Intro to GD-MS

Glow Discharge Mass Spectrometry (GD-MS) is a powerful direct solid state elemental analysis technique that combines a glow discharge ion source with a magnetic sector mass spectrometer. This technique can quantify ultra-trace contaminants of virtually any element. It is ideal for analysing a range of materials, including high purity metals, superalloys, semiconductors and dielectrics.

Modern GD-MS systems are based on technology developed in the early 1970s, with the first commercial GD-MS systems launched in the 1980s. GD-MS systems fall into two categories: fast flow and low flow. This describes the flow rate of plasma gas used in the GD source. Typical flow rates for fast flow are around 500 sccm (standard cubic centimetres per minute), and low flow considerably lower at around 0.5 sccm. Whilst both techniques have their strengths, the Astrum uses a low flow source because of its superior user experience and performance in the most demanding applications, plus the lowest cost of ownership. 

GD-MS uses a solid-state glow discharge source, generating ions directly from the sample surface. This eliminates the need for complex sample preparation, such as acid dissolution or lengthy processes that might introduce contamination. Just quick, clean, simple and safe sample introduction. Other elemental analysis techniques similar to GD-MS are Spark Optical Emission Spectroscopy (Spark-OES), Glow Discharge Optical Emission Spectroscopy(GD-OES) and Secondary ionisation Mass Spectrometry (SIMS)

A glow discharge ion source uses a low-pressure discharge gas in the presence of a high voltage gradient to create a steady plasma consisting of plasma gas and sample ions. The discharge gas of choice is typically argon due to its availability and high first ionisation energy, but low flow ion sources allow other gases, such as krypton and neon, to be used in specialist applications. Low flow GD-MS feature a cryogenically cooled source for ultimate stability, low gas backgrounds and to enable the analysis of low melting point materials such as Ga and In. Glow discharge sources operate on the following principle:

1. A low-pressure plasma gas is introduced into the glow cell in the presence of a high voltage gradient (~1 kV). The sample is the cathode (+5 kV), and cell is the anode (+6 kV). Argon ions generated through electron impact and penning ionisation are accelerated towards the cathode.
2. Upon impact, the ions remove particles from the surface of the sample in a process known as sputtering.
3. The sputtered atoms are ionised in the plasma cloud.
4. Electrostatic forces pull sample ions out of the cell, towards the extraction optics.
5. Once outside the cell, the high positive potential of the cell (+6 kV) accelerates the sample ions towards the mass spectrometer.
6. In the mass spectrometer, sample ions are separated based on their mass and charge ratio before quantification. 

In the mass spectrometer, precision ion optics focus and shape the ion beam to enable the highest resolution for the best resolving power. A large electromagnet separates ions based on their mass and charge before quantification. GD-MS uses two detectors: a faraday cup for measuring high intensity signals, and a secondary electron multiplier for measuring the smallest signals from ultra-trace sample elements. 

GD-MS is a high-resolution technique, capable of reaching resolving powers exceeding 10,000. This ensures most interferences are sufficiently resolved for accurate quantification. Interferences can arise from many sources, with notable examples including CNO-based polyatomics, multiply charged species, and plasma-gas based species. The resolution in GD-MS is primarily controlled by directing the beam through two narrow apertures, one at the source and one at the collector end of the instrument, the source slit and collector slit respectively. These may have fixed or variable width, ranging ~1 mm to <10 µm. 

Tuning is performed with the collector slit open on a flat top peak at ~300RP to maximise ion transmission. Following this, the collector slit is closed to produce a narrow beam, giving a pointed, higher resolution peak. The peak shape is optimised for the highest resolution using the mass spectrometers’ ion optics. Typical tuning routines take <5 minutes, with some higher resolution operating conditions taking a little longer to complete. Most analyses are performed at ~4000 RP: this is the optimum balance between good signal intensity and resolution, since increasing RP usually reduces signal intensity.

GD-MS is a powerful and customisable technique that allows a user to refine analysis conditions to suit their application. For example, ultra-high purity materials benefit from higher signal intensity, enabling lower limits of detection (LOD). For these applications, particularly where interferences are minimal, high resolution is not always required, and a user may wish to operate at ~3000 RP and higher ion transmission, lowering LODs. Conversely, for a complex alloy sample, interferences can be significant. In these applications, purity requirements are often less demanding, and higher resolution will help a user distinguish between interference signals and analyte signals – here it may be useful to tune the instrument to >4000 RP at the cost of some signal loss.

In addition to resolution and LODs, GD-MS allows users to optimise their analysis routines in numerous other ways, allowing for refinements including analysis time/sample throughput, sample pre-sputtering and source temperature. Adjusting these parameters gives the best control and enables GD-MS to seamlessly integrate into their workflow.