Instrumax CAD Performance Evaluation

Objective

Evaluate the performance of the Instrumax CAD detector using caffeine.

Experimental conditions

Instruments: INSTRUMAX Charged Aerosol Detector (CADetector a1), INSTRUMAX Nitrogen Generator g15, Agilent HPLC-1200 liquid chromatograph.

Column: InfinityLab Poroshell 120 EC-C18, 3.0 × 100 mm, 2.7 μm. Reagents: methanol (Merck, HPLC grade) and distilled water (Watson’s). Sample: aqueous caffeine reference solution standards at 5 μg/mL, 25 μg/mL, 125 μg/mL, 250 μg/mL and 500 μg/mL, with certificate.

1. Experimental procedure

3.1 Experimental conditions

Mobile phase: methanol/water = 20:80. Column: InfinityLab Poroshell 120 EC-C18, 3.0 × 100 mm, 2.7 μm. Column temperature: 50 °C. Flow rate: 0.5 mL/min.

CAD conditions: drift tube temperature 35 °C, nebulizing gas flow 3 L/min, charging gas flow 1 L/min, charging current 1 μA, Gain 0.05, RF 2.0.

Test items: noise and drift were acquired for 21 min and evaluated from 1 to 21 min. Signal-to-noise ratio was evaluated using a 5 μg/mL caffeine standard with 10 μL injection. Injection precision was evaluated with 25 μg/mL caffeine standard solution (5 μL), 5 μg/mL caffeine standard solution (10 μL) and 5 μg/mL caffeine standard solution (20 μL). Response linearity was evaluated with five traceable concentration standards.

3.2 Experimental results

3.2.1 Baseline noise and baseline drift

Baseline noise (ASTM): 25.14 fA. Baseline drift: 2.534 fA.

3.2.2 Signal-to-noise ratio

Sample: 5.0 μg/mL caffeine, injection 10 μL, equivalent to 50 ng caffeine. Test result: S/N = 209.3.

3.2.3 Injection precision

For 5 μg/mL caffeine at 10 μL, retention-time RSD was 0.245% and peak-height RSD was 1.123%. For 5 μg/mL caffeine at 20 μL, retention-time RSD was 0.202% and peak-height RSD was 1.201%. For 25 μg/mL caffeine at 5 μL, retention-time RSD was 0.105% and peak-height RSD was 1.142%.

3.2.4 Response linearity

Five traceable standards were used to confirm linear system response. The correlation coefficient between peak area and sample concentration was used as the response-linearity result.

Concentration (μg/mL) / peak area: 5 / 3.638; 25 / 43.116; 125 / 258.658; 250 / 525.608; 500 / 997.079. Response-linearity result: R² = 0.9989, R = 0.9995.