Effect on the detection limit

S. Mathison and E. Bakker, Effect of transmembrane electrolyte diffusion on the detection limit of carrier-based potentiometric ion sensors. Anal. Chem. 70, 303-309 (1998). 

The scope of this article does not permit the consideration of physical vs. empirical models for the calibration curve, nor the effect of new designs on the detection limit, but these are extremely important issues in calibration. For example, it can be shown that with an inadequate design the detection limit (for a 3 0.05) may not even exist (xp - ,)... 

Minimum reportable concentration. The lower concentration limit for a method is usually measured by determining the detection limit. This is basically an instrument signal to noise ratio, and it does not include calibration effects. At low concentrations the calibration process often has a major adverse effect on precision. Detection limits are useful for comparing the inherent sensitivity of methods, but they are not realistic indexes of measurable concentrations in routine analysis. 

However, recent investigations on the effect of the tissue matrix on the detection limits attained by this test have indicated that ceftiofur, sulfonamides, streptomycin, and some macrolide antibiotics cannot be detected in intact meat with the plates and the bacterial strains prescribed in the European four-plate test (81, 82). Two plates of this system were not found suitable for screening sulfamethazine or streptomycin at levels far above the MRL the third plate detected tetracyclines and -lactams up to the MRL levels whereas the fourth was sensitive to -lactams and some but not all macrolides. Detection, on the other hand, of the fluoroquinolones enrofloxacin and ciprofloxacin could only be made possible by an additional Escherichia coli plate not included in the four-plate test. 

In flow analysis with spectrophotometric detection, the Schlieren effect is generally the ultimate limitation on the signal-to-noise ratio and hence on the detection limit. Furthermore, it can lead to systematic measurement deviations, thus degrading accuracy. 

Effects of the transmembrane flux on the detection limits were also studied theoretically (60). The phase boundary potential model in mixed ion solutions as discussed in Section 7.3.1 was extended by implementing the steady-state transmembrane flux to demonstrate that the detection limit of a highly selective electrode, Cj (DL), is given approximately as... 

Even if all possible measures to improve depth resolution are applied, there still exist the effects of the primary ion beam-induced damage experienced by the substrate as a result of the sputtering process. These are discussed in Section 5.3.2.4.1. Crater edge effects and crater base effects can also result in the loss of depth resolution. These are discussed in Sections 5.3.2.4.2 and 5.3.2.4.3, respectively. Dynamic range pertains to the range of concentrations of a specific element or molecule that can be examined in a particular depth profile. As can be envisaged, this depends on the detection limit and on the detector type or combinations thereof (detectors are covered in Section 4.2.3.3). 

The left side shows the mole fraction of primary ion I in the membrane phase boundary relative to its concentration when ion-exchange is absent. This ratio is a direct function of membrane selectivity and sample composition. The effect of ion-exchange on the detection limit of the corresponding electrode is shown in Fig. 9.19. Based on simple ion-exchange, half of the primary ions have been displaced at the detection limit. 

Calibration of an arc or spark source is linear over three orders of magnitude, and detection limits are good, often within the region of a few micrograms per gram for elements such as vanadium, aluminum, silicon, and phosphorus. Furthermore, the nature of the matrix material composing the bulk of the sample appears to have little effect on the accuracy of measurement. 

Samples to be examined by inductively coupled plasma and mass spectrometry (ICP/MS) are commonly in the form of a solution that is transported into the plasma flame. The thermal mass of the flame is small, and ingress of excessive quantities of extraneous matter, such as solvent, would cool the flame and might even extinguish it. Even cooling the flame reduces its ionization efficiency, with concomitant effects on the accuracy and detection limits of the ICP/MS method. Consequently, it is necessary to remove as much solvent as possible which can be done by evaporation off-line or done on-line by spraying the solution as an aerosol into the plasma flame. 

It is significant that in the absence of O2 (solid points in Figure 4) almost no radicals were formed the amount reported is close to the detection limit of the instrument. In one sense, this observation provides an explanation for the positive effect that O, has on the rate of reaction between NO and CH4 [3,4] i.c., O2 enhances CH,- radical formation. However, the results also indicate that NO itself is not very effective in generating active sites which are responsible for CH,- radical production. This means that the reaction of NO with CH4, in the absence of added O, may occur via a nonradical pathway. 

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