An ideal detector is expected to accurately, promptly, and continuously provide feedback on the concentration changes of chromatographic peaks for different samples under varying washing conditions and concentrations.
An ideal detector should possess the following performance characteristics:
- High sensitivity
- Insensitivity to variations in temperature and mobile phase
- Wide linear range
- Low noise and minimal drift
- Low dead volume
- Non-destructive to the samples
- Fast response
- Stable, reliable, and good reproducibility
- Good gas tightness
However, in reality, there is no single detector that can fully meet all of the above conditions. Each detector can fulfill certain major requirements under specific conditions and can be suitable for separation and analysis purposes. The selection of a detector can be based on the separation requirements or by creating conditions that enable the existing detector to meet the demands. Therefore, it is essential to establish consistent performance metrics for evaluating the performance of detectors.
- Noise and drift
Noise and drift are important indicators of the stability of a detector.
Noise: It refers to the variation in the detector’s output signal when no solute is passing through. It can be categorized as short noise and long noise. Short noise, also known as spiking, causes the baseline to appear jagged or fuzzy. Long noise is characterized by random and low-frequency fluctuations in the output signal.
Drift: It refers to the gradual deviation of the baseline in a single direction over time.
Sensitivity is a primary performance indicator of a detector.
The magnitude of the signal produced by a detector when a certain amount of substance passes through it is referred to as the sensitivity of the detector to that particular substance. It is also known as the response value.
3. Detection limit
The detection limit, also known as sensitivity, is defined as the sample amount required to generate a response value that is 2 times (or 3 times) the magnitude of the noise.
It refers to the minimum sample amount, in terms of either volume or time, that needs to be introduced to the detector in order to produce a distinguishable signal against the background noise. The detection limit essentially represents the signal-to-noise ratio, taking into account the influence of noise. Therefore, it provides a comprehensive reflection of the detector’s quality and serves as an important metric for evaluating the detector’s performance. A smaller detection limit indicates a stronger detection capability and better performance of the detector, requiring less sample quantity for detection.
4. Other Parameters
Other parameters include the linear range, minimum detectable quantity, lowest detectable concentration of the chromatographic system, factors affecting peak broadening (such as sample pool volume, connecting tubing, response time of sensors and electronic components), flow rate of the mobile phase, pressure, temperature, gas tightness, reliability, and operability.