Chromatography Theory Chapter 2 – Data Acquisition

Chromperfect
Sep 24
2 min read

Chromatography Software for data acquisition

This chapter explains the math and practice of data acquisition for chromatography: sampling rate, digitization limits, noise vs. drift, converter resolution (bits → µV), and how to maximize dynamic range so both tiny and huge peaks are measured accurately.

What is chromatography data acquisition?

A detector outputs a continuous signal. The chromatography data acquisition system samples it at fixed intervals and stores a digitized raw file:

Discrete in time: each point is the average over the preceding interval.
Discrete in amplitude: each point is rounded to a converter step (quantization).Some information is lost, but with proper settings you capture everything that matters for height, area, and retention time.

Sampling rate

Rule of thumb: ≥10 points across the full baseline width of each peak.

LC: often 1–5 Hz (peaks up to ~1 min).
Capillary GC: 20–200 Hz (sub-second peaks).
Conventional GC: 5–25 Hz.Too slow → flattened apices, shifted RTs, area/height errors. Too fast → larger files (okay, just heavier).

Range (converter limits)

AD/DA converters have fixed upper/lower limits. If the signal hits them:

Upper limit: peak tops clip → height/area lost.
Lower limit: baseline segments clip → small peaks vanish.Fix by adjusting detector zero/attenuation so the signal stays comfortably within range during the run.

Noise and drift (quick recap)

Noise (fast): detector flicker, pump pulsations, electrical hum. Sets the practical detection limit.
Drift (slow): temperature/bleed/etc. Usually benign; handle column bleed via baseline subtraction (see Ch. 3).

Resolution (bits → step size)

Converter resolution = step size of the y-axis. Example: 20-bit over 1 V → ~1 µV/step (~10 µV needed for a 10-step minimum).

Peaks ≥10 steps high quantify reliably.
Quantization adds tiny “round-off noise.” If detector noise ≫ step size, raising bit depth mainly measures noise more precisely (not magical sensitivity).

Maximizing dynamic range (practical)

Use most of the converter span without touching limits.
Avoid feeding a tiny chart-recorder output (e.g., 10 mV max) into a 1–2 V ADC—this wastes ~99% of range and hurts low-level linearity.
If detector outputs 5–10 V, use a simple resistive attenuator to match ADC range, then recalibrate.

Quick setup checklist

Choose sampling rate for ≥10 points across your narrowest expected peak.
Set detector zero/attenuation so baseline and tallest expected peak sit well within ADC limits (allow headroom for drift).
Confirm bit depth / range → smallest peaks are ≥10 steps.
Verify noise & drift on a blank; if needed, address pump/electrical noise and plan baseline subtraction (see Ch. 3).
Archive size okay? If not, reduce rate where peaks are wide.