DETECTORS

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DETECTORS
As the name implies the detector detects the
presence of compound/s after the compound/s
presence in the gas or liquid stream is eluted
from the column

• Importance Detector Characteristics
• Sensitivity ------? implies detector response
  with the
• change in the concentration/amount of the
  analyte.

-?slope of the calibration curve. Which detector
in the Figure to the right is the most
sensitive?
2. Universal Response ---? implies detector
respond to any Component that is passed through
the column (e.g., TCD detector in GC)
3. Fast Response ----? implies how quickly
detector responds to the presence of analyte .
Sharper the peak, faster the response

• Linear Dynamic Range--? The detector response is
  said to be linear if the
• difference in response for two or more
  concentrations of a given compound is
• Proportional to the difference in concentration
  of the two sample. Such response
• appears as a straight line in the calibration
  curve

The linear dynamic range of a detector is the
maximum linear response divided by the detector
noise. Most detectors eventually become
non-linear as sample size is increased and this
upper point is usually well defined. The
chromatographer should know where this occurs to
avoid errors in quantification. Linear range
1M/10-6M 107
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5. Good Stability-----? implies less electronic
noise and less baseline drift
Types of Noise Short term noise consists of
baseline perturbations that have a frequency that
is significantly higher than that of the eluted
peak. Short term detector noise can be easily
removed by appropriate noise filters without
affecting the profiles of the peaks. Its source
is usually electronic
Long term noise is perturbations that have a
frequency that is similar to that of the eluted
peak. Long term noise is the most damaging as it
is indiscernible from very small peaks in the
chromatogram.
Long term noise cannot be removed by electronic
filtering. In figure 13, the peak profile can be
discerned from the high frequency noise, but not
from the long term noise. Long term noise arises
from temperature, pressure and flow rate changes
in the sensing cell or irregular column bleed.
Long term noise ultimately determines the limits
of detector sensitivity or the minimum detectable
concentration.
Drift results from baseline perturbations that
have a frequency much smaller than that of the
eluted peak. Drift can be due to changes in
ambient temperature, changes in mobile flow rate,
or column bleed in GC in LC drift can be due to
pressure changes, flow rate changes or
variations in solvent composition. A combination
of all three sources of noise is shown by the
lowest in the above Figure
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Background Noise ----? implies signal produced
by the detector in the absence of a sample or
solute
Background noise is the constant signal apparent
above a zero level baseline in the chromatograph
chart. The sensitivity of a GC analysis is
judged by the signal to noise ratio (S/N) If
the background noise increases, this in turn
reduces the sensitivity of analysis. -?major
source of GC chromatogram background noise is
from the GC injection port and its various
components. GC septa have long been recognized
as an important source of GC background noise.
Manufacturers have developed and now offer a
wide variety of Low bleed septa to reduce this
source of background noise. Manufacturers of
these septa have also developed elaborate
cleaning and conditioning procedures to
minimize septa bleed.
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• Reliability of a detector -? implies how rugged
  the detector is for longer period
• of use. Can we use detector for longer period of
  time without having baseline drift

• Ease of Operation--? implies how easy it is to
  operate and understand a
• detector without having baseline drift

Some of these characteristics are complementary
to each other High senstivity
------------------? Low background noise, fast
response Hiigh stability ---------------------?
Low background noise
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Signal The detector output is called
Singal. Magnitude of signal is measured as (a)
peak area and (b) peak height which is
proportional to concentration of analyte
Noise is the signal produced by the detector in
the absence of a a sample. Noise is Caused by
random fluctuations,electronic detector
component, stray light, dust contaminations
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Detection Limits and Signal-to-Noise Ratio
-?aka.limit of detection (LOD) is the lowest
concentration in a sample that can be detected,
but not necessarily quantitated under the
experimental conditions.

• -? Since the LOD is dependent on the S/N, it can
  be improved by enhancing the analyte signal or
  reducing the detector noise
• The signal can be improved by increasing the peak
  sharpness (efficiency).
• increasing the injection volume or mass injected
• The signal in GC can be increased by working at
  optimum flow rate and high temp (without column
  bleed)
• The signal in HPLC can be increased by optimizing
  the mobile phase (liquid) composition or longer
  path lengths (if UV detector is used)

?Noise can be reduced by (a) high sensitivity
detector with low noise, (b) low drift (c)
slower detector response, (d) mobile phase with
low UV absorbances and pump with low pulsation in
HPLC
Unit of LOD GC g/sec (mass flow) or
g/mL(conc flow) HPLC mM or mg/mL
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How Minimum Detectable Levels be Measured
without doing more experiments?
--Suppose the peak corresponds to its height (S)
and width of the peak at the base 20s Area of
the peak represent the concentration 570
pg Signal S 2A/Wb 2 x 570 pg/20 57 pg
If the S/N of the peak shown above is 18, what is
the mass flow rate corresponding to the ratio of
2? For S/N 18 (shown in the
chromatogram) ---? 57 pg of analyte is present
For S/N 2 -------------------? 57/18
x2 6.3pg/sec (MDL)
--- Some detectors (e.g., TCD) that respond to
the concentration of the analyte In the detector
rather than quantity. With these detectors the
MDL is dependent on the column flow rate. For
example
Same quantity of sample is going through as
before but there is less carrier gas diluting
it, so detector performs with higher sensitivity
with 1/8
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Baseline Drift
Slow and constant change of baseline over time

• What causes baseline drift?
• Changes in temperature and changes in flow rate
  of the gas (GC) or liquid (HPLC)
• Impurity in Gas or liquid mobile phases
• c) Incomplet column equilibration

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Time Constant (t) is a measure of the speed of
response of a detector.
More specifically time a detector takes to
respond to a sudden change in a signal. 1 T --?
63 of the full response 4 T ? 98 of the full
response
How do you select the value of Time constant to
be set on the detector? --A typical
recommendation is that time constant should be
less than 10 of the peak width at half height
(wh). Consider a peak with a peak volume of 50
mL then t V/flow rate 50 mL/1000mL/min
0.05 min 3sec

• --So how does it affect the chromatographic data
• that is generated?
• Retention time is difficult to measure accurately
  (because of peak dispersion)
• As the peak width gets wider with increasing T ,
  N decreases

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Sensitivity Defined as signal output per unit
concentration or per unit mass of an analyte in
the carrier gas or liquid M.P
Sensitivity m slope of the calibration plot m
DS/DC
Note that the straight line in the above figure
falls off and become non-linear at
high concentration. The point at which
linearity falls off is called Upper limit of
dynamic range The lowest point of the
concentration (x-axis) represents minimum
detectability
A mixture of 2-compounds were separated and then
detected on two different TCD obtained from two
different manufacturer. Which of the two
detector will provide better LOD?
In chromatogram A ( to the right ) the species
display a larger response than B. However, MDL
will be lower in B (left Figure) than A because
of much lower background noise in B. S/N
decreases with increase in Noise In this case
inrease in Noise gtgt increase in Signal
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Linearity is defined as ability of a detector to
respond directly (or after mathematical
transformation) propor-tional to the
concentration of the analyte. Linearity Upper
limit of linear range
Minimum detectability
Is there a difference between dynamic range and
linear range of a detector?
Yes, there is because dynamic range is not always
the same as linear range. The lower limit of the
dynamic range is the minimum detectability. The
upper limit is the highest concentration
(mass-flow rate) will still give an observable
increase in detector signal, but the signal may
not be within linearity limits. Since the
dynamic range is the ratio of the upper and lower
limits, the dynamic range will be greater than
the linear range
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We learned that the desirable characteristics of
a detector system as follows High sensitivity,
Universal response, Wide linear dynamic
range Good stability, Low background noise,
Reliability, Ease of operation
Q1. Which of the above characteristics do you
consider to be most important in Quantitative
Analysis?
Wide linear dynamic range Good stability because
we do not want peak areas to change from
day-to-day
Environmental Analysis
implies trace work
High Sensitivity Low Background Noise (gives
lower LOD) Good Stability
In a Laboratory with a Very Heavy Work Load
Busy Lab
Ease of operation Relaibality
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Classfication of Chromatographic Detectors
Analog Vs. Digital
Concentration Vs. Mass Flow Rate
Bulk Property Vs. Specific Property
Concentration Vs.Mass Flow Rate
Destructive Vs. Non-destructive
Concentration vs. Mass Flow Rate ---Detector that
produces signal which is proportional to the
concentration (mass/ Volume) of the analyte.
Examples of concentration type detector are In
GC----? Thermal Conductivity Detector (TCD),
Electron Capture Detector (ECD) In HPLC-?
UV-Vis, Fluorescence, Refractive Index,
Conductivity, Amperometric
Mass Flow Rate Type Detector that produces
signal which is proportional to the mass flow
(absolute amount/time) of the analyte
irrespective of the carrier gas volume.
In GC--? Flame Ionization Detector(FID) In
HPLC--? Mass Spectrometer (MS)
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Concentration Vs. Mass Flow Rate Type Detector
Effect of stopping the carrier
Effect of Flow rate on peak sizes gas
flow (M.P) on two types of
on two types of detector detector
TCD FID
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Bulk Property Vs. Specific Property Detector
Bulk Property Detector Constantly measuring a
particular property exhibited by both carrier
gas (M.P) and the analyte.
For example TCD in GC is constantly measuring
TC of carrier gas (M.P) RI detector in HPLC is
constantly measuring the RI of the solvent
(M.P) Therefore, analyte appear in the detector
as -----? D TC or DRI
Example of Bulk Property GC detectors are TCD,
ECD Example of Bulk Property HPLC detectors are
RI
Advantage ---? Can be used to detect all
solutes, qualitative screening of new samples
with unknown composition Disadvantage---?
Insensitive Inherently
Specific Property Detector Produces no signal
when no sample is present (aka analyte property
or solute property.
--Appearance of sample/analyte in a detector
produce a larger signal compared to zero signal
for baseline Example of specific property
detector in GC FID, FPD Example of specific
property detector in HPLC UV-Vis, Fluorescence,
Amperometric
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Selective vs. Universal
---Selective Detector Detector which responds
to a selective type of compound only
For example Electron capture detector (ECD)
responds to all compounds (polychlorinated
biphenyl congeners) in GC capable of capturing
electrons Example of selective HPLC detector is
Fluorescence
---Universal Detector Detect all samples
Example of universal GC detectors are TCD,
FID Example of universal HPLC detectors are RI

Destructive vs. Non-Destructive --Destructive
Detectors Sample cannot be recollected Example
of Destructive GC detector are FID and
MS Example of Destructive HPLC detector are MS
and Light scattering detector
--Non-destructive Detectors Sample can be
recovered. Most of the HPLC detector are
non-destructive.

Analog vs. Digital Analog Detectors
Most detector are analog detector. They
contain signal generated which is digitized
before they can be manipulated by digital
computer. Example of Analog GC detectors are
FID, TCD and ECD Exmaple of Analog HPLC detectors
are UV-Vis, Fluorescence, etc---
Digital Detectors Provide digital response
directly Example Radioactive Detector
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Type of GC Detectors
Other Detectors
Most commonly used GC detector with reducing cost
of MS, it may compete with FID
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Thermal Conductivity Detector
What is Thermal Conductivity (l)? --Ability of a
material to transfer heat when subjected to a
temperature difference
What does Thermal Conductivity Detector
Measures? --Changes in l of the carrier gas
caused by the presence of eluted solute --Since
Dl l analyte -l M.P --Therefore, direction of
peak may be positive or
negative
What type of carrier gas is commonly used with
TCD and why? ---He is the most commonly used
carrier gas of choice when chosing TCD because
of its high thermal conductivity
 
Working (a) Body of detector is thermostated
and two small filaments are mounted on 2 flow
channels (a) Analyte (sensor) filament and (b)
reference filament
Solute elutes from the column and enters the
analytical (sensor) channel, upon entering
the Channel TC of M.P decreases and temperature
of the wire filament increase --? increase
resistance -? decreases thermal conductivity
Reference Cell is used through which only
M.P passess ---? corrects for (see next page)
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What is the purpose of a reference cell in
TCD? ?Reference cell is used through which only
M.P passes and it corrects For (a) variations in
flow rate, (b) electric power surges, (c) carrier
gas Pressure. All of these three factors can
change the filament resistance
We have learned that in TCD response depends on
change in thermal Conductivity (Dl) or changes
in resistance DRf between the carrier gas
molecules and the solute molecules.
Which analyte will provide the lowest LOD using
heliumas carrier gas?
Clearly chloroform
Response Equation STC a C Vmol DT
Cv C analyte concentration DT
temp difference b/w hot filament and cell Cv
specific heat Vmol molecule velocity
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Advantages of TCD Responds to all compounds
(Universal detector) Increasing sensitivity with
decreasing temperature, flow rate and currrent
Good linear and adequate sensitivity for many
compounds Adequate sensitivity to many compounds
Simple cheap robust and easy to
use Nondestructive (posible to isolate solutes
with a post dedector cold trap
Why the peak for H2 is negative?
Disadvantages Poor LOD Less sensitive than FID
by atleast 103 orders of magnitude Poor
senstivity when used in combination
with capillary columns
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Flame Ionization Detector (FID)
CHO radicals are reduced at a cathode and
electrons move towards anode, which produces a
current proportional to the radical quantity.
About 10-12A
Potential difference had to be maintained
between the jet and the collector electrode. But
why?
Burning the analyte in flame generates ions
and elctrons but unless there is a potential
difference between the jet and the collector
electrode no electrons will flow --? and no
ionization current is detected.
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What do you think the sensitivity of FID depends
on?
Ion production is directly proportional to the
number of carbon atoms entering the flame. For
example C6H14 gt C5H12
(A)
Hence, response to specific organic depends on
the number of organic carbons (shown on the right)

• Specific for organic carbon, insensitive to
• inorganics, CO2, SO2 etc (see Figure B on the
  right)

Modified form of FID --FID can be modified to
respond to gases listed In Table B only if we
work in hydrogen-rich mode with oxygen to
support combustion. This Can be accomplished in
most commercial FIDs By (a) introducing O2 with
the carrier gas (b) Introducing H2 fuel using air
inlet This modified form of FID is called
Hydrogen- Atmosphere flame ionization detector
(HAFID) With LOD 10-8---10-11 g/sec
Do you think FID can be useful for analysis of
Atmospheric and aqueous environmental samples?
If yes, why?
Yes, because most of the gases (e.g., H20, CO2,
H2S)
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Advantages of FID
1. Almost universal response to all organic
compounds
Mostly carbon containing compounds except those
with carbonyls and carboxyl gps

• Wide linear dynamic range makes FID an excellent
  detector for accurate
• Quantitative analysis

---Upto 107 orders of magnitude in dynamic range.

• Higher sensitivity Generally DL are 100 x time
  less than the TCD. The DL Is about 10-12 g/sec

Limitation of FID Water, inorganic gases, HCOOH,
HCHO sameple cannot be done and they
are Destroyed.
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Application Analysis of Gasoline Using FID
A typical example of ubiquitous Hydrocarbon
analysis in gasoline is shown on the right. The
column was 100 m long, 250 mm I.D carrying a
film of S.P (0.5 mm) Thick. The S.P was
poly(di- methyl siloxane) that was intra- column
polymerized and bonded to the surface. The
column was held at 3500C after injection for 15
min and then programmed to 200 0C at 200C/min and
finally held at 2000C for 5 min.
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Electron Capture Detection ECD

• A) The detector consists of a pair of electrodes
• Outer source electrode()
• Central collector electrode (-)

B) Central collector electrode consist of a
b-radiation emitter (e.g., 63Ni), which emits
electrons Ni-63
gt e-
Principles of Operation Carrier gas (e.g.,
N2)when flows through the detector is ionized by
the emitted electrons resulting in the production
of additional electrons as shown below e- N2
gt 2e- N2 The production of electron result
in a constant current that is detected by the
collector electrode (anode)
--When solute molecules with a high affinity for
capturing electrons elutes from the column and
enter the detector cell the electric current is
decreased. Why? This is because solute molecules
(e.g., AB) capture electrons as they pass through
the detector cell AB e-
----------?AB-
This negatively charged molecular ion (AB-) is
then neutralized by the ionized nitrogen (N2)
AB- N2 -------------------? ABN2
(neutral molecule) ---As a result there is a
decrease in current, which serve as an anaytical
signal
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Sensitive to electron withdrawing groups
especially towards organics containing F, -Cl,
-Br, -I also, -CN, NO2. Therefore, the detector
is very sensitive to Halogens, nitriles, carbonyl
and nitro compounds
Best for detection of solutes with electronegative
functional groups
Not senstive to amine, alchohols and aliphatic
hydrocarbons

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• ECD Characrtersitics (the good)
• VERY low DLWidely used for the determination of
  pesticides, herbicides and PCBs in environmental
  samples. Thus, widely used for detected species
  10-15g/s of many halogenated substances (PCB, DDT
  etc).

OK dynamic range of 104. Non-destructive

• The bad)
• Radioactive Ni-63 source
• EASILY contaminated with O2, H2O, sample
  overloading. High maintenance device.
• Highly variable response to halogenated
  substances, see table on previous page

Can be a real headache when method developing a
specific analysis, e.g. CH2Cl2 in the presence
of CCl4.. For some other halogenated chemicals
( 1ppm hexachlorocyclohexane) could ruin the
detector by contaminating it with excess of
analyte. Sometimes complementary information from
FID helps.
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