Gossman Consulting, Inc.
April 1993

Rational for the Use of XRF as a Screening Method for Controlling and Monitoring BIF Metal Concentrations in Cement Kiln Process Feeds

BIF compliance efforts require the control of metal input in the process feeds. Monitoring the metal concentrations in these feed streams has been problematic. Specifically, problems with digestion methods and instrumentation, particularly ICP, have limited proper QA/QC performance on test data. In addition, excessive manpower and time has been required to utilize these methods. Alternatives are desirable.

EPA itself has acknowledged in the "EPA Proposed Rules - Preamble to SW-846 3rd edition", printed in the Federal Register dated February 8th, 1990, that SW-846 methods are not available.

"Sample preparation methods are not currently available in SW-846 to render non-aqueous liquids in a form that can be analyzed by the atomic absorption or inductively coupled plasma atomic emissions (ICP) type analytical methods for six important elements. These elements are: mercury, arsenic, selenium, lead, barium, and silver."

EPA further indicates in this same document that alternate methods can be used.

"This notice, or the subsequent final rule, should not be construed to require the use of SW-846, Third Edition methods except where specifically prescribed by regulation."

"Except for those situations where the RCRA regulations specify use of a particular method, it is appropriate for the chemist to use judgment, tempered by experience, in selecting an appropriate set of methods from SW-846 or the scientific literature for preparing and analyzing a given sample."

XRF is an established technology for which there are a number of ASTM methods. Numerous technical studies support XRF efficacy.

XRF Technology and Methodology

X-ray Fluorescence (XRF) Spectroscopy using both energy and wavelength modes has for some time been used as the method of choice for determining percentage levels of metals in inorganic matrices such as alloys and cement. Recent technological advances that enhance both the resolution and data process capabilities now allow XRF to perform ppm level determinations on a wide range of sample matrices. The primary focus of sample preparation is to achieve homogeneity. Samples are then loaded into a sample cup with a thin mylar film and placed in the instrument. Depending on specific metals, the run time is 10-30 minutes for an energy dispersive system. Quantitation limits are generally 10-20 ppm for electrically cooled XRF units and the added stability of a liquid nitrogen unit results in an approximate increase in sensitivity of around 25%.

XRF Advantages

There are two key advantages to XRF methods. Sample preparation is generally quick and easy since homogeneity is the primary consideration. Actual analysis is performed in 10-30 minutes during which the unit may be unattended. XRF is fundamentally an elemental technique operating on the atomic level. Although there are matrix effects that must be compensated for, the actual compound form of the metal is inconsequential to the determination.

XRF Disadvantages

There are two significant disadvantages to XRF methodologies. As previously indicated, quantitation limits for electrically cooled XRF units are 10-20 ppm with detection limits generally between 2 and 8 ppm. This may be inadequate for selected metals depending on how the BIF compliance testing was performed. Use of a liquid nitrogen unit would provide about a 25% greater sensitivity which tempers this disadvantage. Plus, even though this is a potential disadvantage, the stability of an energy dispersive unit such as XRF over a wavelength unit such as an AA is a distinct advantage. The other disadvantage is that beryllium (Be) cannot be determined using XRF. For HWF, the solvent dilution AA technique has usually proven quite adequate for Be.

QA/QC Requirements

XRF QA/QC requirements are no different than when performing determinations with AA or ICP. Blanks, duplicates, standards, spikes and/or standard reference materials are analyzed as available and appropriate. Calibration of an XRF is more akin to the less frequently performed calibration of a GC rather than that used with an AA or ICP. In addition, the stability of an XRF unit as compared to the normal drift of an AA/ICP unit is a definite advantage. While the XRF requires a daily energy calibration, it takes only ten minutes or less to perform that calibration.