Current testing methods for element analysis are known for being time consuming, requiring many steps, and the use of hazardous chemicals and expensive gases. All this has changed with the introduction of the Micral™ LIBS solution that dramatically speeds up testing capacity. However, we know that changing to a new method has a big impact on daily operations in a busy laboratory. You might be thinking, our current lab set-up is slow, but it works, so is it worth the change?
In this Q & A article, Senior Scientist, Dr. Daniel Adén shares his insights into the benefits and limitations of Laser Induced Breakdown Spectroscopy (LIBS) for element analysis and explains how LIBS compares to traditional methods like Inductively Coupled Plasma (ICP).
Q: What are the benefits of LIBS as a method?
A: Laser Induced Breakdown Spectroscopy (LIBS) has many advantages for quantitative analysis of elements in samples. LIBS is in general very fast. Pulsed LIBS lasers today deliver hundreds, or even thousands, of high energetic pulses each second towards the sample surface, where each pulse results in a luminous plasma. Thus, within an analysis time of a minute, you will end up with thousands of sub-spectra that represent the elemental composition of the sample material.
Compared to Inductively Coupled Plasma (ICP), the most significant advantage is the easy sample preparation. Time to answer (not considering drying and grinding of the sample) is only a few minutes for LIBS, whereas for ICP it could be more than a day depending on sample digestion method. Furthermore, no harsh chemicals or expensive gases, e.g., argon or helium, are involved in the LIBS sample preparation and analysis. This yields a low cost per analysis. Finally, the element coverage for LIBS is very good, e.g., boron and sodium, which are troublesome for XRF, and carbon, which is not possible with ICP or XRF.
Q: What are the limitations of the method?
A: As for all spectroscopic analytical techniques, LIBS suffers from matrix effects. Different compositions of the sample material can introduce a bias for the line strengths of the element emission lines of interest. Additionally, depending on instrumental setup, LIBS is not optimal for the analysis of liquids. Fat and moisture are known to attenuate the plasma emission. Thus, for Micral, only solid samples are possible. Finally, historically, LIBS has been known for its poor repeatability. However, with proper engineering, optimized sample presentation, and high repetition rate lasers, repeatability can be improved substantially.
Q: Why are these limitations not a problem?
A: The sample matrix effect for plant-based samples is minimized using a groundbreaking sample preparation step known as sample surface searing/charring. Searing is carried out automatically, and it only consumes a few seconds for each sample. Interestingly, from my point of view, we’re not really sure why it works so well. Future research might be able to shed further light on what’s going on in a scientific sense. Additionally, searing greatly improves the level of quantification by enhancing the element emission line strengths.
Repeatability for LIBS can be improved with engineering and sample preparation. For Micral, we use a method called autofocus which utilizes the LIBS system itself to accurately predict where to position the sample surface relative to the laser focusing lens. This procedure is carried out automatically prior to each sample measurement. The final sample spectrum is based on 6000 plasmas distributed across the sample surface, which further improves the repeatability for heterogenic samples.
Q: How can you benefit from changing from an established/respected method such as ICP to LIBS?
A: The greatest advantage with Micral LIBS is the simple sample preparation procedure, which affects the overall throughput organization requirement for a lab. A single operator, with little training, can easily prepare a full autosampler cassette (60 samples) within an hour. The instrument will automatically measure all 60 samples in about an hour, which is enough time to prepare the next cassette. ICP is on another level when it comes to sample preparation, which is not so surprising since you have to translate your solid sample into a liquid. It is easy to underestimate the added complexity, and organizational requirements, when several sample preparation steps are involved. For ICP measurements you need to weigh the sample, add strong acid, digest, cool, dilute, filter, and finally analyze it. For Micral you pelletize and sear, which only consumes a few minutes, before the LIBS analysis. Indeed, earlier this year we took a Micral to an agricultural lab in the US. During only two days we were able to measure 600 samples, and based on those measurements construct a prediction model for the DCAD index (a linear combination of several elements) which was loaded onto the instrument.
Q: What to consider before making the change?
A: The way that we carry out element analysis with Micral is not absolute, i.e., Micral is dependent on a reference method in order to calibrate the predictions of element concentrations. This is carried out during installation, and if the predicted concentrations drift significantly over time the procedure has to be repeated. Thus, at this point in time, Micral LIBS is not a full replacement for ICP for agricultural labs. Instead, it is to be primarily considered as an analytical unit working alongside a reference method, relieving the lab of a substantial amount of work. Of course, the reference analysis could be carried out at another element analysis lab. Additionally, so far Micral is only eligible for forage samples, e.g., hay, silage, alfalfa, etc. Future work will tell what other applications that we are able to enable for LIBS. Our PhD students have been successful exploring Micral LIBS for plant leaf tissue, soil, and other sample types, however they are not officially supported.
Q: What will it take for LIBS to be acknowledge in the lab industry?
A: LIBS will most likely experience the same journey as other analytical methods, e.g., NIR, X-ray, and ICP, where a certain degree of skepticism existed as the technologies were introduced. When NIR took off in the 1970s, it was primarily intended for grain and forage analysis, and it came with a range of problems of its own. Today, NIR is a force to be reckoned with for the analysis of pet food, flour, fish meal, sugarcane, milk, oil, ice cream, etc. To experience the same journey, Micral has to be adopted by some of the big agricultural labs where throughput is key. Additionally, acknowledgement for LIBS would be further accelerated if research-oriented labs, together with FOSS, explored future applications.
