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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
Q

What is the most universal detector type (s)?

A

MS (tunable for any species)

Thermal conductivity

26
Q

what types of compounds can be analyzed with electron capture?

A

halogenated compounds

27
Q

Eletrolytic conductivity detectors can detect what types of compounds?

A

compounds with halogens, sulfur, or nitrogen

28
Q

compounds ionized by UV radiation can be detected by ____ detectors

A

photoionization

29
Q

FTIR is compatible with ____ compounds

A

organic

30
Q

What is known as the ‘universal detector?’ how does it work?

A

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
Q

What does FID detect? How?

A

detects carbon

combust organic molecules in flame -> generates ions -> collect ions -> produce current (proportional to # of C)

32
Q

What does ECD detect? How?

A

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
Q

What is the common B-source in ECD? What does it do?

A

radionuclide 63 Ni

emit electrons to ionize makeup gas

34
Q

main parts of ECD:

A 
ECD Cell:
B-source (Nickel 63 foil)
Carrier gas
makeup gas
electron cloud
ANODE + CATHODE

Frequency/voltage converter

35
Q

ECD is insensitive to:

A

amines
alcohols
hydrocarbons

36
Q

T/F: ECD is expensive

A

False; relatively cheap

37
Q

What is olfactometry?

A

detection of odors; using human assessors (sniff gases)

38
Q

is olfactormetry detection qualitative or quantitative?

A

qualitative

39
Q

What is the ‘peak valley criterion?”

A

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
Q

What is ‘aquisition speed?’

A

how frequent a signal is measured in the detector (# data points in a period of time)

41
Q

How does aquisition speed requirements differ for identification vs quantification purposes?

A

identification: 10 points per peak is sufficient
quantification: 15-20 points per peak recommended

42
Q

The (greater/less) the aquisition speed, the more precise the peak shape

A

greater

43
Q

How does aquisition speed and run time compare for HPLC, UHPLC, and ultra-fast UHPLC?

A

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
Q

Is IDL the same thing as sensitivity?

A

no

45
Q

What is IDL?

A

Instrument detection limit

smallest amount of material detectable (3 sigma criterion) in matrix (express in atomic/mole/weight fraction)

46
Q

IDL is expressed by the ____ ratio. What is the cutoff?

A

S/N (signal to noise)

S/N >/= 3

47
Q

What is the ‘dynamic range?’

A

ratio between max usable indication and minimum usable indication (IDL)

48
Q

What happens to the signal/instrument response as the concentration is increased?

A
  1. signal increases linearly (up to a point)
  2. eventually starts plateauing (non-linear range)
  3. saturation (max signal already reached)
49
Q

Compare linear dynamic range vs dynamic range:

A

linear dynamic range: @ lower concentrations; response directly proportional to conc

dynamic range: INCLUDES linear portion +non-linear response at higher conc

50
Q

What happens at concentrations above the dynamic range?

A

saturation

51
Q

In the linear dynamic range, response = ____ x ______

A

K (response factor) x concentration

52
Q

What types of detectors can be used as serial detectors?

A

non-destructive

53
Q

considerations for selecting a detector:

A 
compatibility w/ sample matrix and mobile phase
acquisition speed
sensitivity
selectivity
linearity
destructive vs. nondestructive
stability
reproducibility
chemical usage &amp; cost
54
Q

What is sensitivity?

A

quantity of analyte in sample vs. noise

Separation Techniques (12 decks)

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