Time of Flight Mass Spectrometry Using the TDC-GPX2

February 12th, 2020

Different chemicals have different masses that impact their time of flight as will be described below.  A mass spectrometer can be used to determine what chemicals are present in a sample.  When viewed in the time domain, peaks will occur at specific points relative to the masses of their molecules.  Mass Spectrometry can also be helpful in determining the purity of the sample and the structure of the molecules.

A time-of-flight mass spectrometer (TOFMS), like all mass spectrometers, consists of an ion source (sample), a mass analyzer, and a detector.  There are a couple of different configurations of the mass spectrometer.  The TOF ion sources can be pulsed or continuous.  The TOF mass analyzer can be a linear flight tube or a reflectron, as shown in the simplified diagram below.  The ion detector usually has a microchannel plate detector or a delay line detector and a time-to-digital converter (TDC).

Simplified Mass Spectrometer Diagram

Ionization is the procedure of converting an atom or molecule into an ion by changing the balance of the number of protons to the number of electrons.

A positive charge is produced if an atom or molecule loses an electron.  The loss of an electron can be initiated by providing enough energy for it to escape the electric potential barrier that held it in place.  A laser can be used to provide enough energy for an electron to escape.

Conversely, a negative electric charge is produced when a free electron collides with an atom and is trapped inside the electric potential barrier, releasing any excess energy.  Electron bombardment can be used to produce negative ions.

A third method is to have two surfaces close together with a high voltage difference.  This causes the gas between them to break down into a plasma cloud (ion cloud).  This is how the Jacobs ladder (an arc traveling up a pair of v-shaped electrodes seen in the old science fiction movies) works.

Ionization is the first step in time of flight (TOF) mass spectrometry.  If a positive ion is placed between a positive electrode and a negative electrode the ion will be repelled by the positive electrode and move in the direction of the negative electrode. 

If the positive electrode is replaced by a plate and the negative electrode is replaced by a grid then this is the beginning of a mass spectrometer.  The grid is called the extraction grid and the plate is called the repeller plate.

Mass spectrometry requires that the “start” time to be precisely controlled since it is a time of flight measurement.  A laser pulse can be controlled to get an accurate “start” time. After leaving the extraction grid, the ion is accelerated by a grid having 10 times the voltage potential than that of the repeller plate, so much greater attraction. Next, the ions are put on the desired course by repelling them with steering plates (or Ion guides).  The diameter of the ion packet is reduced by a cone-shaped orifice called a skimmer. Once the ions pass the beam shaper, they enter the flight tube.  At the end of the flight tube, there is a reflectron that acts like a mirror alters the ion course and sends it to a detector that gives the “stop” time.

How can time-to-digital converters measure the mass of a molecule?

In a mass spectrometer, we know the electrical charge of the Ion and we know the electrical field strength.  This electrical field causes an acceleration of the ion.  This acceleration results in all ions having the same kinetic energy (same charge).  Since Kinetic Energy= ½ m∙v2, the lighter ions go faster. The faster ones arrive sooner and therefore the time can be correlated to the mass.  In other words, the velocity of the ion will depend on its mass to charge ratio (See Below).

Mass Spectometry Formula

The precision of the TOF spectrometer depends greatly on the precision of the electronic data acquisition used.  In principle, TOF spectrometers allow the complete mass distribution to be measured with one single shot.  With this method, huge amounts of data are created within a very short time interval.  The challenge in TOF data acquisition is to design a system that combines extreme velocity (time resolution < 1 ns), minimum dead time and high data rates.  Depending on the ionization scheme TOF spectra are either measured using very fast AD converters or fast ion counting techniques such as a TOF/multi-channel analyzer using time-to-digital converters.  In the case of laser ionization, one laser pulse can create hundreds of ions which arrive in a pulse a few nanoseconds wide at the detector.  The total time of flight is typically in the range of several microseconds.  To improve statistics many spectra are recorded and averaged.  Typically, the data transfer rate is what limits the spectrometer repetition frequency.

The TDC-GPX2 is well suited for this measurement task.  The GPX2 has two different modes to choose from.  Both modes fit this application very well.  The four-channel operation gives you 20ns pulse-to-pulse spacing on the same channel, with a maximum data rate of 35MSPS.  Two-channel mode reduces the pulse-to-pulse spacing to 5ns and allows a maximum data rate of 70MSPS.  The 5ns pulse-to-pulse spacing allows simultaneous detection of molecules with only a small difference in mass-to-charge ratio.  Typically, two peaks in the spectrum can be separated when they are 2 sigmas apart.

There are also two resolution modes to pick from.  The Single Shot resolution is 20 picoseconds per channel.  With the High-Resolution option turned on, the resolution is 10 pico-seconds.  The measurement range is 0 to 8 seconds.

Since the molecules have the same start time and travel at different speeds, the stop times will be different for each type of molecule.  Because of this, the need for multi-hit measurement is essential.  The TDC-GPX2 has in principle unlimited multi-hit capacity, limited only by the pulse-to-pulse spacing on the chosen resolution, the FIFO depth, and the maximum readout rate of the interface.

The user should be aware of the differential non-linearity of digital TDCs which is typical for all TDCs. Because of the measuring principle, wide and narrow BINs follow each other due to the difference in rising and falling times of the gates used in the ring oscillator.  But this DNL is highly systematic and can be corrected by software.