Introduction to LC-MS Part5

Until now, we have described mass spectrometry as a method used to measure the mass of atoms and molecules, but how does it actually measure mass? Starting with this page, we will spend then next few pages describing the mass spectrometer unit of LC-MS systems.

Role of the Mass Spectrometer Unit

Normally when we measure mass, we use a scale or balance, which relies on the Earth's gravity. So, how do you measure the mass of a molecule, which is so extremely small that its gravitational force is almost too small to measure?
The English physicist J.J. Thomson utilized the fact that the flow of charged particles bends in an electric or magnetic field to develop an instrument that could separate charged particles by mass number.

In his instrument, which used a cathode ray tube, cations with identical ratios of charge e and mass m converged along the same parabola. When he measure the neon gas molecule, the parabolas for ²⁰Ne and ²²Ne (both are monovalent cations) were slightly different, which proved the existence of isotopes (1912). If this electromagnetic interaction is utilized, ionized compounds can be separated and measured according to mass-to-charge (m/z) ratio.

Types of Mass Spectrometer Units

As described earlier, the key components of a mass spectrometer consist of a sample introduction unit, ionization unit, mass analyzer unit, and ion detection unit. In recent years, most LC-MS system use atmospheric pressure ionization methods, such as ESI or APCI.
However, a wider variety of models have been used for the mass spectrometer unit, which separates the ions. In the past, magnetic sector models,quadrupole models, and time-of-flight models were frequently used for measuring organic compounds, but the relatively cheaper quadrupole models have gradually been increasing their share.

In addition, an ion-trap MS system that temporarily accumulates ions of a selected range before separating them by mass, and a tandem or hybrid MS system that combines multiple MS units have been developed as well (Table 1). These various types of MS systems each take advantage of their respective features and are used according to analytical objectives.

Magnetic Sector MS

Magnetic sector MS has been used historically the longest (Figure 1).

2 to 8 kV of high voltage is applied to ions generated in an ionization unit to accelerate them into a magnetic sector. According to Fleming's left-hand rule, ions are accelerated in a direction perpendicular to velocity v and the magnetic field, resulting in a curved path.

(Fleming's left-hand rule: At the entrance to the magnetic field in Figure 1, positively-charged ions move toward the right (direction of current flow), so the middle finger of the left hand points to the right. The magnetic field flows toward the viewer, so the index finger also points toward the viewer. That means the resulting force (direction of thumb) is downward, which is toward the right in relation to the forward movement of the positively-charged ions.)

Ions experience a Lorentz force f₁ from the magnetic field that can be calculated according to equation (1).

(where, B: magnetic flux density, z: charge number, e: elementary charge, and v: velocity)

As the direction of an object's movement changes, a centrifugal force f₂, expressed by equation (2), acts on the object.

(where, m: mass and r: path radius)

For the ion to pass through the magnetic field region and reach the detector, it must travel along a curved path of a given radius. In other words, it must move with forces f₁ and f₂ in balance, as described by equation (3).

Meanwhile, the kinetic energy of ions accelerated by voltage V is described by equations (4)

and equation (5) results from equations (3) and (4).

In this case, the motion of ions satisfy equation (5). By keeping the ion acceleration voltage V constant and varying the magnetic flux B (or keeping B constant and varying V) in equation (5), a detector placed on the corresponding path radius r could detect any particular mass m.
In actual magnetic sector mass spectrometers, only one ion detector is used and both the acceleration voltage and curve radius are kept constant while the magnetic flux is scanned. This means that ions with different masses all pass along the path through the magnetic field, one after another, and reach the detector. One mass spectrum is obtained from each scan of the magnetic field.

Characteristics of Magnetic Sector MS

The measurement range of magnetic sector MS systems typically is about 10 to 10,000, though it depends on the acceleration voltage V and instrument design. Resolution of about 2000 can be obtained using a single-focusing magnetic sector model or several tens of thousands using a dual-focusing magnetic sector model. Before high-performance time-of-flight MS and ion cyclone MS systems appeared in recent years, the dual-focusing magnetic sector spectrometer was the only type capable of such high-resolution measurements. In magnetic sector MS systems, the strength of the magnetic field has a major affect on performance, which means larger and larger systems were required to increase performance.
Since magnetic sector MS systems require an extremely high vacuum level of 10-7 Pa, they are difficult to interface with the LC unit, and they also have the disadvantage of a slower scan speed than other MS systems. Therefore, they are now rarely used as LC-MS systems. On the other hand, they interface relatively easily to a GC unit. Consequently, such GC-MS systems are used for dioxin analysis due to the outstanding high-resolution selected ion monitoring (HR-SIM) capability of magnetic sector MS.
Time-of-flight and quadrupole type MS systems will be discussed on the next page.