• Lori N Willhite

How I measure elements in rocks using a laser-ablation inductively coupled plasma mass spectrometer

First let us walk through the name of this instrument and what each piece of the name really means.

Laser: The word laser is actually an acronym for the expression Light Amplified by Stimulated Emission of Radiation. Basically, a laser is a highly focused, single wavelength, high energy beam of light.

Ablation: In this context, ablation means to remove. Usually laser-ablation is hyphenated to combine the two words to mean “laser removal of particles”. This is how atoms are removed from the sample to be analyzed by the instrument.

Induction: In the phrase “inductively coupled plasma”, “inductively coupled” describes the plasma. Induction is a mechanism of heating something up by using a conductive metal coil that has an alternating current running through it. To put it simply, this induction is a way to heat and charge something (in this instrument, induction is used to heat and charge a gas to form the plasma).

Coupled: This word is used a lot in science, but it is not often defined well. Generally, coupled means connected or linked. Sometimes the word coupled implies that two things affect each other, interact, or are both experiencing similar processes. To keep is simple, in this context coupled indicates that the plasma is affected by the induction coil. In other words, any changes in the current running through the coil will also cause change to plasma. We can use this phenomenon to change the temperature and characteristics of the plasma by controlling the current in the induction coil.

Plasma: A plasma is one of the states of matter. Plasmas are made of atoms that have had their electrons stripped off and free electrons. You might be familiar with some natural examples of plasma, like the Aurora Borealis (aka Northern Lights) or Aurora Australis (aka the Southern Lights). The plasma in my instrument is extremely hot, reaching temperatures of about 10,000 °C! In this instrument, the plasma is made of argon gas that is inductively heated and charged.

Mass: Technically this is property that quantifies the amount of matter in an object. It is often thought of as similar to weight, except that weight can be affected by gravity and mass is not. In mass spectrometry, mass is usually referring to the atomic or molecular mass (e.g., carbon-12 is 12 atomic mass units).

Spectrometer: Generally, spectrometry refers to techniques that separate and measure things. Spectrometers can separate anything on a “spectrum”. A spectrum can be any sort of continuous scale. Light-based spectrometers will separate light by its wavelength in the electromagnetic spectrum. Mass spectrometers separate things by mass. In this case the spectrum is mass. We will look as some mass spectra later.

Combine it all: Laser-ablation inductively coupled mass spectrometer = an instrument that uses a high energy column of light to zap and remove material from an object. The removed material is then ionized in a super-hot combination of ionized gas and free electrons that was heated by induction. The ionized material is then separated by mass and measured to provide the user with a mass spectrum.

This is me and our LA-ICP-MS, the Attom.

Mass spectrometry:

I used to think that chemists could put objects in a magical box and it would print out a list of the elements that make up the object along with each element’s respective concentrations. During my undergraduate career, I learned about instruments called mass spectrometers. In the simplest terms, mass spectrometers measure masses. Because different elements have different masses, mass spectrometers can measure the different masses and tell us which masses are present in a sample. A big challenge for mass spectrometrists is to separate elements and molecules that weigh almost the exact same mass. Mass spectrometer instruments can have a hard time separating things that are really similar in mass, but that is not the topic of this blog post today. Today I want to try to explain a technique called laser ablation inductively coupled mass spectrometry (usually referred to as LA-ICP-MS) in a simple and clear way, so that even if you do not use this technique (yet?), you can understand how it works! I'm still new to this technique as well, so I still have a lot to learn!

A laser ablation inductively coupled plasma mass spectrometer uses a laser to zap the target. In this case, the target is whatever you want to analyze; for me, the target is usually a rock. So, a laser zaps a rock and a tiny explosion occurs on the surface of the rock and particles fly off. Usually, we polish the rock first to create a very flat surface. Bumps and topography in the surface of the rock can cause the laser to preferentially remove certain parts of the surface over others. To be as “fair” as possible to the rock’s surface, we polish it first!

The auto-polisher shown in action. We polish rock fragments that are held in epoxy resin (red in this photo). I've polished a lot of rocks by hand, but this is MUCH faster so I Iove it.

Here is one of the lasers we use in our lab! The laser light is covered so that we do not have to wear laser safety glasses to protect our eyes.

A line of helium gas is flowing over the rock, and carries the ablated particles away. The helium “carrier gas” carries the rock particles to the instrument itself. Before entering the instrument, helium is met by another gas line of argon. The argon is really important for creating the plasma!

Ok, so our rock particles are being carried to the instrument by helium, and eventually argon joins in and both gases work together to carry the particles into the plasma. The plasma ionizes pretty much everything. This means the plasma strips electrons off the atoms that come in from the rock. Once stripped of their electrons, these atoms are now positively charged ions, or cations. Ions are great for mass spectrometers because charged particles can be influenced by magnetic fields and electrostatic fields. This means we can use magnets and/or electrodes to steer and guide the ions into our detectors. Here is a really important point: ions with different masses are affected differently by magnetic and electrostatic fields.

Here is a photo of the plasma in our instrument. The plasma is contained in the "torch", which is shown in the movie below.

Time-lapse of the torch (glass tube in the foreground) moving back into place after cleaning and replacing the cones (darker metal cone in the background). In reality, the torch moves really slowly back into place, so I am showing a time-lapse to speed it up!

This is the underlying concept behind a lot of mass spectrometers. Particles with different masses can be separated by magnetic and/or electrostatic fields once the particles are ionized. It is important to note that the charge matters too, not just the mass. For example, if all the ions in the instrument are a +1 charge, then only mass would matter. But some of the ions could have +2 charge, or more (although >+2 becomes less likely). Inside of a mass spectrometer, an ion with a +2 charge and mass 100 would look the same as an ion with a +1 charge and a mass 50. So instead of just mass, in mass spectrometry we often talk about “mass-to-charge” as a ratio. We usually call is m/z, where m is mass and z is charge. So m/z of 50 could be an ion with mass 100 and charge of 2, or mass 50 and charge of 1.

Alright, at this point in the instrument the particles are ionized in the plasma and need to travel to our detectors to be “measured”. From the plasma, the ions are attracted to a metal cone called the sampler cone. This cone has a tiny hole that some of the ions can travel through, the rest of the ions hit the cone and do not continue on the journey. The ions that make it through the sampler cone are accelerated by the sudden drop in pressure. From the plasma side of the sampler cone to the other side, there is a huge pressure difference. The drop is pressure causes the ions to undergo supersonic expansion and create a free jet. The next cone is called the skimmer cone. Some of the ions from the free jet will be travelling in the correct direction to enter the skimmer cone and experience another major drop in pressure. The ions that make it through the skimmer cone will then be directed and steered by either a magnetic field or an electrostatic field or a combination of both. The strength of the magnetic field and/or electrostatic field(s) will determine which m/z (mass-to-charge) will be detected. Heavier masses will not be deflected as much by the electrostatic or magnetic field, compare to light masses. For an analogy, think about how far a baseball would travel when hit by a bat. Now think about how far a bowling ball would travel if hit with the same amount of force… not as far!

Here, my lab mate Kiran and I are taking out the cones to clean them. My hand (foreground) is holding the sampler cone.

Skimmer and sampler cones (from a different LA-ICP-MS instrument than shown in all of the other photos).

Finally, the ions will be measured with detectors. The type of detectors used in an instrument can vary. For LA-ICP-MS instruments the most common detectors are electron multipliers and Faraday cups. In general, these detectors have some things in common. An ion hits the detector and the ion’s charge affects the detector and causes electrons to react and create a voltage. The voltage will be proportional to how many ions hit the detector. This is how abundances of elements are measured. Elements that are more abundant will create a larger voltage signal than those that are less abundant. Usually, the user has some knowledge about which elements will be in the object they are analyzing. For rocks, I usually have at least a guess about which elements are in my rock and their approximate relative abundances. Some prior knowledge about your sample can be really helpful to set up the best method for your overall goal.

This is one example of a detector called an electron multiplier (my photo, but the electron multiplier belongs to a different lab run by my colleague, Richard Ash). I will do a separate blog post for detectors! So check there for more detail about how these work.

Usually, I would not measure all the elements in the periodic table at one time. I would select a small range of elements (i.e., a small range of masses). Measuring only a small range at a time helps us get accurate and precise data. Using detectors, the relative amounts of different masses hitting the detectors can be recorded to produce a mass spectrum! This means I will tell the instrument a mass range that I am interested in before I zap the rock with the laser. The instrument needs something called a mass calibration. The mass calibration is done by scanning the magnet to find different elements and telling the instrument to remember the magnet settings that match each element. For example, the magnet will scan until the element lithium is detected, then it will remember the settings for lithium. Generally, lithium is the lightest element in the mass calibration. Then the magnet will continue to scan through the masses of elements until the heaviest element, uranium, is detected. The instrument will remember the settings that matched uranium. Now the magnet can adjust itself to steer any mass between lithium and uranium into the detectors. Think of this analogy: if you wanted to perfectly steer different sized vehicles (all travelling the same speed) around the same bend in the road, you would need to steer a small sports car much less than a semi-truck. If you systematically steer different sizes of vehicles around the bend, you could remember how much force it requires for different sizes. You would need to try steering the smallest possible vehicle and also the largest possible vehicle. The more vehicles you steer in between those two end-members, the more accurately you would know the force required to steer each different size. Then, after you have calibrated the steering force for different vehicle sizes, you would know how much force to use to steer any size of vehicle in between the smallest and largest previous vehicle you have steered. This is how the mass calibration works.

In practice, I would choose a mass range that I am interested in and set up the instrument to measure those masses. I will enable the laser, zap my rock, and then elements from the rock will enter the mass spectrometer where they will be ionized, steered, and eventually detected. The detectors will produce signals that can be recorded to tell me which elements (and/or their isotopes) were detected and their relative abundances. This information can illuminate important geologic processes that the rock has experienced!


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