Friday, June 19, 2009

Analyzing The Samples...

Author: Hong Ting (Sam) Chiu

We collected 156 samples last week from the batch experiment and were going to analyze them for the arsenic and calcium concentration this week. We have to use Atomic Absorption Spectrometer (AAS) for analysis. There are two AAS at UNESCO-IHE: AAS Flame, and AAS Graphite Furnace. AAS Flame is designed for the mg/L range, which can be used to determine the calcium concentration; AAS Graphite Furnace is designed for the μg/L range, which can be used to determine the arsenic concentration.

For the AAS Flame, I would say it is quite fun and easy to use (and look at when it is on). Before playing with the machine, I have to first create the standard solution for calibration. And since the samples' concentration is higher than the standard, we then diluted the samples before analysis. Lastly, we put in the lamp and turned on the machine and the fuel gas. A flame would light up when everything are on (and I think it is really cool to look at). Then, I just have to insert a tube into the diluted samples and wait; the reading will then appear on the screen afterward. It is easy enough to use :)


AAS Flame.


AAS Flame.
The flame would light up at the gray slit if the power and fuel gas were on.


In contrast, the AAS Graphite Furnace has given us much trouble. We have tried to analyze the arsenic concentration the last few days; but in one way or the other, it fails to give us the results... Some of the problems are: the injection isn't at the center (which resulted in inaccurate reading), the failure at the calibration curve (UNESCO-IHE lab staff prepares the standard solutions), the analysis program shut off for no reason, etc. Also, it takes about 6 hours for one batch of samples, which includes 6 standards and 55 samples. To make the situation worse is that the booking list of this machine is pretty busy. It has already been reserved for the next 7 business days (luckily, we put our name on next Monday too).


AAS Graphite Furnace.


Problem with the calibration curve.
In this trial, the signal for 50 mg/L is almost the same as 40 mg/L.


We have to wait for the result before we can move on with the batch experiment. It is because we will use the same procedures, but with different adsorbent. If the procedures are wrong or if there is a problem with the method, then all the results will be useless. Therefore, I have to ensure this batch produces good result and the procedures are worth repeating. I am thinking of start planning and setting up the rapid small scale column test (RSSCT), which are independent from the batch experiment, next week. Hopefully, the AAS Graphite Furnace will give some accurate results on Monday.

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