Electron probes vs ion probes

CJ asked me to explain what this so-called ultraprobe is all about, so here’s a brief rundown on the difference between electron probes and ion probes, and why it might be useful. The ultraprobe is an electron probe.

An electron probe bombards a sample with electrons with energies in the low tens of KeV. At these energies, these electrons can dislodge inner shell electrons in the material being analyzed. When an outer shell electron then decays to fill that vacancy, an Xray is produced, and the energy of that X-ray is characteristic to each element. So by attaching a bunch of X-ray spectrometers to an electron gun, you can determine the elemental composition of the target by analyzing the X-rays produced by electron bombardment. X-ray diffractometers basically use crystals with known lattice parameters to diffract the X-rays according to the X-ray wavelength.

Since X-rays are photons, the wavelength and energy are related by the equation e=k/lambda, where e is energy, lambda is wavelength*, and k is a constant, 1243 eV/nm. This constant can be derived by multiplying the speed of light (in nm) by planks constant (an obscure, extremely small physics number that relates fundamental physics properties to each other), and dividing by the number of joules per eV (not many).

Electron probes are great for determining the major and minor elemental composition of minerals, but they give no information on isotopic composition, since isotopes all have the same electron configurations. Since the only things being moved are electrons, this technique is non-destructive, unless you turn the power up too high and melt your sample by overheating it.

Ion probes are Secondary Ion Mass Spectrometers. They bombard a sample with ions, and those ions then ionize the target. The ions from the target are then accelerated into the mass spectrometer, sorted by mass. The ionization efficiency of ions is different for different elements, but the same for isotopes of the same element. Since the ions are sorted by mass, isotopes can be separated and their rations are easy to determine. Elemental ratios can also be determined if a standardization method to determine relative ionization efficiencies is used.

U/Th/Pb geochronology determines radiometric ages by using one or more of the following decay chains: ²³²Th -> ²⁰⁸Pb, ²³⁵U -> ²⁰⁷Pb, and ²³⁸U -> ²⁰⁶Pb. Comparing the results of two or more of these chains, allows a geochronologist to determine whether or not the geologic material being dated has lost or gained U, Th, or Pb since crystallization, since elemental loss or gain will disturb one or more of these systems. We can also measure ²⁰⁴Pb, which is not a decay product, to estimate how much initial Pb was in the material. Thermochronic may have explained this in more detail.

Because the electron probe cannot measure isotopes, it cannot determine whether or not a sample has lost or gained U, Th, or Pb, and it cannot identify the presence of common Pb. Thus electron probe dating has to assume closed system behavior and low common Pb. It is also limited to minerals with a fairly high U, Th, and Pb content, since the detection limits on the electron probe are generally in the tens of ppm. On the other hand, they are easier to use, more common, and non-destructive. And if you choose your geologic question such that Pb loss and common Pb are unlikely to occur, and high precision is not necessary, you can get useful numbers quickly and easily.

The article mentions a potential snowball earth application, so my guess is that they plan to use this thing for sedimentary survey work. As Brian previously demonstrated, sediments contain datable minerals (e.g. zircon, rutile, or monazite) of various ages, but the sediment cannot be older than the youngest grain it contains.

If some future civilization wanted to determine if the Mississippi delta sediments were Quaternary (that’s our current geologic epoch), they could dig out a whole lot of grains, and analyze them. The youngest monazites in the sediment will be from the Yellowstone Hot Spot, which is a quaternary rhyolitic volcano. Trouble is, the grains from that volcano are only a tiny portion of the total sediment load of the Mississippi, so in order to find them, you need to survey a huge number of grains.

The electron probe can be used to eliminate grains that can’t possibly be the right age- a grain with too much Pb is either too old or contains common Pb, while a grain with not enough Pb has either lost Pb or is too young. Since electron probe work is non-destructive, the grains identified with the electron probe can then be retrieved an analyzed using a more precise method, to get a high-confidence, precise date. So I suspect that is the Neoproterozoic application that this machine will have.

Neither probe technique possesses the precision of isotope-dilution mass-spectrometry, but isotope dilution cannot be performed in-situ.

It’s too late to proofread, so if there are any glaring fuck-ups, comment and I’ll fix them later.

* Usually, lambda is the decay constant, but scientists like to use the same Greek letters for a million different unrelated quantities, just to piss everyone off.

Ten years ago today...

March 14, 1997:
I am now in Australia. It is dry and partly forested, and actually reminds me of central California. I could swear we flew over a normal-fault scarp on the way here, but maybe I'm oversensitized. Everything (cars, toilets, shadows, etc.) goes the wrong way. The stars are amazing- I can see the Milky Way from the suburbs. Orion is upside down, and I saw the Southern Cross. I'm living in a house with three other students- Matthew, Richard, and Shin. A Korean and two Tasmanians. We had a party tonight- met both native and foreign students.

Drinking sweet holy Jesus

Shelley at scienceblogs has a post about bottled holy water. This reminds me of the after-dinner back-of-the-napkin calculation that my dad showed me as a kid, in order to demonstrate the size of a mole. The calculation is this:

Determine how many molecules of Jesus are in the glasses on the dinner table.

Assume the following:
That Jesus was a person who actually existed, that he died about 2000 years ago, and that the molecules of his body still exist on Earth.

If you believe in the literal ascension of Jesus to a heaven that is not chemically mixed with the Earth’s atmosphere, that’s fine too; keep following the math and I’ll get back to your point of view at the end.

Please also assume that Jesus weighed 65 kilograms (143 pounds), and that 60% of his body weight was water. This is not an unreasonable bodyweight for that time period, and it makes the math easy.

At present, we are only concerned with the water molecules in Jesus. How many are there? This is determined by dividing the total mass of water by the formula weight, then multiplying by Avogadro’s number.

65kg *0.6 water content = 39 kg of water
39 kg = 39000 g water
39000g / 18 g per mol = 2167 moles water.
2167 moles * 6.022e23 molecules per mole = 1.3e27 total molecules of H2O in Jesus.

So, where is this water?

Assuming that he evaporated during ascension, or was buried in a tomb that allowed evaporation or groundwater flow, then it is likely that Jesus’ water has found its way to the ocean. The ocean has a mixing time of approximately 1000 years, so since Jesus died almost 2000 years ago, it is a reasonable assumption that the water molecules of his body have been homogenously diluted by the entire volume of all the world’s oceans.

What is that volume? According to various sources, it is approximately 1.3 billion cubic kilometers. Because a cubic kilometer is a trillion liters, the total volume of the oceans is 1.3e21 liters.

So, to determine the Jesus dilution factor, we divide the total number of Jesus molecules by the total volume of the oceans.

1.3e27 molecules / 1.3e21 liters = 1e6 molecules per liter.

Each liter of ocean water (and rain water, and tap water, and wine) contains a million molecules of Jesus.

In other words, when multiplied by Avogadro’s number, Jesus is bigger than all the world's oceans- by six orders of magnitude.

For the information of any Satanists who wish to avoid drinking Jesus, I should point out that there are a few sources of water that are isolated from the hydrological cycle for timescales longer that the Reign of our Lord. Fossil aquifers such as the Ogallala or the Great Artesian Basin contain groundwater that fell as rain tens of thousands of years ago, well before Jesus’ time. The Greenland and Antarctic ice sheets sequester ice for hundreds of thousands of years, so only the top layer will contain molecules of Jesus. And fossil fuels, which have been preserved in the sedimentary record for tens to hundreds of millions of years, can be burned to produce CO₂ and water vapor, which will be devoid of Jesus’ water molecules.

So what does this mean?

First of all, transubstantiation is moot. The wine already contains a million molecules per liter that derive from Jesus’ blood via the hydrologic cycle. So no further change is necessary. In fact, a miracle is only necessary if, as mentioned above, you believe that Jesus’ body literally disappeared from the Earth during ascension. And even then, the math will still get you.

Because even if a liter of water- or communion wine- doesn’t contain a million molecules of Jesus, it does contain an equal number of molecules of Judas*. And Pontius Pilate*. And of every other scoundrel, heathen, and prehistoric caveman ever to walk the face of the planet. So you’ll probably be needing that miracle you pray for.

The oceans may seem large. People may seem like insignificant specks on the surface of this pale blue dot. But Avogadro’s number is big enough to make up the difference and more. 6.022x10 ²³ is a very big number.

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* This assumes that Judas and Pilate had the same stature and body mass index as Jesus, and that neither of them bodily ascended to heaven upon their demise. I think these are safe assumptions.

What is geochemistry?

Over at Green Gabbro, Yami says that she gets all technical and jargony whenever she doesn’t want to talk about work to normal people. I don’t do that. I can turn people off just by being very basic.
“I determine the chemical composition of rocks.”
My, what interesting shoelaces we all have.

As Chris recently posted, though, it isn’t about the rocks. Don’t get me wrong, geologists appreciate a pretty rock when we see it. But that isn’t why we study them. We study rocks because they tell us stories. And the stories are very cool.

Take geochemistry, for example. Actually knowing the composition of a rock is in fact pretty dull, if it’s just a list of elements and concentrations. The reason we study them, then, is to discover the process that lead to the composition that we measure in lab.

Different processes change chemical composition in different ways, so by measuring various elemental ratios, we can determine what a rock has been through. To start at the beginning, though, we need to acknowledge the sub-field of cosmochemistry.

Cosmochemistry is generally not the chemistry of the cosmos, it is actually the chemistry of the solar system. We have very few mineralogical materials that predate the formation of the solar system. The vast majority of non-terrestrial rocks that we have access to are from various solar system bodies.

The chief goal of cosmochemistry is to determine how the planets formed and what their composition is. Once the bulk chemistry of the Earth is determined, various processes that act on Earth can then be studied using geochemistry. The processes include, but are not limited to, the formation of continents, the evolution of the atmosphere and ocean, ancient and modern climate, the creation of economically significant ore bodies, and the pollution that results from the exploitation of such deposits. Some of these processes are pretty cool, which is why I like my job.

Paper predicament

A co-author wants the experimental & analytical methods section from me in a few weeks. That's fine, except that I used a new method that hasn't been described in the literature before. So theoretically, I should publish the method paper first.

Writing a section in a few weeks is easy. Writing the paper it's based on? Let's just say that I should spend a bit less time on blogs and beer drinking over the next few weeks.

The Pseudoscientific Method

Click for larger print.

Hat tip to Janet for starting the flowchart frenzy last week.

Related post: Women in pseudoscience.

Baby steps

As I mentioned last week, we’re expecting a baby soon. Which means, after the insomnia and the thousands of diapers, we’ll be hearing the pitter patter of tiny little feet. Only they won’t be. I’ve got size 14’s, so if genetics still works the way they taught me in high school, we should be prepared for little footprints more like these:

Paper pause

Expect light blogging for a while, I’m trying to write up the erotic alkali stuff- the project from which I presented a data datum point at the Goldschmidt conference. As you can see from the figure below, I collected too much many data. So now I have to make sense of it them, or at least come up with good excuses to throw the crappy stuff out.



For example, excluding the days when we were running with the designated high alkali cones gives something like this- which almost looks vaguely half-under control for some of the time.



Even after I get a handle on all this, I still need to write the damn thing- and I suck bigtime at writing papers. So the lounge may be a bit quiet for a while.