Chip-based microfluidics on-line with inductively coupled plasma - mass spectrometry for standard dilution analysis

Microfluidics coupled on-line with ICP-MS detection can be combined with powerful quantitation procedures that take advantage of internal standardization and standard additions, such as the recently introduced Standard Dilution Analysis (SDA). Although so far used at mL min^-1 flow rates, here we demonstrate that SDA can be conveniently employed with a microfluidic chip-based ICP-MS system to improve determination accuracy for various sample types, including water, biological and cell digest samples, analyzed at μL min^-1 flow rates. The efficient coupling of a microfluidic chip to ICP-MS was accomplished using a combination of commercially available components, including a pneumatic high-efficiency nebulizer and a spray chamber designed to allow for the addition of a laminar flow makeup gas. The addition of the makeup gas was crucial in order to avoid detrimental suction effects that can disrupt the operation of the microfluidic chip and cause signal instability, while it still allowed for the highly sensitive detection of metal isotopes by using ICP-MS. All mixing and dilution operations of the sample with the two calibration solutions required for SDA were performed in an automated and highly reproducible fashion on the microfluidic chip with the assistance of an external distributor valve. High average recoveries (97.4-100.1%) and low average relative standard deviations (2.9-4.8%) were achieved for the determined elements (Cd, Co, Pb, Cr) across several spiked matrices and certified reference materials, whereas only 140 μL of sample is required for SDA in triplicate or 40 μL for a single analysis. Hence, accuracy, precision, limited sample consumption, and the elimination of the need for manual sample dilution and mixing manipulations are some of the advantages of this newly developed chip-based microfluidic SDA ICP-MS technique.