Probes direct sampling

Another development arising from FAB has been its transformation from a static to a dynamic technique, with a continuous flow of a solution traveling from a reservoir through a capillary to the probe tip. Samples are injected either directly or through a liquid chromatography (LC) column. The technique is known as dynamic or continuous flow FAB/LSIMS and provides a convenient direct LC/MS coupling for the on-line analysis of mixtures (Figure 40.2). 

Direct-inlet probe. A shaft or tube having a sample holder at one end that is inserted into the vacuum system of a mass spectrometer through a vacuum lock to place the sample near to, at the entrance of, or within the ion source. The sample is vaporized by heat from the ion source, by heat applied from an external source, or by exposure to ion or atom bombardment. Direct-inlet probe, direct-introduction probe, and direct-insertion probe are synonymous terms. The use of DIP as an abbreviation for these terms is not recommended. 

In the direct insertion technique, the sample (liquid or powder) is inserted into the plasma in a graphite, tantalum, or tungsten probe. If the sample is a liquid, the probe is raised to a location just below the bottom of the plasma, until it is dry. Then the probe is moved upward into the plasma. Emission intensities must be measured with time resolution because the signal is transient and its time dependence is element dependent, due to selective volatilization of the sample. The intensity-time behavior depends on the sample, probe material, and the shape and location of the probe. The main limitations of this technique are a time-dependent background and sample heterogeneity-limited precision. Currently, no commercial instruments using direct sample insertion are available, although both manual and h ly automated systems have been described. ... 

In the z-direction (depth-direction) the resolution is determined by the confocal instrument settings. While it is essential to be aware of the limitations of confocal measurements, as mentioned above, it is possible to create three-dimensional maps. That means probing a sample in the x, y and z-directions. In Figure 2 the data cube of a two-dimensional map is shown. One element (row) of the cube has been picked out and its content enlarged on the right side of the figure. It is evident that each row therefore contains the information of a whole spectrum. 

Fig. 10.19. IDR-HSQC-TOCSY spectrum of the complex marine polyether toxin brevetoxin-2 (7). The data were recorded overnight using a 500 pg sample of the toxin (MW = 895) dissolved in 30 pi of d6-benzene. The data were recorded at 600 MHz using an instrument equipped with a Nalorac 1.7 mm SMIDG probe. Direct responses are inverted and identified by red contours relayed responses are plotted in black. The IDR-HSQC-TOCSY data shown allows large contiguous protonated segments of the brevetoxin-2 structure to be assembled, with ether linkages established from either long-range connectivities in the HMBC spectrum and/or a homonuclear ROESY spectrum.

The fast atom bombardment ionization (FAB) technique is a soft ionization method, typically requiring the use of a direct insertion probe for sample introduction in which a high energy beam of Xe atoms, Cs+ ions, or massive glycerol-NH4+ clusters sputter the sample and matrix from the probe surface (Figure 8). 

If, however, solid electrolytes remain stable when in direct contact with the reacting solid to be probed, direct in-situ determinations of /r,( ,0 are possible by spatially resolved emf measurements with miniaturized galvanic cells. Obviously, the response time of the sensor must be shorter than the characteristic time of the process to be investigated. Since the probing is confined to the contact area between sensor and sample surface, we cannot determine the component activities in the interior of a sample. This is in contrast to liquid systems where capillaries filled with a liquid electrolyte can be inserted. In order to equilibrate, the contacting sensor always perturbs the system to be measured. The perturbation capacity of a sensor and its individual response time are related to each other. However, the main limitation for the application of high-temperature solid emf sensors is their lack of chemical stability. 

Some analyzers do not use a sampling system the analyzer probe is put directly into the process line. Many spectroscopic analyzers can be operated with a probe directly interfaced with the process (called in-line analysis). Some concerns with taking this approach are calibration, and how to introduce a real standard to the system for instrument verification. These are questions one must be asking. 

Other types of reactions which have been studied using FABMS include those catalyzed by enzymes. This application is particularly interesting because it represents for the first time a generally useful and molecularly specific probe with which to measure a wide variety of enzyme substrates and products. Two approaches have been successful, one in which the reaction is followed by the removal of aliquots of sample taken at timed intervals with subsequent analysis by FABMS and the other allowing the reaction to take place in a glycerol-water mixture on the probe directly inside the mass spectrometer. The choice of either method depends upon the application. If the prime interest is to analyze a substrate, for example, monitoring the release of amino acids from a polypeptide using an exopeptidase, then direct analysis inside the spectrometer may be preferred. If, on the other hand, the prime interest lies... 

The ionization methods reported for IMS included MALDI [41,76-80], Secondary Ion Mass Spectrometry (SIMS) [19, 81-86], Matrix-enhanced (ME)-SIMS [87, 88], Desorption Electrospray Ionization (DESI) [89-99], Nanostructure Initiator Mass Spectrometry (NIMS) [100-102], Atmospheric Pressure Infrared MALDI Mass Spectrometry (AP-IR-MALDI-MS) [103], Laser Ablation-inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) [104-106], Laser Desorption Postionization (LDPI) [107], Laser Ablation Electrospray Ionization Mass Spectrometry (LAESI) [108, 109], and Surface-assisted Laser Desorption/ioniza-tion Mass Spectrometry (SALDI) [110-112], Another method was called probe electrospray ionization (PESI) that was used for both liquid solution and the direct sampling on wet samples. 

To understand the mechanism of water oxidation, it is necessary to characterize each of the S states. A variety of spectroscopic methods have been brought to bear on this problem. Electron paramagnetic resonance (EPR) and X-ray spectroscopies have been especially useful because these techniques allow the Mn complex to be probed directly. EPR spectroscopy has the restriction that the Mn complex must be paramagnetic to be studied. The S2 state is an odd-electron state, and EPR spectroscopy has been used extensively to study the Mn complex in the S2 state. X-ray spectroscopy has the advantage that any state of the Mn complex is observable. However, the successful application of EPR and X-ray spectroscopies requires that a specific S state be prepared in high yield in highly concentrated samples. 

Is the probe direct, or inverse The former is good for direct observation with or without INEPT enhancement. The latter will give poor signal-to-noise in direct experiments since the sample does not fill the coil space, but is much preferred for indirect detection via, for example, a heteronnclear mnltiple qnantum coherence (HMQC) or heteronnclear single qnantum coherence (HSQC) experiment. 

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