A Microfluidic Chip for ICPMS Sample Introduction

Summary

We present a discrete droplet sample introduction system for inductively coupled plasma mass spectrometry (ICPMS). It is based on a cheap and disposable microfluidic chip that generates highly monodisperse droplets in a size range of 40−60 µm at frequencies from 90 to 7,000 Hz.

Abstract

This protocol discusses the fabrication and usage of a disposable low cost microfluidic chip as sample introduction system for inductively coupled plasma mass spectrometry (ICPMS). The chip produces monodisperse aqueous sample droplets in perfluorohexane (PFH). Size and frequency of the aqueous droplets can be varied in the range of 40 to 60 µm and from 90 to 7,000 Hz, respectively. The droplets are ejected from the chip with a second flow of PFH and remain intact during the ejection. A custom-built desolvation system removes the PFH and transports the droplets into the ICPMS. Here, very stable signals with a narrow intensity distribution can be measured, showing the monodispersity of the droplets. We show that the introduction system can be used to quantitatively determine iron in single bovine red blood cells. In the future, the capabilities of the introduction device can easily be extended by the integration of additional microfluidic modules.

Introduction

Elemental analysis of liquid samples by inductively coupled plasma mass spectrometry (ICPMS) is commonly carried out using nebulizers in combination with spray chambers as introduction system¹. In this sample introduction system the sample is sprayed by a nebulizer to generate a polydisperse aerosol. A downstream spray chamber is used to filter out large droplets. This method is associated with high sample consumption (>0.3 ml min^-1)² and an incomplete sample transport. Thus, it becomes impractical for applications where only microliter sample volumes are available, as in biological, forensic, toxicological and clinical studies

Protocol

1. SU-8 Master Fabrication (Figure 2)

NOTE: Perform the fabrication of the SU-8 master molds in a clean room to prevent defects caused by dust particles. Two wafers are needed for the fabrication, one wafer with microfluidic features and one without.

1. Prepare the master molds for the microfluidic chip. First apply an adhesion layer to the silicon wafer.
   1. Dehydrate a silicon wafer for 10 min at 200 °C. Cool the wafer down to RT and load it on to a spin coater and spin coat it with SU-8 2002 with the following protocol.
   2. Dispense about 3 ml resist onto the wafer.
   3. Spin the wafer at....

Results

The presented system can be employed to measure small volumes of solutions or suspensions containing cells or nanoparticles. Examples of a measurement of a standard solution and characterization of single cells are shown here. More examples can be found in Verboket et al.²².

Typically the signal of a single droplet of a solution is a very short event. It usually lasts for a few 100 µsec²⁶. With the ICPMS used here (dwell time 10 msec) short signals like these can.......

Discussion

Although the fabrication of the chips is very reliable there are some critical points during the fabrication that require special attention. First, cleanliness during the assembly is highly important to prevent contamination of the chip by dust. The dust can block the channels and prevent a stable droplet generation. Second, it is especially important that the tip is cut orthogonal to the nozzle channel. The angle of the cut strongly influences the ejection angle. If the liquid is ejected at an angle it can cause a loss .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Silicon wafer 100 mm	Si-Mat (Kaufering, Germany)
SU-8 2002	Microchem Corp. (Massachusetts, U.S.A.)
SU-8 2050	Microchem Corp. (Massachusetts, U.S.A.)
Acetone	Merk VWR (Darmstadt, Germany)	100014
MR-developer 600	Microresist Technology GmbH (Berlin, Germany)
Isopropanol	Merk VWR (Darmstadt, Germany)	109634
1H,1H,2H,2H-perfluorodecyltrichlorosilane	ABCR-Chemicals (Karlsruhe, Germany)	AB111155
Sylgard 184 silicone elastomer kit (PDMS)	Dow Corning (Michigan, U.S.A.)	39100000
Perfluorohexane 99%	Sigma-Aldrich (Missouri, U.S.A.)	281042
FC-40	ABCR-Chemicals (Karlsruhe, Germany)	AB103511
Phosphate-buffered saline	Life Technologies (Paisley, U.K.)	10010-015
Red blood cells in phosphate-buffered saline	Rockland Immunochemicals Inc. (Pennsylvania, U.S.A.)	R400-0100
Single-element standard solutions Na, Fe	Inorganic Ventures (Virginia, U.S.A.)
Multielement standard solution	Merck Millipore (Massachusetts, U.S.A.)	IV
Nitric acid	Sub-boiled
Ultrahigh-purity water	Merck Millipore (Massachusetts, U.S.A.)
Hot plate HP 160 III BM	Sawatec (Sax, Switzerland)		used for wafer preparation
Spin modules SM 180 BM	Sawatec (Sax, Switzerland)		used for wafer preparation
High resolution film photomask	Microlitho (Essex, U.K.)
Step profiler Dektak XT advanced	Bruker  (Massachusetts, U.S.A.)
Hot plate MR 3002	Heidolph (Schwabach, Germany)		used for replica molding
1.5 mm biopsy puncher	Miltex (Pennsylvania, U.S.A.)	33-31AA/33-31A
Spin coater  WS-400 BZ-6NPP/LITE	Laurell (Pennsylvania, U.S.A.)		used for adhesive bonding
Syringe pump neMESYS	Cetoni (Korbussen, Germany)
1 ml syringe	Codan (Lensahn, Germany)	62.1002
5 ml syringe	B. Braun (Melsungen, Germany)	4606051V
PTFE tubing	PKM SA (Lyss, Switzerland)	PTFE-AWG-TFT20.N
Quadrupole-based ICPMS ELAN6000	PerkinElmer (Massachusetts, U.S.A.)
Membrane desolvator CETAC6000AT+	CETAC Technologies (Nebraska, U.S.A.)		only the desolvator unit is used
High speed camera Miro M110	Vision Research (New Jersey, U.S.A.)
Data analysis program Origin pro	OriginLab Corp. (Massachusetts, U.S.A.)	version 8.6
Microscope	Olympus (Tokyo, Japan)	IX71