Hydrocarbon-Type Analysis of Base Oils and Other

Hydrocarbon-Type Analysis of Base Oils and Other

Heavy Distillates by Thin-Layer Chromatography with

Flame-lonization Detection and by the Clay-Gel Method

Bhajendra N. Barman

Texaco Inc., Research and Development, P.O. Box 1608, Port Arthur, Texas 77641

Abstract

Thin-layer chromatography (TLC) using quartz rods coated with

sintered silica particles and flame-ionization detection (FID) is

applied for the rapid determination of saturates, aromatics, and

polar compounds in petroleum heavy distillates. A procedure is

described by which hydrocarbon classes are baseline separated,

and quantitative results are obtained by the measurement of peak

areas. As different hydrocarbon groups are fully resolved from each

other and replicate analysis can be carried out rapidly, accurate

determination of hydrocarbon types is possible by the T L C - F ID

technique. T L C - F I D results are compared with those obtained by

the ASTM D2007 method, the latter being based on gravimetric

determination of separated components by open-column clay-gel

chromatography. It is found that the ASTM method does not always

yield pure fractions of each hydrocarbon type, and crosscontamination

introduces considerable inaccuracies in the results.

Besides superior hydrocarbon type results, T L C - F I D is shown to

offer several operational advantages such as short analysis time,

small sample requirement, simultaneous analysis of multiple

samples, less solvent consumption, and reusable sorbent.

Introduction

Hydrocarbon-type analysis of petroleum-derived lubricant base

stocks and other heavy distillates is useful for monitoring the

quality of these products and determining their suitability for

specific use or compatibility with lubricant additives. Moreover,

such analysis provides data that are important for upgrading and

processing high-boiling refining intermediates and products.

A handful of methods are available for the group-type or

compositional analysis of heavy distillates using both chromatographic

and nonchromatographic techniques (1-4). Perhaps

two of the most commonly used methods are ASTM

Method D2007, which is based on clay-gel adsorption chromatography

(5), and thin-layer chromatography with flame-ionization

detection (TLC-FID) (6-11). The ASTM method has

been in use since the late 1960s. Since its initial development

in the late 1960s and early 1970s (12-14), TLC-FID has been

applied extensively for the analysis of numerous samples including

lipids, natural fats and oils, drugs, surfactants, lubricant

additives, crude oils, heavy petroleum fractions, coal-derived

liquids, asphalts, and environmental samples (6-20).

The TLC-FID technique employs reusable thin quartz rods

coated with micrometer-sized adsorbent particles. A layer of an

adsorbent, such as silica or alumina, is sintered to each quartz

rod. A set of 10 such "chromarods" (6,7) are generally mounted

in a metal rack. Usually, 1 μL of a sample solution (equivalent to

a few micrograms of sample) is spotted on each rod. These rods

are developed simultaneously with appropriate solvents to

achieve the separation of saturates, aromatics, and polars. Then

the rack is placed on a moving frame within a scanner unit

where each chromarod is individually passed lengthwise through

a stationary oxygen-hydrogen flame. The ions produced due to

the pyrolysis of separated sample components are collected by a

metal cylinder, and the current is amplified and recorded.

ASTM D2007 is based

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