This latest blog is being planned as I travel to China. During this trip I’m going to be talking to several groups of scientists who are using (or want to use) stable isotope MS instruments (also known as IRMS) for the measurement of noble gas isotope ratios. Hold on a minute, I hear you say, isn’t there already a solution for that… they’re called noble gas mass spectrometers! But it’s true, there are several groups around the world who have turned to IRMS instruments for their noble gas measurements.
This is a little out of my wheelhouse, but it is clear to see what the benefit is: if you have sufficient sample, you can bracket your sample measurements with measurements of a known standard, potentially giving you much better precision and accuracy. This process can even be automated, using a device called a dual inlet (essentially a set of valves to automatically switch between your sample and your standard). Turns out that our SIRIX stable isotope MS is especially well suited to this, producing extremely consistent data and capable of achieving low single figure ppm precisions – much better than values you could achieve with a “traditional” static vacuum noble gas MS, with the premise that you have sufficient sample. For restricted sample sizes, of course a noble gas MS remains the gold standard for isotope ratios of those elements.
Considering this rather uncommon application of an IRMS, I got to thinking about some of the other lesser-known applications, including unusual sample introduction techniques and unusual sample types. Probably my first taste of the unexpected was in around 1998. A wonderful gentleman by the name of Oliver Donard arrived at my place of work, VG Elemental. He had a gas chromatograph (GC) in the back of his car and had driven all the way from Pau, France.
Olivier was a very early advocate of speciation ICP-MS and went on to become one of the undisputed leading figures in the world on this subject. Way back in 1998, hooking a GC up to an ICP-MS would not have been commonplace. At the time I was working on the Plasma 54 and helping with the development of its eventual replacement, the VG Axiom MC. I’d just been promoted to Product Manager and therefore it was my responsibility to help Olivier make these two instruments connect somehow.
Fortunately, we had access to an abundance of pressure fittings, connectors and valves, so after a day or two we’d managed to get the Axiom MC-ICP-MS to accept gaseous samples from Olivier’s GC. I will freely admit that Olivier did all the work, I just found the connectors he asked for and kept him fueled with huge amounts of black coffee. And ultimately… it worked! We were able to measure the isotope ratios in the mercury in our speciated samples (digested marine organisms). Just as Olivier hoped, there were measurable differences in some of the isotope ratios, and this went on to become a useful technique.
The next uncommon application that I experienced, about a year later, again involved mercury. This element has not been my favourite, mainly due to the fact that it is as sticky as a sticky thing, stubbornly refusing to be washed out of the mass spectrometer sample introduction system. In this instance I was asked to measure the isotope ratios in mercury samples using a thermal decomposition sample introduction technique. In the briefest of terms, you slowly heat up a mercury-gold amalgam and the resultant mercury vapour is released into the mass spectrometer; easy peasy.
The problem here was that I really had no idea how slowly to ramp up the temperature. My naive plan was to watch the signal intensities of several of the mercury isotopes as I manually turned up the wick. I clearly was not doing it slowly enough, as at a certain temperature the background levels of the monitored mercury isotopes jumped almost instantly to ten volts (saturation). Oooops. I was cleaning mercury off the torch, cones and even the first lens stack for days afterwards.
It was probably at about this time that I realized I might be better having a “hands off” role in analytical chemistry and shortly afterwards moved into a marketing role. Best for everyone really. But I still encounter and appreciate the uncommon applications. A more recent one was being involved with the use of a noble gas mass spectrometer to measure tritium at very low levels in marine organisms and sea water. Tritium is hard to quantify at the best of times, particularly when it is at low levels, but there is a technique that has become quite well established, it’s just…. very slow!
The technique is helium ingrowth and utilizes the fact that tritium decays to 3He with a fairly short half-life (about 12 years). The ultra-dumbed down description of this technique is to take a fixed volume of sample, let’s use potentially polluted seawater as an example. You remove any helium from the vessel and seal it, then you wait, usually for some months. As you’d expect, the level of 3He gradually increases as atoms of tritium in the seawater decay into atoms of 3He. Eventually there is enough 3He to make an accurate measurement. But I have oversimplified the sample preparation by several orders of magnitude. The lab I visited that is using the technique had a sample prep system that was ten times larger (and probably five times more expensive) than the mass spec. Obvious to see why it is an uncommon technique!
More positively, the less common techniques do drive the vendors to produce ever more powerful instrumentation. I remember some years ago talking to a government lab about their trace actinide requirements. They wanted to measure certain actinides – you know the ones, high 230’s or low 240’s m/z, but not uranium. The samples were environmental, and you would like to think these nasty actinides were present at very, very low levels. Turns out they were, in some cases with resultant ion beams smaller than one count per second. Clearly that is a major analytical challenge, and yet with the latest generation of multiple ion counting systems we can do just that, with a precision good enough to make a worthwhile measurement. That would have been impossible when I started in this business, so the need really has driven the development.
That’s all for this month, but if you’ve got any stories about the lesser spotted analytical techniques then please let me know, I’d be keen to hear (Stephen.guilfoyle@isotopx.com). More next time.
