ISSN 10637842, Technical Physics, 2009, Vol. 54, No. 11, pp. 1714–1720. © Pleiades Publishing, Ltd., 2009. Original Russian Text © V.T. Kogan, D.S. Lebedev, Yu.V. Chichagov, I.V. Viktorov, I.T. Amanbaev, S.A. Vlasov, 2009, published in Zhurnal Tekhnicheskoі Fiziki, 2009, Vol. 79, No. 11, pp. 153–160.
SHORT COMMUNICATIONS
Ion Source with Electron Ionization for a Portable Mass Spectrometer V. T. Kogan, D. S. Lebedev, Yu. V. Chichagov, I. V. Viktorov, I. T. Amanbaev, and S. A. Vlasov Ioffe PhysicoTechnical Institute, Russian Academy of Sciences, ul. Politekhnicheskaya 26, St. Petersburg, 194021 Russia email:
[email protected] Received December 8, 2008; in final form, February 26, 2009
Abstract—An ion source with electron ionization is considered. The chargedparticle flow at the exit from this source has a cross section of ~0.1 × 0.1 mm, an angle spread of 2° × 2°, a relative energy spread of <0.5%, and an energy range of 0.5–3.0 keV. This ion source is intended for systems where an ion beam is focused in two mutually perpendicular directions. The ion source design makes it possible to ionize a sample locally (in a volume of ~10 mm3), where the concentration of the particles under study exceeds the concentration aver aged over the volume of the vacuum chamber of the mass spectrometer by two to three orders of magnitude. The ion–optical properties of the source are numerically simulated, and the optimum parameters of the source are chosen. Examples of the application of the ion source are given for the massspectrometric determination of metal salts in aqueous solutions and of gases and volatile compounds in samples in various phase states. PACS numbers: 41.75.i, 41.75.Ak, 41.75.Cn, 41.85.p, 41.85.Ar, 41.85.Ct DOI: 10.1134/S1063784209110280
INTRODUCTION Ion sources of various types and designs are used in mass spectrometry [1, 2]. Portable devices intended for outoflaboratory monitoring are equipped mainly with ion ionization (“electron impact”) sources. This is explained by the simple design of such sources, their relatively high efficiency, and the possibility of ion generation directly in the vacuum chamber of a mass spectrometer at a low pressure, which softens the requirements imposed on evacuation systems. Among the electron impact sources, the Nier source has the maximum efficiency due to the intro duction of a weak magnetic field, which substantially increases the electron free path in an ionization cham ber. The ionization chamber volume in such a design is ~1 cm3, and the direct introduction of a sample into this chamber increases the signaltonoise ratio by an order of magnitude. However, when portable mass spectrometers are designed, especially stringent requirements are imposed on their sensitivity, and the measures taken to decrease the background are insuf ficient. At the same time, the power of the vacuum pumps used in such devices is limited, which results in a limited sample flow and, on the other hand, a signif icant background signal of a residual gas. The beam at the exit from the Nier source has a rib bon shape: one of the dimensions of its cross section is small (~0.1 mm), and the other is about 1 cm. Sources with a high ion generation efficiency form beams of this shape, which can hardly be used in ion–optical systems with focusing in two mutually perpendicular directions.
In this work, we describe an ion source design that can solve the problems described above. ION–OPTICAL SCHEME Description. Numerical Simulation Figure 1a shows the ion source design. It consists of axisymmetric focusing electrodes 1–4 and emission filament 5. Ionization chamber 6 is inside electrode 1. Its volume is separated from electrode 2 by a window 1.0–1.5 mm in diameter, through which an electron flow from the emission filament is introduced and the flow of the ions forming in the ion source is simulta neously removed. On the one hand, this solution allows the vapor and gas concentration of a sample in the ionization chamber to be much higher than in the interelectrode gaps; on the other hand, it simplifies the supply of removable samples to the source when deter mining the composition of a solid extract. Under the action of the electric field of the source, the electron flow emitted by heated emission filament 5 is directed into the ionization chamber, where the ions of the compounds under study form. The electrodes then form an ion beam that has a small angle and energy spread and diverges from a compact area. The numerical simulation of ion trajectories in the source was performed with the SIMION 7 software package. The electrode dimensions, the interelectrode distances, and the potentials applied to them (Fig. 1a) were chosen so that the angle and coordinate spreads of ions in mutually perpendicular directions at the exit from the source are minimal when forming a beam of
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ION SOURCE WITH ELECTRON IONIZATION (a) 3 mm
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Fig. 1. Ion source: (a) ion–optical scheme and (b) angle α distribution of ions at the exit from the source ((solid line) calculation, (dashed line) experiment at U1 – U2 = 80 V).
ions with various energies. These spreads are <±2° and ~0.1 × 0.1 mm (which is the cross section of the ion beam in the plane of the source exit window), respec tively. Figure 1b shows the numerical simulation results. TECHNICAL PHYSICS
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VACUUM PATH Description. Estimation of the Pressure Distribution Figure 2 shows the schematic diagram of the vac uum path along which a sample flow introduced into
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l = 1 mm
d = 1 mm
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S = 70 1/s MA
I k J
Fig. 2. Schematic diagram of the vacuum path in the mass spectrometer.
the device passes. Ions mainly form in the ionization zone of source I, which is provided by (i) a nonuniform distribution of the concentration of sample molecules on the path of an electron beam; and (ii) an increase in the electron energy from the ring filament to volume I. The substantial increase in the partial pressure or the concentration of sample molecules in the ioniza tion zone as compared to the pressure in the mass ana lyzer chamber during an analysis of solids (which are nonvolatile under normal conditions) and the vapors of volatile substances is explained by different factors. In the former case, sample vapors flying from the extractor that restricts the ionization zone and is heated to a temperature required for the thermal des orption of the compounds under study (up to 1500°C) become volatile only on the path to the first design ele ment, which has a substantially lower temperature. As a result of heating of the extractor, the partial pressure of the sample increases significantly only near the des orbed surface. This pressure depends on the total amount of the extracted sample, the extractor temper ature, and the sample vapor pressure at this tempera ture. In the latter case, the partial pressure of the sample in the ionization zone increases as compared to the pressure in the vacuum chamber of the mass analyzer due to the introduction of volatile substances directly into this zone. The ratio of these pressures can be esti mated using an equivalent scheme of the vacuum path of the mass spectrometer along which a sample flow passes from the introduction system to the highvac uum pump (see Fig. 2). As follows from a continuity condition for a steady sate process, sample flow J through the cross section of the path shown in the scheme is the same at any site. Thus, flow J is supplied from the introduction system to ionization zone I. The same flow J comes from zone I: J = (PI – PMA)k, where PI is the gas or vapor partial pressure in the ionization zone, PMA is the gas or vapor partial pressure in the mass analyzer chamber, and k is
the capacity of the channel that connects the ioniza tion zone to the mass analyzer chamber. The value of k can be determined from the following expression [3] for the calculation of the capacity of a cylindrical channel for the case of air under molecule passage conditions for ionization zone sizes d1 = 3 mm and l1 = 2 mm: 1/k (l/s) = 0.0825l (cm)/d3 (cm3) + 0.11/d2 (cm2). When substituting the values from Fig. 2 into this expression, we obtain k = 5 × 10–2 l/s. In addition, the gas flow coming from the mass analyzer is controlled by the evacuation rate of the high vacuum pump S (we used an ATH+ (ALCATEL) turbo molecular pump with an air evacuation rate of ~70 l/s) and by the gas pressure in the mass analyzer PMA: J = PMAS = (PI – PMA)k, or (PI – PMA)/PMA = S/k. We substitute the values of S and k and find that, when an air mixture is let in the device, its partial pressure in the ionization zone is more than three orders of magni tude higher than the partial pressure in the mass ana lyzer chamber if we do not take into account the design elements that are situated in this chamber and hinder evacuation. In a real mass spectrometer system, an electrostatic lens of the source is located between the ionization source chamber and the highvacuum pump; it decreases this pressure ratio to ~102–103. Thus, the main pressure drop takes place in the channel located between the ionization zone and the mass analyzer chamber, and the number of ions formed in the ionization zone substantially exceeds the number of ions formed outside this zone. This is favored by both the nonuniformity of the substance concentration distribution in the mass spectrometer chamber and the difference in the ionizingelectron energies on the path from the emission filament to the ionization zone. EXPERIMENTAL In our experiments, we used a portable double focusing mass analyzer [4] and the ion source described above. For testing, we employed samples produced in Russia (VNIIM) and standards prepared in the People’s Republic of China (PRC). Choice of Parameters The results of experimental testing of the operation of the ion source agree qualitatively with the calcula tion data at a fixed ionizingelectron energy (e(U1 – U2) = 80 eV) and various potential ratios U3/U1 (Fig. 1b). The small quantitative discrepancy is explained by the difference between the simplified computation model and the real design. We took into account these results and chose the source design and the potentials applied to its elec trodes (U3/U1 = 0.8). Figure 1b shows the properties of this source at various potentials U1. When applying a portable mass spectrometer with the embedded designed ion source, we were able to TECHNICAL PHYSICS
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FeCl+ FeCl+2
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Sample 2.5 μl H2O + 1% HCl + 0.01% Fe Fig. 3. Ion source and the system of introducing a sample for the massspectrometric determination of metal salts in water. MS stands for mass spectrometer. (inset) Fragments of the mass spectrum of iron chloride extracted from a water sample recorded at various extractor heating currents I(A) and the total mass spectrum (sum).
achieve a high sensitivity when determining the metal salt concentration in water [5] and to significantly decrease the threshold of detectability of gases and volatile compounds. TECHNICAL PHYSICS
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Determination of Metal Salts in Water To increase the sensitivity of the mass spectrometer for determining metal salts in a liquid sample, we
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KOGAN et al. 1000 V
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Sample introduction (b)
Fig. 4. Ion source for the massspectrometric determination of gases and volatile compounds: (a) ion–optical scheme and the trajectories of charged particles and (b) the appearance of the source.
retained the basic parameters of the ion source and suggested a composite design. In the ionization cham ber, the filaments of the heated extractor were mobile and removable and the other elements were assembled into one stationary system (Fig. 3). When a salt from a
sample is extracted on one of the movable filaments, the other filament and the stationary ion–optical sys tem are combined into a highefficiency ion source. In this system, the electron flow from the emission fila ment is moved directly to the surface of the movable TECHNICAL PHYSICS
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Fig. 5. Ion source with a system of Finzel lenses and a possible arrangement of the mass analyzer elements: (a) ion–optical scheme and (b) halfwidth of an image (z direction) at a distance of 120 mm from the source ((solid line) numerical simulation results, (dashed line) experimental results).
filament with a sample extract; when it is heated, salt molecules are desorbed. As a result, the source forms an ion flow that is sufficient for ~10–8 g compound per 1 g solution to be detected at a sample volume of 2.5–25.0 µl. Once the composition of the next portion of an extractor sample is determined, the extractors (movable filaments) are changed over and the next portion is analyzed. Samples are supplied to the extractors with a jet injector. Figure 3 shows the schematic diagram of the source, the sample inlet system, the appearance of the source, and the mass spectra of salt FeCl2 from an TECHNICAL PHYSICS
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aqueous solution recorded at various extractor heating currents. Detection of Gases and Volatile Compounds To determine the concentrations of gases and vola tile compounds, we used the designed source made in the form of an indivisible construction. The design of the ion source is shown in Fig. 4a, and its appearance is shown in Fig. 4b. For the mass spectrometric detec tion of a substance to be analyzed, this design increases the ratios of the peak intensities correspond
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ing to the components let in the source to their back ground intensities by more than an order of magnitude without increasing the total flow rate of a sample as compared to traditional Niertype systems. This fact was experimentally supported when a fixed airmix ture flow (~10–5 l Torr/s) was alternately let in mass spectrometers with a Nier source and with the designed source. The ratio of the nitrogen peak inten sity in the mass spectrum obtained when an air mix ture was let in to the background level for the mass spectrometer with our source was 14 times higher than in the mass spectrometer with a Nier source. Source for a Mass Spectrometer with Focusing in Energy and Two Mutually Perpendicular Directions For the traditional mass analyzer in an unmovable magnetic doublefocusing mass spectrometer, one of the crosssection dimensions of the beam emitted from an ion source must be small in order to ensure a given mass resolution. The application of a source at the exit from which the crosssection dimension is also small significantly simplifies the introduction of elec trodes, which should provide the focusing of an ion beam in both the plane where it is necessary to achieve a high mass resolution and the mutually perpendicular plane (z focusing), into a mass analyzer. As a result, we can use a coordinatesensitive detector with an elec trostatic filter in an unmovable mass spectrometer in order to pass to the mode of mass analysis of multi component mixtures without scanning [6]. To provide z focusing, we supplemented the ion source with two Finzel lenses. With the chosen geom etry of the lenses, we can arrange them between the main elements of the mass analyzer and only weakly change its mass resolution. To this end, we calculated and tested the focusing properties of the source with a set of Finzel lenses in the z direction irrespective of the basic systems of the mass analyzer but with allowance for subsequent arrangement of the Finzel lenses between its elements (Fig. 5a). In Fig. 5b, the calcula tion results are compared with the experimental data. The difference in these results is explained by the fact that the size of the ion generation zone in the numeri cal model exceeds its real size. When simulating the operation of the ion source and the system of Finzel lenses, we found that the beam focusing in the z direction only weakly affects
the ion–optical properties of the system in the xy plane, which is ensured by the choice of the voltages applied to the Finzel lenses (UL/U1 = 1/2) and the interelectrode distance in each set of lenses (~1 mm). CONCLUSIONS We described an ion source with electron ioniza tion. The electrode geometry and potentials were cho sen so that this ion source has advantages over a stan dard Niertype ion source and allows one (i) to form an ion beam with small coordinate and angle spreads in mutually perpendicular directions and a small energy spread and (ii) to increase the concentration of the compo nents to be analyzed in an ionization chamber by more than an order of magnitude at the same flow rate of the sample. The application of this ion source makes it possible to pass to the mode of mass analysis of multicompo nent mixtures without scanning, to increase the sensi tivity, and to decrease the measurement time. REFERENCES 1. F. A. White and G. M. Wood, Mass Spectrometry: Appli cation in Science and Engineering (Wiley, New York, 1987). 2. A. T. Lebedev, Mass Spectrometry in Organic Chemistry (Binom, Moscow, 2003) [in Russian]. 3. W. Heinze, Introduction to Vacuum Technique (Einfu hrung in die Vakuumtechnik) (Technik, Berlin, 1955; Gosenergoizdat, Moscow, 1960), Vol. 1, translated from German. 4. V. T. Kogan, A. K. Pavlov, Yu. V. Chichagov, Yu. V. Tu bol’tsev, G. Yu. Gladkov, A. D. Kazanskii, V. A. Niko laev, and R. Pavlichkova, Field Anal Chem. Technol. 1, 331 (1997). 5. V. T. Kogan, A. K. Pavlov, Yu. V. Chichagov, Yu. V. Tubol’tsev, M. I. Savchenko, O. B. Smirnov, O. S. Viktorova, I. V. Viktorov, S. A. Vlasov, B. M. Duben skii, V. Nedvigin, and Ya. Gao, Zh. Tekh. Fiz. 77 (12), 73 (2007) [Tech. Phys. 52, 1604 (2007)]. 6. V. T. Kogan, S. A. Manninen, D. S. Lebedev, O. S. Vik torova, and I. T. Amanbaev, Zh. Tekh. Fiz. 75 (6), 121 (2005) [Tech. Phys. 50, 794 (2005)].
Translated by K. Shakhlevich
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