Questions: Calibration Strategies: External Standards, Internal Standards, and Standard Addition

Question 1

An analyst measures lead in river water using ICP-MS and finds that the river water suppresses the lead signal by 30% compared to aqueous standards. Which calibration method best addresses this?

• A. External standard method with aqueous standards
• B. Method of standard addition
• C. Blank subtraction followed by external standards
• D. Increasing the number of calibration points

Answer: (B)

Standard addition overcomes matrix effects by spiking known amounts of analyte directly into the sample itself. The matrix suppression affects both the native analyte and the spikes equally, so the slope of signal vs. added concentration reflects the true sensitivity in that matrix. Extrapolating back gives the original concentration. External standards prepared in clean water give the wrong slope because they are not affected by the suppressive matrix.

Question 2

The limit of detection (LOD) for an analytical method is the lowest concentration point on the calibration curve.

• T. True
• F. False

Answer: False

The LOD is calculated from blank statistics, not from the calibration range. Specifically, LOD = 3σ_blank / slope, where σ_blank is the standard deviation of replicate blank measurements and slope is the calibration sensitivity. The LOD is typically far below the lowest calibration standard and represents the smallest signal distinguishable from noise at roughly 99% confidence — a property of the method's precision, not of how many standards were prepared.

Question 3

An internal standard corrects for variable injection volumes in chromatography. Why does using the analyte-to-internal-standard signal ratio outperform using the raw analyte signal alone?

Model answer: If injection volume varies, both the analyte peak and the internal standard peak change by the same factor. Taking the ratio cancels that variation: (k × analyte) / (k × IS) = analyte/IS, which is unchanged even when absolute signals fluctuate. The ratio is a stable measure of relative concentration as long as the internal standard behaves identically to the analyte throughout the procedure.

Internal standards work because they experience all the same procedural variations as the analyte — injection volume, instrument drift, extraction efficiency — and therefore track those variations. By normalizing to the internal standard, systematic multiplicative errors cancel. This is why the internal standard must be chemically similar to the analyte and added at a constant amount to every sample and standard.