In recent years, mass spectrometers have undoubtedly been recognized as the fastest-growing analytical instruments. According to statistics from the American Chemical Society (ACS), seven of the top ten instrument manufacturers worldwide are engaged in the research and production of mass spectrometers. In China, the demand for mass spectrometers is also growing rapidly.
Mass spectrometry is an analytical method used to measure the mass-to-charge ratio of ions. First, the components in the sample are ionized by an ion source, and then in a high vacuum mass analyzer, the ions are separated primarily based on their mass-to-charge ratio (mass of charged ions/number of charges) under the influence of an electromagnetic field, producing distinguishable signals on the detector. Different components generate different ions upon ionization, which is why mass spectrometry can be used to identify different components in a sample.
Schematic Diagram of Basic Structure of Mass Spectrometer
Mass spectrometry technology has developed for over a century, with researchers building on each other’s work to elevate a simple physical phenomenon to its current theoretical and practical heights, making it one of the most important methods in the field of analysis. Today, mass spectrometry is not only a crucial tool in conventional chemical analysis but is also increasingly being applied in popular fields such as life sciences, homeland security, food safety, clinical medical testing, and space technology.
Applications of Mass Spectrometry Technology are Expanding
However, we know that traditional laboratory benchtop mass spectrometers are expensive, energy-consuming, require gas line connections, need powerful vacuum pumps, and often necessitate front-end separation systems, resulting in bulky and heavy instruments. To be applicable in on-site measurement scenarios such as clinical settings, airport security, and food safety, the instruments must be miniaturized.
But saying miniaturization is one thing; have you consulted the vacuum system?
Indeed, the biggest challenge in designing miniature mass spectrometers lies in the vacuum system. As mentioned in the introduction to how mass spectrometers work, “vacuum” is a necessary condition for the internal operation of a mass spectrometer. Maintaining a high vacuum can prevent collisions between molecules, ions, and electrons, avoiding the generation of noise. This means that the higher the vacuum, the better the signal-to-noise ratio of the mass spectrometer.
Unfortunately, vacuum systems tend to be bulky, and miniature mass spectrometers can only opt for smaller vacuum pumps, which results in a decrease in pumping speed, directly leading to a reduction in system vacuum. This can severely impact the normal operation of the mass analyzer and detector. Current research indicates that background noise in mass spectrometry primarily originates from the detector end, due to a phenomenon known as ion feedback.
Common mass spectrometers (such as MCP, electron multipliers/EM) convert ions into electrons; the electrons are accelerated, multiplied, and ultimately detected. However, the accelerated electrons collide with residual gas molecules, generating positive ions. These positive ions move in reverse in the electric field and strike again to produce electrons, a process referred to as ion feedback (ion feedback, IFB).
Since the reverse motion of positive ions takes time, the signal generated by ion feedback does not overlap with the actual signal, instead becoming a significant source of noise and spurious peaks.
Illustration of the Ion Feedback (IFB) Process
In low vacuum conditions, a higher concentration of gas molecules is objectively present. Therefore, rather than controlling ion generation, a more sensible approach is to control the trajectory of the generated ions. However, the electron multipliers (EM) commonly used in quadrupole and ion trap mass spectrometers cannot solve this problem.
The emergence of new detector technology has become a key to the miniaturization of mass spectrometers.
Don’t panic, the Hamamatsu Gen3 MCP is here for miniature mass spectrometers.
The microchannel plate (MCP) is also a commonly used detector in mass spectrometers, especially in TOF-MS. However, traditional two-plate structures of MCP (see below figure a) and other traditional mass spectrometer detectors like electron multipliers (EM) also experience ion feedback due to residual gas molecules undergoing ionization and generating positive ions that return to the MCP.
However, Hamamatsu’s latest triode MCP, with its three-tier structure, successfully addresses the aforementioned issues by implementing a strategy to control the trajectory of ions.
Comparison of the Structure and Potential of Traditional Two-Plate Structure (Bi-Planer Mode) and Hamamatsu’s Latest Triode Structure (Triode Mode) MCP
Hamamatsu’s Latest Gen3 MCP for Miniature Mass Spectrometers
The Hamamatsu Gen3 MCP adopts the following structural design:
A grid electrode is added between the MCP outlet and the anode to form a triode structure, where the grid electrode acts as the anode (grounded in negative high voltage mode), and the rear anode and the MCP inlet are set to equipotential. This allows positive ions generated from residual gas molecules to move from the grid electrode towards the anode and be captured by the anode. This innovative triode structure design can prevent ion feedback from returning to the MCP, thereby addressing the dark current issue at the source.
The following figure compares the experimental data of the triode structure Hamamatsu Gen3 MCP and the traditional two-plate MCP under different vacuum conditions.
Comparison of Measured Noise (Dark Current) Between Traditional Two-Plate Structure (Bi-Planer Mode) and Hamamatsu’s Latest Triode Structure (Triode Mode) MCP
It is evident that at a gain of 105, the current output component of the traditional two-plate MCP will experience ion feedback when the vacuum level exceeds 10-3 Pa. In contrast, for the triode structure Gen3 MCP, even when the vacuum level drops to 1 Pa, ion feedback does not occur.
We also compared the Gen3 MCP with continuous electron multipliers (CEM), which are currently widely used in miniature ion trap mass spectrometers.
Comparison of Measured Data Between Continuous Electron Multiplier (CEM) and Hamamatsu’s Latest Triode Structure (Triode Mode) MCP Under Different Vacuum Conditions (M. Hayashi et al., HEMS Workshop 2017)
It can be seen that at a vacuum level of 0.4 mTorr, CEM will experience ion feedback. In contrast, for the Gen3 MCP, even when the vacuum level drops to 1.5 mTorr, the spectrum remains clean, even outperforming CEM’s performance at a high vacuum level of 0.06 mTorr.
With its outstanding performance at low vacuum levels and compact size (effective area diameter: 14mm), the Hamamatsu Gen3 MCP will greatly free developers from the constraints of the mass spectrometer’s vacuum system, facilitating the development of more flexible, portable, lower power consumption, and more suitable miniature mass spectrometers for field use.
Effective Area Diameter of Hamamatsu Gen3 MCP: 14mm
(Source: Hamamatsu Photonics Trading (China) Co., Ltd.)
For more details, please click the “Read Original” button at the bottom left.