DYNAMIC CONTROL OF GAS CHROMATOGRAPHIC SELECTIVITY

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DYNAMIC CONTROL OF GAS CHROMATOGRAPHIC SELECTIVITY DURING THE ANALYSIS OF ORGANIC BASES Ernest Darko, and Kevin B. Thurbide Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00703 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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Analytical Chemistry

DYNAMIC CONTROL OF GAS CHROMATOGRAPHIC SELECTIVITY DURING THE ANALYSIS OF ORGANIC BASES by Ernest Darko and Kevin B. Thurbide* Department of Chemistry, University of Calgary, 2500 University Drive, NW, Calgary, Alberta T2N 1N4, Canada.

Submitted for publication as a Research Article in: Analytical Chemistry

*Corresponding Author Phone: (403) 220-5370 Fax: (403) 289-9488 E-mail: [email protected]

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ABSTRACT

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A novel method for controlling selectivity during the gas chromatographic (GC) analysis

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of organic bases is presented. The technique employs tandem stainless steel capillary columns,

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each coated with a pH adjusted water stationary phase. The first is a 0.5 m trap column coated

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with a pH 2.2 phase, while the second is an 11 m analytical column coated with a pH 11.4 phase.

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The first column traps basic analytes from injected samples, while the remaining components

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continue to elute and separate. Then, upon injection of a volatile aqueous ammonia solution, the

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basic analytes are released as desired to the analytical column where they are separated and

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analyzed. Separations are quite reproducible and demonstrate an average RSD of 1.2% for

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analyte retention times in consecutive trials. Using this approach, the retention of such analytes

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can be readily controlled and they can be held in the system for periods of up to 1 hour without

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significant erosion of peak shape. As such, it can provide considerable control over analyte

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selectivity and resolution compared to conventional separations. Further, by employing a third

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conventional GC column to the series, both traditional hydrocarbon and enhanced organic base

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separations can be performed. The method is applied to the analysis of complex mixtures, such

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as gasoline, and much less matrix interference is observed as a result. The findings indicate that

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this approach could be a useful alternative for analyzing such samples.

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Keywords: gas chromatography; water stationary phase; trap column; selectivity; organic bases

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Analytical Chemistry 3

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INTRODUCTION

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Gas chromatography (GC) continues to be a very widely used and reliable tool in

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analytical separations, partly due to its exceptional sensitivity and performance in the analysis of

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complex mixtures 1,2. Accordingly, GC is routinely employed for the analytical needs of several

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sectors including the oil and gas, food and beverage, and pharmaceutical industries, among

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others

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technology over the years and they continue to evolve.

3-7.

Owing to its broad applicability, many advances have been made in GC separation

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One area that holds great importance in this regard is developing and improving GC

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column selectivity. Indeed, the benefits of ‘tuning’ the selectivity of a GC analysis to suit

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particular sample needs have long been noted 8, since such efforts can strongly impact the

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resolution and speed achieved by a method and greatly facilitate the analysis of complex

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mixtures. While sample preparation and detection can assist in enhancing selectivity, one of the

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most common routes for this is to alter the separation column. For instance, adjusting column

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selectivity can be achievable by several means 8, including varying the analysis temperature 9-11,

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altering the columns employed 12-16, and creating new stationary phases 17-20. Of note on the latter

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front, novel coatings based on ionic liquids

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benzothiadiazoles

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have also been some reports demonstrating that humidified carrier gases can cause a water layer

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to adhere to the surface of stationary phase particles and provide a means of separation

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These efforts successfully showed that water could uniquely alter analyte retention in such

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packed column GC systems, using either pure or higher ionic strength coatings on the particles.

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However, the main challenges included relatively low column efficiencies and variable analyte

23

21,

metal-organic frameworks

22,

and dithienyl

are being explored for GC use. Previously, in packed column GC, there

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retention due to evaporation of the stationary phase 27. Related explorations in capillary column

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GC are scarce, if any, perhaps due to the fused silica degradation that occurs in heated water 28,29.

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Recently, we introduced a novel method of using water as a stationary phase in capillary 30.

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GC

With this technique, a water coating is established on the inner wall of a narrow bore

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stainless steel tube and forms a stationary phase that can be successfully utilized for GC

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separations. The method can directly analyze various compounds in both aqueous and organic

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samples, and demonstrates good efficiency and reliable performance over a range of conditions.

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Further, the water stationary phase offers unique selectivity for polar over non-polar solutes

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These GC attributes were also similar to our earlier work using pure water as a stationary phase

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in capillary supercritical fluid chromatography 31.

30

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Ionizable analytes such as organic bases are particularly interesting in this GC system

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since they often largely exist in a charged state in the neutral pH water stationary phase 32,33. As

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such, they are heavily partitioned into the phase and very difficult to elute/observe by this

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method. In contrast to this, when the water stationary phase pH is suitably raised enough to

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suppress analyte ionization, then organic bases can be readily and directly analyzed by this

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approach with good sensitivity and peak shape

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such bases is often hampered by unfavorable column active site interactions, which lead to poor

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peak shapes and require laborious, error-prone derivatization steps to help overcome this 34,35.

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32.

This is very useful since the GC analysis of

Certainly the GC analysis of organic bases is of great concern in many important areas 36,37

and pharmaceutical production

38-40.

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like oil and gas development

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involved are often complex and can interfere with analyses, many efforts are being made to

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improve the separation of organic bases, including using novel coatings such as graphitized

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carbon or porous polymer packings, and invoking various derivatization and column deactivation

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Since the matrices

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schemes 41,42. Therefore, the continued exploration of new ways to analyze such bases in GC by

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more direct, simple, and highly selective means is of great interest.

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Here, we describe a novel method for analyzing organic bases in GC, which involves two

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columns placed in series, each coated with a water stationary phase of different pH. The first

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column is at a low pH to selectively trap organic bases from other injected sample components,

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while the second column is at a high pH to provide good analytical separation of the bases

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without prior derivatization. By raising the pH of the first trap column in-situ through injecting a

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volatile base, analytes are released on demand to the second analytical column for separation and

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their selectivity over other components is hence controlled externally. The general method

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operating features, separation characteristics, and application to complex mixtures is presented.

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EXPERIMENTAL

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Instrumentation and Operation

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An HP 5890-Series II GC instrument (Agilent, Palo Alto, CA, USA) equipped with a

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flame ionization detector (FID) was employed. High purity Helium (Praxair, Calgary, Canada)

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was used as the carrier gas and was saturated with water vapour using an ISCO model 100DX

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syringe pump (Teledyne ISCO, Lincoln, NE, USA) that supplied water through a valco zero

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dead volume tee union (Vici-Valco, Houston, TX, USA). The outlet of the tee union was

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connected to a 1 m stainless steel (SS) pre-heating coil (1/16” O.D. x 250 µm I.D.;

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Chromatographic Specialties, Brockville, ON, CAN) and both were kept inside the oven. The

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coil outlet led into the injector. This arrangement helped to prevent evaporation of the water

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stationary phase downstream during separations and provided stable analyte retention times as a

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result

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dependent on carrier gas flow and pressure, temperature had the greatest impact in this regard.

32.

While the amount of hydrating water required to maintain the stationary phase was

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Analytical Chemistry 6

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For instance, at 70 oC, 1 µL/min of water was sufficient to maintain the phase, and every 10 oC

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increase required another 1 µL/min. Above 100 oC, this increase required near 9 µL/min to

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stabilize the system. As well, due to the minor flow of water vapor through the column, FID

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response is little impacted and the detector still provides reasonable performance. Of note, the

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system yields an LOD of 12 pg C/s (S/N=2) and an LOQ of 60 pg C/s (S/N=10). Further, it

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yields a strongly linear response (R2 value of 0.9997) over the 3 orders of magnitude investigated

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up to µg quantitites of analyte injected, with no signs of signal saturation. Figure 1 illustrates

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some response profiles obtained for different injected amounts of analyte in the water stationary

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phase system.

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FID Response (mV)

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Figure 1: Demonstration of FID response using the GC water stationary phase system. The panels (L-R) are peaks obtained for 91, 45, and 4.5 ng of hexanoic acid injected on-column.

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Two columns (a trap and an analytical column) were placed in tandem to one another.

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The trap column (1/16” O.D. x 250 µm I.D. x 0.5 m; Chromatographic Specialties) was

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connected to the injector and its outlet was joined to the analytical column (1/16” O.D. x 250 µm

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I.D. x 11 m; Chromatographic Specialties) via a zero dead volume union (Vici-Valco, Houston,

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Analytical Chemistry 7

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TX, USA). Unless stated otherwise, about 24 cm/s of carrier gas was typically used with a 5:1

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split ratio. The analytical column outlet was joined to a SS capillary restrictor (620 µm O.D. x 75

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µm I.D. x 0.5 m) via a zero dead volume union (Vici-Valco). This restrictor provided system

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backpressure and promoted phase stability at higher temperatures. It was positioned inside the

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FID jet at the burner surface where effluent was deposited directly into the detector flame. An

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injector/detector temperature of around 220 oC was maintained during the experiments. High

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purity Hydrogen and Air (Praxair) were used to support the detector flame at respective flows of

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90 and 350 mL/min. In some experiments, a conventional DB-5 GC column (95% methyl / 5%

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phenyl polysiloxane stationary phase; 250 µm I.D. x 30 m; 0.25 µm thick) was also used.

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Stationary Phase Preparation

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The stationary phase preparation used here has been described previously 32,33. Briefly, a

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1 M base stock solution of sodium hydroxide prepared in HPLC-grade water was used to adjust

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the pH of the water supply to be employed as a stationary phase in the separation column.

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Similarly, a 0.02 M stock solution of sulfamic acid was prepared in HPLC-grade water and used

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to adjust the pH of the trap column. A calibrated pH meter was employed to measure the pH of

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the stirred solutions as stock was added. These pH-adjusted solutions were then used

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immediately to coat the SS capillary columns as described previously

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columns were mounted inside the GC oven for separation use.

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Chemicals and Reagents

30.

Once finished, the

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HPLC-grade water (Honeywell Burdick & Jackson, Muskegon, USA) was used for

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column coating solutions. Analytes, including ethylamine (70% in water; Arkema Inc, Canada),

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butylamine, pentylamine, hexylamine, heptylamine and octylamine (all  98%; Sigma-Aldrich,

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Canada) were used to prepare standard solutions (10 μg/μL) in heptane (>99%; Sigma-Aldrich).

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A mixture of hexanol, ethanol, butanol, propanol (at 6 μg/μL), acetone, methanol, cyclopentanol

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(at 9 μg/μL), propylamine, butylamine, pentylamine, heptylamine (at 10 μg/μL), 1,4-dioxane,

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and verbenol (at 14 μg/μL) was prepared in heptane (all analytes ≥ 99% except hexanol (≥ 98%)

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and verbenol (≥ 97%); all Sigma−Aldrich). A solution (10 μg/μL) of propylamine, butylamine,

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pentylamine, and heptylamine was also prepared in automotive fuel purchased from a local

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vendor. A standard solution of butylamine, pentylamine (at 10 µg/µL), 1,4-dioxane, and ethanol

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(at 1 µg/µL) was prepared in heptane. All other details are described in text.

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RESULTS AND DISCUSSION

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General Operating Characteristics

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Initial experiments were pursued to establish if basic analytes could be sufficiently ionized

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and secured on the ‘trap’ column as a means of manipulating retention behavior. Preliminary

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attempts at this employed pure water as the stationary phase. However, similar to previous

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observations

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eluted quite slowly with poor peak shape. Subsequently an acidic water stationary phase was

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investigated for this purpose. A non-volatile acid was required for this purpose since it needed to

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remain in the trap and maintain the pH therein. While earlier work found that sulfuric and

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phosphoric acids could be useful for this, they were also found to erode the SS tubing33.

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Subsequently, sulfamic acid was identified as an ideal choice without detrimental side effects in

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the system33. Incorporating the latter information, it was found that when a pH 2.2 sulfamic acid

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phase was instead used here, basic analytes were well retained on the trap column as a result of

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their increased ionization and were not observed to elute at all. Thus, acidic phases were used for

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trapping.

32,

basic analytes were often not held static enough at neutral pH and frequently

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Analytical Chemistry 9

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Next it was examined if the trapped basic analytes could then be released in-situ by

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introducing a base into the system. Earlier work found that sodium hydroxide was very non-

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volatile and remained in the column over time, whereas ammonia solution readily volatilized and

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was ejected from the system in a short period32. Therefore injection of aqueous ammonia was

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employed here for releasing basic analytes since it was volatile enough to readily mobilize

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through the system and also yielded a low FID response relative to organic compounds. The

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primary aim of this approach was to dynamically neutralize the ionized analytes on the acidic

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trap column and enable their subsequent elution. However, since the acidic phase itself also

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consumed the injected ammonia, the process had to be optimized with respect to trap column

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length, ammonia concentration, and the injected volume. Further, it should also be noted that

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while attempts were made to trap, release and separate analytes using only the single (i.e. trap)

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column, it was found that analytes did not separate well in this format. Thus, a tandem dual

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column system was adopted where analytes were first trapped on a short acidic phase coated

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column, released by subsequent injection of aqueous ammonia, and then separated on a final

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longer basic phase coated analytical column.

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The main parameters that were explored for optimization in this regard were ammonia

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concentrations of 1 to 10 M, injected base volumes of 1 to 9 µL, and trap column lengths of 0.5

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to 1 m. In general, the main challenge with this setup was found to be that if the trap column

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was too long, or if the amount of base used for release was too low, then all of the analyte could

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not be mobilized in one step and separations were incomplete. For example, using a 1 M

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ammonia solution frequently failed to quantitatively recover the analyte from the trap column in

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one step and so 10 M concentrations were explored. With a 2 m trap column, 1, 3, and 9 µL

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injections of 10 M ammonia solution were found to recover about 75, 80, and 83% of the analyte

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Analytical Chemistry 10

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respectively in one step. However, in the latter case, the volume was so large that it generated an

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FID signal that could interfere in analyses. As such, 3 µL of 10 M ammonia solution was further

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explored. It was found that when using this to release analytes, recoveries increased about 20%

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when moving from a 2 to a 0.5 m trap column. Therefore, ultimately, a 0.5 m long trap column

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and 3 µL of 10 M ammonia was found to be optimum in most cases here for releasing trapped

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solutes. Using this approach, little to no residual organic base remained in the system and 99% of

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the injected analyte was normally recovered in this single step. Alternatively, for certain complex

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samples, a 1 m trap was optimal and employed where noted.

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Figure 2 demonstrates the dual column system under optimum conditions with a

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chromatogram for the injection of pentylamine and butylamine. Without using a trapping

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column, these analytes would normally appear between about 3 and 5 minutes. However, as can

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be seen with the trapping system used in figure 2, after the heptane solvent appears upon

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injection of the sample, the analytes do not elute in this time frame since they are trapped on the

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FID Response

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0

5

Time (min)

10

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Figure 2: Separation of pentylamine and butylamine (in order of elution) in heptane on dual column system (0.5 m trap with pH 2.2 sulfamic acid coating and 11 m analytical column with pH 11.4 NaOH coating). The analytes were released (indicated by the arrow) at 5 minutes with 3 µL of 10 M aqueous NH3. Column temperature is 100 oC and the carrier gas velocity is 24 cm/s.

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first acidic phase column. Subsequently though, when they are released after injection of NH3

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solution at the 5 minute mark, they appear well resolved and with good peak shape near 10

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minutes. Therefore, the system can be dynamically used to delay and control analyte elution.

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It should be noted that consecutive injections could be performed reproducibly if the acidic

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trap column phase was replenished in between runs. This was accomplished here with the

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injection of 3µL of 1 M formic acid after each trial. Note that formic acid was chosen for this

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since it was fairly volatile and could be readily mobilized through the system to re-acidify the

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trap. In contrast to this, sulfamic acid was primarily used to coat the trap column since it was

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non-volatile and would stably remain in place to hold the trap at an appropriate pH level during

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trials33. It was found that this adequately acidified the trap column for reuse and the system

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provided comparable performance in each run as a result. For example, in terms of

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reproducibility, consecutive trials of the separation shown in figure 2 yielded analyte retention

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times and peak areas with average RSD values of 1.2% and 0.8% respectively. As such, the

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optimum operating conditions above were used going forward in this study to further

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characterize the method.

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Effect of Time on Analyte Elution In order to get a better idea of the capacity of the dual column system to hold analytes, a

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study was conducted to examine the effect of time between trap and release of the analyte. The

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typical results obtained are demonstrated below in figure 3. The chromatograms shown are from

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a series of experiments where the injection of an amine pair was performed followed by their

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release at consecutively later points in time for each trial. As seen in figure 3, the analytes can be

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readily trapped for significant periods of time within the considerable range investigated. For

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instance, no significant peak distortion or degradation of the separation is observed for trapping

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periods investigated up to 1 hour. However, it should be noted that the peak resolution appeared

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Analytical Chemistry 12

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FID Response

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FID Response

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Figure 3: Chromatograms of pentylamine and butylamine (in order of elution) on dual column system with analytes released as a function of time. Analytes were released (indicated by arrow) at 10, 20, 30, 40, and 60 minutes. Other conditions are as in figure 2.

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Analytical Chemistry 13

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to decrease slightly after this time. None the less, this demonstrates that elution of such analytes

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can be delayed on-column for significant periods of time. As such, this can potentially allow for

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adjustable selectivity of these organic bases to be achieved, which can in turn facilitate resolving

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them from other sample components as desired.

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Altering Selectivity and Resolution

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In order to investigate this further, various separations were carried out with organic bases

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among other analytes. The first of these was the separation of a sample containing amines,

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ethanol, and 1,4–dioxane using a trapping and non-trapping water stationary phase system. The

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results are shown in figure 4 and, as can be observed, the analytes readily elute on the single

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column system with pentylamine and butylamine appearing at 2.1 and 2.7 minutes respectively,

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while ethanol and 1,4-dioxane are retained for 6.3 and 8 minutes respectively (figure 4A).

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However, when using the dual column system (figure 4B), the retention of the amines can be

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increased such that they elute at much different times. Of note, the analytes now elute at 13.1 and

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13.6 minutes respectively. Conversely, the retention of ethanol and 1,4-dioxane is largely

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unaffected in the dual column system apart from a minor increase in elution time owing to the

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extra 0.5 m trap column length employed. As a result, the formal selectivity realized changes

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dramatically. Specifically, the selectivity obtained in figure 4A for 1,4-dioxane/pentylamine is

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about 5.4, whereas in the dual column system of figure 4B it is 0.7 (or formally 1.4 for the new

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reversed pentylamine/1,4-dioxane elution order of the pair as shown in the figure). Therefore,

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using this approach it is possible to reverse the selectivity observed for certain pairs of analytes.

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α4,1 = 5.4

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1 2 FID Response

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α1,4 = 1.4

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Figure 4: Chromatograms of 1) pentylamine, 2) butylamine, 3) ethanol and 4) 1,4-dioxane on A) single analytical column (11 m, pH 11.4 NaOH coating) and B) dual column system (0.5 m trap with pH 2.2 sulfamic acid coating and 11 m analytical column with pH 11.4 NaOH coating). Analytes in B are released at the 12 minute mark. Other conditions are as in figure 2.

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Inherent to this, the resolution between analytes can also be significantly altered. Figure 5

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shows an example of this with the comparative separations of acetone, butylamine and

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pentylamine on both a single analytical column and the dual column system. As shown in figure

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5A, pentylamine appears first in the chromatogram on the single analytical column, while

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acetone and butylamine co-elute and almost entirely overlap due to similar retention times in this

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system. As a result, the resolution obtained for this analyte pair is only about 0.3. However,

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when the same mixture is analyzed on the dual column trap system (figure 5B), acetone again

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appears near the 5 minute mark as before while the two bases are held on the acidic trap

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Analytical Chemistry

R2,3 = 13.4

0

5

10

25

30

364 365 366 367 368 369 370 371 372

Figure 5: Chromatograms of 1) pentylamine, 2) acetone and 3) butylamine on A) single analytical column (11 m, pH 11.4 NaOH coating) and B) dual column system (0.5 m trap with pH 2.2 sulfamic acid coating and 11 m analytical column with pH 11.4 NaOH coating). Analytes in B are released at the 17 minute mark. Column temperature is 80 oC. Other conditions are as in figure 2.

373

column until they are released for analysis on the basic analytical column after 17 minutes.

374

Consequently, a near 40 fold increase in resolution to a value of 13.4 is achieved for the

375

overlapping analyte pair. Therefore the ability to significantly control basic analyte retention in

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this system could potentially facilitate analyzing such analytes within more complex mixtures.

377

Application to Complex Mixtures

378

In order to further examine the potential analytical utility of the dual column system,

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more complex mixtures were next studied. The first was a 13 component mixture of oxygenated

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and basic analytes that was again analyzed on the single analytical column and then compared to

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that of the dual column trap system. The typical results are demonstrated in figure 6. Similar to

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Analytical Chemistry 16

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2 1

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386 387 388 389

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Page 16 of 25

3 5

7 i

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i i

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4 6

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391 392 393 394 395 396 397 398 399 400 401 402

Figure 6: Chromatograms of an analyte mixture on A) single analytical column (11 m, pH 11.4 NaOH coating) and B) dual column system (1 m trap with pH 2.2 sulfamic acid coating and 11 m analytical column with pH 11.4 NaOH coating). Analytes in B are released at the 17 minute mark. Analytes in order of elution are 1) heptylamine, 2) pentylamine, 3) hexanol, 4) butylamine, 5) acetone, 6) propylamine, 7) cyclopentanol, 8) butanol, 9) propanol, 10) ethanol, 11) 1,4dioxane, 12) verbenol, and 13) methanol. Unknown impurities are denoted by ‘i’. Other conditions as in figure 2.

403

the results shown in figure 5A, on the single analytical column the basic analytes elute amidst

404

various oxygenated compounds with significant interference and poor resolution in some cases

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(figure 6A). However, when employing the dual column system (figure 6B), the oxygenates elute

406

similarly while the basic analytes are effectively held on the trap column until they are later

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released at a more opportune time in the chromatogram. When this is then done at the 17 minute

408

mark, the analytes readily appear well resolved and with sharp prominent peak shape.

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Analytical Chemistry 17

409

Additionally, a few unknown impurities were also revealed as a result of modifying the elution

410

time of the interfering amines to an alternate retention window. Therefore the system can

411

potentially facilitate sample analyses in such circumstances by providing a means of changing

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resolution as desired. It should be noted that the shift in retention time that can be seen for the

413

oxygenates in figure 6B is ascribed to the extra retention of these analytes on the 1 m trapping

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column in place for those trials. This is because the trapping column is also coated with a water

415

stationary phase and these analytes continue to partition within it while traversing the trap

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column. Thus, the analyte velocities established in figure 6A can be extended to an extra 1 m

417

length of travel in figure 6B. In doing so, the predicted retention times for figure 6B differ from

418

the actual retention times by an average of only 3%. Still, further experimentation would be

419

required to establish if other interactions could also make minor contributions to this as well. As

420

such, the slight delay in retention appears to be primarily a function of the extra column length.

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To examine if analyte resolution can be controlled further, the system was applied to the

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analysis of a commercial gasoline sample containing amines. Analyses of this nature are

423

routinely performed using GC and are important since such compounds can inhibit the quality of

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petroleum products 43,44. Figure 7A demonstrates the analysis of this sample using a conventional

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GC column. As seen, the sample is quite complex and numerous hydrocarbons are present as

426

evidenced from the large number of peaks eluting in the first 20 minutes. Further, the target

427

amines present in the sample elute in the first 10 minutes, greatly overlapping with these

428

hydrocarbons, and are obscured from being clearly observed as a result. Figure 7B then shows

429

the analysis of the same sample on the dual column trap system. As observed, a much clearer

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chromatogram is produced. Of note, all of the hydrocarbon matrix components are now

431

unretained in the water stationary phase and rapidly elute in the void volume as a result.

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Analytical Chemistry 18

433 434

FID Response

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1-4

435

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441 442 443 444 445

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Page 18 of 25

C

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446 447 448 449 450 451 452 453 454 455 456 457

Figure 7: Separations of a gasoline sample containing 1) heptylamine, 2) pentylamine, 3) butylamine, and 4) propylamine on A) conventional DB-5 column (80 oC, 50 cm/s carrier gas), B) dual column system (1 m trap with pH 2.2 sulfamic acid coating and 11 m analytical column with pH 11.4 NaOH coating; 100 oC, 18 cm/s carrier gas, analytes released at 30 minutes) and C) tandem system with DB-5 column from A joined with dual column from B (80 oC for 30 minutes then increased to 100 oC, 13 cm/s carrier gas, analytes released at 45 minutes). Other conditions as in figure 2.

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Analytical Chemistry 19

458

Conversely, the amines are retained until they are released from the trap column at 30 minutes

459

and then appear with sharp prominent peaks for each thereafter. Therefore, these features of the

460

dual column system can greatly increase analyte selectivity and simplify the analysis of such

461

samples. In particular, it can remove matrix interference and allow direct analysis of such bases

462

without the need for lengthy derivitization steps that are often required in order to attain

463

appropriate peak shapes for basic analytes 35.

464

Granted, while the results in figure 7B are advantageous for analyzing bases alone in

465

such samples, it can be seen that information regarding the hydrocarbon component of the

466

sample is simultaneously diminished as a result of their complete lack of retention in the

467

system.Indeed, in certain applications it may be desirable to maintain the ability to monitor the

468

hydrocarbon and organic base constituents in the same run. Therefore, in a final set of

469

experiments, it was examined if placing a conventional GC column upstream before the acidic

470

trap column could help facilitate this. Figure 7C shows the analysis of the same sample with a

471

conventional non-polar GC column ahead of the dual column trap system. As seen, when using

472

this now triple column system, the hydrocarbon portion of the sample is reasonably retained and

473

separated on the first column and this is preserved when passing through the remaining two

474

water-based columns since the non-polar analytes have no retention therein. Conversely, the

475

amines are held static upon leaving the first conventional column and entering the acidic trap

476

column. There, they are retained until the interfering matrix components are finished eluting and

477

then released at the 45 minute mark where they appear without being obscured by the matrix.

478

Therefore, this approach can afford the necessary resolution between the bases and the gasoline

479

matrix, and also support the separation of the matrix itself. Accordingly, it could be beneficial for

480

such analyses. It should also be mentioned here that for simplicity in these trials, the same input

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Page 20 of 25 20

481

carrier gas pressure was applied in each case of figure 7 and the resulting carrier gas velocity was

482

subsequently used. As such, in figure 7A, the conventional 30 m column yielded a higher

483

velocity than the 12 m trap system in figure 7B, which also employed a linear restrictor at the

484

outlet. Accordingly, when combining these in figure 7C, the flow was further reduced. With

485

regard to the DB-5 column used, it is also useful to point out that in using it for such analyses

486

over the course of a year with multiple injections of concentrated ammonia and formic acid, no

487

deterioration in its performance was noted and separations before and after such exposure

488

appeared unchanged.

489

It is worth noting that while many of the separations in this work were carried out at 80 to

490

100 oC, this was done to maximize speed while maintaining analyte resolution of the test bases

491

utilized. None the less, with adequate hydration the water stationary phase has been shown to be

492

stable to temperatures investigated up to 160 oC 32. Further, since analyte boiling point does not

493

correlate strongly with retention on this phase, many larger non-volatile solutes can still be

494

successfully eluted within this operating window 32. As well, while not required here, it should

495

also be pointed out that temperature programming with this phase is also readily achievable and

496

does not display any adverse effects on separations 30, 31. Finally, the system is interesting in this

497

regard for triple column applications since target base analytes are trapped initially while the

498

conventional GC column separation takes place. In this way, temperatures can be readily

499

manipulated for the primary column, while analytes remain held in the trap. Then, after the initial

500

separation occurs, the trap and analytical water phase columns can be further manipulated at

501

different temperatures since the release solvent serves to ‘re-inject’ the analyte portion of the

502

sample. Thus, the system can also afford certain flexibility in optimizing temperatures.

503

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Analytical Chemistry 21

504

CONCLUSION

505

A tandem GC column system employing pH-adjusted water stationary phases has been

506

studied and characterized for use in the analytical separation of organic bases. This technique

507

involves the use of a short trap column that can be coated with an acidic stationary phase

508

followed by a long analytical column coated with a basic phase, and employs the differential

509

partitioning behaviour of analytes between the two. The study showed the ability to dynamically

510

and externally control analyte selectivity in GC. As a result, organic bases were reproducibly

511

analyzed with greatly improved selectivity and resolution from other components. Finally, the

512

dual column trap system described here enabled the analysis of organic bases in a gasoline

513

matrix with high selectivity, and the additional separation of the hydrocarbon components when

514

coupled with a conventional GC column (i.e. triple column system). Therefore the versatility of

515

the technique is interesting and potentially beneficial for such analyses. A similar exploration for

516

the analysis of organic acids is being investigated.

517 518 519 520

ACKNOWLEDGEMENTS The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for a Discovery Grant in support of this research.

521 522 523

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[11] Benická, E.; Krupčík, J.; Répka, D.; Sandra, P. J. Chromatogr. 1992, 33, 463-466.

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[12] Baycan-Keller, R.; Oehme, M. J. J. Chromatogr. A 1999, 837, 201-210.

[1] Cazes, J. Ewing’s Analytical Instrumentation Handbook; Marcel Dekker: New York, 2005. [2] Veen, R.F. Principles and Practice of Bioanalysis, 2nd ed.; CRC Press - Taylor & Francis Group: Florida, 2008. [3] James, A. T.; Martin, A. J. P. J. Biochem. 1952, 50, 679-690. [4] Liska, I.; Slobodnik, J. J. Chromatogr. A 1996, 733, 235-258. [5] Kidwell, D. A.; Kidwell, J. D.; Shinohara, F.; Harper, C.; Roarty, K.; Bernadt, K.; McCaulley, R. A.; Smith, F. P. Forensic Sci. Int. 2003, 133, 63-78. [6] Conway, J. M.; Birnbaum, A. K.; Marino, S. E.; Cloyd, J. C.; Remmel, R. P. Biomed. Chromatogr. 2012, 26, 1071-1076. [7] Scott, S. A.; Douglas, G. S.; Uhler, A. D.; McCarthy, K. J.; Emsbo-Mattingly, S. D. J. Environ. Claim. 2005, 17, 71-88. [8] Freeman, R. R.; Kukla, D. J. Chromatogr. Sci. 1986, 24, 392-395. [9] Ettre, L. S. Introduction to Open Tubular Columns; Perkin-Elmer Corp: Norwalk, 1978. [10] Marriott, P. J.; Kinghorn, R. M. Anal. Sci. 1998, 14, 651-659.

[13] Smith, H.; Sacks, R. Anal. Chem. 1997, 69, 5159-5164. [14] Habram, M.; Slemr, J.; Welsch, T. J. High Resolut. Chromatogr. 1998, 21, 209-214. [15] Sharif, K. M.; Kulsing, C.; Marriott, P. J. Anal. Chem. 2016, 88, 9087-9094. [16] Krupcík, J.; Spánik, I.; Benická, E.; Zabka, M.; Welsch,T.; Armstrong, D. W. J. Chromatogr. Sci. 2002, 40, 483–488. [17] Matzke, C. M.; Kottensetette, R. J.; Casalnuovo, S. A.; Frye-Mason, G. C.; Hudson, M. L.; Sasaki, D. Y.; Manginell, R. P.; Wong, C. C. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3511, 261268. [18] Greally, B. R.; Nickless, G.; Simmonds, P. G.; Woodward, M.; de Zeeuw, J. J. Chromatogr. A 1998, 810, 119-130.

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[19] Robson, M. M.; Bartle, K. D.; Myers, P. J. Microcol. Sep. 1998, 10, 115-123. [20] Dorman, F. L.; Schettler, P. D.; English, C. M.; Patwardhan, D. V. Anal. Chem. 2002, 74, 2133-2138. [21] Pacholec, F.; Butler, H. T.; Poole, C. F. Anal. Chem. 1982, 54, 1938-1941 [22] Chen, B.; Liang, C.; Yang, J.; Contreras, D. S.; Clancy, Y. L.; Lobkovsky, E. B.; Yaghi, O. M.; Dai, S. Angew. Chem. Int. Ed. 2006, 45, 1390–1393. [23] Sun, T.; Tian, L.; Li, J. M.; Qi, M. L.; Fu, R. N.; Huang, X. B. J. Chromatogr. A 2013, 1321, 109-118. [24] Phifer, L. H.; Plummer, H. K. Anal. Chem. 1966, 38, 1652-1656. [25] Karger, B. L.; Hartkopf, A.; Pasmanter, H. J. J. Chromatogr. Sci. 1969, 7, 315-317. [26] Karger, B. L.; Hartkopf, A. Anal. Chem. 1968, 40, 215-217. [27] Berezkin, V. G. Russ. Chem. Bull. 1999, 48, 1807-1816. [28] Karásek, P.; Planeta, J.; Roth, M. Anal. Chem. 2013, 85, 327-333. [29] Pollock, R. A.; Gor, G. Y.; Walsh, B. R.; Fry, J.; Ghampson, I. T.; Melnichenko, Y. B.; Kaiser, H.; DeSisto, W. J.; Wheeler, M. C.; Frederick, B. G. J. Phys. Chem. C 2012, 116, 2280222814. [30] Gallant, J. A.; Thurbide, K. B. J. Chromatogr. A 2014, 1359, 247-254. [31] Fogwill, M.; Thurbide K. B. Anal. Chem. 2010, 82, 10060-10067. [32] Darko, E.; Thurbide, K. B. J. Chromatogr. A 2016, 1465, 184-189. [33] Darko, E.; Thurbide, K. B. Chromatographia. 2017, 80, 1225–1232. [34] Kataoka, H. J. Chromatogr. A 1996, 733, 19-34. [35] Ferreira, A. M. C.; Laespada, M. E. F.; Pavon, J. L. P.; Cordero, B. M. J. Chromatogr. A 2013, 1296, 70-83. [36] Li, N.; Chen, C.; Wang, B.; Li, S. J.; Yang, C. H.; Chen, X. B. Appl. Petrochem. Res. 2015, 5, 285-295. [37] Grimmer, G.; Naujack, K. W. Fresenius Z. Anal. Chem. 1985, 321, 27-31. [38] Ternes, T. A. TrAC, Trends Anal. Chem. 2001, 20, 419-434.

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[39] Dugal, R.; Masse, R.; Sanchez, G.; Bertrand, M. J. J. Anal. Toxicol. 1980, 4, 1-12. [40] Basheer, C.; Lee, J.; Pedersen-Bjergaard, S.; Rasmussen, K. E.; Lee, H. K. J. Chromatogr. A 2010, 1217, 6661- 6667. [41] Breitbach, Z. S.; Weatherly, C. A.; Woods, R. M.; Xu, C.; Vale, G.; Berthod, A.; Armstrong, D. W. J. Sep. Sci. 2014, 37, 558-565. [42] Kataoka, H.; Yamamoto, S.; Narimatsu, S. Part III Amines: Gas Chromatography, Encyclopedia of Separation Science; Academic Press: Massachusetts, 2000. [43] Altgelt, K. H. Composition and Analysis of Heavy Petroleum Fractions; 1st ed.; CRC Press - Taylor & Francis Group: New York, 1993. [44] Thornson, J. S.; Green, J. B.; McWilliams, T. B.; Yu, S. K. - T. J. High Resolut. Chromatogr. 1994, 17, 415-426.

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Analytical Chemistry

254x190mm (96 x 96 DPI)

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