Download 高等食品分析(Advanced Food Analysis) VII. GAS

高等食品分析(Advanced Food Analysis)
VII. GAS CHROMATOGRAPHY
*Based on the stationary phase: Gas-solid chromatography (GSC),
GLC, Bonded phase GC (bonded or cross-linked phases).
Sample is injected and vaporized onto the chromatographic
column and eluted by the flow of an inert gaseous mobile
phase.
The mobile phase does not interact with molecules of analyte and
its function is to transport the analyte through the column.
Solve the conventional distillation problem: Benzene (b.p. 80.1)
and cyclohexane (80.8°C). => Easily separated by GC.
*Gas-solid chromatography (GSC): Based on adsorption of
gaseous substances on solid surfaces.
Useful for separation of permanent gases and low boiling
materials, up to a molecular weight of about 150.
Examples: Air, hydrogen sulfide, carbon disulfide, nitrogen oxides,
carbon monoxide, carbon dioxide and the rare gases.
Performed with both packed and open tubular columns.
Porous layer open tubular (PLOT) columns: A thin layer of
adsorbent is affixed to the inner walls of the capillary.
Adsorbents: Alumina, silica gel, carbon, molecular sieves and
porous polymers.
Quite limited in applicability: Due to mainly the tailing caused by
non-linear adsorption isotherms and partially excessive
retention of reactive gases and surface catalysis.
* Adsorbents (solid phases):
1)
Alumina: Modification needed due to its high polarity and
highly catalytic activity.
Alumina coated with 10% Na2SO4, Na2MoO4, NaCl or Al2 (SO4)3
were found to be useful for separation of cis-trans isomers,
chlorobenzene and various dichlorobenzenes.
Alumina/KCl porous layer open tubular (PLOT) columns are
excellent for separating C1-C10 hydrocarbons.
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2) Silica gel: Modification needed due to its high polarity and
highly catalytic activity.
Porasil is a porous spherical bead form of silica.
Zipax is a related material in which the beads have impervious
silica cores surrounded by a porous layer.
Silica surfaces were modified by salt, by chemical bonding of
materials such as silanes and by hydrothermal treatment.
Hydrothermal treatment involves contact with steam at 850°C for
about 24 hr to enlarge the pores.
Chemical modification:
i) Silanization of the surface hydroxyl groups with chlorotrimethyl- or dichlorodimethylsilane renders the surface
non-polar and non-specific.
ii) Esterification: MeOH and EtOH were bonded to the surface.
Example: Modified Porasil => Durapak, stable up to 150°C,
similar as the liquid stationary phase Carbowax 400.
XAD amberlite resins are agglomerated microspheres, beads of
nearly continuous solid phase and pore phase, and widely
used for sampling and trapping organics.
3) Carbon: Active carbons are the most difficult solids to
prepare in reproducible form due to its non-polar surface as
well as microporosity that leads to tailing.
Graphitization overcomes both polarity and microporosity
problems and gives C with a high homogeneity of surface.
The surface of carbon can be modified with phthalcyanins to
reduce the elution time.
Packed capillary of graphitized C is non-specific, has high
permeability, small pressure drop across the column and high
selectivity for certain systems, e.g. geometrical and structural
isomers such as o-, m- and p-cresols and xylenes and polar
compounds such as alcohols, and isotopic systems e.g.
deuteroacetone and acetone.
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4) Molecular sieves: Aluminum silicate ion exchanger, whose
pore size depends on the kind of cation present.
Particle sizes available from 40-60 to 100-120 mesh.
Pore size of commercial molecular sieves: 4, 5, 10 and 13Å.
Example: A 6-ft, 5Å packing at room temperature will separate a
mixture of helium, oxygen, nitrogen, methane and carbon
monoxide in the order given.
5) Porous polymers: Beads of uniform size manufactured from
styrene cross-linked with divinylbenzene.
Useful for separation of gaseous polar species such as hydrogen
sulfide, oxides of nitrogen, water, carbon dioxide, methane
and vinyl chloride.
Tenax-GC is a new porous polymer packing material based on
2,6-diphenyl-p-phenylene oxide and suitable for separation of
high boiling polar compounds such as alcohols, polyethylene
glycols, diols, phenols, amines, ethanolamines, amides,
aldehydes and ketones.
*Gas chromatograph:
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*Carrier gas supply and controls:
Viscosity and thermal conductivity of common carrier gas
Gas
Mol. wt
Viscosity
Thermal conductivity
P, × 106
cal/sec•cm [(°C/cm) × 106]
44.01
189
49
CO2
Ar
39.95
269
50
O2
32.00
256
77
N2
28.01
219
73
He
4.00
228
388
H2
2.02
108
490
Carrier gas: Must be chemically inert, including He, Ar, N2, CO2
and H2 and supplied from gas cylinder.
The gases with smallest diffusion coefficients will give the best
column performance. High molecular weight gases, N2, Ar,
CO2, give lower flow rates than hydrogen and helium.
For fast analysis, the ratio of viscosity to diffusion coefficient
should be minimal, and therefore H2 and He are ideal, while
to reduce diffusion effects that cause peak broadening, higher
flow rates are used.
Hydrogen: Fire- and explosion-hazard, and chemical reactivity
with reducible and unsaturated samples.
In practice, N2 for flame ionization detector (FID) and nitrogenphosphorus detector (NPD); Ar for electron capture detector
(ECD); He or H2 for katherometer detector or thermal
conductivity detector (TCD).
Additional gases may be required for detectors, e.g. air or O2 and
H2 for FID.
Associated with gas supply are pressure regulators, gauges and
flow meters. The carrier gas system contains a molecular
sieve to remove water or other impurity.
Flow rate is measured using a soap-bubble flow meter or linear
velocity is measured by injecting the butane from the lighter.
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*Sample introduction/injector system:
Use of syringe to inject a liquid or gaseous sample through septum
into an injection port or the use of valve to introduce sample.
Sample sizes: Few tenths of l to 20 l for packed, or ~10-2 l for
capillary columns without split.
For packed columns: On-column and glass insert system.
For capillary column: Split/splitless, direct injection (DI) and
on-column (OC) systems.
The temperature of the injection port is normally higher (30-50°C)
than the final programmed temperature of the oven (column)
to ensure the vaporization of the sample prior to getting onto
the column.
On-column (OC) injection: For heat sensitive samples.
Sample is directly injected onto the column without the heated
injection port.
Direct injection: Sample is vaporized inside the injection port but
with no split.
Split or splitless: Sample amount < 0.2 l without splitting.
If flow-splitter is used, about 30-90% of sample is vented, while
the remainder goes onto the column.
The vaporized volume of 1.0 l liquid with a mol. wt of 85 is
about 388 l at 250°C injection port and head pressure 10 psi.
If the head pressure is 5 psi, then the vaporized volume will
double.
Sample and solvent expansion volume = nRT/P
Wheren: # moles of solvent and sample.
T: Absolute temperature of injector.
P: Column head pressure (atm) + 1 atm.
R: Gas constant = 82.06 cc atm/mole degree.
(1)
Column head pressure:increased => Lower the retention time;
decreased => May be better the resolution.
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*Derivatization of samples: For non- or low volatile compounds.
Reasons for derivatization:
i) To increase volatility of the sample.
ii) To reduce thermal degradation of the sample by increasing
thermal stability.
iii) To increase detector response by incorporating into the
derivative functional groups which produce higher detector
signals such as CF3 for ECD.
iv) To improve separation and reduce tailing.
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*Derivatization of samples:
Most derivatives are thermal stable, although trimethylsilyl
derivatives may be decomposed on the stainless steel of an
injector port at > 210°C.
Pyridine is the commonly used solvent for derivatization, and act
as an acid scavenger and basic catalyst if required. DMF,
toluene and methanol are also used.
a) Silylation: Involving the replacement of labile acidic
hydrogen in -OH, -COOH, -SH and -NH2 groups with an
alkylsilyl group, e.g. SiMe3.
The derivatives are generally less polar, more volatile and more
thermally stable.
1)
Using trimethylchlorosilane (TMCS):
R-OH + Cl-SiMe3 => R-O-SiMe3 + HCl
2)
Using hexamethyldisilazane (HMDS):
2 R-OH + Me3Si-N=N-SiMe3 => 2 (R-O-SiMe3) + N2
Silylation reactions generally proceed very rapidly (within 5 min)
with pyridine as the most frequently used solvent.
Other silylating agents: Substituted acetamides, e.g.
i) BSTFA (N,O-bis(trimethylsilyl)-trifluoroacetamide),
ii) BSA (the non-fluorinated analog).
Both react rapidly and quantitatively under mild conditions using
pyridine or DMF as solvent, forming esters, ethers or N-TMS
derivatives.
The main advantage of BSTFA over BSA is that the by- products
are more volatile and often elute with the solvent front.
R-OH + BSTFA => R-O-SiMe3 + CF3-CO-NH-SiMe3
or CF3-CO-NH2
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b) Acylation:
1) Forming perfluoroacyl derivatives of alcohols, phenols or
amines, mainly for enhanced detector performance using an
ECD. An added benefit is the increased volatility.
Using trifluoroacetyl from trifluoroacetic anhydride, TFAA:
R-OH + O-(COCF3)2 —> R-O-CF3
2) N-Fluoroacyl-imidazoles react smoothly to acylate hydroxyl
groups and secondary or tertiary amines.
No acids are produced which could hydrolyze the products. The
imidazole produced as by-product is relatively inert.
Using N-trifluoroacetylimidazole:
N
R-OH + CF3-CO- N
=> R-O-COCF3 + imidazole
c)
Alkylation: Addition of alkyl group to an active group.
Esterification to form methyl ester (methylation) is the most useful
reaction since they are more volatile.
A number of reagents are available such as diazomethane, BF3MeOH, H2SO4-MeOH, HCl-MeOH and sodium methoxide
(CH3ONa), but boron trifluoride in MeOH is commonly used.
RCOOH + BF3/MeOH => RCOOMe
Other reagents commonly used include pentafluorobenzyl bromide
developed for the analysis of acids, amides and phenols using
an electron capture detector for enhanced sensitivity.
Dialkylacetals of DMF react instantaneously and quantitatively
with acids, amines and amides, barbiturates and on-column
derivatization is possible.
Methyl and butyl alkyls are available.
The reaction mixture is injected immediately onto the GC without
washing, extraction and also contain no water.
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Flash alkylation: High temp of the injection port is used to form
derivatives on the injection of sample together with an
appropriate reagent.
i) Quaternary alkylammonium hydroxides, e.g. tetrabutyl
ammonium hydroxide (TBAH) (as 0.2 M solution in MeOH):
Mainly for low mol. wt acids to increase retention times.
ii) General purpose reagent trimethylanilinium hydroxide
(TMAH): Used when normal methylation might cause
confusion with naturally occurring methyl derivatives in
biological systems.
*Chromatographic column and oven:
Separation process occurring in the column involves equilibrium
established by solutes between stationary and mobile phases.
Relative retention of two components will decrease as the
stationary phase (column) temperature increases.
Packed columns with larger amounts of stationary phase on the
support material will require higher temperature to obtain
elution times equivalent to lower stationary phase loadings.
Decreasing the amount of stationary phase and reducing the
column temperature results in the peaks eluted first having
poorer separation.
A balance of stationary phase loadings: 10-15% w/w loading for
small molecules up to C8 and up to 3-5% w/w for C9-C20.
Once the column was chosen, temperature and carrier gas flow
rate are two variables that can be modified to optimize the
separation of components.
Electronic pressure control (EPC) for HP 5890 Series II GC can
control the flow rate for automatic optimization.
Control of temperature is important in order to obtain reproducible
chromatograms.
Oven is operated from ca. 10°C above ambient temp up to 450°C.
Temperature programming: Isothermal at programmed, e.g.
50°C/10 min, 5°C /min, 220°C /30 min.
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*Packed columns: Consist of a glass or metal (stainless steel) tube
packed with adsorbent particles (GSC) or stationary coated
particles (GLC), typically 60-80 or 80-100 mesh size range.
*Supports for packed columns: To provide a uniform, inert
support for the stationary phase, and include diatomaceous
materials and polytrifluoroethylene (PTFE).
Diatomaceous materials:
1) Pink firebrick-derived materials, e.g. Chromosorb P or Gas
Chrom R, suitable for non-polar hydrocarbon compounds.
2) White diatomaceous materials derived from filter aids, e.g.
Chromosorb W, Gas Chrom Q.
These support materials contain mineral impurities that promote
catalytic reaction, and silanol (Si-OH) groups that are
reactive, polar, forming H-bonds with suitable components.
The supports are then treated with HCl to remove minerals and
silanized using dimethyldichlorosilane (DMDCS) or
hexamethyldisilylazane (HMDS) to block the Si-OH groups
with methylated siloxane bonds (Si-O-Si).
PTFE supports, e.g. Chromosorb T, are extremely inert and used
for corrosive materials often with fluorocarbon oil (e.g. Kel-F,
Fluoropak-80), polyethylene glycol and squalane stationary
phase.
*Stationary phases for packed columns: Liquid at the operating
temperature of GC columns.
The stationary phase should be thermally stable, unreactive, have
negligible volatility, and have a reasonable column life
(generally 2 years) over the operating temperature range.
The life of a stationary phase can be extended if use at the temp.
of 20-50°C below the recommended maximum.
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*Stationary phases for packed columns:
Polar stationary phases: -CN, -CO and -OH.
Hydrocarbon-type stationary phases and dialkyl siloxanes are
nonpolar, while polyester phases are highly polar.
Polar analytes include alcohols, acids and amines; species of
medium polarity include ethers, ketones and aldehydes.
When the polarity of stationary phase matches that of the sample
components, the order of elution is determined by the boiling
point of the eluants.
Bonded stationary phase: Stationary phase is bonded to the
support through silyl ether linkages, prepared by reaction of
the support silanol groups (Si-OH) with chlorosilanes.
*Capillary or open tubular columns: Long narrow tubing coated
on the inner surface with about 1 m of stationary phase.
1) WCOT (wall-coated open tubular) columns:
The stationary phase is coated directly on tubing surface.
2) PLOT (porous layer open tubular) columns: For GSC.
Adsorbent such as Porapak, molecular sieve, aluminum oxide.
3) Micropacked columns: Packed column < 1 mm ID.
4) SCOT (support coated open tubular) columns:
Stationary phase is coated on solid support to increase loadings.
5) FSOT (fused-silica open tubular) or new WCOT columns:
Fused silica column is preferred due to its low catalytic activity.
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*Capillary columns:
The efficiency of SCOT is less than that of WCOT but
significantly greater than that of packed columns.
The length of capillaries can be 15, 30, 60 and 120 m.
180, 250 and 320 m capillaries: A sample splitter must be used to
reduce injected sample size.
530 and 750 m capillaries, megabore or wide bore columns:
Tolerate sample sizes that are similar to those for packed
columns.
The performance characteristics of megabore OT columns are not
as good as those of smaller diameter capillaries but are
significantly better than those of packed columns.
*Column conditioning:
Condition when a column is first installed or after solvent rinsing.
Condition periodically to remove high b.p. components.
Overnight conditioning at the maximum operating temp. or 20°C
below provides most stable baseline.
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*Capillary vs. packed columns:
Advantages:
a) Greater separating power: Narrow peaks and faster RT.
b) Versatility: One capillary replaces many packed columns.
c) Higher reproducibility: More controlled and individually
tested.
d) Greater inertness: No support effects.
e) Bonded stationary phases: minimal bleed, better longevity,
rinseable.
f) Improved accuracy: Symmetrical peaks easier to integrate.
g) Lower detectability: Narrower peaks = more response/time.
h) Less leakage: Operates at lower pressure.
Disadvantages:
a) Lower sample capacity: ng vs. g.
b) Smaller injections required: Lower operating pressure.
c) Less tolerance to dirty samples: Required packed inlets/guard
columns.
d) Require more knowledge: Such as inlets, pneumatics,
make-up, seals.
e) Make-up gas required and high dead volume.
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Packed
Capillary
Length, m
1-6
5 - 105
I.D., mm
2-4
0.2 - 0.75
Flow, ml/min
10 - 60
0.5 - 15
Pressure drop, psi
10 - 40
3 - 40
Total effective plates 5,000 (2 m)
180,000 (60 m)
Effective plates/m
2,500 (2 mm ID) 3,000 (0.25 mm ID)
Capacity
10 g
50 ng (0.25 mm ID)
Film thickness, m
1 - 10
0.1 - 7.0
*Guard columns (untreated or deactivated columns):
To increase separation efficiency, decrease maintenance
requirements and minimize peak splitting.
To protect columns from high mol. wt compounds, inorganic
residues, pyrolyzates, particulates, other contaminants, needle
damage and water. => Increase the life of column.
*Detectors: A detector is used to monitor the GC column effluent;
it does not identify the components except in the case of
specific detectors.
When a component is eluted and detected, the signal produced is
proportional to the concentration or mass of that component.
The temp of the detector port is normally higher (30-50°C) than
the final programmed temp of the oven (column) to ensure
that column effluent will get into the detector.
*Characteristics of the ideal detector:
1) Adequate sensitivity. Sensitivity of current detectors lies in
the range of 10-8 to 10-15 g analyte.
2) Good stability and reproducibility.
3) A linear response to analyte extending over several orders of
magnitude.
4) A temperature range from room temp. to at least 400°C.
5) A short response time independent of flow rate.
6) High reliability and ease of use.
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7) Similarity in response to all analytes or to one or more classes
of analytes.
8) Nondestructive of sample.
*Compatible unretained compounds for various detectors:
1) TCD/FID—CH4 (methane)
2) ECD—CH2Cl2 (methylene chloride) vapor
3) NPD—Acetonitrile vapor
4) ELCD—Dichlorodifluoromethane vapor
5) MS—O2 or N2 (air)
6) PID—Ethylene or acetylene
Comparison of GC detectors
IRD
MSD
AED
TCD
FID
ECD
NPD(N)
NPD(P)
FPD(S)
FPD(P)
ELCD (C)
PID
-15
10
fg
1 ppt
-12
10
pg
1 ppb
-9
10
ng
1 ppm
Sensitivity
-6
10
痢
0.1%
-3
mg
100%
10
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*Minimum detection level (MDL): S/N ≥ 2
The level of sample measured by the detector at the peak
minimum, i.e. minimal concentration when the detector
signal (S) is at least twice the mean noise signal level (N).
The MDL of the common detectors is as follows:
NPD: 10-14 -13 g/ml
FID: 10-12 -11g/ml
FPD: 10-11
TCD: 10-9 -8
ECD: 10-14 -13
*Type of GC detectors:
a) Thermal conductivity detector (TCD) or katherometer.
A heated thin-wire filament or thermistor (semi-conductor of
fused metal oxides) is positioned in the path of the effluent
gas from the column; the other is positioned only in the path
of the carrier gas.
Both filaments need the carrier gas to flow in, otherwise ≠ !
2 or 4 resistive elements form the arms of Wheatstone bridge.
When a sample component is eluted from the column, the thermal
conductivity of the gas in the detector cell changes, which
causes a change in the temperature and resistance of the
detecting element, and hence a change in the electrical
current flowing in the bridge circuit.
The heated element may be a fine platinum, gold or tungsten wire,
or alternatively a thermistor of higher sensitivity.
The advantages of TCD: Its simplicity; large linear dynamic range
(~105); reproducible response to all types of organic and
inorganic compounds including those not detector by FID;
and non-destructive character, which allows collection of
solutes after detection.
H2 and He carrier gases are preferred with TCD due to their high
thermal conductivity.
However, the sensitivity of TCD is low => 1 ng. 10 ng
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b) Flame ionization detector (FID).
Most widely used and generally applicable GC detectors,
particularly with capillary columns due to its great sensitivity.
The effluent from the column is mixed with hydrogen and air and
then ignited electrically.
High sensitivity to virtually all organic compounds in proportional
to the number of carbons.
Little or no response to water, CO2, the common carrier gas
impurities, giving a zero signal when no sample is present.
Gives a stable baseline as it is not significantly affected by
fluctuations in temp or carrier gas flow rate and pressure.
Has good linearity over a wide sample conc’n range (about 107).
Functional groups such as carbonyl, alcohol, halogen and amine
yield fewer ions or none at all in a flame.
FID is 1000-fold more sensitive than TCD. (1 pg vs. 1 ng).
Disadvantage of FID: Destructive and only for organic comp’ds.
Detection limits is lowered using Ar or He as the carrier gas, using
O2 instead of air for combustion, or providing pure carrier
gas (make-up gas) directly into the detector (scavenging).
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c) Nitrogen-phosphorus detector (NPD), alkali flame ionization
detector (AFID), flame thermionic detector (FTD), or
thermionic detector (TID).
Commonly used in the detection of pesticide residues.
The ionization processes in the flame were modified by the
presence of an alkali metal salt which brings an enrichment of
the detector response towards P (500 folds) and N (50 folds).
Its response to a P atom is 10 x greater than to a N atom and
104~106 × larger than to a C atom.
To improve detector stability, modified design of NPD uses a glass
bead as the alkali source which contains an essentially
non-volatile, stable rubidium silicate.
d) Flame photometric detector (FPD).
Applicable in the detection of pesticides containing P (40 pg) and
S (200 pg).
The detector consists of a hydrogen-air burner with a
photo-multiplier detector optically coupled to it.
P and S in the flame produce emissions at 394 (HPO*) and 526
nm (S2*), respectively. Interchangeable optical filters permit
selection of one or the other of the two elements.
e) Electron capture detector (ECD).
Non-destructive detector that utilizes the ability of compounds to
capture free electrons.
Radiation source: Titanium foil containing tritium (T or 3H) or
63Ni are used as -ray sources, due to easy shielding against
radiation hazard.
ECD is highly sensitive to electrophilic molecules such as
halogens, peroxides, quinones and nitro groups, insensitive to
amines, alcohols and hydrocarbons.
An important application of ECD is for the detection of
chlorinated pesticides, such as DDT and lindane.
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f) Atomic emission detector (AED).
The eluent is introduced into a microwave-energized helium
plasma that is coupled to a diode-array optical emission
spectrometer, which is capable of detecting emitted radiation
from ca. 170 to 780 nm.
The plasma atomizes all the elements in a sample and excites their
characteristic atomic emission spectra.
Used to detect elements including C: 495.7 nm, H: 486.1 nm, O:
777.2 nm and Cl: 479.5 nm, and to establish the chemical
formula of compounds.
Sensitive to water, S and organo-metals.
g) Electrolytic conductivity detector (ELCD) or Coulson
conductivity detector (CCD).
Based on measurement of the electrolytic conductivity of water.
Organic substances in the column effluent are combusted in a
stream of O2 and air over a platinum catalyst; the combustion
products, such as SO2, SO3 and HCl, are dissolved in water;
and the conductivity of the water is measured.
Specific for X-, S- and N-containing compounds.
CO2 is not quickly absorbed by water.
h) Photo-ionization detector (PID).
The column eluent is irradiated with an intense beam of UV
radiation varying in energy from 8.3 to 11.7 eV (149 to 106
nm), which causes ionization of molecules.
Application of a potential across a cell containing the ions leads to
an ion current, which is amplified and recorded.
For the detection of impurity (organic and inorganic gases) in high
purity gases.
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i) Electron ionization detector (EID).
GCD system provided by Hewlett Packard is based on HP 5890
GC equipped with a single, built-in electron ionization
detector (EID) that is a compact HP 5970 MSD (mass
selective detector).
GCD provides information of retention times, area for each
chromatographic peak and (mainly EI) mass spectrum.
j) Mass spectrometric detector (MSD) in GC/MS.
k) Infrared Detector (IRD) in GC/FTIR.
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