Detectors Flashcards
IUPAC definition of a chromatographic detector:
a device that measures change of composition of the effluent
True/False: a detector can provides both qualitative and quantitative information
true
What qualitative info can be given by the detector?
chemical identification (from retention time + detector specific criteria - wavelength, m/z, etc…)
What quantitative information can be given by the detector?
mass/concentration of chemicals entering
signal intensity vs amount
what separation techniques are usually coupled to detectors?
LC
ion chromatogaphy
capillary electrophoresis
GC
Typical HPLC detectors (commerically available): (8)
absorbance fluorescence electrochemical refractive index conductivity MS FTIR light scattering
3 HPLC detectors that are not commercially available:
optical activity
element selective
photoionization
which detector has the lowest mass LOD?
fluorescence (10 fg)
typical mass LOD in detectors?
10 fg to 1 microgram
The detectors with the highest mass LOD?
FTIR
light scattering
T/F: MS detectors have a higher LOD typically than absorbance detectors
False
absorbance: 10pg
MS: <1pg
In order to be compatible with absorbance detectors, the analyte requires a ______
chromophore
molecule that absorbs UV/vis radiation
T/F: vanillic acid can be analyzed with absorbance
True (it is a chromophore; has a ring)
range of UV-Vis:
190nm - 600nm
What is the law (relationship) that is needed for using absorbance detectors for quantification?
Beer’s Law
A = ELC
A (absorbance)
E (molar extinction coefficient)
L (path length)
C (molar concentration)
What are the various system types for absorbance UV-vis detectors?
- one fixed wavelength (1 setting)
- one variable wavelength (can adjust wavelength)
- multiple variable wavelengths (DAD/PDA)
examples of compounds analyzed with absorbance detectors:
proteins/peptides anthocyanins + other flavonoids/phenolics carotenoids colorants water-soluble vitamins preservatives mycotoxins
What is the principle of fluorescence detectors?
measures OPTICAL EMISSION of light by molecules, after they are excited at higher energy wavelength
Compounds that naturally exhibit fluorescent properties have ____ fluorescence. Can compounds without this property still be used with a fluorescence detector?
native fluorescence
Yes; but need to derivatize into a fluorescent compound first
What does a conductivity detector measure?
electric conductivity (presence of ions) of the solution eluting from the column
2 electrodes from detector measure solution
Conductivity detectors are commonly used for what compounds?
inorganic ions, small organic substances (organic acids/amines)
disadvantages of conductivity detectors: (3)
limited working range
low sensitivity/specificity
strong dependence on temperature
examples of gas-chromatography coupled detectors: (8)
flame ionization thermal conductivity electron capture MS thermionic electrolytic conductivity (hall) photoionization FTIR
What samples are applicable for flame ionization detectors?
hydrocarbons
What is the most universal detector type (s)?
MS (tunable for any species)
Thermal conductivity
what types of compounds can be analyzed with electron capture?
halogenated compounds
Eletrolytic conductivity detectors can detect what types of compounds?
compounds with halogens, sulfur, or nitrogen
compounds ionized by UV radiation can be detected by ____ detectors
photoionization
FTIR is compatible with ____ compounds
organic
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
What does FID detect? How?
detects carbon
combust organic molecules in flame -> generates ions -> collect ions -> produce current (proportional to # of C)
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)
What is the common B-source in ECD? What does it do?
radionuclide 63 Ni
emit electrons to ionize makeup gas
main parts of ECD:
ECD Cell: B-source (Nickel 63 foil) Carrier gas makeup gas electron cloud ANODE + CATHODE
Frequency/voltage converter
ECD is insensitive to:
amines
alcohols
hydrocarbons
T/F: ECD is expensive
False; relatively cheap
What is olfactometry?
detection of odors; using human assessors (sniff gases)
is olfactormetry detection qualitative or quantitative?
qualitative
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)
What is ‘aquisition speed?’
how frequent a signal is measured in the detector (# data points in a period of time)
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
The (greater/less) the aquisition speed, the more precise the peak shape
greater
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
Is IDL the same thing as sensitivity?
no
What is IDL?
Instrument detection limit
smallest amount of material detectable (3 sigma criterion) in matrix (express in atomic/mole/weight fraction)
IDL is expressed by the ____ ratio. What is the cutoff?
S/N (signal to noise)
S/N >/= 3
What is the ‘dynamic range?’
ratio between max usable indication and minimum usable indication (IDL)
What happens to the signal/instrument response as the concentration is increased?
- signal increases linearly (up to a point)
- eventually starts plateauing (non-linear range)
- saturation (max signal already reached)
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
What happens at concentrations above the dynamic range?
saturation
In the linear dynamic range, response = ____ x ______
K (response factor) x concentration
What types of detectors can be used as serial detectors?
non-destructive
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
What is sensitivity?
quantity of analyte in sample vs. noise
