SPEX Analysis Exam Flashcards

During SPEX inductively coupled plasma (ICP) analysis, a matrix-matched calibration is performed, yet results for a high-salinity water sample still show a 12% positive bias compared to certified reference values. Which mechanism is MOST likely responsible?

Answer: Space-charge effects from high dissolved solids suppressing analyte ion transmission in the mass spectrometer interface

High dissolved solids create space-charge effects at the skimmer/sampler cone interface of an ICP-MS system. The abundant matrix ions repel analyte ions, reducing transmission efficiency. A positive bias of this magnitude in high-salinity matrices is the hallmark of space-charge suppression being under-compensated — matrix-matched calibration can partially address it but may not fully eliminate the effect without dilution or matrix separation.

A SPEX certified reference standard for multi-element ICP analysis is prepared at 1000 µg/mL in 2% HNO₃. An analyst observes that calcium results drift upward by 8% over a 6-hour analytical run despite successful initial calibration. The most probable cause, before blaming instrument drift, is:

Answer: Evaporative concentration of the working standard due to uncapped containers in a heated spray chamber environment

Working standards prepared from SPEX concentrates and left uncapped near a heated spray chamber are subject to evaporative loss of solvent (water/acid), which concentrates the solution over time. A progressive 8% upward drift over 6 hours is consistent with evaporation rather than instrument drift (which typically appears as both positive and negative variation). Proper practice requires capping standards and tracking the mass of calibration vessels.

When performing SPEX flame atomic absorption spectroscopy (FAAS) for lead in a soil digest, the analyst uses an air-acetylene flame but obtains severely depressed signals even after standard additions. A nitrous oxide-acetylene flame corrects the issue. This is BEST explained by:

Answer: Formation of refractory lead phosphate or sulfate compounds in the cooler air-acetylene flame that resist atomization

Lead in complex soil digests can form refractory compounds — particularly lead phosphate, lead sulfate, or lead silicate — that do not dissociate efficiently in the relatively cool air-acetylene flame (~2300°C). The hotter nitrous oxide-acetylene flame (~2900°C) provides sufficient thermal energy to break these bonds and atomize the lead. This is a known matrix interference in environmental analysis and explains why standard addition alone does not correct it when the interference is atomization-based rather than spectral.

In SPEX X-ray fluorescence (XRF) analysis of thin geological samples, the analyst notices that iron results are consistently lower in samples with high silica content compared to fusion bead preparations of the same samples. This discrepancy is PRIMARILY due to:

Answer: Mineral grain-size effects causing preferential X-ray absorption by coarse silica particles before iron fluorescence can escape the sample

In pressed powder XRF analysis, heterogeneous grain size causes a 'mineralogical effect' or 'particle size effect.' Coarse silica grains can shield iron-bearing minerals, and the iron Kα X-rays generated at depth within silica-rich particles are differentially absorbed before reaching the detector. Fusion beads dissolve all minerals into a homogeneous glass, eliminating this matrix effect entirely. This is a classic error source in direct pressed-powder XRF of geological materials and is why fusion beads are the gold standard for accurate major-element analysis.

An analyst is validating a SPEX multi-element ICP-OES method for drinking water. The method detection limit (MDL) study yields an MDL of 0.8 µg/L for arsenic. However, during routine sample analysis, replicate measurements of a 2 µg/L arsenic QC standard show a %RSD of 22%. The MOST likely cause of this inconsistency is:

Answer: The MDL study was performed on reagent blank matrix while real samples have a suppressive matrix effect increasing noise at low concentrations

MDL studies are typically conducted using reagent-grade water (blank matrix) spiked at low concentrations, which does not reflect the matrix of real drinking water samples. When real samples contain dissolved minerals, competing species, or organic matter, they suppress or increase noise in the analytical signal. A 22% RSD on a sample at 2.5× the MDL — which should theoretically be achievable at <10% RSD if the matrix matched — indicates the MDL is artificially low because it was determined in a cleaner matrix. Proper validation requires matrix-specific MDL determination.

A forensic laboratory uses SPEX-certified standards to calibrate an ICP-MS system for trace metals in blood. Indium (In-115) is used as the internal standard. The analyst notices that mercury (Hg-202) results are 40% lower than expected on human blood samples, but spike recoveries in aqueous matrix are 98%. Which scenario BEST explains this pattern?

Answer: Mercury volatilization from blood samples during sample preparation at room temperature, with aqueous spike recoveries unaffected because mercury remains in ionic form in dilute acid

Mercury is notorious for volatilization losses during biological sample preparation. Organic mercury and elemental mercury species in blood can be lost if samples are not acidified immediately with oxidizing acids (e.g., HNO₃/H₂O₂) or if preparation involves any heating step without a sealed environment. Aqueous matrix spike recoveries are unaffected because the spike is added in dilute acid, where mercury is stabilized as Hg²⁺ ion. This is why blood mercury analysis requires cold vapor atomic absorption (CVAA) or careful microwave digestion protocols, and why spike recovery in aqueous matrix does not validate recovery from biological matrices.