Analytical Instrumentation


21


Figure 3. Mean peak areas of the enriched TPH mixture run using helium carrier gas (brown bars), hydrogen cylinder carrier gas (red bars) and hydrogen generator carrier gas (white bars).


Gasoline analysis


GC analysis is used across the oil and gas industry for hydrocarbon analyses using techniques such as detailed hydrocarbon analysis (DHA) which is a separation technique used by a variety of laboratories to analyse and identify individual gasoline components as well as for bulk hydrocarbon characterisation. Bulk analysis looks at gasoline composition of PONA components (Paraffins, Olefins, Naphthalenes and Aromatics) and other fuels in the C1-C13 range as this gives an indication of overall sample quality.


The analysis of gasoline for spark ignition components is essential for quality control. Because of the complex nature of gasoline samples, resolution between eluents is required and long columns, typically 100m in length, are used. Several methods are routinely used for DHA which differ in their oven temperature ramp rates or in the length of column used. Each method has advantages and disadvantages with some improving peak resolution of low boiling compounds while others provide better resolution of heavier, later eluting compounds. The complex methodology along with the use of long columns means run times can easily exceed 120 minutes when using helium carrier gas. The use of hydrogen, however, can greatly increase sample throughput because of its efficiency at higher linear velocities4


. Of course having faster run times


counts for nothing if critical separations cannot be achieved, and use of hydrogen carrier gas has been shown to increase run time efficiency whilst maintaining critical DHA separations4. This will appeal to oil analysis laboratories since faster throughput of sample means increased profitability. The benefits of using hydrogen in terms of improved chromatography combined with the increasing cost of helium along with supply issues means that laboratories switching from helium to hydrogen can become much more profitable whilst conforming to industry standards.


Hydrogen vs helium carrier gas


We assessed the performance of hydrogen compared with helium and cylinder hydrogen for carrier gas in GC-FID using a Total Petrolium Hydrocarbon Mixture (TPH Mixture 1, Supelco). The TPH mixture contained 10 alkanes which we enriched with two polar compounds, 1-Octanol and 2-Nonanone, in order to assess both polar and non-polar compounds using the different carrier gases. Using a DB-1 column (30m x 0.25mm, 0.25 µm film thickness) we made a split injection (50:1) of 1 µl enriched TPH mixture.


Comparison of the results shows that hydrogen offered a 25% reduction in run time compared with helium (figure 2), with a reduction from 12 minutes to 9 minutes for analysis. Despite the faster sample throughput, there was no loss of resolution, in fact peak shape improved when using hydrogen and analysis showed that hydrogen from both generators and cylinders produced consistently larger peaks than helium (figure 3). We found that in general hydrogen produced significantly larger peaks than helium carrier gas for each of the 12 compounds analysed (data not shown). When looking at the sample variance, results of analysis using carrier gas from a hydrogen generator showed much more uniform results than either cylinder hydrogen or helium (figure 4).


These results clearly show that hydrogen offers a number of advantages over helium for carrier gas in FID analyses. Throughput is increased, peaks are larger and when using a gas generator, results are more consistent. Consistency of results is very important to chromatographers, since samples are not always replicated and it is important to be sure that results from sample to sample are comparable. The results show that hydrogen from generators can produce more


Figure 4. Variance of mean peak areas of the enriched TPH mixture using helium carrier gas (brown bars), hydrogen cylinder carrier gas (red bars) and hydrogen generator carrier gas (white bars).


consistent chromatographic results than hydrogen or helium from cylinders.


The helium shortage has prompted a number of laboratories to switch from helium to hydrogen for carrier gas. Those who make the switch can see benefits in reduced cost, superior chromatography and faster throughput.


Experimental: Reagent: Total Petrolium Hydrocarbon Mixture 1 (Sigma-Aldrich cat. No. 861424-U)


GC Conditions: Carrier Gas Carrier flow Column Inlet


Oven initial temperature


Generator hydrogen 3.6 mL/min


DB-1 (30m x 0.25mm, 0.25 µm film thickness)


Split (50:1) 60°C (1 min hold) Oven heating rate 40°C /min to 280°C Run time GC References


1: Schroeder & Hotappels (2005). Explosion characteristics of hydrogen-air and hydrogen-oxygen mixtures at elevated pressures. International conference on hydrogen safety, Pisa. CD-ROM publication, Paper 120001.


2: Yamada et al. (2011). Mechanism of high-pressure hydrogen auto-ignition when spouting into air. International Journal of Hydrogen Energy 36(3), PP 2560–2566


3: Bruker Application Note # 1820230: Performance of Method 8270 Using Hydrogen Carrier Gas on Bruker SCION™ GC-MS


4: https://theanalyticalscientist.com/issues/0713/detailed-hydrocarbon-analysis-dha/ 9 min Agilent 6890 with FID Cylinder hydrogen 3.6 mL/min DB-1


(30m x 0.25mm, 0.25 µm film thickness)


Split (50:1) 60°C (1 min hold) 40°C /min to 280°C 9 min Cylinder helium 1.6 mL/min DB-1


(30m x 0.25mm, 0.25 µm film thickness)


Split (50:1) 60°C (1 min hold) 40°C /min to 280°C 16.5 min Agilent 6890 with FID Agilent 6890 with FID


All of our articles are online! To view and download them, visit: www.petro-online.com


AUGUST / SEPTEMBER 2013 • WWW.PETRO-ONLINE.COM


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52