Mass direct sample introduction system

The physicochemical aspects of the ionization process in general, ion internal energy, and the principles determining the reaction pathways of excited ions have already been addressed (Chap. 2). After a brief repetition of some of these issues we will go more deeply into detail from the analytical point of view. Next, we will discuss technical and practical aspects concerning the construction of El ion sources and sample introduction systems. Finally, this chapter directly leads over to the interpretation of El mass spectra (Chap. 6). 

Mass spectra can only be obtained from compounds which are in the vapor-phase. The vapor pressure required to obtain a spectrum depends on the kind of sample introduction system if the sample is first evaporated in the gas container of the spectrometer and from there introduced into the ion source, a vapor pressure of about 10-2 mm Hg is necessary, while for direct introduction of the substance into the ion source a vapor pressure of only 10-6 mm is needed,2 usually sufficient to obtain spectra of very polar and nearly nonvolatile compounds, e.g., amino acids. Therefore, direct introduction systems2-9 (see also Biemann,10 p. 33) available since the pioneering work of Reed2 in all commercial instruments should be used in spite of experimental difficulties, if thermal or catalytic decomposition of the sample is to be expected. If the vapor pressure is so low that the sample cannot be vaporized sufficiently in the ion source, protecting of polar OH and NH groups by methylation or acetylation may produce a derivative of volatility enough to obtain a spectrum. 

Figure 4. Direct inlet sample introduction system for a mass spectrometer...

The sample introduction system used will depend on the type of sample and whether a chromatographic separation is required. The usual technique for separated and relatively pure samples is the direct insertion probe which carries a small amount of solid sample through an air lock into the high vacuum of the mass spectrometer. The sample would then be vapourised by applying gentle heat, and the vapours ionised in an ionisation source. Samples already in the gas phase, such as vapours or the eluent from gas chromatography, can be introduced directly into the ionisation source at low flows ( 1 mLmin ). Usually two ionisation techniques are used for these samples, electron ionisation (El) or chemical ionisation (Cl). 

Mass spectrometers with simple sample introduction systems are not effective when analyzing complex mixtures, although multianalyzer systems can address this problem to some extent (Section 3.3.2). Samples of biological or environmental origin nsnally require chromatographic separation with subsequent, sequential introduction of the constituent species into the ion source. Gas chromatography (GC), liquid chromatography (variously abbreviated as LC, HPLC, and UPLC), and capillary electrophoresis (CE) can be interfaced directly with mass spectrometers (Section 2.1.2). 

Briefly, LA ICP-MS, SIMS, and GDMS are suitable for the direct analysis of trace uranium in solid samples. RIMS offers near complete suppression of interferences. Quadrupole ICP-MS is relatively low cost and the most available. In sector field ICP-MS, the mass resolution can be increased up to 0.02 of a mass unit, which provides an inherent separation of many interferences. All types of ICP-MS can be coupled with a range of sample introduction systems to improve the delivery of sample to the plasma, and thereby increase the sensitivity of the measurement. TIMS provides the best isotope ratio measurements, with typical precisions of 0.25%, and AMS offers the lowest detection limits, in the region of 10 atoms, and effective reduction of interferences. 

Mass spectrometry is a sensitive analytical technique which is able to quantify known analytes and to identify unknown molecules at the picomoles or femto-moles level. A fundamental requirement is that atoms or molecules are ionized and analyzed as gas phase ions which are characterized by their mass (m) and charge (z). A mass spectrometer is an instrument which measures precisely the abundance of molecules which have been converted to ions. In a mass spectrum m/z is used as the dimensionless quantity that is an independent variable. There is still some ambiguity how the x-axis of the mass spectrum should be defined. Mass to charge ratio should not lo longer be used because the quantity measured is not the quotient of the ion s mass to its electric charge. Also, the use of the Thomson unit (Th) is considered obsolete [15, 16]. Typically, a mass spectrometer is formed by the following components (i) a sample introduction device (direct probe inlet, liquid interface), (ii) a source to produce ions, (iii) one or several mass analyzers, (iv) a detector to measure the abundance of ions, (v) a computerized system for data treatment (Fig. 1.1). 

MAT 731 double-focussing mass spectrometer, a combined EI/FI/FD/FAB ion source and an AMD Intectra direct introduction system, has been previously described in detail (12). The samples were heated linearly from 50 C to 750 C at a rate of 100 K/min. In general, 35 FI mass spectra were recorded in the m/z 50-900 mass range. 

Even a technique as complicated as direct liquid-introduction mass spectrometry has been coupled with reactor systems to provide real-time compositional analysis, as described in a series of articles by Dell Orco and colleagues.32-34 In their work, these authors used a dynamic dilution interface to provide samples in real time to un-modified commercial ionization sources (electrospray (ESI) and atmospheric pressure chemical ionization (APCI)). Complete speciation was demonstrated due to the unambiguous assignment of molecular weights to reactants, intermediates, and products. 

Another MS technique used in connection to pyrolysis is MIMS (membrane introduction mass spectrometry). MIMS is in fact a special inlet for the mass spectrometer, where a membrane (usually silicone, non-polar) lets only certain molecule types enter the Ionization chamber of the MS. This allows, for example, direct analysis of certain volatile organic compounds from air. The system makes possible the coupling of atmospheric pyrolysis to a mass spectrometer [61a] allowing direct sampling of the pyrolysate. Other parts of the mass spectrometer do not need to be changed when using MIMS. 

In pyrolysis-mass spectrometry (Py-MS) the pyrolysate is directly transferred to a mass spectrometer and analyzed, generating a complex spectrum. The sample introduction can be done using various techniques. One simple technique is the direct insertion probe (DIP) where the sample is deposited on an insert that has the capability of heating the sample and of introducing the pyrolysate directly into the ion source of the mass spectrometer (see e.g. [1]). Another technique is the Curie point Py-MS where an attachment to the mass spectrometer allows the sample to be placed in a radio frequency (RF) region continued by an expansion chamber connected to the ion source. The sample is pyrolyzed and the pyrolysate ionized and analyzed in the MS instrument. A schematic diagram of a Curie point Py-MS system is shown in Figure 3.3.2. 

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. 

---