The Linear Cycloid MS Technology Platform

Early mass spectrometry development between 1930 and 1950 was oriented toward low molecular weight gas analysis.  This was a reflection on the state of development in the mass spectrometer equipment, as well as limited sample introduction methods for large molecules.  During these formative years in mass spectrometry, mostly fixed gases and light hydrocarbons were studied up to around 200 amu.  Development was accelerated by a high priority application, oil refining.

A classic review of early development in mass spectrometry was written by Professor John H. Beynon entitled, “Mass Spectrometry and Its Applications to Organic Chemistry”.  The following excerpt is particularly relevant.

A most important feature in the design of a mass spectroscope, and one which it is convenient to use for distinguishing between different types of machine, is the method used to focus the ion beam.  Focusing improves the degree of separation between adjacent masses, increases the intensity of the beam to be measured, and thus makes measurements of the strength and position of the beam more precise.  The range of problems to which any particular mass spectroscope is suited is controlled to a large extent by the efficacy of the focusing method.  Types of focusing which can be used to concentrate a beam of ions all of the same mass are direction focusing in which the ion beam is focused for a number of different initial directions when all the ions are moving at the same speed, and velocity focusing in which the ion beam is focused when it contains ions traveling with a range of different speeds, provided they are all moving in the same initial direction, and double focusing in which the ion beam of varying initial speed and direction is brought to a focus.

Beynon concludes this short introduction by stating that “perfect” double focusing and instruments using such systems have been constructed, referring to a cycloid design.  In the real world, ions formed in any source generally have a significant spread in both direction and velocity.

One way to provide perfect focus with spatial mass dispersion, is to inject a beam of positively charged ions into a crossed combination of homogeneous magnetic and electric fields.   This method was first used by Bleakney & Hipple in 1938.  The equations for motion for an ion in this environment describe a trochoid trajectory where the ions are brought to a perfect focal point as they pass the same x-axis plane which defined the location of the ion source.  The distance along this x-axis, known as the pitch, is described by the following equation:

        A = (2 x p x E/H²) x m/e          Eq. 1

where: A = pitch (the distance from “B” to “A” below)

             E = electric field strength (in the vertical direction below)

             H = magnetic field strength (into the page below)

             m = mass of ion

             e = charge on ion

The following diagram depicts the cycloid design, where ions formed in a source at “B” are detected with an ion collector at point “A.”  This patented linear cycloid design is used in our Series 3000 instruments.

Figure 1: Monitor Linear Cycloid

Since neither the initial velocity nor direction of motion appear in Equation 1, initial values of these parameters do not affect the ion focus.  This relationship forms the basis for describing the cycloid as a “perfect” double focusing mass spectrometer: the term coined by Prof. Beynon to describe a design which was rigorously independent of both direction and energy ion spread.

It is useful to examine other basic relationships about ion trajectories in light of miniaturized mass spectrometer design objectives: low cost, ease of use, small size and superior analytical performance.  The relationship between radius of curvature (size) and ion energy is shown in Equation 2.

          R = (2mV/e)^1/2/H                  Eq. 2

        where: R = radius of curvature

                  V = ion acceleration voltage

                   m = mass of ion

                   e  = charge on ion

                    H = magnetic field strength

A small magnetic deflection mass spectrometer implies low ion energies. For example, in order to decrease the radius of curvature of a given ion by 10, the ion kinetic energy must be decreased by a factor of 100.  Magnetic instruments with a radius of curvature of 1-2 cm have ion energies on the order of 10’s of volts.  Therefore small variations in ion energy, due to thermal energy spread for example, become relatively important components to the energy of the ion beam.  This relationship was recognized by Robinson & Hall at CEC when they commercialized the first cycloid mass spectrometer.  In the words of Beynon, “an instrument with perfect focusing properties for both velocity and direction is particularly desirable as the scale of the instrument is reduced.”  This is the fundamental design premise of the Monitor linear cycloid mass spectrometer.

In the following SIMION trajectory plots (Figures 2 and 3), ions formed in a micro electron impact ionizer located near a ground plane are focused through an entrance slit into a uniform electric field produced by two sets of positive and negative plates. The two outer plates are solid structures while the inner plates are cut out to form an annular space in which the ions fly their trajectories. The magnetic field is in the z-axis (into the plane of the paper). The focal point is at the ground plane where the ions pass through an exit slit and are collected. The distance between the entrance and exit slits is referred to as the pitch. Ions with the same mass to charge (m/e) have the same pitch regardless of ion energy or injection angle.

Figure 2: A single mass to charge ion is shown in the crossed magnetic and electric field of the cycloid mass spectrometer. The trajectory profiles are shown with each ion represented as a line with different injection angle and energy. The total energy and angular spread combine to form the width of the black area.

The Monitor linear cycloid analyzer can be supplied with either a single ion collector configuration or in a dispersive multiple collector configuration.  In the former, the ions are detected sequentially by scanning the electric field.  This more versatile configuration, depicted in Figure 2, is the one most commonly used.  The dispersive cycloid analyzer is depicted in Figure 3.   It provides continuous detection of pre-determined ions of interest.  This is sometimes a critical performance parameter.

Figure 3: Dispersive, or multiple collector configuration. Ions with m/e 18, 28, and 44 dispersed to three different collector slits and faraday collectors. Unlike the previous plots, each m/e ion is represented by one average line.

There are many applications where the linear cycloid mass spectrometer provides significant advantages.  The perfect focusing properties lead to better mass separation than can be achieved with alternative mass spectrometers –at a fraction of their cost and size.  Furthermore, the superior abundance sensitivity (the spill-over of ion current into an adjacent mass measurement), its inherent stability and the characteristic flat top peaks make this a viable quantitative analyzer. By way of comparison, the Monitor linear cycloid abundance sensitivity is less than 50 ppm; similar instruments typically have at least 100-200 ppm abundance sensitivity.

This technology is a distinct advantage, particularly in

·    Isotope ratio measurements,

·    Quantitative gas stream analyses, and

·    Direct mixture analysis without pre-separation.