Rhenium Coil Field Sampling for Determinations by ICP-AES with Electrothermal Vaporization Introduction

Jan 01, 2014
Volume 29, Issue 1

The authors have been characterizing an electrothermal, near-torch vaporization (NTV) micro- and nano-sample introduction system for inductively coupled plasma (ICP) spectrometry. In this article they demonstrate that portable coiled-filament assemblies can be used for taking part of the laboratory to the sample and concentration determination in a laboratory using NTV-ICP–atomic emission spectroscopy (AES).

Typical inductively coupled plasma–atomic emission spectrometry (ICP-AES) systems have been developed and characterized, and they are frequently used with a pneumatic nebulizer for the introduction of liquid samples into the ICP. Although solid samples are usually acid-digested to a liquid before their introduction into an ICP (also typically using a nebulizer), the focus of this report is on naturally occurring aqueous samples. But before such samples can be introduced into an ICP, they must first be collected in the field.

Conceptually, field sampling involves on-site collection of anywhere between 100 mL to 1 L of various samples followed by their shipment to a laboratory for analysis. There are costs associated with sample transport because of the relatively large volume (and weight) of samples that must be transported from a sampling site in the field to a laboratory for analysis by ICP spectrometry. Extra costs can be incurred if sample storage (sometimes in a cooled or refrigerated unit) is also required before analysis.

Practically, in addition to the costs involved and depending on the analyte of interest, there are chemistry-related issues that must be considered as well. For instance, aqueous samples most likely must be filtered (ideally on-site), typically using a coarse filter. They also must be stabilized (often using a strong acid) before transportation. Sample stabilization is required to minimize analyte loss resulting primarily from adsorption on the walls of the sample container or to prevent precipitation. Safety precautions should also be considered when dealing with the transportation of strong or corrosive reagents to the field (for sample stabilization). Field-stabilized samples are then shipped to a laboratory and are usually stored before analysis.

The elapsed time interval between sample collection and analysis is often critical, especially if there is natural species interconversion — that is, if the species of interest is labile. Let's consider chromium (Cr) as an example: A Cr6+ to Cr3+ interconversion over time has been reported (1). Furthermore, depending on the analyte of interest, acidification may also change the oxidation state of the species of interest. Oxidation-state information (that is, speciation) is important because toxicity is often oxidation-state dependent. Again, chromium is a typical case here: Cr3+ is an essential micronutrient, but Cr6+ is reported as carcinogenic (1). In addition, redox equilibrium is, among others, a function of the final pH of an acidified aqueous sample, the oxygen concentration, and the final ionic strength of the sample. And, it depends on the amount of colloidal and fine particulate material that may still be present in a sample (even after coarse filtering). As already alluded to, the type of acid used to stabilize the sample and the final pH of an acidified sample may shift naturally existing equilibria. It may also help to release particle-bound species or dissociate ligand-bound species (likely both), thus altering the naturally occurring species distribution in the sample. In natural waters, for instance, acidification has been reported to induce a reduction of Cr6+ to Cr3+ (2). Because such chemical transformations begin to take place immediately after sampling, species concentrations determined in a laboratory (after unavoidable delays because of transport and possibly storage) may not necessarily reflect actual species concentration in a sample at the time and conditions (for example, temperature) of sampling. Thus, a species concentration "bias" will likely develop. The United States Environmental Protection Agency (US EPA) recommended that unless stabilized with reagents, Cr-containing water samples "must be analyzed within 24 hours of collection" (2). Additional sample handling steps are required if longer transport and storage times are necessary.

Two conclusions can be drawn from the discussion above. First, elemental analysis can be made cheaper if smaller amounts of sample are brought to the laboratory for analysis. Second, concentration information will more accurately reflect species distribution at the time and conditions of sampling if species are collected separately at their naturally occurring oxidation state and are stored by deposition on an appropriate support. Because it is difficult to bring an ICPA-ES system to the field for on-site measurements, concentration determination of the species deposited in the field must be done in a laboratory.

This article describes an approach that uses removable and interchangeable coiled filament assemblies onto which samples are deposited (for example, by pipetting or electrodepositing samples) on-site. The coils with dried sample residues are subsequently transported to a laboratory where they are used with an electrothermal, near-torch vaporization (NTV) sample introduction system and ICP-AES (1).

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