Questions: Inductively Coupled Plasma Spectrometry (ICP-OES and ICP-MS)

Question 1

An analyst needs to measure arsenic (⁷⁵As) in a seawater sample by ICP-MS. The results are consistently higher than expected. What is the most likely cause?

• A. The plasma temperature is too low to fully ionize arsenic
• B. ⁴⁰Ar³⁵Cl⁺ forms in the plasma and registers at mass 75, overlapping with ⁷⁵As⁺
• C. Arsenic evaporates before reaching the plasma
• D. ICP-MS cannot measure arsenic because it is a metalloid, not a metal

Answer: (B)

Seawater contains high chloride concentrations. In the argon plasma, argon combines with chloride to form ⁴⁰Ar³⁵Cl⁺, a polyatomic ion with mass 75 — identical to ⁷⁵As⁺ in the mass spectrum. This is a classic polyatomic interference in ICP-MS and a major challenge for arsenic determination in chloride-rich matrices. Solutions include collision/reaction cells (which break up or react away the polyatomic ion), high-resolution instruments (which resolve the 0.02 Da mass difference), or cool-plasma conditions. This type of interference does not affect ICP-OES, which separates by emission wavelength rather than mass.

Question 2

A laboratory runs a soil digest by ICP-MS and obtains signals that are 30% lower than expected based on calibration standards. No instrument malfunction is found. What is the most likely explanation?

• A. The soil digest contains too many elements, saturating the detector
• B. High total dissolved solids in the digest suppress the analyte signal
• C. The collision cell is removing analyte ions along with interferences
• D. ICP-MS cannot analyze soil samples because of particulate matter

Answer: (B)

High TDS (total dissolved solids) is a primary cause of matrix suppression in ICP-MS. Excess dissolved material affects nebulization efficiency, deposits on the sampling cone, and alters ion transport through the interface — all of which reduce signal for analyte ions. The solution is matrix matching (calibrate in the same matrix), internal standardization (add elements absent from the sample to correct for signal variability), or diluting the sample. This is a fundamental operational challenge that must be addressed before ICP-MS data can be trusted quantitatively.

Question 3

ICP-MS can determine the chemical speciation of arsenic in a sample — for example, distinguishing toxic arsenite (As³⁺) from less-toxic arsenate (As⁵⁺).

• T. True
• F. False

Answer: False

ICP-MS detects ions by mass-to-charge ratio and measures the total amount of an element — it cannot distinguish between different chemical forms of the same element. Both arsenite and arsenate produce ⁷⁵As⁺ ions in the plasma and are indistinguishable by mass. Speciation requires coupling ICP-MS to a separation technique — typically HPLC or ion chromatography — that separates the chemical species before they enter the plasma. This hybrid technique (HPLC-ICP-MS) is now standard for arsenic speciation in environmental and food safety analysis.

Question 4

ICP-OES allows multiple elements to be measured simultaneously in a single sample run, which is one of its main advantages over flame atomic absorption spectroscopy.

• T. True
• F. False

Answer: True

This is one of the defining advantages of ICP-OES over AAS. Flame AAS measures one element at a time, requiring separate lamp changes and multiple sample introductions. ICP-OES uses a polychromator or array detector to capture emission lines across a wide wavelength range simultaneously, allowing 20–70 elements to be quantified from a single one-minute sample run. This multielement capability dramatically improves throughput for environmental, geological, and food analysis where many elements must be screened.

Question 5

ICP-MS achieves far lower detection limits than ICP-OES for most elements. What is the fundamental reason for this, and what is the main analytical trade-off?

Model answer: ICP-MS extracts ions from the plasma and detects them by mass spectrometry, which counts individual ions with extremely high efficiency — achieving parts-per-trillion (ng/L) detection limits. ICP-OES measures emitted photons against a background of other emission, limiting detection to parts-per-billion. The trade-off is polyatomic interferences: argon and matrix elements form molecular ions (e.g., ArCl⁺, ArO⁺) at masses that overlap with analyte ions, requiring collision/reaction cells or high-resolution instruments to resolve.

The ~1000-fold better detection limit of ICP-MS over ICP-OES comes from the fundamental difference in detection: ion counting vs. photon measurement against a noisy optical background. But this sensitivity comes at a cost. The plasma not only ionizes analyte elements — it also forms new polyatomic species that ICP-OES never has to worry about (optical emission lines rarely overlap with molecular emission bands the same way). Managing these interferences is the central analytical challenge of ICP-MS method development.