Unit 1 Overview of Time of Flight Mass Spectrometry

Unit 1 Overview of Time of Flight Mass Spectrometry 

Time of flight Mass Spectrometry after been created for a very long time did not become very well known, until its resolution was further improved upon by orthogonal acceleration, reflectrons and high speed electronics. The improved resolution coupled with appropriate ion sources and quadrupole mass filter technology has helped in no small way for the observation of particles that can be separated by gas chromatography.

The process of separating ions in TOFMS

The time of flight mass spectrometer functions by calculating the mass dependent time for ions whose masses differ from their sources to the detector. This process requires that the time which these ions leave their source is highly characterized. The formation of particles occurs by a pulsed ionization strategy, usually, Matrix Assisted Laser Desorption Ionization (MALDI), or various types of rapid electric filed switching, which aid the discharge of ions from their sources in the shortest available time.

Note: Kinetic energy of an ion discharged

T=eV=mv²/2

Ion velocity (V)=L/t

Substitute this v in to the kinetic energy saver equation,

 m/e=2Vt²/L²,reaarange this equation to obtain the time of-flight: t=L√m/e 1/2V

Reflectron

Ions leaving their sources in a time of flight mass spectrometer have different start times and kinetic energies, efforts have been made to cover up for these differences  by creating a variety of time-of-flight mass spectrometer designs, manufactured to minimize the effects of these differences. A reflectron is an ion optic device which causes a reversal when ions in a time of flight spectrometer go through its mirror, It was invented by (B.A Mamyrin, 1973).

Ions with greater kinetic energies in a linear field reflectron are able to infiltrate much more than those with smaller kinetic energies. It takes a much longer time for those ions that have greater kinetic energy, hence having the ability to penetrate deeper, to come back to the detector. When varying kinetic energies occur in a group of ions with known mass-to-charge ratio, there is a decrease in the spread of the ion flight times, hence, have a positive effect on the time of flight mass spectrometer by improving its resolution. See more : Mergers  Acquisitions and Private Equity Assignment Help

In the instance of a curved field reflectron, it prevents a fluctuation in a perfect detector position for a TOFMS with mass-to-charge proportion, this improves TOFMS resolution.

Flight Time and its Relationship to Mass Equations for time of flight

Every mass has a peculiar time of flight, it occurs when there is a connection between a high voltage pulse and the back plate of the ion pulser, and stops when the detector is struck by the ion,(John Fjeldsted, 2003). To obtain time of flight (t), kinetic energy (E), distance (d) and mass to charge ratio (m) are determined. These time of flight analysis can be solved using two popular formulae:

The first one is the Kinetic energy formula: E = 1/2mv²

which is solved for m as shown below:

m = 2E/v²

and also solved for v as shown below:

v = ` (2E/m)

In the mathematical equation above: in a given Kinetic energy (E), there is a difference  in  velocities as there is in masses, smaller masses have higher velocities while bigger ones have smaller velocities, this is a perfect illustration of the actions in a time-of-flight mass spectrometer. The detector is struck faster by ions with lower masses, it is better to gauge the time an ion strikes the detector, than to determine its velocity.

Figure:3. Time of flight investigations of different masses, each with a single charge. This illustration is presented in a linear time of flight mass spectrometer without an ion mirror for clarity sake.

Remember the second mathematical statement: v=d/t

Consolidating the first and second: m = (2E/d2)t², (Tom Field,2010, p.4)

We are able to derive the time of flight relationship, Kinetic energy (E), with a distance (d), the mass (m) is directly proportional to the square of the time of flight of the ion.

We are able to derive the time of flight relationship, Kinetic energy (E), with a distance (d), the mass (m) is directly proportional to the square of the time of flight of the ion. Much exertion is given to maintaining the values of the E applied to ions, in configuring an oa-TOF mass spectrometer, while the distance (d) the ions travel is constant. This is to ensure that an exact mass value is obtained from an exact estimation of flight time. Therefore: m = At²

This equation is ideal for deciding the ion flight time and mass relationship.  A delay is recorded in practice, from the time control hardware initiates a pulse to the time a high voltage is experienced on the back ion pulser plate, likewise, a delay is also recorded from the arrival of ion at the detector to the digitization of ion by acquisition electronics.  Although, these period of lag are short, however, they are important, since we cannot measure the true time of flight , we adjust the measured time “tm”, this is done by subtracting the aggregate of both the start and stop lag times, which can be given as t=tm-to.  Read more : Mobile Client/Server Application Assignment Help

By substitution, the essential formula that can be applied for geuine estimations then becomes: m=A(tm-2)

TOFMS optimization for mass accuracy, resolution and dynamic range

Mass calibration

To be able to solve for mass (m) from measured flight time (tm), “A” and “to” needs to be settled, therefore, a calibration is carried out. This process is done with by analyzing a solution of compounds of known masses with high accuracy level, a table is drawn of flight times comparing known masses, (John Fjeldsted , 2003, p5). This is illustrated in table 1. Order Now