Ultra-Trace Arsenic Speciation at Columbia Analytical
Ultra-trace speciation of arsenic and other metals is performed using a variety of techniques tailored to the specific combination of species, matrix, and detection limits required. Currently, two analytical systems are applied: Ion Chromatography-Inductively Coupled Mass Spectrometry (IC-ICP-MS) and Hydride Generation-Cryogenic Gas Chromatography-Flame Atomic Absorption Spectrometry (HG-CGC-AAS). Each of these techniques has particular strengths that can be exploited depending upon the scientific question being asked. These techniques are combined with specific extraction techniques in order to maximize speciation integrity and data quality.
Ion Chromatography-Inductively Coupled Mass Spectrometry. IC-ICP-MS is a more generalized analytical technique allowing, through optimization of the ion chromatography parameters, the separation of a wide range of inorganic and organic arsenic species in aqueous, geological, and biological media. This technique allows quantification of As(III), As(V), CH3As2+, (CH3)2As+ (cacodyllic acid), as well as the major forms of arsenic in aquatic organisms: arsenobetaine and arsenocholine at levels less than 0.2 µg/L (ppb) in aqueous solution. After applying gentle, ultra-clean extraction techniques (typically mixtures of aqueous phosphate buffers and methanol) to sediments, soils, or biota, detection limits below 2 ng/g in most solids may be obtained and even lower in biological fluids. Many other arsenic compounds are accessible to analysis by this technique, with the limitation generally being the difficulty of obtaining standards and/or the chemical stability of the compounds. Currently, optimization of this technique has begun for several arsenic compounds of interest to the environmental and food sciences community, including the poultry additives 4-aminobenzene arsenic acid (Roxarsone™) and 4-hydroxy 3-nitrobenzene arsenic acid (para-arsenillic acid), and a natural arsenic metabolic intermediate, tetramethyl arsonium. Additional species, including natural inorganic arsenosulfur compounds and arsenosugars, are also analytes quantifiable by this technique.
Hydride Generation-Cryogenic Gas Chromatography-Atomic Absorption Spectrometry. When lower detection limits are required, HG-CGC-AAS is the method of choice, as sample volumes of up to 100 mL can be concentrated prior to analysis, and high levels of dissolved salts are no problem. Although limited to those species that are inherently volatile or form volatile hydrides, detection limits of <2 ng/L (ppt) are achievable for As(III), total inorganic arsenic (TIAs), CH3As2+, (CH3)2As+, and (CH3)3As (trimethyl arsine, which is difficult to separate at all using IC-ICP-MS). This method is likely most applicable to the low-level quantification of the war gas, Lewisite (2-chloro-ethynl dichloro arsine), which is also inherently volatile at room temperature. Although As(V) is not directly measured using HG-CGC-AAS, it is unambiguously determined as the difference between TIAs and As(III). Thus, it can generally be well quantified in samples where the As(V):As(III) ratio is 0.1 or higher. – Detection limits down to 0.4 ng/g (ppb) are quantifiable in biological and geological solids. In addition, through the use of ultra-clean UV photo-oxidation, you can use this method to determine total arsenic in solution at levels 1-2 orders of magnitude lower than can be obtained using ICP-MS, and with very few interferences.
Speciation of Geological Solids. A variety of sequential selective extraction (SSE) techniques are routinely applied to sediments, soils, and air particulate matter to help further the available speciation information. Comparison of field samples to an array of pure arsenic minerals suspended in kaolin, SSE combined with the speciation methods described above can provide additional insights into solid phase speciation and mineralogy of As in samples such as mine tailings and fly ash. Through a number of collaborative analytical research laboratories throughout the world, more specific solid phase arsenic speciation is available using synchrotron techniques such as EXAFS and XANES, as well as electron microprobe spectrometry (EMPS).
Sampling and Preservation for Arsenic Speciation. As with all trace metals speciation problems, the accurate assessment of arsenic speciation is critically dependant upon the use of the appropriate sampling and preservation procedures. In general, it is important to avoid the use of glass containers (unless inherently volatile species such as (CH3)3Hg are being sought), minimize or eliminate contact of the sample with oxygen and other oxidizers, and analyze the sample or preserve it using low-temperature freezing as soon as possible after sample collection. In general, soil and biota samples should be placed in a to-fit trace-clean plastic container and be frozen until analysis. Aqueous samples are best if they are quick frozen in liquid nitrogen or dry-ice plus methanol in appropriate-sized trace-metal clean polyethylene containers provided by our laboratory, and then stored at below -80 oC until thawed for analysis. Otherwise, it is best to send most water samples by overnight delivery, unpreserved in completely filled trace-clean plastic containers. Never use containers that have been cleaned with HNO3 or BrCl, or preserve samples with these reagents. Oxic aqueous samples (surface waters) may be preserved for As(III), As(V), CH3As2+ and (CH3)2As+ using 0.2% chlorine-free HCl (pre-purged with inert gas) if they will be analyzed using HG-CGC-AAS. Samples of sediment, sediment pore water, groundwater, and/or anoxic water samples have very specific sampling and preservation requirements that should be discussed prior to attempting. It is often best for speciation data quality for your laboratory to provide sampling equipment and personnel for these specialized procedures.