Separations of petroleum products involving supercritical fluid chromatography

Journal of Chromatography A, 1252 (2012) 177–188

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Separations of petroleum products involving supercritical fluid chromatography Didier Thiébaut ∗ CNRS, UMR 7195 PECSA, CNRS, UPMC, ESPCI, Laboratoire des Sciences Analytiques, Bioanalytiques et Miniaturisation, Ecole Supérieure de Physique et de Chimie Industrielles de la Ville de Paris, 10 rue Vauquelin, 75231 Paris Cedex 5, France

a r t i c l e

i n f o

Article history: Received 19 March 2012 Received in revised form 21 June 2012 Accepted 22 June 2012 Available online 29 June 2012 Keywords: Supercritical-fluid chromatography Simulated distillation Group-type separation Petroleum compounds Hyphenated systems GC×GC Lubricant additives

s u m m a r y This paper gives a survey of the most attractive trends and applications of supercritical fluid chromatography in the petroleum industry: simulated distillation, group-type analysis and related applications including the implementation of multidetection in a so-called “hypernated” system, as well as the hyphenation to GC×GC for improved group-type separation, SFC×GC and first promising SFC×SFC results. Some specific technical information related to the use of capillary columns or conventional packed columns in combination with FID (or detectors that require decompression and in some instances splitting of the mobile phase prior detection) is also provided. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Application of supercritical-fluid chromatography (SFC) to the separation of petroleum compounds really started in the 1980s. It is related to the fact that SFC, using carbon dioxide as the mobile phase, combines many advantages of gas chromatography (GC) and liquid chromatography (LC) that are suitable for the separation of hydrocarbons and related compounds: its efficiency is close to LC, it works with LC and GC detectors and involves mobile phases whose solvating power can be tuned as in LC; thus, SFC fills the gap between GC and LC. SFC is a niche for petroleum applications because it can combine GC detectors with a LC-like mobile phase: high molecular-weight compounds that could not elute from a GC column can be eluted in SFC, while the universal and sensitive GC detector, the flame-ionisation detector (FID), can still be used. As a consequence, typical SFC applications of petroleum-related compounds [1–5] are GC-like separations, e.g. simulated distillation (Simdis), and LC-like separations, e.g. hydrocarbon group separations. However, a GC-like separation does not mean that a GC open-tubular column must be used. In fact, both packed LC columns and open-tubular GC capillary columns can be implemented in SFC, both having their advantages and drawbacks. However, in SFC, packed columns are much easier to handle than open-tubular

∗ Corresponding author. Tel.: +33 1 4079 4648; fax: +33 1 4079 4776. E-mail address: [email protected] 0021-9673/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2012.06.074

capillary columns (the main application of SFC is chiral separations using the same packed columns as in LC). Replacing a LC by a SFC method is also worth considering in order to reduce solvent consumption and costs, or for environmental reasons. Most of normal phase LC separations could easily be transposed in SFC with minimum optimisation. In the field of petroleum analysis, Simdis is a routine GC application; however, SFC is very attractive for eluting hydrocarbons having more than 80–100 atoms of carbon: they are difficult to elute in GC or high-temperature GC (HTGC) without cracking and their elution can be obtained in SFC on packed or open-tubular columns at much lower temperatures than in HTGC. As hydrocarbons starting from C20 to more than C130 can also be eluted in SFC, it is an alternative to GC for the Simdis of heavy fractions. The most studied type of LC applications in the petroleum industry is hydrocarbon group-type analysis. SFC’s best features for this application are the detection capabilities of FID and the properties of carbon dioxide as a mobile phase: FID provides easy quantification of hydrocarbon groups because of the similarity of response factors of most hydrocarbons. SFC is registered by ASTM for this application. Group-type application of SFC can be improved via the implementation of different stationary phases in series or in multidimensional mode. Selectivity can also be improved via the implementation of multidetection for group-type analysis by the combination of Ultraviolet detection (UV) and FID. The use of a “hypernated” system including Atomic Emission Detection (AED), Fourier Transform Infrared Spectrometry (FTIR), Mass

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Fig. 1. Scheme of apparatus used in open-tubular capillary SFC for simulated distillation.

Spectrometry (MS), UV and FID will be described as a way to improve the analysis of lubricants. Another interesting feature of using carbon dioxide as a mobile phase lies in the hyphenation with GC: it is very easy to hyphenate SFC to GC or comprehensive two-dimensional GC (GC×GC) because one gets a gas after decompression of the supercritical carbon dioxide. Some recent results demonstrating the features of this strategy for the analysis of hydrocarbon groups in heavy gas oils and vacuum distillates will be presented. A separation according to polarity (number of double bounds, presence of polar moieties. . .) can be performed by SFC prior to the on-line transfer to GC×GC; owing to GC×GC separation performance, it is possible to discriminate each hydrocarbon group type for (i) easier quantitation of group types and (ii) plotting simulated distillation curves per hydrocarbon groups. Last but not least, it seems quite reasonable to imagine a SFC×SFC system that could reach performances in the same range as in GC×GC because long columns can be used in the first dimension and a very fast separation can be obtained in the second dimension. This paper will give an overview of the most attractive SFC trends and applications in the petroleum industry: Simdis, group-type analysis and related applications including the implementation of multidetection, as well as the hyphenation to GC×GC for improved group type separation, SFC×GC and first promising SFC×SFC results. Some specific technical information related to the use of open-tubular capillary columns or conventional packed columns in combination with FID (or detectors that require decompression and in some instances splitting of the mobile phase prior detection) is given in an unusual “experimental part”.

2. Experimental part: SFC–FID The use of SFC with FID deserves a few experimental explanations. Moreover, Simdis can be performed in open-tubular capillary SFC: in these conditions, SFC is a miniaturised separation technique performed under high pressure and temperature conditions; although the SFC system to be used is quite simple (Fig. 1), it requires some precautions. If the detector to be used is a FID, as in Simdis and many applications devoted to the separation of saturated hydrocarbons, a fixed restrictor (integral or frit type [5]) must be connected to the column outlet to transfer and decompress the supercritical fluid (SF) in the chimney of the FID; the outlet of the restrictor is placed a few millimetres under the flame of the detector [6]. Only one pump is required to deliver carbon dioxide. For Simdis, the maximum operating pressure of the pump should be as high as possible, higher than 60 MPa, and pressure programming is mandatory.

- Low ID columns: (i) High pressure syringe pumps can be used without splitting the flow rate before the column owing to the very low flow rate they can deliver accurately (less than 0.1 ␮L/min). The pressure in the system depends on the fixed restrictor: pressure and flow rate in the column are linked and pressure/density gradients are obtained by increasing the flow rate of the pump. (ii) HPLC like pumps available in most of SFC systems can also be implemented using a flow splitter prior to the column; the software-controlled automatic pressureregulator is fed by the diverted flow and enables pressure/density programming [7,8]. However, the pressure regulator and the conventional reciprocating pumps have a quite low pressure resistance, 40 MPa (modified Ultra High Performance Liquid Chromatography (UHPLC) reciprocating pumps can be used to exceed 50 MPa [9]). In all the cases, a test separation or the measurement of the CO2 gas flow-rate in the detector should be performed daily; it is necessary to check the accuracy of the flow-rate in a small ID column because the column flow-rate is negligible versus the diverted flow (generally, a flow higher than ca. 0.5 mL/min is necessary to obtain an accurate behaviour of the pressure regulator). - In the case a HPLC column of regular ID should be implemented, one could use a conventional SFC system (P < 40 MPa) or a modified UHPLC one [9]. A splitter should be placed at the column outlet (see Section 5.1) so that the FID be fed with ca. 10–50 mL of CO2 (gas flow rate measured at the restrictor outlet); air and hydrogen flow rates must be optimised [6]. The same set up can be used to hyphenate detectors that require decompression prior detection such as atomic emission detector, sulphur chemiluminescence detector (SCD). With modern columns (packed or open-tubular capillary), fixed restrictors can deliver a constant flow rate for months [10–12]. Columns, injectors and connection tubings must be compatible with maximum operating pressures of the pump. Owing to the high temperature injection conditions used in Simdis, the rotor seal of the injector must be replaced on a shorter period of time compared with standard operating conditions. Various types of connectors for GC or LC can be used. PEEK tubings should not be used owing to the pressure/temperature conditions. GC ferules can be used if firmly tightened. - Samples: In order to reduce the size of the solvent peak, carbon disulphide (CS2 ) is generally used for applications with FID because it should not respond (like CO2 ). For quantitative purposes depending on the concentration of the samples to be injected, the CS2 response can be neglected. For Simdis, Polywax calibration standards can be dissolved in xylene at 100 ◦ C; to avoid precipitation, the injection device should be at high temperature too. Heavy samples to be analysed in Simdis such as vacuum residues (boiling point >700 ◦ C), can be easily dissolved using such conditions [11]. 3. Simulated distillation Simulated distillation is a standardised GC technique described in ASTM methods. It is widely used in the petroleum industry for the evaluation of many types of samples, feeds and cuts treated in refining and conversion processes. It requires a calibration curve to be established between the boiling point of normal paraffins and their elution temperature or retention time to obtain the hydrocarbon distribution of the sample (in weight percent) versus the boiling range of the fraction (expressed in Atmospheric Equivalent Boiling Point, AEBP). Using proper column and experimental conditions, Simdis results are in agreement

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with the True Boiling Point (TBP) distillation curve described in ASTM D2892 [13]. The final boiling point (FBP) that can be reached with GC simulated distillation is exceeding 538 ◦ C (ASTM D2887 [14]) compared with 400 ◦ C for ASTM D2892 and 565 ◦ C for ASTM D5236 [15] involving lower pressure (ca. 10 Pa). The use of high-temperature GC (ASTM D6352 [16], 7169 [17], 7500 [18]), at oven temperatures up to 450 ◦ C allows the determination of higher AEBP, around 700 ◦ C, in routine analysis using non polar dedicated stationary phases; however, the resistance of high molecular-weight hydrocarbons (HMHs) to cracking reactions is questionable [19], although it could only have a minor effect on results [20]; the use of high stability GC columns is mandatory [21]. Owing to the real solvent strength of a supercritical fluid and the polarity of the most commonly used supercritical fluid, carbon dioxide, SFC provides some advantages over GC techniques. Depending on the operating conditions, the polarity of carbon dioxide varies between that of pentane and, at least, of toluene. This is the reason why CO2 is a good solvent of hydrocarbons and SFC a powerful technique for the elution of HMHs at much lower temperatures than in GC. In SFC Simdis, both open-tubular capillary GC-like columns and LC packed-columns can be implemented.

3.1. Packed columns Implementation of micro or narrow bore packed-column SFC (pSFC) for Simdis application is quite straightforward because no split injection is required and the columns provide a high loadability. The first report in 1988 by Schwartz [22] described a 1-mm ID column packed with a polysiloxane material for the elution of nC108 alkane from polyethylene PE740. Further developments of SFC Simdis were carried out using alkyl-bonded silica stationary phases packed in small ID columns (micro bore columns). Main published results are compiled in Table 1 including some comments. Using alkyl bonded phases, the longer the alkyl chain, the stronger the retention of hydrocarbons; no minimum is observed [24,25]. A stationary phase bonded with a short chain such as butyl was preferred in Ref. [25]; it was used at quite high temperature for SFC conditions: When it was operated at a temperature of 170 ◦ C, it enabled the elution of C136 hydrocarbon instead of C106 at 130 ◦ C. However, the higher the temperature of the mobile phase, the higher the operating pressure required to maintain a high CO2 density in the column during the separation; the pressure limits of the whole system (pump, injector, column) were reached. As described by Shariff et al. [24], the lowest difference between the retention of different compounds having the same boiling point was obtained using phases bonded with long alkyl chains, C8 and more (this can be referred as “Simdis selectivity”; it is undesirable because compounds having the same boiling point should coelute regardless of their structure for a perfect simulated distillation; the “Simdis selectivity” can be expressed in retention time when considering compounds having the same boiling points or in degrees when considering compounds having the same retention time but different boiling point). However, as the retention increased with the length of the alkyl bonded chain, a compromise had to be found: to favour a lower retention of the compounds, the deactivated C4 -bonded silica was used to keep the final operating pressure around 50 MPa (this value still exceeded the pressure limit of many reciprocating pumps that could be used at this time); the “Simdis selectivity” was less than 10 ◦ C. No precipitation of heavy aromatic compounds was reported. Nevertheless, a blank injection following oil or standard samples is highly recommended. SFC results were consistent with GC results [25].

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3.1.1. Concluding comments - Elution of heavier compounds than in GC is obtained at much lower temperatures in SFC. - Non-polar alkyl bonded silica LC stationary phases enable elution of hydrocarbons with minimum “Simdis selectivity”; using a nonaqueous mobile phase, such as CO2 , they can be used at elevated temperatures (more than 150 ◦ C) without evidence of degradation of their chromatographic performances [25] and they are suitable for routine analysis. - However, routine SFC-Simdis requires an apparatus designed for operation at elevated pressures: at least all the parts should withstand 60 mpa, including the pressure regulator, so that working at about 50 MPa would not be a problem; the design of a new injection system allowing sample introduction at high temperature is very desirable (some are available for size exclusion chromatography of polymers). UHPLC injectors and pumps could be implemented as soon as they are modified to withstand carbon dioxide and higher temperature conditions than in LC. Of course, columns (mostly HPLC columns) must also withstand these conditions. - Very recently, the use of sub-2 ␮m packed columns has been shown to allow a very fast separation of the standard alkanes mixture from C16 to C80 [9]. The analysis time was less than 4 min using a C18 column while it was shorten to less than 2.5 min with a C4 bonded phase using a density gradient from 80 bars to 300 bars and a temperature of 100 ◦ C. To our knowledge, the fastest reported GC separation of this type of standard mixture takes 4 min and the last eluting alkane is C60 [26]. Thus, SFC Simdis on sub-2 ␮m packed columns deserves further investigations in order to extend the mass range of eluting compounds by increasing both the temperature and the pressure of the mobile phase. - As it will be shown in the section devoted to group-type separation, the hyphenation of packed column SFC to GC×GC allows Simdis to be performed per hydrocarbon group.

3.2. Capillary columns Generally speaking, open-tubular capillary SFC (cSFC) is not very popular in the SFC community: main SFC applications are carried out using LC-like columns and polar modifiers, generally for the separation of enantiomers in the pharmaceutical industry. Simdis seems to be the last niche for open-tubular capillary SFC applications. The main results obtained in cSFC Simdis and reported in the literature are gathered in Table 2. In every case, non-polar stationary-phases are used, including original octyl phases, in similar conditions as in packed column SFC. A very high pressure is required for eluting HMHs exceeding C100 at constant high temperature (>100 ◦ C). Of course, the density of the mobile phase has to be increased during the separation via pressure programming. As stressed above, the lowest “Simdis selectivity” is required. Comparing the TBP and simulated BP of aromatic hydrocarbons from naphthalene to chrysene, octyl-bonded phase are reported to provide a lower deviation than polydimethylsiloxane and phenylmethylpolysiloxane stationary phases [24,27,28]. The reported heaviest paraffin separated in cSFC Simdis is C126 [11] on a 5% phenylpolydimethylsiloxane stationary phase using CO2 at 160 ◦ C and a pressure programming starting at 10 min. from 100 to 550 bar at 13.3 bar/min (Fig. 2). The identification of paraffins having ca. 120 atoms of carbon was possible. Heavier paraffins eluted but were not separated and could not be identified. Fig. 3 shows the chromatogram of a vacuum distillation residue obtained in the same operating conditions: compared with Fig. 2, elution of HMHs is obtained; compounds having more than 200 atoms of carbon (BP > 900 ◦ C) were eluted; their BP were so high that the calibration was out of range for this application. Thus,

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Table 1 Main published results in packed column SFC simulated distillation. Refs

Column Oven temperature

Carbon # sample

Comments

[7] [23]

Packed column C18 bonded silica C11 bonded silica Packed capillary 180 ◦ C Various chain length Packed capillary 120 ◦ C C4 -bonded silica Custom packed capillary 160–170 ◦ C

C100 Polywax 655 C132 Polywax 1000

FID/UV using flow splitting Injection T: 130 ◦ C

C130 Polywax

Better RSD on C6 Capillary packed column

41.5

C136 Polywax 1000

Routine between C80 and C120 Selectivity is better on long alkyl bonded chains (>C4 ) Retention increases vs alkyl bonded chain length

48

[24]

[25]

Final pressure (MPa)

36 >50

Table 2 Main published results in open-tubular capillary column SFC simulated distillation. Authors (year) [Ref.]

Stationary phase

Column temperature (◦ C)

Maximum pressure (MPa)

Heaviest ref. alkane eluted (BP)

Raynie (1991) [27] Shariff (1994) [24] Bouigeon (1996) [29] Dahan (2002) [30] Dulaurent (2005) [11]

n-octylpolysiloxane n-octylpolysiloxane SB-octyl 5% phenyl-methylpolysiloxane PDMS 5% phenyl-methylpolysiloxane 5% phenyl-methylpolysiloxane

150 NA 180

32 NA 50

160 160

55 55

C100 (719) C90 (700) C96 (712) C92 (704) C108 (732) C120 (750) C126 (759)

the calibration curve was extrapolated via a logarithmic regression [11]. The global calibration curve was the combination of the first part, where standards were available, with the last part, where the retention times of standards were extrapolated (Fig. 4). Moreover, it must be pointed out that most of the BP in this range of high molecular weight (MW) had also to be estimated using a correlation curve [25]. Thus, it was possible to obtain information on the

composition of vacuum residues by comparing the Simdis curves of different heavy samples such as vacuum distillation residues [11]. For samples containing a significant amount of low MW compounds eluting in the tail of the solvent peak, the initial part of the curve (no more than 2 points) was corrected using the curve obtained by GC. Then, the SFC and GC curves could be superimposed. Beyond C120 , no information could be obtained from GC [11]. Recently, Simdis of sulphur-containing compounds was carried out using sulphur-chemiluminescence detection (SCD) hyphenated to open-tubular capillary SFC [30]. The chromatograms of three

Fig. 2. Calibration chromatogram of paraffin standards in open-tubular capillary SFC Simdis of heavy samples. Conditions: Column DB5 m × 0.05 mm, 0.2 ␮m. using CO2 at 160 ◦ C, pressure programming starting at 10 min. from 100 to 550 bar at 13.3 bar/min.

Fig. 3. Open-tubular capillary SFC chromatogram of a vacuum distillation residue. Same conditions as in Fig. 2.

(Reprinted with permission from Ref. [11].)

(Reprinted with permission from Ref. [11].)

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Two other methods, D6550-05 [32] and D7347-07 [33], are devoted to the determination of olefin content in gasoline by SFC and of denatured ethanol, respectively; they require an improvement of the separation to isolate the olefins from the non-aromatic fraction. 4.2. Practice of SFC group-type separations

Fig. 4. Extended calibration curve used in open-tubular capillary SFC for simulated distillation of heavy fractions. (Reprinted with permission from Ref. [11].)

vacuum residues were obtained using cSFC–SCD. Similarly to classical simulated distillation, the cumulated sulphur content of samples could be plotted versus the BP (or carbon number). The total sulphur content of the investigated residues determined by SFC–SCD was in good agreement with the information provided with the sample, i.e. 2.5–5.6%. In the part of the curve corresponding to low BP values, it was not necessary to use GC information because the solvent peak (xylene) was not detected by SCD; the ratio of the response of sulphur versus that of non sulphur containing species (S/C) was better than 105 . Owing to the high level of sulphur in the samples, classical and sulphur distillation curves were very similar and almost coincided. Thus, it could be assessed that almost all the detected molecules bore an atom of sulphur. 4. Group-type analysis 4.1. Principle of SFC group type-separations Group-type analysis refers to the separation and quantification of the hydrocarbon groups, i.e. saturates, olefins, aromatic hydrocarbons and “polar” compounds, and, possibly, subgroups as saturates can be further separated into paraffins, branched paraffins and cycloalkanes (“naphthenes”), and aromatic hydrocarbons into mono-, di-, tri- and polycyclic aromatic hydrocarbons. The acronyms used are SAR, SARA or SOARA (saturates, olefines, aromatics, resins and asphaltenes). A large number of methods were reported by Eric Robert in Ref. [5]. This chapter will only deal with SFC applications described in ASTM methods and recent trends (i) to obtain better or faster separations, (ii) to extend the application to heavy fractions or (iii) to provide better separations by hyphenating SFC to GC×GC or developing SFC×GC or SFC×SFC. As already pointed out for Simdis application, SFC is very attractive for group type separations because, as in GC, flameionisation detection can be used with no or minor correction of response versus the hydrocarbon type. In LC, refractometric detection requires a calibration that can be inaccurate [5,28]. SFC can be considered for group-type separations of middle distillates and heavy fractions. The main ASTM method, D5186, was released in 1991 (D5186-03) [31] for the separation of hydrocarbon groups in diesel fuels. The goal was the determination of total aromatic and non-aromatic content. Further upgrades of the method were released in 1996 and 2003 (reapproved 2009); the application range was extended to aviation turbine fuels and blend stocks; the goal was the determination of non-aromatic, mono and polyaromatic hydrocarbon groups.

4.2.1. Determination of non-aromatic, mono and polyaromatic hydrocarbon groups in light fractions (gasoline, gas oil, jet fuel) 4.2.1.1. ASTM methods. The requirements of the D5186 ASTM method for a successful separation of non-aromatic, mono and polyaromatic hydrocarbon groups are quite simple. The criterion used for evaluating the performance of the separation is the resolution between test compounds selected as markers of the groups to be separated. In ASTM D5186-03, a resolution of, at least, 4 between the non-aromatics (hexadecane) and monoaromatics (toluene) must be obtained and the resolution between the monoaromatics (tetraline) and polynuclear aromatics (naphthalene) must be at least 2. This means all the compounds eluted before the end of the peak corresponding to hexadecane are assigned to the nonaromatic hydrocarbons. The compounds eluting after hexadecane but prior to the time corresponding to the start of the peak of naphthalene are assigned to monoaromatics; all of the integrated area occurring after the start time of the naphthalene peak through the final return to baseline is assigned to the polynuclear aromatic hydrocarbons. These resolution values can be very easily reached using adsorption chromatography on high specific area bare silica and carbon dioxide at 30 ◦ C [5,34,35]. Examples of adequate stationary phases are indicated in the method. However, it was observed that only a partial resolution between the non-aromatic fraction and monoaromatic hydrocarbons was obtained on gas oils using a single hydrocarbon-group separation column (250 mm × 4.6 mm) [36]. Replacing the pure carbon dioxide with a mixture containing sulphur hexafluoride allowed the possible extension of ASTM D5186 for the determination of all aromatic groups except olefins that can only be partly separated from alkanes using neat SF6 [37]. However, it is not to be recommended owing to the use of a complex mixture involving corrosive SF6 . 4.2.1.2. Improved separation via the coupling of different columns in series. Other attempts to improve the resolution between the groups or to separate more fractions in diesel fuels involved the implementation of two different columns connected in series. A cyanopropyl-bonded silica connected in series to a bare silica column [38,39] was shown to enhance selectivity between aromatic groups. The use of longer columns can also enhance the overall resolution between families and sub groups; the main drawback is related to the increase of separation duration and spreading of bands to the detriment of the ease of quantification. Improvements of aromatic determination via the double detection mode, UV and FID, were also proposed: it can be found in “vintage” applications notes from Hewlett Packard [40], Analytical Control [41] and a US patent [42]. UV detection is also reported to allow the detection of conjugated dienes eluting in the nonaromatic fraction using two silica columns in series [43]; it was also implemented by Paproski et al. [44,45]. In Ref. [44], Paprosky et al. investigated titania, zirconia and silica and their combination for the separation of hydrocarbon groups in diesel fuels according to ASTM D 5186-03. The best results were obtained using a titania column coupled to a silica column. This combination of columns was shown to improve the resolution between families in the heaviest diesel fuel investigated in this work.

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In another paper [45], the same authors investigated the use of monolithic silica (Chromolith) or shorter silica particle columns in order to speed up the separation. Despite a 13-fold reduction of the analysis time, the Chromolith was not considered as an interesting alternative to silica column owing to its lower surface area and the resulting decrease of the resolution between families. Using the same titania-silica combination of stationary phases in shorter columns than in Ref. [44], resolution and sensitivity compared favourably with results from longer columns [45]. 4.2.1.3. SFC high resolution mass spectrometry. The coupling of supercritical fluid chromatography (SFC) with field ionisation timeof-flight high-resolution mass spectrometry (FI-ToF HRMS), in parallel with ultraviolet (UV) detection and flame ionisation detection (FID) was reported in Ref. [46] for the quantitative analysis of petroleum middle distillates. As expected, SFC separated petroleum middle distillates into saturates and 1- to 3-ring aromatics. FI generated molecular ions for hydrocarbon species eluted from the SFC. The high resolution and exact mass measurements by ToF mass spectrometry provided elemental composition of the molecules. However, SFC–FID assisted by SFC–UV was used for quantification purpose to determine the amounts of saturates and aromatic ring types using a carbon-number calibration. 4.2.1.4. Hyphenated multidimensional techniques: SFC–GC×GC.. Interestingly, the separation between saturated and unsaturated hydrocarbons obtained by SFC was used to perform independent analysis of saturates and aromatics fractions by GC×GC [47]. For this purpose, SFC has been recently on-line coupled to GC×GC for the analysis of gas oils. As described by Levy et al. [48,49], the online hyphenation of SFC only required decompression of the dense fluid to the gas state in the GC×GC injector; no dedicated interface was needed; the transfer of solutes from SFC was carried out via the introduction of an integral restrictor in the GC×GC injector; using the cryogenic capabilities of the GC×GC gas chromatograph, the focusing of the solutes was performed in the first dimension column in order to reduce the band broadening during the transfer of fractions from the SFC column to the GC×GC. Group-type analysis by SFC was restricted to the direct elution of the saturated hydrocarbons in a first GC×GC system of columns while the unsaturated fraction was backflushed from the SFC column to a second GC×GC system of columns placed in the same oven. After trapping, the analysis of the two SFC fractions was performed in one run, thanks to this “Twin” GC×GC system; unsurpassed information according to both the carbon number and group type was reported. Indeed, this setup was very efficient for the determination of families that would coelute without the SFC separation, i.e. (i) naphthenic families that would coelute with aromatic hydrocarbons and (ii) olefins that would coelute with alkanes and branched alkanes. 4.2.2. Determination of olefins using Ag modified columns As demonstrated in SFC by Norris and Rawdon on gasoline [50], olefins and aromatic hydrocarbons can be specifically retained on silver-loaded silica because Ag ions form adducts with the ␲-donor olefins and aromatic hydrocarbons. This pioneering work initiated other approaches involving either multidimensional methods [51,52] or carbon dioxide SF6 mixtures [53]. This lead to multidimensional ASTM method D6550 [32] released in 2005 for their determination in gasoline: the method involves a switching valve connected to a bare silica column having a high surface area where aromatic and oxygenate compounds are retained while olefins are trapped on a Ag+ strong cation exchange column or silver-loaded bare silica column that is connected in series to the silica column via a second valve. With appropriate switching of the valves defined by the separation of the so-called performance mixture, saturates elute first from the two columns;

CO2

Injecon FID orGCx

Ag CN

Fig. 5. Multidimensional SFC system used for vacuum residues analysis prior to on-line GC×GC. (Adapted from Ref. [56].)

then aromatics and oxygenates are backflushed from the silica column to the detector; finally, olefins are backflushed from the silver loaded column to be detected. The previous method was modified in ASTM method D734707 [33] in order to enable the separation of olefins in denatured ethanol. A third column packed with polyvinyl alcohol-bonded silica is added to the previous system upstream of the silica and silver loaded columns in order to trap the alcohol while hydrocarbons pass onto the silica column in a forward mode. Thus, alcohols are backflushed to the detector while the hydrocarbons are maintained in the silica column. Then, the flow is resumed in the silica column to proceed as described in the ASTM D6550 method for the separation of saturates, aromatics and olefins groups. The setup of the method is carried out using a performance mixture of hydrocarbons (no more than 2%) prepared in ethanol. 4.2.3. Heavy samples Multidimensional separation is mandatory for group type separation of heavy samples. It is based on the same principles as the previous methods presented for lighter samples and the separation of olefins; thus, it involves 2 or 3 different columns. The main difference lies in the replacement of the bare silica column by, at least, a normal phase column such as cyanopropyl-bonded silica in order to reduce the retention of very polar compounds as shown in the approach developed by Skaar et al. [54] for group-type separation of crude oil. This approach was recently adapted by Dutriez et al. [55,56] for the analysis of vacuum distillates using on-line multidimensional SFC coupled to GC×GC similarly to the system developed by Adam et al. for gas oils (Fig. 5). Two columns were involved in this multidimensional system, a cyanopropyl-bonded and a silver loaded silica that could be commuted by two six-port valves. In a first step, polar compounds were trapped on the cyano-propyl stationary phase while hydrocarbons were eluted and transferred to the silver loaded silica. Once the aliphatic fraction had eluted from the second column, the sixport valves were switched in order to backflush (i) the unsaturated compounds initially adsorbed on the silver column and (ii) the polar compounds retained on the cyanopropyl column. Using this method, a complete separation between aliphatic, unsaturated and polar group types was baseline achieved as illustrated in Fig. 6 for vacuum distillates. As mentioned for the analysis of gas oils, the high separation power of GC×GC applied to each of the transferred SFC fractions enabled a better knowledge of the hydrocarbon distribution in families. By adding another valve to the multidimensional SFC, an aminopropyl-bonded silica column could be added to the setup to perform a partial separation according to the number of aromatic rings on the aromatic fraction after it was backflushed from the silver column. For the subsequent GC×GC analysis of fractions, an interface (Fig. 7) allowing the intermediate sampling of SFC fractions into loops was designed [57]. The interface enabled (i) the temporary

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paramount importance to reduce the complexity of the sample prior to the GC×GC step. 4.2.4. Separation of unusual families 4.2.4.1. Commercial gasoline [60]. A comprehensive SFC×GC instrumentation was developed for the detailed separation of polar compounds in a commercial gasoline. A porous layer open tubular silica gel column (30-m, 0.32-mm-i.d. Chrompack CPsilica PLOT column) was used together with subcritical CO2 as the mobile phase to effect the group separation of polar oxygenate compounds. A resistively heated GC column was used to perform the fast GC separation according to the volatility of compounds in the second dimension. Comprehensive two-dimensional analysis was carried out in stop flow mode; 5-s fractions of the SFC eluent were transferred to the GC column for fast analysis. The chromatogram of a commercial gasoline is shown in Fig. 8. Aliphatic and aromatic compounds were shown to elute together while ethers were fully separated. Results obtained from a test mixture showed ethers would also be separated from compounds containing an aldehyde or alcohol functional group. It was reported that compounds with a carboxylic acid moiety also eluted from the silica gel. This work is an illustration of a valuable orthogonal combination of columns and techniques. 4.2.4.2. Coal tar liquefaction distillates. 4.2.4.2.1. SFC–GC×GC of middle distillates. Coal liquefaction products definitely appear among the new generation of fuel substitutes. These product characteristics are very far from fuel specifications: their paraffin content is very low as they are mainly composed of naphthenes, aromatics, polycondensed naphthenic and aromatic structures, and heteroatomic compounds (bearing nitrogen or oxygen atom). Oxygen is mainly present in alkylated phenolic and furanic structures. Identification and quantification of oxygen-containing species in coal-derived liquids are required in order to understand their behaviours during processing. Owing to the SFC features for separating hydrocarbon groups, SFC could also been implemented in separation schemes prior to GC×GC for the fractionation of compounds according to their polarity. Omais et al. recently investigated the performances of three polar stationary phases for the separation of the families of compounds supposed to be present in these types of samples [56]. A pyridine phase was included in the set of columns. As can be seen in Fig. 9, saturated hydrocarbons rapidly eluted from the column since no strong interaction can occur with these phases;

Fig. 6. Group type separation of heavy fraction using two-dimensional system depicted in Fig. 5. Separation is performed at 250 bars and 65 ◦ C. (Adapted from Ref. [56].)

storage and (ii) the distribution into GC×GC of the collected fractions coming from the SFC part of the system. While one fraction was analysed by GC×GC, other fractions could be stored in the loops. The transfer to the GC×GC columns was similar to the procedure described above for the analysis of saturated and unsaturated fractions of gas oils by SFC–GC×GC. This setup is being investigated for heavy vacuum distillates analysis owing to the recent advances in the separation of heavy boiling point compounds in GC×GC [58,59]. Again, the implementation of a group-type separation by SFC prior to GC×GC is of Restrictor

V3

1

Septum Purge

From SFC Liner V2

Pressure regulator

First GC×GC column

V1

Fig. 7. Schematic of the designed interface for SFC–GC × GC. (Adapted from Ref. [57].)

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Phenols Furans N-compounds

Paraffins Naphthenes

Aromatics

Silica

Pyridine

Cyano

0

10

20

30

40

min

Fig. 9. Elution zones of different chemical families representative of a coal-derived middle distillate. (Reprinted with permission from Ref. [56].)

Fig. 8. SFC×GC analysis of a commercial lead-free gasoline sample showing the presence of TAME, diisopropyl ether, and diisoamyl ether. Conditions: SFC analysis, CO2 at a pressure of 150 atm and a temperature of 28 ◦ C. The flow through the PLOT column was collected for intervals of 5 s. The GC was repeatedly temperature programmed from −50 to +250 ◦ C at 450 ◦ C/min; hydrogen was used as carrier gas. (Reprinted with permission from Ref. [60]. Copyright 2006, American Chemical Society.)

silica, as expected, could provide a partial separation of the test compounds. Cyano-bonded phase was not retentive enough to provide a sufficient selectivity between the different groups of compounds. As expected too, nitrogen-containing compounds were also strongly retained by each phase, especially silica. Interestingly, the pyridine-bonded phase enabled a full separation of phenols owing to a strongest O H· · ·N hydrogen bond compared with O H· · ·O (29 kJ/mol against 21 kJ/mol). As a result, using the SFC–GC×GC system, the phenolic fraction from the real sample

eluted in the 40–46 min section of the chromatogram; it could be collected without any interference from other compounds (Fig. 10). The subsequent GC×GC separation showed nothing else than phenols [56]: it could be optimised in order to provide a full separation and identification of this group of compounds or even replaced by GC! 4.2.4.2.2. SFC×SFC of vacuum distillate. Unlike the pioneering SFC×SFC work presented by Hirata et al. [61], two conventional supercritical fluid chromatographs were hyphenated without decompression via an on-line comprehensive 2D Liquid Chromatography-like interface [62]; it consisted of a two-loop switching valve allowing the collection of the first dimension column effluent, the second dimension separation of a fraction being performed during the time allowed for the collection of the subsequent fraction of the first dimension eluent. Owing to the diameter of the column used in the first dimension, the flowrate had to be split prior to the modulation. Both dimension separations were monitored via UV detection; for the second dimension, the main flow was diverted to implement a FID for the detection of hydrocarbons and the construction of the corresponding colour plots. The first SFC×SFC–FID chromatogram of a vacuum distillate from a coal tar is presented in Fig. 11. Although it is a preliminary work, the results seem very promising; it can be assessed that with optimisation of the transfer between the two columns for promoting peak focusing, the chromatograms could be very similar to GC×GC ones.

Fig. 10. SFC–FID chromatogram of coal-derived middle distillates. (Reprinted with permission from Ref. [56].)

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Fig. 11. Comprehensive 2D SFC chromatogram of a coal tar vacuum distillate and pressure program profile in the two dimensions. Conditions: Mobile phase: carbon dioxide pressure programmed, 60 ◦ C; 1st dimension, 1 3 Ascentis Express fused core columns (each 15 cm × 4.6 mm, 2.7 ␮m); second dimension column was Syncronis Silica, 5 cm × 2.1 mm, 1.8 ␮m with a CO2 flow rate of 1.6 mL/min. Modulation period was 21 s.

5. Separation of base stocks and additives of lubricants Analysing a lubricant is a complex task since it is composed of both a base stock and a large number of additives that can be added to the base as mixtures (“packages”). Direct spectroscopic methods, such as ultraviolet or infrared spectrometry [63] and NMR [64], on carbon or hetero-elements, offered rapid and general information on the lubricant but these methods generally suffer from low specificity. Chromatographic methods, GC [65–67] and LC [68–70] were used for the separation

Injector

CO2

Packedcolumn

FID

AED

Gaseous phase detectors

MS

PCO2 Effluent

and the identification of additive (most often polymers additives) but they could not directly provide the total determination of the composition of a lubricant [71,72]. Elution of heavy compounds in GC is difficult or impossible and degradation of some additives is likely to occur, while HPLC lacks resolution and both universal and sensitive detector. Thus,

UV

FTIR

Make-up pump

Dense phase detectors Fig. 12. Scheme of the Hypernated SFC system used for lubricant analysis.

Fig. 13. Hypernated SFC separation of ester hemi-synthetic base stock Column Capcell Pak C18 (250 mm × 4.6 mm), 80 ◦ C, Pressure program from 100 to 300 bars at 20 bars/min, CO2 flow rate 2 mL/min. (From Ref. [78].)

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P (253 nm) P (255 nm)

Zn (213 nm) 200

210

220

245 250 255 260 265 nm

nm

FTIR : Gram-Schmidt

UV (254 nm)

FTIR : 1041-948 cm-1 AED S (181 nm) 2

4

6

8

10

12

min

Fig. 14. Hypernated SFC separation of a reference package of additives: overlay of UV-FTIR-AED signals. Same conditions as in Fig. 13. AED 400 ◦ C, S at 181 nm. (From Ref. [78].)

SFC, owing to its versatile mobile phase and its multiple hyphenation possibilities is a powerful tool for such an application. In the same approach as mentioned above for group-type separation, a two-dimensional column switching system was implemented to (i) trap polar additives on a first polar stationary phase (bare silica) while the apolar base stock was partially separated and (ii) to backflush the additives and transfer them on an apolar stationary phase (octadecyl bonded silica) [12]. It must be pointed out that an encapsulated silica C18 column such as Capcell Pak was required for eluting polar additives without adding polar modifiers. Unfortunately, co-elution could not be avoided. Thus, specific detection was implemented in a “hypernated” SFC system to improve the comprehensive analysis of a lubricant.

The hyphenation of AED is well described in [10] for Microwave Induced Plasma. Briefly, the principal parameters to adapt were the position of the restrictor, the flowrates (effluent, plasmagen gases, He and Ar, reactant gases O2 , H2 ), and the wavelength; other parameters were generally set as for GC hyphenation. The main drawbacks were (i) the signal depletion induced by the diminution of the global energy of the plasma owing to the introduction of the CO2 , (ii) specific emissions from the CO2 that perturbed signal acquisition for some elements, principally in the range 170–200 nm. For example, the detection of N had to be carried out at 388 nm via the emission of CN formed in the plasma rather than at 174 nm [10]. The addition of argon in proportion close to 10–15% of the He flow rate was shown to reduce the CO2 interference [10,73]. For MS implementation in the hypernated system, atmospheric pressure chemical ionisation (APCI) was used as ionisation mode using a make-up fluid [74]. When no make-up fluid was added, ionisation could occur because traces of water in the CO2 were able to form (M+H)+ adducts or charge transfer with the CO2 could lead to (M)•+ ions [75–77]. Via a special device the system allowed MS

5.1. Multi hyphenated system Further to FID and UV detection, SFC was hyphenated to AED, MS and FTIR. For FTIR, a high pressure cell was adapted to the detector [5].

Reference Additive

Lubricant

9.4 min

TIC C m/z : 328-330

TIC C m/z : 328-330 m/z : 514-516

m/z : 514-516 0 1 2 3 4 5 6 7 8 9 10 11 min (M+H)+ 515.2

+O .

O S

9.4 min

0 2 4 6 8 10 12 14 16 min (M+H)+ 515.2

O S

O C12H25

397.3

O C12H25

M : 514 u

329.0 100 200 300 400 500

m/z

329.2 200

300

400

500

m/z 600

Fig. 15. Hypernated SFC/MS analysis of Lowinox DLTDP reference additive and of a formulated lubricant. Same conditions as in Fig. 13 MS: APCI in positive mode, methanol added at 200 ␮L/min in the transfer line. The MS spectra correspond to the chromatographic peak at 9.4 min. TIC is the Reconstructed Total Ion Chromatogram. (From Ref. 78.)

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parameters tuning in the supercritical fluid: the tuning compound could be introduced in the supercritical CO2 rather than in the liquid phase used in LC coupling [74,78]. Doing so, the detection sensitivity was improved by a factor varying from 1.5 to 7.7 depending on the compound investigated. The hypernated system FID–UV–FTIR–AED–MS – SFC is presented in Fig. 12 [78,79]. 5.2. Analysis of base stock The ASTM Method D 5186 [31], described above, was not resolutive enough for group type separation in the mineral base stock [73,74,78]. Improvements involving silver-loaded silica and multidimensional system, as presented in the group type part, should be applied. In the synthetic base stock, using bare silica for group type separation, a partial resolution of PolyAlphaOlefins and aliphatics could be achieved under pressure programming and a raw estimation of their amount could be calculated using reference samples [73,74,78]. For the monitoring of base stocks containing esters (Fig. 13), FTIR via specific chromatograms (“chemigrams”) reconstructed using the carbonyl band stretching vibration at 1736 cm−1 demonstrated that it can distinguish esters from the other components of the base stock [78,79]; it was sensitive enough to allow their easy quantification. 5.3. Additives The additives are added to the base stock as mixtures of various compounds having selected properties in order to trap particles, improve viscosity at high and low temperatures, improve resistance to oxidation, limit wear of the engine, etc. Their identification is required for (i) a better understanding of their behaviour, (ii) studying their structure–relationship properties, and (iii) monitoring the ageing of the lubricant. The direct analysis of additives could be performed in one run using the hyphenation properties of SFC. Using AED capabilities, Zn, S, P and N (via CN) could be monitored [10,78–80] to define elution zones prior to further investigations using FTIR and/or MS spectra. For example, on the chromatograms presented in Fig. 14, early eluting peaks could be identified as a major anti-oxidant family. The UV chromatogram also exhibited two intense elution zones of more retained compounds that could be attributed to nitrogencontaining compounds. MS was of paramount importance in the hypernated system because it could give structural information or even identification of compounds as shown in Fig. 15. The hypernated system also proved to be valuable to monitor the ageing of lubricant under standardised ageing conditions. For example, chromatograms of used and unused lubricant exhibited strong differences with the UV detector at 254 nm; in the used lubricant, Irganox L57, or Zn-dithiophosphates also identified with MS, disappeared while other more retained compounds were generated during ageing standardised tests.

general and specific information on the sample composition can be obtained with hypernated SFC – FID–UV–FTIR–MS. The hyphenation of FID is of paramount importance for petroleum compound analysis. SFC is a routine technique for hydrocarbons group-type analysis based on ASTM methods. Recently, online SFC fractionation of gas-oils prior to GC×GC and SFC×GC results demonstrated another of its best features related to its easy hyphenation to GC after decompression. Work is on progress to improve the group type separation of heavier fractions using multidimensional SFC; the recent results obtained in SFC–GC×GC on vacuum distillates and alternative fuels are very promising for the enhancement of the resolution between the families while GC×GC compensates for the lack of resolution between some groups and provides a distribution according to the carbon number. For samples that are not amenable to GC×GC, SFC×SFC is also very desirable and under development for full on-line implementation.

Acknowledgements D.T. wish to thank all the persons who participated with him to some of the experiments compiled in this paper. He is particularly indebted to the students who were or are still involved in SFC experiments, Frédérick Adam, Thomas Dutriez, Badaoui Omais, Laure Mahé at IFP Energies Nouvelles, Rachid M’Hamdi, Vy-Khanh Huynh, Fabrice Bertoncini, Gwenaelle Lavison, Laure Dahan, Alina Dulaurent, Jiri Urban, Cédric Sarazin, Pierre Guibal at ESPCI and their supervisors at IFP Energies Nouvelles, Fabrice Bertoncini, Marion Courtiade, Hughes Dulot, Cyril Dartiguelongue, Jeremie Ponthus and Vincent Souchon, and his colleagues at ESPCI, Patrick Sassiat and Jérôme Vial.

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