Particle transport in a He-microchip plasma atomic emission system with an ultrasonic nebulizer for aqueous sample introduction

The transport efficiency of dried particles generated from an ultrasonic nebulizer (USN) was studied to improve the analytical performance of a lab-made, He-microchip plasma system, in which a quartz tube (~ 1 mm i.d.) was positioned inside the central channel of a poly(dimethylsiloxane) (PDMS) polymer chip. The polymer microchip plasma has the advantages of low cost, small size, easy handling and design, and self-ignition with long stabilization (> 24 h). However, direct introduction of aqueous solution into the microplasma for the detection of metals remains problematic due to plasma instability. In addition, the much smaller size of the system can cause signal suppression due to low transport efficiency. Therefore, knowledge of particle transport efficiency in this microplasma system is required to enhance the sensitivity and stability. The weight of transported particles in the range of 0.02 to 10 mg m− 3 was measured using a piezobalance with a precision of 0.4–17.8%, depending on the operating conditions. The significant effects of the USN operating conditions and the physical properties of the tubing, namely, length, inner diameter and surface characteristics, on the number of particles transported from the nebulizer to the microplasma were studied. When selected metals, such as Na, Mg and Pb, at a concentration of 5 mg L− 1 were nebulized, transported particles were obtained with a mass range of 0.5–5 mg m− 3, depending on atomic weights. For application of the He-rf-microplasma, the atomic emission system was optimized by changing both the radio frequency (rf) power (60–200 W) and cooling temperature of the USN (− 12–9 °C). The limits of detection obtained for K, Na and Cu were 0.26, 0.22, and 0.28 mg L− 1, respectively. These results confirmed the suitable stability and sensitivity of the He-rf-PDMS microchip plasma for application as an atomization source.