5.5.4 Time-of-Flight (TOF) mass filter
While time-of-flight mass filters were one of the first MS systems to be developed, they had limited use due to their need for very fast electronics to process the data. Developments in fast electronics and the need for mass filters capable of resolving high mass ranges (such as in MALDI systems) has renewed interest in time of flight systems. TOF is used exclusively with MALDI systems and also has other applications, as in HPLC where high molecular weight compounds are encountered.
Entry into the TOF mass filter is considerably different than with other mass filters. The entry has to be pulsed or intermittent in order to allow for all of the ions entering the TOF to reach the detector before more ions are created. With sources that operate in a pulsing fashon such as MALDI or field desorption, the TOF functions easily as a mass analyzer. In sources that continually produce ions such as a GC system or an EI source, the use of a TOF is more difficult. In order to use a TOF system with these continuous sources, an electronic gate must be used to create the necessary pulse of ions. The gate changes the potential on an accelerator plate to only allow ions to enter the TOF mass filter in pulses. When the slit has a positive charge, ions will not approach the entryway to the mass analyzer and are retained in the ionization chamber. When all of the previously admitted ions have reached the detector, the polarity on the accelerator(s) is again changed to negative and ions are accelerated toward the slit(s) and into the TOF mass analyzer. This process is repeated until several scans of each chromatographic peak have been measured. (This type of ionization and slit pulsing will be shown in the animation below). The other way to interface EI with TOFs is to operate the EI source in a pulsing mode. This is achieved by maintaining a constant negative polarity on the accelerator plate/slit, and pulsing the EI source. This method can also periodically introduce packets of ions into the TOF mass filter.
Whichever type of ionization and entry into the TOF mass filter is used the remainder of the process is the same. Prior to developing the mathematics behind TOF separations a simple summary is useful. Mass to charge ratios in the TOF instrument are determined by measuring the time it takes for ions to pass through the “field-free” drift tube to the detector. The term “field-free” is used since there is no electronic or magnetic field affecting the ions. The only force applied to the ions occurs at the repulsion plate and the acceleration plate(s) where ions obtain a similar kinetic energy (KE). All of the ions of the same mass to charge ratio entering the TOF mass analyzers have the same kinetic energy and velocity since they have been exposed to the same voltage on the plates. Ions with different mass to charge ratios will have velocities that will vary inversely to their masses. Lighter ions will have higher velocities and will arrive at the detector earlier than heavier ones. This is due to the relationship between mass and kinetic energy.
The kinetic energy of an ion with a mass m and a total charge of q = ze is described by:
where VS is potential difference between the accelerator plates, z is the charge on the ion, and e is the charge of an electron (1.60 x 10-19 C). The length (d) of the drift tube is known and fixed, thus the time (t) required to travel this distance is
By solving the previous equation for v and substituting it into the above equation we obtain
In a TOF mass analyzer, the terms in parentheses are constant, so the mass to charge of an ion is directly related to the time of travel. Typical times to traverse the field-free drift tube are 1 to 30 ms.
Advantages of a TOF mass filter include their simplicity and ruggedness and a virtually unlimited mass range. Additionally, virtually all ions produced in the ionization chamber enter the TOF mass filter and traverse the drift tube. However, TOF mass filters suffer from limited resolution, related to the relatively large distribution in flight times among identical ions (resulting from the physical width of the plug of ions entering the mass analyzer). Animation 5.8 illustrates how a pulsed accelerator plate/slit acts as a gate for a TOF mass filter system.
Animation 5.8. Illustration of a Traditional TOF Mass Filter.
Animation 5.9 illustrations how a pulsed accelerator plate/slit acts as a gate for a reflective TOF mass filter system. (The system shown is actually for the analysis of metal isotopes with an Inductively Coupled Plasma (ICP), but the reflective TOF works the same for organic analytes.
Animation 5.9. Illustration of a Reflective TOF Mass Filter
Ion Mobility Mass Spectrometry:
If you have been in an airport recently you have seen or your luggage has been analyzed by an Ion Mobility Spectrometer (IMS). Although originally developed by Earl W. McDaniel of Georgia Institute of Technology in the 1950s, IMS systems have gained popularity recently due to their versatility—they can designed for specific classes of compounds, they have excellent detection limits, and they can be manufactured to be light-weight and mobile.
The basic design is similar to the TOF mass filter. Important differences are that they use an easier ionization source, they can be operated at atmospheric pressure and therefore do not necessarily require pressurized gases or high vacuum pumps, and as a result of their atmospheric pressure sample introduction they have superior detection limits. Samples are introduced at atmospheric pressure and ionized by corona discharge, atmospheric pressure photoionization (APPI), electrospray ionization (ESI), or a radioactive source such as a small piece of 63Ni or 241Am, similar to the thoses used in ionization smoke detectors or GC electron capture detectors. The ionized analytes are then introduced to the drift tube by a gate valve similar to the one described earlier in this section for TOF mass filters. However, the IMS drift tube is different in that it can be operated at atmospheric pressure and is a counter current environment. The analytes travel from left to right in the one-meter drift tube due to a 10-30 kV potential difference between the inlet and exit. As the analytes are mobile due to the potential they travel through a buffer gas that is passed from right to left in the drift tube (and atmospheric gases are commonly used). Separation of different analtyes is achieved due to each ion having a different mass, charge, size and shape (the ion mobility). As the ions are electrically drawn toward the detector, the ion’s cross sectional area strikes buffer gases and its velocity is reduced based on its size and shape. Larger ions will collide with more buffer gas and be impeded, travel slower, and arrive at the detector after longer times in the drift tube. Detectors for IMS are usually relatively simple Faraday cups but better detection limits can be obatined with an EM.
The most common use of IMS is for volatile organic molecules. IMS has been expanded for use in gas, liquid, and super critical fluid chromatography.
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