We present an atomization system for atomic absorption spectrometry comprised by a stainless steel furnace heated by Joule effect by means of its intrinsic resistance. This new kind of furnace does not require any gases during operation. The sample is introduced with an independently controlled thermospray injector. The device outperforms conventional FAAS (Flame Atomic Absorption Spectrometry) for many analytes, giving a very safe, compact and inexpensive alternative for many analytical determinations. Full characterization of the system is presented, and theoretical simulations are contrasted with experimental data.
Atomic Absorption Spectrometry (AAS) is one of the most widespread techniques for determination of trace elements since Alan Walsh introduced Flame Atomic Absorption Spectrometry (FAAS) in 1955.
Since then, many changes of the original idea have been investigated and several new versions of AAS have been proposed with diverse success.
The use of a flame in FAAS implies the acceptation of two drawbacks: a complex gas management system able to ensure a safe operation under every condition, and a short residence time of the sample into the optical path4 , given the fact that the flame is an open environment.
Electrothermal atomizers5-8, mainly graphite furnaces9,12 , or tungsten coils13-16 in a lower extent, became an excellent alternative to overcome the problems depicted above. Nonetheless, for these alternatives and regardless the employment of standardized temperature platform furnaces, transverse heating or matrix modification17,18, high purity argon gas flow for protection of the heated cell is mandatory.
Another strategy to confine the sample into the optical path was proposed recently by Berndt19 and refined by several groups. 20-25 This arrangement, known as thermospray flame furnace atomic absorption spectrometry (TS-FFAAS), allows the introduction of the totality of a liquid sample into a tube (or furnace) mounted over a combustion flame in the optical path of an atomic absorption spectrometer. The sample is injected via a peristaltic pump and transported to the flame furnace (FF) through a ceramic capillary directly heated by the flame, producing the so called “thermospray” (TS). The combination of the whole sample introduction and the confinement of the atomized sample has proven to optimize sensitivity for volatile elements when compared to FAAS with no need of major changes in the basic instrument.
Inspired in FFAAS and ETAAS, we present the Electrothermal Metal Furnace Atomic Absorption (EMFAAS) approach that takes advantages of both techniques. EMFAAS employs a stainless steel alloy tube as furnace. This tube of relatively high electric resistivity is very well suited to be heated by conducting a high current through its body.
Thus, the high temperature of the furnace is attained with no need of burning or protecting gases. The total liquid sample introduction is performed with the assistance of a peristaltic pump and a ceramic thermospray injector which is heated electrically and independent from the furnace heating.
In this way, EMFAAS combines electrical heating of both the FF and the TS injector, keeping away from pressurized gases either for combustion, protection or typical pneumatic nebulisation of liquid samples.
Materials and methods
All solutions were prepared with analytical grade chemical reagents and double deionized water (DDW) obtained from a Milli-Q purification system (Millipore, Bedford, MA, USA). All glassware was washed with EXTRAN (Merck) 1% v/v and kept in 10% (v/v) HCl with further cleaning with DDW. Standard solutions of all the analytes were prepared by proper dilution of 1,000 g L-1 stock solutions (Merck Darmstadt, Germany). A digital camera Samsung NX 1100 was used to take all the temperature images. The heating of the cell was performed by means of a commercial 700 W microwave oven transformer, in which the original secondary wiring was replaced by a twoturns coil of 7 mm diameter copper multifiber cable (automotive, for batteries).
An atomic absorption spectrometer Shimadzu AA6800 (Shimadzu, Kyoto, Japan), hollow cathode lamps (Hamamatsu, Japan) and a deuterium lamp for background correction were employed throughout the measurements. Other instrumental conditions were those provided by the manufacturer.The EMFAAS system was assembled with a peristaltic pump of eight channels and six rollers (IPC, Ismatec, Glattbrugg-Zürich, Switzerland), a six-ports rotatory valve VICI (Valco Instruments, Houston, TX, USA), 0.5 mm i.d. PTFE® tubing, a ceramic capillary (0.5 mm i.d., 6 cm length) and the metallic flame furnace atomizers placed in the optical path of the spectrometer with the assistance of a homemade holder.
In a typical experimental procedure, 500 µL of standard solution was introduced in a carrier stream (DIW) and injected into the atomization cell at a flow rate of 1.1mL min-1. A Nichrome coil wounded around the ceramic injection capillary was used to heat the solution above the vaporization temperature prior to entering the cell. The heating was controlled by changing the applied voltage to the coil between 10 and 24 V to ensure full vaporization of the sample at different flow rates.