Overview of Time of Flight Mass Spectrometry

Overview of Time of Flight Mass Spectrometry (TOFMS) 

TABLE OF CONTENTS

INTRODUCTION

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 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.

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.

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

Labeled picture of Time of Flight Mass Analyzer

Figure 1: Labeled picture of Time of Flight Mass Analyzer

TOFMS optimization for mass accuracy, resolution and dynamic range

Mass calibration

To be able to solve for mass (m) from measured flight time ™, “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.

Table 1. TOF Mass Calibration

Calibrant compound mass (u)          Flight time(μsec)
49.9962613.2900
68.9946615.8984
130.9914720.8786
218.9850826.6954
263.9865629.2101
413.9769736.3206
501.9705839.8914

“m” and “tm” have known values over the mass range, the calculation to decide “A and to” is done by a computer that accepts data from the instrument. Non linear regression method is used to discover the values, so that the right hand side and left hand side of the  equation match as much as obtainable for each of the eight of the mass values in the calibration.

It is important to have another step of calibration, as the earlier derivations, though accurate are not precise enough. After resolving “A and to” there is a comparison between the real mass values for the calibration masses and their calculated values qualities fro equation. There is usually a deviation of just a few parts per million(ppm). A second pass correction is carried out to obtain a better mass calibration, since the deviations obtained are not so great and are relatively consistent.

Reference mass correction

To deliver exact mass estimations, it is important to establish a very precise mass calibration, A slight change in energy applied to ions brings about a detectable mass movement, this defeats the objective of achieving mass accuracies at or even below 3 ppm. It is possible to counterbalance these effects by employing the use of reference mass correction.

Dynamic Range

Dynamic range can be measured in various methods; probably the most accurate definition for mass spectrometry is the in-scan condition,(TOFMS,2011, P.12). It is the dynamic range within a single spectrum, defined as the ratio in signal abundance of the biggest and smallest useful mass peaks. Despite the confinement to the in-scan meaning of dynamic range, the boundaries of this confinements must be characterized, which include the hypothetical and practical confinements.  In the hypothetical confinement, it is conceivable to single out a lone ion, however for all intents and purposes, this is obscured and considered as low level in a chemical background. For practical confinements, it is more dependent on the application, for instance: the lower limit is set by the base example sum, where exact mass estimations can be retrieved, when the instrument is used to obtain precise mass estimation.

Taking in to consideration the confinement of ion measurements, the base example sum is decided. With a goal of achieving an objective of 5 ppm mass precision, with a certainty level of 67% considering a lone unaveraged spectrum allowing for 1ppm of calibration error, the 1s=4 ppm.  According to the presumption of 10,000 resolving power, approximately 200 ions will be required for this estimation.

This data is autonomous of acquisition innovation, and depends on determining power and particle ion statistics, the estimation is unaffected by back-ground pollution as it is of the assumption that there is a note worthy sensitivity(signal to –noise).

 Appropriate ion sources for TOFMS

Electrospray ionization and Matrix Assisted Laser Desorption Ionization (MALDI) are new strategies for ionization; they are better suited for substantial organic molecules, as they have an advantage over the old technique called the “Electron gun”, which has destructive effects  like complete fragmentation of a molecule, leaving no molecular ion.

Electrospray Ionization (ESI): it often creates charged ions, whose quantities are directly proportional to atomic weight increase. A sample solution is showered from a needle in to an orifice in the interface over a high potential difference, the ion existing in the sample solution are broken by applying heat and gas streams.

+ good for charged, polar or basic compounds, m/z alright for most MS, best for multiple charged ions, low foundation, controlled fragmentation, MS/MS compatible

– integral to APCI: not useful for uncharged, non-fundamental, low-polarity compounds,

low ion currents

mass range <200’000Da, (Pierre-Alain Binz,2004, p.4)

Overview of Time of Flight Mass Spectrometry

Fig.2 : Electrospray Ionization process

MALDI: Matrix-Assisted Laser Desorption Ionization: analyte co-cystallized in matrix.

The matrix chromophore ingests and appropriate the energy of a laser, created plasma, vaporizes and ionize the sample.

+ rapid, convenient for molecular weight (lone charged ions for the most part)

– MS/MS difficult almost not compatible with LC coupling <500’000Da, (Pierre-Alain Binz, 2004, p.5).

Overview of Time of Flight Mass Spectrometry

Fig.3: Example of matrices used in MALDI

Detection System

A detection apparatus is very sensitive; Ions isolated by mass to charge proportion are easily detected. Under low pressure, with the aid of electrostatic lenses, ions are gathered to the analyzer, the sensitivity level have improved with advances in technology.

Advantages of Time of Flight Mass Spectrum

  • It is ranked the highest mass range of all the Mass Spectrum analyzers
  • It has a very high ion transmission
  • It is the fastest known MS analyzer
  • It is appropriate for pulsed ionization methods
  • MS/MS information from post-source decay

 Disadvantages

  • It has a limited precursor- ion selectivity
  • It requires pulsed ionization method
  • TOF fast digitizers have limited dynamic range