A dried and finely ground plant or soil sample is wrapped in tin foil. For plants, 85 mg are used. For soil, 150 mg are used. Phenylalanine (for CN) or sulfanilamide (CNS) are used to compute the daily factor that changes based on ambient temperature, pressure, and humidity. Douglas fir and NAPT standards are used as check samples for plant and soil, respectively. Check samples are used for every 15-20 unknown samples. Sixty samples can be loaded into the autosample carousel. Each sample takes 8-15 minutes to process. The machine can be left on with samples loaded overnight. 

Purpose

The Elementar Vario MACRO cube analyzes a sample for total carbon and total nitrogen content.

Equipment

  • Vario MACRO cube
  • 35 mm tin foil square
  • Metal spatula

Consumables

  • Phenylalanine
  • Douglas fir and/or NAPT standards
  • Ethanol

Procedure

Getting the machine ready

  1. Turn on the machine (green button on the bottom of the right side of the machine)
  2. Open up the software
  3. Wait until the TCD temp is at least 50°C (can take about a half hour)
  4. Once the TCD temp is at least 50°C, go to Options -> Settings -> Parameters -> Temperatures and set the temperatures as follows:
  5. Combustion tube: 960
  6. Reduction tube: 830
  7. Post combustion tube: 900
  8. It can take up to two hours for the tubes to get up to the required temperatures. The machine will not allow an analysis to begin if the temperatures are not in range. 

Checking the maintenance status and performing a leak test

Emptying the ash crucible is the only maintenance you might be expected to perform. To check its status, click on the colored indicator bar labelled “maintenance” at the bottom of the screen. On the left side of the window that pops up, click on “ash crucible (steel)”. If the bar is green, and the number under “standing” is low, you do not need to empty the crucible. If the number is high, you might need to, depending on how many samples you are going to run. The ash crucible fills up much faster when soil samples are analyzed than plant samples. If only plant samples are being processed, the crucible can go about 150 samples before needing to be emptied. If only soil samples are being analyzed, it is best not to go above 70. Emptying the ash crucible only takes a few minutes, and doesn’t use up any product, so it is generally a good idea to empty the crucible before starting a new batch, regardless of how many other samples have been processed. If you empty the crucible, change the “standing” number to 0. If you are going to empty the ash crucible, first go to Options -> maintenance -> replace parts. Wait to do anything in the furnace area until the software tells you to. 

To empty the ash crucible

Open the combustion tube flange

Remove the L-shaped tube near the top front of the furnace compartment

Unclamp the reduction tube (on the left) from the side of the machine

Gently pull out the furnace compartment. It will be very hot, so be careful and grip it more towards the bottom. While wearing a heat-resistant glove, gently remove the oxygen flange from the top of the combustion tube. Pull it directly up so as not to damage the oxygen lance that is attached to it. Place the flange upside down on a non-combustible surface. Grab the ash tongs (very long and skinny) and the crucible tongs (look like scissors, have a bulge). Pinch the ash tongs together and insert into the combustion column until the leading edge is inside of the ash crucible. Release the tension on the tongs, and carefully lift them back out. It should bring the red-hot ash crucible with it. Use the crucible tongs to grab the crucible and hold it more stably, and take out the ash tongs. Empty the crucible onto the metal tray on the counter. If the ash is relatively loose, some of it will fall out after a simple inversion. More likely, you will have to tap it a few times and use the metal hex wrench near the tray to scrape out the ash. Remember that the crucible will stay hot, even after the redness fades! To put the crucible back in, replace the ash tongs and gently replace the crucible in the combustion column. Check the bottom of the oxygen lance to make sure it isn’t clogged. Reinsert the flange, being very careful that the oxygen lance goes into the ash crucible, and doesn’t get stuck on the lip or outside of the crucible. Push the furnace unit back into the cube. Close the flange lever. Replace all clamps and tubes.

Check the drying tube

On the left side of the inside of the unit, there is a clear quartz drying tube with a red top and bottom, filled with desiccant. If the desiccant is dry, it is white. If it is not, it is green/blue/purple. If less than a quarter of the column is white, the tube needs to be changed. Ask a CAL employee for help. The drying tube needs to be visually inspected every day.

Perform a leak test

If any maintenance was performed, a leak test needs to be performed. Go to options -> diagnostics -> rough leak test. Before beginning the test, follow the instructions on the screen. The oxygen tank needs to be turned off, and tube 9 needs to be unclamped and blocked with a blocking piece from the leak test kit that is kept in the second drawer to the left of the computer. Follow the instructions on the screen. If the leak test passes, or if the leak test fails, but does so fairly late (40 seconds or later in the “test” phase), it’s good to go. Turn the oxygen back on and reattach tube 9. If the leak test fails, check the connections around whatever part was changed/maintained. If you can hear gas escaping, that’s your leak. If you can’t fix the leak very easily, ask a CAL employee for help.

Preparing a sample

Plant and soil samples should be dried and ground. The finer the grind, the fewer problems will occur in sample prep.

Preparing the machine and computing the daily factor

Make sure that there are no samples from a previous run left in the carousel. Go to system -> carousel position and check the box and type “30” in the “carousel position” box, and click “ok” (don’t hit “enter”!). If the number 1 hole isn’t the next in line, you will need to do a reference run, and then set the location to 30. In the spreadsheet-like part of the software, enter “Blnk” as the name of the first four “samples”. Enter the weight as 100.0 mg. For the method, choose “Blnk with O2”. Then click on the button that looks like a pill that is bright green on both sides to start the auto analysis. It’ll take about ten minutes per sample, so there will be plenty of time to get samples in before the machine catches up.

The next line should be labelled “RunIn”. For the method, choose “phenyl3”. Wear gloves. Clean the metal spatula with ethanol and wipe dry with a KimWipe. Take one of the pressed tin foil squares and gently place it on the balance plate. Close the doors and hit “rezero”. Once it is at 0, use the clean spatula to put approximately 70 mg of phenylalanine in the foil. Anywhere between 67 and 73 is fine. Once the weight has stabilized with the doors closed, hit the “print” button to automatically enter the displayed weight into the next line in the software. If you somehow get out of order, you can manually adjust the weight. The line highlighted in pale yellow will be the one to receive the next weight from the balance.

Gently grab the foil containing the phenylalanine and fold it in such a way that the phenylalanine is completely encased by the foil and is relatively compact. If the foil splits, but you know that know product has spilled, you can wrap another piece of foil around it. You do not need to adjust the weight in the software. Once the foil is all compact, drop it into the small metal piston-like packing device. Press the top part down and lift up the top and middle parts. The sample should now be a little foil disk. Gently pick it up and place it in the appropriate hole in the carousel. All samples are packed in this way, but the mass needed will depend on the type of sample. Dried plant samples should be around 85 mg, and soil samples should be around 150 mg. The absolute minimum for a sample weight is 50 mg. To finish completing the daily factor, prepare three samples named “phenylalanine” that contain 70 mg of phenylalanine. The next sample should be a check sample – either douglas-fir (plants) or the NAPT soil (soil). You can then begin to weigh and add your samples. A check sample should be added every 15-20 samples. The ideal C and N values are written on their containers, along with acceptable variation. Variation will be lower when the samples are dry. The check samples can be placed in the plant drying oven for a few hours before testing if desired. Do not touch the samples with ungloved hands. Do not touch your face with gloved hands. If the sample falls on the floor or anywhere that you didn’t just clean, throw it out.

Samples generally take between 7 and 15 minutes each. You can load as many samples as you can into the carousel, and the numbers on the software will just keep going up. If you stop the analysis (by pushing the red and green pill button) to empty the ash crucible, the first sample that goes in after you restart will not be analyzed accurately, so you will need to redo it or build in blank spots.