Optimum Radius Size between Cylindrical Ion Trap and Quadrupole Ion Trap

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  • ABSTRACT

    Quadrupole ion trap mass analyzer with a simplified geometry, namely, the cylindrical ion trap (CIT), has been shown to be well-suited using in miniature mass spectrometry and even in mass spectrometer arrays. Computation of stability regions is of particular importance in designing and assembling an ion trap. However, solving CIT equations are rather more difficult and complex than QIT equations, so, analytical and matrix methods have been widely used to calculate the stability regions. In this article we present the results of numerical simulations of the physical properties and the fractional mass resolutions m/Δm of the confined ions in the first stability region was analyzed by the fifth order Runge-Kutta method (RKM5) at the optimum radius size for both ion traps. Because of similarity the both results, having determining the optimum radius, we can make much easier to design CIT. Also, the simulated results has been performed a high precision in the resolution of trapped ions at the optimum radius size.


  • KEYWORD

    Quadrupole ion trap , cylindrical ion trap , optimum radius size , fifth order Runge Kutta method , stability regions , ion trajectory , fractional mass resolution

  • Introduction

    Ion trap mass spectrometry has been developed through several stages to its present situation of relatively high performance and increasing popularity. Quadrupole ion trap (QIT), invented by Paul and Steinwedel,1 has been widely applied to mass spectrometry,2-11 ion cooling and spectroscopy,12 frequency standards, quantum computing,13 and so on. However, various geometries has been proposed and used for QIT.14

    An ion trap mass spectrometer may incorporate a Penning trap,15 Paul trap16 or the Kingdon trap.17 The Orbitrap, introduced in 2005, is based on the Kingdon trap.18 Also, the cylindrical ion trap CIT has received much attention in a number of research groups because of several merits. The CIT is easier to fabricate than the Paul ion trap which has hyperbolic surfaces. In addition, the relatively simple and small sized CIT make it an ideal candidate for miniaturization. Experiments using a single miniature CIT showed acceptable resolution and sensitivity, and limited by the ion trapping capacity of the miniature device.19-21

    With these interests, many groups such as Purdue University and Oak Ridge National Laboratory have researched on the applications of the CIT to miniaturize mass spectrometer.22,23

    Electric field inside CIT

    In CIT, the hyperbolic ring electrode,24 as in Paul ion trap, is replaced by a simple cylinder and the two hyperbolic end-cap electrodes are replaced by two planar end-plate electrodes.25 The potential difference applied to the electrodes24-26 is:

    image

    with

    image

    Where, Udc is a direct potential, Vac is the zero to peak amplitude of the RF voltage, is RF angular frequency, and z1 expresses the distance from the center of the CIT to the end cap and r1 the distance from the center of the CIT to the nearest ring surface. The electric field in a cylindrical coordinate (r, z,θ) inside the CIT can be written as follows:

    image

    here, ▽ is gradient. From Eq.(3) (grad), the following is retrieved:

    image

    he equation of the motions10,21,24,25 of the ion of mass m and e can be written as

    image
    image

    and the following

    image

    Where J0 and J1 are the Bessel functions of the first kind of order 0 and order 1, respectively, whereas ch is the hyperbolic cosine function, mir is the roots of equation J0 (mir) = 0. To obtain λis the Maple software was employed to find J0 (λi) = 0 roots. Eqs.(5) and (6) are coupled in u and v (respective r and z), and thus, can only be treated as a rough approximation.21,25 Therefore, studies on CIT equations are more difficult and complex compared to QIT equations. As stated earlier, the optimum radius size between CIT and QIT helps us to study QIT instead of CIT.27

    The motions of ion inside quadrupole ion trap

    A hyperbolic geometry for the Paul ion trap was assumed;

    image

    Here, z0 is the distance from the center of the QIT to the end cap and r0 is the distance from the center of the QIT to the nearest ring surface. In each of the perpendicular directions r and z, the ion motions of the ion of mass m and charge e5,24,28,29 may be treated independently with the following substitutions:

    image
    image

    CIT and QIT stability parameters

    If the ions the same species are taken into consideration and the same potential amplitude and frequency, the following relations has been obtained:

    image

    From Eq.(9) and one can obtain

    image

    The optimum radius between cylindrical ion trap and quadrupole ion Trap

    In some papers,21,24 stability parameters have been used to determine the optimum radius size for cylindrical ion trap compared to the radius size for the quadrupole ion trap, as following:

    image
    image

    Eqs.(11) and (12) are from Refs,21,24 respectively. In this study, Eqs.(5), (6) and Eqs.(7), (8) were used for the same propose to find optimum radius size for cylindrical ion trap compared to the radius size for quadrupole ion trap, as:

    image
    image

    with u' = r/r0 and v' = z/z0. Where α and χ are the trapping parameters, which λi is the root of equation J0 (mir1)=0. Eqs.(13) and (14) are true when (α,χ) and (az,qz) vqlues belong to stability regions. In this case, u = u(0) = c1, v = v(0) = v2, u' = u'(0) = c3, and v = v(0) = v4, were assumed. Here, u(0), v(0), u'(0), v'(0) are the initial values for u,v,u' and v', respectively. Now, from Eqs.(13) and (14) with d2u/2 = d2c1/2 = 0, d2v/2 = d2c2/2 = 0, d2u'/2 = d2c3/2 = 0 and d2v'/2 = d2c4/2 = 0, the following can be obtained:

    image
    image

    In adding Eqs.(15)and (16), we have:

    image

    After substituting α,χ,αz and qz we have,

    image

    with Eq.(18) gives the optimum value of z1 and z0 for CIT and QIT with conditions c1 = c3 and c2 = c4. After substituting λi',s,c1 = c3 = 0.01 and c2 = c4 = 0.01 ; in Eq.(18) and by simplification, we have:

    image

    Therefore, using the Maple software we will have,

    image

    In Eq.(20), z1 = 1.01978z0 is the optimum radius size between the quadrupole and the cylindrical ion traps. For any initial conditions we can obtain same answer with Eq.(20) almost. This optimal radius size (z1 = 1.04978z0) is almost comparable with the optimal radius size in Eq.(11)(z1 = z0) when χ = qz21,24 For the various u; v; r; z when (α, χ) and (az,qz) belongs to the stability regions, we found almost the comparable optimum values equivalent to Eq. (20) was found. For example with c1 = c3 = 0.005, c2 = c4 = 0.01 and c1 = c3 = 0.05, c2 = c4 = 0.01, we have z1 = 1.4976z0. and z1 = 1.05024z0.

    Numerical results

      >  Stability regions

    There are two stability parameters which control the ion motion for each dimension z (z = u or z = v) and (z = z or z = r), and az, qz in the case of cylindrical and quadrupole ion traps,24 respectively. In the plane (az, qz) and for the z axis, the ion stable and unstable motions are determined by comparing the amplitude of the movement to one for various values of az, qz.26,30 To compute the accurate elements of the motion equations for the stability diagrams, we have used the fifth order Runge-Kutta numerical method with a 0.001 steps increment for Matlab software and scanning method.

    Figure 1 (a) and (b) shows the calculated first and second stability regions for the quadrupole ion trap and cylindrical ion trap,31) black line (solid line): QIT and blue line (dash line): CIT with optimum radius size z1 = 1.04978z0, (a): first stability region and (b): second stability region. Figure 1 shows that the apex of the stability parameters az stayed the same and the apex of the stability parameters qz decrease for CIT to compare with QIT. Area of first stability regions for QIT and CIT are almost same, as 0.4136 and 0.4087, respectively. Figure 1 reveals almost a comparable stability diagram two methods.

      >  Phase space ion trajectory

    Figure 2 shows evolution of different values of the phase ion trajectory for ξ0 with red line : QIT with z0 = 0.82 cm and blue line: CIT with the optimum radius size

    The results illustrated in Figure 2 show that for the same equivalent operating point in two stability diagrams (having the same βz), the associated modulated secular ion frequencies behavior are almost same for the quadrupole and cylindrical ion traps with the optimum radius size z1 = 1.04985z0. Table 1 presents the values of for the quadrupole and cylindrical ion traps, when az = 0 and α = 0 with the optimum radius size z1 = 1.04985z0, respectively for βz = 0.3.;0.6;0.9. For the computations presented in Table 1, the following formulas were used:

    image

    and

    image

    for QIT and CIT, respectively. Hence, it is important to know that βz point are the equivalent points; two operating points located in their corresponding stability diagram have the same βz.31 For the same 0< βz <1 we have, Here, 1.35 and 1.23 are maximum values of stability diagrams for QIT and CIT when az = 0 and α = 0 respectively. Therefore, for βz = 0 we have and for βz = 1 we have for QIT and CIT, respectively. To compute Table 1, Maple software have been used.

      >  The effect of optimum radius size on the mass resolution

    he resolution of a quadrupole ion trap9 and cylindrical ion trap mass spectrometry in general with optimum radius size z1 = 1.04985z0, is a function of the mechanical accuracy of the hyperboloid of the QIT Δr0, and the cylindrical of the CIT Δr1, and the stability performances of the electronics device such as, veriations in voltage amplitude ΔV, the rf frequency ΔΩ,9 which tell us, how accurate is the form of the voltage signal.

    Table 2 shows the values of qzmax and Vzmax or the quadrupole ion trap and cylindrical ion trap with optimum radius size size z1 = 1.04985z0 in the first stability region

    when az = 0, respectively. The value of Vzmax has been obtained for 131Xe with = 2Π × 1.05 × 106 rad/s, U = 0 V and z0 = 0.82 cm in the first stability region when az = 0.

    To obtain the values of Table 2 we suppose Vzmax as function of for QIT and CIT with z1 = 1.04985z0, respectively as follows,

    image
    image

    Now, we use Eqs.(23) and (24) to calculate VzmaxQIT and VzmaxCIT for 131Xe with = 2Π×1.05×106 rad/s, z0 = 0.82 cm and z1 = 1.04985z0 as follows,

    image

    To derive a useful theoretical formula for the fractional resolution, one has to recall the stability parameters of the impulse excitation for the QIT and CIT with z1 = 1.04985z0, respectively as follows,

    image
    image

    By taking the partial derivatives with respect to the variables of the stability parameters qzQIT for Eq.(25) and qzCIT for Eq.(26), then the expression of the resolution Δm of the QIT and CIT, respectively are as follows,

    image
    image

    Now to find the fractional resolution we have,

    image
    image

    Here Eqs.(29) and (30) are the fractional resolutions for QIT and CIT with optimum radius size z1 = 1.04985z0, respectively.

    For the fractional mass resolution we have used the following uncertainties for the voltage, rf frequency and the geometry; ∆V / V = 10-15, ∆ / = 10-7, ∆r0 / r0 = 3×10-4. The fractional resolutions obtained are m/∆m = 1638.806949;1638.398047 for QIT and CIT with optimum radius size z1 = 1.04985z0, respectively. When optimum radius size z1 = 1.04978z0 is applied, the rf only limited voltage is increased by the factor of approximately 2.6893, therefore, we have taken the voltage uncertainties as ∆VCIT / VCIT = 2.6893 × 10-5. From Eqs.(29) and (30) we have (m/∆m)QIT = 1638.8069 and (m/∆m)CIT = 1598.6598 for QIT and CIT with optimum radius size z1 = 1.04985z0, recpectively. When these fractional resolutions are considered for the 131Xe isotope mass m = 3.18, then, we have ∆m = 0.001940436 and 0.001994156 for QIT and CIT with optimum radius size z1 = 1.04978z0, respectively. This means that, as the value of m/∆m is decreased, the resolving power is increased due to increment in ∆m. Experimentally, this means that the width of the mass signal spectra is better separated.

    Discussion and conclusion

    In this study, the behavior of the quadrupole and cylindrical ion traps with the optimum radius has been considered. Also, it is shown that for the same equivalent operating point in two stability diagrams (i.e. having the same βz = 0.3), the associated modulated secular ion frequencies behavior are almost the same with a suitable optimum radius size z1 = 1.04978z0 with This optimal radius size (z1 = 1.04978z0) is almost comparable with the optimal radius size in Eq.(11) (z1 = z0) when x = qz22,25

    Table 1 also indicate that for the same equivalent operating point, almost a comparison physical size between two ion traps are shown; z1 = 1.04978z0 = 0.86 cm and z0 = 0.82 cm. The CIT has a smaller trapping parameter compared to QIT; for example for βz = 0.3 we have a difference of 0.0564 higher for the QIT.

    This difference in trapping parameters indicates that for the same rf and ion mass values, we need more confining voltage for CIT than QIT (see Table 2). So, higher fractional resolution can be obtained; higher separation confining voltages, especially for light isotopes9,31 (see Figure 3).

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  • [Figure 1.] The first and second stability regions, black line (solid line): QIT with z0 = 0.82 cm and blue line (dash line): CIT with optimum radius size z1 = 1.04985z0 and (a): first stability region and (b): second stability region.
    The first and second stability regions, black line (solid line): QIT with z0 = 0.82 cm and blue line (dash line): CIT with optimum radius size z1 = 1.04985z0 and  (a): first stability region and (b): second stability region.
  • [Figure 2.] The evolution of the phase space ion trajectory for different values of the phase ζ0 for red line: QIT with z0 = 0.82 cm and blue line :CIT with optimum radius size z1 = 1.04985z0, and z = r1v.
    The evolution of the phase space ion trajectory for different values of the phase ζ0 for  red line: QIT with z0 = 0.82 cm and blue line :CIT with optimum radius size z1 = 1.04985z0,  and z = r1v.
  • [Table 1.] The values of for the quadrupole ion trap and cylindrical ion trap when az = 0 and α = 0 with z0 = 0.82 cm and optimum radius size z1 = 1.04985z0 and for βz = 0.3,0.6,0.9
    The values of  for the quadrupole ion trap and cylindrical ion trap when az = 0 and α = 0 with z0 = 0.82 cm and optimum radius size z1 = 1.04985z0 and  for βz = 0.3,0.6,0.9
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  • [Table 2.] The values of qzmax and Vzmax for the quadrupole ion trap with optimum radius size z1 = 1.04985z0, respectively in the first stability region when az = 0
    The values of qzmax and Vzmax for the quadrupole ion
 trap with  optimum radius size z1 = 1.04985z0, respectively in the
 first stability region when az = 0
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  • [Figure 3.] The resolution of ?m as function of ion mass m for 131Xe with Ω = 2Π × 1.05 × 106 rad/s, z0 = 0.82 cm and z1 = 1.04985z0, dash line: for CIT and dash point line: for QIT.
    The resolution of ?m as function of ion mass m for 131Xe with Ω = 2Π × 1.05 × 106 rad/s, z0 = 0.82 cm and z1 = 1.04985z0, dash line: for CIT and dash point line: for QIT.