Laser diode thermal desorption
Ionization technique using laser heat for mass spectrometry analysis.
Laser diode thermal desorption (LDTD) is an ionization method used with mass spectrometry, paired with atmospheric pressure chemical ionization (APCI). It uses a laser to heat and release analytes from a stainless steel sample plate called a LazWell. This technique is applied in forensics, pharmaceuticals, environmental testing, food analysis, and clinical studies. LDTD works for small molecules from 0 to 1200 Da and for certain peptides, like cyclosporine.
Phytronix Technologies Inc., based in Quebec, Canada, filed a patent for the LDTD ion source in 2005. In 2016, the Luxon Ion Source, which uses the same technology, was released.
For operation, a sample volume of 1 to 10 μL is pipetted into a well of the metal holder and dried at room temperature up to 40 degrees Celsius. Once dry, the holder goes into the ion source. Unlike methods such as DESI, DART, or MALDI, where droplets, gas, or laser directly contact the sample, LDTD transfers heat through the metal surface. An infrared laser diode array (980 nm) heats the back of the holder, desorbing molecules. The resulting neutral gas-phase molecules travel through a transfer tube, which moves pneumatically and sequentially into each well, with a carrier gas into a corona discharge region for atmospheric pressure ionization. Ions then enter the mass spectrometer for detection. The process takes 0.7 to 10 seconds, depending on the laser pattern and user method. The carrier gas is compressed air with a water content of 3 to 1800 ppm, which enables efficient protonation. Besides software-controlled parameters, users can adjust carrier gas flow, laser power, and laser gradient to improve sensitivity or reproducibility. Common sample preparation methods include liquid-liquid extraction, protein precipitation, solid phase extraction, or dilution.
Because LDTD is always coupled with APCI, the ionization mechanism is the same as standard APCI, but no solvent or mobile phase is present; protons come from water in the carrier gas. Ionization can be done in positive or negative mode. For some applications, like analyzing tacrolimus in whole blood, ammonium hydroxide is added to the carrier gas to alter ionization.
The sample holders, called LazWell, come in 96, 384, or 1536-well plates. Different coatings are available depending on the molecules. The hexagonal well shape concentrates the sample in the laser’s path fo
- field
- Analytical chemistry / mass spectrometry
- known_for
- Laser diode thermal desorption ionization for mass spectrometry
- first_patent_filed_by
- Phytronix Technologies Inc., Quebec, Canada
- patent_year
- 2005
- commercial_product
- Luxon Ion Source (2016)
Lore & Background
In 2005, a patent for the LDTD ion source was filed by Phytronix Technologies Inc. from Quebec, Canada. In 2016, the Luxon Ion Source, based on the same technology, was put on the market. The technique uses an infrared laser diode array (980 nm) to heat the back of a metal sample holder, causing desorption of molecules without direct contact with the sample.
Reader's Guide
LDTD-APCI is significant as a soft-ionization technique that eliminates the need for a solvent or mobile phase, resulting in highly efficient protonation and strong resistance to ionic suppression. It reduces analysis time compared to traditional LC-MS and uses low sample volumes, making it valuable for limited or difficult-to-acquire samples. The technique can be attached to various mass spectrometers, including triple quadrupole, time-of-flight, and orbitrap. However, because no chromatographic separation occurs, interferences from isobaric compounds may arise in heavily charged matrices, which can be addressed with differential ion mobility spectrometry or high-resolution mass spectrometry. The manual sample placement can cause variation in results, and the lack of chromatography prevents analysis of isomers.
Did You Know?
- LDTD uses a 980 nm infrared laser diode array to heat the back of the sample holder.
- The carrier gas is compressed air with a water concentration between 3 and 1800 ppm for efficient protonation.
- Sample holders named LazWell are available in 96, 384, or 1536-well plates with hexagonal well shapes.
- The analysis process takes between 0.7 and 10 seconds depending on the laser pattern and method.
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