Name: a microfluidic chip for icpms sample introduction
Uploaded: 2015-03-05T14:00:00
Description: Watch this Scientific Journal Video about A Microfluidic Chip for ICPMS Sample Introduction at JoVE.com

This work was supported by the European Research Council (ERC Starting Grant n&#956;LIPIDS, No. 203428) and ETH Zurich (project number: ETH-49 12-2). The authors of this manuscript would like to thank Bodo Hattendorf for help with the ICP-MS and F. Kurth for cell counting. The authors also would like to thank Christoph B&#228;rtschi and Roland M&#228;der for their support with building the mechanical setup. The clean room facility FIRST at ETH Zurich is acknowledged for support in microfabrication.

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.
<ol>
	<li>Prepare the master molds for the microfluidic chip. First apply an adhesion layer to the silicon wafer.
	<ol>
		<li>Dehydrate a silicon wafer for 10&#160;min at 200&#160;&#176;C. Cool the wafer down to RT and load it on to a spin coate...

<ol>
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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 ...

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 (&#62;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 studies3. To reduce the sample consumption, nebulizers with smaller nozzle dimensions were developed3. However, the reduced nozzle size increases the risk of clogging when samples of undigested biological fluids or concentrated salt solutions have to be analyzed3.
A different approach for sample introduction was proposed by Olesik et al.4. The authors injected a liquid into ICPMS in the form of monodisperse discrete microdroplets, which were produced by a piezo-electrically driven micropump. Even though this very system did not find wide application, it initiated the further development of the concept of discrete droplet introduction in ICPMS. Today, piezo-electrically driven dispensing systems, which can generate droplets in size of 30, 50, 70 and 100 &#181;m and at frequencies of 100-2,000 Hz, can be purchased. The droplets can be transported into ICPMS with close to 100% efficiency5. These microdroplet dispensers have been applied for quantitatively measuring single nanoparticles5,6 as well as characterizing individual biological cells7. A similar system based on thermal inkjet technology8 was tested for analysis of biological samples9. Although the available single droplet introduction systems are very efficient, can be used for small sample volumes and are promising for the analysis of nanoparticles and cells, they have several limitations. For a fixed nozzle size, the droplet size can be varied only slightly (unless custom settings are used10). Changes of the physical properties of the liquid (pH, salt content) can alter the droplet characteristics (size, injection speed). Also, these devices are rather expensive, prone to clogging and are difficult to clean.
Another method to generate droplets is known in the field of droplet microfluidics11. In recent years droplet microfluidics has gained interest for (bio-)chemical reactions12-15 and for single cell studies16,17. Additionally, this technique was applied for introducing samples in electrospray ionization mass spectrometry18,19 and for preparing samples in matrix-assisted laser desorption/ionization mass spectrometry20,21.
Recently, we introduced a microfluidic based system for sample introduction in ICPMS22. The key component of our introduction system is the liquid assisted droplet ejection (LADE) chip. This chip consists completely of poly(dimethylsiloxane) (PDMS). In the first channel junction flow focusing is used to generate monodisperse droplets of an aqueous sample solution (Figure 1). For this purpose the highly volatile (boiling point of 58-60&#160;&#176;C23) and immiscible carrier phase perfluorohexane (PFH) is used (Figure 1). These PFH properties enable a stable droplet generation and fast removal of the carrier phase. Changes in the properties of the sample liquid influence this generation method less, compared to other droplet generators. The droplet size is adjustable over a wide range by changing the flow rates of the aqueous phase and the PFH. In a downstream secondary junction, more PFH is added to increase the flow speed to at least 1 m&#160;sec-1. At this speed the liquid can be ejected from the chip in stable and straight jet (Figure 1) without droplet destruction (Figure 1 inset). This double-junction design allows controlling the jet stability independent of droplet generation. The droplets are transported to the ICPMS with a customized transport system. This system comprises a falling tube and a membrane desolvator to remove the PFH. The dried residues of the aqueous droplets are subsequently ionized in the plasma of the ICPMS and a mass detector measures the ions. The front part of the chip is barrel-shaped to ensure a tight connection with the droplet transport system. The ejection of the aqueous sample as droplets in PFH is beneficial, because contact with the nozzle is avoided. This considerably lowers the risk of nozzle clogging, which can be a problem when working with cell suspensions or concentrated salt solutions. The LADE chips, fabricated by PDMS soft lithography, are cheap (material cost approximately $2 per chip), disposable and easy to modify. In combination with the fabrication that requires only a small amount of manual work each experiment can be performed with a new chip. Therefore, a laborious cleaning is not needed and cross contamination is minimized.
Here, the fabrication of the LADE chip by soft lithography and its application for ICPMS are described. Examples of measurements with an aqueous solution and a cell suspension are presented.

<table border="0" cellpadding="0" cellspacing="0" width="919">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Name of Reagent/ Equipment</td>
			<td style="width:320px;">Company</td>
			<td style="width:147px;">Catalog Number</td>
			<td style="width:183px;">Comments/Description</td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Silicon wafer 100 mm</td>
			<td align="left" style="width:320px;">Si-Mat (Kaufering, Germany)</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">SU-8 2002</td>
			<td align="left" style="width:320px;">Microchem Corp. (Massachusetts, U.S.A.)</td>
			<td style="width:147px;">n.a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">SU-8 2050</td>
			<td align="left" style="width:320px;">Microchem Corp. (Massachusetts, U.S.A.)</td>
			<td style="width:147px;">n.a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Acetone</td>
			<td align="left" style="width:320px;">Merk VWR (Darmstadt, Germany)</td>
			<td style="width:147px;">100014</td>
			<td></td>
		</tr>
		<tr height="42">
			<td align="left" height="42" style="height:42px;width:269px;">MR-developer 600</td>
			<td align="left" style="width:320px;">Microresist Technology GmbH (Berlin, Germany)</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Isopropanol</td>
			<td align="left" style="width:320px;">Merk VWR (Darmstadt, Germany)</td>
			<td style="width:147px;">109634</td>
			<td></td>
		</tr>
		<tr height="42">
			<td align="left" height="42" style="height:42px;width:269px;">1H,1H,2H,2H-perfluorodecyltrichlorosilane</td>
			<td align="left" style="width:320px;">ABCR-Chemicals (Karlsruhe, Germany)</td>
			<td style="width:147px;">AB111155</td>
			<td></td>
		</tr>
		<tr height="42">
			<td align="left" height="42" style="height:42px;width:269px;">Sylgard 184 silicone elastomer kit (PDMS)</td>
			<td align="left" style="width:320px;">Dow Corning (Michigan, U.S.A.)</td>
			<td style="width:147px;">39100000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Perfluorohexane 99%</td>
			<td align="left" style="width:320px;">Sigma-Aldrich (Missouri, U.S.A.)</td>
			<td style="width:147px;">281042</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">FC-40</td>
			<td align="left" style="width:320px;">ABCR-Chemicals (Karlsruhe, Germany)</td>
			<td style="width:147px;">AB103511</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;">Phosphate-buffered saline&#160;</td>
			<td align="left">Life Technologies (Paisley, U.K.)&#160;</td>
			<td style="width:147px;">10010-015</td>
			<td></td>
		</tr>
		<tr height="42">
			<td align="left" height="42" style="height:42px;width:269px;">Red blood cells in phosphate-buffered saline</td>
			<td align="left">Rockland Immunochemicals Inc. (Pennsylvania, U.S.A.)&#160;</td>
			<td style="width:147px;">R400-0100</td>
			<td></td>
		</tr>
		<tr height="42">
			<td align="left" height="42" style="height:42px;width:269px;">Single-element standard solutions Na, Fe</td>
			<td align="left" style="width:320px;">Inorganic Ventures (Virginia, U.S.A.)</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;">Multielement standard solution&#160;</td>
			<td align="left" style="width:320px;">Merck Millipore (Massachusetts, U.S.A.)</td>
			<td align="left" style="width:147px;">IV</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Nitric acid</td>
			<td align="left" style="width:320px;">Sub-boiled</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;">Ultrahigh-purity water</td>
			<td align="left" style="width:320px;">Merck Millipore (Massachusetts, U.S.A.)</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;"></td>
			<td style="width:320px;"></td>
			<td style="width:147px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Name of Equipment</td>
			<td style="width:320px;"></td>
			<td style="width:147px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Hot plate HP 160 III BM</td>
			<td align="left" style="width:320px;">Sawatec (Sax, Switzerland)</td>
			<td style="width:147px;">n. a.</td>
			<td align="left">used for wafer preparation</td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Spin modules SM 180 BM</td>
			<td align="left" style="width:320px;">Sawatec (Sax, Switzerland)</td>
			<td style="width:147px;">n. a.</td>
			<td align="left">used for wafer preparation</td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">MA-6 mask aligner</td>
			<td align="left" style="width:320px;">Su&#776;ss MicroTec (Garching, Germany)</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">High resolution film photomask</td>
			<td align="left" style="width:320px;">Microlitho (Essex, U.K.)</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Step profiler Dektak XT advanced</td>
			<td align="left" style="width:320px;">Bruker&#160; (Massachusetts, U.S.A.)</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Hot plate MR 3002</td>
			<td align="left" style="width:320px;">Heidolph (Schwabach, Germany)</td>
			<td style="width:147px;">n. a.</td>
			<td align="left">used for replica molding&#160;</td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">1.5 mm biopsy puncher</td>
			<td align="left" style="width:320px;">Miltex (Pennsylvania, U.S.A.)</td>
			<td style="width:147px;">33-31AA/33-31A</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Spin coater&#160; WS-400 BZ-6NPP/LITE</td>
			<td align="left" style="width:320px;">Laurell (Pennsylvania, U.S.A.)</td>
			<td style="width:147px;">n. a.</td>
			<td align="left">used for adhesive bonding</td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Syringe pump neMESYS</td>
			<td align="left" style="width:320px;">Cetoni (Korbussen, Germany)</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;">1 mL syringe&#160;</td>
			<td align="left">Codan (Lensahn, Germany)&#160;</td>
			<td style="width:147px;">62.1002</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;">5 mL syringe&#160;</td>
			<td align="left">B. Braun (Melsungen, Germany)&#160;</td>
			<td style="width:147px;">4606051V</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;">PTFE tubing&#160;</td>
			<td align="left">PKM SA (Lyss, Switzerland)&#160;</td>
			<td style="width:147px;">PTFE-AWG-TFT20.N</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;">Quadrupole-based ICPMS ELAN6000</td>
			<td align="left">PerkinElmer (Massachusetts, U.S.A.)&#160;</td>
			<td>n. a.&#160;</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;">Membrane desolvator CETAC6000AT+</td>
			<td align="left">CETAC Technologies (Nebraska, U.S.A.)&#160;</td>
			<td style="width:147px;">n. a.</td>
			<td align="left">only the desolvator unit is used</td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;">High speed camera Miro M110</td>
			<td align="left">Vision Research (New Jersey, U.S.A.)</td>
			<td style="width:147px;">n. a.</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Data analysis program Origin pro</td>
			<td align="left" style="width:320px;">OriginLab Corp. (Massachusetts, U.S.A.)</td>
			<td align="left" style="width:147px;">version 8.6</td>
			<td></td>
		</tr>
		<tr height="21">
			<td align="left" height="21" style="height:21px;width:269px;">Microscope</td>
			<td align="left" style="width:320px;">Olympus (Tokyo, Japan)</td>
			<td align="left" style="width:147px;">IX71</td>
			<td></td>
		</tr>
	</tbody>
</table>

a microfluidic chip for icpms sample introduction

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 &#181;m and from 90 to 7,000&#160;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.

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 &#181;sec26. With the ICPMS used here (dwell time 10 msec) short signals like thes...

Watch this Scientific Journal Video about A Microfluidic Chip for ICPMS Sample Introduction at JoVE.com

A Microfluidic Chip for ICPMS Sample Introduction

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&#8722;60 &#181;m at frequencies from 90 to 7,000 Hz.

This protocol discusses the fabrication and usage of a disposable low cost microfluidic chip as sample introduction system for inductively coupled plasma ...

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