Optimised for Speed and Accuracy:  ASTM D2887 Option b

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^® 690 Gas Chromatograph (GC) with wide range flame ionization detector (FID) was used in these experiments.  TotalChrom^® 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 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.
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).
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 2 represents a chromatogram from RGO #2 on the narrow bore column. This chromatogram is presented without baseline subtraction to demonstrate raw data on this thin film column.
Table 3 demonstrates excellent performance for boiling point accuracy using the RGO #2 for both columns with all boiling points passing criteria.  The narrow bore column did out perform on initial boiling point (IBP) and final boiling point (FBP).  Five injections of the RGO #2 was performed over a three-day period using the same retention time calibration file with very similar results to those listed in Table 2.  One example is provided.
Table 4 tabulates the results from two Canadian cross check samples analysed in this research. The average temperature and the standard deviation from the proficiency data are presented.  
The temperatures from the PerkinElmer Clarus 690 GC using the narrow bore column are documented. The % deviation from the average temperature from the CC samples compared to the  temperatures from the narrow bore column are recorded. The temperature is in degrees Celsius.
Future work
Additional work has been performed on enhancing analysis time on the narrow bore column. A solution with an analysis time under 5.5 minutes and an inject to inject time of under 7.5 minutes using conventional columns and a conventional GC has been achieved. This research continues with validation on these new parameters including reference standards and cross check samples. Also, Interlaboratory Study (ILS) sample results will be submitted.

Conclusion
The narrow bore column demonstrates excellent performance for skewness, retention time precision and resolution. These results outperform the requirements in ASTM D2887 with multiple injections over a period of months. The results of the three reference standards is further testament to the accuracy of this fast method.
The ability for fast cooldown and fast heating rates enhances sample throughput for the need of this industry for quick results.

Acknowledgements
The author would like to acknowledge Tom Kwoka and Leeman Bennington, Sr Field Application Specialists, and Tony Rhoden, Account Manager, PerkinElmer, and for their contribution developing a fast method for diesel range hydrocarbons which provided a starting point for this development. Also, Jay Ferraro, Scientist, PerkinElmer and Tom Grills, Envantage, in their assistance in optimizing software parameters. In addition to Miles Snow, Sr Scientist, PerkinElmer and Chris Goss, InnoTech Alberta, for their review.

References
1. ASTM Method D2887, Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography1,2, https://www.astm.org/Standards/D2887.htm
2. Using Hydrogen For Gas Chromatography, http://www.restek.com/Landing-Pages/Content/gen_B008