On the Limitations of UV−Fluorescence Spectroscopy in the Detection of High-Mass Hydrocarbon Molecules

Morgan, Trevor; Millán, Marcos; Behrouzi, Mahtab; Herod, Alan A.; Kandiyoti, R.

doi:10.1021/ef049885g

Cited by 70 publications

(101 citation statements)

References 18 publications

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“…2 does not show this effect since the spectra have been height-normalized to emphasize the relative positions of the peaks. We have shown elsewhere 16 that typically only the fluorescence from molecules below 3000 g mole À1 can be detected. Fluorescence from earlier eluting molecules was too low to be detected.…”

Section: Resultsmentioning

confidence: 85%

Molecular mass ranges of coal tar pitch fractions by mass spectrometry and size‐exclusion chromatography

Karaca

Morgan

George

et al. 2009

Rapid Comm Mass Spectrometry

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A coal tar pitch was fractionated by solvent solubility into heptane-solubles, heptane-insoluble/toluene-solubles (asphaltenes), and toluene-insolubles (preasphaltenes). The aim of the work was to compare the mass ranges of the different fractions by several different techniques. Thermogravimetric analysis, size-exclusion chromatography (SEC) and UV-fluorescence spectroscopy showed distinct differences between the three fractions in terms of volatility, molecular size ranges and the aromatic chromophore sizes present. The mass spectrometric methods used were gas chromatography/mass spectrometry (GC/MS), pyrolysis/GC/MS, electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) and laser desorption time-of-flight mass spectrometry (LD-TOFMS). The first three techniques gave good mass spectra only for the heptane-soluble fraction. Only LDMS gave signals from the toluene-insolubles, indicating that the molecules were too involatile for GC and too complex to pyrolyze into small molecules during pyrolysis/GC/MS. ESI-FTICRMS gave no signal for toluene-insolubles probably because the fraction was insoluble in the methanol or acetonitrile, water and formic acid mixture used as solvent to the ESI source. LDMS was able to generate ions from each of the fractions. Fractionation of complex samples is necessary to separate smaller molecules to allow the use of higher laser fluences for the larger molecules and suppress the formation of ionized molecular clusters. The upper mass limit of the pitch was determined as between 5000 and 10,000 u. The pitch asphaltenes showed a peak of maximum intensity in the LDMS spectra at around m/z 400, in broad agreement with the estimate from SEC. The mass ranges of the toluene-insoluble fraction found by LDMS and SEC (400-10,000 u with maximum intensity around 2000 u by LDMS and 100-9320 u with maximum intensity around 740 u by SEC) are higher than those for the asphaltene fraction (200-4000 u with maximum intensity around 400 u by LDMS and 100-2680 u with maximum intensity around 286 u by SEC) and greater than values considered appropriate for petroleum asphaltenes (300-1200 u with maximum intensity near 700 u).

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Section: Resultsmentioning

confidence: 85%

Molecular mass ranges of coal tar pitch fractions by mass spectrometry and size‐exclusion chromatography

Karaca

Morgan

George

et al. 2009

Rapid Comm Mass Spectrometry

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“…All LDI results that do not account for these experimental factors are likely invalid (Mullins et al in process). Gas-phase aggregation in LDI can cause misinterpreted data to yield apparent huge asphaltene molecular weights (Trevor et al 2005;Herod et al 2007).…”

Section: Introductionmentioning

confidence: 99%

“…What is clear is that polystyrene, which is commonly used as an SEC standard, has very different adherence properties than asphaltenes and is not a suitable standard. Certain SEC experiments obtain giant molecular masses for asphaltenes (megaDalton) (Trevor et al 2005;Herod et al 2007) and employ eluting solvents such as N-methyl pyrrolidinone, which flocculate up to ½ the asphaltene sample (Mullins et al in process). The giant asphaltene molecules are likely the expected asphaltene flocs (Mullins et al in process).…”

Section: Introductionmentioning

confidence: 99%

Review of the Molecular Structure and Aggregation of Asphaltenes and Petroleomics

Mullins

2008

SPE Journal

115

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Summary Tremendous strides have been made recently in asphaltene science. Many advanced analytical techniques have been applied recently to asphaltenes, elucidating many asphaltene properties. The inability of certain techniques to provide correct asphaltene parameters has also been clarified. Longstanding controversies have been resolved. For example, molecular structural issues of asphaltenes have been resolved; in particular, asphaltene molecular weight is now known. The primary aggregation threshold has recently been established by a variety of techniques. Characterization of asphaltene interfacial activity has advanced considerably. The hierarchy of asphaltene aggregation has emerged into a fairly comprehensive picture, essentially in accord with the Yen model with the additional inclusion of certain constraints. Crude oil and asphaltene science is now poised to develop proper structure-function relations that are the defining objective of the new field: petroleomics. The purpose of this paper is to review these developments in order to present a more clear and accessible picture of asphaltenes, especially considering that the asphaltene literature is a bit opaque. Introduction The asphaltenes are a very important class of compounds in crude oils (Chilingarian and Yen 1978; Bunger and Li 1981; Sheu and Mullins 1995; Mullins and Sheu 1998; Mullins et al. 2007c). The asphaltenes represent a complex mixture of compounds and are defined by their solubility characteristics, not by a specific chemical classification. A common (laboratory) definition of asphaltenes is that they are toluene soluble, n-heptane insoluble. Other light alkanes are sometimes used to isolate asphaltenes. This solubility classification is very useful for crude oils because it captures the most aromatic portion of crude oil. As we will see, this solubility defintion also captures those molecular components of asphaltene that aggregate. Other carbonaceous materials such as coal do possess an asphaltene fraction, but that often will not correspond to the most aromatic fraction. Petroleum asphaltenes, the subject of this paper, can undergo phase transitions that are an impediment in the production of crude oil. Fig. 1 shows a picture of an asphaltene deposit in a pipeline; obviously, asphaltene deposition is detrimental to the production of oil. Immediately it becomes evident that different operational definitions apply for the term asphaltene in the field vs. the lab. Indeed, the field deposit is very enriched in n-heptane-insoluble, toluene-soluble materials, but this field asphaltene deposit is not identically the standard laboratory solubility class. It is common knowledge that a pressure drop on certain live crude oils (containing dissolved gas) can cause asphaltene flocculation, the first step in creating deposits that are seen in Fig. 1. Highly compressible, very undersaturated crude oils are most susceptible to asphaltene deposition problems with a pressure drop (de Boer et al. 1995). In depressurization flocculation, the character of the asphaltene flocs is dependent on the extent of pressure drop, suggesting some variations in the corresponding chemical composition (Hammami et al. 2000; Joshi et al. 2001). Comingling different oils can result in asphaltene precipitation that can resemble solvent precipitation. Asphaltenes are hydrogen-deficient compared to alkanes; thus, either hydrogen must be added or coke removed in crude oil refining to generate transportation fuels. Thus, asphaltene content lowers the economic value of crude oil. Increasing asphaltene content is associated with dramatically increasing viscosity, especially at room temperature; again, this is of operational concern. The strong temperature dependence of viscosity of asphaltic materials is one of their important properties that make them useful for paving and coating; application of asphaltic materials is facile at moderately high temperatures, while desired rheological properties are obtained at ambient temperatures.

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“…Different authors have suggested models with one or the other being predominant for different asphaltene samples (e.g., refs. [19][20][21][22][23][24][25] favour the archipelago architecture and [26][27][28][29][30][31] favour the continental) or asphaltene fractions [32], notwithstanding that the reliability of some common experimental methods may be a matter of concern [33][34][35]. A predominant molecular structure for asphaltenes supporting the continental archetype has been identified in the Yen-Mullins model [36][37][38] (cf.…”

Section: Introductionmentioning

confidence: 99%

Thermodynamic modelling of asphaltene precipitation and related phenomena

Forte

Taylor

2015

Advances in Colloid and Interface Science

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On the Limitations of UV−Fluorescence Spectroscopy in the Detection of High-Mass Hydrocarbon Molecules

Cited by 70 publications

References 18 publications

Molecular mass ranges of coal tar pitch fractions by mass spectrometry and size‐exclusion chromatography

Molecular mass ranges of coal tar pitch fractions by mass spectrometry and size‐exclusion chromatography

Review of the Molecular Structure and Aggregation of Asphaltenes and Petroleomics

Thermodynamic modelling of asphaltene precipitation and related phenomena

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