A versatile and low-cost chip-to-world interface: Enabling ICP-MS characterization of isotachophoretically separated lanthanides on a microfluidic device

Microfluidics offer novel and state-of-the-art pathways to process materials. Microfluidic systems drastically reduce timeframes and costs associated with traditional lab-scale efforts in the area of analytical sample preparations. The challenge arises in effectively connecting microfluidics to off-chip analysis tools to accurately characterize samples after treatment on-chip. Fabrication of a chip-to-world connection includes one end of a fused silica capillary interfaced to the outlet of a microfluidic device (MFD). The other end of the capillary is connected to a commercially available CEI-100 interface that passes samples into an inductively coupled plasma mass spectrometer (ICP-MS). This coupling creates an inexpensive and simple chip-to-world connection that enables on-chip and off-chip methods of analyzing the separation of rare earth elements. Specifically, this is demonstrated by utilizing isotachophoresis (ITP) on a microfluidic chip to separate up to 14 lanthanides from a homogenous sample into elementally pure bands. The separated analyte zones are successfully transferred across a 7 nL void volume at the microchip-capillary junction, such that separation resolution is maintained and even increased through the interface and into the ICP-MS, where the elemental composition of the sample is analyzed. Lanthanide samples of varying composition are detected using ICP-MS, demonstrating this versatile and cost-effective approach, which maintains the separation quality achieved on the MFD. This simple connection enables fast, low-cost sample preparation immediately prior to injection into an ICP-MS or other analytical instrument.

Design and optimization of a fused-silica microfluidic device for separation of trivalent lanthanides by isotachophoresis

Elemental analysis of rare earth elements is essential in a variety of fields including environmental monitoring and nuclear safeguards; however, current techniques are often labor intensive, time consuming, and/or costly to perform. The difficulty arises in preparing samples, which requires separating the chemically and physically similar lanthanides. However, by transitioning these separations to the microscale, the speed, cost, and simplicity of sample preparation can be drastically improved. Here, all fourteen non-radioactive lanthanides (lanthanum through lutetium minus promethium) are separated by ITP for the first time in a serpentine fused-silica microchannel (70 µm wide × 70 µm tall × 33 cm long) in <10 min at voltages ≤8 kV with limits of detection on the order of picomoles. This time includes the 2 min electrokinetic injection time at 2 kV to load sample into the microchannel. The final leading electrolyte consisted of 10 mM ammonium acetate, 7 mM α-hydroxyisobutyric acid, 1% polyvinylpyrrolidone, and the final terminating electrolyte consisted of 10 mM acetic acid, 7 mM α-hydroxyisobutyric acid, and 1% polyvinylpyrrolidone. Electrophoretic electrodes are embedded in the microchip reservoirs so that voltages can be quickly applied and switched during operation. The limits of detection are quantified using a commercial capacitively coupled contactless conductivity detector (C⁴ D) to calculate ITP zone lengths in combination with ITP theory. Optimization of experimental procedures and reproducibility based on statistical analysis of subsequent experimental results are addressed. Percent error values in band length and conductivity are ≤8.1 and 0.37%, respectively.