Radioisotopes
Please observe the following:
1. Insure that the exterior of all vials are free from chemical or radioisotope
contamination.
2. Remove your sample vials from the counting rack and from the equipment room
when finished counting.
3. Form feed the printer paper and tear at the perforation to prevent jams from
occurring.
4. Leave the command cards in the rack.
Liquid Scintillation Counting
Interested students are referred to a number of extensive references on liquid scintillation counting theory and practice. What follows is a brief introduction to the instruments we have and features in common.
LSC is dependent on the interaction of an energetic particle with an organic compound, the scintillant. This interaction results in a photon(s) which are detected up by a photomultiplier(s) and usually displayed as counts per minute (CPM). Both the LS5801 and 6500 instruments count samples in a liquid format, either in the standard 20 ml glass or plastic vial or in a 6 ml miniature vial. Only the Model 6500 can be programmed to accurately count dry samples, such as the Beckman ReadyCap. If you are using these insure that you have programmed the instrument properly.
Parameters that you should be prepared to enter include the count time, sample repeat and count channel as well as the radioisotope you are observing. Programs are selected by the use of command cards which are inserted in the first rack. Racks may also be numbered, but this is optional, by the use of special rack number cards. It is possible that a suitable program exists for your application, but confirm that you understand what the program is counting and any calculations it may be performing on your data.
Command cards take a beating by repeated insertion and removal. Please take care when inserting or removing them. Load racks into the right hand side of the counter. Counting is initiated after loading racks on the right side of the carrier by pressing the Start button on the LS6500 or AutoCount on the LS5801
Beckman LS5801
This instrument can be operated in either a manual or automatic mode. Only the automatic mode is discussed. All racks following will be counted with the protocol defined by the first encountered command card. Counting will stop if the STOP protocol card is encountered. Usually this card is placed into a red rack but the color of rack is immaterial. If a rack is encountered with another command card, before the STOP rack, the new protocol will be used.
To edit user programs, select the appropriate user number after pressing the blue User Number key. While in this screen you may edit count time, sample repeat, select channel to count and set window.
Standard window settings, expressed in channel units are:
| Tritium | 0-400 |
| I125 | 0-567 |
| C14 | 0-670 |
| S35 | 0-688 |
| P32 | 0-945 for samples in liquid scintillation media. |
on the H3 channel (0-400 channels) at an efficiency of about 45% in water.
Beckman LS6500
Functionally very similar to the LS5801 with additional sophistication. Selection of programs is also accomplished through the use of command cards, but the process of editing user programs is somewhat different.
Additional functionality for the 6500 include improved counting efficiency of dry scintillant (eg ReadyCaps); a reversible belt carrier system that allows for more convenient recounting of samples; counting single racks without command card (under interrupt in the Multi Task menu), single photon counting, and dual label studies. Entering user programs involves selection of isotopes to set spectrometer windows rather that actual window energies. Thus, if you wish to count tritium in channel 1 you would select 3H for isotope one (actually this is the default). If you wish to manually set the window you may do so by entering Manual in this field. Please refer the the manual for detailed instructions.
It is important to note that if are doing a dual labeling study you should choose Isotope 1 as the lower energy isotope and Isotope 2 as the higher energy isotope. The lower limit for Isotope 2 is set the same as the upper limit for Isotope 1. Don't define multiple isotopes in singlely labeled studies as it will result in lowered counts.
Packard TopCount
This instrument counts either dry or liquid samples in a microtitre format. In either case you must seal the plate with tape which you may obtain from Packard Instruments. You may count a single plate in which case you do not need to use the stacking carriers. However, if you do count multiple plates be sure to install the loading and unloading carriers.
Packard Cobra Gamma Counter
Gamma counting differs from LSC in that no scintillation media is used in the sample vial. The gamma ray is detected after it has passed out of the sample vial and impinges on a crystal of NaI(Tl) which is the scintillator. A photomultiplier tube "sees" this light, amplifies it and sends the electrical signal to be further processed. The energy of the gamma ray emitted from the decaying isotope varies but is more energetic than the alpha or beta particles normally detected by LSC. Polypropylene tubes are recommended for maximum counting efficiency.
The Cobra also uses protocol clips (analogous to command cards in the LSC) to define counting conditions. All functions are accessed from the Status Screen. Use the directory screen to access the protocol you want to define (edit). You are referred to the manual (Getting Started) for brief details on how to define count conditions and data reduction parameters.
