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Data Acquisition with the PowderPump™

This graph shows a typical plot of emission intensity versus time when powder sample introduction is used. For illustration purposes, we will assume that 10mg of material was introduced over the time shown. In this case the background emission intensity (on peak background) without powder flowing is 0.5. The average emission intensity while powder is flowing through the plasma is around 7.5. This sort of graph could be obtained from the spectrometer with "time domain" data acquisition.

The second graph (above) shows the same data as a histogram. Even though the integration periods are 10 seconds, the background level of 0.5 is still evident. This sort of data could be obtained from the spectrometer with using many repeated on-peak integrations for data acquisition. The spectrometer normally does this sort of data acquisition for liquid sample introduction. It is also typical for the spectrometer software to process the intensity data statistically to give standard deviation of the signal and number of integrations. This statistical information, when using the PowderPump, is of little value since we want the baseline intensity and the total intensity numbers for our calculations.
To calculate the integrated emission due to the powder introduced, it is necessary to subtract the background emission from the emission due to the analyte. In this case, the background intensity is 0.5 and the test ran for 150 seconds. This gives an integrated background intensity of 75 volt seconds. The total integrated emission intensity here is 715 volt seconds. Thus, the emission due to the analyte for this line is 640 V s. In effect, we have subtracted the area beneath the dashed line from the total to get the emission due to the analyte. The final result is that the emission given off by 10 mg of powder was 640Vs/.010g = 64000 V s/g. For a photon counting spectrometer, the units of measurement would be counts/g.

Dividing (normalizing) the emission intensity by the mass of sample consumed is practical for the PowderPump™ and improves precision of analysis. The mass of sample consumed is measured by weighing the vial before and after sample delivery. Normally, all of the sample removed from the vial goes through the plasma. Less than 100% transport efficiency is possible if some powder sticks to the inside of the transfer tubing. Weighing accuracy typically causes larger errors than powder sticking to the wall of the transfer tube. Since the vial weighs 12g and sample consumption is normally in the range of 10 to 50mg, it is best to have a balance that reads in grams to four digits after the decimal. You should obtain a small plastic base that will hold the vial upright in the center of the balance pan to eliminate weighing error caused by off-center loading of the balance.