Principle
The first application of photoionization detection was as a gas chromatography (GC) ion detector. In a photoionization detector high-energy photons, typically in the ultraviolet (UV) range, break molecules into positively charged ions. As compounds elute from the GC's column they are bombarded by high-energy photons and are ionized when molecules absorb high energy UV light. UV light excites the molecules, resulting in temporary loss of electrons in the molecules and the formation of positively charged ions. The gas becomes electrically charged and the ions produce an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current.
The current is amplified and displayed on an ammeter. It is widely held that the ions recombine after passing the detector to reform their original molecules, however only a small portion of the airborne analytes are ionized to begin with so the practical impact of this (if it occurs) is probably negligible.
Read more about this topic: Photoionization Detector
Famous quotes containing the word principle:
“Now, what I want is, Facts. Teach these boys and girls nothing but Facts. Facts alone are wanted in life. Plant nothing else, and root out everything else. You can only form the minds of reasoning animals upon Facts: nothing else will ever be of any service to them. This is the principle on which I bring up my own children, and this is the principle on which I bring up these children. Stick to Facts, sir!”
—Charles Dickens (1812–1870)
“I sincerely believe ... that banking establishments are more dangerous than standing armies, and that the principle of spending money to be paid by posterity, under the name of funding, is but swindling futurity on a large scale.”
—Thomas Jefferson (1743–1826)
“Thanks to all. For the great republic—for the principle it lives by, and keeps alive—for man’s vast future,—thanks to all.”
—Abraham Lincoln (1809–1865)