Saturday, June 13, 2009
Part of the audit entails doing calculations that are present in the standard operating procedure. These are calculations that, normally we do not have to do because the instrument software or the reporting software does them for us. My instrument takes the intensity of photons it detects from a sample and from there uses the calibration data to calculate the concentration. Some of the elements that we scan for use a linear calibration curve(y = mx + b) and some use a quadratic curve(y = ax^2 + bx + c). Where y is the intensity of the sample, x is the concentration and the rest are different characteristics of the line (eg, m = slope). The calibration data gives 2 curve coefficients for linear elements and 3 curve coefficients for quadratic. The SOP tells how to solve these problems and for the quadratic elements it says that a = curve coefficient 1, b = curve coefficient 2 and c = curve coefficient 3.
Now I had never actually calculated a sample concentration before but I had remembered reading that and knew how to solve a quadratic equation and so when my manager asked if I wanted to do a linear or quadratic one, I said that I would do a quadratic one. Well, my answer came out wrong. I was shocked! My manager said, that's okay it will just go as a finding and you will all have to write a math quiz. I didn't think it was okay at all. I was puzzled as to how I could have gotten this wrong. So I took some calibration and sample data home with me to work on because I couldn't just let this sit.
Well, it didn't take long for me to discover what my problem had been. The SOP is wrong! Ha! I knew it couldn't have been me. Where is said that a = curve coefficient 1, it should be a = curve coefficient 3 and c = curve coefficient 1. I emailed my manager the next morning at work. I didn't hear anything from her, but I am hoping that she makes the correction in her audit findings. If she doesn't I will point it out to her again.
I bet we will still probably have to write a math quiz though. They like to make us do things like that.
Since I was looking for a post to make, I thought that I would share the theory behind what I do at work.
The instrument I work on is called an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). First, plasma is a state of matter where the atoms are highly ionized. Which means that they have lost most or all of their electrons. So plasma is a mixture of positive ions and free electrons. An inductively coupled plasma is a plasma that is generated using an alternating magnetic field, which is produced by an alternating electric field, which, in my case is produced by a radio frequency generator. Our plasma is made from argon gas. The plasma has a temperature between 6000-10000K. That is as hot as the surface of the sun! The plasma is normally a green-white colour. But sometimes when I get a sample that has high concentrations of certain metals, the plasma turns all kinds of neat colours.
What happens with my instrument is that when the sample makes contact with the plasma, it is super heated. The electrons in the atoms get excited and jump to the next energy level, but then almost right away they fall back down to their original energy level. When they do that, they release a photon. The photon has a wavelength characteristic with a certain type of metal and energy level. There is a photo detector that detects the photons. The concentration of metal in any given sample is calculated from the intensity of the photons of a given wavelength picked up by the photo detector. That's the optical emission spectroscopy part.
It's all very interesting. I especially like that the theory is stuff that I learned in school! Optical emission spectroscopy is used to help classify stars. Newer generations of stars tend to be made up of larger amounts of heavier metals (a metal in astronomy being anything heavier then helium). One can look at the spectrum coming off of the star and determine what concentration of what elements are present in the star.