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]

ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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47

ABSTRACT

48

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

49

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

50

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

51

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

52

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

53

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

54

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

55

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

56

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

57

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

58

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

59

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

60

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

61

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

62

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.

64 65 66 67

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


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Analytical Chemistry 3

68

INTRODUCTION

69

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

70

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

71

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

72

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

73

others

74

technology over the years and they continue to evolve.

3-7.

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

75

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

76

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

77

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

78

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

79

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

80

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

81

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

82

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

83

front, novel coatings based on ionic liquids

84

benzothiadiazoles

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

86

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

87

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

88

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

89

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


24-26.


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

91

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

92

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

93

GC

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

94

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

95

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

96

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

97

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

98

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

99

in capillary supercritical fluid chromatography 31.

30

100

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

102

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

104

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

105

approach with good sensitivity and peak shape

106

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

107

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

108

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.

109

like oil and gas development

110

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

112

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


Since the matrices

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113

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

114

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

115

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

116

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

117

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

118

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

119

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

120

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

121

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

122

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

123 124

EXPERIMENTAL

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Instrumentation and Operation

126

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

127

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

128

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

129

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

130

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.;

132

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

133

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

134

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

135

result

136

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



137

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

138

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

139

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

140

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

141

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

142

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

143

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

144

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

145

phase system.

146 147 148 149 150 151

FID Response (mV)

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30000

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10000

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152 0

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0

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5

8

2

5

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0 2

5

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Time (min) 154 155 156

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.

157

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

158

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

159

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

160

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


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

162

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

163

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

164

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

165

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

166

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

167

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

168

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

169

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

170

Stationary Phase Preparation

171

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

173

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

175

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

176

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

177

immediately to coat the SS capillary columns as described previously

178

columns were mounted inside the GC oven for separation use.

179

Chemicals and Reagents

30.

Once finished, the

180

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

181

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

182

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

183

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

185

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

186

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

187

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

188

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

189

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

190

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

191 192

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

195

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

196

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

197

observations

198

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

199

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

200

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

201

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

202

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

203

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

204

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

205

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

206

trapping.

32,

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


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

208

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

209

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

210

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

211

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

212

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

213

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

214

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

215

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

216

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

217

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

218

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

219

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

220

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

221

longer basic phase coated analytical column.

222

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

223

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

224

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

225

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

226

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

227

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

228

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

229

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



230

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

231

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

232

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

233

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

234

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

235

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

236

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

237

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

239

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

240

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

241

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

242

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

243 244 245 246 247 248 249 250 251 252 253 254 255 256 257

FID Response

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

5

Time (min)

10

15

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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258

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

259

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

260

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

261

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

262

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

263

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

264

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

265

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

266

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

267

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

268

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

269

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

270

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

271

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

272

characterize the method.

273 274

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

275

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

276

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

277

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

278

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

279

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

280

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

281

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



283 284

FID Response

282 10 min

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30 40 Time (min)

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FID Response

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FID Response

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FID Response

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FID Response

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60 min

0

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30 40 Time (min)

50

60

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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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308

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

309

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

310

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

311

them from other sample components as desired.

312

Altering Selectivity and Resolution

313

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

314

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

315

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

316

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

317

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

318

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

319

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

320

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

321

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

322

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

323

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

324

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

325

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

326

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

327

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

328 329 330



331 332

334

2

A

FID Response

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1

3

0

335

α4,1 = 5.4

4

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15 Time (min)

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336 337 338 339 340

1 2 FID Response

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Page 14 of 25

α1,4 = 1.4

3 4 0

5

B

10

15 Time (min)

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25

30

341 342 343 344 345 346 347

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.

348

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

349

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

350

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

351

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

352

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

353

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

354

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

355

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


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15

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FID Response

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A R2,3 = 0.3

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FID Response

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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

376

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,

379

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

380

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

381

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



382

384

FID Response

383

2 1

4 6 35

A 7

8

9 10

11

385

0

13 12

i

5

10 Time (min)

15

20

386 387 388 389

B

FID Response

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

3 5

7 i

390

0

8 9

i i

5

10

11 12

2

4 6

1

13

10 Time (min)

15

20

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

405

(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

407

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.


Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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

412

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

414

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

416

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.

421

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

422

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

424

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

425

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

430

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.



433 434

FID Response

432

A

1-4

435

0

10

20

30 Time (min)

40

50

60

50

60

436 437

439

FID Response

438

3

440

4

B

2 1

0

10

20

30 Time (min)

40

441 442 443 444 445

2 3 4

FID Response

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

C

0

10

20

30 Time (min)

40

1

50

60

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.


Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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



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


Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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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DYNAMIC CONTROL OF GAS CHROMATOGRAPHIC SELECTIVITY

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