Continuous Sampling Modes

The reaction manifold describing the automated determination of ammonia is shown in Fig. 6.1. Two alternative modes of sampling are shown discrete and continuous. Discrete 5 ml samples contained in ashed (450 °C) glass vials are sampled from an autosampler (Hook and Tucker model A40-11 1.5 min sam-ple/wash). For high-resolution work in the estuary, the continuous sampling mode is preferred. The indophenol blue complex was measured at 630 nm with a colorimeter and the absorbance recorded on a chart recorder. 

Both intermittent and continuous sampling modes can be incorporated into a system, which was the case with one of the earliest systems described. The... 

Brookhaven National Laboratories (BNL) has constructed an electron-capture detector capable of direct air sampling. The electron capture detector follows a catalytic reactor that contains H2 and Pd/Molecular Sieve 5A at 140°. This reactor effectively removes oxygen from the air sample stream by forming water. Additionally, chlorofluorocarbons are combusted and reduced to HCl and HF. These acids, along with HjO, are removed from the sample stream via a desiccant. However, such gases as SFg and per-fluorinated cyclic aliphatics [perfluorodimethyl-cyclobutane (PDCB), perfluoromethylcy do hexane (PMCH), and perfluorodimethylcyclohexane (PDCH)] survive the reactor and are detected via an electron capture device. These chemical compds survive due to their unusual thermal stability. BNL has demonstrated a lower limit of detection of these compds with their breadboard instrument (on a continuous sampling mode) of 0.1 parts per trillion (Ref 12)... 

In 1998, Anderson and Busch presented an offline TLC-MS probe composed of an array of microcapillaries [9]. This device lacked the general sensitivity of Luftmann s device because it only sampled the uppermost layer of the TLC plate. Nevertheless, it also planted the idea of MS imaging of sample spots from the developed TLC plates by an elution-based approach. Van Berkel further developed this concept in 2002 [10]. A liquid microjunction surface sampling probe enabled stepwise as well as continuous sampling mode of operation, which theoretically could be used for imaging analyses of whole TLC plates. The spatial resolution was a function of scan speed and the nature of the eluting solvent. This device underwent many improvements in the following years however, it has been never commercialized. 

In a continuous infusion mode, the mass spectrometer acts as the detector for target-specific small molecules exiting the sample chamber. Each compound s intrinsic unique mass results in specific elution profile that is recorded for quanti-... 

Fig. 24.4. Study of the biosensor stability with biological sample. Arrows indicate where (i.e. biological sample) a solution obtained by dialysing a human serum with the microdialysis probe was flowed in the biosensor cell. At the beginning a perfusion solution and control solution (glucose 5 mmol l-1) were used instead of the serum to test the biosensor response. Control solution of glucose was also used during and at the end of the experiment to evaluate the stability of the biosensor. Continuous flow mode 10 pi min-1). Applied potential —50 mV vs. int. ref. Reprinted from Ref. [59] with permission from Elsevier.

In the conventional measuring mode the sample stays in the NMR tube, and thus in the radiofrequency Helmholtz coil all of the time. In the continuous-flow mode it resides within the NMR detection coil only for a distinct time of some few seconds (Figure 1.2). This residence time t is dependent upon the volume of the detection cell and the employed flow rate (Table 1.1). For example, a detection volume of 120 jjlI, together with a flow rate of 0.5ml/min, results in a residence time of 14.4 s, while with a detection volume of 8 jjlI the residence time is only 0.96 s. A shorter residence time t within the NMR measuring coil results in a reduction of the effective lifetime of... 

This example demonstrates that high-quality 13C NMR spectra can be obtained in the continuous-flow mode. Thus, it is possible to monitor electrochemical reactions of high-concentration compounds (0.1 M solutions) [3], or even to use diluted samples with 13C-labelled positions [4-6],... 

Recently, NMR spectrometers directly coupled with LC systems have become commercially available. Spectra can be acquired in either of two modes, continuous or stopped flow. In continuous flow mode the spectrum is acquired as the analyte flows through the cell. This method suffers from low sensitivity since the analyte may be present in the cell for only a brief period of time, but it has the advantage of continuous monitoring of the LC peaks without interruption. Fig. 12A shows a contour plot of the continuous flow NMR analysis of a mixture of vitamin A acetate isomers.Fig. 12B shows the spectra taken from slices through the contour plot. These plots highlight the olefinic region of the spectra which provided ample information for the identification of each of the isomers. With very limited sample quantities, the more common method of LC-NMR analysis is stopped flow. Here the analyte peak is parked in the flow cell so any of the standard NMR experiments can be run. 

Lasers for solid sampling should always be operated in the pulsed mode. The continuous, CW mode, as often used for drilling and welding in engineering, would have serious drawbacks. The application of a CW laser provides much lower intensities than the peak intensities of pulsed laser. There would be continuous removal from the sample, which is closer to ordinary evaporation than the explosion-like ablation process of pulsed lasers. 

Alternatively, nano-ESl is used in continuous-flow mode by injecting samples in a mobile-phase flow or in on-line combination with nano-LC (Ch. 17.5.2) and other low-flow separation methods (Ch. 6.6.2 and Ch. 17.5.5-6). As such, nano-ESl plays an important role in the progress in the field of protein characterization. 

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