Study of thermospray and evaporation effects in a flame-on-a-chip

Highlights

• Thermospray and evaporation effects were combined in a flame-on-a-chip using 2 μL sample volume.

• The combined mechanism of the thermospray and evaporation was firstly studied.

• 2 μL of sample volume provided a very efficient sample introduction.

• This combined sample introduction method can be utilized in atomic spectroscopy.

Abstract

In the present work, a simple thermospray sample introduction applied in the recently developed μFAES system (where a microburner and flame were created on a microchip (flame-on-a-chip)) was extended with a process based on combined effects of thermospray and evaporation of the formed solid salts. In the used microfluidic system, the mechanisms of the thermospray and evaporation exhibit interesting and new features, which can be utilized in atomic spectroscopy. The effects of several parameters (like flow rates, sample volume and inner diameter of the thermospray capillary) of the sample introduction were investigated.

Introduction

The miniaturization of classical analytical methods created important and promising research fields in analytical chemistry. The largest level of miniaturization could be achieved by the lab-on-a-chip (LOC) technology, which offers several advantages over the common, classical instrumentations like the necessity of minimal volume of sample and reagents, portability and rapid analysis [1]. Although many analytical instruments have been already miniaturized into microchip scale, atomic spectrometry is hardly affected. A few works were published about miniaturized plasmas (inductively coupled plasma (ICP) [2], capacitively coupled plasma (CCP) [3], microwave-induced plasma (MIP) [4] or dielectric barrier discharge (DBD) [5,6]), but these were not spread. Up to now, neither atomic absorption methods nor graphite furnace technique can be miniaturized.

However, recently our group showed the first flame-on-a-chip, that is a low-cost microfluidic flame atomic emission spectrometer (μFAES), which incorporates a microburner and flame [7]. The microfluidic FAES system operated with a cheap, easily available commercial cigarette lighter including butane or propane/butane gas, with several hundreds of Celsius degrees lower flame temperature compared to those flames (acetylene/air) commonly used in flame atomic spectrometry. The lower flame temperature makes the excitation of the most easily excitable atoms like alkali metals possible only. This simple device showed proper linearity in the range of 5–100 mg/L for Li, Na, K, and it was tested by analyzing various real samples. The merits and limitations of the microfluidic flame atomic emission device were demonstrated and discussed [7]. The central part of this device is a simple thermospray (TS) system that can be applied for effective (total) sample introduction. This TS system is more easily miniaturizable and integrable than the conventional nebulization methods.

Firstly, Vestal et al. [8] described the thermospray generation, which was applied as an interface between liquid chromatography (LC) and mass spectrometry (MS). Later, it was used also for atomic spectrometry (ICP/AES) [9,10]. Although, in general the thermospray systems are used in complicatedly controlled electrical heating systems to maintain a constant temperature for the sample vaporization, in thermospray flame furnace (TSFF) AAS technique a TS capillary is simply heated only by the flame itself when the liquid is pumped through the glowing metal capillary tip [11]. In TSFF methods, the TS capillary is heated not solely by the flame, but by the wall of the flame furnace, as well. In the developed μFAES system a small sample liquid volume (~1 μL) is transported toward the heated end of the capillary [11,12]. Firstly, small bubbles and then larger droplets are formed in this glowing tip, that are finally vaporized to a fine, gaseous jet of sample.

In the present work a simple thermospray method applied in the recently developed μFAES system [7] was extended to a combined sample introduction technique based on thermospray effect and evaporation of the formed solid salts. We demonstrated that the evaporation effects appear if equal or larger than 2 μL volume is introduced into the thermospray capillary. This new sample introduction method of atomic spectrometry was investigated.