## 6 Discussion
### Pros and Cons of radiocarbon Methods
#### 6.3.2 Liquid Scintillation Counting  
An advantage of the LSC direct method is that it is much cheaper and more simple than AMS. The Institut Ruder Boskovic in Croatia have been investigating the use of the LSC method to detect bio-content in liquid fuels for several years. They have produced peer reviewed papers on the subject (for, e.g., [45], and in addition their laboratory carries out LSC testing of fuel samples commercially. They indicate the cost of LSC analysis on a liquid fuel sample is EUR 110 per sample [53]. A second option would be to install an LSC machine in the refinery and carry out testing in-house. The cost of a new LSC capable of measuring biofuel at a concentration less than 1% is -EUR 100,000 [54]. One of the issues with LSC direct is that it works best with clear fuels, as the flashes of light during scintillation need to be detected by the LSC machine. Darker liquids (especially those coloured yellow or brown) absorb some of the flashes before they can be detected, hence causing an under-estimation of the bio-content. This effect is called \"colour quenching\" but there are methods to greatly reduce this problem. A further difficulty with using LSC would be how to consider the production of Bio-LPG (liquified petroleum gas, the bio-fraction is in the propane of the LPG). Gases are considered unsuitable for testing with LSC using the direct method. Regarding precision, at 5% bio-content in a clear liquid, [PERSON] et al. [45] report a 95% confidence range of \(\pm\)0.17%. Absolute error is approximately proportional to the square root of the bio-content, so that at 10% bio-content the 95% confidence interval after 10 h counting should be \(\pm\)0.24%. However, at very low bio-contents, the uncertainty in the background counts of a 14C-free sample becomes significant, so that the 95% detection limit is \(\sim\)0.46%. This means that LSC gives a large proportional error if a given amount of bio-feedstock is spread at low concentration over many batches. A graph showing the precision of the LSC method with increasing bio-content is provided in Figure 3. It shows the measured bio contents using LSC for mixtures of vegetable oils and fossil fuel and compares it to the nominal (actual) bio-contents. The diagonal line in the middle of the graph shows the test results are accurate with respect to actual values. The blue line relates to the degree of quenching (colouring) which the fuel blends caused; there was little movement in the values as the quenching due to the slight colour in both the vegetable oil and the fossil test fuels [45]. [PERSON] et al. [55] also worked on LSC to determine the bio component in liquid fuels. Their method aimed to reduce the background signal and maximize the detection efficiency of 14C, through better choice of scintillation cocktail, increasing the analysis time as well as the type of LSC instrumentation used [55]. A statistical analysis of their results shows a good correlation between their LSC method and AMS, with a LSC detection limit between 0.53% and 0.72% depending on sample type. Figure 4 shows the results of AMS and LSC measurements for gasoline with repeatability error bars. In summary, both AMS and LSC allow the direct quantification of the bio-component in co-processed fuels. AMS is the more expensive and slower method of the two, but is said to be more accurate, at least at very low bio-concentrations in fuels (below 1%). Significant improvements in the costs and accuracies of both methods, and in particular for the testing of clear liquid fuels with the LSC method have occurred. These improvements do not yet appear to have been reflected in fuel testing standards and may not yet be fully understood by the co-processing industry. This is especially pertinent given that specialized laboratories (e.g., recent work by [53; 55]) have improved testing accuracy and reduced costs, most notably for liquid fuel tests using the 'direct' LSC method.