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GC GC With Valve Modulation

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					GC x GC With Valve-Based Modulation

          John Seeley
          Oakland University
          Department of Chemistry
          Rochester, MI 48309
          seeley@oakland.edu
            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
The Key Characteristics of a GC x GC Separation
I.    A GC x GC separation is a normal GC separation (the
      primary separation) followed by a steady repetition of
      secondary GC separations.

II.   The selectivity of the 1o stationary phase and 2o stationary
      phase are different.

III. The timescale of the 2o separations (the modulation period)
     is small enough to not substantially diminish the resolution
     achieved by the 1o separation.

IV.   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.

V.    The width of the pulses entering the 2o column should be
      much less than the modulation period.
 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.
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.
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 <= 1.
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 Shannon’s 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 < 1 impose a new and significant
 limitation on GC x GC modulation?
    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 > 4 (3 to 4 peaks)   Maintenance of 1o Separation
Murphy et al. (1998)     MR > 2                  Keep 1o Broadening below 30%
Seeley (2002)            MR > 2.67               Accurate Quantitation For Duty Cycles < 1
                                                 Accurate Quantitation For Trace Compounds
Marriott el al. (2006)   MR > 3.0                With Duty Cycles = 1
              Modulation Ratio And 1o Peak Broadening


The classical requirement
of MR > 4 keeps 1o
broadening less than 10%.

The Murphy restriction
for MR > 2 allows for
broadening up to 30%.

The low duty cycle
restriction (MR > 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).
       Modulation Ratio And Inconsistent 1o Peak Transfer




MR > 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 > 2).

Complete 1o sampling (i.e., d =
1) is much more robust
toward under-sampling.
  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.
 Direct Diversion Modulation
With A 4-Port Diaphragm Valve




  Low duty cycles required (d < 0.1).

  Diaphragm valves impose temperature limitations.
Differential Flow Modulation




 Higher duty cycles possible (d = 0.9).
 Diaphragm valves impose temperature limitations.
 Higher secondary column flows are required.
Modulating Diaphragm Valve
Dual-Secondary Column Comprehensive Two-
Dimensional Gas Chromatography (GC x 2GC)
 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).
Breath

         1.5 L sampled
         Full Scale = 500
What Are The Strengths of Differential Flow Modulation?

• It’s 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
A Fluidic Modulator To Address
Temperature Limitations




        • Three-port valve is outside oven.

        • F2’’ > F1 > F2’ > 0

        • Simultaneous fill and flush.

        • Generates pulses by switching valve.

        • Minimal pressure disturbances.

        • No inherent temperature limitations.
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
                  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 = 1:1

2o Injection Period = 1.5 s
          Gasoline Aromatic Analysis




Accuracy and precision similar to GC-MS and thermal
modulation GCxGC
1 L of Northern Michigan Air
A Simpler Fluidic Modulator
Pure Diesel
B100 Soy
B5 Soy
                                                                              Quantitative Precision for
                                                                           B20 Commercial Biodiesel Blend
Calibration for B1 to B20 Soy Biodiesel Blends

                                                    y = 47.651x - 3.2694           Run Day    % FAME (vol)
                                 Total FAMES
                                                                               1    11-May        19.4
                                                        R20.9999 =
                                                                               2    11-May        19.7
                  1000
                   900                                                         1    12-May        19.9

                   800                                                         2    12-May        20.0
Total Peak Area




                   700                                                         1    12-May        20.2
                   600
                                                                               2    12-May        20.2
                   500
                                                                               1    15-May        19.8
                   400
                   300                                                         2    15-May        20.3

                   200                                                         1    15-May        20.4

                   100                                                         2    15-May        20.8
                     0
                                                                                       Avg        20.1
                         0   5     10          15      20           25
                                                                                    Std Dev        0.4
                                     Vol % FAME
                                                                                        RSD        1.9
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.
Our Experimental Studies Confirm Our Original Theoretical Analysis:

Low duty cycle modulation is quantitative provided MR > 2.5.
High speed separations with low modulation ratio (MR = 1.5) show
increased area fluctuations.

Standard speed separations have accuracy and precision
comparable to thermal modulation GC x GC, DF-GCxGC, and GC-MS
0.2% Diesel Fuel in Hexane
0.2% Diesel Fuel in Hexane
                                  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 (we’ve 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.
                      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.

				
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