SPEX Exam SPEX Analysis Exam 6

Question 1

During ICP-OES analysis using a SPEX multi-element standard, a laboratory observes that the measured concentration of arsenic (As) is consistently 8% higher than the certified value when analyzed in a nitric acid matrix. After verifying instrument calibration, what is the MOST likely cause of this systematic positive bias?

Accepted Answer

An argon emission line at 193.759 nm causing spectral overlap with the As 193.696 nm line in a high-matrix sample

Answer

Argon has an emission line at 193.759 nm that can overlap with the arsenic primary analytical line at 193.696 nm. In high-matrix or high-solvent-load samples, argon emission intensity can increase due to altered plasma conditions, creating a consistent positive bias on As measurements. Chlorine does have a nearby line but requires chloride-containing matrices. This is why analysts often switch to the secondary As line at 188.979 nm or use background correction when chlorine or argon interference is suspected.

Question 2

A SPEX certified reference standard for a mixed-metal solution shows a certified value of 10.0 ± 0.2 mg/L for lead (Pb). When analyzed by GFAAS, a laboratory consistently obtains results of 9.1 mg/L after following all standard procedures. The lab's Z-score is –4.5. Before declaring the reference material out of specification, which investigation step is most critical to perform FIRST?

Accepted Answer

Check the expiration date and storage conditions of the SPEX standard, as Pb can precipitate or adsorb to container walls at low pH below 1%

Answer

For aqueous lead standards analyzed by GFAAS, adsorption of Pb²⁺ onto container walls is a significant and frequently overlooked source of negative bias, especially if the solution pH is not sufficiently acidic (ideally <1% HNO₃). Before blaming instrument or method parameters, confirming the standard's integrity—its storage conditions, container type (HDPE vs. glass), and acidification—is the most critical first step. Pb is one of the most prone elements to container adsorption losses. A Z-score of –4.5 indicates a large systematic deficit that is more consistent with analyte loss from the standard than instrument drift.

Question 3

In a QC program using SPEX CertiPrep standards, a laboratory prepares a calibration verification standard (CVS) and obtains a percent recovery of 101.5%. However, the method blank shows a signal equivalent to 0.8 µg/L for the target analyte. The MDL for this method is 0.3 µg/L. Which statement BEST describes the corrective action required under a rigorous ISO 17025 QC framework?

Accepted Answer

Investigate and eliminate the blank contamination source before re-running the batch; if blank signal exceeds 1/3 of the lowest calibration standard, all affected samples must be flagged and potentially re-analyzed

Answer

Under ISO 17025 and most environmental methods (e.g., EPA 200.8), a method blank signal above the MDL is a contamination flag requiring investigation. The key threshold is whether the blank signal is significant relative to sample concentrations or calibration range—commonly flagged when it exceeds 1/3 of the lowest calibration standard or the reporting limit. Blind blank subtraction without identifying the contamination source is not acceptable under rigorous QA frameworks. Simply passing the CVS recovery does not override a contaminated blank; the root cause must be investigated and eliminated before results can be reported without qualification.

Question 4

A laboratory uses a SPEX 1000 mg/L single-element Hg standard to prepare a 10 µg/L working calibration standard for cold vapor atomic fluorescence spectrometry (CVAF). After two weeks of daily use, the working standard shows a 15% decline in measured response compared to freshly prepared standards. The working standard is stored at 4°C in an amber HDPE bottle with 0.5% HNO₃. What is the MOST likely cause of degradation, and what is the correct remediation?

Accepted Answer

Mercury is volatilizing from the solution as elemental Hg⁰ due to photoreduction by ambient light; increase HNO₃ to 2% and add 0.05% K₂Cr₂O₇ as an oxidizing preservative

Answer

Mercury in dilute aqueous solutions is highly susceptible to photoreduction: Hg²⁺ is reduced to volatile Hg⁰ by UV/visible light, especially at low concentrations and in dilute acid matrices. Ambient laboratory lighting through an amber bottle can still cause this over time. The standard remediation for Hg CVAF working standards is to increase nitric acid to at least 2% and add a small amount of potassium dichromate (K₂Cr₂O₇, typically 0.05–0.1%) as an oxidizing agent to keep mercury in the Hg²⁺ oxidation state and prevent photoreduction. HDPE is actually preferred over glass for Hg (glass adsorbs Hg more strongly), and bacterial activity at 4°C is negligible for this mechanism.

Question 5

When preparing a SPEX multi-element standard for ICP-MS analysis at 1 µg/L in a 2% HNO₃ matrix, an analyst notices that the measured signal for ⁵¹V (vanadium) is approximately 30% higher than expected based on the certified concentration. The instrument's internal standard (⁴⁵Sc) shows normal recovery at 98%. Which polyatomic interference is MOST likely responsible, and what is the best instrumental correction approach?

Accepted Answer

³⁵Cl¹⁶O⁺ at m/z 51 from residual chloride in the nitric acid; use collision cell technology (CCT) with helium to dissociate the polyatomic ion

Answer

³⁵Cl¹⁶O⁺ (m/z = 35 + 16 = 51) is the most common and significant polyatomic interference on ⁵¹V in ICP-MS. Even trace chloride contamination in reagents or from the sample matrix can produce substantial ClO⁺ signal at m/z 51. The scandium internal standard at m/z 45 would not correct for this because the interference is matrix-specific rather than a global sensitivity change. The preferred solution is collision cell technology (CCT) with helium as the collision gas, which reduces polyatomic ions through kinetic energy discrimination without the reaction byproduct issues of hydrogen-based reaction gases for this specific interference. Cool plasma can reduce some interferences but is not effective for ClO⁺.

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SPEX Exam SPEX Analysis Exam 6