OPTIMISED FOR SPEED AND ACCURACY: ASTM D2887 OPTION B


Simulated distillation determines the boiling point distribution of a petroleum product by gas chromatography rather than physical distillation. It is a well-established method for several matrices including crude and finished products.


One of the main advantages of simulated distillation is speed; the ability to automate the process; and to provide a quick finished report. Laboratory throughput requirements have increased in a very high demand industry for fast results.


Study Objectives


In keeping with this need and the desire to use conventional instruments and columns to achieve greater efficiency and higher throughput, ASTM has supported development of an accelerated method as part b in D2887 (1).


An advantage to this solution is that detailed separation is not required as it is in a detailed hydrocarbon analysis which allows for fast chromatography.


The data presented here demonstrates pairing the accelerated chromatography with a gas chromatograph designed to provide maximum throughput and accuracy.


Instrumentation The Clarus®


Table 1: GC oven conditions used on the narrow bore column


Steps Initial


1 2 3 4 5


Ramp (o C/min)


140 105 85 55 35


Temp (o 40


70


115 175 300 340


C) Hold (min) 1


0 0 0 0


0.9


mm x 0.18 µm which will be compared to an Elite 1: 10 m x 0.53 mm x 1.5 µm (PerkinElmer). They will be referred to as the narrow bore and wider bore columns, respectively. The


wide bore, 0.53 mm id, is one of the columns suggested in the method.


Boiling point time assignments were made using ASTM D2887- 12 Calibration Standard containing 20 n-paraffins (Restek). The concentration of this stock solution is 1% of the paraffins in carbon disulfide (CS2) that was diluted 1 to 10.


690 Gas Chromatograph (GC) with wide range flame ionization detector (FID) was used in these experiments. TotalChrom®


Three reference fluids were included in this study to qualify the boiling point assignments. Reference Gas Oil (RGO) #2 (Spectrum Quality Solutions) and two Canadian Proficiency (cross check or CC) Samples D282 and D283 (InnoTech Alberta, Edmonton, AB).


chromatography data system (CDS) was used for


instrument control and data collection (Shelton, CT). Dragon® simulated distillation software from Envantage®


(Cleveland, OH/


Houston,TX) was used to calculate and report the boiling point distribution from the TotalChrom result file.


Developing a fast method with a conventional GC


A GC method has been developed with a focus on speed for both rapid heating to enhance chromatography (GC) runtime, and rapid cooling to minimise delay for the analysis of the next sample. In this development, ASTM D2887 requirements have been maintained.


The Clarus 690 GC has the ability for rapid heating while maintaining stringent retention time repeatability. An additional criterion of the new oven design was ensuring temperature uniformity to eliminate hot or cool zones. This aids in retention time stability which is critical for accuracy in this solution. Oven temperature uniformity eliminates what is known in the industry as “Christmas tree” effects on chromatographic peaks. Narrow Gaussian peak shapes are achieved even for the most volatile compounds.


In addition to fast GC runtimes sample throughput is optimised by fast temperature oven ramp profiles including the unique ability to rapidly cool and re-establish the oven initial temperature. The GC oven cools from 350 to 40 degrees in less than 1.5 minutes. During this time, the syringe is prepared for the next injection further enhancing sample throughput.


Experiment


In addition to an optimised GC, hydrogen was used as the carrier gas in this experiment. According to Van Deemter equation (2), hydrogen is the best carrier gas to use when fast column carrier flows are desired. Hydrogen has the highest efficiency at these flow rates; therefore, optimizing resolution while attaining faster chromatography.


A narrow bore capillary column was investigated to determine if this column could provide faster results while improving the rigorous accuracy required by this solution. The phase and dimensions of this test column was an Elite 1ms: 14 m x 0.18


Figure 1b. Chromatography from the wide bore column AUGUST / SEPTEMBER • WWW.PETRO-ONLINE.COM


Reference samples were not diluted and a 0.1 µL injection was made. Blanks were made without a solvent injection for subtraction.


Discussion and results


Table 1 displays the oven profile used on the 14 m x 0.18 mm x 0.18 µm. Figure 1a and 1b are the chromatograms collected on the narrow bore, 0.18 mm id (run time 6.2 min) and megabore, 0.53 mm id (run time 6.6 min) columns, respectively. The peak shapes and separations are acceptable in both chromatograms; however, the peak efficiency is much improved on the narrow bore column. Because of enhanced resolution, the run time of 6.2 minutes on the narrow bore column can be improved.


The results of skewness, resolution and retention time repeatability for the calibration standard are presented in Table 2. For the narrow bore column, precision was accomplished over eight consecutive runs. For the wide bore column, the calibration standard for precision was acquired at five-point intervals during a nine-hour batch of samples. Even though both are acceptable, the narrow bore column demonstrate improvement in efficiency.


The peak resolution of both narrow and wide bore columns is acceptable (meet the criteria in ASTM D2887 of greater than 3 for C16 and C18 resolution); however, the peak shape (skewness) of the narrow bore column is much improved. ASTM D 2887 recognises that peak skewness results in a distortion of the peak apex, therefore distorting the retention time, and hence creating an error in the boiling point calibration. The sharper and narrower the peaks, the more accurate the retention time calibration. Skewness in section B is listed as between 0.8 and 1.3 section 18.3


Figure 1a. Chromatography from the narrow bore column


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