Topics Map > SCS CORES > Microanalysis
MICROANALYSIS - Elemental Analysis Method for CHN via CHN/O/S Elemental Analyzer CHN440
This method covers the analysis of Carbon, Hydrogen, and Nitrogen via the Exeter 440 instrument, which is a horizontal-movement system. While the instrument can be configured to run O or S, we do not do so. ICP analysis can be done for S percentages and we do not have the capabilities for running O.
The CHN instrument detects only Carbon, Hydrogen and Nitrogen, all during the same analysis, from the same sample aliquot. To detect these elements, the sample needs to be broken down into its atomic components and then separated. The sample is broken down via combustion in an oxygen atmosphere at 1000°C. At this temperature, the desired elements react with the oxygen to form CO2, H2O, N2, and NxOy. Reagents in the combustion tube scrub out interfering components of the samples. The gases are then carried via a stream of helium gas through a reduction chamber, then to a homogenization chamber, then to a thermal conductivity detector. The detector reports a result to the computer, which compares it to the known result of a standard. The gases are then run through CO2 and H2O traps in the process of detection, with the differential signal being proportional to the concentration of gases removed by the scrubbers, which is a function of the amount of the desired element in the original sample. The weight of the sample is then used with the result to calculate the percent of each desired element by weight.
For solid samples, an aliquot is collected and placed in a tin capsule, which is then weighed, crimped/sealed and then placed in a nickel sleeve for placement in the instrument’s 64-position auto sampler wheel. The sleeve prevents the tin of the capsule from sticking to the quartz ladle. These client samples, set-up samples and calibration standards are designated as ‘SHA’. The use of tin capsules are to aid in the combustion phase, as the use of this type of capsule can increase the temperature in the combustion tube to over 1700°C. (The temperature of the oven should not be set at temperatures higher than 1000°C, as a too-high final temperature will have a detrimental effect on the instrument and its components.)
Semi-solids, liquids and air/extremely moisture-sensitive samples are considered ‘non-routine’ and are designated ‘drop-in’s’ as they are run immediately after weighing and are ‘dropped into’ the instrument. For liquid samples, an aliquot is aspirated up a capillary glass tube and placed into the tin capsule, which is then sealed via cold welding, placed in a nickel sleeve and then immediately run. Semi-liquid samples are usually transferred to the tin capsule via capillary glass tube or the cleaned-off end of an unused straightened paper clip, sealed via crimping or cold-welding (depending on the sample characteristics) and then run the same as a liquid sample. ‘Drop-in’s’ are designated as ‘SSI’ in the system. SSI runs can be activated via the SSI option on the control panel. SSI causes the system to pause after the current run finishes and the system will not continue until told to by the operator. After they have been run, if the samples have been sealed via cold-welding, the results should be recalculated with the blank N amount of a previously known amount from a cold-welded sealed blank. The control panel can also tell the system to enter Overnight mode once the entire run set is finished. (The Standby button.)
The nickel sleeve with the tin capsule and sample is then placed in the ladle within the instrument, either via the autosampler wheel or via the technician ‘dropping – in’ the sleeve, capsule and sample via tweezers into the drop-in port. With drop-in samples, the ladle is moved slightly (via the ladle switch on the control box) so that the sleeve drops flat onto the glass, enabling the ladle to move freely and prevent ladle jams. Once the run has been started, the ladle is driven by the instrument program into the furnace where combustion takes place and the analysis described above in the Summary continues. Liquid or possibly volatile samples cannot be used with the autosampler wheel, in case the seal on the capsule is not air-tight, thus preventing contamination of the entire wheel and its samples.
Duplicate runs of solid samples are the standard procedure, as long as enough sample can be recovered. Triplicate runs of liquid/semi-liquid samples are the standard procedure in case of lack of precision between only two runs. Lack of precision could be caused by lack of homogeneity in aliquots, sample being above the cold-welded seal, volatile components being lost, etc. In cases where the air-sensitive capsules are filled by the client, they decide how many jars they wish to fill and submit.
Turn off overnight standby option via the menu options. Review the previous day’s print out to ensure the run was completed and that the Acet calibration runs were within spec. (If there were problems, consult the Troubleshooting SOP.) On the print-out document of the sample run, record the current total & when-to-replace total counts for: combustion/reduction tubes, CO2/H2O traps, He/O2 scrubbers. (This information can be found in the Service: Maintenance Schedule section of the instrument program.)
If the traps, tubes and/or scrubber have been changed, record that in the EA 400 Maintenance Log. Also record if gas cylinders have been changed, ink cartridges have been changed and other problems such as ladle jams. If the ladle has been jamming, see the Maintenance SOP for how to change the ladle. Occasionally inspect the ladle for bumps or residue that may prevent the removal of old sleeves/capsules. It may be possible to remove these with a tool, scraping along the glass. If the ladle breaks in the combustion tube, both the combustion and reduction tubes may need to be replaced or, if the reduction tube is still new-ish, just replace the combustion tube.
Select the Service option from the instrument program and then the Bridges option. This lets the detector bridges ‘warm up’. The bridges need to be running about 15 minutes for the system to stabilize if the instrument has been used the day before. If the instrument has not been used for a number of days, it is better to have the bridges warming up for at least an hour. If extra COND runs are being run after the replacement of the combustion/reduction tubes, the bridges do not need to have been running that long. If the bridges have not warmed up enough, the detectors will not be stabilized by the time the calibrations are run, leading to incorrect calibration values and K’s.
The bridge values should be between 3000-5000, although it is usually kept at about 4000. If necessary, the detectors can be adjusted with the small screwdriver from the tool drawer. This should be done carefully, as the adjustment screws for the detectors can be very sensitive. Remove the cover of the instrument. Four detector adjustment screws should be visible. The identification of which detector belongs to which screw are labelled on the outside cover and visible with the cover off. The orientation to turn the screw as to increase or decrease the bridge value is also noted on the cover and visible with the cover off. Clockwise decreases the value, counter-clockwise increases. (Note: the C screw is very sensitive and finicky; the slightest change may throw it way off of where it was originally. Even after the value is in the right range when you adjust it, it may stabilize out of the desired range.)
The instrument must be set-up anew after sitting idle for more than ten minutes. The set-up runs consist of a set of blanks, conditioners and standards, before the SSI or SHA samples are then run. When the combustion/reduction tubes have been replaced, additional conditioner runs should be run. Standards should always be fresh. But they can be weighed out and stored overnight in the vacuum desiccator above the balance, along with stable non-volatile weighed sample capsules. Don’t allow standard to sit over a weekend in the tin capsule. Standard calibration runs are considered ‘in spec’ if the results are between these ranges: %C: 70.69 - 71.49, %H: 6.31 - 7.11, %N: 9.96 - 10.76 .
Once the standards and samples for the autosampler wheel have been weighed out, select the Start a New Run option from the instrument program. The start of the wheel can be delayed by indicating the time you want the wheel to start running. A list for the samples should appear. Insert the weight and identification of each autosampler wheel run in the sample position as where you will place that specific sleeve and capsule. Once the weights and labels have been entered, click Okay. This will bring up a new instruction block.
Setting up the wheel:
Open the hand valve to release any pressure inside the autosampler wheel box. Open the spent capsule door, under the autosampler wheel box. Remove the spent capsules from the spent capsule chamber box, using a brush and very small dustpan type item; disposing of the spent capsules into a container to be put into the chemical waste when full. Clean the chamber out thoroughly with a kimwipe. If needed, grease the opening of the chamber with a small amount of grease onto the chamber door. Put the cover back on the spent capsule chamber box, making sure that the seal is tight.
Open the autosampler wheel box, diagonally unscrewing the screws until the top is able to be opened. Remove the sample wheel from the box. Remove the pin from the center hole location of the sample wheel and put it in the hole on the side. Reset the sample wheel by turning counterclockwise until it can’t move anymore. This sets hole #1 as the first sample, when the hook inside the autosampler box is set at the mark. Carefully transfer the weighed capsules and sleeves from the holding blocks into the appropriate numbered hole. Make sure that the correct sleeve goes into the same hole as in the list you entered! The system uses the weight you entered, for that run, to calculate the results! If the weight is not correct, the results will be off! Put the filled wheel into the autosampler box, matching up the hook with the marked place on the wheel. Take the metal pin from before and replace it into the center of the wheel. The lid will not close if the pin is in the side of the wheel. Make sure that the o-ring for the autosampler box is intact and looking well. (Occasionally grease the o-ring with a very thin layer of vacuum grease if needed.) Close the autosampler box lid and tighten the screws in a diagonal fashion until tight. Close the hand valve and then click the Okay options on the instruction box on the monitor. The instrument should start purging the autosampler box for three minutes. Once that is done, the system will either wait until the delay time is reached or will immediately start running the programmed list of runs.
The program will print out the programmed list once you have entered your whole list. It will also print out about four runs per page. Keep an eye on the toner for the laser printer and give the toner cartridge an occasional shake to redistribute the toner evenly.
Instrument set-up runs:
The wheel can only hold 64 sleeves. Using SSI, more than 64 runs can be performed over the course of an entire run set. The spent capsule chamber can hold about 100 spent sleeves, after which, the ladle may not be able to move well, as it can catch on spent capsules sticking out of the chamber.
1. Blank - no sleeve or capsule, just an unfilled spot. Weight should be 0.0ug. If a blank is necessary later in the run but you do not want the Blank Values to change (for example, making sure that an overweight sample has completely run through the system, leaving nothing behind, or right before a sample looking for trace carbon amounts), set the weight to 100.0ug and the label to MT (empty).
2. COND - weight should be greater than 1900 and and less than 2200ug.
** If this is the first set-up run after replacing the combustion/reduction tubes, add three (in addition to the above two) COND runs before the second Blank.
4. Blank - nickel sleeve and squished tin capsule. This sets up the correct blank N amount for samples in the autosampler wheel. If necessary, a cold-weld sealed capsule can be run to find the blank N value to be used in recalculations. If this is to be run, run it first, before the regular Blank, as the system will use the blank values of the last blank run for its parameters. (Which is also why run #1 is a Blank.) Weight is 0.0ug.
6. STD1 - These are the runs which set the KC/KH/KN. If the weights are off or something else is wrong and the difference between the current STD1 and the previous one are too great, the K’s will not average between the two values and thus will not be correct, which will probably cause the Acet values to be off-spec. It is possible to recalculate these K values and recalculate the results after the STD1. Weight should be greater than 1900 and and less than 2200ug.
7. STD1 - The K values for this second STD1 run should be different than run #6, but not different enough that the instrument thinks that something is wrong and thus does not change the K values. K values are the average between the K’s of run #6 and run #7. Later STD1 runs will be the average of the K values of that run and the previous STD1 run.
8 Acet - Weight should be greater than 1900 and and less than 2200ug. This standard run is considered ‘in spec’ if the results are between these ranges: %C: 70.69 - 71.49, %H: 6.31 - 7.11, %N: 9.96 - 10.76 .
It is very important for the instrument to be
calibrated before sample analysis occurs.
If the values of run #8 is not within spec, the samples should not be run until the problem is fixed. Usually this is because the K values did not change or have a problem. Options are to recalculate the K values and see if the Acet values are corrected, weigh out another standard capsule and insert as a drop-in Acet run and see if the weight of run #8 was incorrect or weigh out another two standard capsules and run them as STD1 and Acet and see if the second Acet works. (See how to insert drop-in runs.)
Calibration runs prove that the instrument was working within acceptable parameters, especially in case there is a question about a sample result. The run program can be stopped by selecting the K/B’s button on the run panel. This will pause the run system until the K’s and B’s of the run have been okayed by the technician. Once okayed, the system will continue.
After 6-9 sample runs (usually depending on whether SHA or SSI. SHA samples are run in duplicate, SSI in triplicate), three calibration runs (Acet/STD1/Acet) are run to check that the instrument is still in spec, slightly adjust the instrument if needed (as over time, the components in the combustion/reduction tubes will work in slightly different levels; along with how the H K-values especially will change over time while running) and then show that the instrument is still in spec. The STD1 K values will change, based on these component changes, which will then be used in the calculations of the runs until the next STD1 run.
Acet runs bracket each set of samples, so as long as one Acet run’s values are within specification, the samples are considered to be ‘in spec’. If both are ‘out,’ then the samples will need to be rerun or the weights of the Acet runs need to be rechecked and possibly recalculated. The sample runs right before an Acet run can affect the values of the calibration: samples with low H values can cause the H value of the Acet to read too low, samples that were overweight can cause results of the Acet run to be too high. If the N values rise drastically over the course of the run, especially between two runs of the same sample, the reduction tube is probably exhausted and needs to be replaced.
How to insert drop-in runs:
Drop-in runs are inserted at points that the instrument is paused. This is done by selecting the Standby option on the run panel. These can be the sealed N-blank run, blanks/empties, COND, STD1, Acet or sample runs. Once the instrument is paused, input the weight of the drop-in and the label of the run. Select Okay. Then open the drop-in port, twisting the port key and lifting it out of the port. Set the key in a safe place. Transfer the drop-in sleeve into the port hole, onto the ladle. Using the ladle power switch, move the ladle back and forth a little until the sleeve falls flat, allowing the ladle to move freely. This prevents ladle jams. Replace the port key, making sure that the key is twisted to lock. Select Okay on the instructions until the run starts.
Sample and Standard preparation
Samples and standards must be weighed on the microbalance, protected from drafts and vibration, in micrograms. The weights should then be recorded in the run notebook and on the printed sample submission form. Autosampler runs will be weighed prior to being placed in the wheel, while the drop-in runs will be weighed just prior to dropping into the instrument. Samples should be within 1500ug (1.5mg) and 3000ug (3mg) for accurate results.
Sometimes, not enough sample is provided by the client. Then, try to proportion the weights to at least 1500ug for each duplicate run or, if there is not enough to do that, weigh out as much as possible from the sample provided, under 3000ug. If the amount of recoverable sample is not enough, make sure to notify the client of this in the results email, especially if not enough sample was provided for even the minimum weight for one run. If the client did not provide enough sample for the minimum weight, tell them that their results may be affected.
Soil or trace carbon samples: If the compound is a soil sample with low or trace carbon percents, increase the weight of the sample in the capsule, to provide a great enough weight difference to give an accurate result. This is usually 5mg, but it can go up to 15-20mg. It depends on the sample theoreticals and how much can fit into the tin capsule. Be careful of samples that claim trace theoreticals of carbon, as some clients do not know what their theoreticals actually are and just put a random number onto the submission form. When in doubt, stick with below 3000ug of sample, as it is easier to run more samples of a greater weight at a later time than have to deal with an overload of the system.
Cleaning of the area and tools can be done with kimwipes, acetone and an air puffer. Make sure that the sample prep area and the balance pan are kept clean! If acetone was used to clean the tools or the pan, make sure that it has completely evaporated away before weighing a new sample/standard, or the weight may not be accurate.
Solid samples and standards will be transferred using a clean metal spoon/spatula into a tin capsule, which will then be crimped with the tweezers to seal and then be placed into a nickel sleeve. Make sure that the outside of the tin capsule does not have sample/standard clinging to it, as those bits may fall off when the capsules/sleeves are moved and not be analyzed, changing the weight of what was actually run.
Make sure that there is no transfer/contamination between different samples!
Make sure that the weights are accurate and not drifting from loss or gain of moisture from the air!
If the weight drifts, make the decision as to whether a tighter crimping of the capsule is needed or whether the sample should be run as a drop-in.
Moisture-/air-sensitive (non-jar protocol):
Sometimes samples will be submitted that change or degrade upon exposure to air, but the client cannot or will not use the jar protocols. This is more often seen in off-campus clients. These samples are treated the same as drop-ins.
Liquid samples will be aspirated up a capillary glass tube and placed into the tin capsule, which is then sealed via cold welding, placed in a nickel sleeve and then immediately run. Aqueous solutions will not be run! Aqueous solutions have the possibility of overloading the water traps and exhausting the components prematurely. Also, given usual concentration of non-water-compounds in aqueous solutions, results would be more likely to show simply water than anything else. Semi-liquid samples are usually transferred to the tin capsule via capillary glass tube or the cleaned end of a new straightened paper clip, sealed either by crimping or cold-welding (depending on the sample characteristics) and run the same as a liquid sample. Once again, make sure that the outside of the tin capsule does not have sample clinging to it, as those bits may rub/blot off and not be analyzed, changing the weight of what was actually run.
Samples sealed by cold welding will have trapped air within the capsule. You can indicate on the instrument as to whether a run is sealed ‘yes’ or ‘no’, but there is no actual change in the calculations due to switching this value from no to yes. The technician must make sure to recalculate the results using a sealed blank run previously. Usually a new sealed blank run is performed after the helium tank has been changed, in case there is a leak where air is being introduced, changing the N blank value of the system.
Air/moisture sensitive samples (jar protocol):
Samples may be submitted in the air-sensitive jars, via the laboratory procedures. Each plastic jar is given a unique designation. This designation is written as well onto an empty glass vial with a sharpie (the color depends on the type of capsule that will be going into the vial. Red is for tin capsules (CHN, Aluminum ICP analysis, Fluorine halide analysis), blue for aluminum capsules (all other ICP analyses, Bromine/Chlorine/Iodine halide analysis). Aluminum capsules must not be used in the CHN instrument!. The balance is tared and an empty capsule is weighed. This weight is recorded into the Air Sensitive Capsule Notebook, correlated to the unique designation.
A client will then take a specific jar and add their sample to the capsule. Remind clients that the weight needs to be between 1500ug and 3000ug. The capsule is then closed by the client, usually just by crimping, which does not completely seal the capsule, but as most clients prepare these samples in a glove box environment (which the laboratory does not do, mostly because the balance would not be able to be accurate in a place with drafts) and bring the capsules back in the glass vials filled with an inert gas, the exposure of the sample to air is minimal. With the purchase of a second cold-welder, we can offer to lend one out to on-campus clients for preparing their samples. (Remind clients who borrow the cold-welder that if the welder cuts the top off of the capsule, this piece must be brought back in the vial as well, or the weight will be useless.) The capsule is placed back into the designated glass vial, then the glass vial is placed back into the respective plastic jar and brought back to the laboratory. The client is asked to talk to the technician at least a day prior to when they want to have their samples analyzed, so that the instrument is available and the sample does not sit and deteriorate.
When the instrument is stopped for a drop-in, via the Standby command, the plastic jar is opened and the balance is tared to zero. Once the balance is stable at zero, the glass vial is opened and the capsule is transferred via tweezers to the balance pan. Once the weight is stable, the weight of the capsule is recorded on the submission form. (If the weight is not stable, it is probably because the client did not seal the capsule well enough or there is sample on the outside of the capsule which is degrading upon the exposure to the air. Work as quickly as possible and make sure to notify the client that the weight was not stable and that this may have affected their results.) Subtract the original weight of the capsule as listed in the Air Sensitive Capsule Notebook from the filled weight for the weight of the sample. Record this sample weight onto the submission form and use it in the sample weight section when identifying the sample in the instrument. Also record the sample weight into the run notebook. If the client provided less than 1500ug in the sealed capsule or provided more than 3000ug, tell them that their results may be affected.