AxisPharm Services

Principle of time-of-flight mass spectrometry and ionization process

Time of Flight Mass Spectrometer is a very commonly used mass spectrometer.

The mass analyzer of this mass spectrometer is an ion drift tube. The ions generated by the ion source are collected first. All ion velocities become 0 in the collector. Accelerated with a pulsed electric field, it enters the field-free drift tube and flies towards the ion receiver at a constant speed. The larger the ion mass, the longer it takes to reach the receiver; the smaller the ion mass, the shorter the time it takes to reach the receiver. According to this principle, ions of different masses can be separated according to the m/z value. The time-of-flight mass spectrometer can detect a large molecular weight range, has a fast scanning speed, and has a simple instrument structure.

The main disadvantage of traditional time-of-flight mass spectrometers is the low resolution, because the initial energy of the ions when they leave the ion source is different, so that the ions with the same mass-to-charge ratio reach the detector with a certain time distribution, resulting in a decrease in the resolving power. One of the improved methods is to add a set of electrostatic field mirrors in front of the linear detector to push back the ions in free flight. The ions with high initial energy have a long distance into the electrostatic field mirror due to their fast initial velocity. The return distance is also long, and the return distance of the ions with small initial energy is short, so that they will be focused at a certain position on the return distance, thereby improving the resolving power of the instrument. Such time-of-flight mass spectrometers with electrostatic field mirrors are called reflection time-of-flight mass spectrometers/Reflectron time-of-flight mass spectrometers.

The time-of-flight mass spectrometer has two flight modes, parallel flight mode and vertical flight mode. In modern mass spectrometry products, most of them have adopted vertical flight mode. Especially in the field of atmospheric chemistry, the scientific research team in the United States is mainly based on mass spectrometers, while in Europe, the instruments for measuring particle size are mainly used. Among them, Aerodyne INC., Ionicon GmbH, THS INC. have become industry leaders in recent years.

Mass spectrometers need to operate under vacuum to protect the detectors and improve measurement accuracy. In the instrument, the gas sample is first sampled through the micropore, and then reaches the ion source, where a pulsed electric field is fed into the time-of-flight module. The ions are then accelerated using a pulsed electric field perpendicular to the feed direction. The main purpose of this is to make sure that all ions have no initial velocity in the horizontal direction. After the V or W flight, the sensor is reached.
Different ions arrive at the sensor at different times, thereby deriving m/z. The common assumption is that ions can only have one charge, so the obtained signal directly corresponds to the relative atomic mass of the detected ion, so in most mass spectrometry charts, the x-axis unit is Atomic Mass Unit (AMU).

After the great development of electronic technology, the resolution of ToF-MS has been greatly improved. Since it is necessary to resolve the time for the ions to reach the sensor, the sensor signal is continuously scanned to reduce the averaging time. This process has very high requirements on the sampling rate of the Digital-to-Analog Converter (ADC). In Aerodyne and Ionicon products, 4-way digital-to-analog converters are used. The sampling rate of each channel is 500 MS/s (500 M samples/sencond, 500 M samples/s). Through synchronous coordination, the sampling rate of 4 channels is equivalent to 2GS/s. The instrument resolution (M/dM) can reach 0.1amu. There are commercial PTR-TOF MS with the highest resolution up to 15000.

Comparison of ToF-MS and Quadrupole Mass Spectrometry
During the sampling process of Quadru Pole Mass Analyzer Mass Spectrometer (QMA-MS), only a specific m/z is allowed to pass through each time, so if you want to obtain a complete mass spectrum, you need to perform different m/z measurements. Scan continuously. The major producers of quadrupole mass spectrometry in the field of atmospheric chemistry are Aerodyne’s ACSM products and THS. On the other hand, ToF-MS can collect all m/z in the sample in full spectrum during each injection, which improves the data collection efficiency and allows retrospective analysis.
The amount of data obtained by quadrupole mass spectrometry for 24-hour detection is usually about 2MB, while the data collected by ToF-MS in one day can reach 10GB.

The ionization process of a mass spectrometer

chemical ionization mass spectrometry
Chemical ionization mass spectrometer (CIMS) is a common soft ionization (Soft Ionization) method in the atmospheric field. The advantage of using chemical ionization is that there is no ion fragmentation, and real-time analysis can be performed on-line. At present, the most common reagents used in the field of atmospheric chemistry are nitric acid, ethanol and water. Chemical ionization mass spectrometry is highly selective compared to other ionization methods. For example, the proton transfer reaction requires specific conditions, and these ionization means are called proton transfer reaction (PTR). Among them, the nucleophilic energy (Proton affinity) of water is 707kJ/mol, and ethanol is 770kJ/mol. This energy gap allows the PTR-MS mass spectrometer with hydrated ions as the reactant to detect most organic compounds, while the mass spectrometer with ethanol or ammonia as the reactant specifically detects amines with higher nucleophilicity. Chemical ionization mass spectrometry can be used for quantitative analysis because of its strong selectivity and fast ionization process.

Electron ionization
Electron ionization is a strong ionization method, which is different from soft ionization, but a strong ionization method. Electrons are generated using a tungsten wire, and the sample is bombarded directly with the electron beam. The advantage of this method is that the ionization efficiency is higher. The downside is that it creates a lot of debris. These fragments are used to analyze the components in the sample. However, this method can obtain detailed sample information when using a single substance as a sample. When the sample becomes a mixture, such as atmospheric observations, this method can only qualitatively analyze sulfate, nitrate, and organic matter, but cannot quantify it. .

No ionization
There are a large number of molecular ions in the atmosphere. Based on the charge selection principle of the mass spectrometer, these already charged molecular ions can be directly fed into the mass selector. So a method called Atmospheric Pressure interface (APi) was born. This mass spectrometer without an ion source obtains the components of the ions by directly screening the ions.

AxisPharm Laboratories is equipped with a Voyager DE MALDI-TOF system made by Applied Biosystems. We are routinely using this instrument in analyzing polymers, oligonucleotides, peptides, proteins, and many other macro-molecules that would be difficult or not possible to be detected with API sources or any other mass spec techniques. Masses of peptides (low molecular weight, 750-4,500) can be determined on low picomol quantities with an average mass accuracy about 0.2%. Under optimum conditions, the limit of sensitivity of tryptic peptides (below 4,000) is in the lower femtomol range. Masses can potentially be obtained on numerous biopolymers including oligosaccharides, nucleotides and proteins that range from ~600 to 750,000 Daltons.

Maldi-TOF Analysis