engineering

Laser diode thermal desorption

Laser diode thermal desorption is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Laser diode thermal desorption rather than just read about it. In short: Laser diode thermal desorption (LDTD) is an ionization technique that is coupled to mass spectrometry to analyze samples with atmospheric pressure chemical ionization (APCI). It uses a laser to thermally desorb analytes that are deposited on a stainless steel sheet sample holder, called LazWell.

Key takeaways

Laser diode thermal desorption belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Laser diode thermal desorption to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Laser diode thermal desorption from memory before moving on to harder problems.

Reference excerpt

Laser diode thermal desorption (LDTD) is an ionization technique that is coupled to mass spectrometry to analyze samples with atmospheric pressure chemical ionization (APCI). It uses a laser to thermally desorb analytes that are deposited on a stainless steel sheet sample holder, called LazWell. The coupling of LDTD and APCI is considered to be a soft-ionization technique. With LDTD-APCI, it is possible to analyze samples in forensics, pharmaceuticals, environment, food and clinical studies. LDTD is suitable for small molecules between 0 and 1200 Da and some peptides such as cyclosporine.

History In 2005, a patent was filed by Phytronix Technologies Inc., from Quebec, Canada, for the LDTD ion source for mass spectrometry. In 2016, the Luxon Ion Source, based on the same technology, was put on the market.

Principle of operation

An aliquot of the sample preparation between 1 and 10 μL is deposited with a pipette into the well of a metal sample holder and dried with a temperature between room temperature and 40 degrees Celsius. After the sample is completely dried, the sample holder is inserted into the ion source. Compared with desorption electrospray ionization (DESI), direct analysis in real time (DART) and matrix-assisted laser desorption/ionization (MALDI), where the droplets, gas or laser come into direct contact with the sample, LDTD relies on heat transfer through a metal surface. An infrared laser diode array (980 nm) is collimated to heat the back of the sample holder causing the desorption of the molecules. The gas-phase neutral molecules are then transported through a transfer tube, which is pneumatically and sequentially inserted into each well, with a carrier gas into a corona discharge region to undergo an atmospheric pressure ionization. The ions enter the mass spectrometer through the inlet to be measured by the detector. This whole process takes between 0.7 and 10 seconds depending on the laser pattern and the method created by the user. The carrier gas used is compressed air which contains a concentration of water between 3 and 1800 ppm to be able to efficiently protonate the molecules. Adding to the mass spectrometer's software-controlled parameters, three other parameters can be varied to achieve a higher sensibility or reproducibility: the carrier gas flow, the laser power and the laser gradient. An important part of the analysis is also the sample preparation. The most common sample preparation methods used with LDTD are liquid-liquid extraction (LLE), protein precipitation, solid phase extraction (SPE) or a dilution.

Ionization mechanism Since LDTD is always coupled to APCI, the same ionization mechanism happens. The main difference is that no solvent or mobile phase is available and the protons come from the water content of the carrier gas. A water concentration between 3 and 1800 ppm is recommended. The ionization can be done in negative or positive mode. In some applications, such as the analysis of tacrolimus in whole blood, ammonium hydroxide is added to the carrier gas to modify the ionization process.

Sample holder The sample holders that can be inserted in LDTD ion sources are named LazWell and are specially designed 96, 384 or 1536-well plates. Different coatings can be applied depending on the molecules being analyzed. The hexagonal well shape is designed to concentrate the sample in the path of the laser for an optimal desorption.

Advantages Since no solvent or mobile phase carries the sample, this technique is characterized by a highly efficient protonation and a strong resistance to ionic suppression. This and the fact that no needle touches the samples adds the benefit of eliminating carry over between the different wells of the plate. The technology is also a good alternative for the traditional LC-MS users since the results give a similar peak shape as in liquid chromatography and it significantly reduces the analysis time. It also uses low volumes of samples, which is an asset in applications where the available sample volume is limited or difficult to acquire. In addition, it is deemed to be an environmentally friendly alternative to LC-MS/MS. The ion sources, LDTD and Luxon Ion Source, can be attached to different mass spectrometers with its adapted source housing, available for multiple manufacturers, such as triple quadrupole, time-of-flight, and orbitrap mass spectrometers. On the downside, since no chromatographic separation is done, interferences coming from isobaric compounds may occur in heavily charged matrices. Differential ion mobility spectrometry-mass spectrometry (DMS-MS) or high-resolution mass spectrometry (HRMS) can be used in tandem with LDTD to eliminate these interferences.

Disadvantages While it only requires a small volume of sample, this technique results in a destruction of that sample. The manual sample placing required can cause a variation in results achieved. Care must be taken when designing methods within an experiment using this technology as the lack of chromatography can cause the inability to analyze isomers.

References

Illustrations

Worked examples

Example 1 — a first encounter with Laser diode thermal desorption

Start with the simplest possible case. Write down what Laser diode thermal desorption claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Laser diode thermal desorption before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Laser diode thermal desorption ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Laser diode thermal desorption

In research: Laser diode thermal desorption appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Laser diode thermal desorption in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Laser diode thermal desorption is common in secondary-school and first-year university syllabi. It links to neighbouring topics Analytical chemistry, Canadian inventions, Food safety, so understanding it makes those chapters shorter.
In everyday life: Look for Laser diode thermal desorption outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Laser diode thermal desorption in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Laser diode thermal desorption means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Laser diode thermal desorption out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Laser diode thermal desorption in simple terms?

Why does Laser diode thermal desorption matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Laser diode thermal desorption?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Laser diode thermal desorption.