We have constructed, and tested as mass filters, linear quadrupoles with added hexapole fields of 4%, 8%, and 12%, with and without added octopole fields. A hexapole field can be added to the field of a linear quadrupole by rotating the two y rods toward an x rod. This also adds an octopole field which can be removed by making the x rods greater in diameter than the y rods. In comparison to conventional quadrupole mass filters these rod sets have severely distorted quadrupole fields, with a mix of both even and odd higher spatial harmonics. They allow evaluating the performance of rod sets with strong geometric and field distortions as mass filters. Conventional mass analysis at the tip of the stability diagram has been compared to mass analysis using islands of stability. The stability islands are produced by applying an auxiliary quadrupole excitation field to the quadrupole. We show that with normal mass analysis at the tip of the stability diagram, the transmission, resolution, and peak shapes are relatively poor in comparison to a conventional rod set. However, the use of islands of stability dramatically improves the resolution and peak shape, and in some cases ion transmission, suggesting that mass analysis with islands of stability may provide a method to overcome a wide range of field imperfections in linear quadrupole mass filters.
Mass analysis with linear quadrupole mass filters is possible by forming "islands" in the stability diagram with auxiliary quadrupole excitation. In this work, computer simulations are used to calculate stability boundaries, island positions, and peak shapes and ion transmission for mass analysis with linear quadrupole mass filters that have added octopole fields of about 2 to 4%. Rod sets with exact geometries that have quadrupole and octopole fields only in the potential, and round rod sets, with multipoles up to N ϭ 10 (the twenty pole term) included in the calculations, show the same stability boundaries, island positions, and peak shapes. With the DC voltage applied to the rods so that the Mathieu parameter a Ͻ 0, conventional mass analysis is possible without the use of an island. With the DC polarity reversed so that a Ͼ 0, the resolution and transmission are poor preventing conventional mass analysis. In principle, mass analysis in an island is possible with operation at either of two tips. Provided the correct island tip is chosen for mass analysis, peak shapes comparable to those with a Ͼ 0 and no excitation are possible, both with a Ͼ 0 and with a Ͻ 0. In the latter case, the use of an island of stability allows mass analysis when the added octopole otherwise prevents conventional mass analysis. As with three-dimensional Paul traps [8], the addition of field distortions to a linear quadrupole ion trap can improve MS/MS efficiency [9] or give faster ejection of ions at a stability boundary [10]. The field distortions are described mathematically by the addition of higher spatial harmonics or multipole fields to the quadrupole field. Methods to add octopole [9c, 11] or hexapole [12] fields to linear quadrupoles have been described.In some applications [3], it is desirable that a linear trap used for MS/MS is capable of mass analysis. Various methods of mass analysis with linear quadrupoles have been described. Conventionally, DC and RF potentials can be applied between the rod pairs [13] to place ions to be mass analyzed at a tip of a stability region to produce a mass filter. The addition of higher multipoles to the field has, in the past, been expected to degrade the performance of a linear quadrupole operated as a mass filter in this way [14]. Nevertheless, it has been found that linear quadrupoles with added octopole [15] and hexapole fields [12a] can in fact be operated as mass filters, provided the DC voltage is applied to the electrodes with the correct magnitude and polarity. Ions can also be mass analyzed in a linear quadrupole by radial ejection through slots in the rods [2]. Alternatively, axial ejection can be used for mass analysis with a linear quadrupole, either with or without trapping of ions. Ions within the quadrupole are excited by dipole or quadrupole excitation, gain sufficient kinetic energy to overcome a potential barrier at the quadrupole exit, and are ejected [3, 16]. Preliminary experiments show that this method can be used for mass analysis with a linear quadrupole that h...
Mass analysis with islands of stability has been investigated with three linear quadrupole mass filters: two with 4% added hexapole fields constructed with equal diameter (quadrupole 4A) and unequal diameter (quadrupole 4B) rods, and a conventional round-rod quadrupole that has apparently been slightly damaged. Islands are formed by applying auxiliary quadrupole excitation. With the Mathieu parameter, a Ͻ 0, mass analysis with both quadrupoles with hexapole fields operated normally, i.e., without islands, gives only low resolution. A factor of 10 or more increase in resolution is possible with the use of stability islands. With a Ͼ 0, when quadrupole 4A is operated normally, peak shapes similar to that of a conventional quadrupole can be obtained at resolutions higher than 850. At lower resolutions, peaks are split. When quadrupole 4B is operated without islands, resolution up to 2000 is possible, but there are low mass tails and structure is formed on the peaks. With mass analysis with an island of stability, both quadrupoles 4A and 4B show peaks free of structure and without tails. Ion transmission is also improved with some operating conditions. With the conventional round-rod quadrupole, mass analysis with islands of stability increases the limiting resolution from 2500 to 4360. At a resolution of 2500, the transmission is increased by about two orders of magnitude. These results show that the use of islands of stability improves mass analysis with quadrupoles with distorted fields, and may, in the future, allow use of quadrupoles constructed with at least some lower mechanical tolerances. In some instruments, a quadrupole operated as a linear ion trap (such as Q3 in a triple quadrupole mass spectrometer) must also be capable of mass analysis [9].In general, the potential of a linear quadrupole with field distortions can be written as [10]where x and y are Cartesian coordinates, r 0 represents the distance from the central axis to an electrode for an ideal quadrupole and otherwise is a normalization factor, Re͓f͑x ϩ iy͔͒ is the real part of the complex function f(x ϩ iy), i 2 ϭ Ϫ1, A N is the dimensionless amplitude of a multipole (2N-pole), and (t) is a timedependent voltage applied to the electrodes. An ideal linear quadrupole has A 2 ϭ 1 and all other multipoles zero. A practical linear quadrupole usually has A 2 Ϸ 1, and other higher order multipole amplitudes in the range 10 Ϫ5 to 10 Ϫ3 [2-4]. Methods of adding an octopole field of 2% to 4% [11], i.e., A 4 ⁄A 2 ϭ 0.02 Ϫ 0.04, and a hexapole field [12] of up to 12% (A 3 ⁄A 2 Յ 0.12) to linear quadrupoles constructed from round rods have been described. (Hexapole and octopole fields are the next two higher multipoles after the quadrupole in the expansion of eq 1.) A hexapole field is added by rotating the two y rods towards an x rod through a small angle . The amplitude of the hexapole, A 3 , is approximately proportional to . This method also adds other higher multipoles, including an octopole field [12]. Addition of these field distortions might be expe...
Conventional mass analysis has been investigated experimentally with six quadrupole mass filters with added hexapole fields; three with added hexapole fields of 4%, 8% and 12% with equal diameter rods, and three with added hexapole fields of 4%, 8% and 12% with unequal diameter rods to remove an added octopole field. Compared with conventional quadrupoles, these rod sets have very large field distortions. With the positive resolving dc applied to the y rods (Mathieu parameter a(x) < 0) only low resolution (10-100) and low transmission are seen. With the polarity reversed (a(x) > 0) much higher resolution (> or = 1000) and transmission are possible. Increasing the magnitude of the added hexapole field decreases the limiting resolution at m/z 609. Removing the added octopole field increases the limiting resolution. In some cases structure is formed on the peaks. For a given scan line slope, U/V(rf), the resolution decreases as the amplitude of the added hexapole field increases. These results are consistent with changes to the stability diagrams, calculated here. With a(x) > 0, adding a hexapole field causes the x stability boundary to move outward with all rod sets. With a(x) < 0, the boundaries become diffuse and the tip of the stability diagram becomes rounded, limiting the resolution to ca. 10-100. Where comparisons are possible, experiments show the rod sets with added hexapole fields have transmission 10-300 times less than a conventional quadrupole. Thus these quadrupoles are less useful for mass analysis than conventional quadrupoles. However, it is surprising, given the highly distorted fields, that some of the quadrupoles give resolution of 1000 or more.
Most Czerny-Turner based LIBS systems are used widely. In the present work, an instrumental setup based on Paschen-Runge is introduced, and analytical parameters are optimized for LIBS. With optimum parameters, analytical performance measures, such as precision, accuracy and detection limit, are compared for LIBS and Spark-OES. The results show that the analytical accuracy obtained by LIBS is similar to that obtained with Spark-OES, while the precision and detection limit are notably close to those of Spark-OES. Thus, LIBS can be used satisfactorily for process analysis in the metallurgy industry.
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