WHAT IT IS
Source frequency in Spark-OES and GD-OES refers to the rate at which electrical pulses or current cycles are applied to the sample during excitation. It determines how often the energy source (spark or glow discharge) is turned on and off or how fast it alternates, typically measured in hertz (Hz).
The source frequency affects the characteristics of the excitation plasma, including energy transfer, temperature, and stability. It is a critical parameter that influences the precision, sensitivity, and speed of the analysis.
HOW IT WORKS
Pulsed Spark (Spark-OES) – In Spark-OES, high-voltage pulses are applied to generate short-duration sparks. The frequency of these pulses controls how rapidly sparks occur over time.
Glow Discharge (GD-OES) – In GD-OES, a direct current (DC) or pulsed direct current (pulsed-DC) power supply creates and maintains a low-pressure plasma. In pulsed modes, the frequency controls how often the discharge is turned on and off.
Excitation Dynamics – Higher frequencies produce more consistent plasma excitation, leading to better atomization and excitation of sample atoms. Lower frequencies may allow more cooling time but can reduce signal stability.
Elemental Response – Some elements respond better at specific frequencies due to differences in excitation efficiency. This makes frequency selection part of method optimization.
Depth Profiling (GD-OES) – In GD-OES, frequency influences sputtering rate and resolution. Pulsed operation can improve control over ablation and minimize heat buildup.
TYPES OF FREQUENCY SETTINGS
Low Frequency (<100 Hz): Used for traditional arc discharge or coarse excitation. Less common in modern instruments.
Mid to High Frequency (1–1000 Hz or kHz): Spark-OES: Typically operates between 100 Hz to several kHz, depending on the system and application. GD-OES: Pulsed-DC frequencies range from a few Hz to tens of kHz, depending on the desired sputtering characteristics.
Continuous vs. Pulsed Operation: Continuous: Maintains a stable plasma but may cause more thermal stress. Pulsed: Allows cooling between pulses and better control over excitation energy.
IMPACT ON PERFORMANCE
Signal Stability: Higher frequencies generate more consistent plasmas, improving signal stability and reducing noise.
Sensitivity: Optimized frequency settings enhance the excitation efficiency for trace elements, improving detection limits.
Resolution and Depth Control: In GD-OES, pulse frequency impacts the precision of depth profiling by regulating the rate of material removal.
Sample Heating: Pulsed frequencies reduce excessive heat buildup on the sample surface, preserving sample integrity and improving reproducibility.
Measurement Speed: Frequency affects the speed at which usable analytical signals are generated, influencing total analysis time.
CHALLENGES AND LIMITATIONS
Optimization Required: The ideal frequency varies based on matrix, element, and instrument. Incorrect settings can reduce sensitivity or cause instability.
Hardware Limitations: Not all systems support a wide range of frequencies. Some instruments are limited to fixed or narrow frequency settings.
Thermal Effects: At very high frequencies, heat can accumulate on the sample or in the discharge area, potentially distorting results or damaging components.
Signal Overlap or Noise: Poorly tuned frequencies can lead to overlapping pulses or unstable plasmas, causing higher background noise or poor resolution.
Power Supply Stress: High-frequency operation can increase wear on electronic components and require more robust power management systems.