TECHNICAL NOTE: Analysis of volatile fatty acids in rumen fluid by gas chromatography mass spectrometry using a dimethyl carbonate extraction

Abstract

Analysis of rumen fluid volatile fatty acids (VFA) is typically conducted by injecting acidified aqueous rumen fluid into a gas chromatograph (GC) with a flame ionization detector (FID). Aqueous samples are highly problematic because of the large vapor volume that can lead to poor peak shape and contamination of inlets, potentially causing sample carryover. Methods using aqueous samples are not well suited for use in a mass spectrometer (MS) detector system. The objective of this project was to validate a dimethyl carbonate (DMC) extraction process and GCMS method for rumen VFA analysis. To perform the extraction, 100 µL of sample, KHSO4 (500 g/L), and 2-ethylbutyrate (internal standard; 8.5 mM) were added to a microcentrifuge tube (in order) followed by 1 mL of DMC. The mixture was thoroughly vortexed and centrifuged. The organic layer (top) was removed and placed in a GC vial. The DMC extract was injected (0.5 µL) into an Agilent 5977B GCMS (8:1 split injection) with a polar DB-FFAP column. The column was held at 105 °C for 5 min, increased at 10 °C/min to 150 °C, then 65 °C/min to 240 °C, and held constant for 10 min. The peak area of acetate relative to the internal standard is linear from approximately 2 mM to at least 130 mM and encompasses the expected values of rumen concentrations for the other VFA. Recovery of VFA from spiked rumen fluid was tested at three concentrations in rumen fluid from steers fed a finishing diet or grazing wheat pasture. Recovery was not affected by the diet of the animals (P > 0.10) or the amount of VFA spiked (P > 0.19) for acetate, propionate, isobutyrate, or butyrate. There was an interaction of amount of VFA spiked and the diet of the animal (P = 0.021) for valerate and a tendency for an interaction (P = 0.051) for isovalerate, due to the recovery of the VFA being lower in the medium spike amount in rumen fluid from cattle on wheat pasture. Overall, recovery was greatest for propionate (101.9 ± 1.67%) and lowest for valerate (95.7 ± 1.95%). Including the 10-min hold at 240 °C at the end of each run prevented carryover from sample to sample. This method appears to perform well in a GCMS system and accurately and precisely quantifies rumen fluid VFA.

Keywords: VFA analysis; gas chromatography; mass spectrometer.

Figure 1.

A representative chromatogram from the analysis of volatile fatty acids in rumen fluid of a steer consuming a high-concentrate diet.

Figure 2.

Effect of carryover of acetate with repeated injection of a water blank, rumen fluid sample, and followed by water blank injections. Water blank and rumen fluid samples were extracted using the dimethyl carbonate method and analyzed with a gas chromatogram with a mass spectrometer detector (GC-MSD). Initial GC-MSD conditions showed increasing acetate concentrations with repeated injections and were sustained with 5 blank injections. Inclusion of a 10-min hold at the final temperature removed the carryover of acetate.

Figure 3.

Effect of time after extraction of rumen fluid on detector response (left y-axis) and concentration (right y-axis) of acetate (A), propionate (B), isobutyrate (C), butyrate (D), isovalerate (E), valerate (F), and the internal standard 2-ethylbutyrate (G). Time points within panel lacking a similar letter differ P < 0.05.