The correlation between ion production and emission intensity in the laser-induced breakdown spectroscopy of liquid droplets

Laser-induced breakdown spectroscopy (LIBS) is applied to detect trace metals contained in liquids. The sample in the form of liquid droplets is generated with an electrospray ionization needle. The microdroplets are interacted with an impinging laser pulse approximately 2 mm downstream from the needle tip. A sequence of single-shot time-resolved LIB emission signals of Na, K and Al is detected, respectively. The LIB signal intensity integrated within a gate linearly correlates with the plasma current obtained simultaneously on a single-shot basis. The correlation plot exhibits a straight line, of which the slope increases with the sample concentration. Given the calibration curves and the focused cross-sections of the incident laser beam, the detection limits may be determined to be 0.63±0.02 (0.3 pg), 1.2±0.1 (0.5 pg), and 43±5 mg/l (21 pg) for Na, K, and Al, respectively. As compared to the correlation methods applied, our treatment is more straightforward to yield concentration information and the resulting detection limits are comparable to those reported previously. The dependence of the correlation plot on the adopted laser energy and wavelength is also characterized. The detection limits tend to be improved by applying a laser with larger pulse energy but shorter wavelengths.