XRF SQX Analysis

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The objective of this study was to compare ICPMS and XRF semi-quantitative analysis, particularly for a high silica sample which is difficult to dissolve for ICPMS. XRF has three advantages: (1) the sample does not need to be put into solution as in ICPMS , (2) no standards are required for semi-quantitative (SQX) analysis, and (3) XRF SQX analysis is about half the price of a Metals Screen by ICPMS. An NIST certified Standard Reference Material (SRM) 1c, Argillaceous Limestone, was selected for comparison. The results are summarized in Tables 1 and 2 below.

From the study of this material and several other reference materials, the conclusion is that both techniques produce outstanding semi-quantitative results where the observed value is within 50-200% of the true value. XRF is a more economical test for samples where composition down to 0.01% (100 ppm) is desired. But ICPMS is required to reach down to sub-ppm levels in solids or sub-ppb levels in liquids, although some solid samples may be very difficult to dissolve.

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Conclusions:

Both ICPMS and XRF produce excellent Semi-Quantitative Results
XRF range is 0.01%-100%
ICPMS range is 0.01 ppm-100%
XRF-SQX is less expensive
XRF is for solids while samples for ICP-MS must be dissolved

XRF SQX Analysis

The sample for XRF was mixed with an organic binder and pressed into a pellet for analysis. In the SQX calculation the metals were calculated as metal oxides, although they are reported here as the metal, and balance was assumed to be CO₂ because the sample is know to be predominantly carbonates. This is one of the more

difficult SQX calculations because the determination of C by XRF has a large error. For samples of metal oxides or pure metals, the SQX results are generally better than shown here for a carbonate based sample. Note that while liquids can be analyzed by XRF, there are generally better approaches available.

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	Certified Value	ICPMS	XRF SQX	XRF Quantitative
Aluminum	6,880	6,500	7,900	6,900
Calcium	359,500	370,000	400,000	363,440
Iron	3,900	4,300	5,000	4470
Magnesium	2,500	2,510	3,300	2920
Manganese	194	144	170	130
Phosphorus	175	302	440	180
Potassium	2,300	2,450	2,700	2420
Silicon	32,000	32,000	28,000	33,470
Sodium	150	304	ND	450*
Strontium	250	245	290	190*
Titanium	420	192	440	370
		Non-Certified Elements
Chromium		16	260
Sulfur		NA	0.68	1,360
Zirconium		11	40
ND - Not Detected. * Near the LOD.

ICPMS Metals Screen

In this comparison, the sample for ICPMS was digested in nitric and hydrofluoric acids to completely dissolve the silicate matrix. Low results for Al, K, Si, and Ti are observed if only nitric acid is used for digestion. The ICPMS Metals Screen is not just a semi-quantitative technique. Standards are run for more than 20 elements including all those listed in Table 1, except zirconium. The other elements are estimated based upon their response relative to an internal standard. For more information on this, visit our web page on ICPMS Metals Screen.

Table 2 shows the complete results for the Metals Screen. Many more elements are determined and at much lower detection limits than are possible with XRF.

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Table 2. Metals Screen by ICPMS
Sample ID: NIST SRM 1c (Nitric acid/hydrofluoric acid prep, 1:5 dilution)

		Detection			Detection
	ppm	Limit		ppm	Limit

Aluminum	6500	20	Molybdenum	ND	0.6
Antimony	0.13	0.06	Neodymium	3.9	0.03
Arsenic	1.72	1	Nickel	ND	30
Barium	75	0.04	Niobium	0.67	0.03
Beryllium	0.12	0.02	Osmium	ND	0.02
Bismuth	ND	0.08	Palladium	0.27	0.1
Boron	ND	10	Phosphorus	302	200
Bromine	ND	200	Platinum	ND	0.02
Cadmium	0.23	0.02	Potassium	2450	200
Calcium	370000	50	Praseodymium	0.95	0.02
Cerium	5.4	0.02	Rhenium	ND	0.02
Cesium	1.03	0.02	Rhodium	0.02	0.02
Chromium	16.2	1	Rubidium	16.4	0.04
Cobalt	7.7	0.04	Ruthenium	ND	0.02
Copper	4.5	0.1	Samarium	0.73	0.02
Dysprosium	0.61	0.02	Selenium	ND	2
Erbium	0.37	0.02	Silicon	32000	4000
Europium	0.2	0.02	Silver	0.03	0.02
Gadolinium	0.85	0.02	Sodium	304	200
Gallium	1.01	0.02	Strontium	245	0.1
Germanium	ND	2	Tantalum	ND	0.5
Gold	ND	0.4	Tellurium	ND	0.2
Hafnium	ND	0.2	Thallium	0.06	0.02
Holmium	0.14	0.02	Thorium	0.91	0.06
Iodine	ND	8	Thulium	0.05	0.02
Iridium	ND	0.02	Tin	0.75	0.06
Iron	4300	300	Titanium	192	0.6
Lanthanum	3.5	0.02	Tungsten	ND	40
Lead	2.73	0.2	Uranium	1.34	0.02
Lithium	5.2	0.06	Vanadium	11.9	3
Lutetium	ND	6	Ytterbium	0.3	0.02
Magnesium	2510	2	Yttrium	4.4	0.03
Manganese	144	0.2	Zinc	18.4	0.6
Mercury	ND	0.3	Zirconium	11.1	6
Date Analyzed: 09-29-03
Elements Not Analyzed: All Gases, C, S, Sc, In, Tb

XRF Quantitative Analysis

For Quantitative XRF analysis, the sample is fused at 1000-1100^oC with a lithium borate flux. A calibration curve was prepared from several high purity calcium carbonate samples and limestone Standard Reference Materials. This method is not quite as sensitive as pressed pellet methods because the sample is more diluted by fusion with a flux. However, this minimizes matrix and particle size effects.

Quantitative applications currently include: