Sample signal losses

The magnitude of a method s relative error depends on how accurately the signal is measured, how accurately the value of k in equations 3.1 or 3.2 is known, and the ease of handling the sample without loss or contamination. In general, total analysis methods produce results of high accuracy, and concentration methods range from high to low accuracy. A more detailed discussion of accuracy is presented in Chapter 4. 

Normal geometry Term used to describe the construction of a conventional dual/multi channel probe. Since the X nucleus is a far less sensitive nucleus than 1H, a normal geometry probe has the X nucleus receiver coils as close to the sample as possible to minimise signal loss and the XH receiver coils outside the X nucleus coils (i.e., further from the sample). This design of probe is thus optimised for X nucleus sensitivity at the expense of some XH sensitivity. 

FAB ionization has been used in combination with LC/MS in a technique called continuous-flow FAB LC/MS (Schmitz et al., 1992 van Breemen et al., 1993). Although any standard HPLC solvent can be used, including methyl-ferf-butyl ether and methanol, the mobile phase should not contain nonvolatile additives such as phosphate or Tris buffers. Volatile buffers such as ammonium acetate are compatible at low concentrations (i.e., <10 mM). Continuous-flow FAB has also been used in combination with MS/MS (van Breemen et al., 1993). The main limitationsof continuous-flow FAB compared to other LC/MS techniques for carotenoids, such as ESI and APCI, are the low flow rates and the high maintenance requirements. During use, the 3-nitrobenzyl alcohol matrix polymerizes on the continuous-flow probe tip causing loss of sample signal. As a result, the continuous-flow probe must be removed and cleaned approximately every 3 hr. 

Although conventional mirrors have long been used to redirect transmitted laser light back into the sample as a way of increasing the intensity of Raman signal [48] and to reduce photon loss near the laser radiation coupling zone [49] such elements do not prevent photon loss at what is often the most critical area, the delivery zone of laser radiation into the sample. This loss becomes more marked in applications where safety or other limits prevent the laser radiation from being concentrated onto a small area. Examples include the illumination of human skin or applications in explosive powder environments in the pharmaceutical industry. The solution presented here is fully compatible with the defocused laser beams used in such conditions. 

Cho et al. [280] compared dc and rf gas jet-boosted GD-AES for the analysis of steel in terms of the voltage-current relationship they studied the effects of the gas flow-rate (0-800 ml/min) and pressure (3-5 torr) on the dc bias potential, sample weight loss and emission intensity. The use of a simultaneous spectrometer for the rf mode proved dispensable by virtue of the high stability (variations less than 0.3%) for both matrix and trace elements. Both the rf and the dc mode provided calibration graphs that were linear over two or three decades (or even more if the analytical signal was normalized to the signal of a matrix component) however, the limits of detection obtained in the rf mode for many trace elements in steel were at the level of tens of ppb, which was an order of magnitude better than in the dc mode. 

Fig. 10.3.7 Pulse sequence for spin-diffusion imaging with ID spatial resolution [Wei8] and effect of mobility filters, (a) The magnetization source is selected by the dipolar filter which suppresses the magnetization in the sink. During the spin-diffiision time the magnetization dif ses from the source to the sink, (b) The dipolar filter selects magnetization from chain segments which are highly mobile and intermediately mobile. By use of a lineshape filter the signal loss is analysed only for the mobile components. IP(Tc) is the probability for a particular correlation time to arise in the sample. It is essentially the spectral density of motion.

Many pulse sequence suppression schemes exist and these can be loosely classified into three broad classes (i) saturation based, (ii) magnetization destruction based, and (iii) methods avoiding solvent saturation. An ideal method would suppress the solvent only with no other effect on the spectrum. In reality this is never the case. For example, some suppression sequences which involve frequency-selective excitation result in frequency-dependent artifacts such as phase shifts and amplitude responses. Sometimes the applicability of a suppression method depends on the sample. For example, in addition to creating artifacts, caution must be taken when using suppression methods that involve saturation since cross-relaxation can result in significant signal loss, especially with large molecules. 

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