), and be applied at precise delay intervals to ensure that mass resolution is not compromised, and to allow for the unimpeded passage of the previous and subsequent masses. Also, the... 

This design also utilized two sets of beam steering plates. One set, labeled D1 and D2 in Fig. 12.9, constitute a pair of deflection plates employed to remove Ar+ from the extracted ion packets. A 230-V pulse of variable duration delayed from the repeller pulse eliminated more than 99% of these ions and prevented detector saturation. 

Figure 1 gives a schematic drawing of the basic setup used in the GISAXS experiments. The two-dimensional detector is only recording the intensity reflected above the sample surface. The direct beam is not recorded with the detector to avoid detector saturation as several orders of magnitude in intensity separate the incoming intensity from the reflected one. In addition the specularly reflected peak (condition af = at) is shielded with a beam stop to... 

Another approach, which is used in this experiment, is to develop the analytical separation on the high-surface-area packing and increase the amount injected into the column to determine the loading level for preparative work. A common problem in preparative LC is detector saturation. Detector saturation occurs when the concentration of sample eluting from the column is so high that the detection system is electronically overloaded. The result of detector saturation is loss of the ability to observe the peaks. This is demonstrated in this experiment when the spectrophotometer is saturated, and the refractive index detector is not. 

Linear dynamic range the concentration range of the sample that is detectable and where the detector response is linear (between the MDL and detector saturation). 

Rux is much less of a problem at X-ray sources indeed sometimes it needs to be reduced to avoid detector saturation. The whole field of protein crystallography is heavily dependent on the use of synchrotron sources, an enormous area of application which has made a spectacular impact - and no doubt will continue to do so. The high flux additionally means that the application of real time studies is much further advanced for both the wide and small angle regimes. Additionally, X-rays are much more easily focussed, and can therefore be used at high spatial resolution to produce microfocus data. For biological systems this has already led to some interesting studies, e.g. on starch " and flax. ... 

If desired, or required for a particular application, repeat step 4 for a range of integration times, from near the detection limit to near detector saturation. The detector response should be linear with integration time. For an FT-Raman system, a similar test may be performed by varying the laser power to check photometric linearity. 

Adding too much or too little IS can also limit the dynamic range of the assay, as the comparison of very large ion currents (detector saturation) with very small ion currents (poor ion statistics) will greatly increase the variance of the assay. A good guide is a threefold excess of the IS over the analyte but this may take a few trials to establish. 

Practical understanding and appreciation of detector saturation limits are critical when using any of the aforementioned detectors. The saturation limit can be defined as the point at which a nonlinear detector response is observed with an increase in ion concentration. The saturation characteristics of a detector can be proportional to concentration up to a critical point, at which no additional signal can be obtained, or can be nonlinear, where detector response changes with concentration but in a nonlinear way. 

Detector saturation can effect both quantitative and qualitative data analysis, and each of these effects should be appreciated. The effect on sample quanti-tation is intuitive, where for instance a twofold increase in sample concentration produces a less than twofold increase in response. This will cause a flattening of calibration curves at higher concentrations. For API techniques, source saturation (or ion suppression) is another source of response saturation independent of detector saturation. Detector saturation can also effect qualitative measurements such as mass accuracy and isotope ratio calculations. In the former, when a mass spectral peak that has some finite resolution stalls to saturate the detector the peak-top calculations that provide the m/ measurement of the peak will become ambiguous. Likewise, it is possible that as one isotope of an ion starts to saturate the detector, adjacent isotopes in the distribution will still provide a linear response. The result of this is that incorrect isotope ratios will be obtained. Changes in relative isotope ratios of individual spectra across a chromatographic peak is an indicator of possible detector saturation. 

Figure 6.11. An example of a composite assay (combining both assay and impurity testing in one method) for a drug substance in early development. Note that the absorbance of the API must be <1.5 absorbance units (AU) to prevent detector saturation. Since the method has some deficiencies (e.g., partial resolution between several peaks), an improved gradient method was thus developed (Chapter 8).

---