Cromatografia líquida com espectrometria de massas para a determinação de ácidos orgânicos e ânions em água separada de emulsões de petróleo

Abstract

The main difficulty in relation to the methodology development in crude oil separated water is related to the high concentration of salts (up to 150 g L-1 of Cl-) of this type of sample, which can cause interferences in the chromatographic separation stage. In this work, ion chromatography with conductivity and mass spectrometer (MS) detectors was proposed for determination of anions and organic acids in crude oil separated water. Initially, the separation in the ion exchange column it was optimized, with optimal separation using a KOH mobile phase gradient (1 to 60 mmol L-1), column temperature of 35 ºC and 50 min chromatographic run. The organic acids C1 to C8 and the anions F- , Cl- , Br- , NO3 - , CO3 2-, SO4 2- and PO4 3- were separated with good resolution limits of quantification (LOQs) ranging from 10 to 500 µg L-1, for anions and organic acids, respectively. Due to overlapping signals for high chloride content, it was not possible to quantify anions and organic acids simultaneously by ion chromatography in samples with more than 100 mg L-1 of Cl- . Most samples need to be diluted, increasing the LOQs in the same order. As an alternative, the organic acids separation it was evaluated using ion-exclusion chromatography and conductivity detection, using heptafluorobutyric acid 1 mmol L1 as mobile phase in the isocratic mode, 35 ºC and chemical suppression with external regeneration of tetrabutylammonium hydroxide 0,5 mmol L-1. It was possible to separate and quantify the organic acids C1, C2, C3, C4 e C5 with limits of quantification of 2.5, 5, 8, 5 and 5 mg L-1, respectively. However, as with the ion-exchange column, the samples needed to be diluted to Cl- concentrations below 5000 mg L -1 to minimize signal overlap. C1 and C2 are presented in most samples with concentrations ranging from bellow of 2.5 to 1680 mg L-1 and bellow of 5 to 15310 mg L-1, respectively. For the acids C3, C4 and C5, due to the need for sample dilution and consequent increase in LOQs proportionally to the dilution factor used it was bellow of LOQ with exception of C3 in only one sample, with 352 mg L-1. Twodimensional ion chromatography with ion-exclusion pre-column was used to eliminate the Cl- from the matrix in the first dimension and to separate organic acids in the second dimension (with ion-exclusion column) and minimize the need for sample dilution. It was necessary 3,8 min in the first dimension to eliminate the most of Cl- of the sample matrix and enable quantification of C3, C4 and C5 organic acids of the water samples separated from emulsions of crude oil injected directly into the chromatographic system, without the need sample dilution, with LOQs up to 30 times lower (samples with 150 g L-1 of Cl- ). The quantification of C1 and C2 organic acids it was not possible in this method because most of them are eliminated with the sample matrix in the first dimension. The organic acids C5 to C10 were quantify using ion-pairing chromatography and MS detection, with acetonitrile, methanol and water acid mobile phase, 35 min of run and 25 ºC. The separation of C1 to C5 it was not quantify because they was eliminated in the Cl- matrix removal step using C18 SPE cartridges (washing with water and eluting with 75% acetonitrile + 25% methanol). The eluted samples were injected directly into the chromatographic system and the LOQs were 0.5 mg L-1 for C5, C6 and C7 and 1.0 mg L-1 for C8, C9 and C10. The values found for organic acids C5, C6 and C7 ranged from 0.557 to 22.1 mg L-1, from 0.686 to 5.06 mg L-1 and 0.685 to 1.4 mg L-1, respectively. For C8, C9 and C10, the values were lower than LOQ (1 mg L-1). It should be noted that, for all methods used, recovery tests were performed using a reference solution and the recoveries were about 80 to 110%. In addition, the accuracy was evaluated using certified reference material (CRM 46975) and the results were in agreement with the reference values (85 to 110%).