GC x GC With Valve-Based Modulation

1
GC x GC With Valve-Based Modulation

• John Seeley
• Oakland University
• Department of Chemistry
• Rochester, MI 48309
• seeley_at_oakland.edu

2
Seminar Structure

1. The Nature of a GC x GC Separation
2. An Examination of Low Duty Cycle Modulation
3. Direct Diversion Modulation With Multiport Valves
   
4. Differential Flow Modulation With Multiport
   Valves
5. Differential Flow Modulation With Fluidic Devices
6. Direct Diversion Modulation With Fluidic Devices
7. Summary

3
The Key Characteristics of a GC x GC Separation

• A GC x GC separation is a normal GC separation
  (the primary separation) followed by a steady
  repetition of secondary GC separations.
• The selectivity of the 1o stationary phase and 2o
  stationary phase are different.
• The timescale of the 2o separations (the
  modulation period) is small enough to not
  substantially diminish the resolution achieved by
  the 1o separation.
• A consistent portion of each peak emerging from
  1o is transferred to the secondary column and the
  total area of the 2o is representative of the
  component concentration.
• The width of the pulses entering the 2o column
  should be much less than the modulation period.

4
What is Valve-Based Modulation?

• Rough Definition GC x GC modulation through the
  precise control of flow using one or more valves.
• Contrast To Thermal Modulation
• Valve-Based Modulation does not involve
  concentrating the primary effluent thus,
  manipulating temperature is unnecessary.
• If done correctly, valve-base modulators should
  be simpler, less costly, smaller, and more rugged
  than thermal modulators.
• High 2o resolution with valve-based modulation
  involves a loss of analyte and/or non-optimal
  flows.

5
But Is It Comprehensive GCxGC?

• Interfaces based upon a series of valves that
  produce similar results also have been developed.
  Because these valve-based interfaces vent
  primary-column effluent to a certain extent, they
  violate the first rule of a comprehensive 2-D GC
  separation having the entire sample undergo
  separation in both dimensions and reach the
  detector. Thus, strictly speaking, they are not
  comprehensive 2-D GC interfaces. LCGC, 20 (9),
  2002
• Other authors and manuscript reviewers made
  similar statements with these key objections
• Valve-based GC x GC is not quantitative.
• Valve-based GC x GC is only capable of analyzing
  standard mixtures.
• Valve-based GC x GC is only capable of analyzing
  VOCs.

6
(No Transcript)
7
What is the impact of the missing effluent?
Duty Cycle Fraction of modulation period where
1o effluent is sampled. Thermal modulators have
a duty cycle of 1. Valve-based modulators have
duty cycles lt 1.
8
Constraints On The Modulation Period
The corrupting influence of the missing effluent
can be minimized by sampling the primary effluent
more frequently (this is essentially Shannons
Sampling Theorem). For GC x GC, this means
ensuring that the modulation period, PM, is not
too large. However, the modulation period is
already constrained by the desire to maintain the
primary separation, and this constraint is
imposed upon all modulation methods. So the
question is Do duty cycles lt 1 impose a new and
significant limitation on GC x GC modulation?
9
Constraints On The Modulation Period
For optimum GC x GC performance, a constraint is
placed on the modulation period. This constraint
is best quantified by the ratio of the modulation
period to the primary peak width. tz PM /
1s Seeley (2002) MR 4 1s / PM 4/ tz
Modulation Ratio Marriot et al. (2006)
Modulation Constraint Cause of Constraint
GCxGC Lore (19??) MR gt 4 (3 to 4 peaks) Maintenance of 1o Separation
Murphy et al. (1998) MR gt 2 Keep 1o Broadening below 30
Seeley (2002) MR gt 2.67 Accurate Quantitation For Duty Cycles lt 1
Marriott el al. (2006) MR gt 3.0 Accurate Quantitation For Trace Compounds With Duty Cycles 1
10
Modulation Ratio And 1o Peak Broadening
The classical requirement of MR gt 4 keeps 1o
broadening less than 10. The Murphy restriction
for MR gt 2 allows for broadening up to 30. The
low duty cycle restriction (MR gt 2.67) keeps 1o
broadening less than 20. Using lower duty
cycles actually reduces the average 1o
broadening. This observation was also made by
Bartle et al. (2003).
11
Modulation Ratio And Inconsistent 1o Peak Transfer
MR gt 2.67 essentially eliminates area
fluctuations due to low duty cycle
modulation. However, low duty cycle modulation
can exhibit large area fluctuations if the
primary dimension is under-sampled (i.e., MR gt
2). Complete 1o sampling (i.e., d 1) is much
more robust toward under-sampling.
12
Summary of Modulator Requirements
The additional constraint placed on RM from low
duty cycle modulation does not create the need
for vastly different modulation
periods. However, low duty cycle modulation can
lead to a substantial loss in signal when
compared to thermal modulation.
13
(No Transcript)
14
Direct Diversion ModulationWith A 4-Port
Diaphragm Valve
Low duty cycles required (d lt 0.1). Diaphragm
valves impose temperature limitations.
15
Differential Flow Modulation
Higher duty cycles possible (d 0.9). Diaphragm
valves impose temperature limitations. Higher
secondary column flows are required.
16
Modulating Diaphragm Valve
17
(No Transcript)
18
(No Transcript)
19
Dual-Secondary Column Comprehensive
Two-Dimensional Gas Chromatography (GC x 2GC)
20
2-D Chromatograms from a mixture of alkanes,
2-ketones, 1-alcohols, 2- alcohols,
2-methyl-2-alcohols, acetates, alkyl aromatic,
and aldehydes (all straight chain).
21
Breath
1.5 L sampled Full Scale 500
22

• What Are The Strengths of Differential Flow
  Modulation?
• Its Simple mostly off-the-shelf parts, no
  consumables
• High sample transfer between 1o and 2o columns
• Good resolution with thin-film secondary
  columns
• Best suited for high-speed separations with low
  modulation periods
• What Are The Limitations Of Differential Flow
  Modulation?
• High secondary flows limits the direct
  implementation of MS
• Flow disturbances upon switching
• Temperature limitations due to diaphragm valve

23
A Fluidic Modulator To Address Temperature
Limitations

• Three-port valve is outside oven.
• F2 gt F1 gt F2 gt 0
• Simultaneous fill and flush.
• Generates pulses by switching valve.
• Minimal pressure disturbances.
• No inherent temperature limitations.

24
Modulation of a Pentane Peak
F1 1.0 ml min-1 F2 20.0 ml min-1
Peak widths near the theoretical limit are
observed
25
Our Most Common GC x 2GC Setup
H2 Carrier Gas Constant Flow Mode 1o flow 1.0
cm3 min-1 2o flow 20.0 cm3 min-1 2o Column
split 11 2o Injection Period 1.5 s
26
(No Transcript)
27
(No Transcript)
28
Gasoline Aromatic Analysis
Accuracy and precision similar to GC-MS and
thermal modulation GCxGC
29
1 L of Northern Michigan Air
30
A Simpler Fluidic Modulator
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
Pure Diesel
36
B100 Soy
37
B5 Soy
38
Quantitative Precision for B20 Commercial
Biodiesel Blend
Calibration for B1 to B20 Soy Biodiesel Blends
Run Day FAME (vol)  
  1 11-May 19.4  
  2 11-May 19.7  
  1 12-May 19.9  
  2 12-May 20.0  
  1 12-May 20.2  
  2 12-May 20.2  
  1 15-May 19.8  
  2 15-May 20.3  
  1 15-May 20.4  
  2 15-May 20.8  
    Avg 20.1  
    Std Dev 0.4  
    RSD 1.9  
39
A Microfluidic Deans Switch As A GC x GC
Modulator Agilent has a Deans switch etched on a
plate. It is a rugged device with a very wide
temperature range. Direct diversion modulation
with no temperature restrictions.
40
(No Transcript)
41
Our Experimental Studies Confirm Our Original
Theoretical Analysis Low duty cycle modulation
is quantitative provided MR gt 2.5.
42
High speed separations with low modulation ratio
(MR 1.5) show increased area fluctuations. Stan
dard speed separations have accuracy and
precision comparable to thermal modulation GC x
GC, DF-GCxGC, and GC-MS
43
0.2 Diesel Fuel in Hexane
44
0.2 Diesel Fuel in Hexane
45
Summary

• A variety of simple valve-based modulators have
  been developed. Fluidic devices seem to have the
  greatest flexibility.
• Valve-based modulation is as quantitative as
  GC-MS or Thermal Modulation GC x GC.
• Valve-based modulation can analyze a wide range
  of compounds from permanent gases to barely
  volatile compounds (weve looked at C40). The
  column stationary phase is the source of the
  temperature constraint.
• Valve-based modulation does not require
  substantial additional consumables (perhaps a
  little more carrier gas).
• Differential flow modulators should produce a
  similar sensitivity enhancement as thermal
  modulation if an FID is used. This would not be
  the case for mass spectrometric detection.
• Thermal modulation should always generate better
  2o resolution. But the difference will get
  smaller as the speed of the separation is
  increased.

46
Some of Our Publications On GC x GC
Differential Flow Modulation With Diaphragm
Valve J. V. Seeley, F. Kramp, C.J. Hicks,
"Comprehensive Two-Dimensional Gas Chromatography
Via Differential Flow Modulation", Analytical
Chemistry, 72, 4346-4352, 2000. J. V. Seeley, F.
J. Kramp, and K. S. Sharpe, A dual-secondary
column comprehensive two-dimensional gas
chromatograph for the analysis of volatile
organic compound mixtures, Journal of Separation
Science, 24, 444-450, 2001. J.V. Seeley, F.J.
Kramp, K.S. Sharpe, and S.K. Seeley,
"Characterization of gaseous mixtures of organic
compounds with dual-secondary column
comprehensive two-dimensional gas
chromatography", Journal of Separation Science,
25, 53-59, 2002. Theoretical Aspects of GC x
GC J.V. Seeley, "Theoretical Study Of Incomplete
Sampling Of The First Dimension In Comprehensive
Two-Dimensional Chromatography", Journal of
Chromatography A., 962, 21-27, 2002. J.V. Seeley,
N.J. Micyus and S.K. Seeley, A Method for
Reducing the Ambiguity of Comprehensive
Two-Dimensional Chromatography Retention Times,
Journal of Chromatography A, 1086, 171-174, 2005.
Differential Flow Modulation With The Dual-Loop
Flow Switching Modulator P.A. Bueno, Jr. and J.V.
Seeley, A Flow-Switching Device for
Comprehensive Two-Dimensional Gas
Chromatography, Journal of Chromatography A,
1027, 3-10, 2004. R.W. LaClair, P.A. Bueno, Jr.
and J.V. Seeley, A Systematic Analysis of A
Flow-Switching Modulator for Comprehensive
Two-Dimensional Gas Chromatography, Journal of
Separation Science, 27, 389-396, 2004. J.V.
Seeley, N.J. Micyus and J.D. McCurry, Analysis
of Aromatic Compounds in Gasoline with
Flow-Switching Comprehensive Two-Dimensional Gas
Chromatography, Journal of Chromatography A,
1086, 115-121, 2005. Differential Flow
Modulation With The Simple Flow Switching
Modulator J.V. Seeley, N.J. Micyus, J.D. McCurry,
and S.K. Seeley, Comprehensive Two-Dimensional
Gas Chromatography With a Simple Fluidic
Modulator, American Laboratory News, 38, 24-26,
2006. Low Duty Cycle Modulation With A Deans
Switch J.V. Seeley, N.J. Micyus, S.V. Bandurski,
S. K. Seeley, J.D. McCurry, Microfluidic Deans
Switch for Comprehensive Two-Dimensional Gas
Chromatography Analytical Chemistry, 2007, In
Press.