Fast speciation analysis of inorganic arsenic by using frontal chromatography for separation

Since the total concentration of trace elements does not provide good enough information about the mobility, transport, fate and impact of the elements in environmental and biological systems, speciation analysis has become increasingly important. Unfortunately, most element selective detection techniques are not species selective and therefore require to be combined with a separation technique.

The hyphenated technique is not only more complex and costly, but also provides only a reduced sample throughput, making speciation analysis costly. While ICP-MS can do multielement determinations at a sampling rate of 60 samples per hour and higher, the coupled technique HPLC-ICP-MS requires several minutes for the species determination of a single element. For this reason, the optimization of the separation method with respect to analysis time is a prerequisite to enhance the acceptance of speciation analysis in the routine environment.

For cases in which the species of interest are extremely different in some characteristics such as ionic/neutral, cationic/anionic, organic/inorganic the required separation power is limited. In such cases of "binary" speciation also non-chromatographic separation methods have been used (see the Brief Summary).

In case that a chromatographic column is used for such separation task, a very short column with a very low number of theoretical plates is potentially suitable. Moreover, if the back pressure caused by the short column is sufficiently low, its feeding may be simply carried out with a peristaltic pump. Methods using such low pressure have been reported in the literature and columns for it have been provided commercially.     

Now a team of Italian researchers is going one step further in simplifying the separation technique by proposing to use frontal chromatography that is avoiding the use of an injection valve.

Figure: Principal instrumental setup for frontal chromatography coupled to ICP-MS

In frontal chromatography, or frontal development, the sample is also used for elution. Thus, the sample feed is continuously applied to the column and the sample components will displace each other in order of decreasing affinity for the chromatographic medium (this is called sample self-displacement). The least retained solute will be obtained in a pure form (i.e. depleted of the more strongly retained components) until the other solutes break through. Eventually the column will be saturated with the strongest retained component and the effluent will have the same composition as the feed. Thus, even the strongest retained component may be obtained after washing the column and a desorption step.

The researchers tried two different setups using two peristaltic pumps. In the optimized setup, the first pump is used to load the sample onto the column, while the second pump is used to add  nitric acid to the eluent and to split the eluent towards the mixing point.

This very simple configuration enabled an efficient separation of the fronts of the two species, showing up in two distinct sigmoidal curves. The first sigmoidal curve is related to As(III), which is not retained by the strong anionic exchange resin, whereas the second curve is due to the presence of As(V) which interacts with the stationary phase. The analysis time to record the two signals was optimized to about 160 s.

For signal evaluation the researchers used the slopes of the frontal curves over the inflection points by evaluating two non-overlapping zones of the chromatogram around 25 and 100 s. Moderately accurate quantification of the two species were obtained by using 9 single species standard solutions for the calibration. The mean relative absolute errors in prediction were 3.5% for As(III) and 6.6% for As(V). Unfortunately, approx. 3-times higher errors are observed, when low levels of one species have to be detected in presence of high concentrations of the other species.

Therefore this approach for determining the two inorganic arsenic species in less than 2 minutes can be an option, if fast analysis with moderate precision is fit for purpose and sufficient sample amount is available. 

The original publication:

Davide Spanu, Damiano Monticelli, Laura Rampazzi, Carlo Dossi, and Sandro Recchia, Introducing Frontal Chromatography-ICP-MS as a fast method for speciation analysis: the case of inorganic As, Anal. Chem., 91/21 (2019) 13810-13817. DOI: 10.1021/acs.analchem.9b03279

  B.P. Jackson, Fast Ion Chromatography-ICP-QQQ for Arsenic Speciation. J. Anal. At. Spectrom. 2015, 30 (V), 1405–1407. DOI: 10.1039/c5ja00049a
J. Morton, E. Leese, Arsenic speciation in clinical samples: Urine analysis using fast micro-liquid chromatography ICP-MS, Anal. Bioanal. Chem, 399 (2011) 1781-1788. DOI: 10.1007/s00216-010-4180-z

Brief summary: Non-chromatographic separation techniques for speciation analysis Brief summary: LC-ICP-MS: The most often used hyphenated system for speciation analysis

Brief summary: Speciation and Toxicity
Brief summary: Standard methods for arsenic speciation analysis
Link database: Toxicity of arsenic species
Link database: Analytical Methods for Arsenic Speciation Analysis

last time modified: July 22, 2020