Article pubs.acs.org/ac
Development and Characterization of a Capillary Gap Sampler as New Microfluidic Device for Fast and Direct Analysis of Low Sample Amounts by ESI-MS Volker Neu,† Roger Steiner,‡ Stephan Müller,‡ Christof Fattinger,‡ and Renato Zenobi*,† †
Department of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland F. Hoffmann-La Roche AG, pRED, Pharma Research & Early Development, Discovery Technologies, Grenzacherstrasse 124, CH-4070 Basel, Switzerland
‡
S Supporting Information *
ABSTRACT: Direct hyphenation of miniaturized sampling devices to electrospray ionization-mass spectrometry (ESI-MS) is attractive because ESI-MS is compatible with microfluidics and allows comprehensive sample analysis, yielding information that is orthogonal to that available from optical methods. We present a “capillary gap sampler” as a platform for directly connecting microfluidics to μ-ESI-MS. The sampler was designed to be robust, light and compact, and to allow precise and fast liquid handling. Sample introduction in the range of a few nanoliters is performed via an open liquid bridge as a new microfluidic element. This allows minimum contact of the sample with system surfaces during the infusion process. The system shows good performance characteristics such as symmetrical peak shapes, low sample carryover (below 1%), and total injection cycle times of less than 15 s. This new device thus has the potential for rapid analysis of biomedical and pharmaceutical samples with limited sample amounts in a high-throughput mode.
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amounts in the nanoliter range.1 The sample is usually introduced by one or several sample lines connected to the chip, which present additional surfaces that can cause undesired analyte adsorption. An elegant way to generate defined sample plugs on the chip has been developed by Yin et al. using an integrated trapping column.7 Even in this case, lines for sample introduction are still required. As examples of commercialized chip-based devices which use integrated trapping columns, the High Performance Liquid Chromatography (HPLC)-Chip/MS system from Agilent and the recently introduced Trizaic system from Waters should be mentioned.8,9 Alternatively, there are platforms for handling small liquid volumes that are not based on chips. Established systems for interfacing with MS are the Nanomate developed by Advion as well as the RapidFire merchandised by Agilent.10,11 The Nanomate enables sample loading of a few to several microliters, is cross contamination-free due to the use of disposable pipet tips, and allows pneumatically assisted spraying through a microfabricated ESI chip for up to several minutes. The device is designed for automated nano-ESI-MS analysis, which is easy to handle. The Rapidfire is an integrated platform with focus on applications in high-throughput screening (HTS). It provides automated online solid phase extraction (SPE) of sample volumes of ≥10 μL and sample spraying using
irect interfacing of microfluidic devices to electrospray ionization-mass spectrometry (ESI-MS) has drawn much attention over the last 15 years.1,2 This is based on several characteristics of ESI-MS as an analytical technique, including compatibility with low flow rates in the nL/min range, high sensitivity and selectivity, and the capability of label-free and simultaneous detection of multiple compounds. Moreover, ESIMS delivers information that is orthogonal to that available from optical methods such as fluorescence spectroscopy, which is usually label-dependent.3 Among the existing microfluidic systems, chip-based ESI-MS devices have been studied most extensively due to a substantial progress in chip manufacturing processes and materials. Starting from electrospray ionization directly from the chip edge, the field has been developed toward the use of inserted capillaries and to spraying via integrated emitters, meaning that both microfluidic channel and emitter are fabricated from one piece.1,2 This has, on the one hand, improved the quality of the electrospray in terms of avoiding hardly controlled surface wetting at the chip edge and, on the other hand, addressed problems with dead volumes and capillary mounting when using inserted capillaries. Several chip configurations have been developed, including multiplexing of sample channels for parallel analysis, the creation of micro mixers with defined reactant channels (e.g., for reaction monitoring of bioassays)4 and the integration of multiple microfluidic devices on a single chip such as pumps, columns, microbioreactors, and ESI-MS interfaces.5,6 Nevertheless, one of the major problems of chipbased platforms is the need for introducing low sample © 2013 American Chemical Society
Received: January 28, 2013 Accepted: April 9, 2013 Published: April 9, 2013 4628
dx.doi.org/10.1021/ac400186t | Anal. Chem. 2013, 85, 4628−4635
Analytical Chemistry
Article
Figure 1. (a) Concept and (b) design of the capillary gap sampler. The sampler design consists of (1) the micro delta robot, (2) the sampling tool held by the robot arms, (3) a microwell plate for sample storage, (4) optics for real-time monitoring of the liquid bridge, (5 and 6) the injection port motor plus axis, (7) the injection port, (8) the ESI spray tip, and (9) the pressure chamber.
a standard ESI-MS interface. Typically, flow rates of 1 to 1.5 mL/min are applied, which translates into total injection cycle times of 6−13 s. In both systems, vacuum has to be used for sample uptake, and gas or liquid flow driven overpressure for sample infusion. An interesting alternative for introducing small sample volumes into an analysis system is the use of liquid bridges/ junctions. They can be formed in the gas phase (e.g., air) or within another, nonmiscible liquid phase, such as an inert oil. The latter case was described for microdroplet manipulation, where liquid bridges were used for controlled drop coalescence of two reactant solutions.12 However, the coupling of an inert oil as a medium to separate small “chemical reactors” with ESIMS analysis is still challenging and creates problems with the spray stability. Possibilities to circumvent these problems include on-chip extraction of the microdroplet/sample plug by controlled coalescence with a flow stream of aqueous solvent,13−15 the use of an air gap as a spacer to separate the sample plugs,16,17 or direct spraying from oil-segmented sample plugs.18,19 In contrast, the coupling of a liquid bridge/junction in air with ESI-MS is much easier to handle and has in fact been described as a part of several system configurations: Wachs et al. initially described a miniaturized ion sprayer device coupled to a special sampling probe, consisting of two coaxially arranged
capillaries, for direct surface sampling via a liquid junction.20 The concept of this so-called liquid microjunction surfacesampling probe (LMJ-SSP) was used and further developed for several applications by Van Berkel and his group.21−25 The principle of LMJ-SSP is also known from Nanomate applications by creating a liquid microjunction between the pipet tip and the sample surface.26,27 Roach et al. created a liquid bridge on a sample surface between two probes when performing nanospray desorption electrospray ionization (nano-DESI) but used it for continuous sample extraction rather than for discrete sample introduction.28 The same principle, but using only a single probe for sample extraction and ionization, was applied by Otsuka et al., when performing so-called scanning probe electrospray ionization (SPESI).29 In consideration of the size of sampling devices described in the literature and the associated size of liquid junctions of several hundreds of micrometers, methods based on liquid extraction surface sampling appear to be limited in spatial resolution. In this context, Ovchinnikova et al. showed the possibility to couple laser ablation of a sample from a surface with liquid extraction of the sample plume.30,31 An alternative device developed by Park et al. uses a partially open liquid channel for discrete sample introduction after laser ablation.32 In both systems, the sample plug formed can either be analyzed directly 4629
dx.doi.org/10.1021/ac400186t | Anal. Chem. 2013, 85, 4628−4635
Analytical Chemistry
Article
the injection port with a tolerance of only a few micrometers. The entire sampling tool is moved by a light microrobot (model PocketDelta, Asyril, Villaz-St-Pierre, Switzerland) that provides a fast, precise, and pulsation-free operation. Thanks to (i) a dedicated step-control of the injection cycle (see Operation Cycle of a Sample Injection) that includes sealing the injection port by the metal pin sleeve, (ii) the implementation of a gas reservoir (see Pressure and Buffer Flow Regulation), and (iii) a precisely manufactured injection port and port axis made of ceramics, the pressure drop during sample infusion monitored by the chamber pressure gas regulator is
