Sensitivity Considerations

Two-dimensional NMR experiments are sometimes thought to be much less sensitive than their one-dimensional analogs. However, the sensitivity of 2D experiments can be quite high with well-executed experimental procedures. In this section we discuss briefly several of the general factors that determine 

In a regular ID FT NMR experiment, the signal-to-noise ratio for a single time domain data point can be very poor, but the Fourier transformation takes the signal energy of all data points and usually puts it into only a few narrow resonance lines. Similarly, in a 2D experiment a spectrum with poor S/N may be recorded for each q value, but the Fourier transformation with respect to q combines the signal energy of a particular resonance from all spectra obtained for different q values and concentrates it into a small number of narrow lines in the 2D spectrum. 

As we have seen in the examples in Section 10.2, most 2D experiments involve transfer of polarization or coherence, sometimes in multistep processes in which the efficiency of each is far less than 100%. Thus, great care must be used in designing such experiments to craft each step carefully to optimize the final signal. Also, extensive phase cycling that is required in some multidimensional NMR experiments extends the minimum time required for the study. If signals are weak and extensive time averaging is needed anyway to obtain adequate 

In EPR, like H NMR, signal intensity is directly proportional to the number of unpaired electrons giving rise to the signal. However, we learned in Chapter 2 that different isotopes have different inherent sensitivity toward generating an NMR signal (Table 2.1). In addition, we saw how the intensity of an NMR signal is very sensitive to saturation effects because of the small differences in spin state populations. Do you expect saturation problems to be as important a consideration in EPR  

This ratio indicates that at equilibrium 50.0379% of the electrons are in the lower (ms = -j) state, while 49.9621% are in the upper state. Or, to put it another way, of one million electrons, there are 758 more in the lower state than in the upper state. Although this may seem like a very small difference, it is 38 times greater than the difference in proton spin state populations, a difference of only 20 nuclei per million at 2.35 T (Example 2.8)  

The fact that the population difference between spin states is greater for electrons than for nuclei means that EPR spectroscopy is much more sensitive than NMR. Thus, while modern NMR methods (Section 3.3) still require sample concentrations of at least 0.01 M (moles per liter), EPR signals can be detected from radicals in as low a concentration as 10-8 M at room temperature (10 12 mol in a sample volume of 0.1-0.2 mL). 

In Chapter 1 we discussed the concept of spectroscopic time scale. Because EPR involves frequencies on the order of 109 Hz (and a resulting time scale of 10-9 s), it takes a much faster snapshot of dynamic systems than does NMR. As a result, EPR can generate information about chemical processes that are too fast to study by NMR. 

Especially notable are nitroxide radicals such as 11-1. Molecules of this class are sufficiently stable to survive a variety of chemical reactions, allowing them to be covalently bonded to other molecules (e.g., biopolymers such as proteins and nucleic acids). They can then serve as spin labels, transmitting information (via their EPR spectra) about the molecules to which they are attached. 

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One type of ec detector (the coulometric detector) reacts all of the electroactive solute passing through it. This type has never become very popular (there is only one on the market at the moment). Another type (the amperometric detector) reacts a much smaller quantity of the solute, less than 1%. The currents observed with these detectors are very small (nanoamps), but such currents are not too difficult to measure and the detector has a high sensitivity, considerably higher than that of uv/visible absorbance detectors although not as good as fluorescence detectors. Noise equivalent concentrations of about 10, 0g cm-3 have been obtained in favourable cases. Another advantage of these detectors is that they can be made with a very small internal volume. 

Common colds can influence sensitivity considerably as we all know, but other illnesses which are accompanied with a change of body temperature may act in this respect as well. Since the nose has a very generous bloodsupply, changes in this supply may influence olfaction also. 

By modulating the electric field and using phase-sensitive detection methods, Uehara et al. 8 ) were able to increase the sensitivity considerably and they could even detect Stark splittings of less than the doppler width of the components. Fig. 3 shows the Stark spectrum of HDCO for different electric field strengths. Because of the Stark modulation technique the absorption lines appear differentiated the zero points represent the center of each line. 

Lietal [42] demonstrated that, whereas AAS is usually insensitive to organotin compounds, the addition of tributyl phosphate enhances sensitivity considerably. Tributyltin at 1000 °C converts organotin to SnP207 and Sn2P2C>7. 

The next most important factor is to bring the capacity factors into the optimum range. At the same time or immediately thereafter, we should try to optimize the selectivity (a). Both are very important stages in the method development process, because no separation will be obtained if either k=0 or a= 1 (see section 1.5), no matter how efficient the column and how good the instrument. Very large k values should also be eliminated at this stage, because of both time and sensitivity considerations (see e.g. figure 6.1b). 

The acid-free product is very stable, but exceedingly sensitive to impact. The transportation of nitroglycerine and similar nitrate esters is permitted only in the form of solutions in non-explosive solvents or as mixtures with fine-powdered inert materials containing not more than 5% nitroglycerine. To avoid dangers, internal transport within the factories is made by water injection (- Water-driven Injector Transport). Transport of pure nitroglycerine and similar products outside factory premises is difficult in the U.S., special vessels have been developed in which the oil is bubble-free covered with water without air bubbles which raise the impact sensitivity considerably. The nitroglycerine produced is ideally processed immediately to the products (e.g., explosives double base powders). 

Thus the concentration of anion A in an elution peak reduces the benzoate concentration by an equivalent amount. However, the eluent equilibrium is dynamic, and it is always 20 % ionized. This means that 80 % of the hydrogen and benzoate ions in the exchange reaction come from molecular benzoic acid and are converted into highly ionized H A. This effect enhances the detection sensitivity considerably because the counterion of the A has an unusually high equivalent conductance. A discussion of the phenomenon is given in Section 6.3.3.6. 

Garrett, P.E., and Krouwer, J.S. (1985) Method evaluation II precision and sensitivity considerations. Journal of Clinical Immunoassay, 8, 165 168. 

From a practical point of view this equation is still not very helpful as Ci depends on conversion by being proportional to the critical tube diameter. Sensitivity considerations on the influence of the safe distance between true and critical tube diameter on Cl and on the Ci dependent term of Equ.(4-148), however, show, that a further simplification is justified. These considerations are presented graphically in Fig. 4-37. 

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