The Direct Inlet System

The Hitachi RMH-2 mass spectrometer was employed, appropriately modified to provide chemical ionization spectra. A differential pump (pumping rate 250 If sec) was added to the ion source region to provide adequate removal of the solvent vapor. The column eluent passed into the ion source via a glass capillary (0.076 mm I.D.) which passed through the center of a PTFE rod. The PTFE rod was inserted through the vacuum lock provided for probe injection 

In 1982 Krien et al. (16) examined the use of microbore columns in LC/MS systems. The advantages of micrdbore columns are very apparent when the amount of sample is limited, for example, for those samples of biological or biochemical origin, as the small bore column allows relatively high concentrations of solute to be eluted even when small masses of the individual solutes are available. 

The column was terminated by a 1/2 iim porosity filter and a short piece of PTFE tubing. Subsequent to the tubing, was a Vespel ferrule carrying a length of quartz tubing 0.3 mm O.D., 50 pm I.D. that projected into the ion source of a quadrupole mass spectrometer. 

The volatilization of the mobile phase from a quartz capillary was examined by Bruins and Orenth (17) who used an apparatus very similar to that of Krien et al. (16). It was shown that significant heat transfer was necessary to support the evaporation of 

It consisted of a central stainless-steel tube 0.004 in I.D. that passed through a heated copper block which acted as the vaporizer and was heated by two 50 W cartridge heaters. A thermocouple was also included in the block to monitor the temperature. At the end of the tube, prior to a removable cap was a stainless-steel pin hole diaphragm. The detection limits of the system was about 100 ng when employed in the thermospray mode for involatile labile compounds and when used with 2 mm I.D. small bore LC columns operated at a flow rate of 150 il/min. Practical detection limits obtained in general use were reported to be 250 ng in the direct liquid inlet mode and about 100 ng in the thermospray mode. 

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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. 

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. 

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. 

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. 

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 ... 

Figure 4-4 shows a typical system under positive pressure. It differs from the vacuum system in that the material enters from one source and is distributed directly to several tanks. In this case no cyclone separator is used the air laden with solids enters the process bins directly. The decrease in velocity of the stream and its change in direction will cause most of the solids to drop out. For this system each receiver must have a filter to remove the remaining solids. Note that the blower is placed at the air entrance, instead of after the filter as in the vacuum system. Should a bag in the fiber filter break, no dust will get into the blower or its motor. Another advantage is that no contaminants from the atmosphere can enter the system when it is under positive pressure, except through the air inlet system. 

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. 

The coupling of a GLC column with the sample inlet system of a mass spectrometer is relatively easy, as the effluents are already in gaseous form. The main problem is the relatively high pressure at which these effluents reach the spectrometer and the excess of carrier gas in the stream. Several experimental devices now allow separation of the sample from the carrier gas, either by an effusion process or with the help of a thin, semi-permeable membrane222,353. The use of capillary columns permits direct insertion of the GLC effluent into the ion source without overtaxing the pumping capacity of the mass spectrometer 311 3 5 5 >3 5 6. 

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 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... 

Instrumentation. Hambitzer and Heitbaum described a coupling of the thermospray inlet system with an electrochemical cell [851, 852]. The electrolyte solution is pumped into the cell. The working electrode was initially mounted as a coil of platinum wire around the exit bore connecting the cell to the interface. Thus the liquid in which the reaction products are present was transferred directly to the thermospray interface. An advanced version that could be used with flat electrodes instead of a wire is depicted in Fig. 5.136. 

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. 

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

For the purpose of sample introduction, any sample introduction system (also sample inlet system or inlet) can be employed that is suitable for the respective compound. Hence, direct probes, reservoir inlets, gas chromatographs, and even liquid chromatographs can be attached to an El ion source. The preferable inlet systems is to be preferred depends on the type of sample to be analyzed. Whatever type the inlet system may be, it has to manage the same basic task, i.e., the transfer of the analyte from atmospheric conditions into the high vacuum of the El ion source Table 5.1 provides an overview. 

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