Agilent GC Detector Sampling (Data) Rate

Answer

This Information Applies To: Agilent GC Systems

Issue

The data rate determines the number of sampling points across the chromatographic peak and the sample background. The data rate is based on the width of the analyte speak (in minutes) and the ratio of the analyte signal to the background noise. 

Background

The data rate unit is hertz (Hz), where 1 Hz is one cycle per second. The data rate determines the number of data points taken across the width of the peak, and in the background. The ratio of the signal peak height to the background peak height determines the detection limit. For more information on calculating and checking the signal-to-noise see How to Check the signal-to-noise ratio with Agilent OpenLab CDS (version 2.x) .

• Faster data rates (higher Hz):
  • Takes more readings per second, increasing the peak height, peak symmetry, noise, and the sample data file size. 
  • Improves signal-to-noise
• Slower data rates:
  • Decrease background noise, peak height and peak symmetry, the noise ratio and the data file size. 
  • Decreases signal-to-noise.

The impact on signal-to-noise ratio (relative noise level) is demonstrated in (Figure 1):

Figure 1. Impact on data rates on signal-to-noise
1 signal-to-noise, 2 Faster data rates, 3 Slower data rates
 - - -  signal-to-noise, — Excess noise due to flow, oven temperature, detector temperature 

Agilent recommends 10 to 20 data points across a peak to optimize peak height and signal-to-noise. Use the guidelines in Table 1 to assist selecting the data rate. 

Table 1. Date rate guidelines.

* The thermal conductivity detector (TCD) switching rate is 5 Hz. A setting of less than 5 Hz can cause tail ringing (Figure 2):

Figure 2. TCD tail ringing

Learn how to effectively operate your Agilent GC System: GC-0GEN-1013s - GC Detectors Theory and Operation GC-0GEN-1051r - FID Theory and Operation e-learning courses available from Agilent education