ASTM E355-96
(Practice)Standard Practice for Gas Chromatography Terms and Relationships
Standard Practice for Gas Chromatography Terms and Relationships
SCOPE
1.1 This practice deals primarily with the terms and relationships used in gas elution chromatography. However, most of the terms should also apply to other kinds of gas chromatography and are also valid in the various liquid column chromatographic techniques, although at this time they are not standardized for the latter usage.
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Standards Content (Sample)
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Designation: E 355 – 96
Standard Practice for
Gas Chromatography Terms and Relationships
This standard is issued under the fixed designation E 355; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope components to emerge in the normal order, that is, least-to-
most strongly absorbed.
1.1 This practice covers primarily the terms and relation-
2.7 Isothermal Gas Chromatography is the version of the
ships used in gas elution chromatography. However, most of
technique in which the column temperature is held constant
the terms should also apply to other kinds of gas chromatog-
during the passage of the sample components through the
raphy and are also valid in the various liquid column chro-
separation column.
matographic techniques, although at this time they are not
2.8 Programmed Temperature Gas Chromatograp-
standardized for the latter usage.
hy (PTGC), is the version of the technique in which the
2. Names of Techniques
column temperature is changed with time during the passage of
the sample components through the separation column. In
2.1 Gas Chromatography, abbreviated as GC, comprises all
linear PTGC the program rate is constant during analysis.
chromatographic methods in which the moving phase is
Isothermal intervals may be included in the temperature
gaseous. The stationary phase may be either a dry granular
program.
solid or a liquid supported by the granules or by the wall of the
2.9 Programmed Flow, Pressure, or Velocity Gas Chroma-
column, or both. Separation is achieved by differences in the
tography is the version of the technique in which the carrier
distribution of the components of a sample between the mobile
gas flow, pressure, or velocity is changed during analysis.
and stationary phases, causing them to move through the
2.10 Reaction Gas Chromatography is the version of the
column at different rates and from it at different times. In this
technique in which the composition of the sample is changed
recommended practice gas elution chromatography is implied.
between sample introduction and the detector. The reaction can
2.2 Gas-Liquid Chromatography, abbreviated as GLC, uti-
take place upstream of the column when the chemical compo-
lizes a liquid as the stationary phase, which acts as a solvent for
sition of the individual components passing through the col-
the sample components.
umn differs from that of the original sample, or between the
2.3 Gas-Solid Chromatography, abbreviated as GSC, uti-
column and the detector when the original sample components
lizes an active solid (adsorbent) as the stationary phase.
are separated in the column but their chemical composition is
2.4 Gas Elution Chromatography utilizes a continuous in-
changed prior to entering the detection device.
ert gas flow as the carrier gas and the sample is introduced as
2.11 Pyrolysis Gas Chromatography is the version of reac-
a gas or a liquid with a finite volume into the carrier gas stream.
tion gas chromatography in which the original sample is
If the sample is introduced as a liquid, it is vaporized in the
decomposed by heat to more volatile components prior to
system prior to or during passage through the separation
passage through the separation column.
column.
2.5 Gas-Frontal Chromatography is a technique in which a
3. Apparatus
continuous stream of carrier gas mixed with sample vapor is
3.1 Sample Inlet Systems, represent the means for introduc-
instantaneously replaced by a continuous stream of carrier gas
ing samples into the separation column, including the heated
containing sample vapor at a different concentration. The
zones permitting the vaporization of the introduced liquid
concentration profile is therefore step-shaped at the column
samples prior to their passage through the column. Sample
inlet.
introduction can be carried out by introduction of a liquid,
2.6 Gas-Displacement Chromatography employs a desor-
solid, or gas into the carrier-gas stream. The sample may be
bent as the carrier gas or in the carrier gas to displace a less
vaporized before or after introduction into the column.
strongly held solute from the stationary phase which in turn
3.1.1 Direct Inlets, rapidly vaporize the sample prior to
displaces the next less strongly held one etc., causing the
entering the column. All of the sample vapor enters the column.
3.1.2 On-Column Inlets, introduce a liquid sample into the
column. The sample vaporizes as the column section contain-
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
ing the liquid heats up after injection.
Spectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-
tography. 3.1.3 Split Inlets, rapidly vaporize the sample prior to
Current edition approved April 10, 1996. Published June 1996. Originally
entering the column. A defined fraction of the sample vapor
published as E 355–68. Last previous edition E 355-77 (1989).
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 355
enters the column; the remainder leaves the inlet through a vent 4. Reagents
at a flow rate F . The ratio of the total inlet flow (F + F )to
v v c
4.1 Carrier Gas is the Mobile Phase used to sweep or elute
the column flow ( F ) is called the split ratio (s):
c the sample components through and from the column.
F 1 F 4.2 The Stationary Phase is composed of the active immo-
v c
s 5 (1)
F bile materials within the column that selectively delay the
c
passage of sample components by dissolving or adsorbing
3.1.4 Splitless Injection, utilizes a split inlet wherein the
them, or both. Inert materials that merely provide physical
split vent flow is blocked during the injection period such that
support for the stationary phase or occupy space within the
most of the sample vapor enters the column. The injection
column are not part of the stationary phase.
period is typically one minute. The split vent flow is reestab-
4.2.1 Liquid Stationary Phase is one type of stationary
lished afterward usually for the remainder of the run.
phase which is dispersed on the solid support or the inner
3.1.5 Programmed-Temperature Vaporizers (PTV), accept a
column wall and causes the separation of the sample compo-
liquid sample that vaporizes as the inlet system heats up after
nents by differences in the partitioning of the sample compo-
injection. A PTV may operate in either a split, splitless,
nents between the mobile and liquid phases.
on-column, or direct mode.
4.2.2 An Active Solid is one that has ab- or adsorptive
3.1.6 A Retention Gap, is a section of tubing inserted
properties by means of which chromatographic separations
between the inlet and the analytical column proper. The
may be achieved.
retention gap may have an inner diameter different than the
4.3 The Solid Support is the inert material that holds the
analytical column. The retention gap has significantly lower
stationary (liquid) phase in intimate contact with the carrier gas
retaining power than the analytical column; in practice the
flowing through it. It may consist of porous or impenetrable
retention gap is deactivated but not coated.
particles or granules which hold the liquid phase and between
3.2 Columns, consist of tubes that contain the stationary
which the carrier gas flows, or the interior wall of the column
phase and through which the gaseous mobile phase flows.
itself, or a combination of these.
4.4 The Column Packing consists of all the material used to
3.2.1 Packed Columns, are filled with granular packing that
is kept in place by gas-permeable plugs at both ends. fill packed columns, including the solid support and the liquid
phase or the active solid.
3.2.2 Open-Tubular Columns, have unobstructed central
4.4.1 The Liquid-Phase Loading describes the relative
gasflow channels.
amount of liquid phase present in a packed column when the
3.2.2.1 Wall-Coated Open-Tubular Columns, abbreviated
column packing consists only of the liquid phase plus the solid
WCOT columns, have the liquid phase coated directly on the
support. It is usually expressed as weight percent of liquid
inside, relatively smooth wall of the column tubing.
phase present in the column packing:
3.2.2.2 Porous-Layer Open-Tubular Columns, abbreviated
Liquid2phase loading, wt%
PLOT columns, have a solid porous layer present on the tube
~amount of liquid phase!3 100
wall but still maintain the unobstructed central gas-flow
5 (2)
~amount of liquid phase 1 amount of solid support!
channel. This porous solid layer can either act as an adsorbent
4.5 Solutes are the introduced sample components that are
or a support which in turn is coated with a thin film of the
delayed by the column as they are eluted through it by the
liquid phase, or both. The solid layer can either be deposited on
carrier gas.
the inside tube wall or formed by chemical means from the
4.6 Unretained Substances are not delayed by the column
wall.
packing.
3.2.2.3 Support-Coated Open-Tubular Columns, abbrevi-
ated SCOT columns, refer to those PLOT Columns where the
5. Gas Chromatographic Data
solid layer consists of the particles of a solid support which
were deposited on the inside tube wall. 5.1 A Chromatogram is a plot of detector response against
time or effluent volume. Idealized chromatograms obtained
3.3 Detectors, are devices that indicate the presence of
with differential and integral detectors for an unretained
eluted components in the carrier gas emerging from the
substance and one other component are shown in Fig. 1.
column.
5.2 The definitions in this paragraph apply to chromato-
3.3.1 Differential Concentration Detectors, measure the in-
grams obtained directly by means of differential detectors or by
stantaneous proportion of eluted sample components in the
differentiating the records obtained by means of integral
carrier gas passing through the detector.
detectors. The Baseline is the portion of the chromatogram
3.3.2 Differential Mass Detectors, measure the instanta-
recording the detector response in the absence of solute or
neous rate of arrival of sample components at the detector.
solvent emerging from the column. A Peak is the portion of the
3.3.3 Integral Detectors, measure the accumulated quantity
chromatogram recording the detector response while a single
of sample component(s) reaching the detector.
component is eluted from the column. If two or more sample
3.3.4 Spectrometric Detectors, measure and record spectra
components emerge together, they appear as a single peak. The
of eluting components, such as the mass spectrum of the
Peak Base, CD in Fig. 1, is an interpolation of the baseline
infrared spectrum.
between the extremities of the peak. The area enclosed between
3.4 Traps, are devices for recovering sample components the peak and the peak base, CHFEGJD in Fig. 1, is the Peak
from the mobile phase eluting from GC columns. Area. The dimension BE from the peak maximum to the peak
E 355
FIG. 1 Typical Chromatogram.
base measured in the direction of detector response is the Peak
Relative retention = (AB) /(AB) or (AB) /(AB)
j i i s
2@~OB! – ~OB! #
Peak resolution =
j 1
Height. Retention dimensions parallel to the baseline are
=
KL! 1 KL!
~ ~
i j
termed as the peak widths. The retention dimension of a line
~OB! – ~OB!
j i
parallel to the peak base bisecting the peak height and
~KL!
j
terminating at the inflexion points FG of the tangents drawn to
the inflection points (= 60.7 % of peak height) is the Peak
Subscripts i, j, and s refer to any earlier peak, any later peak,
Width at Inflection Points, w . The retention dimension of a line
i
and a reference peak, respectively.
parallel to the peak base drawn to 50 % of the peak height and
terminating at the sides HJ of the peak is the Peak Width at
7. Presentation of Isothermal Retention Data
Half Height, w . The retention dimension of the segment of the
h
7.1 Retention values should be reported in a form that can
peak base KL intercepted by the tangents drawn to the
be applied for a specific stationary phase composition in
inflection points on both sides of the peak is the Peak Width at
different apparatus and for different conditions of column
Base or Base Width, w .
b
length, diameter, and inlet and outlet pressures, and for
5.3 The following definitions apply to chromatograms ob-
different carrier gases and flow rate. When the solid support is
tained with integral detectors, or by integration of the records
inert, its particle-size range and distribution, and (within limits)
obtained by means of differential detectors. As sample compo-
the amount and mode of deposition of the liquid phase, may be
nents pass through the detector the baseline is displaced
varied also. While the solid support is commonly assumed to
cumulatively. The change in baseline position as a single
be inert, often this is not so. The physical disposition of the
sample component is eluted is a Step. The difference between
liquid phase may also affect retention values (1). Conse-
straight line extensions of the baselines on both sides of the
quently, all components of the column packing and the
step, measured in the direction of detector response, is the Step
procedure for combining them must be fully specified to enable
Height, NM.
other workers to prepare identical compositions.
6. Retention Parameters 7.2 Retention in gas-liquid chromatography can be ex-
pressed on an absolute basis in terms of the partition coefficient
6.1 Retention parameters are listed in Table 1. The interre-
or specific retention volume of a substance (tacitly assuming an
lations shown apply only to gas elution chromatography
inert solid support). Relative retentions are more conveniently
columns operated under constant conditions and for which the
determined, however, and they should be expressed relative to
partition coefficients are independent of concentration. Fig. 1
a substance which is easily available and emerges relatively
can be used to illustrate some of these parameters:
close to the substance of interest.
Gas holdup time = OA
7.3 Retention index is another retention parameter. It is
Retention time = OB
Adjusted retention time = AB
Partition (capacity) ratio = AB/OA
Peak width at half height = HJ
Peak width at base = KL
The boldface numbers in parentheses refer to the list of references at the end of
2 2
Number of theoretical plates = 16 (OB/KL) = v 5.54 (OB/HJ)
this practice.
E 355
defined relative to the retention of n-alkanes, and represents the
nu
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