Chris Johnson

Lotos Chen

Luke Cotton

Tiffany Melvill

Specimen Pictures

Our PowerPoint Presentation - pdf

Sample Spread Sheet for Soil - pdf
1/22/04

Plant Growth Information - pdf
3/10/04

How to Interpret the INAA
Information Results

INAA Sample from Sweet Home 1/22/04	INAA Sample from Crescent Valley 1/22/04
INAA Results from Sweet Home 1/22/04	INAA Results from Crescent Valley 1/23/04

Comparative INAA Analysis Techniques

TERMS:

Activity (A): # of counts / elapsed time (this is usually then divided by the mass of the sample to obtain the activity on a per-gram basis).

Concentration (C): the concentration of an element within a sample or standard (ppm).

Standard: a sample with a known concentration of a certain element(s). The standard(s) are measured out and sealed in vials just as unknown soil samples are. The standard(s) is then irradiated with the unknown samples.

: Percent error associated with the number of counts measured by the detector. The greater the number of counts, the smaller the “counting” error. This is calculated by dividing the square root of the number of counts in the sample by the total number of counts.

: Percent error associated with balance accuracy. Typically, the balance can be read to within . Also, this is usually the largest source of error.

: Percent error associated with the measurement of time by the computer.

: Final percent error to be associated with INAA results. . Note here that you will actually have six terms for there will be corresponding terms for both the sample and the standard.

Decay Factor: A factor added to account for the fact that each sample and standard will be “counted” at a different time relative to the time the sample or standard was removed from the reactor (i.e. when irradiation was terminated).

COMPARITIVE INAA ANALYSIS

Once an unknown sample and a standard have been irradiated, at some point the gamma radiation given off by both the sample and the standard will be “counted”. The activity for both the sample and the standard at a given energy will be determined as defined above. If the concentration (ppm) and the mass (in grams) are known for the standard, and if the mass of the unknown sample is known, the concentration of the element in the unknown sample can be calculated. This is done by equating the ratio of the concentration of an element in the unknown and the standard to the ratio of the activities of the unknown and the standard.

EXAMPLE PROBLEMS:

1. A soil sample was measured for 3600 seconds on a gamma ray spectrometer and has 4245 counts in the photopeak of Arsenic-75. The standard was measured (i.e. counted) for 1200 seconds and had 3445 counts. The concentration of Arsenic-75 is known to be 45 ppm. Both samples have the same mass. What is the concentration of arsenic in the soil sample?

2. A vegetation sample (0.754 g) was measured for 1200 seconds on a gamma ray spectrometer and had 125 counts in the photopeak of Arsenic-75. The standard (0.545g), having a known arsenic concentration of 250 ppm, was measured for 600 seconds and had 15433 counts. What is the concentration of arsenic in the vegetation sample?

DETERMINATION OF UNCERTAINTY (% ERROR) IN RESULT

To determine the error associated with the Problem #2 above:

FOR THE UNKNOWN SAMPLE	FOR THE STANDARD

This would be interpreted to mean that there is an 8.98% error associated with our reported result of .73 ppm. We should then report our result as .73 ppm.066 ppm or .73 ppm 8.98%