GC-FID Method Note – C7-C40 Saturated Alkanes
Analysis of C7-C40 saturated alkanes standard in the miniGC
Introduction
In this app note we run a standard in the miniGC containing saturated alkanes from C7 (Heptane) to C40 (Tetracontane). The compounds are the constituent compounds of gasoline, diesel fuel, kerosene, and motor oils, therefore it is an important test for a GC to be able to separate the compounds, produce good peak shapes for these compounds, and be able to see peaks for the longer chain, less volatile compounds, like C40. It is recognized as a good test for a GC ensuring that the system can get to the elevated temperatures needed in every part of the system and that the system does not have cold spots, which would result in not seeing the later eluting compounds or seeing poor peak shapes.
In this app note we simply show three injections of a C7-C40 standard and qualitatively assess the overall chromatograms and how repeatable they are. Further studies beyond this are planned.
The standard we are using contains C7 (heptane), C8 (octane), C9, all the way through C40 (Tetracontane) in 1000ug/mL (approximately 1000ppm) concentration each in hexane. It is C7-C40 Saturated Alkanes Standard from Millipore Sigma (SKU: 49452-U).
Our method calls for injecting 1uL of the sample using a method with a 100:1 split (1/100 of the injected sample goes onto the column). This means we are injecting 1000ng of each alkane into the system and putting 10ng of each alkane onto the column.
miniGC Method 1
| Column | MXT-5, 30 m, 0.25 mm ID, 0.25 µm (cat.# 70223-LUC25) |
| Sample | C7-C40 Alkanes standard from Millipore Sigma (PN: 49452-U) |
| Injection | |
| Inj. Vol.: | 1.0 µL split (split ratio 100:1) |
| Liner: | 4 mm low pressure drop precision inlet liner w/ wool (cat.# 23309) |
| Inj. Temp.: | 300°C |
| Oven | |
| Oven Temp.: | Stage 1: Hold at 40°C for 1.0 min Stage 2: Ramp to 350°C at 30°C/min and hold for 11.0 min |
| Carrier Gas | He, constant pressure |
| Linear Velocity: | 40.7 cm/sec (2.00mL/min) |
| Detector | FID @ 325°C |
| Instrument | Lucidity miniGC |
We injected the standard 3 times in a row all at the same concentration to test repeatability.
Figure 1 shows an overlay of the 3 injections.
Figures 2-4 show the 3 injections individually.
Conclusions
This is a very cursory study only qualitatively evaluating the chromatograms produced in the miniGC from an alkanes standard that includes alkanes up to C40. The injections show a good overall chromatogram indicating that the system is capable of running to higher temperatures without leaving cold spots in the sample pathway. Each compound from C7 to C40 come out in very regular intervals right where you would expect them and with good size, meaning the detector is reasonably sensitive. Even C40 comes out where it should and with good peak shape and height. In addition all three injections overlay on each other very nicely showing good retention time repeatability and good peak area repeatability. The clean baselines after the C40 and the clean baselines throughout the run seem to suggest that the miniGC is also able to get all the components all the way through the system not leaving any compounds on the column or elsewhere in the system that might show up in subsequent runs as garbage or unexpected peaks.
Further work is planned to do a more quantitative study as well as to explore some real world petroleum standards.
More results from the miniGC can be found here: https://luciditysystems.com/products/minigc/minigc-results/
More information on the miniGC can be found here: https://luciditysystems.com/products/minigc/
To see the miniGC in operation go here: https://luciditysystems.com/products/minigc/minigc-interface/