Hyphenated systems

Gc/ms is an example of a hyphenated instmment consisting of two techniques that provide orthogonal (independent) information. If two independent techniques are linked in tandem, a large number of data resolution elements is obtained from each. The more dimensions present, and the higher the available resolution along each dimension, the greater is the obtainable information (19). Because hyphenated systems generate an immense... 

Economic Analysis. Costs of ms/ms instmmentation remain at the high end of the scale for hyphenated systems. Because more powerhil computer systems are becoming available at lower cost and improvements are being made in the less expensive ms hardware, the trend in instmmental cost is downward. The current range for ms/ms systems extends from about 350,000 to about 1.4 million where the ms components are equipped with higher mass and resolution capabiUties. 

FTIR in multiply hyphenated systems may be either off-line (with on-line collection of peaks) [666,667] or directly on-line [668,669]. Off-line techniques may be essential for minor components in a mixture, where long analysis times are required for FT-based techniques (NMR, IR), or where careful optimisation of the response is needed. In an early study a prototype configuration comprised SEC, a triple quadrupole mass spectrometer, off-line evaporative FTIR with splitting after UV detection see Scheme 7.12c [667]. Off-line IR spectroscopy (LC Transform ) provides good-quality spectra with no interferences from the mobile phase and the potential for very high sensitivity. Advanced approaches consist of an HPLC system incorporating a UV diode array, FTIR (using an ATR flow-cell to obtain on-flow IR spectra), NMR and ToF-MS. 

Applications Ideally, multiply hyphenated systems should be assembled rapidly in response to real need. Access to these means is restricted to a few laboratories only. Multiple LC hyphenations have been used to analyse test mixtures of polymer additives see Table 7.74. The relative ease with which SEC-UV using CDCI3 as a solvent can be coupled to on-line 1II NMR and an in series off-line FUR (Scheme 7.12b), has been shown for a mixture of polymer additives (BHT, Irganox 1076, DIOP) [666]. Figure 7.35 shows representative spectra for on-flow NMR and MS and off-line FTIR of 2,6-di-f-butyl-4-methoxyphenol. 

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 general, MS performance should not be compromised by a static magnetic field. Many factors such as the design of ion optics, selection of interface, and type of MS analyzer should be considered with regard to the construction and configuration of the double hyphenated system. The optimum design should overcome some of the mutual incompatibilities of LC-NMR and LC-MS systems. For LC-MS, ionization propensities vary considerably depending upon solvent, ionization source type, and complex matrix effects. Most NMR analyses, however, are not affected by variations... 

In all these hyphenated techniques, many different experimental parameters affecting the chromatographic separation, the interface and the detector should be carefully optimised. The use of mathematical approaches for adequate optimisation and development of the hyphenated systems is unavoidable (see Chapter 2 for many examples). 

Calculation of the timings for the movements of the peak between the different positions in the hyphenated system. The time taken for the peak to reach the NMR unit or the storage loop depends on the void volume between the LC detector and the NMR spectrometer loop. This is also a function of the flow rate. For precise and reproducible positioning of the peaks, the software must allow interactive selection of the peaks from the chromatogram and the automatic calculation of these time delays based on the actual parameters. 

The control of the hyphenated system, comprised of an Agilent 1100 HPLC, a Bruker Avance 600 MHz NMR spectrometer and a Bruker Daltonics Esquire ion-trap mass spectrometer, was achieved by using HYSTAR software from Bruker Analytik. Peaks were selected for analysis using both UV and mass detection and were stored in the Bruker BPSU-36 loop storage unit prior to NMR analysis. A schematic diagram of the system is shown in Figure 4.9. 

Figure 8.2.20 Schematic of the potential development of parallel CHPLC/NMR hyphenation systems. CHPLC is used for high separation efficiency, with a number of separations carried out simultaneously. Each separation is monitored either by using on-flow or stop-flow NMR detection using an RF microcoil, with each coil feeding data to a separate receiver...

To obtain a comprehensive analysis of carbohydrate mixtures from biological matrices that display a high degree of compositional and structural heterogeneity, a multidimensional approach is required. The combination into a so-called hyphenated system, of liquid chromatography (LC) or CE with MS provides the advantage of selective and efficient separation with the mass specificity, sensitivity, and structural information gained from MS. 

A rapid and simple MW-assisted digestion method with alkaline solution (TMAH or methanolic KOH solution) was developed for speciation analysis of inorganic Hg and methyl-Hg in biological tissues [41]. Extracts with quantitative recoveries of Hg species after the alkaline dissolution of the sample were directly analyzed by an automated on-line hyphenated system incorporating aqueous HG, cryogenic trapping, GC, and detection by A AS. The proposed method was validated by the analysis of biological CRMs (CRM 463, DORM-1, TORT-1) and one BCR sample from an interlaboratory study (Tuna Fish 2). 

Iodine and Se speciation in breast milk provides an example of the use of CE in hyphenated systems with ICP-MS detection. By employing CE, Michalke and colleagues determined selenoaminocids and identified two chemical forms of iodine, I- and thyroxine, which were present in comparable amounts in milk [115-117]. Other authors used SEC and IEC for I speciation in various types of milk and infant formulae (see Table 8.3) and found I- as the main species, with the exception of breast milk and formulae. The latter were found to contain less I than commercial and human milk, and mostly as an unidentified macromolecular compound. 

Analytical methods for aluminum can be divided into those that give information about its localization, and those that inform about its quantity. In the first group are nuclear techniques, involving the Al-26 tracer and the Al-27 nuclear magnetic resonance (NMR). In the second group is the well-established technique for aluminum quantification, ETAAS, and a new generation of chromatographic separations coupled with spectrometric detection that allow compartmentalized quantification, the hyphenated systems. 

The use of filament-on thermospray LC-MS in environmental pesticide analysis is a valuable technique with points of similarity in the ionization process with other hyphenated systems such as direct liquid introduction (DLI) LC-MS and chemical ionization GC-MS. The relative merits of ammonium formate as ionizing additive in PI and NI modes TSP LC-MS for three different... 

Brief Outline of Atomic Spectroscopy Hyphenated Systems... 

The concentration of metals that are detrimental to catalysts added can vary between 20.0 ppm for Fe to 100 ppm for Ni and lOOOppm for V. The presence of these metals necessitates the need for analysis of these metals to determine their concentrations prior to the cracking process. The best method to analyse these oil samples needs to be rapid and accurate. Careful selection of the method either from experience or by trial and error may be applied depending on the metal and the concentration. Sample dissolution in a solvent or solvent mixture is considered the easiest but may not be suitable for low limits of detection. Destructive sample preparation methods, i.e. oxygen bomb combustion, microwave acid digestion followed by pre-concentrating may be required for trace analysis and/or with the aid of a hyphenated system, e.g. ultrasonic nebuliser. Samples prepared by destmctive methods are dissolved in aqueous solutions that have very low matrix and spectral interferences. 

In the field of sample introduction, hyphenated systems and devices allowing online preconcentration, automated sample introduction or speciation will become more and more important. 

An HPLC detector measures the concentration (or mass) of eluting analytes4 by monitoring one of their inherent properties, such as UV absorbance. A detector can be universal to all analytes or specific to particular classes of analytes. Common detectors and their attributes are listed in the Table 4.2. Early HPLC detectors are spectrometers equipped with small flow cells however, most modern units are compact and designed solely for HPLC. The ubiquitous UV/visible variable wavelength absorbance and the photodiode array detectors (PDA) are covered in more depth in this section. Note that mass spectrometers (MS) and nuclear magnetic resonance spectrometers (NMR) are discussed in the section on hyphenated systems. 

Figure 4.13. Pictures of hyphenated systems and systems for prep LC and bio-purification. Note that the magnet is not shown in the Bruker LC/NMR system.

Several other hyphenated systems are LC coupled with infrared spectroscopy (IR) for compound identification and LC couple with inorganic spectroscopy such as atomic absorption (AA) or inductively coupled plasma (ICP) for studies of metal speciation in samples. 

Capillary electrophoresis-mass spectrometry (CE-MS) combines the separating power of CE with the uniquely powerful detection capabilities of MS. This hyphenated system enables the analyst to separate, identify and quantify components in a mixture. As with GC-MS and LC-MS instruments, there is a variety of mass spectrometric ionisation techniques and analysers available. 

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