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This is the author version of an article published as: Volk, Herbert; Fuentes, David; Fuerbach, Alexander; Miese, Christopher; Koehler, Wolfgang; Bärsch, Niko and Barcikowski, Stephan 2010 First on-line analysis of petroleum from single fluid inclusions using ultrafast laser ablation, Organic Geochemistry, Vol. 41, Issue 2, p.74-77

Access to the published version: http://dx.doi.org/10.1016/j.orggeochem.2009.05.006 Copyright: 2009 Elesevier

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First on-line analysis of petroleum from single inclusion 1 using ultrafast laser ablation 2 3

Herbert Volk1*, David Fuentes1, Alexander Fuerbach2, Christopher Miese2, Wolfgang 4 Koehler3, Niko Bärsch4 and Stephan Barcikowski4 5 6 1) CSIRO Petroleum, North Ryde, Sydney, NSW 2113, Australia. 7 2) MQPhotonics Research Centre, Macquarie University, Sydney, NSW 2119, 8

Australia. 9 3) Femtolasers Produktions GmbH, Fernkornagasse 10, 1100 Wien, Austria. 10 4) Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany. 11 12 Submitted to Organic Geochemistry, Special Issue: AOGC2008 13 14 *Corresponding Author, E-mail: Herbert.Volk@csiro.au, Telephone +61 2 9490 8954, 15 Facsimile +61 2 9490 8197 16 17

Abstract 18

For many years, geochemical analysis of petroleum from single fluid 19

inclusions has been a challenging objective for fluid inclusion studies. In 20

this study, individual petroleum inclusions have been selectively opened and 21

analysed, for the first time, by coupling an on-line femtosecond laser with a 22

gas chromatograph – mass spectrometer (GC-MS). 23

GC-MS chromatograms show straight-chained, branched and cyclic 24

alkanes and aromatic hydrocarbons with carbon numbers ranging from 4 25

(iso-butane) to 19 (pristane). The distribution of these compounds is similar 26

to that observed by on-line bulk crushing, and pyrolysis artefacts such as 27

alkenes and ketones were not detected. Hydrocarbons with higher carbon 28

numbers appear to have remained in the extraction chamber, a limitation 29

that may be overcome by improvements to the inlet system. 30

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This pilot study proves that ultrashort laser pulses can be used to liberate 31

unaltered oil from individual inclusions, which opens exciting research 32

opportunities to deconvolute information on the nature of different 33

hydrocarbon palaeo-fluids from single minerals. 34

Keywords 35

Fluid inclusion; petroleum inclusion; femtosecond laser; laser ablation 36

37

Introduction 38

Petroleum fluid inclusions are tiny, usually <10 µm-sized vacuoles that are 39

filled with petroleum and their occurrence in rocks and minerals is well 40

documented (Burruss, 1981). They can often be found in quartz crystals and 41

sand grains of petroliferous basins, where they occur in healed fractures and 42

along crystal growth zones. The petroleum in these perfectly sealed vessels 43

provides a snap-shot of the fluid composition in the geological past, as it is 44

shielded from contamination and alteration that usually affects 45

hydrocarbons over time. Thus, unique information on the fluid evolution can 46

be obtained by analysing the chemical composition of the trapped 47

hydrocarbons. To date, the chemical composition of petroleum trapped in 48

fluid inclusions can be determined by inference from spectroscopic (e.g. 49

measurement of epifluorescence when excited with ultraviolet light, Raman 50

spectroscopy, Fourier-transform infrared spectroscopy) and 51

microthermometric methods (Munz, 2001) and by mechanical crushing or 52

thermal decrepitation of bulk samples followed by solvent or thermal 53

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extraction for analysis by gas chromatography – mass spectrometry (GC-54

MS) (George et al., 2007). However, fluorescence microscopy and heating – 55

freezing experiments commonly indicate that these crystals contain several 56

fluid inclusion assemblages that hold information from different geological 57

events (e.g. Schubert et al., 2007). While spectroscopic methods, sometimes 58

combined with volumetric measurements and modelling (e.g. Thiéry et al., 59

2000), can constrain the composition of hydrocarbons in fluid inclusions 60

without loosing the spatial resolution provided by a microscope, detailed 61

molecular and isotopic data that hold the key to source and maturity of the 62

oil can so far only be obtained by using the crushing / GC-MS analysis 63

approach. However, as the samples are extracted in bulk, distinguishing the 64

geochemical signature of different fluid inclusion assemblages in the same 65

mineral is not possible. 66

The extraction of fluid inclusions using laser systems can potentially 67

bridge this gap. In some previous studies lasers have been used to extract oil 68

from fluid inclusions, yet these approaches were not selective enough to 69

differentiate individual inclusions. Greenwood et al. (1998) analysed 10-100 70

random inclusions using thermal decrepitation induced by an infrared 71

Nd:YAG laser beam with a wavelength of 1064 nm coupled on-line to a GC-72

MS system. Hode et al. (2006) extracted fluid inclusion oil in a vacuum 73

chamber using an infrared Er:YAG laser with a wavelength of 2940 nm and 74

condensation of this oil onto a cold finger. The condensed oil was 75

subsequently dissolved in hexane and analysed off-line by GC-MS. While 76

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their approach could extract oil from a group of inclusions and individual 77

inclusions could be opened with this laser, it is not possible to analyse 78

volatile or gaseous hydrocarbons in this manner. Moreover, the infrared 79

wavelengths of both the Nd:YAG and Er:YAG lasers couple with the 80

petroleum trapped in the fluid inclusion rather than with the transparent 81

host mineral, such that the inclusions are explosively decrepitated. We 82

found decrepitation of selected inclusions using a Nd:YAG laser hard to 83

control and suspect that the decrepitation mechanism is likely to cause 84

other inclusions to open, as probably occurs in healed, yet still structurally 85

defective zones of the mineral. 86

In the present work, individual petroleum inclusions have been selectively 87

opened and analysed for the first time, by coupling a femtosecond laser on-88

line to a GC-MS instrument. 89

90

Experimental setup 91

The laser extraction GC-MS setup was a modified version of that used by 92

Greenwood et al. (1996). It comprises a Hewlett Packard 6890 gas 93

chromatograph (GC) interfaced to a Hewlett Packard 5973 mass selective 94

detector (electron energy 70 eV, electron multiplier 1200 V, source 95

temperature 250 ºC, 0.1 amu resolution) and an Olympus BX60M system 96

microscope with a custom-built laser chamber and inlet system. The 97

microscope was used with a long-working-distance objective (20x with 98

numerical aperture of 0.4). The Nd:YAG laser of this existing laser 99

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micropyrolysis (LaPy) – GC-MS system was replaced by a high-energy 100

femtosecond oscillator system (Femtosource scientific XL 200, Femtolasers 101

GmbH). The pulses emitted by this laser have a spectral width of about 50 102

nm centred in the near-infrared around 800 nm. In contrast to continuous-103

wave radiation or q-switched (nanosecond) laser pulses, the energy in this 104

case is concentrated into an extremely short, i.e. only about 50 femtoseconds 105

long time interval. This results in an ultrahigh peak intensity at the laser 106

focus (>1014 W/cm2) that induces nonlinear (multiphoton) absorption of the 107

radiation, a process that is virtually independent on the actually centre 108

wavelength of the laser (Lenzner 1999, Martin et al., 2003). 109

The minimum energy that is required to exceed the ablation threshold and 110

to drill micro holes into quartz can be reduced if ultrashort laser pulses are 111

used. For pulses with a duration of 50 fs, the threshold fluence is only about 112

3 J/cm2 (Lenzner 1999). Combined with the fact that with the long-working-113

distance objective used in our setup the laser can be focussed to a minimum 114

spot diameter of about 2 µm, a pulse energy in the order of 100 nJ is 115

required. However, standard femtosecond oscillators (laser systems that are 116

relatively inexpensive, compact and user friendly) only offer pulse energies 117

of up to about 10 nJ. To our knowledge only femtosecond amplifier systems 118

have been employed for the ablation of transparent dielectric media so far. 119

Such laser systems, however, are expensive, complex and require a well 120

controlled laboratory environment, with minimal variations in temperature 121

and humidity, and have to be set-up on vibration-free optical tables. 122

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Moreover, it has been shown that the required spatial resolution in the few-123

micrometer range can only be achieved in the ablation process if the pulse 124

energy is adjusted just above the actual ablation threshold of the material to 125

be processed (Gattass and Mazur, 2008). This implies that the output of 126

femtosecond amplifiers, typically in the mJ range, has to be strongly 127

attenuated for high precision micromachining, which greatly reduces the 128

penetration rate. 129

In this work we have chosen to use a novel and unique laser system that 130

avoids these drawbacks (Fig. 1). It can deliver pulse energies in the order of 131

hundreds of nanojoules while offering the same excellent performance in 132

terms of compactness and user friendliness as standard oscillators. By 133

introducing a Herriot-type multipass cell into the resonator of a standard 134

femtosecond oscillator, its repetition rate can be reduced by more than an 135

order of magnitude. At constant average output power, this results in a 136

substantially increased energy per laser pulse that can be sustained inside 137

the cavity by utilising an innovative dispersion management scheme 138

(Fuerbach et al., 2005). 139

The GC-MS was operated in full-scan mode (50-550 amu) and 140

chromatography was carried out using a DB-5 column (J&W, 25 m , 0.32 141

mm I.D, 0.52 �m film thickness) with helium as a carrier gas (1.5 mL/min). 142

The sample chamber was held at ca. 100°C, and helium at 100 mL/min 143

flushed petroleum liberated by the laser ablation process into a coiled nickel 144

loop immersed in liquid nitrogen. After 2 minutes of trapping and 145

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cryofocussing, the helium flow through the nickel trap was reduced to 1 146

mL/min by switching a 6-port Valco valve, the cryotrap was removed and 147

the nickel trap heated to 320 ºC. Products were then re-focussed in a loop of 148

GC column immersed in liquid nitrogen. Once this trap was removed, the 149

GC oven was programmed from an initial temperature of 40 ºC (2 min. 150

hold), followed by heating at 4 ºC/min to 310 ºC (30 min. hold). 151

152

Sample 153

Our new analytical approach was trialled on a well-characterised 154

idiomorphic clear bipyramidal quartz crystal of ca. 3 mm size that was 155

isolated from a volcanic dyke cross-cutting organic-rich Silurian graptolite 156

shales of the Kopanina Formation at the quarry Kosov in the Barrandian 157

Basin (Czech Republic). Petroleum inclusions in this crystal are abundant 158

and can reach a large size of up to 100 µm in diameter. Inclusions are 159

dominated by abundant blue-fluorescing inclusions with some larger and 160

irregular-shaped inclusions showing a yellow fluorescence colour. More 161

detailed descriptions of the fluid inclusion assemblages in the test sample 162

(KQ7-Qz) and geochemical results from on-line bulk crushing are provided 163

in Volk (2000) and Volk et al. (2002). 164

165

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Results 166

In our first attempt to selectively open a single inclusion we have 167

specifically targeted a small blue fluorescing (<10 µm sized) inclusion. We 168

focused laser pulses with an energy of 160 nJ and a pulse duration of about 169

50 fs at a repetition rate of 11 MHz onto the crystal, thereby ablating the 170

quartz and successfully opening the inclusion. This did not appear to affect 171

nearby inclusions. However, the amount of petroleum that was liberated 172

was too small for confident detection on our GC-MS system. 173

We then focused on a different and larger (> 50 µm) blue fluorescing 174

inclusion that is shown in Fig. 2a. Again, we extracted the petroleum from 175

this single inclusion without affecting adjacent inclusions, including one 176

with a more yellowish fluorescence colour deeper in the crystal (see Fig. 2b). 177

The extraction hole ablated into the quartz crystal had a diameter of about 2 178

µm (Fig. 2c). For this larger inclusion, the amount of extracted hydrocarbons 179

was sufficient to produce a clear signal on the GC-MS. The resulting 180

chromatograms show straight-chained, branched and cyclic alkanes and 181

aromatic hydrocarbons with carbon numbers ranging from 4 (iso-butane) to 182

19 (pristane) (Fig. 2d), while gases were not monitored with the chosen GC-183

MS scan rate. The distribution of low- to mid-weight molecular compounds 184

is similar to that observed by on-line bulk crushing (Volk et al., 2002), 185

however, C19+ compounds that were detected up to n-C31 in the on-line 186

crushing approach could not be detected using our laser approach. Similar 187

to previously reported laser extraction methods (e.g. Greenwood et al., 1998, 188

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Hode et al., 2006) pyrolysis artefacts such as alkenes and ketones were also 189

not detected when using the pulsed laser system. This proves that the laser 190

pulses themselves do not alter the oil composition and do not introduce any 191

thermal artefacts. This can be explained by the fact that due to their 192

extremely high peak intensities, femtosecond laser pulses transfer their 193

energy to the sample via nonlinear optical processes rather then via thermal 194

absorption and thus alter the material by ‘cold ablation’ (Keller, 2003). More 195

detail on the distribution of low-molecular-weight hydrocarbons in the C5-196

C9 range is provided in Fig. 2e. 197

198

Outlook 199

It appears as if hydrocarbons with carbon numbers > 20 have remained in 200

the extraction chamber, possibly due to the relatively cool thermal 201

extraction temperature (ca. 100 ºC) and/or cold spots in the transfer line. 202

Improvements to the inlet system may overcome these problems, and thus 203

enhance analytical sensitivity and the recovery of high–molecular-weight 204

recovery. While increasing the thermal extraction temperature may improve 205

extraction efficiency, it may lead to unintentional thermal decrepitating of 206

some of the inclusions. Coupling fluid inclusion extraction with time of flight 207

– secondary ion mass spectrometry for the detection of biomarkers as 208

proposed by Siljeström et al. (2009) may be another way of significantly 209

enhancing the sensitivity of the system, although this approach will lack the 210

chromatographic resolution currently used to obtain a maximum of 211

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information on petroleum. Nevertheless, this pilot study proves the concept 212

that ultrashort laser pulses can be used to liberate unaltered oil from 213

individual fluid inclusions, which opens exciting research opportunities to 214

de-convolute information on the nature of different hydrocarbon palaeo-215

fluids from single minerals. 216

217

Acknowledgments 218

We want to thank Neil Sherwood and Roger Boudou (CSIRO Petroleum, 219

Sydney) for their assistance in microscopy and photomicrography and V. 220

Suchý (Czech Academy of Science) for sample material. The paper benefited 221

significantly from comments of journal reviewers Paul Greenwood 222

(University of Western Australia) and Jonathan Watson (Open University, 223

UK), and Guest Editor David McKirdy . 224

225

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femtosecond laser pulses. Laser and Particle Beams 23, 113-116. 233

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Gattass, R.R., Mazur E., 2008. Femtosecond laser micromachining in 234

transparent materials. Nature Photonics 2, 219-225 235

George, S.C., Volk, H., Ahmed, M., 2007. Geochemical analysis techniques 236

and geological applications of oil-bearing fluid inclusions, with some 237

Australian case studies. Journal of Petroleum Science and 238

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Greenwood, P.F., George, S.C., Wilson, M.A., Hall, K., 1996. A new 240

apparatus for laser micropyrolysis gas chromatography mass 241

spectrometry. Journal of Analytical and Applied Pyrolysis 38, 101-242

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Greenwood, P.F., George, S.C., Hall, K., 1998. Applications of laser 244

micropyrolysis - gas chromatography -mass spectrometry. Organic 245

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Hode, T., Zebuhr, Y., Broman, C., 2006. Towards biomarker analysis of 247

hydrocarbons trapped in individual fluid inclusions: First extraction 248

by ErYAG laser. Planetary and Space Science 54, 1575-1583. 249

Keller, U., 2003. Recent developments in compact ultrafast lasers. Nature 250

424, 831-838. 251

Lenzner, M., 1999. Femtosecond laser-induced damage of dielectrica. 252

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Martin S, Hertwig A, Lenzner M,: Krüger, J., Kautek, W., 2003. Spot-size 254

dependence of the ablation threshold in dielectrics for femtosecond 255

laser pulses. Applied Physics A – Material Science & Processing 77, 256

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analytical methods and applications. Lithos 55, 195-212. 259

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280

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Figure captions 280 Figure 1: Cross-plot of dependency of work per pulse and pulse frequency. 281

The use of a Herriott-type multipass cell leads to an increase of the 282

cavity length of a femtosecond oscillator and hence the energy per 283

pulse. 284

Figure 2: Petroleum trapped in fluid inclusions in quartz sample KQ7-Qz 285

before (a) and after (b) ultrafast laser ablation. UV epifluorescence 286

photomicrographs. (c) Extraction crater ablated into quartz, scanning 287

electron microscope photomicrograph. (d) Total ion chromatogram 288

(TIC) of the extracted petroleum. nC10 = n-decane etc., iC15 = 289

isopentadecane etc., Pr = pristane). (e) TIC and selected m/z 290

chromatograms providing more detail for compounds in the C5-C9 291

elution range. MCP = methylcyclopentane, B = benzene, MH = 292

methylhexane, DMCP = dimethylcyclopentane, MCH= 293

methylcyclohexane, T= toluene, Alk-CH = alkylcyclohexanes, EB = 294

ethylbenzene, m+p+oX = meta-, para- and orthoxylene.295

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296

Figure 1 297 298

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300

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Figure 2 301

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