Ion source and inlet system

The MS ion source where electron impact is utilised is maintained at a pressure of 10 r (torr). The sample inlet system is designed to release the analytes into the central region of the source at a carefully controlled rate. This is dependent on the concentration of the analyte and its physical properties. In chromatography interfaces a short length of heated silanised silica or stainless-steel capillary tubing is used to transfer the eluant directly 

There are many methods used to produce ions in the MS ion source, with the electron impact (El) process being the most widely used. Databases containing standard 70 eV El spectra for over 140000 compounds are available. Chemical ionisation (Cl) and field ionisation (FI) have attracted a specialised interest. 

Electrons emitted at a hot filament are accelerated and the resulting electron beam traverses the ion chamber to the collector anode. Interaction between the electron beam and the organic molecules (M) results in an energy transfer of over 20 eV which is sufficient to ionise most molecules in many cases the molecular ion is unstable and subsequently undergoes 

Technique Column i.d. (mm) Mobile phase Gas/vapour flow-rate (mlmin ) 

HPLC normal column 4.6 Water/ methanol 1.5= 1860mlmin vapour 1.5 = 825mimin vapour 

---

TOF mass spectrometers are very robust and usable with a wide variety of ion sources and inlet systems. Having only simple electrostatic and no magnetic fields, their construction, maintenance, and calibration are usually straightforward. There is no upper theoretical mass limitation all ions can be made to proceed from source to detector. In practice, there is a mass limitation in that it becomes increasingly difficult to discriminate between times of arrival at the detector as the m/z value becomes large. This effect, coupled with the spread in arrival times for any one m/z value, means that discrimination between unit masses becomes difficult at about m/z 3000. At m/z 50,000, overlap of 50 mass units is more typical i.e., mass accuracy is no better than about 50-100 mass... 

The choice of a mass spectrometer to fulfill any particular task must take into account the nature of the substances to be examined, the degree of separation required for mixtures, the types of ion source and inlet systems, and the types of mass analyzer. Once these individual requirements have been defined, it is much easier to discriminate among the numerous commercial instruments that are available. Once suitable mass spectrometers have been identified, it is then often a case of balancing capital and running costs, reUability, ea.se of routine use, after-sales service, and manufacturer reputation. 

The result of the above process means that sample molecules dissolved in a solvent have been extracted from the solvent and turned into ions. Therefore, the system is both an inlet and an ion source, and a separate ion source is not necessary. 

Sample introduction system. A system used to introduce sample to a mass spectrometer ion source. Sample introduction system, introduction system, sample inlet system, inlet system, and inlet are synonymous terms. 

Table 1. Compatible Inlet Systems, Ion Sources, and Mass Analyzers...

The essential components of a mass spectrometer include a sample inlet system, an ionization source and acceleration chamber where sample molecules are ionized, fragmented and accelerated into an analyser or separator, and an ion detection and recording system (Figure 9.51(a)). 

A mass spectrometer consists of the following major parts (a) inlet system (b) ion source (c) analytical system, and (d) amplifier-registration system. 

For LC-MS to become a reality an interface had to be designed which was capable of providing a vapour sample feed consistent with the vacuum requirements of the mass spectrometer ion source and of volatilising the sample without decomposition. Various enrichment interfaces have been developed such as the molecular jet, vacuum nebulising, the direct liquid introduction inlet and thermospray systems. 

The detector, mass analyzer and parts of ion source are maintained under vacuum. The instrument control system monitors and controls all parts of the instrument. Details of the sample inlet, ion source, and mass anlayzer define the type of instrument and the capabilities of that system. 

In terms of the hardware, TRMS methods described in this book use most common types of ion sources and analyzers. Electrospray ionization (ESI), electron ionization (El), atmospheric pressure chemical ionization (APCI), or photoionization systems, and their modified versions, are all widely used in TRMS measurements. The newly developed atmospheric pressure ionization schemes such as desorption electrospray ionization (DESI) and Venturi easy ambient sonic-spray ionization (V-EASI) have already found applications in this area. Mass analyzers constitute the biggest and the most costly part of MS hardware. Few laboratories can afford purchasing different types of mass spectrometers for use in diverse applications. Therefore, the choice of mass spectrometer for TRMS is not always dictated by the optimum specifications of the instrument but its availability. Fortunately, many real-time measurements can be conducted using different mass analyzers equipped with atmospheric pressure inlets - with better or worse results. For example, triple quadrupole mass spectrometers excel at quantitative capabilities however, in many cases, popular ion trap (IT)-MS instruments can be used instead. On the other hand, applications of TRMS in fundamental studies often require a particular type of instrument (e.g., Fourier transform ion cyclotron resonance mass spectrometer for photodissociation studies on trapped ions). 

The availability of these data collections is of considerable value, permitting the identification of organic acid spectra by comparison with reference spectra in the collection. As variations in ion intensities can arise when spectra are acquired on different instruments with differing ion source conditions and inlet systems, many laboratories prefer to obtain authentic reference spectra on their own instruments for matching purposes. Such personalized collections are usually limited and do not have the benefit of input of less commonly encountered acid spectra from contributors working in related but different areas. 

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

Having considered the various parts of a dynamic-FAB system (atom gun, ionization, and matrix), it is now necessary to see how these are put together in a working inlet/ion source interface. 

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

It is worth noting that some of these methods are both an inlet system to the mass spectrometer and an ion source at the same time and are not used with conventional ion sources. Thus, with electrospray, the process of removing the liquid phase from the column eluant also produces ions of any emerging mixture components, and these are passed straight to the mass spectrometer analyzer no separate ion source is needed. The particle beam method is different in that the liquid phase is removed, and any residual mixture components are passed into a conventional ion source (often electron ionization). 

---