Universal calibration for analysis of organic solutions by inductively coupled plasma atomic emission spectrometry

The petrochemical industry needs analytical methodology for the determination of trace elements in a variety of hydrocarbons and organic solvents, some highly volatile. The authors have previously demonstrated the use of an ultrasonic nebulizer (USN) coupled to a microporous membrane desolvator (MMD) for direct ICP-AES analysis of hydrocarbons and solvents of boiling-point well below 100 degrees C. Removal of the organic matrix by the USN/MMD was sufficient not only for good ICP operability but for the nearly complete elimination of calibration bias between the various solvent systems. This suggested the concept of 'universal calibration', a single analytical calibration valid for ICP-AES analysis of multiple organic solvent systems. Universal calibration is important for the analysis of refinery/chemical plant streams which demonstrate temporal variability in composition and volatility. The present study extended the scope of previous work by using a longer (140 mm) MMD for more complete matrix removal. With this system, direct analysis of pentanes is practical with sample uptake rates of 1-3 ml min(-1) and detection limits similar to aqueous solution analysis with USN, Using 140 mm MMD USN/ICP-AES it is possible to establish a single calibration for direct trace element analysis of gasoline, refinery naphtha streams and volatile organic solvents. Corrections for differences in sample nebulization efficiency can be made by prior calibration or with the use of an internal standard.