Volatile Species Generation for Trace Element and Speciation Analysis – Current State and Future Perspectives

Abstract

The current concept of trace element analysis relies mainly on liquid nebulization to atomic spectrometric detectors characterized by a low sample introduction efficiency, typically reaching 5–8%. This is the bottleneck of all the common nebulizers regardless of the detector employed. As a consequence, more efficient approaches to analyte introduction into element-specific detectors, including atomic absorption (AAS), atomic fluorescence (AFS) and inductively coupled plasma (ICP) with either optical emission (OES) or mass spectrometry (MS) detection, have been sought. One of the strategies is volatile species generation (VSG) – a group of techniques based on analyte derivatization in order to form a volatile compound prior to spectrometric detection.1 Selective analyte conversion from liquid to gas phase results not only in enhanced analyte introduction efficiency but also in separation of the analyte from the sample matrix, leading to a reduced risk of interference.1 Additionally, VSG can employ substantially higher sample introduction flow rates than nebulization, further improving the resulting detection power. In principle, conversion of an analyte to the corresponding volatile compound can be achieved in three ways: chemically (C-VSG),1,2 electrochemically (Ec-VSG)3 or photochemically (P-VSG).4 Presently, hydride generation (HG) is the dominant and most explored VSG technique. However, HG is restricted to hydride-forming elements only, including thus ca eight analytes such as As, Se, Sb, Bi, Pb, Sn, Ge and Te.1 C-VSG, i.e. chemical reduction by means of NaBH4, is the most common approach to HG. Under the optimized conditions, the efficiency of chemical hydride generation (C-HG) reaches 100%, making this approach attractive for routine measurements. Owing to the benefits of the HG technique, effort has been made to expand the number of elements detectable by means of VSG to include volatile compounds other than binary hydrides. Generation of cold mercury vapors,1 i.e. free Hg atoms, is another example of a routinely used VSG technique, the popularity of which is comparable to that of HG. VSG-based approaches have been intensively explored in the last 15–20 years in order to make use of the benefits offered by VSG for elements other than hydride-forming elements and mercury. C-VSG and P-VSG have been employed as the most dominant strategies.2 Presently, successful VSG of more than 40 elements including transition and noble metals and even non-metals (S, P, Si, F, Cl, Br, I) has been reported.2 The volatile species generated are of different chemical structures including, e.g., carbonyls (Fe, Co, Ni, Mo, W), alkyl-halides (Cl, Br, I), free atoms (Cd), nanoparticles (Ag, Au, Cu, Pd), chelates (Pd) and oxides (Os). The recent challenges in the field of total element content determination at ultratrace levels by means of VSG lie in: 1) extending the VSG technique to new elements; 2) identifying the structure of the volatile species generated; 3) understanding the mechanisms of the VSG step; and 4) reliably quantifying the generation efficiency. The importance of understanding the VSG mechanisms must be highlighted. The insights into VSG processes not only allow further optimization of the VSG step if necessary but also lead to trouble-free applications of VSG-based methods including interference control in real sample matrices. Efficiency of the VSG step is a crucial parameter characterizing VSG-based methods. Its accurate and reliable quantification is absolutely crucial for assessing the performance and competitiveness of the VSG method. There are several approaches to quantifying VSG efficiency.5 The simplest one is based on determination of residual analyte in the liquid waste after the VSG step. However, it must be emphasized that this method can significantly overestimate the results since it is assumed that all the analyte not found in the waste liquid has been converted to the gas phase. This is not true if significant losses (deposits) of analyte occur on the inner surfaces of the volatile species generator, waste tubing, etc. It has been proven many times that the deposited fraction might reach tens of %, thus bringing huge uncertainty to the VSG efficiency results and leading to overestimation of VSG step performance. For that reason, other approaches are recommended to quantify VSG efficiency correctly. One of them is the comparison of VSG with liquid nebulization, both to be coupled to the same detector, most commonly ICP-MS. Providing the same sample loop volume is used and plasma conditions are identical for both approaches, i.e. VSG and nebulization are run simultaneously, one being used for analyte introduction while the other one operates with a blank, reliable data can be obtained. Using this approach, the efficiency of the nebulizer must be determined at first. Subsequently, a sensitivity enhancement factor is determined for the VSG step compared to liquid nebulization. VSG efficiency can be easily calculated as the product of these two values. The use of a radioactive indicator is another way to quantify VSG efficiency reliably. It not only allows the determination of VSG efficiency but also brings a detailed insight into the distribution of the fraction of analyte not converted into a volatile species between the liquid waste and apparatus components. Recently, VSG of cadmium was optimized using NaBH4 as a reductant in the presence of Cr3+ and KCN as additives. Efficiency of the VSG step reached 60% as proven by comparison of VSG and liquid nebulization as well as using a 115mCd radioactive indicator.