When analyzing samples by a analytical instrument such as gas chromatography, the peak areas of some of the analytes are less than that of LOD and thus they could not be reported quantitatively. Please, get me a reference describing such a situation.
As per my knowledge maximum reference says....report any peak above LOQ but you are asking about below LOD. Below LOD, any peak should be discarded and reported as below LOQ. so before reporting make sure that they are the peaks of your interest and not any baseline noise or spike.as per reporting is concern just calculate the %of each peak against main peak /against standard peak and report as unknown .
Hi! You can report peaks that are above LOD, as detected. If they are also above LOQ you can assign a concentration to the peak.
In the case when the peak is higher than LOD but lower than LOQ, you can say that you can detect the peak but you cannot quantitate it, usually you say "traces". But, by definition of LOD, anything below LOD (limit of detection) is NOT detectable.
Hi Esmaeel.
I think the normative 2002/657 of European Community explain about decision limit and detection capability. I don't know if this is your problem. But read the normative, it's available in internet. May be help you.
Regards.
You should check the manuscripts in Environ Sci and Technol. listed below and perform "forward searches" for these articles. First rule of thumb is that estimators can be used to justify parametric statistics if no more that 30% of data are BDL.
Environ. Sci. Technol. , 1988, 22 (8), pp 856–861. DOI: 10.1021/es00173a001.
Environmental Science & Technology 2011 45 (4), 1481-1489
Esmaeel
Another good reference for using censored data is "Quantitative Ecotoxicology, 2nd ed" by Michael Newman (2013, CRC press: see chapter 2).
You may find this reference helpful too: http://cfpub.epa.gov/ordpubs/nerlpubs/recordisplay.cfm?deid=62970 Methods of Dealing With Values Below the Limit of Detection Using SAS by Croghan, C., and P. P. Egeghy 2003 (http://analytics.ncsu.edu/sesug/2003/SD08-Croghan.pdf)
In contrast to Mr. Carrasco's answer, clearly peaks can be detected below the LOD. The LOD is a statistical measure of the lower limit of RELIABLY reporting data; not a measure of the actual ability of the instrument to detect a finite concentration of a compound. Reporting data as "detected, below LOD" flags the compound as probably present without attempting to quantify the concentration present. Depending upon your method of assessing the LOD this may be a common occurrence. For example, USEPA 40 CFR 136 Appendix B, which is widely used to assess the LOD, assumes a 95% statistical certainty factor for "detected"; this is normally significantly higher than what you can reliably detect with a given instrument. In this case you will almost certainly encounter situations where you detect a compound below the artificially-derived "LOD"; it would certainly be valid to report the compound as present without providing quantification.
This is a common problem faced by researchers analyzing data in the environmental and occupational health fields. There are quite a lot of references such as,
"Less than obvious: statistical treatment of data below the detection limit. Environ Sci Technol 24(12)1766-1774, 1990."
"Interpolation of groundwater quality parameters with some values below the detection limit, Hydrology and Earth System Sciences Discussions. 05/2011; 8(3):5263-5299. DOI: 10.5194/hessd-8-5263-2011"
Any competent quality assurance plan will address this. Look at ISO 17025 criteria, USFDA GLP or any of the EU plans. Typically you report a datapoint and then flag it as less than the LOD, but this depends on your specific QA plan.
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