Analysis of Samples Stored as Individual Plugs in a Capillary by Electrospray Ionization Mass Spectrometry

Abstract

Droplets or plugs within multiphase microfluidic systems have rapidly gained interest as a way to manipulate samples and chemical reactions on the femtoliter to microliter scale. Chemical analysis of the plugs remains a challenge. We have discovered that nanoliter plugs of sample separated by air or oil can be analyzed by electrospray ionization mass spectrometry when pumped directly into a fused silica nanospray emitter tip. Using leu-enkephalin in methanol and 1% acetic acid in water (50:50 v:v) as a model sample, we found carry-over between plugs was < 0.1% and relative standard deviation of signal for a series of plugs was 3%. Detection limits were 1 nM. Sample analysis rates of 0.8 Hz were achieved by pumping 13 nL samples separated by 3 mm long air gaps in a 75 μm inner diameter tube. Analysis rates were limited by the scan time of the ion trap mass spectrometer. The system provides a robust, rapid, and information-rich method for chemical analysis of sample in segmented flow systems. SirMultiphase flow in capillary or microfluidic systems has generated considerable interest as a way to partition and process many discrete samples or synthetic reactions in confined spaces. 1-4 A common arrangement is a series of aqueous plugs or droplets separated by gas or immiscible liquid such that each plug can act as a small, individual vial or reaction vessel. 4,5 Methods for formation and manipulation of plugs on the femtoliter to nanoliter scale have recently been developed. [2][3][4][6][7][8][9][10][11] The sophistication of these methods has rapidly increased so that it is now possible to perform many common laboratory functions such as sampling, 12 splitting, 13-15 reagent addition, 16, 17 concentration,18 , 19 and dilution 20 on plugs in microfluidic systems. A frequent emphasis is that such manipulations can be performed automatically at high-throughput. These miniaturized multiphase flow systems have roots in the popular technique of continuous flow analysis (also known as segmented flow analysis) which uses air-segmentation of samples for high-throughput assays in clinical, industrial and environmental applications. 21,22 A limiting factor in using and studying multiphase flows is the paucity of methods to chemically analyze the contents of plugs. Optical methods such as colorimetry 22 and fluorescence are most commonly used. 20 Systems for electrophoretic analysis of segmented flows have recently been developed. 23,24 Drawbacks of these methods are that they require that the analytes be Mass spectrometry (MS) is an attractive analytical technique for analysis of segmented flows because it has the sensitivity and speed to be practically useful for low volume samples analyzed at high-throughput. Mass spectrometry has been coupled to segmented flow by collecting samples onto a plate for MALDI-MS26 or a moving belt interface for electron impact ionization-MS.27 ICP-MS of air-segmented samples has been demonstrated on a relatively large sample format (0.2 mL samples).28 MS analysis of acoustically levitat...

Rapid and label-free screening of enzyme inhibitors using segmented flow electrospray ionization mass spectrometry

“…This concentration range was tested based on the amount of choline formed during the enzyme assay. (Our previous work has suggested that lower concentrations can be used for assays that require it as the segmented flow electrospray method has high sensitivity [17].) As shown in Figure 4b, choline signal increased with its concentration nonlinearly while chlormequat signal decreased with increasing choline concentration.…”

“…Air-segmented sample plugs were generated using the system illustrated in Figure 1, similar to that recently described [17]. To fill the tube with air-segmented samples, a computer-controlled xyzmicropositioner (built in-house from XSlide assemblies; Velmex Inc., Bloomfield, NY, USA) was used to move the inlet of the Teflon tubing from sample-to-sample on the multiwell plate while the pump was operated at a fixed aspiration rate.…”

“…The last few years have seen a resurgence of interest in segmented flow with the advent of sophisticated microfluidics that allow miniaturization (femtoliter to nanoliter samples) and new methods for manipulating sample plugs and droplets [14 -16]. We have recently demonstrated the feasibility of directly pumping segmented flow through an ESI emitter tip to obtain mass spectrometric analysis of discrete sample plugs at high-throughput (0.8 Hz analysis rate) with low carry-over [17]. Other groups have also reported ESI-MS of droplet samples [18,19].…”

A Microfluidic Chip for ICPMS Sample Introduction

“…In recent years droplet microfluidics has gained interest for (bio-)chemical reactions [12][13][14][15] and for single cell studies 16,17 . Additionally, this technique was applied for introducing samples in electrospray ionization mass spectrometry 18,19 and for preparing samples in matrix-assisted laser desorption/ionization mass spectrometry 20,21 .…”

Robust Extraction Interface for Coupling Droplet-Based and Continuous Flow Microfluidics

“…ESI-MS analysis of aqueous droplets has been achieved by directly coupling a segmented flow system with MS (Pei et al, 2009), but this method has some potential risks including instability of the electrospray due to oil accumulation at the tip, introduction of chemical background, and contamination of the MS instrument. Hence, it is generally necessary to extract the aqueous droplet from the oil phase for further separation or MS analysis.…”