Researchers from the Czech Republic propose to use selenium as an internal standard to correct ionization interferences degrading the determination of arsenic and its species.
Arsenic determination by inductively coupled plasma-mass spectrometry (ICP-MS) is hampered by both spectral and non-spectral interferences. While polyatomic interferences such as those resulting from the presence of 40Ar35Cl+ can be eliminated by either high resolution MS or by the use of collision/reaction cell technology, non-spectral interferences have to corrected by an appropriate calibration strategy.
One of the non-spectral interferents affecting the ionization of arsenic in the plasma is carbon. The presence of carbon in the sample is influencing the plasma conditions leading to a signal enhancement for Arsenic. The enhancement effect is resulting from both the reduction of AsO by neutral carbon and the following ionization of As by C+ species.
Another effect reducing the signal intensity is produced by the presence of easily ionized elements such as sodium and potassium. Since the two effects act against each other, the final resulting interference depends on the ratio of the two making it difficult to quantify them in real samples. These interference may affect the determination of total arsenic content obtained by conventional direct liquid sample introduction and also the determination of As species after separation by HPLC. The new study:
Researchers from the University of Prague now found a way to correct for these interferents, by using external calibration with an internal selenium standard.
Total arsenic determination
The researcher could demonstrate that selenium as internal standard added in the form of selenite is able to completely correct the increase in As signal intensity up to a carbon content of 600 mg/resulting in an overestimation of 5% for a carbon content of 3000 mg/l.
The correction of the signal suppression by sodium is more critical. Here the internal selenium standard works satisfactory only to a maximum concentration of about 18 mg/L Na. The overall approach was tested by analysing real samples of canned fish. Accurate results were obtained, event when residual carbon survived a short microwave digestion.
The authors also admit that the use of selenium as an internal standard has its limitations, since selenium content in food samples is very variable. However, for most samples the selenium concentration added by the internal standard is significantly higher than the selenium present naturally.
As speciation analysis
For speciation analysis, trimethylselenonium iodide served as an internal standard and was added to the mobile phase. This was possible, since the internal standard does not interact with the stationary phase used for anion-exchange chromatography. For speciation analysis using anion-exchange chromatography the carbon interference is less critical, since carbon does not coelute with the As species. The situation is different for sodium, which is eluted mainly in the first half of the chromatogram, interfering with the determination of AB, DMA and MA. Since the chromatographic conditions lead to a dispersion of the original sample, much higher Na concentrations can be tolerated in presence of the Se internal standard. Therefore, the study revealed that the Se internal standard nicely corrects for the signal suppression caused by Na up to 3540 mg/L.
The researchers also found that the natural Se content of samples is less a problem for As speciation analysis using Se as an internal standard, because the most prominent Se species (selenomethionine) does not coeleute with the As species. However, the authors admit, that some samples could contain other or unknown As or Se species or high amount of known species, which may overlap. Anyhow, it could be demonstrated, that common samples containing typically up to 1 µg/g Se can be analysed without any problem. The original study:
The authors concluded that their method can be used for the analysis of As and its species in undiluted biological and environmental samples with high sodium and/or carbon content, such as sea water, urine or plasma without any sample pre-treatment.
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