Interfaces first commercial availability

A solid or liquid sample can be introduced to the analyzer by thermal desorption. The resultant vapors are swept through the inlet by the carrier gas and ionized by a radioactive 63Ni source. Discreet packets of ions are then pulsed down the flight tube under a controlled potential. The arrival of the ions at the detector is inversely proportional to the mass of the molecule. Thus, the smaller ions arrive at the detector first, and the larger ions arrive later. At that point the signal is amplified and read out via an appropriate computer interface. Both commercially available instruments are capable of generation and detection of both positive and negative ions. 

There are numerous ionization methods that allow formation of ions to carry out mass spectrometry however, in this chapter we will only focus on those most common in LC-MS. The challenge in coupling HPLC to mass spectrometry is that the chromatography operates with liquids and under high pressure, while the detector operates under high vacuum. The device between the chromatograph and the mass spectrometer is called the interface. Here, ionization and transition from liquid to gas phase of the compounds occur. The development of the first commercial available interfaces started as early as in the 1970s. Since then numerous interfaces have been introduced. Table 3.6 shows a list of current interfaces and their acronyms. 

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. 

The advantages and disadvantages of this type of interface, particnlarly in comparison to the moving-belt interface which was available at the same time, are listed below. This was one of the first LC-MS interfaces to be made commercially available and, although used in a number of laboratories, its development was halted premamrely by the introduction of the thermospray interface (as we shall see later). 

Sample molecules that are too large to enter the pores of the support material, which is commercially available in various pore dimensions, are not retained and leave the column first. The required elution volume Ve is correspondingly small. Small molecules are retained most strongly because they can enter all the pores of the support material. Sample molecules of medium size can partly penetrate into the stationary phase and elute according to their depth of penetration into the pores (Fig. 7.3). No specific interactions should take place between the molecules of the dendrimer sample and the stationary phase in GPC since this can impair the efficiency of separation by the exclusion principal. After separation the eluate flows through a concentration-dependent detector (e.g. a UV/VIS detector) interfaced with a computer. One obtains a chromatogram which, to a first approximation, reflects the relative contents of molecules of molar mass M. If macromolecules of suitable molar mass and narrow molar mass distribution are available for calibration of the column, the relative GPC molar mass of the investigated dendrimer can be determined via the calibration function log(M) =f( Vc). 

The first commercial LC-MS interface, available in 1977, was the moving-belt interface, which was a modification by MacFadden et al. [36] of the moving-wire system described by Scott et al. [35], The moving-belt interface, discussed in Ch. 4.4, was capable of introducing up to 1 ml/min of mobile phase and achieving solvent-independent analyte ionization by El or CL A similar system was described by Millington et al. [72]. 

Liquid chromatography-mass spectrometiy (LC-MS) based on atmospheric-pressure ionization (API) was demonstrated as early as 1974 (Ch. 3.2.1). However, it took until the late 1980 s before API was starting to be widely applied. Today, it can be considered by far the most important interfacing strategy in LC-MS. More than 99% of the LC-MS performed today is based on API interfacing. In this chapter, instrumentation for API interfacing is discussed. First, vacuum system for MS and LC-MS are briefly discussed. Subsequently, attention is paid to instrumental and practical aspects of electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), and other interfacing approaches based on API. The emphasis in the discussion is on commercially available systems and modifications thereof. Ionization phenomena and mechanisms are dealt with in a separate chapter (Ch. 6). Laser-based ionization for LC-MS is briefly reviewed (Ch. 5.9). 

Next to conventional (vacuum) MALDI, atmospheric-pressure MALDI interfaces have been described, especially to enable MS-MS on MALDI-generated ions by ion-trap and (J-TOF instmments [145-146]. Atmospheric-pressure MALDI sources are commercially available from all major instrument manufacturers. First results on-line LC-atmospheric-pressure MALDI were reported as well [147]. 

The book is intended to give a state-of-the-art overview of the recent achievements in the area of piezoelectric sensors. The focus lies on TSM resonators, since this class of piezoelectric devices is most frequently used in physical and chemical sensor and biosensor apphcations, and they are largely commercially available. The book is divided into three parts. The first four chapters cover the physical background of piezoelectric devices. While Ralf Lucklum and Frank Eichelbaum discuss different interface circuits to drive a TSM resonator in the first chapter, Diethelm Johannsmann provides a comprehensive picture of how to treat different load situations of the quartz crystal microbalance (QCM) in the second, including rather new development in the area of con-... 

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