ANALYSES BY INSTRUMENT

Index:

I     Costech Elemental Analyzer

II    Thermo TC/EA

III    OI Analytical TOC Analyzer

IV    GasBench

V    GC-C and GC-TC

VI    Dual Inlet

I.    COSTECH ELEMENTAL ANALYZER

The Costech EA was originally developed for the determination of elemental composition of a sample. This device generates CO₂ from carbon, N₂ from nitrogen, H₂O from hydrogen, and SO₂ from sulfur (special furnace packing is needed for this element). The sample gases are chromatographically separated using a PoraPlot column and then individually passed over a TCD detector and quantified. The effluent is then vented from the room. For isotope analysis, some modifications are made. H₂O is a terrible compound to get into an isotope ratio mass spectrometer. Hence, a magnesium perchlorate water trap is used to remove water from the gas stream. The effluent from the instrument is sent to a Conflo Interface so that a small amount of this gas stream is sent to the mass spectrometer for isotopic analysis. With the EA we can thus measure ¹³C, ¹⁵N, and ³⁴S isotope ratios. It must be remembered that this instrument works by pulsing some O₂ into a hot (1000°C furnace) for combustion. Hence we cannot measure oxygen isotopes with this device.

The most common sample types we look at with the EA are soils, plant matter, and organic matter collected onto glass or quartz filters.

II.    Thermochemical Elemental Analyzer (TC/EA)

The TC/EA is basically a high temperature (1350°C to 1450°C) furnace packed with glassy carbon chips to form a strongly reducing environment. When organics are introduced into the furnace all carbon is deposited as graphite, organic oxygen reacts with glassy carbon to form CO, hydrogen is converted to H₂, and sulfur is (presumably) reduced to a sulfide (S^2-). The instrument can perform these conversions on organics and also on some inorganics. For example, oxygen isotope ratios may be measured in dry, solid sulfates, phosphates, and nitrates. Additionally, some work has shown that inorganic nitrogen isotope ratios can also be measured from nitrate and ammonium salts.

In our current lab configuration the TC/EA is coupled to a MAT 253. This instrument has faraday cups for hydrogen so that we can measure both H and O isotope ratios of waters and solids.

III.    OI ANALYTICAL TOC ANALYZER

The TOC analyzer is designed to determine the total amount of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in an aqueous sample. It works by initial addition of 10% phosphoric acid to a sample solution at close to 100°C. All dissolved inorganic carbon (in the form of bicarbonate or carbonate) is driven off as CO₂. The CO₂ driven off is removed by a continuous flow of helium and streamed past an infrared detector. Sodium persulfate (a peroxide: ie. a very strong oxidant) is then added to the solution and all organic carbon is oxidized to CO₂. This second pulse of gas is also carried past the IR detector for quantification. This instrument operates at near ambient pressures (ie. gauge pressures of 1 to 2psi) and hence is difficult to couple to a Conflo interface for IRMS measurements. However, Gilles St. Jean (from the Hatch Lab at U. of Ottawa) developed an interface that cleans up the gas stream from the TOC analyzer and allows isotope analysis to be performed on the effluent. The interface is shown here:

IV.    GASBENCH AND HEATING BlOCK

Front view showing the heating block and PAL autosampler (by CTC Analytics). The GasBench interface is the box behind the PAL. In the back is Brandon McLean, a graduate student in Everett Shock's lab. In our current setup we have the gasbench plumbed into the deltaplus for use for either carbonate C and O analysis or headspace gas analysis. Our deltaplus advantage does not have collectors for H.		View inside of the GasBench interface. In center back is the reference gas open split. To the left of that is the sample gas open split. On the right side is the 8-port valve with sample loop. The box on the far right is a small GC oven. In front are three valves for reference gas control and one valve for overall helium pressure. There are additional inlets for the flush gas on the left side. We have two cryotraps installed on the gabench as well.

V.    GC-C AND GC-TC

The GC-C/TC is for compound-specific analysis of C, N, O, and H from whatever you can get to make it through a GC.

VI.    DUAL INLET

We have a dual inlet on the MAT 253 for isotope ratio measurements from pure gases. We have a suite of certified gases from Oztech Trading Corp for this application.

Home

Page last updated: April 16, 2007