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Home Applications 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.

Figure 1. Basic functional elements of a time-of-flight mass spectrometry system.

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).

Figure 2. DC offset is a crucial feature needed to fully utilize the digitizers input range while avoiding overflow/saturation.

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).

Figure 3. Uncorrected baseline fluctuation/drift may result in missed pulses (top). With DBS this is corrected (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.

Figure 4. The onboard FPGA is crucial for reducing the data rate so that it matches the link capacity without loss of signal information.

Precise triggering when using accumulation

Averaging involves accumulating a large number of waveforms (records) and subsequently scaling by the total number of accumulated waveforms. The accumulation functionality is available through the FWATD firmware option. This process effectively suppresses random noise and enhances systematic (periodic) signals.

However, when a large number of records are accumulated, systematic noise may become apparent. Refer to the figure below, where the left-hand red curve illustrates accumulated systematic noise at 6 µV. This type of noise is amplified when the trigger is correlated with the noise source.

To mitigate this, decorrelating the trigger frequency from the noise source can suppress the systematic noise. The green and blue curves in the figure demonstrate the resulting noise suppression.

Decorrelation can be achieved through various methods, though care must be taken to avoid introducing jitter. In the right-hand figure, the green pulse exhibits jitter and is broadened as a result. In contrast, the red and blue pulses remain sharp, indicating the absence of jitter.

The ADQ35 and ADQ35-PDRX digitizers incorporate a fractional-N PLL-based trigger source, which enables both systematic noise suppression and low jitter performance, as demonstrated by the blue curves in both plots.

For further details, refer to the application note 25-3162 ADQ35-PDRX FWATD trigger

The application note outlines a method for achieving noise suppression during waveform accumulation. It is recommended for use with the ADQ35 and ADQ35-PDRX devices when accumulating a large number of waveforms.

The method relies on a Fractional-N PLL, which requires the ADQ35 to act as the timing master within the system. This may necessitate some system-level redesign. For scenarios where only the pulse source needs to be triggered, figure 7 in the application note provides a viable solution. If more precisely controlled signals are required, this approach remains applicable but with minor modifications.

In this configuration, the Fractional-N PLL triggers the pulse source and simultaneously serves as a clock reference for an FPGA or microprocessor, the generation of states and control signals as illustrated below.

If the trigger rate is too low to support PLL operation, a higher frequency can be used, with a gating mechanism applied to set the desired trigger rate for the pulse source.

This setup ensures synchronization across all system components, enhancing timing precision and signal integrity.

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.
With LICPDRX, customers gain access to the DSP blocks utilized in PDRX. By utilizing two 12-bit analog-to-digital converters we have achieved a dynamic range equivalent to commercial 16-bit converters. This optional paid license enables PDRX as an add-on to existing firmware options (FWDAQ, FWPD, etc.) for selected dual-channel digitizers.

Want to learn more about the technical details and advantages? Download our white paper below!

Hardware options	Resolution [bits]	Channel Count	Sampling rate [GSPS]	Waveform Averaging	FPGA	Interface
ADQ7DC	14	2 1	5 10	Optional	AMD KU085	PCIe, PXIe, USB3.0, MTCA.4, 10 GbE
ADQ14	14	4 2 1	1 2 2	Optional	AMD K325T	PCIe , PXIe, USB3.0, MTCA.4, 10 GbE
ADQ32	12	2 1	2.5 5	Optional	AMD KU040	PCIe
ADQ33	12	2	1	Optional	AMD KU040	PCIe
ADQ32-PDRX ¹	12	1	2.5	Optional	AMD KU040	PCIe
ADQ33-PDRX ¹	12	1	1	Optional	AMD KU040	PCIe
ADQ35	12	2 1	5 10	Optional	AMD KU115	PCIe
ADQ35-PDRX¹	12	1	5	Optional	AMD KU115	PCIe

¹ PDRX utilize a dual-gain configuration in order to achieve a dynamic range equivalent to 12 ENOB.

Firmware options	Comment
FWATD	Optional waveform averaging firmware.
FWPD	Optional pulse detection firmware.
DEVDAQ	Optional FPGA development kit based on FWDAQ.