TOF Technology

Basics

BenchTOF-dx
TOF Principle (www.kore.co.uk)

In the time-of-flight mass spectrometry (TOF MS), the ion's mass-to-charge ratio (m/z) is determined by measuring the time required for the corresponding ion to reach a detector at a known distance. The ion is accelerated by an electric field of known strength, wherefore the ion obtains the same kinetic energy as any other ion that has the same charge. However, the velocity of the ion depends on the mass-to-charge ratio and, therefore, the time to reach detector will increase with an increasing mass-to-charge ratio. The time determined to reach the detector and the known distance between the ion source and detector, as well as the applied acceleration energy, are sufficient to calculate the mass-to-charge ratio of the ion.

 

Ion Source

EI Source (Almsco BenchTOF-dx)
Combined EI/FI/FD Source (JEOL AccuTOF)
Schematic of the DART Source (JEOL)

The TOF MS can be coupled as detector to a chromatographic system (e.g. GC, HPLC) or can be combined with a matrix-assisted laser desorption/ionization (MALDI) source or an atmospheric pressure ion source, e.g. the direct analysis in real time (DART™) ion source.

When coupling a TOF MS on a chromatographic system, different ionization sources are available. Beside the most widely-used electron impact ionization (EI) and chemical ionization (CI), various soft ionization techniques like the field desorption (FD), field ionization (FI), and liquid injection field desorption ionization (LIFDI) are used. The direct probe (exposure or insertation) allows the fast measurement of liquid or solid samples without previous chromatographic separation of the different components present.

MALDI is a soft ionization technique for mass spectrometry and used to analyse molecules which tend to be fragile when ionized by more conventional ionization methods (e.g. EI).  Biomolecules, e.g. biopolymers such as DNA, proteins, peptides and sugars, and large organic molecules, such as polymers, dendrimers or other macromolecules, are just some examples of such molecules.

The DART™ ion source allows to probe samples without preparation by instantaneously ionizes gases, liquids and solids in open air under ambient conditions.

 

Mass Analyzer

Different Flight Pathes

A TOF MS typically consists of an ion source (pulsed or continuous), a mass analyzer and a detector. The mass analyzer used in TOF-MS can be either be a linear flight tube, applying a bented geometry the the flight path to reduce the influence of neutral particles (Poschenrieder type) or a employ one (V-type) or more reflectrons (W-type) to enhance the path lenght within a given flight tube.

The usage of one reflectron approximately doubles the flight path, triples the flight time due to a deceleration by the ion's entry and an acceleration by it's exit of the reflectron, and corrects the energy spread by a first order time focusing which finally increases the resolution of the TOF MS. The energy spread leads to a spread in velocity and the resulting peak broadening calls for a spread in the apparent mass.

 

Detector

Microchannel Plate MCP (www.dmphotonics.com)

Ions were typically detected using microchannel plate detectors (MCP) or a fast secondary emission multipliers (SEM). The electrical signal created by those detectors is recorded by means of a time to digital converter (TDC) or a fast analog-to-digital converter (ADC). TDCs are fast ion counting detectors with a limited dynamic range. The use of multichannel detectors expands the dynamic range by assambling an array of mini-anodes to a common MCP stack with multiple TDC. Here, every TDC records signals from an individual mini-anode. The required intensity is received by adding up hundreds of individual mass spectra (hystograming). An individual spectrum on a TOF MS takes some hundred microsecondes, depending on the length of the flight path. The high counting rate is realized by a very high repetition rate of the ion extractions to the TOF tube is used (approx. 10 kHz).

Modern ADC digitize the pulsed ion current from the MCP detector at discrete time intervals, up to 250 picoseconds which correspondes to 4 GSample/sec. In addition, 8- or 10-bit 4 GHz ADC have much higher dynamic ranges than the comparable TDCs.

The mass resolution using TOF technology depends on the time resolution of the used converter. Therefore, the mass resolution with ultra-fast ADC can be improved by using MCP detectors with shorter response times.


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