Tandem system

The most common modes of operation for ms/ms systems include daughter scan, parent ion scan, neutral loss scan, and selected reaction monitoring. The mode chosen depends on the information required. Stmctural identification is generally obtained using daughter or parent ion scan. The mass analyzers commonly used in tandem systems include quadmpole, magnetic-sector, electric-sector, time-of-flight, and ion cyclotron resonance. Some instmments add a third analyzer such as the triple quadmpole ms (27). 

Advantages and disadvantages of chromatographic detectors and tandem systems are given in Table 4.13, whereas Table 4.14 gives a breakdown of the various detectors over the main chromatographic techniques. Simultaneous detection is possible, e.g. SCD/CLND. 

Table 4.14 Characteristics of chromatographic detectors and tandem systems...

Linking TLC with a tandem instrument differs from combining GC or LC with an appropriate spectrometer. Hyphenation of planar chromatographic techniques represents a niche application compared to HPLC-based methods. Due to the nature of the development process in TLC, the combination is often considered as an off-line in situ procedure rather than a truly hyphenated system. True in-line TLC tandem systems are not actually possible, as the TLC separation must be developed before the spots can be monitored. It follows that all TLC tandem instruments operate as either fraction collectors or off-line monitoring devices. Various elaborate plate extraction procedures have been developed. In all cases, TLC serves as a cleanup method. 

In our previous indole oxidation experiments, H202 has been added continuously with a flow rate of 10 pmol min 1 to a buffered indole solution in a batch reactor. In this case a constant maximum conversion at pH values between 3.0 and 8.0 was observed, whereas the indole conversion of the tandem system is limited by the H202 formation rate. At pH... 

Figure 1. An overall schematic of our present Tandem system.

A key issue is the improvement of solar light harvesting. New active materials with high optical absorption in the visible and good photostability are needed. Implementation of carrier multiplication through impact ionization in quantum dots arrays could mitigate the losses related to carrier thermalization. The alternative approach is the development of vertically stacked tandem systems of increasing band gap active materials, which effect H+ reduction and water oxidation on opposite sides. 

Natsume, T., Yamauchi, Y, Nakayama, H., Shinkawa, T., Yanagida, M., Takahashi, N., and Isobe, T., A direct nanoflow liquid chromatography—Tandem system for interaction proteomics. Analytical Chemistry 74(18), 4725 733, 2002. 

The Tandem system (Bruker Optics) is available for use during tablet production to measure tablet weight, thickness, hardness, and diameter, as well as online NIR content uniformity. The system can provide online analysis for drug substance uniformity, moisture content, and excipients. The advantage of systems like this is that the necessary data are available immediately to make adjustments to the production parameters in order to improve product rmiformity. Therefore, adjustments can be made to tablet weight in real time in order to achieve 100% of the label claim. 

Apart from LC-MS, TSP has been also used in LC-MS-MS systems. With such a tandem system, residues of chloramphenicol have been detected in milk and fish by CID on the basis of the mass spectrum of the chloramphenicol protonated molecule (101). In both food commodities, chloramphenicol residues could be identified unequivocally at the 500 ppb level. 

Battista, M., A. Di Corcia, and M. Marchetti (1989). Extraction and isolation of triazine herbicides from water and vegetables by a double trap tandem system. Anal. Chem., 61 935-939. 

Immobilized CPO and glucose oxidase (GOx) on SBA-15 were mixed for tandem catalysis By the use of the tandem system, a conversion of 92% of indole was reached at pH = 5.5. Also, the mixture of immobilized CPO and GOx was recycled several times without significant loss of activity [9]... 

H-NMR spectroscopy. The resulting dynamic one-pot tandem system, generated from these compounds and acetylthiocholine, reached equilibrium in short time. 

Tandem systems can operate in two ways and the method selected depends on the relative speed of sample generation from the chromatograph and the speed at which the sample can be scanned in the spectrometer. If the spectroscopic data can be obtained rapidly, as with a fast scanning mass spectrometer, then the column eluent or a portion thereof, can be fed directly into the spectrometer, assuming certain precautions are taken and the appropriate interface is used. 

It should be pointed out that, although tandem systems are very effective and extremely exciting to operate, they are quite complex and also very expensive. If the analysis is not repetitive, there is adequate sample, and only one substance in the mixture being chromatographed is of consequence, then an off-line spectroscopic examination may be the more practical alternative. For single solutes it may be nearly as fast to collect the component as it is eluted and examine it spectroscopically offline, as it would be to employ a tandem system. In fact, it is probable that better quality spectra will be obtained and the solute identity confirmed with greater confidence. The off-line approach, however, is not practical for multi-component mixtures or for repetitive analyses where the results may be required for forensic purposes. 

In general, tandem systems involving the combination of the liquid chromatograph in-line with the infrared spectrometer have not been very successful and most IR spectra of LC eluents are obtained by what are, in effect, off-line procedures, as in the example given above. The FTIR spectrometer, in its present form, demands too large a sample size and is too insensitive for successful in-line association with modem high-efficiency microbore LC columns. Fortunately, the demand for in-line production of IR spectra from LC eluents is not great and, in most cases, the off-line methods are quite satisfactory for the majority of LC/IR applications. 

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