SPEX Analysis Exam Flashcards

During ICP-OES analysis using SPEX CertiPrep certified standards, a laboratory notices that the calcium signal at 317.933 nm is consistently 8% higher than expected even after matrix-matching. After ruling out contamination, what is the most likely root cause?

Answer: Spectral overlap from vanadium emission lines near the calcium wavelength

Vanadium has several emission lines in the 317–318 nm region that can cause spectral interference on the Ca 317.933 nm line, especially in samples with elevated vanadium content. This is a classic non-spectral vs. spectral interference distinction: the 8% positive bias and the exclusion of contamination and matrix mismatch point directly to a spectral overlap. SPEX analysis protocols require interference correction factors (ICFs) for known overlaps, and the Ca 317.933 nm line is a documented overlap risk when vanadium >10× calcium concentration.

A SPEX-based multi-element standard containing both arsenic and chloride is prepared in 2% HNO₃ for ICP-MS analysis. The laboratory observes an anomalously high arsenic (m/z = 75) signal. Which polyatomic interference is most responsible, and what is the correct mitigation?

Answer: ⁴⁰Ar³⁵Cl⁺ at m/z 75; use a collision/reaction cell with hydrogen or helium gas to dissociate the polyatomic ion

⁴⁰Ar³⁵Cl⁺ is the dominant polyatomic interference on ⁷⁵As in ICP-MS because both argon (the plasma gas) and chloride (from the HCl matrix or chloride-containing samples) are abundant. The argon-chloride dimer forms at m/z = 40+35 = 75, directly overlapping the only stable isotope of arsenic. A collision/reaction cell (CRC) using H₂ or He effectively dissociates or retards this polyatomic ion, restoring accurate arsenic quantification. The other options describe real but secondary interferences or incorrect element combinations.

When preparing a series of calibration standards from a SPEX 1000 mg/L single-element stock for trace selenium analysis by GFAAS, a chemist uses a Pd/Mg(NO₃)₂ chemical modifier. At low concentrations (≤5 µg/L), precision dramatically degrades despite stable lamp energy and proper background correction. Which factor is the MOST likely cause?

Answer: Adsorption of selenium onto the graphite tube walls is saturating active sites at low concentrations

At sub-µg/L to low µg/L concentrations, selenium adsorption onto active sites on the graphite tube wall becomes a dominant source of imprecision. When active sites are not uniformly conditioned or become saturated differently between injections, the analyte signal varies non-reproducibly. The Pd/Mg modifier helps stabilize selenium thermally, but does not eliminate wall adsorption effects at trace levels. Rigorous tube conditioning (pre-firing with modifier blanks) and use of platforms (L'vov platform) are the corrective approaches. The other options describe real phenomena but are not the primary driver of poor precision at trace levels.

A SPEX-certified 10 mg/L mixed anion standard is used to calibrate an ion chromatography system. The method detection limit (MDL) study for nitrite shows the MDL is acceptable, but during routine sample analysis, nitrite peaks are systematically wider and show tailing not present in standards. The most diagnostically important next step is:

Answer: Inject a sample blank spiked with the SPEX nitrite standard at the MDL level and compare peak shape to the neat standard

Systematic peak tailing and broadening in real samples but not standards is a classic matrix effect signature. The correct diagnostic step is to perform a matrix spike using the certified SPEX standard added to the actual sample matrix (at MDL level), then compare peak morphology. If tailing reproduces in the spiked sample but not the neat standard, it confirms a matrix-induced effect (e.g., co-eluting organic acids, pH mismatch, or ionic strength differences). This isolates whether the problem is sample-specific before making any instrumental changes. Changing suppressor flow or eluent strength alters the chromatographic conditions globally and could mask rather than diagnose the root cause.

In SPEX-based XRF analysis, a certified reference soil standard is used to validate a pressed-pellet method for arsenic. Results are consistently 12–15% low compared to the certified value, even though results for iron, calcium, and zinc are within ±3%. The most probable explanation is:

Answer: Matrix absorption enhancement by iron is suppressing the arsenic Kα fluorescence signal because iron's K-edge energy is just above the arsenic Kα emission

In iron-rich soil matrices, the high iron concentration creates a significant X-ray absorption effect on arsenic Kα fluorescence (~10.5 keV). Iron's K-absorption edge (7.11 keV) is below the As Kα energy, meaning iron atoms efficiently absorb arsenic's fluorescent X-rays before they can exit the sample and reach the detector. This matrix absorption effect causes a systematic negative bias for arsenic specifically in iron-rich samples — elements with higher-energy characteristic lines (like zinc Kα at ~8.6 keV relative to the iron matrix) are less severely affected. The correct remedy is to use matrix-matched calibration standards or apply fundamental parameter corrections accounting for the iron content.

A laboratory uses SPEX CertiPrep aqueous standards for EPA Method 200.8 (ICP-MS) trace metal analysis of drinking water. Internal standards include ¹⁰³Rh and ²⁰⁹Bi. During a sample batch, Rh recovery drops to 72% while Bi remains at 98–102%. The analyst should FIRST:

Answer: Suspect a physical/dissolved solids issue causing mass discrimination below m/z ~110, and check TDS of the affected samples

The divergence between ¹⁰³Rh (mid-mass, ~103 Da) and ²⁰⁹Bi (high-mass, ~209 Da) internal standard recoveries is the diagnostic key. When a lower-mass internal standard shows suppression while a high-mass one remains stable, this is a strong indicator of mass-dependent matrix effects, most commonly caused by elevated total dissolved solids (TDS) or high concentrations of a specific matrix element. High TDS causes space-charge effects in the ion beam that disproportionately suppress lighter ions. The analyst should check TDS of the flagged samples and, if elevated, dilute and re-analyze. Bi is heavy enough to be less affected by space-charge suppression.