Direct inlet system

LC-MS interfaces generally produce ions with a relatively wide energy and spatial distribution. Table 7.49 lists the main LC-MS interface types. The most important types of contemporary LC-MS interfaces are direct inlet systems PB, TSP, API, ICPI and MIP (the latter two for plasma source detection, cf. Section 7.3.3.5). Three main types of LC-MS coupling systems are usually distinguished ... 

The most straightforward tool for the introduction of a sample into a mass spectrometer is called the direct inlet system. It consists of a metal probe (sample rod) with a heater on its tip. The sample is inserted into a cmcible made of glass, metal, or silica, which is secured at the heated tip. The probe is introduced into the ion source through a vacuum lock. Since the pressure in the ion source is 10-5 to 10-6 torr, while the sample may be heated up to 400°C, quite a lot of organic compounds may be vaporized and analyzed. Very often there is no need to heat the sample, as the vapor pressure of an analyte in a vacuum is sufficient to record a reasonable mass spectrum. If an analyte is too volatile the cmcible may be cooled rather than heated. There are two main disadvantages of this system. If a sample contains more than one compound with close volatilities, the recorded spectrum will be a superposition of spectra of individual compounds. This phenomenon may significantly complicate the identification (both manual and computerized). Another drawback deals with the possibility of introducing too much sample. This may lead to a drop in pressure, ion-molecule reactions, poor quality of spectra, and source contamination. 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

Franzen, J. Ktiper, H. Riepe, W. Henne-berg, D. Automatic Ion Current Control of a Direct Inlet System. Ira. J. Mass Spectrom. Ion Phys. 1973,10, 353-357. 

Traldi, P. Vettori, U. Dragoni, F. Instra-ment Parameterization for Optimum Use of Commercial Direct Inlet Systems. Org. Mass Spectrom. 1982,17,587-592. 

The chemical ionization mass spectrum of cimetidine and the major fragmentation ions are presented in Figure 8 and Table 4. The spectrum was obtained using a Finnigan lYbdel 3200 quadrupole mass spectrometer fitted with a chemical ionization source. The sample, applied to the probe from an acetone solution, was introduced via the direct inlet system. Methane was used as the reactant gas. 

Azine formation, shown in reaction (3), took place at 175-250° in the heated inlet system of a mass spectrometer but was not observed with a direct inlet system (Blythin and Waight, 1967 Nakata and Tatematsu, 1967). However, earlier it had been observed that similar azine formation occurred in sulphonylhydrazones (4) even with a direct inlet system... 

The mass spectra of quinones often give rise to M -(-1 and M -(- 2 ions by addition of hydrogen and this has been considered to be due to reaction with water on the surfaces of a hot inlet system (Aplin and Pike, 1966). Similar results, which were temperature- but not pressure-dependent, have been obtained with a direct inlet system (Ukai et al., 1967 Dean and Houghton, 1968 Oliver and Rashman, 1968). The mass spectra of vinylchlorins (Budzikiewicz and Drewes, 1968) and of s-tetrazines (Yates et al., 1968) all showed M-f2 ions, whilst the strongest peaks in the mass spectrum of N-bromosuccinimide were due to succinimide (Bentley and Johnstone, 1968b). 

The spectrum was recorded (Fig. 64) on a R1010 Ribermag quadrupole mass spectrometer using a direct inlet system. 

The interfaces that effectively replaced the transport system were the thermospray and electrospray sample introduction systems. The thermospray interface, a diagram of which is shown in figure 23, is a development from the direct inlet system of McLafferty. The successful use of the thermospray interface was first reported by... 

A direct investigation of unsubstituted A-imines by mass spectrometry is impossible since these compounds are stable only in solution. Unsubstituted pyridine A-imines are formed on heating A-aminopyridinium chlorides in the direct inlet system at 200°, as was shown by Tamura and co-workers.126 The mass spectrum of the unsubstituted A-imines (see Scheme 8) contains peaks due to the loss of 15 (NH) and 16 (NH2) mass units from the molecular ion in a ratio of 2.5 1. HCN is also eliminated from the molecular ion to a small extent, the reaction presumably proceeding via the sequence molecular ion — 85 —> 86 see also thermal A-imine diazepine tautomcrism in Section II, A,3. 

When a direct inlet system is used, spectra can be obtained of the free bile acids. This is not possible with a gas chromatographic inlet. The simplest derivative which can be analyzed with the combination instrument is the methyl ester. Valuable information on the nature of the fragment ions can be obtained by analysis of both the methyl and ethyl esters. Particularly for gas chromatographic reasons it is better, however, to protect hydroxyl groups by acetylation, trifluoroacetylation, or trimethylsilylation. 

MS, however, also has certain advantages over other spectroscopic methods that in some ways makes it an ideal technique to combine with LC. Mass spectra can be obtained rapidly, only sub->ig amounts of material are required to provide satisfactory spectra and the data produced is highly informative with respect to molecular structure. There are two well established methods that can be used to interface a liquid chromatograph with a mass spectrometer. Firstly, the direct inlet system developed by McLafferty and co-workers (9-11) and secondly, the wire transport system developed by Scott et al. (12,13). The former takes a proportion of the column eluent and passes it directly into a conventional mass spectrometer volatilizing both solvent and solute into the ion source. The latter employs the wire transport system in the normal way and the solvent is evaporated from the wire after passage through the column eluent stream. The wire, coated with the residual solute. 

The direct inlet system for introducing a portion of the column eluent directly into a mass spectrometer was first devised by McLafferty and co-workers (9-11) and a diagram of their interface is shown in Figure 9. 

Figure 10. Chromatograms from the LC/MS direct inlet system.

A small lichen sample is introduced into a mass spectrometer by a direct inlet system. The sample is heated, and many lichen substances sublime readily at the very low pressure (about 10 torr) in the mass spectrometer. Mass spectra of the subliming compounds may then be recorded and used for tentative identification. (For a general introduction to mass spectrometry, see Beynon et al, 1968). 

To determine a gas-phase spectrum from a sample that is solid under normal conditions, the sample must be volatile. If the volatility is fairly high, the same inlet can be used as for liquids. For samples with low vapor pressure, a direct inlet system can be used. A small capillary is filled with the sample and placed in a heatable sample holder, and the opening of the capillary is then brought close to the PIR. Molecules evaporating from the capillary reach the PIR directly, and difficulties with deposition at narrow or cold parts of the inlet system do not arise. In addition, the molecules do not come into contact with heated metal parts, which often leads to catalytic decomposition. The amount of substance needed for the measurement of a gas-phase PE spectrum is about 20 mg, and it cannot be recovered. 

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