241 Exam 2 Detectors

What are the 3 most common GC detectors?

ECD — Electron Capture Detector | FID — Flame Ionization Detector | TCD — Thermal Conductivity Detector

What are the 3 most common LC detectors?

UV-VIS Absorbance | Refractive Index (RI) | Fluorescence

Which detectors are used for GC? Which are used for LC?

GC: ECD / FID / TCD. LC: UV-VIS / RI / Fluorescence. Both GC and LC: Mass Spectrometry (MS)

Which is the most commonly used GC detector?

FID (Flame Ionization Detector)

Which is the most commonly used LC detector?

UV-VIS Absorbance

Rank the 3 common GC detectors from MOST to LEAST sensitive.

ECD — most sensitive | FID — middle | TCD — least sensitive

Rank the 3 common LC detectors from MOST to LEAST sensitive.

Fluorescence — most sensitive | UV-VIS — middle | RI — least sensitive

Which GC detector is the most sensitive?

ECD (Electron Capture Detector)

Which LC detector is the most sensitive?

Fluorescence

Which LC detector is the least sensitive?

RI (Refractive Index)

Which GC detector is the least sensitive?

TCD (Thermal Conductivity Detector)

Which of the 3 common GC detectors is universal?

TCD — it detects any compound with a different thermal conductivity than the carrier gas

Which of the 3 common LC detectors is universal?

RI — it detects any compound with a different refractive index than the mobile phase

Is the FID universal?

No — it only detects organic (carbon-hydrogen containing) compounds. It cannot detect water or CO₂ or inorganic gases etc.

Is the ECD universal?

No — it is highly selective. It only detects electronegative compounds (halogens and nitro groups etc.)

Is the UV-VIS detector universal?

No — it only detects compounds that have a chromophore (a UV/Vis-absorbing functional group)

Is the fluorescence detector universal?

No — it only detects compounds that fluoresce (or have been tagged with a fluorescent label)

What does "universal" mean for a detector?

It responds to ALL compounds regardless of their chemical structure or functional groups

What is a chromophore?

A functional group that absorbs UV or visible light. Examples: aromatic rings / C=O (carbonyl) / conjugated double bonds / nitro groups (–NO₂)

What is required for a compound to be detected by the fluorescence detector?

The compound must fluoresce — it must absorb light at an excitation wavelength (λex) and emit light at a longer emission wavelength (λem)

Which of the 3 common GC detectors is destructive?

FID — it burns the sample in a hydrogen/air flame

Which of the 3 common GC detectors are NON-destructive?

ECD and TCD — the sample passes through without being chemically destroyed

Are the 3 common LC detectors (UV-VIS / RI / Fluorescence) destructive or non-destructive?

All three are non-destructive — the sample passes through the detector flow cell unchanged

How does the TCD work?

Measures the thermal conductivity of the gas stream. When an analyte exits the column it changes the thermal conductivity of the gas → changes how fast heat is lost from a heated filament → resistance change is measured as the signal

How does the FID work?

Burns the column effluent in a hydrogen/air flame. Organic compounds produce ions and electrons when burned → these are collected at an electrode → the resulting current is the signal

How does the ECD work?

A radioactive ⁶³Ni source creates a constant stream of electrons (standing current). Electronegative compounds entering the detector capture electrons → reduce the current → the drop in current is the signal

How does the UV-VIS detector work?

Shines UV or visible light through a flow cell. Compounds with a chromophore absorb some of the light. The detector measures how much light is absorbed (Beer-Lambert Law: A = εbc). More analyte = more absorbance

How does the RI detector work?

Compares the refractive index of the column effluent (sample cell) to pure mobile phase (reference cell). When an analyte is present it changes the RI of the mobile phase → a light beam is deflected differently → deflection is measured as the signal

How does the fluorescence detector work?

Shines excitation light (λex) on the sample. Fluorescent compounds absorb the light and re-emit light at a longer wavelength (λem). This emitted light is detected at 90° to the excitation beam. More analyte = more emitted light

Which LC detector CANNOT be used with gradient elution?

RI (Refractive Index)

Why can't the RI detector be used with gradient elution?

Because changing the mobile phase composition changes the RI of the mobile phase itself → creates a constantly drifting baseline → impossible to distinguish the baseline drift from an actual analyte signal

Which LC detectors CAN be used with gradient elution?

UV-VIS and Fluorescence (and MS and EC and ELSD)

What is the ECD best for detecting?

Electronegative compounds — especially halogenated compounds (Cl / F / Br / I) and nitro groups (–NO₂). Classic use: chlorinated pesticides and PCBs

What is the FID best for detecting?

Virtually all organic (carbon-hydrogen containing) compounds — hydrocarbons / alcohols / drugs / petroleum products etc.

What is the TCD best for detecting?

Any compound but especially used when you need to detect inorganic gases (N₂ / CO₂ / CO / O₂ / H₂O) that FID cannot detect

What is the RI detector best for detecting in LC?

Compounds with no UV chromophore and no fluorescence — especially sugars / carbohydrates / polymers / lipids

What is the fluorescence detector best for?

Trace-level detection of naturally fluorescent compounds (PAHs / certain drugs / biological molecules like tryptophan and riboflavin). Best choice when maximum sensitivity is needed

What is the UV-VIS detector best for?

General-purpose LC detection of any compound with a chromophore. Best balance of sensitivity and practicality and versatility for everyday LC work

Universal vs. most commonly used — which GC detector is universal and which is most commonly used? Are they the same?

No — they are different. Universal: TCD. Most commonly used: FID. TCD is universal but has the worst sensitivity. FID is most used because it's sensitive and responds to nearly all organics but is NOT truly universal.

Universal vs. most commonly used — which LC detector is universal and which is most commonly used? Are they the same?

No — they are different. Universal: RI. Most commonly used: UV-VIS. RI is universal but has the worst sensitivity and can't be used with gradients. UV-VIS is most used because it's more practical.

ECD vs. FID — which is more sensitive? Which detects more compound types?

More sensitive: ECD (femtogram range vs. nanogram range for FID). Detects more compound types: FID (nearly all organics vs. only electronegative compounds for ECD)

UV-VIS vs. Fluorescence — which is more sensitive? Which is more universal?

More sensitive: Fluorescence (100–1000× lower detection limits). More universal: UV-VIS (more compounds have chromophores than fluoresce)

If a compound has BOTH a chromophore and fluoresces which LC detector should you use for trace-level analysis and why?

Fluorescence — because it is far more sensitive (detects against a dark background vs. measuring a small decrease in a large light beam). Better signal-to-noise = lower detection limits

Why is fluorescence detection more sensitive than UV-VIS detection?

UV-VIS measures a small decrease in a large beam of light → limited by noise on that large signal. Fluorescence measures light emitted against a dark (near-zero) background → much better signal-to-noise ratio → far lower detection limits

A sample has no UV chromophore and does not fluoresce. Which LC detector do you use?

RI (Refractive Index) — the only common LC detector that doesn't require a chromophore or fluorophore

You need to run an LC gradient. Which detector is NOT an option?

RI (Refractive Index) — cannot be used with gradient elution

Which GC detector requires a radioactive source? What isotope is used?

The ECD (Electron Capture Detector) uses ⁶³Ni (nickel-63) a beta emitter

Which GC detector is best for detecting trace chlorinated pesticides in an environmental sample?

ECD — chlorinated compounds are highly electronegative (halogenated) → ECD detects them with exceptional sensitivity at trace (ppt) levels

True or False: The FID can detect water and CO₂.

False. The FID cannot detect water or CO₂ or any compound without C–H bonds. Only the TCD (among common GC detectors) can detect these.

True or False: The RI detector can be used with any mobile phase gradient in LC.

False. The RI detector CANNOT be used with gradient elution because changing the MP composition changes the RI of the MP itself making it impossible to detect analytes against the drifting baseline.

For each detector is it Universal (U) or Selective (S)? ECD / FID / TCD / UV-VIS / RI / Fluorescence

ECD → S | FID → S | TCD → U | UV-VIS → S | RI → U | Fluorescence → S

For each detector is it Destructive (D) or Non-Destructive (ND)? ECD / FID / TCD / UV-VIS / RI / Fluorescence

ECD → ND | FID → D | TCD → ND | UV-VIS → ND | RI → ND | Fluorescence → ND

Which single detector among ALL detectors discussed (GC and LC) is the most sensitive overall?

ECD and Fluorescence/LIF compete for this title. ECD detects femtograms of halogenated compounds. LIF in CE can detect single molecules. For practical LC: Fluorescence is most sensitive. For GC: ECD is most sensitive.

What does the Beer-Lambert Law state and which LC detector is it the basis for?

A = εbc (Absorbance = molar absorptivity × path length × concentration). It is the basis for the UV-VIS absorbance detector. Since A is proportional to c the detector has excellent linearity.

What are the 3 CE detectors you need to know?

UV-VIS — most commonly used in CE (on-column detection through capillary window) | Fluorescence/LIF (Laser-Induced Fluorescence) — most sensitive CE detector | Mass Spectrometry (CE-MS) — provides structural identification