Hyphenated methods

However, mass spectrometry itself offers two additional degrees of freedom . One can either resolve the complexity of a sample by going to high or even ultra-high mass resolution or one can employ tandem MS techniques to separate the fragmentation pattern of a single component from that of others in a mixture. [2,3] In practice, the coupling of separation techniques to mass spectrometry is often combined with advanced MS techniques to achieve the desired level of accuracy and reliability of analytical information. [1,7,24-27] 

This chapter is about extending the range of samples that can be analyzed by mass spectrometry and about increasing the specificity of analytical information thereof. It briefly explains the basic concepts and methodologies such as handling of chromatograms, quantitation, as well as GC and LC interfaces. 

In Fig. 12.20 are shown schematically the chief hybridizations possible between chromatography and various spectrometric techniques. The simplest configuration (Fig. 12.20a) involves the linkage between the column and the spectrometer detection zone via a suitable interface —the information is obtained from the spectrometer only. This configuration is one of the commonest in GC-MS [42-44], HPLC-MS [45], HPLC-plasma emission (ICP, MIP) [46], SFC-MS [47] and HPLC-NMR [48] hybridizations. In the configuration In Fig. 12.20b, the typical non-destructive detector (thermal conductivity In GC and UV-visible In HPLC) of the chromatograph provides an ordinary chromatogram  

Nicolet 60SX FT-IR Split 200 1 s Nicolet FT-MS 1000 mass spectrometer 

Mass Spectrometry, 2nd ed., DOI 10.1007/978-3-642-10711-5 14, Springer-Verlag Berlin Heidelberg 2011 

Note From an MS-centered view, any chromatographic system simply is another type of sample inlet, whereas from the chromatographer s point of view mass spectrometers are just detectors for their separation technique. Here, we deal with the peculiarities of those analyte-separating inlets and the associated implications for the operation of the attached mass spectrometers. 

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

Multidimensional techniques are used extensively in all of these areas, with GC-GC being only one of the many commonly used hyphenated methods. Reviews of each of these application areas is discussed in greater detail in Chapters 10, 13 and 14. The remaining sections of this present chapter, however, will use some selected GC-GC applications to demonstrate how such techniques in particular have been applied in practice. 

T. Hu schfeld, Hyphenated methods . Awn/. Chem. 52 297A-312A (1980). 

J. B. Phillips and J. Beens, Comprehensive two-dimensional gas chromatography a hyphenated method with strong coupling between the two dimensions , J. Chromatogr. 856 331-347 (1999). 

In this section we focus on methods for the quantitation of a compound in the presence of an unknown interference without the requirement that this interference should be identified first or its spectrum should be estimated. Hyphenated methods are the main application domain. The methods we discuss are generalized rank annihilation method (GRAM) and residual bilinearization (RBL). 

CHAPTER 9. HYPHENATED METHODS FOR IDEV TFXCATIOH AFTER CHROMATOGRAPHIC SEPARATION... 

Various ancillary GC techniques are headspace GC (Section 4.2.2), thermal desorption GC, pyrolysis GC, hyphenated methods (Chapter 7), multidimensional techniques (Section 7.4.1) and process GC. 

Table 7.1 Some selected principles of hyphenated methods...

Table 10.32 is a shortlist of the characteristics of the ideal polymer/additive analysis technique. It is hoped that the ideal method of the future will be a reliable, cost-effective, qualitative and quantitative, in-polymer additive analysis technique. It may be useful to briefly compare the two general approaches to additive analysis, namely conventional and in-polymer methods. The classical methods range from inexpensive to expensive in terms of equipment they are well established and subject to continuous evolution and their strengths and deficiencies are well documented. We stressed the hyphenated methods for qualitative analysis and the dissolution methods for quantitative analysis. Lattimer and Harris [130] concluded in 1989 that there was no clear advantage for direct analysis (of rubbers) over extract analysis. Despite many instrumental advances in the last decade, this conclusion still largely holds true today. Direct analysis is experimentally somewhat faster and easier, but tends to require greater interpretative difficulties. Direct analysis avoids such common extraction difficulties as ... 

In contrast to combined systems, hyphenated techniques consist of two or more analytical systems each of which is independently applicable as an analytical technique. Usually, the connection is realized by means of an interface and the system is controlled by a computer. With regard to integrated sample treatment, separation and transfer, hyphenated methods like GC-MS, HPLC-MS, GC-IR, GC-IR-MS, GC-AAS, GC-ICP-MS, MS-MS, and... 

LC-NMR plays a central role in the on-line identification of the constituents of crude plant extracts (Wolfender and others 2003). This technique alone, however, will not provide sufficient spectroscopic information for a complete identification of natural products, and other hyphenated methods, such as LC-UV-DAD and LC-MS/MS, are needed for providing complementary information. Added to this, LC-NMR experiments are time-consuming and have to be performed on the LC peak of interest, identified by prescreening with LC-UV-MS. NMR applied to phenolic compounds includes H NMR,13 C NMR, correlation spectroscopy (COSY), heteronuclear chemical shift correlation NMR (C-H HECTOR), nuclear Overhauser effect in the... 

Fujiwara et al. [94] found that, when present as a heteropolyacid complex, molybdenum(VI), germanium(IV), and silicon(IV) produced CL emission from the oxidation of luminol, and similar CL oxidation of luminol was observed for arsenic(V) and phosphorus(V) but with the addition of the metavanadate ion to the acid solution of molybdate. A hyphenated method was therefore proposed for the sensitive determination of arsenate, germanate, phosphate, and silicate, after separation by ion chromatography. The minimum detectable concentrations of arsenic(V), germanium(IV), phosphate, and silicon(IV) were 10, 50, 1, and 10... 

Hyphenated Method Separation Method Modification Identification Method... 

Chromatographic methods are also often used as part of systems that are called hyphenated methods, (see Chapter 15) where the output of the chromatographic section is used as the input for an identification method such as mass spectrometry. These hyphenated methods are also most often referred to by their acronyms, for example, GC-MS—gas chromatography-mass spectrometry and HPLC-MS—high-performance liquid chromatography-mass spectrometry. Note that although ultraviolet-visible (UV-Vis) is hyphenated, it is not a hyphenated method in that it does not consist of two different methods of analysis. Hyphenated methods will be discussed fully in Chapter 15. 

Hyphenated methods involve both separation and identification of components in one analytical procedure and are commonly used in investigating soil chemistry. These investigations can involve one separation step and one identification step, two separation steps and one identification step, and two separation and two identification steps. Hyphenated analytical method instruments are arranged in tandem, without the analyte being isolated between the applications of the two methods. This leads to a very long list of possible combinations of instrumentation and, potentially, any separation method can be paired with any identification method. The list of hyphenated methods is long, although only a few methods are commonly used in soil analysis as can be seen in the review by DAmore et al. [1],... 

Hyphenated methods can be divided into two types those that do and those that do not destroy the sample in the process of analysis. Spectrophotometric methods, thermal conductivity, and refractive index methods of detection do not destroy the sample. Chromatographic methods using flame ionization and similar detection methods destroy the sample as it is detected. Any hyphenated method that involves MS or thermal analysis (TA) will also destroy the sample. In most cases, the identification of the components in soil is most important, so the destruction of the analyte is of less importance. 

The best-known and the most commonly used hyphenated method is GC-MS more specifically, and most commonly, capillary column GC combined with quadrupole MS. This type of instrumentation is controlled by computer and data collected and analyzed by dedicated computer programs. The mass spectra produced by the analytes can be compared to those in a library of mass spectra of known compounds using a computer search algorithm. The computer program finds known compounds that best match the spectra of the analytes of interest. 

Sample preparation for analysis by hyphenated methods requires some additional planning when compared to nonhyphenated methods. All steps, extraction, concentration, and final solvent selection must take into consideration and be compatible with all the components of the hyphenated instrumentation. For gas chromatographic methods, all the components in the mixture must be in the gaseous state. For liquid chromatography (LC) or high-performance liquid chromatography (HPLC), the samples of the analytes of interest can be solids or liquids, neutral or charged molecules, or ions, but they must be in solution. If the follow-on analysis is by MS, then each of the analytes may require a different method of introduction into the MS. Metals and metal ions may be introduced by HPLC if they are in solution but commonly are introduced via AAS or inductively coupled plasma (ICP). Other analytes may be directly introduced from HPLC to MS [2],... 

There are four basic hyphenated methods that result in the sample being destroyed. These are GC-MS, HPLC-MS, AAS/ICP-MS and TA/DTA-MS. All mass spectroscopic methods destroy the sample after separation however, both AAS and ICP destroy the sample no matter what follow-on method of analysis is used. In most cases, TA and differential thermal analysis (DTA) will also destroy the sample. The follow-on methods then analyze the components that result from this decomposition. DTA may also be used to follow transitions in the sample without destroying it. Because the sample is identified, there is typically no reason to collect the analyte of interest, and so destruction is not of concern. However, if there is a limited amount of sample, care should be taken in using one of these methods. 

In a typical analysis, one approach would be to carry out the analysis by first using Cl and quadrupole MS. The fragments from this first MS would then be directed to an El and a TOF mass spectrometer. Different fragments will be observed and this will yield additional information about the sample. In many cases, the MS-MS analysis is applied to samples eluting from either LC, HPLC, or GC chromatographic separation techniques. For additional information on this topic, see Triple Hyphenated Methods. ... 

Nondestructive hyphenated methods of analysis require follow-on spectro-photometric methods. The most common spectrophotometric analysis is FTIR, and it is most commonly associated with GC. It is, however, possible to combine it with other separation methods under specific conditions. 

It is also possible to carry out a GC-GC-FTIR analysis (see Triple Hyphenated Methods ). FTIR is very rapid, which means that several scans of the eluting material can usually be obtained. These are automatically combined by the spectrometer to give the final spectrum. The more scans that can be obtained, the more noise can be reduced, producing a cleaner spectrum. 

Triply hyphenated methods are not common. However, they do exist and have been used in certain applications to elicit information about soil chemistry. LC linked to ICP spectroscopy linked to MS is one example. The eluent from a liquid chromatograph is easily directed into an ICP torch and the gases from the ICP are then directed into an MS. Because the components from the LC are converted to gases in the ICP torch, they are thus easily analyzed by the MS [14]. 

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