WHAT IT IS
The optical system in Spark-OES and GD-OES is responsible for capturing, dispersing, and detecting the light emitted by excited atoms in the sample. This light, characteristic of each element, is used to determine elemental composition. The design and performance of the optical system directly influence the instrument’s resolution, sensitivity, and analytical accuracy.
Optical systems typically consist of light collection optics, dispersive elements (such as gratings), and detectors. They operate under vacuum or inert gas environments to prevent interference from atmospheric gases and ensure high throughput of emitted light.
HOW IT WORKS
Light Collection – Emission from excited atoms in the plasma (in Spark-OES) or glow discharge (in GD-OES) is collected through a lens or mirror system and directed into the spectrometer.
Wavelength Dispersion – A diffraction grating or prism separates the incoming light into its component wavelengths. Each wavelength corresponds to a specific elemental emission line.
Wavelength Selection – Optical systems are configured to isolate wavelengths of analytical interest while minimizing overlap from other lines or background emission.
Detection – Dispersed light is focused onto a detector array (such as a photomultiplier tube or CCD), which converts light intensity into an electrical signal for quantitative analysis.
Data Processing – The software processes the electrical signals, applies background corrections, and matches wavelengths to specific elements based on pre-defined analytical lines.
TYPES OF OPTICAL SYSTEMS
Paschen-Runge (Rowland Circle) Systems: Common in Spark-OES, this optical configuration aligns the grating and detectors on a circular arc for fixed, simultaneous multi-element detection.
Echelle Spectrometers: Used for high-resolution applications, echelle systems combine high-dispersion gratings with cross-dispersers to cover wide spectral ranges. Suitable for both Spark and GD-OES.
Polychromators vs. Monochromators: Polychromators detect multiple wavelengths simultaneously, increasing throughput and making them suitable for routine, multi-element analysis. Monochromators allow scanning of individual lines, offering flexibility but slower analysis speed.
Vacuum vs. Purged Optics: Vacuum optics eliminate atmospheric absorption (especially in UV range) and improve signal quality. Purged optics use inert gases like nitrogen to prevent air interference while simplifying maintenance.
IMPACT ON PERFORMANCE
Resolution: High-resolution optics allow separation of closely spaced spectral lines, reducing spectral overlap and enhancing analytical precision.
Sensitivity: Efficient light collection and high-transmission optics maximize the signal reaching the detector, improving detection limits for trace elements.
Stability and Reproducibility: Well-aligned and thermally stable optical systems provide consistent results across multiple analyses.
Wavelength Range: Broad spectral coverage enables detection of a wide array of elements across ultraviolet and visible ranges.
Speed: Simultaneous detection with polychromators or CCD arrays allows rapid multi-element analysis, especially important in quality control workflows.
CHALLENGES AND LIMITATIONS
Spectral Interference: Overlapping emission lines or matrix effects can cause misidentification or inaccurate quantification without high-resolution optics.
Alignment and Calibration: Optical components require precise alignment and routine wavelength calibration to maintain accuracy.
Thermal Stability: Temperature fluctuations can cause optical drift, especially in systems without active temperature control.
Cost and Complexity: Advanced optical systems such as echelle spectrometers or vacuum setups increase instrument cost and require specialized maintenance.
Aging of Components: Optical coatings, gratings, and detectors degrade over time, potentially reducing transmission efficiency and requiring replacement or recalibration.