The Thermo-Chemical or high Temperature Conversion Elemental Analyzer (TCEA) is a peripheral device which decomposes organic and inoganic solid and liquid compounds by pyrolysis in a carbon reducing environment. This device is coupled to an isotope ratio mass spectrometer and used to measure:

δ¹⁸O and δD of a wide range of inorganic and organic compounds.

At UCSC, the TCEA is primarily used to measure δ¹⁸O of Silver Phosphate which has been chemically prepared from phosphorus bearing compounds.

Solid or liquid samples are analyzed for δ¹⁸O and/or δD isotopes using a Finnigan high temperature conversion elemental analyzer (TCEA) interfaced to a ThermoFinningan Delta Plus XP isotope ratio mass spectrometer (IRMS). Silver encapsulated samples undergo pyrolysis that breaks molecular bonds, in an oxygen free carbon reducing environment, at ca. 1350-1450°C to generate hydrogen and carbon monoxide gas. The pyrolysis reaction occurs in a column containing vitrous ("glassy") carbon tube filled with glassy carbon grains and sheathed in an alumina tube, and using a reverse He flow configuration. The pyrolysis products then pass through a ascarite (NaOH on vermiculite)/magnesium perchlorate trap to remove water and contaminants before entering a 90°C molecular sieve gas chromatographic column that separates the hydrogen and carbon monoxide gases for introduction into the IRMS.

During analysis, calibrated in-house standards preceed and are interspersed between samples to correct for linearity (size) effects, source stretching, and drift. A second calibrated laboratory standard is run "as-a-sample" to monitor quality control and long term performance. These laboratory standards, which are selected to be compositionally similar to the samples being analyzed, have been previously calibrated against NIST Standard Reference Materials (IAEA-601, IAEA-602 Benzoic Acid).

Corrected delta values are expressed relative to international standards PDB (PeeDee Belemnite) for δ¹⁸O and SMOW for δD.

Note (6 May 2009): The material below was copied from the Yale Isotope lab website as a starting point for the development of the content for this website. The content belongs to the Yale Isotope lab and their personnel. This content will reside here for a short period as this page is developed.

Samples

Sample Size: >The amount of sample required depends on the amount of carbon and nitrogen in the material. A sample should contain between 20-150µg N (60µg optimal size) and 200-2000µg C (60µg optimal). The instrument can detect down to ~2 umole gas, i.e. 2 umole O as CO and 4 umole H as H2. However you should use at least twice as much sample as that for consistent and accurate results--signals should be greater than 2000 mV. For simultaneous &delta:D and δ¹⁸O analysis, the limiting material will depend on the sample type. If there are significant differences between amounts of H and O, weigh out enough material to accommodate the limiting factor, then set the method to dilute the other one if needed. It is always best to run a series of test samples to find the best amounts and settings for your material, as well as for to check for sample homogeneity.

Sample Types

Phosphates and Sulfates: These samples are wet chemically converted as silver phosphate and barite for δ¹⁸O analysis. These samples are usually dried after isolation by heating up to 500oC under vacuum on our vacuum lines. They run fairly cleanly at ~200ug/run, and can be run with added H2 in the carrier gas stream (use the Aux gas on the TC/EA) to maintain reducing conditions. When run neat, barites give close to 100% yields (when compared to benzoic acid), phosphates a bit less, with consistent isotopic values, though there may be slight memory effect. Note: You cannot run phosphates samples after running barites. After running barites it is necessary to use a new or cleaned reaction tube, otherwise poor performance will be observed. After ~150 runs (three autosampler loads), the crucible needs to be changed. After ~300 total runs, the reaction tube needs to be changed/cleaned.

Organic material: Unless youre interested in trapped/absorbed water, these must be dried before being run. The drying is sample dependent. The IAEA/NBS polyethylene foil can be run neat--it doesnt trap water. Pure amino acids samples may crystallize out as hydrates, and that water would need to be removed. Dried vs undried samples can also be run to determine value of the trapped water--just remember to propagate the error. These samples tend to leave a fair amount of residue behind, and the crucible has to be swapped out after every autosampler run, ~50 samples for plant material--though certain materials swamp the reaction column much sooner (<30 samples). This also depends on sample size and homogeneity, and amounts can range from 200ug to >1 mg depending on actual samples. Remember, the hydrogens on -OH and -NH2 groups are exchangeable and the samples have to be dry for good results.

Nitrates: Run as dried silver nitrate for δ¹⁸O analysis. Dry samples, ~200ug, can be run at slower than normal carrier gas flow rates and lower GC temperatures. Nitrates will breakdown in the reaction tube to N₂ and CO, though you can generate some CO₂ if flow rates are too high. The lower GC temperature is needed to give clean separation between N2 and CO2. You may be able to analyze both the N2 and the CO for 15N and 18O together, though that has proved to have some problems here. The N2 peak can interfere directly with the CO peak, and even if there is baseline separation for the main peaks, N2 can form NO in the source that does not pump away quickly, and it will interfere with 18O determination. The work around is to use small samples, dilute the N2 peak as it elutes to minimize NO formation, then re-run the same sample for N2 analysis (EA or TC/EA). For the 18O analysis, H2 in the carrier gas stream can help (i.e. use the Aux gas for the carrier gas on the TC/EA). Crucible is cleaned every 150 samples, reaction column every 300.

Hydrous Minerals: These samples work well for δD analysis. Dry samples, amount can be >2mg depending on quantity of -OH in sample, and 18O analysis can also be done at the same time. This will give total labile H and O, which should be all the H but only a fraction of the O, depending on the mineral. Silica and alumina do not freely release their O under standard conditions. Other types of minerals may or may not release all or part of their O, and there may be isotopic exchange between oxygens at 1450oC prior to release. Crucible is cleaned after ~50 samples, depending on amount of material needed per run.

Silicates, and other materials run with additives. Studies, see Werner, R., Isotopes Environ. Health Stud., Vol. 39, No. 2, June 2003, pp. 85-104, have shown that you can look at silicate O by adding either KF and/or -C2F4- (ptfe, aka Teflon) to the sample. That fluorinates the silicate, releasing the O that then forms CO in the TC/EA pyrolysis/carbon reduction tube. Care must be taken to trap any reactive compounds, especially HF, before the cold trap. The gas coming out of the TC/EA reaction furnace passes through a chemical trap: ascarite (sodium hydroxide on vermiculite, an acid and CO2 trap) and magnesium perchlorate (water trap) loaded into a quartz tube (6mm OD, 35-40 cm long, or larger). Sodium bisulfite (halogen trap) may also be added before the ascarite. The chemical trap is then connected to 1/8 stainless steel tubing, looped back on itself, for the cold trap (liq N2), which is then connected to the switching valve--see discussion below. The cold trap itself is there to keep fluorinated material, e.g. Si F6?, Cx Fy?, etc., from making it to the mass spectrometer.

Typically we use 100-200ug silicate, 300-400 ug KF or 1-1.5mg PTFE (thin slice of teflon tubing), and run at a reduced flow rate. Remember, the KF is hydroscopic so work in a dry box if possible. Also, the PTFE + KF combo (200ug + 750ug to start) seem to work better than either alone. The sequence starts with blank (empty capsule) and reference (benzoic acid) runs, followed by a couple of conditioning/clean up runs (KF or PTFE alone), a fluorinated blank (KF or PTFE alone), then the samples. Please note, each sample usually requires one or two clean up runs (KF or PTFE alone) afterwards, and even then you may still see a memory effect. Fluoridated graphite (powder--dont breath it in) can be used instead of PTFE, but it is much easier to slice off a fixed size of teflon tubing we have and place that into the silver capsule. Both PTFE and the fluoridated graphite are permeable to O2 from the air, which may add to the background. The KF must be dry. These runs should be done in a dedicated reaction tube, and a freshly packed tube will require extra conditioning/clean up runs. The crucible will have to be replaced after each PTFE sequence (49 runs) and every other KF sequence--tube cleaning with every second crucible change. Attempts to run at temperatures <1450 using PTFE did not do well. Initial tests combining KF with PTFE in a single run (say 200-400 ug each), seem to indicate theyll work well together, as long as the KF is dry. Hydroscopic additives, KF, Ag F2?, etc., are difficult to work with without a dry box.

Also, moving to the new He flow regime for the TC/EA (IVA carries new fittings) may be better at minimizing reaction of fluorine/fluorinated compounds with the alumina tubing or quartz wool since the sample only sees the inside of the glassy C tube in the reactor.

Liquids injected via PAL autosampler. Available with some modifications of TC/EA set up. The graphite cone that sits on top of glassy C tube in reactor is replaced by a piece of glassy C tube that brings the total height of the glassy C tubing almost to the top of the alumina tube. A ball of silver wool in placed on top of this, and the injection port is sealed on top of reactor--in place of the zero blank autosampler. A 500 nL syringe (SGE, plunger in needle) is mounted on the CTC-Pal autosampler. Move the zero-blank autosampler out of the way, copy tray 5 to tray 1 so youll use the small sample tray, attach the wash and waste tray and double check positions of injection port, sampler tray and waste tray. Use autosampler method 2, which is for 500 nL SGE syringes, injects 400 nL, and havs the needle wash protocol (5-10x with DI water, 2-3x with acetone that is injected to waste vial that is under vacuum). This gives both DH and 18O results, but with larger error than the H-device and CO2-water equilibration. Expect +/- 2 per mil for H and +/-0.3 per mil for O on good days. Please note, this should give values for the total H and total O in the sample, so it probably would not be the way to follow exchange reactions with a labeled solute.

Standard samples and blank runs: There are three main types of runs in a sequence: sample, reference (for percent composition, reference, start reference mean, add reference mean, start reference regression and add reference regression), and blank (subtracts out areas found in blank runs from succeeding samples to give blk corrected isotopic ratios, blank, start blank mean, and add blank mean). You also need to run an isotopic ratio standard (run as sample type) in the middle and/or at the end of a sequence, which may or may not be the same material you use for the percent composition reference, to account for any drift during the sequence. The best reference is something that is close in composition and size to your sample. The blank should be a clean capsule folded up like the sample capsules. Please note, the first run in a sequence is a junk run used to condition and clean instrument. The autosampler (solid samples) has 49 usable positions, and with one junk, two blanks, two references, and six to eight standards, you have approximately 37 slots available for your samples which you may want to run in duplicate or triplicate. Please see the discussion on setting up a sequence below. Also, note the special situations mentioned under sample preparation, e.g. silicate runs.

TCEA Supplies

SiC heating elements: Kanthal's part number for the SiC element in the TCEA - GB102947. Detailed product number and description: GMO-1-29-150-300-25-4.00-1515-ZG4154 Silicon Carbide Heating element

We have had a devil of a time with the brass connectors that Thermo supplies with their TC-EA furnaces. We'd always find the brass collars all pitted when disassembled. In consultation with Kanthal technical staff about this pitting problem they expressed surprised that Thermo was using brass connectors. It turns out, at high current and temperatures, copper leaches out of brass and creates a galvanic battery effect between the contact of the brass collar and the SiC element. This battery effect causes local overheating at the contact, which converts the aluminum metal sputtered onto the SiC element to aluminum oxide. Hence creating a local insulator and generating more local heating. Ultimately one of our elements started arcing badly and the TCEA started blowing fuses. Kanthal recommended that we switch to aluminum collars, instead of brass. We've had our machine shop make up a collar in aluminum. So far, with limited experience (a couple week-long TCEA sessions), we've been doing better with the aluminum collar. One warning is that you need to add a little thermal blanket insulating material right below the aluminum collar. Heat rising in the chimney of the TC-EA furnace can overheat the Al collar from below and oxidize it. A little insulation can protect the collar from overheating due to the chimney effect.

Glassy Carbon Tubes and Bottom Feeder Adaptors from IVA in Germany (Prices of May 2007):

Item	Description	Catalog #	Price ea	Qnty.	Total
1	Bottom Feeder adaptor for TC/EA	IVA350101008BFA	$337.15	1 EA	337.15 USD
2	Glassy Carbon tube OD 12 mm, ID 7 mm with one side drilled ot 8,6 mm.	IVA127453	1,099.11	3 EA	3,297.33 USD
3	O-ring seals for TC/EA bottom feed adaptor	IVA29000063	39.78	3 SET	119.34 USD

Ceramic tubes:
International Ceramic Engineering
235 Brooks Street
Worcester, MA 01606
Phone: 508-853-4700
Fax: 508-852-4101
Email: sales@intlceramics.com

Material: 99.8% Alumina, Delivery: 6-8 weeks ARO

17.48 mm OD x 12.7mm ID x 47cm long Tube: $43.95/ea
Tolerances* to be: OD: ± 0.87mm, ID: ± 0.64mm, Length: ± 0.38mm

*Technically, at the extremes of these tolerances, the ceramic tubes could in some instances not fit in the furnace, or the glassy carbon wouldn't fit in the ceramic tube.