Download Low Thermal Mass Gas Chromatography: Principles and Applications

Journal of Chromatographic Science, Vol. 44, May/June 2006
Low Thermal Mass Gas Chromatography:
Principles and Applications
Jim Luong1, Ronda Gras2, Robert Mustacich2, and Hernan Cortes3
1Dow
Chemical Canada, P.O. BAG 16, Highway 15, Fort Saskatchewan, Alberta, Canada, T8L 2P4; 2RVM Scientific, Inc.,
5511 Ekwill St. # A, Santa Barbara, CA, 93111-2398; and 3Dow Chemical USA, Midland, MI, 48667
Abstract
In gas chromatography (GC), temperature programming is often
considered to be the second most important parameter to control,
the first being column selectivity. A radically new GC technology
to achieve ultrafast temperature programming with an
unprecedented cool down time and low power consumption has
recently become available. This technology is referred to as low
thermal mass GC (LTMGC). Though the technology has its roots in
resistive heating, which forms the basis of principle and design
concept, the approach taken to achieve ultrafast heating and cool
down time by LTMGC represents a significant break-through in
GC. Despite some rectifiable shortcomings, LTMGC has proven to
be an ideal methodology to deliver near/real time GC data, high
precision, and high throughput applications. It is a new approach
for modern high-speed GC. This paper documents the fundamental
design principles behind LTMGC, performance data, and examples
of applications investigated.
Introduction
The process of increasing column temperature during a gas
chromatographic (GC) analysis is referred to as temperature
programming (TPGC). In GC, temperature programming is
often considered to be the second most important parameter to
control, the first being column selectivity (1–3). For a particular solute, TPGC leads to a decrease in retention volume and
retention factor. The benefits of TPGC include better separation
for solutes with a wide boiling range, improved detection limit,
and improved peak symmetry, especially for solutes with high
retention factors (1). In addition, TPGC is essential for the
removal of unwanted heavier materials that might otherwise
compromise the integrity of a chromatographic system.
A new, recently introduced instrument incorporates technology to achieve ultrafast temperature programming with an
unprecedented cool down time and a power consumption of
approximately 1% of conventional GC. The technology is
referred to as low thermal mass GC (LTMGC). In addition, this
LTMGC technology is designed as an accessory for commercially available GCs.
* Author to whom correspondence should be addressed: email [email protected].
This paper documents the fundamental principles behind
LTMGC, performance data, and examples of applications investigated using the RVM LTMGC accessory (RVM, Santa Barbara,
CA) for the Agilent HP-6890 series GCs (Wilmington, DE).
Experimental
An RVM LTMGC accessory for Agilent HP-6890 GC series
model LTM-A68 was used. The LTM-A68 was integrated with an
Agilent HP-6890A GC equipped with a vent exhaust deflector,
Transcendent Enterprise heated pressurized liquid injector
(HPLIS)–pressurized liquid injector (PLIS), split/splitless
injector, programmable temperature vaporizer injector, flame
ionization detector (FID), and Valco pulsed discharge detector
(VICI, Houston, TX).
Column modules used for evaluation and applications developments included: (i) 18-m × 0.25-mm i.d., 0.25-µm VF-1MS
column on a 3-inch standard tray; (ii) 5-m × 0.10-mm-i.d.,
0.12-µm CP-Sil 8 CB column on a 3-inch standard tray; (iii)
7-m × 0.53-mm-i.d. CP-Lowox column on a 5-inch wide tray;
(iv) 2-m × 0.10-mm-i.d., 0.12-µm CP-Sil 8 CB column on a
5-inch wide tray; and (v) 5-m × 0.10-mm-i.d., 0.12-µm DB-1
column with no transfer line on a 5-inch wide tray (Varian,
Middleburg, the Netherlands).
In all the GC applications described, hydrogen was used as
the carrier gas, and a flame ionization detector operated with
30 mL/min hydrogen, 25 mL/min nitrogen, and 350 mL/min
air was used as a detector.
The GC conditions for the system suitability test shown in Figures 1–3 (Figure 3, see page 5A) are as follows: the LTMGC
module used was a Varian VF-1MS column (18 m × 0.25-mm i.d.,
0.25 µm). The LTMGC temperature profile was 40°C held for 1
min then increased 100°C/min to 150°C. The host oven temperature was 200°C. The format tray was a standard
3-inch tray. The average linear velocity was 50 cm/s. The injector
was operated in split mode (50:1), and the injector temperature
was 250°C. The transfer line was fused silica, uncoated, but
deactivated. The interface was SGE zero dead volume connector
(Victoria, Australia). The detector temperature was 300°C, and
the injection was carried out manually at 1 µL.
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The GC conditions for the separation of 300 ppm (w/w)
nC14–nC16 in hexanes (Figures 4 and 5) were as follow: the
LTMGC module was a CP-Sil 8CB column (2 m × 0.10-mm i.d.,
0.12 µm). The LTMGC temperature profile was 50°C for 0.5
min then increased 100°C/min to 300°C. The host oven temperature was 250°C. The format tray used was a standard
3-inch tray. The average linear velocity was 100 cm/s, and the
injector was operated in split mode (50:1). The injector
temperature was 250°C. The transfer line was fused silica,
uncoated, but deactivated. Agilent Press-fit connectors were
used as the interface. The detector temperature was 300°C, and
the injection was carried out manually at 1 µL.
The GC conditions for the analysis of volatile alcohols in
hydrocarbons and stacked injection (Figures 6 and 7) were as
follow: the LTMGC module used was a Varian Lowox column
(7 m × 0.53-mm i.d.). The LTMGC temperature profile was
50°C held for 1 min then increased 300°C/min to 325°C and
held for 1 min. The host oven temperature was 250°C. The
format tray used was a 5-inch wide tray. The average linear
velocity was 100 cm/s, and the injector was operated in the split
mode (3:1). The injector temperature was 250°C. The transfer
Figure 1. Chromatogram of a Grob’s Test Mixture by LTMGC–FID. Note the
excellent chromatogram obtained. For the figure: 1-octanol, 1; n-undecane, 2; 2,6-dimethylphenol, 3; octanoic acid methyl ester, 4; 2,6dimethylaniline, 5; naphthalene, 6; n-dodecane, 7; 1-decanone, 8;
n-tridecane, 9; and decanoic acid methyl ester, 10. The concentration
was 0.1% each in cyclohexane.
Figure 2. An overlay of chromatograms of chlorinated phenols with temperature programming rates from 200°C/min to 400°C/min. For the figure:
phenol, 1; 2,4-dichlorophenol, 2; 4-chlorophenol, 3; 2,4,5-trichlorophenol,
4; and 2,4,5,6-tetrachlorophenol, 5. The concentration was approximately
300 ppm (v/v) each in hexane.
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line was fused silica, uncoated, but deactivated. Restek Pressfit connectors were used as the interface. The detector temperature was 300°C. The injection was made with a PLIS valve.
The GC conditions for the extractable chlorinated hydrocarbons analysis, (Figure 8) were as follow: the LTMGC module
was a Varian VF-1MS column (18 m × 0.25-mm i.d., 0.25 µm).
The LTMGC temperature profile was 40°C held for 1 min then
increased 450°C/min to 250°C. The host oven temperature
was 250°C, and the format tray used was a standard 3-inch tray.
The average linear velocity was 200 cm/s. The injector was
operated in the split mode (20:1). The injector temperature was
250°C. The transfer line was fused silica, uncoated, but deactivated. The interface used was an SGE zero dead volume connector. The detector temperature was 300°C, and the injection
was performed manually.
Figure 4. An overlay of chromatograms nC14–nC16 with temperature programming rates of 100°C/min, 300°C/min, and 600°C/min. For the figure:
nC14, 1; nC15, 2; and nC16, 3. The concentration was 300 ppm (w/w)
each in hexane.
Figure 5. An overlay of chromatograms of nC14–nC16 with temperature programming rates of 1200°C/min, 1500°C/min, and 1800°C/min. For the
figure: nC14, 1; nC15, 2; and nC16, 3. The concentration was 300 ppm (w/w)
each in hexane.
Journal of Chromatographic Science, Vol. 44, May/June 2006
The GC conditions for the analysis of organo-metallic tins
(Figure 9) were as follow: LTMGC module was a Varian VF-1MS
column (18 m × 0.25-mm i.d., 0.25 µm). The LTMGC temperature profile was 40°C held for 1 min then increased 450°C/min
to 250°C. The host oven temperature was 280°C. The format
tray used was a standard 3-inch tray. The flow velocity was
200 cm/s, and the average linear velocity was split (5:1). The
injector temperature was 300°C. The transfer line was fused
silica, uncoated, but deactivated. The interface used was an
SGE zero dead volume connector, and an FID was used as the
detector. The detector temperature was 300°C. The injection
was performed manually.
The GC conditions for the analysis of chlorinated phenols
(Figure 10) were as follow: the LTMGC module was a Varian
VF-1MS column (18 m × 0.25-mm i.d., 0.25 µm). The host
Figure 6. A chromatogram of methanol, ethanol, and 2-propanol in hexane
oven temperature was 280°C. The LTMGC temperature profile
by LTMGC–FID. For the figure: methanol, 1; ethanol, 2; and 2-propanol,
was 40°C for 1 min then increased 300°C/min to 250°C. For the
3. The concentration was 100 ppm (v/v) each in hexane.
variable rate for the heating rate test, the temperature programming varied from 200°C/min to
400°C/min. The format tray used was a
standard 3-inch tray. The average linear
velocity was 60 cm/s. The injector was
operated in split mode (20:1), and the
injector temperature was 300°C. The
transfer line was fused silica, uncoated,
but deactivated. The interface was an SGE
zero dead volume connector. The detector
temperature was 300°C, and the injection was carried out manually.
The GC conditions for the analysis of
Norpar 12 (Figure 11) were as follow: the
LTMGC module CP-Sil 8CB column (2 m
× 0.1-mm i.d., 0.12 µm). The LTMGC
temperature profile was 40°C held for 0.5
min then increased at a variable rate to
Figure 7. Stacked injection of 10 ppm (v/v) each of methanol, ethanol, and 2-propanol in hexane by
300°C. The host oven temperature was
HPLIS–LTMGC–FID. Top chromatogram: stacked of seven injections. Bottom chromatogram: stacked of
250°C. The variable rate was from
five injections.
100°C/min to 1000°C/min. The format
tray used was a 5-inch wide tray. The
average linear flow velocity was 100 cm/s, and the injector
was operated in the split mode (10:1). The injector temperature
was 300°C. The transfer line was fused silica, uncoated, but
deactivated. Press-fit connectors were used as the interface. An
FID was used as the detector at a temperature of 300°C. The
injection was carried out manually.
GC conditions for simulated distillation analysis (Figure 12,
see page 5A) were as follow: the LTMGC module was a Varian
VF-1MS column (18 m × 0.25-mm i.d., 0.25 µm). The host
oven temperature was 300°C. The LTMGC temperature profile
was 40°C held for 0.5 min then increased 300°C/min to 300°C
and held for 2 min. The format tray used was a standard 3-inch
tray. The average linear flow velocity was 200 cm/s, and the
injector was operated in the split mode (20:1). The injector
temperature was 300°C. The transfer line was fused silica,
Figure 8. A chromatogram of extractable chlorinated hydrocarbons by
uncoated, but deactivated. An SGE zero dead volume conLTMGC–FID. For the figure: tetrachloroethane, 1; hexacloroethane, 2;
nector was used as the injector. The detector temperature was
hexaclorobutadiene, 3; pentachlorobenzene, 4; and hexachlorobenzene,
300°C, and the injection PLIS.
5. The concentration was 200 ppm (w/v) each in hexane.
Standards used for testing were obtained from Aldrich
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Journal of Chromatographic Science, Vol. 44, May/June 2006
Chemicals (Milwaukee, WI) and VWR Scientifics Products
(Edmonton, Alberta, Canada), and other chemicals were
obtained from various production plants at Dow Chemical
Canada (Western Canada Operations, Fort Saskatchewan,
Alberta, Canada).
100°C/min can be attained. Cool down time, however, is very
long especially when the initial temperature approaches
ambient temperature. Another shortcoming with this design is
that the power consumption reaches the kilowatt level for a
single temperature cycle.
Miniature heating elements with GC columns have been
proven to be the most successful approach for achieving rapid
temperature programming at rates of up to 1200°C/min (3–8).
Results and Discussion
Some typical approaches include resistively heated metal
coated columns, resistively heated metal walled columns,
Principle and design of LTMGC
smaller heater plates, internal heater wire, and a resistively
Temperature programming is often conducted by electronic
heated element with a column in a sheath.
temperature control of an oven that houses the GC column.
In theory, resistively heated temperature programming is
The relatively small thermal mass of the capillary GC column
quite attractive. The fundamental principle behind it is rather
along with the winding of the column on a wire frame support
straightforward, based primarily on Ohm’s law, and the hardprovides extensive surface contact of the column with the
ware involved is inherently space conscious. Despite this
heated air for fast and reproducible temperature equilibrium.
discovery in the early 1950s (9), the deployment and commerUsing this type of arrangement and depending on the size of
cialization of said technique has been protracted. Figure 13
the oven, reproducible temperature programming of up to
shows a chronology of at-column resistive wire heating. Electrical shorting is the major concern of this principle, exacerbated by the inclusion of additional metallic components such
as a heating wire, resistive temperature detector (RTD), or
metal columns in such close proximity to each other. The lack
of an appropriate insulation material that can withstand elevated temperatures and temperature cycles has further aggravated the development (9).
In 2001, Mustacich et al. was granted two patents for the
invention of LTMGC (10,11). This led to the recent commercialization of a radically new disruptive technology to achieve
ultrafast temperature programming and unprecedented cool
down time with a power consumption of only approximately
1% of conventional GC (12–16).
In the context of GC operation, the terminology low thermal
mass (LTM) is used to qualitatively describe GC column assemblies, including both heating and temperature sensing means that
have a very small total mass compared with GC designs that are
Figure 9. A chromatogram of organo-metallic tins by LTMGC–FID. For the
heated by convection means. For temperature programming of
figure: tetratethyl tin, 1; tetrabutyl tin, 2; and tetrapentyl tin, 3. The concentration was 100 ppm (w/w) each in hexane.
the GC column, this results in a very small mass being heated,
hence the LTM description for GC designs of this type. For the
LTM approach to be useful, a certain level of
thermal efficiency is implicit, otherwise the GC
column assembly will not easily heat even
though it has a low total mass. The LTMGC
technology achieves a high heating efficiency
through its reduction of the surface area of the
assembly.
At the heart of LTMGC is the column module
assembly. A typical module assembly consists of
a capillary column—columns of any dimension
or length can be used; a 2-m platinum resistive
temperature detector (RTD) with typical diameter of 0.02 to 0.03 inches; a nickel alloy heating
wire with a typical diameter of 0.08 inch; a
metal tray to support the module, transfer lines,
and for heat dissipation; and a microprocessorFigure 10. A chromatogram of chlorinated phenols by LTMGC–FID. For the figure: phenol, 1;
controlled electric fan to facilitate rapid heat
2,4-dichlorophenol, 2; 4-chlorophenol, 3; 2,4,5-trichlorophenol, 4; and 2,4,5,6-tetrachlorophenol,
removal during the cooling down cycle of a run
5. The concentration was 100 ppm (w/w) each in hexane.
that is mounted at the bottom of the metal tray.
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Journal of Chromatographic Science, Vol. 44, May/June 2006
To employ ultralow thermal mass technology, a highly precise
entire assembly is then twisted into a torus and covered with a
RTD is a necessity in the design. The oven compartment in a
thin aluminum foil as external skin. Figure 14 shows a diagram
conventional GC is relatively massive (i.e., it has a large specific
of the assembly and a cross section of the module.
heat). If the specific heat of the system is large enough relative
The low thermal mass design, coupled with the high surface
to the rates of loss mechanisms, namely convection and conarea, offers a theoretical rapid temperature programming rate
duction, and is relatively constant, it is certainly possible to
of up to 30°C/s, or 1800°C/min, in addition to an unprecedirectly control the delivery of heat and employ a sensorless
dented cooling down time. Power consumption is approxisystem. One must also be confident in such a system that the
mately 1% that of conventional GC, primarily because of the
distribution of heat is not spatially localized because the column
effective heating of interstitial air with the random positioning
and heating element are not the same component and heat is
of the heating wire in the torus and the low thermal mass of
either convected or conducted to the column according to the
the assembly (10).
geometry and design of the apparatus. By this geometry, fast
Heat capacity is the quantity of heat required to increase the
temperature equilibrium and temperature repeatability are
temperature of a system or substance by one degree of temperafacilitated, even though there may be a difference in the actual
ture. It is usually expressed in calories or Joules/°C relative to a
temperature of the column itself.
Even if the size of the oven is significantly
reduced, such as in the case of many small
GCs, the thermal mass is still much larger
than the convective and conductive losses. If
the convective and conductive losses can be
maintained relatively constant through the
design of the apparatus, then it may still be
possible to use a sensorless design.
In the LTM design, the oven is completely
eliminated, and the mass of the system has
collapsed down to the capillary tubing, fine
wires, micron-sized insulation fibers, and thin
aluminum wrapping. Unique to the LTMGC
design, the specific heat is a minute fraction of
the convective and the conductive heat loss.
These losses are relatively insignificant when
compared with a convection oven. The specific
heat may now only be a few percent of the
total power. The overall power requirements
have dropped approximately 100 times when
compared with an oven for temperature proFigure 11. An overlay of Norpar 12 by LTMGC–FID with temperature programming rates of
gramming. This high efficiency results in a
100°C/min, 500°C/min, and 1000°C/min. The concentration was 100 ppm (w/w) Norpar 12 in
cross-over such that natural convection is
hexane.
responsible for most of the power consumption. This process is highly variable and is a
function of many variables such as local flows, convection currents, temperature gradients around the periphery of the
instrumentation, surface area, assembly packing density, specific length of column in the assembly, type of column, specific
wire lengths, and types of wire alloys, as well as others.
This variability means it is not possible to obtain repeatable
heating results with a specific amount of power delivered to the
heating element. For this reason, an ultralow thermal mass,
high precision RTD and an advanced control algorithm are
combined with the column for accurate and highly precise
temperature measurement in LTMGC (14–16).
The column module utilizes an advanced composite material,
commercialized by 3M (St. Paul, MN), to rove the RTD/assembly
and then the heating wire with a specially designed weaving
device. This insulating material advantageously meets the contrary disparative objectives of preventing shorting between
Figure 13. The history of at-column resistive wire heating. Diagram courtesy
components while maximizing the heat conducted between
of Dr. Leslie Ettre.
the heating element and these components. After roving, the
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Journal of Chromatographic Science, Vol. 44, May/June 2006
Figure 14. A diagram of the LTMGC assembly and a cross section of the
module. Picture courtesy of Dr. Robert Mustacich, inventor of LTMGC technology.
unit mass or system mass. For the LTMGC, the system mass is
small and, therefore, the system heat capacity, based on the heat
capacities of the components, is relatively small. This resulted in
a very small quantity of heat required to raise the temperature of
the system when compared with standard GC systems.
The impressive low consumption of power by LTMGC is
clearly illustrated in Figures 15 and 16, respectively. In Figure
15, a conventional GC was subjected to a temperature programming run of 40°C held for 0.5 min then increased
30°C/min to 180°C and held for 3 min. The average power
required for this programming run was approximately 1090 W.
In Figure 16, an LTMGC module with a capillary column (1 m
× 0.25-mm i.d., 0.25 µm) was subjected to a temperature programming run of 40°C held for 0.35 min then increased
60°C/min to 180°C and held for 0.35 min. The average power
required for LTMGC was less than 5 W.
In this specific case, a reduction of power consumption of
approximately 200 times was realized.
The significance of an ultralow power GC is yet to be appreciated. LTMGC not only has the potential to change how GC is
being conducted in different analytical theatres such as laboratories, in-situ, near line, or online, but it also has the potential of becoming a very important technology enabler to other
analytical techniques being practiced. On the basis of principle and design concept, the technology has its root in resistive heating; however, the approach taken to achieve ultrafast
heating and cool-down time represents a significant breakthrough in GC.
Performance
Figure 15. Power profile of a conventional GC. Temperature profile: 40°C
held for 0.5 min then increased 30°C/min to 180°C and held for 3 min.
The average peak power consumption was 1.09 kW.
Figure 16. Power profile of LTMGC. Temperature profile: 40°C held for
0.35 min then increased 60°C/min to 180°C and held for 0.35 min. The
average peak power consumption was 5 W.
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Installation
The LTM-A68 accessory, built on the principle of LTMGC, can
accommodate up to four different column modules in standard
format, two column modules in standard format and one
column module in wide format, or two column modules in
wide format. These modules can be operated independently
from one another. Up to eight methods, each method with a
maximum of 10 temperature programming ramps, can be
stored by the on-board microprocessor.
Installation involves simply replacing the door of the host
oven with the LTM-A68 controller/door and the modules are
then connected to the appropriate injector/detector with fusedsilica transfer lines. The host oven and LTM operation are independent of each other and can be operated as such. No
modification of the electronic system is required for the host
GC to integrate with the LTM-A68, apart from the attachment
of a remote controlled cable. A system set up is relatively
straightforward and takes less than 1 h to complete.
In Figure 17 (see page 6A), the modules attached to the
oven door of the conventional gas chromatograph contain two
different columns: (i) a Varian VF-1MS column (18 m × 0.25mm i.d., 0.25 µm) in a 3-inch standard tray or (ii) a Varian CPLowox (7 m × 0.53-mm i.d.) in a 5-inch wide tray.
System suitability
The LTM-A68 can be interfaced to any injector or detector via
deactivated fused silica column transfer lines. To demonstrate
the performance of the unit, a Varian VF-1MS (18 m × 0.25-
Journal of Chromatographic Science, Vol. 44, May/June 2006
mm i.d., 0.25 µm) in a standard tray was used for evaluation.
Interfacing was carried out using two approximately 100-cm
deactivated 0.25-mm i.d. fused-silica retention gaps.
A standard mixture consisting of 0.1% each of 1-octanol,
n-undecane, 2,6-dimethylphenol, octanoic acid methyl ester,
2,6-dimethylaniline, naphthalene, n-dodecane, 1-decanone,
n-tridecane, and decanoic acid methyl ester in cyclohexane
was analyzed. The GC conditions used were listed in the Experimental section, and Figure 1 shows a chromatogram obtained.
Excellent peak symmetry for all probe compounds was
observed, highlighting the high degree of inertness of the
chromatographic system under the conditions used.
Heating
LTMGC fast temperature capability was tested using the
module described in System suitability section. This module
was not ideal for ultra fast temperature programming because
of the fact that it has a rather long column, but it was useful to
demonstrate the practical aspect of LTMGC being used in conventional settings and conditions. Figure 2 shows an overlay of
chromatograms of chlorinated phenols with temperature programming rates varied from 200°C/min to 400°C/min. Compression of peak capacity can be seen even at 400°C/min.
Figure 3 shows an overlay of chromatograms with temperature
programming rates of 300°C/min, 400°C/min, and 500°C/min;
however, little gain was observed. Clearly, for this module,
the maximum temperature-programming rate is around
400°C/min.
To demonstrate the ultrafast temperature programming
capability of LTMGC, a CP-Sil 8CB module (2 m × 0.1-mm
i.d., 0.12 µm) on a wide tray was used. Figures 4 and 5 show
overlays of chromatograms of a test solute comprising 300
ppm (w/w) each of nC14, nC15, and nC16 hydrocarbons in
hexane with temperature programming rates ranging from
100°C/min to 1800°C/min. The GC conditions used were listed
in the Experimental section. Even at 1800°C/min, peak capacity
compression was observed. This fast programming rate makes
this module ideal for use with hyphenated techniques such as
comprehensive liquid chromatography–GC or GC–GC.
Key learnings from these experiments suggest that although
LTMGC can be programmed at a rate of up to 1800°C/min, its
maximum rate was a function of thermal mass of the column
assembly. That is, for a 2-m × 0.1-mm-i.d. column, 1800°C/min
can be reached without difficulty; whereas for an 18-m × 0.25mm-i.d. column, a limit was reached somewhere between
400°C/min and 500°C/min.
Cooling
Like heating, in LTMGC, cooling rate is also inversely proportional to the column module thermal mass (i.e., a faster
cooling rate is attained when the column module mass is
smaller). In addition, the cooling rate is also a function of the
amount of surface area and the rate of heat being removed by
mechanical fans. Figure 18 (see page 6A) shows a comparison
of cooling rates of four modules to that of an Agilent HP6890A GC equipped with a vent deflector.
As can be seen from the conventional Agilent HP-6890 data,
it takes slightly more than 11 min to cool down from 300°C to
30°C at 22°C ambient temperature. For LTMGC, with the
higher thermal mass module such as that of the 18-m column,
it takes 4 min. With the shorter columns of 2 to 7 m in length,
an average cooling time of only 1.3 min was obtained using
LTMGC. To cool from 300°C to 30°C, depending on the thermal
mass of the modules, LTMGC was 3 to 10 times faster when
compared with conventional GCs.
For a CP-Sil 8CB column (2 m × 0.1-mm i.d., 0.12 µm) in a
wide tray format, the time required to cool from 250°C to
50°C was an unprecedented 25 s without the use of a cryogen,
making the module ideal for use in the second dimension of
comprehensive multidimensional chromatography or where
high throughput or near real time data (or both) are required.
Applications
The following are some industrial applications investigated
using the LTM-A68 unit.
Alcohols in hydrocarbons
Volatile alcohols, such as methanol, are specification items
in hydrocarbon final products. The presence of methanol in
hydrocarbons such as ethylene had a negative consequence
on catalyst performance. The special features of LTMGC in
rapid heating and cooling, combined with a highly selective
column, makes it suitable for improving throughput and sensitivity. Figure 6 shows a chromatogram of 100 ppm (v/v) of
methanol, ethanol, and 2-propanol in hexane with a total
analysis time of approximately 3.5 min. Hexane was used
mainly for ease of handling. The GC conditions used were
listed in the Experimental section.
Table I shows the repeatability of retention time and area
counts obtained. Very respectable chromatographic performance was achieved. Less than 0.1% relative standard deviation
(RSD) at 95% confidence level for retention time and less than
3% at 95% confidence level for area counts was observed
despite the fact that the system was programmed at a very fast
rate of 400°C/min.
Stacked injection, a novel concept of employing a column
stationary phase for both sample enriching and a separating
medium to improve solute detectability, has been described
earlier (17,18). The special features of LTMGC were advantageous for this type of analysis by using a low initial temperature
to remove the matrix and the very fast temperature programming capabilities to elute the trapped solutes and improve
sensitivity and throughput. Figure 7 shows an overlay of
stacked injections of five and seven injections of 10 ppm (v/v)
of methanol, ethanol, and 2-propanol in hexane. The chromatogram shows five solvent peaks for five injections and
seven solvent peaks for seven injections, correspondingly.
Methanol, ethanol, and 2-propanol of these injections were
focused into three discrete peaks, respectively. When compared with conventional GC, a throughput improvement of at
least five times was realized when using LTMGC (18).
Extractable chlorinated hydrocarbons
Extractable chlorinated hydrocarbons such as 1,1,2-tricholoethane, 1,1,2,2-tetrachloroethane, hexachlorobutadiene,
pentachlorobenzene, and hexachlorobenzene are routinely
259
Journal of Chromatographic Science, Vol. 44, May/June 2006
analyzed in environmental applications. Total analysis time
required for this application, including cool down time, was
approximately 35 min (19).
Figure 8 shows a chromatogram of the same sample using
LTMGC. Cool down time included, a total analysis time of 3
min was attained, representing an analytical throughput
improvement of 10 times. The GC conditions used were listed
in the Experimental section.
Organo-metallic tins
Organo-metallic tins are commonly used as anti-coking
agents in various industrial applications. Close monitoring of
the solutes was required to determine dosing efficiency. With
conventional GC, the total analysis time was approximately 30
min. Figure 9 shows a chromatogram of organo-metallic tins in
hexane using LTMGC. A total analysis time of less than 3 min
was attained, representing an analytical throughput improvement of 10 times when compared with conventional GC. The
GC conditions used were listed in the Experimental section.
ASTM-D-2887 with nC44 eluting at approximately 2 min and a
total analysis time of approximately 3 min. This represents an
overall throughput improvement of 10 times over conventional GC. The GC conditions used were listed in the Experimental section.
Limitations/constraints
Some limitations and rectifiable constraints include: the
host oven has to be kept constantly at an elevated temperature.
This might have a negative impact on the capillary tubing used
Retention time (min)
Chlorinated phenols
Figure 19. A chromatogram of Norpar 12 by conventional GC. The conditions were: GC, Agilent HP-6890N; split/splitless injector in split mode;
Chlorinated phenols such as chloro-, di-, tri-, tetra-, and pen15:1 ratio; helium carrier at 21 cm/s; column, CP-Sil5CB MS (30 m × 0.32tachlorophenol are commonly analyzed in environmental applimm i.d., 1 µm). The temperature program was 120°C held for 0 min then
cations by GC. With conventional GC, total analysis time was
increased 20°C/min to 250°C and held for 4 min. The concentration was
approximately 40 min (20).
100 ppm (w/w) Norpar 12 in hexane.
Figure 10 shows a chromatogram of chlorinated phenols in
methanol using LTMGC. A total analysis
time of less than 3.5 min was attained, repTable I. Reproducibility of Retention Times and Area Counts for 100 ppm (v/v)
resenting an analytical throughput
Each of Methanol, Ethanol, and Propanol in Hexane by LTMGC–FID
improvement of approximately 10 times.
The GC conditions used were listed in the
Retention time (min)
Integration area counts
Experimental section.
Run
Norpar 12
Exxon Norpar 12 is a paraffin based
hydrocarbon fluid used for ethylene
recovery. Figure 19 shows a chromatogram of Norpar 12 by conventional
GC. A total analysis time of approximately
12 min was required. Figure 11 shows an
overlay of chromatograms of Norpar 12
by LTMGC using different temperature
programming rates from 100°C/min to
1200°C/min with a total analysis time of
approximately 2 min. Clearly, with
LTMGC, a six times faster analysis can be
conducted. The Experimental section
listed the GC conditions used.
Simulated distillation
Simulated distillation analysis is often
carried out in petroleum industries as per
ASTM-D-2887 for product characterization. A typical analysis, using conventional
GC, takes approximately 30 min for the
elution of hydrocarbons of up to nC44.
Figure 12 (see page 5A) shows an overlay
of chromatograms of two standards for
260
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Average
SD*
%RSD
95% CV†
Methanol
Ethanol
Propanol
Methanol
Ethanol
Propanol
1.891
1.893
1.891
1.891
1.892
1.890
1.892
1.892
1.891
1.892
1.891
1.892
1.891
1.891
1.891
1.891
1.891
1.891
1.892
1.891
1.891
0.0007
0.0355
0.08
1.953
1.952
1.953
1.952
1.952
1.952
1.951
1.951
1.951
1.952
1.952
1.953
1.952
1.953
1.952
1.952
1.952
1.953
1.953
1.952
1.952
0.0007
0.0344
0.08
2.001
2.002
2.001
2.001
2.000
2.001
2.002
2.001
2.001
2.002
2.001
2.002
2.001
2.002
2.001
2.002
2.001
2.002
2.001
2.002
2.001
0.0006
0.0293
0.06
50.3
50.4
49.1
49.7
49.8
48.7
48.9
50.4
50.1
50.4
50.9
50.1
50.9
50.9
50.1
50.5
50.7
50.9
50.6
50.1
50.175
0.656
1.3075
2.9
90.1
90.2
89.1
89.2
89.1
90.3
90.1
90.3
90.1
90.2
92.1
90.0
91.0
90.7
91.3
92.1
91.9
90.8
91.2
91.0
90.54
0.9005
0.9946
2.2
83.9
83.4
83.4
83.2
83.1
83.2
83.1
83.2
83.1
83.5
84.5
84.1
83.9
83.2
83.2
83.2
84.9
82.7
83.9
84.5
83.56
0.5807
0.695
1.5
* SD = standard deviation.
† 95% CV = standard deviation at the 95% confidence interval.
Journal of Chromatographic Science, Vol. 44, May/June 2006
for interfacing the host GC with the LTMGC. For example, if
the carrier gas used contains moisture or air, this might lead
to undesired reactions such as hydrolysis or stationary phase
oxidation resulting in excessive bleed.
Because the host oven has to be kept constantly at an elevated temperature, if LTMGC is used with a cool-on column
injection system or with a splitless injector, an uncoated yet
deactivated transfer line (retention gap) should be employed to
connect the injector with the column module so that proper
chromatographic focusing effect can take place at the inlet of
the column module.
With the host oven being at a higher temperature than the
LTMGC module, especially when the system is in standby mode,
chromatographic impurities such as septum bleed, stationary
phase decomposition, or impurities in carrier gas can accumulate in the LTMGC module. This requires thermal conditioning
of the LTMGC prior to analytical work being conducted.
The interfacing of the LTMGC to the host oven must be conducted in such a fashion that chromatographic fidelity is not
destroyed. Mixing, cold spot, or band broadening can occur if
connection between the analytical columns and transfer lines
are not done properly.
Although fan and assembly trays are user-exchangeable,
the column module assemblies are not. Columns, regardless
of vendor sources, can be coiled only by the manufacturer.
This requires careful planning and an on-hand inventory of
appropriate column modules in applications that are deemed
critical.
Conclusion
Though the technology has its root in resistive heating, the
approach taken to achieve ultrafast heating and cool down
time by LTMGC represents a significant breakthrough in GC.
Innovations in the domain of oven design and thermal management resulted in a fast heating rate of up to 1800°C/min and
unprecedented cool down time in seconds. LTMGC is ideal for
use in applications that require near or real-time analysis and
is a mission critical component for high-speed GC. The
ultralow power consumption of LTMGC not only has the potential to change the way GC is currently being practiced, it may
be a potent technology enabler to other analytical techniques
in analytical chemistry.
In the applications evaluated, when compared with conventional GC, LTMGC shows markedly improved chromatographic
performance and sample throughput. With the advent of
LTMGC, new techniques such as stacked injection (18), pyrolysis LTMGC, LTMGC×LTMGC can be developed to further
enhance the applicability of GC in analytical chemistry.
Acknowledgments
Special thanks to Dr. James Griffith, Dr. Terry McCabe, Rony
Van Meulebroeck, Patric Eckerle, Myron Hawryluk, Lyndon
Sieben, Vicki Carter, and Dr. Don Patrick for their contributions in the implementation of LTMGC technology. Credit
must be given to Dr. Mary Fairhurst and the Leveraged Technology Separations Leadership Team for their support of this
project. The authors would also like to express their appreciation to the editors and reviewers for their assistance and advice
in preparing the manuscript. This project was partially funded
by Analytical Science’s Corporate Innovation Funds.
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Journal of Chromatographic Science, Vol. 44, May/June 2006
Low Thermal Mass Gas Chromatography: Principles and Applications
Figure 3. An overlay of chromatograms of chlorinated phenols with temperature programming rates of 300°C/min, 400°C/min, and 500°C/min. For
the figure, 300°C/min (green trace), 400°C/min (red trace), and 500°C/min
(blue trace). From left to right: phenol, 2,4-dichlorophenol, 4-chlorophenol,
2,4,5-trichlorophenol, and 2,4,5,6-tetrachlorophenol. The concentration
was approximately 300 ppm (v/v) each in hexane.
(see pp. 253–61)
Figure 12. An overlay of nC8–nC44 hydrocarbons (simulated distillation
standards) by LTMGC–FID.
5A
Journal of Chromatographic Science, Vol. 44, May/June 2006
Figure 17. A Picture of LTM-A68 interfaced with an Agilent HP-6890A
GC. In this picture, an LTM-A68, shown connected to an Agilent HP6890A GC, is equipped with two column modules: (top) a Varian CPLowox (7 m × 0.53-mm i.d.) in a wide tray and (bottom) a Varian
VF-1MS column (18 m × 0.25-mm i.d., 0.25 µm) in standard tray.
6A
Figure 18. A comparison of cool down rates between an Agilent HP-6890A
GC and various LTMGC modules. A comparison of cooling rates between
conventional GC to LTMGC modules. RVM-1-ST: Varian VF-1 column (18
m × .25-mm i.d., 0.25 µm). RVM-2-ST: Varian CP-Sil8 CB (5 m × 0.1-mm
i.d., 0.12 µm). RVM-3-WT: Varian Lowox (7 m × 0.53-mm i.d.). RVM-4-WT:
Varian CP-Sil8 CB (2 m × 0.1-mm i.d., 0.12 µm). For the figure, Blue: oven
temperature program: 30°C held for 0.5 min then increased 30°C/min to
250°C and held for 1 min. Red: oven temperature program: 40°C held for
0.5 min then increased 30°C/min to 250°C and held for 1 min.