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Separation Techniques > Detectors > Flashcards

Detectors Flashcards

1
Q

IUPAC definition of a chromatographic detector:

A

a device that measures change of composition of the effluent

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2
Q

True/False: a detector can provides both qualitative and quantitative information

A

true

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3
Q

What qualitative info can be given by the detector?

A

chemical identification (from retention time + detector specific criteria - wavelength, m/z, etc…)

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4
Q

What quantitative information can be given by the detector?

A

mass/concentration of chemicals entering

signal intensity vs amount

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5
Q

what separation techniques are usually coupled to detectors?

A

LC
ion chromatogaphy
capillary electrophoresis
GC

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6
Q

Typical HPLC detectors (commerically available): (8)

A 
absorbance
fluorescence
electrochemical
refractive index
conductivity
MS
FTIR
light scattering
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7
Q

3 HPLC detectors that are not commercially available:

A

optical activity
element selective
photoionization

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8
Q

which detector has the lowest mass LOD?

A

fluorescence (10 fg)

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9
Q

typical mass LOD in detectors?

A

10 fg to 1 microgram

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10
Q

The detectors with the highest mass LOD?

A

FTIR

light scattering

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11
Q

T/F: MS detectors have a higher LOD typically than absorbance detectors

A

False
absorbance: 10pg
MS: <1pg

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12
Q

In order to be compatible with absorbance detectors, the analyte requires a ______

A

chromophore

molecule that absorbs UV/vis radiation

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13
Q

T/F: vanillic acid can be analyzed with absorbance

A

True (it is a chromophore; has a ring)

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14
Q

range of UV-Vis:

A

190nm - 600nm

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15
Q

What is the law (relationship) that is needed for using absorbance detectors for quantification?

A

Beer’s Law

A = ELC

A (absorbance)
E (molar extinction coefficient)
L (path length)
C (molar concentration)

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16
Q

What are the various system types for absorbance UV-vis detectors?

A
  1. one fixed wavelength (1 setting)
  2. one variable wavelength (can adjust wavelength)
  3. multiple variable wavelengths (DAD/PDA)
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17
Q

examples of compounds analyzed with absorbance detectors:

A 
proteins/peptides
anthocyanins + other flavonoids/phenolics
carotenoids
colorants
water-soluble vitamins
preservatives
mycotoxins
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18
Q

What is the principle of fluorescence detectors?

A

measures OPTICAL EMISSION of light by molecules, after they are excited at higher energy wavelength

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19
Q

Compounds that naturally exhibit fluorescent properties have ____ fluorescence. Can compounds without this property still be used with a fluorescence detector?

A

native fluorescence

Yes; but need to derivatize into a fluorescent compound first

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20
Q

What does a conductivity detector measure?

A

electric conductivity (presence of ions) of the solution eluting from the column

2 electrodes from detector measure solution

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21
Q

Conductivity detectors are commonly used for what compounds?

A

inorganic ions, small organic substances (organic acids/amines)

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22
Q

disadvantages of conductivity detectors: (3)

A

limited working range
low sensitivity/specificity
strong dependence on temperature

23
Q

examples of gas-chromatography coupled detectors: (8)

A 
flame ionization
thermal conductivity
electron capture
MS
thermionic
electrolytic conductivity (hall)
photoionization
FTIR
24
Q

What samples are applicable for flame ionization detectors?

A

hydrocarbons

25
What is the most universal detector type (s)?
MS (tunable for any species) | Thermal conductivity
26
what types of compounds can be analyzed with electron capture?
halogenated compounds
27
Eletrolytic conductivity detectors can detect what types of compounds?
compounds with halogens, sulfur, or nitrogen
28
compounds ionized by UV radiation can be detected by ____ detectors
photoionization
29
FTIR is compatible with ____ compounds
organic
30
What is known as the 'universal detector?' how does it work?
thermal conductivity detector (TCD) wire/thermistor measure thermal conductivity of 2 gas flows (pure reference gas vs mobile phase from GC) the power needed to keep the wire temp. constant will change in presence of the sample
31
What does FID detect? How?
detects carbon combust organic molecules in flame -> generates ions -> collect ions -> produce current (proportional to # of C)
32
What does ECD detect? How?
electronegative halogen-containing organics B-source (63 Ni) emit electrons -> ionize make-up gas -> more electrons electrons accelerate towards ANODE (create current) electroneg. groups in sample will capture/absorb electrons -> decrease current (conc. is proportional to degree of electron capture)
33
What is the common B-source in ECD? What does it do?
radionuclide 63 Ni | emit electrons to ionize makeup gas
34
main parts of ECD:
``` ECD Cell: B-source (Nickel 63 foil) Carrier gas makeup gas electron cloud ANODE + CATHODE ``` Frequency/voltage converter
35
ECD is insensitive to:
amines alcohols hydrocarbons
36
T/F: ECD is expensive
False; relatively cheap
37
What is olfactometry?
detection of odors; using human assessors (sniff gases)
38
is olfactormetry detection qualitative or quantitative?
qualitative
39
What is the 'peak valley criterion?"
if valley height (between 2 peaks) is <25% or 50% of height of smaller peak, then integrate them by dividing where the valley is (separate peaks) otherwise, integrate together (as 1 peak)
40
What is 'aquisition speed?'
how frequent a signal is measured in the detector (# data points in a period of time)
41
How does aquisition speed requirements differ for identification vs quantification purposes?
identification: 10 points per peak is sufficient quantification: 15-20 points per peak recommended
42
The (greater/less) the aquisition speed, the more precise the peak shape
greater
43
How does aquisition speed and run time compare for HPLC, UHPLC, and ultra-fast UHPLC?
HPLC: 70 bar (lower pressure); longer runtime, slower aquisition speed UHPLC: 600 bar, faster, need faster aquisition speed ultra-fast UHPLC: 970 bar, fastest (<1 min); need fastest aquisition speed
44
Is IDL the same thing as sensitivity?
no
45
What is IDL?
Instrument detection limit smallest amount of material detectable (3 sigma criterion) in matrix (express in atomic/mole/weight fraction)
46
IDL is expressed by the ____ ratio. What is the cutoff?
S/N (signal to noise) | S/N >/= 3
47
What is the 'dynamic range?'
ratio between max usable indication and minimum usable indication (IDL)
48
What happens to the signal/instrument response as the concentration is increased?
1. signal increases linearly (up to a point) 2. eventually starts plateauing (non-linear range) 3. saturation (max signal already reached)
49
Compare linear dynamic range vs dynamic range:
linear dynamic range: @ lower concentrations; response directly proportional to conc dynamic range: INCLUDES linear portion +non-linear response at higher conc
50
What happens at concentrations above the dynamic range?
saturation
51
In the linear dynamic range, response = ____ x ______
K (response factor) x concentration
52
What types of detectors can be used as serial detectors?
non-destructive
53
considerations for selecting a detector:
``` compatibility w/ sample matrix and mobile phase acquisition speed sensitivity selectivity linearity destructive vs. nondestructive stability reproducibility chemical usage & cost ```
54
What is sensitivity?
quantity of analyte in sample vs. noise

Separation Techniques (12 decks)

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