Inductively Coupled Plasma-Mass Spectrometry (ICP-MS): a Personal Odyssey III

Abstract

This essay follows from a Heritage lecture which I gave at the Winter Conference on Plasma Spectrochemistry, in Tucson, Arizona, on January 10, 2012, and is a sequel to my earlier “Odyssey” papers. This essay attempts to describe the joy and frustration of “ten years of wandering in the wilderness”, in the early days of ICP-MS which started for me in 1983. Soon after the installation of our first ICP-MS our research group adapted Laser Ablation (LA) sample introduction which started a second Odyssey. The goal of this essay is to encourage others to engage in the next new multi-elemental procedure when it appears. Finally some “words to live by” will be mentioned, hoping these will also be a guide and inspiration for others.

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Introduction to ICP-MS

My introduction to ICP-MS took place almost thirty years ago, in May of 1983 at the American Society of Mass Spectrometry convention in Boston. There Alan Gray and Alan Date, v.i., had a poster presentation on a prototype ICP-MS instrument, which evolved into the VG instrument. They had on their poster a mass spectra of what was clearly a solution of equal concentrations of the Rare Earth Elements (REE). Knowing that Alan Date was a geologist, I asked him to show me a spectra of a real rock, and from his briefcase he pulled out, as if by magic, a spectra of a Canadian syenite rock reference material, which they later presented in a lecture. At that time, I thought to myself, and while I do not think that I ever wrote it down, have said it many times, that “if ICP-MS only does the REE's it will be a success”. To this day the REE's are still one of the best groups of elements to determine using ICP-MS.

Wandering in the wilderness

The ICP-MS developed very quickly, with the landmark paper being published¹ in 1980, just a few years before the announcement of commercial production of ICP-MS instruments at PittCon in March 1983. The time between the first paper and the delivery of commercial instruments was a spectacularly short time in comparison with the usual decade used for earlier new techniques. Sam Houk recently described, from his point of view, the development of ICP-MS² in these early days. The word “Odyssey” is from Homer's epic poem describing the ten years Odysseus spent wandering in the wilderness. Odyssey, still seems to be a good title, describing the early years following the December 1984 installation in our laboratory of a first generation SCIEX model 250 ICP-MS. I have written two other Odyssey papers, the first being about the first decade when the solution nebulisation procedures were developed while the instrument characteristics were being investigated.³ This was followed by a second Odyssey dealing with Laser Ablation (LA) sample introduction,⁴ which was later reprinted.⁵ While these Odysseys were indeed often frustrating they were continually filled with success and joy.

We wrote the proposal to fund the purchase of an instrument in the fall of 1983, with funding granted in the spring of 1984 at about the time when the first SCIEX commercial installation took place in Ottawa. Just before Christmas of 1984, we took delivery and installed the tenth purchased instrument from SCIEX. We were the first academic geo-science department to install an instrument. Other geo-science laboratories, including the United States Geological Survey (Denver and Boulder), X-Ray Assay Laboratories, Geological Survey of Canada, and the Ontario Geological Survey also installed instruments, along with several other laboratories, including the Ontario Ministry of Environment and Agriculture Canada.

Many, if not most of the geo-science community thought we were somewhere between “crazy” and worse, to try something which was essentially totally unproven, since many unproven things have failed in the past. We had interesting advantages and disadvantages over most of the other new users as we came from a mass spectrometric (Thermal Ionisation) background, and not only did we really not understand what ICP was, but until the Winter Conference on Plasma Spectrochemistry in January of 1984, I had never actually seen one. At that time, there was great skepticism about calibration to which I answered many times that we could simply do “Isotope Dilution”, which is the procedure used by Thermal Ionisation-Mass Spectrometers (TI-MS) when quantitative analysis is required. Interestingly this only rarely happened.

Early literature

In our first published paper on ICP-MS⁶ there were among the references cited, nine which were about the ICP-MS instrument, including the paper by Houk, et al. (USA) previously cited. Some of the papers were from the group of Alan Grey and Alan Date in the U.K. which led to the VG ICP-MS instrument. The remaining ICP-MS papers were from the group of Douglas and French, et al. in Canada⁷ which lead to the SCIEX instrument. The interesting point is that at that time (our paper was submitted on March 8, 1985) these citations include, what was to the best of our knowledge, all of the pertinent papers on ICP-MS. In hard copy, all of these papers fit into a regular manila folder, with room to spare. Today the list and volume of papers on ICP-MS boggles the mind. The ability of a researcher to keep up and read all the papers today is indeed a herculean task. This leads to the problem of “re-inventing the wheel” syndrome, which is unfortunately very prevalent among new users.

The ICP-MS was the multi-elemental analysis instrument of the 80's. This followed other important multi-elemental analysis instrumental techniques including (in alphabetical order): Arc/Spark Emission and MS, Atomic Absorption (flame and furnace), Neutron Activation, Plasma (DCP, ICP, MIP, GD, flame) Emission, and X-Ray Fluorescence. A new technique developed approximately every decade. Thus today it is well past the time for the next new and innovative technique to appear and be introduced at the next conference. My message to the reader, especially to younger readers, is that the early days of ICP-MS were very exciting, and a lot of fun with new information appearing almost daily, while conference presentations and discussions often brought forth immediate new understanding. If the reader of this essay is there when the next new instrument is introduced, “go for it”, because the experience of being involved in the beginnings of a new and rapidly developing technique is a marvellous one, even if it is sometimes a frustrating (challenging) one.

ICP-MS, the marriage of the ICP ion source and the quadrupole mass spectrometer

One of the very attractive attributes of what was then the new, ICP-MS instrument was that the ICP was essentially identical, and still is, to the ICP used for several decades in ICP-OES instruments including a decade of commercial instrument production. The marriage was the coupling of the ICP, which had long been known to be a good source of ions as many of the superior emission wavelengths used in OES instruments were known to be from ions in the plasma. In the mass spectrometer instruments, the ICP was coupled to quadrupole mass analysers which were used in both of the first generation instruments. Quadrupole mass analysers have a history which goes back to the 1940's, hardly a new component. The use of ICP sector instruments was later introduced commercially in the early 1990's, and was followed in the late 1990's by time-of-flight instruments. The link, the marriage or interface, was and still is a very simple device, being two cones with orifice diameters of around one mm spaced about ten mm, with the region between the two cones being evacuated using a conventional rotary vacuum pump. Following the second cone one or two lower pressure vacuum regions are found. The cones are machined usually of nickel, although aluminium, copper, or platinum are also used. The requirement is only that the metal has a high thermal conductivity, thus alloys such as stainless steel are not suitable.

The point of this discussion of the fundamentals of the ICP-MS interface is that this meant that essentially all of the sample introduction devices which were then available and used on ICP-OES instruments were expected to be used on the new mass spectrometers. This was true, with the note that the total mass transferred to the plasma needed to be reduced. For example, whole rock acid digest solutions prepared for analysis on OES instruments needed to be diluted by approximately a factor of ten for the MS to allow for acceptably small matrix correction effects. The basic simplicity of the interface, along with a long history of ICP's and quadrupole mass spectrometers suggested to us that this new instrument, while expected to be a challenge, was destined, in our opinion, to be a success.

ICP-MS sensitivity

By the time that our first ICP-MS instrument was delivered and installed the sensitivity (signal per unit concentration) had increased ten fold from the specifications which were used in our funding proposal. When we installed our second instrument⁸ in 1994, sensitivity increased an additional five hundred fold. Current generation instruments have a sensitivity which is additionally approximately five times higher. To reach single ion detection, that is to detect every ion which enters the sampler cone, an additional increase of approximately four hundred fold would be needed. For solution analysis for a huge majority of samples our first generation instrument was more than satisfactory with solution detection limits for ideal (e.g., mono isotopic REE, all of which have low ionisation potentials) elements around 0.01 ng g⁻¹, which for solid rock samples analysed at a dilution of one thousand, gave detection limits⁹ in the solid of 10 ng g⁻¹. However, for laser ablation there is no limit to the sensitivity which could be usefully used, as the present day typical ablation pit diameters of 50 μm, could easily be reduced by an order of magnitude or more, which recognising the cubic relationship of volume to diameter would produce a thousand fold fewer ions.

In conclusion while increased sensitivity would be only of minor use for solution nebulisation, for LA there is no limit on the useful sensitivity. Noting as well the incredible increase in sensitivity over the years, the theoretical maximum has yet to be reached. The time when we “found” our second generation instrument was another one of the special days.

Horlick mountains

An early paper by Horlick and his research group¹⁰ which was presented at the Canadian Institute of Chemistry convention in June 1985, described the variation of the signal of singly charged ions, but also included the important poly atomic (not molecular ions) oxide ions, and doubly charged ions as a function of the nebuliser gas flow (a Horlick mountain). Douglas and French¹¹ showed a determination of the plasma temperature from measurements of the REE degree of formation of poly atomic oxide ions from mono elemental single charged ions as a function of oxide ion bond strength. In later papers our group further used the plasma temperature as a measure of ICP conditions as various ICP parameters were varied¹² including most importantly, as shown by Horlick's group, the effect of the nebuliser gas (central or sample carrier). Shown also was the changes of ICP temperature with changes of the other two ICP gas flows, the RF applied power, the torch position, liquid sample uptake, and in a later report, the spray chamber temperature.¹³ A paper which considered the effect of various solvents including nitric acid, acetic acid, and ethanol also showed a large change in ICP conditions¹⁴ due to changes in the quantity of volatile components introduced into the ICP, which in turn changes plasma temperatures.

The point of this discussion is firstly to highlight the great awakening moment that came when hearing Horlick’s lecture in 1985. At that time, our observations of instrument behaviour obtained over the previous six months came together and were made clear. These “magic moments” of “eureka” seldom, if ever, happen any more, but in the early days of ICP-MS they happened often and are still remembered. And secondarily, to emphasise the importance for new users to be aware of the older literature. Many new users have not read this literature, partially due to the lack of easy access, but also because of a lack of appreciation of this work. It is very important to understand that “the ICP is not one-dimensional”. A change in any one parameter of the ICP changes the signals of the elemental ions and poly atomic ions as a function of the apparent oxide temperature. Any optimisation of the ICP can not be done without appreciating the interrelationship of these parameters.

Laser Ablation (LA) coupled to the ICP-MS

Soon after the introduction of the ICP-MS instrument, the first report of coupling a Laser Ablation (LA) sample introduction system to a ICP-MS was published in 1985.¹⁵ Based upon other literature, we perceived that much smaller sample volumes could usefully be ablated. After securing funding in 1989, and putting together our in-house laser system, our landmark paper was published¹⁶ in the Canadian Mineralogist which received the Howley Award for the best paper of the year. While this paper did not make any fundamental advances, we demonstrated to the geo-science community the usefulness of this new technique (LA-ICP-MS) for which we initially used the acronym (LAM-ICP-MS) where the M stood for Microprobe which emphasised that this was for micro analysis as opposed to bulk analysis. For better or worse, the “M” has not survived the passage of time. Possibly the most important algorithm developed in this paper was the use of a “naturally occurring internal standard” which was a major element contained in both the sample and the calibration material. This internal standard is usually measured using an electron probe X-Ray analyser or, in simpler cases, using known stoichiometry. It was not until some years later when after a general lecture, I was asked what a “naturally occurring internal standard” was, that I realised that we probably had invented this term. It is a great pleasure to be associated with bringing this micro elemental technique to where it is today, with a very large number of users.

A surprise laser ablation paper

Some time after our work on the establishment of LA as a tool for micro analytical geoscience, one of our group asked me how the various results for LA analysis were calculated, including calculations of concentrations and detection limits. As I often do, I wrote up an “occasional essay”. Having done that, we decided that other users may find this useful, and in any case having such a document in the literature makes it easy for users of our facility to easily cite a single reference for documentation. So we decided to submit this for publication, after adding citations and discussion concerning the choice of instrument data acquisition parameters. The surprise, to us, is that this report (not really a “full” paper, but “only” a “laboratory note”), turned out to be the sixth most cited paper in JAAS in the first twenty years of the journal publication.¹⁷ Knowing and recognising that the number of citations is not necessarily important, this same review ranked this report as the twelfth most impactful paper. Also remembered was a reviewer's comment that “everyone should know this, but they do not”, to which I responded to the editor “that was why we wrote and submitted the paper”.

The important conclusion is that you never know what paper you write will be important and which ones will be ignored. Another earlier paper on REE analysis,¹⁸ which I thought might be a “landmark” is ignored. Conclusion is that we are probably not very good judges of our work, and should publish regardless of our personal opinions and let history decide.

Words to live by

I almost always conclude general lectures with some “words to live by”, and the heritage lecture which this essay is based on was no exception. Occasionally I am very gratified when other scientists adopt and use these words.

No method is a panacea

Often scientists expend a large amount of work to make their instruments do what it really can not do well, when there are other developed techniques that could do the task very easily. The conclusion is that one should be aware of the many techniques (elemental analysis in this essay) which are available in the “tool box” of the analytical chemist. This is not to suggest that research should not be done to improve and develop, but more to be aware of what is already developed. And not to “reinvent the wheel”.

Suitability for purpose

This is one of my most popular sayings. It is not necessary that all procedures and methods be the best possible, with the best detection limits, the best precision, and the best accuracy. The reason for this is because of costs and time, which have increased in importance with the passing of the decades, noting that time and cost are often highly correlated. A new method which does not produce the best results can be most useful in the “real world” of analytical chemistry. At one time, I wanted to cite this expression to James Holcombe¹⁹ who wrote an editorial in Applied Spectroscopy. However, after some research and discussion, it was concluded that the exact expression was not his, but the thought certainly was, and this was expressed in his editorial.

One of these things is not like the other

A well known quote from Sesame Street which for our clients is essentially always their goal, even though they do not usually recognise that. The application of our analytical results is to allow the discrimination of various samples. Does a food sample meet maximum legally limited concentration values for an analyte? Is one rock older than another? Do they have the same age? Do these samples come from the same place?

In conclusion, these “sayings” are interrelated, but should be guiding principles for our research, development, and real world applications.

Acknowledgements

None of the work which was accomplished, and for which I get some credit, would have taken place without the efforts and support of a large number of persons over several decades. I have implied throughout this discussion, using the normally unacceptable first person, that this was always a group effort. All of these persons are not mentioned here, but it is important to mention the major players, especially in the early years. When I returned to university after the ASMS convention in Boston, I mentioned to my colleague, Brian Fryer, that this ICP-MS looks like the best thing “since sliced bread”. When I asked him if I should call or write for more information, he said “call”, noting that this was in the days when writing was cheaper than phoning. David Strong then became the Principle Investigator and put together, with contributions from another faculty, a top ranked proposal to fund the instrument. Before the funding was granted, it was necessary that a “high level” committee visit us to evaluate our proposal, and at that time, I knew more than they did, and basically they only knew what we had told them. This situation, for ICP-MS does not occur now as there are ICP-MS experts everywhere. Another reason for the reader to “go for” the next new “toy on the block”. A year after the installation, Simon Jackson, joined our group. Over the years the four of us moved on, being a mobile group of researchers, continually looking for new challenges, such as ICP-MS provided for us. David became a university president in British Columbia, Canada, Brian became a Dean in Ontario, Canada, I took voluntary early retirement to become an “itinerant professor and long distance hiker”, and soon after, Simon moved to Australia. Over the decades many assistants, post doctoral fellows, visitors, and students came and went, and all of these contributed to making our facility and the science of ICP-MS in our laboratory a success. Especially noted were the geologists who continually provided new challenges. Mention must be made of Detlef Günther, who spent one year with us, and has remained in special contact over the years, and has contributed significantly to LA. Funding was provided over many years in many different programs by the Natural Sciences and Engineering Research Council of Canada. Funding for the ICP-MS instruments and the laser ablation system was provided as well as research grants to individual researchers. Visitors from other universities in Canada also contributed to the operation through their individual grants. Also under different programs, funding was provided for operating expenses including salaries and funds to facilitate work by visiting scientists. Memorial University also in many ways contributed to the success of the operation, including, notably, the services of the Technical Services which provided electronic, machine shop, glass blowing, and welding services.

References

1. R. S. Houk, V. A. Fassel, G. D. Flesch, H. J. Svec, A. L. Gray and C. E. Taylor, Inductively Coupled Argon Plasma as an Ion Source for Mass Spectrometric Determination of Trace Elements, Anal. Chem., 1980, 52, 2283–2289.
2. R. S. Houk, Thirty years ago this month, J. Anal. At. Spectrom., 2010, 25, 1801–1802.
3. H. P. Longerich, Inductively Coupled Plasma Mass Spectrometry: a Personal Odyssey, ICP Inf. Newsl., 1995, 20(10), 703–705.
4. H. P. Longerich, Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS): a Personal Odyssey, Newsletter of the Mineralogical Association of Canada, 2001, 65, 1–6.
5. H. P. Longerich, Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS): a personal Odyssey, ICP Inf. Newsl., 2004, 30, 501–507.
6. D. F. Strong and H. P. Longerich, Machinations: the Inductively Coupled Plasma/Mass Spectrometer (ICP-MS), Geoscience Canada, 1985, 12, 72–75.
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14. H. P. Longerich, The effect of nitric acid, acetic acid, and ethanol on Inductively Coupled Plasma-Mass Spectrometric (ICP-MS) ion signals as a function of nebulizer gas flow, with implications on matrix suppression and enhancements, J. Anal. At. Spectrom., 1989, 4, 665–667.
15. A. L. Gray, Solid sample introduction by laser ablation for Inductively Coupled Plasma Source Mass Spectrometry, Analyst, 1985, 110, 551–556.
16. S. E. Jackson, H. P. Longerich, G. R. Dunning and B. J. Fryer, The application of laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) to in situ trace element determinations in minerals, Can. Mineral., 1992, 30, 1049–1064.
17. D. W. Koppenaal, JAAS—20 years of manuscripts, citations, andscientific impact, J. Anal. At. Spectrom., 2006, 21, 259–262.
18. H. P. Longerich, B. J. Fryer, D. F. Strong and C. J. Kantipuly, Effects of operating conditions on the determination of the rare earths by inductively coupled plasma-mass spectrometry (ICP-MS), Spectrochim. Acta, Part B, 1987, 42, 75–92.
19. J. Holcombe, Does your luggage fit in the overhead bin on the airplane, Appl. Spectrosc., 1996, 50(12), 10A–11A.

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