CPC Chemical Instrumentation & Calibration 2 — Questions and Answers
Question 1: In ICP-OES, which type of interference occurs when an analyte emission line overlaps with a line from a matrix element?
- Chemical interference
- Spectral interference (Correct answer)
- Physical interference
- Ionization interference
Correct answer: Spectral interference
Spectral interference in ICP-OES occurs when emission lines of different elements overlap, causing inaccurate analyte signal readings.
Question 2: What is the purpose of a Certified Reference Material (CRM) in analytical chemistry?
- To clean instrument optics before use
- To provide a matrix-matched blank solution
- To establish traceability of measurements to SI units (Correct answer)
- To calibrate the injection volume of autosamplers
Correct answer: To establish traceability of measurements to SI units
CRMs provide metrological traceability by linking measurements to SI units through an unbroken chain of calibrations.
Question 3: Which detector is most commonly used in modern gas chromatography for the detection of halogenated compounds at trace levels?
- Flame ionization detector (FID)
- Thermal conductivity detector (TCD)
- Electron capture detector (ECD) (Correct answer)
- Nitrogen-phosphorus detector (NPD)
Correct answer: Electron capture detector (ECD)
The ECD is highly sensitive to electronegative halogenated compounds because they capture electrons from a radioactive source, reducing the standing current.
Question 4: In UV-Vis spectrophotometry, what does the term 'stray light' refer to?
- Light reflected off cuvette walls back into the detector
- Radiation reaching the detector at wavelengths other than those selected by the monochromator (Correct answer)
- UV radiation leaking through visible-light filters
- Fluorescence emitted by the sample interfering with absorbance
Correct answer: Radiation reaching the detector at wavelengths other than those selected by the monochromator
Stray light is any radiation outside the selected bandpass that reaches the detector, causing errors particularly at high absorbance values.
Question 5: A pH meter is calibrated with buffers at pH 4.00 and 7.00. When measuring a sample at pH 10, what is the primary concern?
- Junction potential errors are eliminated at two-point calibration
- The calibration range does not bracket the sample pH, reducing accuracy (Correct answer)
- The slope correction will be automatically applied by the firmware
- Buffer ionic strength will cause a positive bias in the reading
Correct answer: The calibration range does not bracket the sample pH, reducing accuracy
Measurements outside the calibrated range are extrapolations and less accurate; a three-point calibration including pH 10.00 buffer should be used for alkaline samples.
Question 6: In atomic absorption spectroscopy (AAS), what is the role of the hollow cathode lamp?
- To atomize the sample in a high-temperature flame
- To emit narrow-line radiation characteristic of the analyte element (Correct answer)
- To ionize the analyte atoms for detection
- To modulate the background continuum for Zeeman correction
Correct answer: To emit narrow-line radiation characteristic of the analyte element
The hollow cathode lamp emits element-specific narrow spectral lines that are absorbed by ground-state atoms of the same element in the atomizer.
Question 7: Which calibration approach corrects for both instrument drift and matrix effects by adding a known amount of analyte directly to the sample?
- External standard calibration
- Internal standard calibration
- Standard addition method (Correct answer)
- Isotope dilution calibration
Correct answer: Standard addition method
The standard addition method adds known analyte increments to the sample itself, so matrix effects and drift affect both analyte and spikes equally, allowing accurate quantitation.
CPC Chemical Instrumentation & Calibration 2
In ICP-OES, which type of interference occurs when an analyte emission line overlaps with a line from a matrix element?