Intro to Noble Gas MS

The noble gases are extracted from solid samples via heating (furnace or laser), laser ablation, crushing (to release gases from fluid inclusions), or from fluids. Extraction systems are very diverse and frequently custom built by laboratories. Regardless of extraction method, it is essential to clean the sample by removing non-noble species, so that — as far as possible — only noble gases are admitted to the mass spectrometer. The different noble gases are often separated cryogenically before admission. 
 

Occasionally, if extremely large amounts of sample are available (e.g. from fluids or ice cores), Ar or Kr isotopes are measured dynamically by IRMS. Conversely, N2 isotopes are sometimes measured statically by NGMS, if the sample size is very small (e.g. N2 extracted from rock samples). 
 

Measurement Concepts

Static Measurement

The rarity - and hence small sample size – of the noble gases mandates that almost all measurements are made in static mode: the sample is admitted to the mass spectrometer after isolation from pumping. This has several important consequences. 

All commercial NGMS use electron impact ionisation. Sensitivity may be defined as signal for a given pressure (or gas density) in the MS, or signal for a given amount of sample. In the latter case, halving the MS volume will almost double the sensitivity. Sensitivity can also be increased by increasing the electron ionisation current in the source. Both of these may be useful, e.g. in raising the signal above the Faraday noise floor (see below); but both consume the sample faster and reduce the time available for measurement.

 

Mass Resolving Power and Resolution

It is sometimes necessary to measure one ion beam without interference from another of similar mass. The ability to do so is described by the two parameters mass resolution (MR) and mass resolving power (MRP). 

MRP is the more fundamental of the two and describes the ion beam width at the collector. It is usually defined by:
 

MRP = m / Dm_5-95

where  Dm_5-95 is the width of the side of the peak, from 5% to 95%, which corresponds to the ion beam being swept over the edge of the collector slit. 
MR describes the width of the whole peak in the mass scan and depends on both MRP and collector slit width.  It is usually defined by:
 

MR = m / Dm_5-5

where Dm_5-5 is the width across the peak at 5% height. 
These concepts are well illustrated by the following mass scan – obtained with just 150000 atoms of ³He, demonstrating the very high sensitivity of NGMS. 
 

In this example, MR (730) is high enough to fully resolve ³He from the HD interference (m/Dm = 512): there is a valley between the peaks, sometimes called true resolution. In contrast, in the case of HD-H₃, the two peaks overlap (since MR (730) <  m/Dm=1952); despite this, MRP (3800) > m/Dm (1952), hence we can still see the two peaks and there is some ‘flat’ where, in principle, either could be measured free of the other. This is called edge resolution. 

Both true resolution and edge resolution find use in noble gas analyses. The choice between them depends on factors such as relative size of interference, possibility of tailing of one peak into another and amount of peak flat available or required (for a given target peak, interference and MRP, edge resolution gives more ‘flat’ than true resolution). 

The importance of resolving interferences varies between the different noble gases. ³He is almost always resolved from HD but this is not difficult as the mass difference is large. Interferences are most numerous for Ne, including ⁴⁰Ar⁺⁺ at ²⁰Ne⁺, which usually requires that Ar be removed before measuring a Ne sample. Ar is often measured at low resolution, but some laboratories report advantage in resolving ¹²C3⁺ from ³⁶Ar⁺. Hydrocarbon interferences are occasionally encountered at Kr and Xe but are easily resolved, due to the large mass difference.  

Ion beam detection

Ion beams are detected using either an electron multiplier (smaller beams) or a Faraday cup (larger beams). The fundamental limit on measurement precision is set by the shot noise (also called counting statistics) in the ion beam and is frequently achieved for small ion beams with an electron multiplier. Faradays perform well with larger signals, but the Faraday preamplifier always exhibits some noise in the baseline, which limits the performance for small signals. For a Faraday amplifier with purely resistive feedback, the baseline noise arises from Johnson noise in the feedback resistor.  Over the last decade, improvements in Faraday amplifier technology (e.g. 10¹³ Ohm gain Faraday amplifiers) have reduced – but not eliminated – the noise floor. 

It is sometimes useful to consider the ‘cross over’ point for Faraday and IC, i.e. when shot and Johnson noise are equal. For an amplifier with purely resistive feedback, it can be shown that this occurs when the true voltage across the resistor is 52 mV.