Liquid Chromatography-Mass Spectrometry of Arsenic Compounds Using the Electrospray Ionization with Postcolumn Addition of Methanol

Abstract

It has been well known that inorganic arsenic compounds can be carcinogenic to humans.1 Most mammals convert inorganic arsenic compounds to low toxicity monomethylarsonic acid (MMAA), dimethylarsinic acid (DMAA) or trimethylarsine oxide (TMAO) by methylation. 2,3 As the toxicological properties of arsenic compounds depend on their chemical forms, it has been demonstrated that the analysis of every chemical form is very important. Although the LC-ICP/MS is sensitive and precise, the LC-ICP/MS does not give any information on speciation. [4][5][6] On the other hand, the LC/MS is a good tool for qualification and quantification. We have previously reported the appropriate mobile phase and the MS conditions for the arsenic compounds using LC/MS. 7 Since it is known that the addition of an organic solvent is useful to increase the sensitivity on electrospray ionization (ESI), we examined the postcolumn addition of methanol for improvement of the linearity, detection limit and repeatability of the method. ExperimentalOrganoarsenic species MMAA, DMAA, TMAO, tetramethylarsonium (TeMA) iodide, and arsenobetaine (AB), were purchased from Tri Chemical Laboratories Inc. (Yamanashi, Japan). Other reagents (each analytical grade) were purchased from Wako Pure Chemical Industries. Deionized water was obtained from a Milli-Q system (Nihon Millipore, Tokyo, Japan). The LC/MS equipment was Model HP1100 series HPLC with MS (Hewlett-Packard, DE, USA). Electrospray ionization (ESI) mode was selected as the ionization system. A Shodex RSpak NN-614 (Showa Denko, Tokyo, Japan) packed with a hydrophilic resin-based cation exchange resin (cation exchange capacity of 0.1 meq/g) was chosen as the separation column. The separation conditions were set by referring to those of the previous paper. 7Influence of postcolumn addition of methanol on sensitivity of arsenic compounds Figure 1 shows the relationships between the flow rate of methanol added by postcolumn addition and the peak areas of arsenic compounds. The graph demonstrates that the addition of methanol to the mobile phase was effective for increasing the sensitivity, and that around 0.2 ml/min was a suitable rate for the high sensitive detection. The reason for increasing of sensitivity is that the methanol allows the surface tension of droplet from nebulizer. To decrease is consequently easier to make smaller droplets and to vaporize the liquid of droplet than without the addition of methanol. Inc., Musashino, Japan Keywords HPLC/MS, arsenic compounds, electrospray ionization Fig. 1 The relationships between the flow rates of methanol and the peak areas of arsenic compounds. Conditions: carrier, 8 mM HNO3 / 5 mM NH4NO3, 0.4 ml/min; drying gas, N2 (13 l/min, 350 ˚C); nebulizer, N2 (60 psi); accelerating voltage, 80 V; polarity, positive scan mode; sample, 10 ng/µl each, 10 µl inject; postcolumn, methanol.

Synthesis and characterisation of ferrocenyl-phosphonic and -arsonic acids

“…[43,44] The use of methanolic solvents is commonplace in the reported studies, and methanol is specifically added to increase sensitivity in one study. [45] The positive ion ES behaviour of six organoarsenic acids in a methanol-water solvent system in the presence of acetic acid has been studied. [42] In all cases, the protonated molecule was the most intense ion, however, all spectra contained…”

Understanding arsenic metabolism through spectroscopic determination of arsenic in human urine

“…Table 2 shows the advantages and disadvantages of the different spectroscopic techniques. Table 3 gives selected references that describe these techniques, GF-AAS Total arsenic [57] LC-MS AB, DMA [58] and which have been used for total As and As speciation studies. The steps involved in urine analysis starting from the collection step and ending with its analysis using different spectroscopic techniques are summarised in Fig.…”

“…More investigations by the same group led to a conclusion that a sulfur-containing arsenic (dimethylarsinothioic acid (DMAS)) compound was mistakenly identified as DMA(III) in urine [125]. More applications of HPLC-ESI-MS for As speciation in urine have been reported; AB and DMA have been measured in human urine [126], and TMAO was identified in urine of rats chronically exposed to DMA [127]. The purpose of both studies was to evaluate the capability of this technique for quantification and identification of organoarsenic compounds in urine.…”

Environmental Mass Spectrometry

“…Lindemann et al reported the simultaneous determination of arsenic, selenium, and antimony species using LC/ICPMS (235). Two papers report the determination of arsenic species using either LC/MS with ESI and postcolumn addition of methanol (236) or ion exclusion chromatographyhydride generation ICPMS (237). Szostek and Aldstadt determined organoarsenicals in water, soil, and sediment using SPME-GC/MS (238).…”