High-purity argon is crucial for optical emission spectrometry. The argon used must meet the standards of GB/T 4842-2006 for high-purity argon, which means the purity must be ≥99.999% with strict limits on impurities.
The primary reason for this requirement relates to the two types of spark discharge that can occur in an argon atmosphere: condensed discharge and diffuse discharge.
Condensed discharge focuses the spark on a small area of the sample, which leads to efficient atomization of the sample material. This leaves a small, black spot on the sample surface. Condensed discharge is what you want for accurate analysis.
Diffuse discharge is scattered and inefficient, resulting in poor atomization. It leaves a large, white spot on the sample. This type of discharge is caused by impurities in the argon.
The presence of impurities like oxygen and moisture in the argon can easily react with the sample surface, forming an oxide layer during the spark discharge. This leads to a diffuse discharge, which produces inaccurate results.
For elements like carbon, nitrogen, sulfur, and phosphorus, whose spectral lines are in the ultraviolet range, impurities like oxygen can also absorb the light, affecting the measurement. Additionally, elements such as aluminum, silicon, chromium, molybdenum, titanium, and vanadium have a high affinity for oxygen and are particularly prone to forming these white spots. In contrast, elements like copper, nickel, gold, and silver are more stable and less likely to form an oxide film, making them less sensitive to argon purity.
To ensure a condensed discharge and prevent a diffuse discharge, the argon gas must meet strict impurity limits. Specifically, the acceptable volume fraction of impurities should not exceed:
Oxygen: 5 x 10⁻⁶
Nitrogen: 20 x 10⁻⁶
Moisture: 5 x 10⁻⁶
Other impurities: 5 x 10⁻⁶
For some applications, even stricter limits are required, with oxygen and moisture levels not exceeding 2 x 10⁻⁶. This high level of purity ensures the stable and reliable spark necessary for accurate analysis.
