Source spectrometer

Figure 10. Comparison of the velocity dependence of the disappearance cross-section of CHa+, formation cross-section of CH0 +, and Langevin orbiting collision cross-section, all as a function of reciprocal average kinetic energy of ions in the mass spectrometer source...

The maximum flow rate that can be accommodated while still allowing the mass spectrometer to operate is in the range of 10-20 tilmin" Typical flow rates used in conventional HPLC separations are between 500 and 1000 tilmin and therefore only between 1 and 4% of the column eluate, and therefore ana-lyte(s), enter the mass spectrometer source. The sensitivity, or more accurately the lack of sensitivity, of the DLl interface is one of its major limitations. 

The particles then enter a conventional mass spectrometer source where they are vaporized prior to being ionized using electron impact or chemical ionization. As with other interfaces, this may cause problems during the analysis of thermally labile and highly in volatile compounds. 

Factors may be classified as quantitative when they take particular values, e.g. concentration or temperature, or qualitative when their presence or absence is of interest. As mentioned previously, for an LC-MS experiment the factors could include the composition of the mobile phase employed, its pH and flow rate [3], the nature and concentration of any mobile-phase additive, e.g. buffer or ion-pair reagent, the make-up of the solution in which the sample is injected [4], the ionization technique, spray voltage for electrospray, nebulizer temperature for APCI, nebulizing gas pressure, mass spectrometer source temperature, cone voltage in the mass spectrometer source, and the nature and pressure of gas in the collision cell if MS-MS is employed. For quantification, the assessment of results is likely to be on the basis of the selectivity and sensitivity of the analysis, i.e. the chromatographic separation and the maximum production of molecular species or product ions if MS-MS is employed. 

The problems associated with coupling packed columns to a mass spectrometer are s re severe than those encountered with capillary columns. Conventional pacdced columns are operated at much higher flow rates, 20 to 60 al/ain, and although this diminishes the influence of dead volumes in the interface on sample resolution, it poses a problem due to the pressure and volume flow rate restrictions of the mass spectrometer. The interface must provide a pressure drop between column and mass spectrometer source on the order of 10 to 10, it must reduce the volumetric flow of gas into the mass spectrometer without diminishing the mass flow of sample by the same amount, and it must retain the integrity of the sample eluting from the column in terms of the separation obtained and its chemical constitution [3,25,26]. To meet the above requirements the interface must function as a molecular separator. 

The matrix compound is typically mixed together in an aqueous/ organic solution with the analyte, such that the relative concentration of matrix to analyte is on the order of 5000 or 10,000 to 1. The solution is applied to a surface that will be irradiated by the laser beam and the solvent is allowed to evaporate, leaving a solid, crystalline deposit of matrix and analyte. Many of the original applications described instruments in which the surface containing the dried matrix/ analyte sample was introduced into the vacuum housing of a mass spectrometer source housing for irradiation. However, recently it has been demonstrated that MALDI can be successfully carried out at atmospheric pressure (outside the vacuum chamber of the mass spectrometer), much in the same way as the ESI, APCI and APPI techniques. 

The introductory chapter is brief but provides an ample introduction to mass spectrometry and leaves one comfortable as he/she moves on to the historical and instrumentation chapters that follow. A few of the basic equations are given as part of the review of basic concepts. In these few pages Dr Becker clearly introduces the concepts of atomic mass units relative to carbon, isotopes and isotope abundance. Figures 1.1 and 1.2 go hand in hand in providing the reader with the three major parts of a mass spectrometer (source, ion separation, detection) and show various alternatives for each of these. The subtle use of color in these and subsequent figures adds an attractive benefit for the reader. 

Rudat (29) has taken the nozzle from a Capillaritron gun and mounted it on his spectrometer source. Because of the small distance between gun and target, sufficient ion bombardment is attained at very low gun currents. Closely related to this work is that of McEwen who mounted a cesium gun on his ionization source (30). The gun functioned by using a filament to heat cesium chloride salts that drove off the cesium. Before emerging from the gun toward the target, cesium vapor is surface ionized by the filament, then accelerated by the impressed moltage. 

Additional evidence for the occurrence of gas-phase intermolecular and intramolecular nucleophilic substitution was obtained in an investigation on the reactivity of mono- and dichlorophenols within the high-pressure mass spectrometer source under conditions of argon-enhanced negative ion mass spectrometry282. It was shown that the reactant anions involved in these processes are derived exclusively from the chlorophenols and not from possible impurities such as residual oxygen, water, etc. Thus, for example, the formation of an abundant [2M - H - Cl]" adduct was attributed to an intermolecular nucleophilic Cl displacement by an [M - H]" chlorophenoxide ion282. 

The separation methods furnish a given flow of eluate (liquid or gaseous) generally under atmospheric pressure. One way or another, the eluted substances must find their way into the mass spectrometer source, where a high vacuum is necessary. We saw in the Introduction that the mean free path could be evaluated according to the following equation, where the pressure p is expressed in Pa and the distance L is expressed in centimeters ... 

This coupling consists of having the capillary column directly enter the spectrometer source by a set of vacuum-sealed joints. Its yield can only be 100 %. The pumping is not problematic, for the capillary is necessarily very long. A length of at least 15 m is necessary for a column with an inside diameter of 0.25 mm. 

Different methods are used to tackle these problems [10-13], Some of these coupling methods, such as moving-belt coupling or the particle beam (PB) interface, are based on the selective vaporization of the elution solvent before it enters the spectrometer source. Other methods such as direct liquid introduction (DLI) [14] or continuous flow FAB (CF-FAB) rely on reducing the flow of the liquid that is introduced into the interface in order to obtain a flow that can be directly pumped into the source. In order to achieve this it must be reduced to one-twentieth of the value calculated above, that is 5 pi min. These flows are obtained from HPLC capillary columns or from a flow split at the outlet of classical HPLC columns. Finally, a series of HPLC/MS coupling methods such as thermospray (TSP), electrospray (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) can tolerate flow rates of about 1 ml min 1 without requiring a flow split. Introducing the eluent entirely into the interface increases the detection sensitivity of these methods. ESI can accept flow rates from 10 nl min-1 levels to... 

The skimmed solvent vapour and the helium are mechanically pumped out of the apparatus. This process is repeated in the second pumping stage (about 500 mTorr). Finally, the flow enriched in particles consisting of a narrow particle beam with a diameter smaller than 100 nm is sent into the mass spectrometer source without disturbing the vacuum. In the El or Cl mode, the particles injected into the source are rapidly vaporized before ionization. In the FAB mode, the particle beam is directed onto an FAB nozzle covered by a matrix. Thus the particles collide with the matrix surface and are trapped in it. 

Sample Derivatization. Hie derivatization of nutmeg constituents described by Harvey (7) is designed to increase the volatility and stability of the components so that they can be separated in the gas chromatograph. With direct probe introduction, MS/MS is usually able to deal with samples of lower volatility hence, derivatization is not required. Direct probe temperatures reach as high as 400° C, vaporizing many samples directly into the vacuum of the mass spectrometer source. Derivatization is used in MS/MS for the somewhat different purpose of inparting a specific chemical reactivity to the analyte. 

Fig. 2. Mass spectrum obtained by probe analysis of methyprylone, molecular weight 183, with mass spectrometer source at A, 200° and B, 150°.

The first pump removes air at a rate of 500 1/min and the second at about 300 1/min. The pressure in the first vacuum-lock is maintained at about 1 to 20 torr and that in the second about 0.1 to 0.5 torr. As a consequence the mass spectrometer source can be easily operated at about 10 torr. Flash vaporization of the solute occurs by radiant heating in a small chamber that butts directly onto the solid probe entrance to the ionization chamber and the vapor passes through a small hole directly into the ion source. The flash heater is either a nichrom coil or a quartz heater tube. The slots in the vacuum-locks are made of sapphire strips. An example of the use of the belt interface to monitor the separation of a pesticide mixture is shown in figure 20. 

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