In part 1 of this blog series, I discussed how ICP-MS collision cells work when operated in collision mode (using pure helium as the cell gas) and in simple reaction / collision mode (using a mixture of hydrogen in helium (typically 8% (v/v) H2 in He, as this concentration is below the explosion limit). Here in part 2, I’m going to explore what can be achieved using reactive cell gases.
Why would you want to run the cell with a reactive gas?
Well, although collision cell operation with pure helium on its own is pretty effective, there are a few situations where it is not particularly helpful. Firstly, if the interference is due to an isotope of another element that has the same mass as the analyte with which it interferes (a so-called isobaric interference – an example would be 40Ar interference on 40Ca) then He KED mode has the same effect on both isotopes, because they have similar kinetic energy and collision cross section areas. This leads to no improvement in the signal to interference ratio. Secondly, if the interference signal is very large compared to that of the analyte and the analyte sensitivity is strongly reduced in He KED mode, then the improvement in the signal to interference ratio is very small, if there’s any improvement at all (an example would be O2+ interference on S+).
The first situation can be dealt with by using a reactive gas in the cell to facilitate a charge exchange reaction (see schematic below) to specifically remove the interference while leaving the analyte unaffected.
Charge exchange reaction between an analyte ion and a gas molecule (e.g. H2).
Charge exchange reaction between an analyte ion and a gas atom (e.g. Xe)
The second situation (see schematic below) can sometimes be dealt with by charge exchange or, alternatively, by converting the analyte to a product ion by means of a chemical reaction with a reactive gas and measuring the product ion in place of the original analyte ion.
Product ion generation reaction between an analyte ion and a gas molecule (e.g. O2).
Both approaches are explained in more detail below.
Charge Exchange Reactions
An example of where a charge exchange reaction is very useful is low level analysis of iodine-129. This isotope is mainly formed from the fission of uranium and plutonium in nuclear reactors (significant amounts were released into the atmosphere during weapons testing in the 1950s and 1960s) but some is also produced naturally by the spontaneous fission of natural uranium and by interaction of cosmic rays with xenon in the atmosphere. The problem with 129I in quadrupole ICP-MS is that it is isobarically interfered by 129Xe. Simple reaction of 129Xe with H2 diluted in He doesn’t work in this case (in fact, neither does pure H2), because both I and Xe react to some extent with H2. This causes two problems: firstly, IH and IH2 form along with some XeH and XeH2 which therefore continue to overlap and secondly, 127I in the samples (which is usually present at much higher concentrations than 129I) is partly converted to 127IH2+ thereby increasing the interference on 129I. The solution is to use a different reactive gas, in this case oxygen, to exploit a gas phase ion-molecule reaction which removes the 129Xe signal while leaving the 129I signal unaffected. The reaction in question is a charge exchange process as shown below:
129Xe+ + O2 = O2+ + 129Xe
This particular reaction is a fascinating one as it actually also works in reverse, as indicated by the equals sign in the above equation, such that O2+ interference on sulphur can be removed by using Xe as the cell gas (although pure Xe is very expensive and is a heavy gas so is not ideal for use in a collision / reaction cell). If you’d like to learn more about how to apply this approach in practice, there’s an application note available here. This brings us neatly onto the next type of reaction – chemical reactions which generate product ions that are then analyzed at a different, non-interfered (or at least, less interfered) mass to the original analyte.
Chemical Reactions That Generate Product Ions
Perhaps the best example of using this class of reaction, as mentioned above, is tackling the problem of O2+ interference on S. He KED mode does reduce the (massive) 16O2+ interference on 32S+ but at the same time, the sensitivity of S drops dramatically too (it isn’t that high to start with anyway, as S has a high first ionization potential (10.36 eV)). The outcome is that in He KED mode, there is not much improvement in the signal to interference ratio in this case. The solution, ironically, is to use O2 gas to reduce the O2+ interference problem. When O2 molecules collide with S+ ions, the resulting reaction(s) generate, among other things, SO+ product ions. These ions appear higher up than S in the mass spectrum, with the largest and hence most sensitive signal, 32S16O+, appearing at mass 48. By operating the instrument with O2 cell gas and without applying KED, the SO+ product ions can be transmitted to the quadrupole and detected. As the background at mass 48 is generally much lower than the O2+ signal at mass 32, a large improvement in signal to interference ratio is achieved, as shown in the table below.
* BEC = blank equivalent concentration (i.e. the calibration intercept divided by the calibration slope)
# I.d.l = instrument detection limit (3σ)
I said ‘as the background at mass 48 is generally much lower than the O2 signal’ above because there are potential contributions at mass 48 from 48Ca, 48Ti (which partly reacts with O2 to form TiO2+) and O3+, which means that exceptionally high concentration of either Ca or Ti in the sample would be problematic. Fortunately, the abundance of 48Ca is only 0.18% and Ti is usually present at far lower levels than S in most samples, which reduces the impact of their interference. Operating in non-KED mode (which is required to allow SO+ to be detected, as described above) tends to raise background signals, particularly at lower masses, as this mode of operation allows all ions with lower kinetic energy to reach the quadrupole. Nonetheless, converting S+ to SO+ has been successfully applied to GC-ICP-MS sulphur speciation in fuel, providing much lower detection limits than could be achieved using He KED operation (there are various literature papers on this topic) and my colleagues in Bremen have applied this approach to analyzing protein digests (via S in the protein molecules) using HPLC-ICP-MS. Other gases that can be useful for this approach to interference reduction / removal include NH3 which has the useful quality of forming a wide range of higher mass NH3 clusters with a variety of analyte ions. But, beware: if you’re planning to use pure NH3 as a collision / reaction cell gas, check with your instrument supplier first that this gas can be safely used with the instrument without damaging any vacuum seals and ensure that safety measures are put in place should there be any leakage of NH3 into the laboratory.
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