Micro-bomb Calorimetry Calibration

Micro-bomb Calorimetry Calibration

We found that a temperature recording at 1/64th seconds (0.015625 seconds) ensured that peak temperature after combustion was captured. Maximum temperature occurs <5-seconds after ignition and a data-logger is required as it is unlikely a digital thermometer will capture peak temperature.

The micro-bomb is calibrated using benzoic acid with a known calorific value (6318 cal g-1). Optimum pellet mass is dependent on the size of the bomb for complete combustion to obtain maximum heat given off. For our micro-bomb, benzoic acid pellets >3 mg did not combust completely and therefore underestimated calorific content. That is, although a strong linear relationship was seen when calibrating with benzoic pellets up to 25-mg dry mass (Figure 2, top panel), note the break-point at ≈20 calories (pellet mass ≈3 mg). Therefore, we calibrated with pellets <3 mg (Figure 2, bottom panel).
If you use the expensive platinum wire then heat is not added to the reaction from the burning of the ignition wire. However, for large numbers of analyses, it may be more cost effective to use Ni-Cr alloy fuse wire. In this instance, combust several Ni-Cr wire samples without a sample pellet to determine heat contribution from the wire and obtain the intercept of the calibration line using the mean value.
Calibration lines should be tested with a second external standard. We used NIST Spinach 1570a with a non-certified calorific content of 3500 cal g-1. This check is invaluable and highly recommended to test for complete combustion. For example, a clogged system may still show a linear relationship between pellet mass (energy) and temperature but will be consistently below expected temperatures. The check of the calibration line using spinach with known calorific content confirmed the break point described above (Figure 2, top panel). Energy of spinach was underestimated when using the calibration line from benzoic pellets up to 25 mg in mass. Using a calibration line with benzoic pellets <3 mg, we measured an average of 3536 cal g-1 for n=35 spinach pellets (i.e., +1% of the expected value).
Once you have obtained a calibration equation then you can start the day with a combustion of one standard of known mass (energy) to ensure that the micro-bomb is functioning properly (i.e., it is not clogged and there is complete combustion). Analyzing a standard at regular intervals is a good check for complete combustion. However, a standard indicating incomplete combustion only reveals that there is a problem somewhere between the analysis of this standard and the previous standard. Thus, this requires a re-analysis of all samples between the two standards. We recommend removing the thermocouple and cleaning the system and thermocouple sheath well before the expected time of clogging. A 10-minute cleaning can save hours of re-analysis time.