Hyphenated instrumental techniques

What is a hyphenated instrumental technique Give the most common example of such a technique. 

More recently for ultratrace determination and speciation of antimony compounds the so-called hyphenated instrumental techniques have been applied which combine adequate separation devices with suitable element-specific detectors. They include high-performance liquid chromatography (HPLC) connected on-line with heated graphite furnace (HGF) AAS (HPLC-HGF-AAS), hydride-generation atomic fluorescence spectrometry (HPLC-HG-AFS) or inductively coupled plasma (ICP) mass spectrometry (MS) (HPLC-ICP-MS) capillary electrophoresis (CE) connected to inductively coupled plasma mass spectrometry (CE-ICP-MS) and gas chromatography (GC) coupled with the same detectors as with HPLC. Reliable speciation of antimony compounds is still hampered by such problems as extractability of the element, preservation of its species information, and availability of Sb standard compounds (Nash et al. 2000, Krachler etal. 2001). Variants of anodic stripping voltammetry for speciation of antimony have also been applied (Quentel and Eilella 2002). 

Mixtures can be identified with the help of computer software that subtracts the spectra of pure compounds from that of the sample. For complex mixtures, fractionation may be needed as part of the analysis. Commercial instmments are available that combine ftir, as a detector, with a separation technique such as gas chromatography (gc), high performance Hquid chromatography (hplc), or supercritical fluid chromatography (96,97). Instmments such as gc/ftir are often termed hyphenated instmments (98). Pyrolyzer (99) and thermogravimetric analysis (tga) instmmentation can also be combined with ftir for monitoring pyrolysis and oxidation processes (100) (see Analytical methods, hyphenated instruments). 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

Gas chromatography is a most favourable case for interfacing to a mass spectrometer, as the mobile phases commonly used do not generally influence the spectra observed, and the sample, being in the vapour phase, is compatible with the widest range of mass-spectral ionisation techniques. The primary incompatibility in the case of GC-MS is the difference in operating pressure for the two hyphenated instruments. The column outlet in GC is typically at atmospheric pressure, while source pressures in the mass spectrometer range from 2 to... 

As liquid chromatography plays a dominant role in chemical separations, advancements in the field of LC-NMR and the availability of commercial LC-NMR instrumentation in several formats has contributed to the widespread acceptance of hyphenated NMR techniques. The different methods for sampling and data acquisition, as well as selected applications will be discussed in this section. LC-NMR has found a wide range of applications including structure elucidation of natural products, studies of drug metabolism, transformation of environmental contaminants, structure determination of pharmaceutical impurities, and analysis of biofiuids such as urine and blood plasma. Readers interested in an in-depth treatment of this topic are referred to the recent book on this subject [25]. 

I) Faradaic electrochemical methods. From a general analytical point of view, electrochemical techniques are very sensitive methods for identifying and determining the electroactive species present in the sample and, in addition, they also are able to carry out speciation studies, providing a complete description of the states of oxidation in which the ionic species are present in the object. Other applications and improvements obtained by their hyphenation with other instrumental techniques, such as atomic force microscopy (AFM), will be described in the following chapters. 

Instrumental developments (e.g., of sector field instruments with multiple ion collection, introduced in 1992, or the insertion of collision and reaction cells in order to reduce disturbing isobaric interferences), the progress in applications for ultratrace analysis, also in combination with on line hyphenated separation techniques (HPLC, CE), especially routine capability as well as decreasing price and user friendly maintenance mean that sales are increasing by 10 % every year. To improve the analytical performance of ICP mass spectrometers for precise isotope ratio measurements (e.g., for geochronology or for the study of fine isotope variation in nature) powerful instrumentation with high mass dispersion and multiple ion collector systems instead of single ion collection are commercially available on the analytical market. 

Numerous papers have relied on only UV-visible spectra for their identification of phenolics, but for positive identification purposes, HPLC-mass spectrometry (MS) is another detection mode that can provide detection of all phenolic compounds in foods. This technique involves a hyphenated instrument that uses a mass spectrometer as a detector for HPLC or uses HPLC as cleanup step for mass spectrometry. After preparative HPLC, the MS technique has frequently been employed for structural identification of phenolics in many foods and essential oils because of its sensitivity and selectivity and its ability to provide structural information. 

Hirschfeld, in his article on coupled techniques, defined, a hyphenated instrument as one in which both instruments are automated together as a single integrated unit via a hardware interface. . . whose function is to reconcile the often extremely contradictory output limitations of one instrument and the input limitation of the other (22). Therefore, the key to combining SFE with chromatographic techniques is the interface which should allow the optimum and independent usage of each instrument while the couple still operate as an integrated unit (23). 

Both Soderstrom et al. (25) and Creasy et al. (26) have applied GC/FTIR together with several other hyphenated chromatographic techniques to analyze CWC-related chemicals in complex matrices. The results of the different technique have been combined to unequivocally identify the relevant chemicals in low concentrations. In both studies, IR and MS spectra have been used together in spectral interpretation. Weimaster et al. (70) studied samples collected in Iraq, using a very wide variety of instrumentation, but they could not find any scheduled chemicals. 

Any analytical data obtained by hyphenated instruments or by two-way spectroscopic techniques such as excitation-emission fluorescence spectroscopy are bilinear ones. The bilinear data matrix has a very useful property, namely the rank of such matrix obtained with any chemical mixture is equal to the number of chemical components in the mixture. Thus, theoretically, the rank of a data matrix of any pure chemical component is unit. It can be expressed by the product of two vectors ... 

Analytical techniques that are used to measure therapeutic drugs includes immunoassay and instrumental techniques, such as chromatographic and electrophoretic procedures, and the so-called hyphenated techniques, where chromatographs are coupled with a mass spectrometer. ... 

Since the advent of hyphenated MS techniques, gas chromatography (GC)-MS in the 1980s and LC-MS in the 1990s autosamplers have become a necessity at the front of MS-based instrument systems. Indeed the autosampler is a critical component of any modem LC-MS-based analysis system. Autosamplers have evolved to meet the increased demand requirements of automated well-based MS analysis. The primary figures of merit for autosampling devices are robustness, speed, lack of memory effect, swept volume, plate capacity, integration with MS software, and flexibility ... 

The mass spectrometric detector is often coupled to GC to provide a very sensitive separation and detection instrument in one. This system is discussed in more detail in the section on hyphenated (hybrid) techniques. 

In conclusion, wavelet transforms have been employed by analytical chemists to solve various problems in chromatographic studies. Owing to the popularity of hyphenated instruments, more applications based on two-dimensional wavelet transform (2D-WT) will be developed. The 2D-WT technique is more suitable for processing data produced from such instruments. 

Note that the first three entries in the table are hyphcn-aied methods, combinations of two instrumental techniques that produce analytical results superior in some way to the results from either of the original individual methods. VVe shall encounter a number of hyphenated methods elsewhere in this boyk. 

The possibiHties for multidimensional instrumental techniques are endless, and many other candidate components for inclusion as hyphenated methods are expected to surface as the technology of interfacing is resolved. In addition, ternary systems, such as gas chromatography-mass spectrometry-infrared spectrometry (gc/ms/ir), are also commercially available. 

In 1990, Chemical Abstracts Service listed over 10 million substances in their Registry. Moreover, the growth of new compounds is exponential, leading to a doubling of known chemicals every eleven years. Thus there is an ever increasing need to efficiently identify substances and quantitate material with high confidence. Hyphenated instruments, combinations of accepted instrumental techniques where the sample is passed from one instalment direcdy into another, were developed to aid in solving this problem (1). 

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