watson.fawkes
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Atomization process in ICP-MS apparatus
The recently-closed thread on the topic whose name shall not be uttered (lest it attract attention) brought up an idea that I'd like to ask advice
about. Is it possible that there exist substances for which the atomization process in an ICP-MS apparatus is incomplete? This would lead to undercounting such a species. Suppose, for the purposes of discussion, it were, say, a heavy
metal cluster.
I've never had occasion to use one of these machines, either as operator or client, but I do know some physics and understand the principles of
operation. The sample is injected into an argon plasma, in which two processes occur. The first is atomization; the second is ionization. Atomization
is the breaking of atomic bonds and reduction of compounds to atoms. Ionization strips off an electron (or more). The basic assumption is that
atomization is complete. It's not assumed that ionization is complete. Different atoms ionize at different percentages, depending the ionization
energy, the Ar plasma temperature, and some other things, I'm sure.
Here's the thought. Suppose there's a metal cluster that ionizes far more readily than it dissociates. If it's large enough, it's surface-to-volume
ratio might reduce the per-atom dissociation rate by a few orders of magnitude. On the other hand, the work function for removing an electron from
such a cluster is fairly low and the ionization of such a cluster will occur early. It's almost certain to be multiply ionized. Even if the ionization
ratio (the number of electrons stripped to the number of atoms in the cluster) isn't as high as 1.0, if it's even past 0.5 it will propagate in the
plasma comparably to other ions. So you have a cluster that's moving out a bit slower, if not very much slower, but is dissociating far more slowly.
The next stage of ICP-MS is the quadrupole (or higher order) separator. The important thing about this stage is that it's a rejection stage. At any
point, it's tuned to a particular mass-to-charge ratio. Everything else is rejected. The commercial devices seem to be designed to look only up mass
numbers of 240 or so (for uranium). If so, they'll reject everything with a higher mass number. Such a substance just won't appear in the assay.
So my first question is: Is this a proved phenomenon, a disproved phenomenon, or an unknown one?
The substance in the thread to which I will not provide URL was metal ions in an acidic solution that is slowly titrated back to neutrality. I find
the reported process plausible for the formation of metal clusters in solution. The slowness seems ideal for the formation of clusters. The
agglomeration of metal clusters is diffusion-limited. Fast neutralization would afford an ordinary crystallization process, with nucleation, etc. Slow
neutralization of a solution would lead to an equilibration of clusters with ionization ratios less than 1.0. As solubility of the singly-charged
atomic species reduces, you'd end up with a mixture of cluster sizes and ionizations, changing in composition as the pH increases. The ionization
ratio would have to decrease, which would tend to favor larger and larger cluster sizes. (As an aside, I'd recommend that anybody who wants to try
this use ultrasonic agitation on the solution to increase the reaction rate and drive the cluster sizes to uniformity more quickly.) Finally, you
would end up with neutral nanoscale clusters suspended in solution. They might even be considered to be solvated (perhaps), but in any case you have a
neutralized metal that hasn't formed an ordinary crystal state.
It would seem possible to prove whether this is happening fairly easily. Perform the above process and divide the solution into two aliquots. Submit
one to ICP-MS analysis and smelt the other. As long as you're using base elements (in the metallurgical sense) and a noble metal, you should recover
all the metal easily. This experiment is well within the capacity of an amateur.
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merrlin
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Attached is a short excerpt from a reference on ICP-MS. Based upon the plasma temperature and residence time, my guess is that large particles will
undergo at least partial atomization, allowing representation in the mass spectrum, even if the quantitative results are distorted by incomplete
atomization.
Attachment: ICP-MS.pdf (78kB)
This file has been downloaded 963 times
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watson.fawkes
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Quote: Originally posted by merrlin  | | Attached is a short excerpt from a reference on ICP-MS. Based upon the plasma temperature and residence time, my guess is that large particles will
undergo at least partial atomization, allowing representation in the mass spectrum, even if the quantitative results are distorted by incomplete
atomization. |
As far as results, that's exactly what I was thinking--an undercount. But the reference
provided brought to mind an alternate theory. The lab that did the work was analyzing water for mineral content, which means they were using a
nebulizer for introducing the sample into the plasma chamber. Some of these devices then screen out large particles to promote total atomization. If
the metal cluster is larger than this (effective) screen size, much of the material looked for will be rejected even before it hits the plasma
chamber. And also, because of the cubic power law of mass vs. particle diameter, it doesn't take much of a percentage of particle count in large
diameter to take away most of the sample. For example, a sample with 99% 1 μm particles and 1% 10 μm particles has about 91% of its mass
in the large particles.
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Fleaker
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I have an excellent book on practical ICP-MS the name of which escapes me, but it discusses this in some detail. What you mention is indeed a problem
and limits the accuracy of the instrument.
The idea with ICP is that the atomization should be more or less 'complete' (however we define that term) at the very high temperatures used. An
interesting study would be flame and electrothermal atomization in AA compared to any of the ICP methodologies.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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JohnWW
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| Quote: Originally posted by Fleaker | | I have an excellent book on practical ICP-MS the name of which escapes me, but it discusses this in some detail. |
http://rs109l32.rapidshare.com/files/26551696/crc_press_-_pr... (PDF) 3,056 Kb
http://rs458cg.rapidshare.com/files/160628707/Curso.icp-ms.r... (Português) 1,587 Kb
http://rs557tl.rapidshare.com/files/148861270/30_minute_guid... 370 Kb
http://rs502l34.rapidshare.com/files/148863119/Agilent_7500_... 3,440 Kb - about an ICP-MS instrument
[Edited on 12-7-09 by JohnWW]
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watson.fawkes
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| Quote: Originally posted by Fleaker | | I have an excellent book on practical ICP-MS the name of which escapes me, but it discusses this in some detail. What you mention is indeed a problem
and limits the accuracy of the instrument. |
In the case that spawned this posting, we have at-least-half a
charlatan, charitably, whose had dissolved gold with hydrogen peroxide, neutralized the solution slowly, and then sent it all off to a lab who,
wowie-zowie!, found almost no gold left. This was proof that they had found something Beyond the Explanation of Science.
Well, no.
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unionised
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Gold has a boiling point of about 3,100 K
Typical ICP MS plasma temperatures are about 6,000 to 10,000K
We are talking about 2 or 3 times the boiling point of the bulk metal.
Any "cluster" would boil.
Clusters do form in ICP systems (there aren't many places where argon oxides are a problem) but they are not usually the dominant form.
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Nicodem
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| Quote: Originally posted by watson.fawkes | In the case that spawned this posting, we have at-least-half a charlatan, charitably, whose had dissolved gold with hydrogen peroxide, neutralized the
solution slowly, and then sent it all off to a lab who, wowie-zowie!, found almost no gold left. This was proof that they had found something Beyond
the Explanation of Science.
Well, no. |
Actually, if you check that Excel spread posted by that new age economist, you will notice that he sent to the ICP analysis a sample of table salt
(NaCl). Assuming, the table salt was not taken from the table, but truly obtained by his neutralization of his gold chloride dissolved in HCl with
NaOH and filtering the precipitate (this was, as far as I understood, his "process"), then this is nothing even remotely resembling "something Beyond
the Explanation of Science". All it does prove is that he was intelligent enough to waste some gold by poring it down the sink.
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watson.fawkes
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| Quote: Originally posted by unionised | Gold has a boiling point of about 3,100 K
Typical ICP MS plasma temperatures are about 6,000 to 10,000K
We are talking about 2 or 3 times the boiling point of the bulk metal.
Any "cluster" would boil.
Clusters do form in ICP systems (there aren't many places where argon oxides are a problem) but they are not usually the dominant form.
|
If the cluster doesn't gets into the plasma in the first place, it can't boil. (That's particle size
rejection in the nebulizer.) While the plasma temperature is much higher, the cluster doesn't sit in the plasma indefinitely. If it passes through too
quickly it will only partially vaporize. I don't know which of these two mechanism would cause the greatest error here.
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watson.fawkes
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| Quote: Originally posted by Nicodem | | obtained by his neutralization of his gold chloride dissolved in HCl with NaOH and filtering the precipitate (this was, as far as I understood, his
"process") |
Another systematic error: filtration loss. I had missed that one.
The question here for me is not convincing myself, nor scientists on this forum. What I'm interested in is understanding what happened enough to
explain to the ignorant middle, the general public smart enough to read popular science writing but ignorant enough about the details to accept the
possibility that there's something new here.
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DDTea
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It sounds like this analysis wasn't done properly. The sample should have been spiked with the analyte of interest--or even a series of spiked
solutions for a proper standard addition. That way, if you don't see *any* of your analyte, you know you screwed up (because there should at least be
the added analyte).
I hope that I'm understanding this thread and this problem properly.
"In the end the proud scientist or philosopher who cannot be bothered to make his thought accessible has no choice but to retire to the heights in
which dwell the Great Misunderstood and the Great Ignored, there to rail in Olympic superiority at the folly of mankind." - Reginald Kapp.
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Texium
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Thread Moved
19-11-2023 at 14:56 |
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