Techniques, hyphenated combined

Principles and Characteristics A new level of understanding is achieved when several analytical techniques are combined in a hyphenated approach. On the other hand, in a simple experiment in which spectroscopic analysis is performed as a function of time, concentration, or other additive properties, the output will be multidimensional, and a number of independent variables will determine its dimensions. If... 

Hyphenated techniques like combination of optical detection methods based on reflectometry or refractometry and separation techniques are of future interest. The same is valid for the intention to couple SPR or RIfS with mass spectrometry like MALDI33. 

Hyphenated technique The combination of two analytical techniques (see also Hybrid technique and Tandem technique). 

Temporal resolution is important for obtaining quality output in analyses using hyphenated techniques which combine mass spectrometers with liquid- or gas-phase separation... 

One of the limitations of the above techniques is that either the type of components or the amount of each is known but not both. However, there is a third hyphenated technique that combines all of the above. This technique uses the method described in applications in combination with a gas chromatograph and a mass spectrometer (GC-MS) that is also coupled with an infrared spectrometer (FTIR). Most GC-MS units are already equipped with infrared capabilities. The advantage of this technique is that not only... 

The use of instrumental chromatographic methods was described earlier in the role of hyphenated or hybridized techniques when combined with infrared spectroscopy. In this case, the chromatographic front-end acts as a sophisticated sample preparation system for isolation of specific chemical species in a mixture. In cases in which a particular component needs to be removed or separated, it is not necessary to resort to an instrumental method. In such cases, the sample may be passed through a simple column containing the solid-separation phase. A convenient approach for liquids is to prepare small columns of adsorbent or ion-exchange resin in a Pasteur or dropper pipette. This is ideal as a method for sample cleanup or for selectively removing contaminants or specific chemical components. 

Apart from combined TA techniques (on-line or not) the actual trends in thermal analysis are the introduction of modulated and high-resolution techniques, hyphenated thermal analysis methods e.g. TG-FTIR, TG-MS, DSC-XRD, etc.), alternative heating modes, microthermal analysis methods, industrial standardisation and quality control. Modulation means a periodic perturbation of a temperature program. Temperature modulation finds application in DSC, TG, DETA, TMA and uTA. Temperature-modulated techniques, such as Modulated DSC (MDSC ) and Modulated TGA (MTGATM), broaden the insight into the material properties. The use of modulated temperature programs in thermal methods has been reviewed [37,37a]. 

Spectroscopic techniques used in essential oil analysis comprise ultraviolet and visible spectrophotometry, infrared spectrophotometry (IR), mass spectrometry (MS), and nuclear magnetic resonance spectroscopy (NMR), including the following H-NMR, C-NMR, and site-specific natural isotope fractionation NMR. Combined techniques (hyphenated techniques) employed in essential oil analysis are GC/MS, liquid chromatography/mass spectrometry, gas chromatography/Fourier transform infrared spectrophotometry (GC/FT-IR), GC/FT-IR/MS, GC/atomic emission detector, GC/isotope ratio mass spectrometry, multidimensional GC/MS. 

Software for both controlling the hyphenated combination of techniques and efficient processing of the data is of utmost importance to the breakthrough and general use of a hyphenated technique. In this respect, the development of automatic tuning and calibration software for GC-MS and more recently LC-MS, of computer spectral library searching after electron ionization, and of efficient quantitation software packages after both GC-MS and LC-MS can be considered as important steps in the development of hyphenated techniques. Control of the complete (often multivendor) instrumentation. 

Combining separation and analysis techniques (hyphenated techniques) can produce powerful tools for chtiracteriz-ing viscous oils. Thus, liquid chromatography or gas chromatography can be used to separate a sample for subsequent characterization by mass spectrometry (LC/MS or GC/MS). Research into suitable methods for the analysis of viscous oils is underway, but no standard tests have yet been prepared. Extensive research on both proton and carbon-13 nuclear magnetic resonance spectroscopy shows promise as a tool for the analysis of lubricant base oils and other viscous oils. Both near-infrared spectroscopy (NIR) and Fourier-transform IR (FTIR) are the subjects of active research into methods to characterize hydrocarbons and for quality control during production of petroleum products. Standard test methods using these techniques should become available in the future. 

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

Hyphenated analytical methods provide more complementary information in a shorter time period leading to faster and more reUable results, than data obtained from traditional instmmental methods. The types of analytical instmments that can be joined is very large depending only upon the nondestmction of samples after the initial analytical procedure and the ability of the manufacturer to interface the instmmental techniques. Combinations include separation—separation, separation—identification, and identification—identification techniques (see Analytical methods, survey). 

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). 

The combination of chromatography and mass spectrometry (MS) is a subject that has attracted much interest over the last forty years or so. The combination of gas chromatography (GC) with mass spectrometry (GC-MS) was first reported in 1958 and made available commercially in 1967. Since then, it has become increasingly utilized and is probably the most widely used hyphenated or tandem technique, as such combinations are often known. The acceptance of GC-MS as a routine technique has in no small part been due to the fact that interfaces have been available for both packed and capillary columns which allow the vast majority of compounds amenable to separation by gas chromatography to be transferred efficiently to the mass spectrometer. Compounds amenable to analysis by GC need to be both volatile, at the temperatures used to achieve separation, and thermally stable, i.e. the same requirements needed to produce mass spectra from an analyte using either electron (El) or chemical ionization (Cl) (see Chapter 3). In simple terms, therefore, virtually all compounds that pass through a GC column can be ionized and the full analytical capabilities of the mass spectrometer utilized. 

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