Determination of silicon-aluminum ratio in MFI molecular sieve by inductively coupled plasma tandem mass spectrometry with super microwave digestion
MFI molecular sieve is an important catalyst for catalytic cracking of heavy oil and methanol-to-gasoline,and the silicon-aluminum ratio is a major index influencing its catalytic performance.The sample was treated by super microwave digestion with hydrochloric acid,nitric acid and hydrofluoric acid,which overcame the problem of difficult digestion of high silicon samples.The oxygen mass transfer mode of tan-dem mass spectrometry(MS/MS)was adopted to determine Si with the mass-to-charge ratio of the first mass filter(Q1)of 28 and the second mass filter(Q2)of 60,and the flow rate of oxygen was controlled at 0.40 L/min.The helium collision cell mode of unipolar rod was adopted to determine Al with the flow rate of helium as 3.0 L/min.Sc and Ge were selected as the internal standard element for the correction of Si and Al,respectively.The determination of Si and Al in molecular sieve was realized by inductively coupled plasma tandem mass spectrometry(ICP-MS/MS)with super microwave digestion.Therefore,the silicon-aluminum ratio could be calculated.In order to avoid the volatilization loss of Si,the time for addition of hydrofluoric acid was investigated.It was indicated that the determination result of Si was higher when the hydrofluoric acid was added after microwave digestion than that obtained before microwave digestion.Mo-reover,its corresponding relative standard deviation was lower.Therefore,the addition of hydrofluoric acid after microwave digestion could effectively avoid the loss of Si.Under the optimized experimental con-ditions,the linear ranges of calibration curve for Si and Al were 50-1 000 μg/L and 5-400 μg/L,respective-ly.The correlation coefficients were both 1.000 0.The limits of detection were 0.003 1 μg/g and 0.001 4μg/g,and the limits of quantification were 0.010 2 μg/g and 0.004 5 μg/g,respectively.MFI molecular samples were determined according to the proposed method,and the determination results of Si and Al were basically consistent with those obtained by inductively coupled plasma atomic emission spectrometry(ICP-AES).The relative standard deviations(RSD,n=7)were 2.5％and 3.1％for Si and Al,respec-tively.The recoveries were 95％-110％and 90％-110％,respectively.
inductively coupled plasma tandem mass spectrometry(ICP-MS/MS)super microwave diges-tionsilicon-aluminum ratiomolecular sieve
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