Ultrasonic slurry sampling electrothermal vaporization inductively coupled
plasma mass spectrometry (USS-ETV-ICP-MS) is a very powerful technique for
the direct analysis of solid materials prepared as slurries. The use of iso
tope dilution USS-ETV-ICP-MS (USS-ETV-ID-ICP-MS) for micro-homogeneity char
acterization studies of powdered reference materials based on elemental ana
lyses, was investigated. Slurry analysis conditions were optimized taking i
nto consideration density, particle size, analyte extraction, slurry mixing
, analyte transport and sampling depth. Slurries were prepared using 1-20 m
g of material and adding 1.0 ml of 5% nitric acid diluent containing 0.005%
Triton X-100((R)). Three reference materials were analyzed (RM 8431a Mixed
Diet, SRM 1548a Typical Diet and SRM 2709 San Joaquin Soil). Cu and Ni wer
e determined in each material and Fe was also determined in RM 8431a Mixed
Diet. ETV conditions were optimized and the benefit of using Pd as a carrie
r to enhance transport, combined with oxygen ashing was demonstrated. The a
ccuracy of the method was verified by comparing analytical results with cer
tified values. The precision of the method was demonstrated by comparing R.
S.D.'s for slurry samples and aqueous standards and elemental 'homogeneity'
was quantified based on the slurry sampling variability. The representativ
e sample mass analyzed was calculated taking into consideration extraction
of analyte into the liquid phase of the slurry. Representative sample masse
s of approximately 4 mg of RM 8431a. provided slurry sampling variabilities
of 10% or less for Cu, Fe and Ni. Representative sample masses of approxim
ately 10 mg of SRM 1548a provided slurry sampling variabilities of approxim
ately 10% for Cu and Ni. Representative sample masses of approximately 0.3
mg of SRM 2709 resulted in total analytical variabilities of less than 7%,
highlighting the fact that the San Joaquin Soil is clearly the most homogen
eous of the materials characterized. (C) 2001 Elsevier Science B.V. All rig
hts reserved.