Time-of-Flight Mass Spectrometry (TOFMS)
Time-of-flight mass spectrometry systems are used to analyze the chemical composition of analyte samples. The operating principle is simple - a sample substance is ionized and the ions are accelerated using an electrical field. They travel through the flight tube before they collide with the detector at the end of the tube. The velocity of individual ions depends on their so-called mass-charge ratio (m/z) where ions with smaller m/z travel faster and hence hit the detector earlier. The flight time of each ion is recorded and used to create a mass spectrum on the host PC. The mass spectrum hence illustrates the concentration of specific ions in the sample substance. The basic principle is illustrated in figure 1.
What to consider when selecting a digitizer for mass spectrometry
The list below contains some - but not all - important signal characteristics and system-level aspects that influence the choice of digitizer and accompanying firmware:
- Ions that hit the detector generate short-lived pulses - often in the range of 500 - 700 picoseconds (full width at half maximum, FWHM).
- The total flight time often needs to be recorded and typically vary from 10 µs to 100 µs.
- Acquired pulses are unipolar and mostly negative-only (negative polarity).
- Pulse acquisition is commonly done using a level trigger, where the trigger level is set relative to a baseline (DC level).
- The detector's output often contains a combination of strong and weak signals due to difference in ion concentration within the sample substance.
- Ions with similar m/z will hit the detector almost at the same time, and it is important to be able to distinguish them.
- The acquired signal may contain either dense or spare pulses, depending on the ion concentration.
- The digitizer need to fit within the confined space of the MS system and should be located close to the detector so that short cables helps minimize signal reflections.
- It is beneficial to use digitizers with hardware trigger output so that they can trigger the ion source with accurate timing.
Do you need help with selecting the right digitizer for your MS system? Don't hesitate to contact us.
Commonly used products and features
Mass spectrometry developers use digitizers that combine high sampling rate and high vertical resolution. Sampling rates are typically between 2 to 10 Gigasample per second (GSPS) in combination with 12 or 14 bits vertical resolution. This ensures that each pulse is digitized with multiple samples per pulse, and the high resolution allows weak signals to be detected in the presence of noise. Commonly used products are ADQ7DC, ADQ14, or ADQ32.
These and other products offer a number of useful features that help address the challenges listed above. A few of these features are briefly described below. Please refer to the white paper for additional information.
- Unipolar signals only utilize half (upper or lower part) of the digitizers voltage input range (fig. 2 a). Furthermore, they may exceed the minimum or maximum voltage and therefore saturate the digitizer (fig. 2 b, area marked in red). A user-programmable DC offset is a crucial feature that helps address this so that the digitizer's input range can be fully utilized and saturation avoided (fig. 2 c). It is also common to leave some margin for overshoot/undershoot (fig.2 c, DC offset level - blue dashed line slightly below the maximum input voltage).
- The reference baseline used for triggering may fluctuate due to temperature variation and/or component ageing. If left uncorrected, this fluctuation can result in missed pulses and/or incorrect pulse characterization/analysis (fig. 3, top). Teledyne SP Devices' proprietary digital baseline stabilizer (DBS) corrects the baseline drift continuously and automatically in the background without user interaction. After baseline correction the pulses are correctly captured again (fig. 3, bottom).
- High sampling rate and high resolution are required to capture the short-lived weak MS pulses, but the amount of data generated by such digitizers can easily exceed the capacity of the link to the host PC. It is therefore crucial to utilize the onboard FPGA for real-time signal processing and data reduction. This results in less data being transfered to the host PC and thereby also relaxes the requirements for subsequent post-processing. Teledyne SP Devices offer both stand-alone firmware packages as well as firmware development kits:
- FWATD is an optional stand-alone firmware package that provide extreme dynamic range through noise reduction. It provides four methods for noise reduction; DBS for stable trigger reference (baseline) level, low-pass filter for noise suppression, threshold for detection of rare events, and real-time waveform averaging of repetitive signals for power enhancement. This firmware is commonly used in mass spectrometry and have in some instances helped reduce the output rate from 20 Gbyte/s to 40 Mbyte/s – a reduction of 500 times without loss of signal properties/characteristics!
- FWPD provides pulse detection capabilities including tools for detecting sparse, non-repetitive pulses. It outputs either pulse metadata, raw pulse data captured within a detection window, or both. It also discards unwanted data in order to reduce the overall data rate. In mass spectrometry this firmware is used for determining peak location, pulse width, etc.
- The ADQ Development Kit is a tool for developing custom digitizer firmware. In mass spectrometry it can for example be used to implement real-time curve fitting to distinguish pulses that overlap.
- In some MS systems it is desirable to either post-process the data in graphics processing units (GPUs) or record data to disk storage. In both cases it is beneficial to use peer-to-peer (P2P) technology that enables direct data transfers between the digitizer and GPU or storage. This is a huge advantage compared to conventional solutions that put a lot of stress on both the CPU and RAM of the host PC. With P2P, both the CPU and RAM can instead be used for other tasks. Learn more about peer-to-peer here.
Want to learn more about the technical details and advantages? Download our white paper below!