Unit 2 Overview of Time of Flight Mass Spectrometry

Unit 2 Overview of Time of Flight Mass Spectrometry

Table 1. TOF Mass Calibration

Calibrant compound mass (u)	Flight time(μsec)
49.99626	13.2900
68.99466	15.8984
130.99147	20.8786
218.98508	26.6954
263.98656	29.2101
413.97697	36.3206
501.97058	39.8914

“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. Read more : Five Personality Traits Assignment part 1

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. See more : Business Organization System part 1

+ 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)

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).

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

References

• R PLAB. (2008).Time of flight Mass Spectrometry. ECT workshop Mass Olympics, Trento, Italy, 2-8.

• Peter Van Galen. (2005) .Mass Spectrometry. Organic chemistry department, Nijmegen University, (4-24). Order Now