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Here we take a brief look at the history of the laboratory to help give perspective about ''why'' they're important to modern life.
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{{raw:wikipedia::Detection limit}}
 
==Laboratories: A historical perspective==
 
===Introduction===
 
===Origins of the laboratory===
Among the earliest known organized scientific study was that under the rule of the early Ptolomies of Alexandria in the third century B.C. While little to no evidence seems to exist for public or organized laboratories during this time period, researchers and historians widely accept the idea that at least organized and individual research (meaning "direct personal contact with the objects of study, and by the aid of such appliances as were then available"<ref name="WelchTheEvolution20">{{cite book |url=http://books.google.com/books?id=utc0AQAAMAAJ&pg=200 |chapter=The Evolution of Modern Scientific Laboratories |title=Papers and Addresses by William Henry Welch |author=Welch, William Henry |volume=3 |publisher=The Johns Hopkins Press |year=1920 |pages=200–211}}</ref>) into anatomy, physiology, and medicine took place.<ref name="ZilselTheSocial03">{{cite book |title=The Social Origins of Modern Science |chapter=The Genesis of the Concept of Scientific Progress and Cooperation |series=Boston Studies in the Philosophy of Science |author=Zilsel, E. |editor=Cohen, R.S., Wartofsky, M.W. |publisher=Kluwer Academic Publishers |year=2003 |pages=130–171 |isbn=1402013590}}</ref><ref name="MartinSomeThoughts1888">{{cite book |url=https://books.google.com/books?id=Raw-AQAAMAAJ&pg=PA256 |title=Physiological Papers |chapter=Some Thoughts About Laboratories |author=Martin, H.N. |publisher=The John Hopkins Press |pages=256–264 |year=1895}}</ref><ref name="WelchTheEvolution20" /><ref name="SerageldinAncient13">{{cite journal |title=Ancient Alexandria and the dawn of medical science |journal=Global Cardiology Science & Practice |author=Serageldin, I. |volume=2013 |issue=4 |pages=395–404 |year=2013 |doi=10.5339/gcsp.2013.47 |pmid=24749113 |pmc=PMC3991212}}</ref> Dissections and experiments took place, but certainly not in an organized teaching or research laboratory setting like today. Early twentieth-century philosopher of science Edgar Zilsel suggests that scientific endeavor was non-collaborative in this early era, and the laboratory as a collaborative environment simply didn't exist<ref name="ZilselTheSocial03" />:
 
<blockquote>No publications, no astronomical or geographical investigation which are the work of several collaborating scientists are known. Even the learned compendia of the Roman period (Varro, Pliny, Celsus) and the encyclopedias of late antiquity (Boëthius) were composed by single polyhistors. There is no evidence that the Alexandrian Museum conjointly carried out investigations. Laboratories, the birth places of scientific co-operation in the modern era, existed neither in the Alexandrian Museum, nor in the Academy, nor in the Lyceum. As far as the fellow scholars of the museum did not work each for himself they might have contented themselves with dinners and debates. And of course, there were in antiquity no scientific periodicals in which new findings could have been discussed.</blockquote>
 
With scientific advancement and discovery still largely a personal (i.e, prestigious) goal, even through the Renaissance humanists of the fourteenth through sixteenth century A.D.<ref name="ZilselTheSocial03" />, it would take quite some time for both the private and public laboratory to evolve. To be certain, private laboratories surely existed, from Aristotle<ref name="WelchTheEvolution20" /> (third century B.C.) to the anatomical laboratory — "the first scientific laboratory" — that began to take hold in the late thirteenth to early fourteenth century<ref name="WelchTheEvolution20" /><ref name="WalkerClinical90">{{cite book |url=https://www.ncbi.nlm.nih.gov/books/NBK201/ |title=Clinical Methods: The History, Physical, and Laboratory Examinations |chapter=Chapter 1: The Origins of the History and Physical Examination |author=Walker, H.K. |editor=Walker, H.K.; Hall, W.D.; Hurst, J.W. |edition=3rd |publisher=Butterworths |year=1990 |isbn=040990077X}}</ref>, all the way to the "zenith" of the alchemical research laboratory in the second half of the sixteenth century.<ref name="Martinón-TorresA16th03">{{cite journal |title=A 16th century lab in a 21st century lab: Archaeometric study of the laboratory equipment from Oberstockstall (Kirchberg am Wagram, Austria) |journal=Antiquity |author=Martinón-Torres, M.; Rehren, T.; von Osten, S. |volume=77 |issue=298 |url=http://antiquity.ac.uk/projgall/martinon298}}</ref> But it wouldn't be until the late sixteenth to early seventeenth century that collaboratory science and the first university-affiliated labs would appear.
 
Zilsel claims that Italian polymath Galileo Galilei, while teaching at the University of Padua from 1592 to 1610, founded the first university-affiliated laboratory in his own home, with help from craftsmen who aided in researching architectural and mechanical concepts.<ref name="ZilselTheSocio00">{{cite journal |title=The Sociological Roots of Science |journal=Social Studies of Science |author=Zilsel, E. |volume=30 |issue=6 |pages=935–949 |year=2000 |url=http://www.jstor.org/stable/285793}}</ref> As Galileo was nearing completion of his professorship at Padua, chemist Johannes Hartmann opened up a university laboratory for students at the University of Marburg in 1609, albeit for "instruction not in [chemical] analysis — still in a very rudimentary state — but in pharmaceutical preparations."<ref name="IhdeTheDevelop84">{{cite book |url=https://books.google.com/books?id=89BIAwAAQBAJ&pg=PA262 |title=The Development of Modern Chemistry |chapter=Chapter 10: The Diffusion of Chemical Knowledge |author=Ihde, A.J. |publisher=Dover Publications |pages=259–276 |year=1984 |isbn=0486642356}}</ref> One of the first actual public laboratories dedicated to chemical instruction was founded later that century, in 1683, hosted at the University of Altdorf, created and directed by physician and professor Johan Moritz Hofmann.<ref name="IhdeTheDevelop84" /><ref name="WiechmannChemistry1899">{{cite book |url=https://books.google.com/books?id=z4k-AAAAYAAJ&pg=PA83 |title=Chemistry: Its Evolution and Achievements |author=Wiechmann, F.G. |series=Science Sketches |publisher=William R. Jenkins |location=New York |pages=176 |year=1899}}</ref><ref name="LockemannFriedrich53">{{cite journal |title=Friedrich Stromeyer and the history of chemical laboratory instruction |journal=Journal of Chemical Education |author=Lockemann, G.; Oesper, R.E. |volume=30 |issue=4 |pages=202–204 |year=1953 |doi=10.1021/ed030p202}}</ref> That same year the (Old) Ashmolean played host to Britian's first university laboratory, directed by chemistry chair Robert Plot.<ref name="BowenTheBalliol70">{{cite journal |title=The Balliol-Trinity Laboratories, Oxford 1853-1940 |journal=Notes and Records of the Royal Society of London |author=Bowen, E.J. |volume=25 |issue=2 |pages=227–236 |year=1970 |url=http://www.jstor.org/stable/530877}}</ref><ref name="Martinón-TorresTheArch11">{{cite journal |title=The Archaeology of Alchemy and Chemistry in the Early Modern World: An Afterthought |journal=Archaeology International |author=Martinón-Torres, M. |volume=15 |pages=33–36 |year=2011-2012 |doi=10.5334/ai.1508}}</ref>
 
By the end of the seventeenth century, textbooks on various subjects such as anatomy<ref name="BartholinTheAnat15">{{cite book |url=https://books.google.com/books?id=Y9o_CgAAQBAJ&pg=PA20 |title=he Anatomy House in Copenhagen |author=Bartholin, T. |publisher=Museum Tusculanum Press |pages=222 |year=2015 |isbn=9788763542593}}</ref> and chemistry<ref name="WiechmannChemistry1899" /> were becoming more notable, and numerous vital scientific measurement and observation devices — including astronomy equipment — had been created.<ref name="BronfenbrennerTheRole1913">{{cite book |url=https://books.google.com/books?id=-v4CAAAAIAAJ&pg=PA11 |title=The Role of Scientific Societies in the Seventeenth Century |author=Bronfenbrenner, M.O. |publisher=University of Chicago Press |location=Chicago |pages=308 |year=1913}}</ref> And most importantly, as early twentieth century political science researcher Martha Ornstein put it, after much build-up, finally "the [public] chemical and physical laboratory existed in embryonic form."<ref name="BronfenbrennerTheRole1913" />
 
===Eighteenth- and nineteenth-century laboratories===
 
The teaching of practical or "physical chemistry" — separating itself even further by several decades from alchemical study — first took place in St. Petersburg, Russia in 1751 under the professorship of Mikhail Lomonosov. Two years prior he had built for him a small 15 x 9 meter brick structure where he developed colored glasses for mosaics, but he quickly turned his focus towards teaching students in physical chemistry, "a science which must explain by means of physical laws and experiments the cause of changes produced by chemical operations in composite bodies."<ref name="MenschutkinARussian1927">{{cite journal |title=A Russian physical chemist of the eighteenth century |journal=Journal of Chemical Education |author=Menschutkin, B.N. |volume=4 |issue=9 |pages=1079–1087 |doi=10.1021/ed004p1079}}</ref>
 
In 1806, Friedrich Stromeyer, fresh from being named "extraordinary professor" after the death of Johann Friedrich Gmelin, took over as director of University of Göttingen's chemical laboratory. Stromeyer's strong opinion that students could only learn chemistry best through practice and self-analysis led to a subtle but significant change: the development of one of the first university laboratories in Germany to offer students hands-on chemical analysis.<ref name="LockemannFriedrich53" /><ref name="IhdeTheDevelop84" />
 
===Modern laboratories and their importance===
 
==References==
{{Reflist|colwidth=30em}}
 
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Template:Short description

"LOQ" redirects here. For the company listed as LOQ on the London Stock Exchange, see Lo-Q. For the airport in Botswana with IATA code LOQ, see Lobatse Airport.

The limit of detection (LOD or LoD) is the lowest signal, or the lowest corresponding quantity to be determined (or extracted) from the signal, that can be observed with a sufficient degree of confidence or statistical significance. However, the exact threshold (level of decision) used to decide when a signal significantly emerges above the continuously fluctuating background noise remains arbitrary and is a matter of policy and often of debate among scientists, statisticians and regulators depending on the stakes in different fields.

Significance in analytical chemistry

In analytical chemistry, the detection limit, lower limit of detection, also termed LOD for limit of detection or analytical sensitivity (not to be confused with statistical sensitivity), is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) with a stated confidence level (generally 99%).[1][2][3] The detection limit is estimated from the mean of the blank, the standard deviation of the blank, the slope (analytical sensitivity) of the calibration plot and a defined confidence factor (e.g. 3.2 being the most accepted value for this arbitrary value).[4] Another consideration that affects the detection limit is the adequacy and the accuracy of the model used to predict concentration from the raw analytical signal.[5]

As a typical example, from a calibration plot following a linear equation taken here as the simplest possible model:

where, corresponds to the signal measured (e.g. voltage, luminescence, energy, etc.), "Template:Mvar" the value in which the straight line cuts the ordinates axis, "Template:Mvar" the sensitivity of the system (i.e., the slope of the line, or the function relating the measured signal to the quantity to be determined) and "Template:Mvar" the value of the quantity (e.g. temperature, concentration, pH, etc.) to be determined from the signal ,[6] the LOD for "Template:Mvar" is calculated as the "Template:Mvar" value in which equals to the average value of blanks "Template:Mvar" plus "Template:Mvar" times its standard deviation "Template:Mvar" (or, if zero, the standard deviation corresponding to the lowest value measured) where "Template:Mvar" is the chosen confidence value (e.g. for a confidence of 95% it can be considered Template:Mvar = 3.2, determined from the limit of blank).[4]

Thus, in this didactic example:

There are a number of concepts derived from the detection limit that are commonly used. These include the instrument detection limit (IDL), the method detection limit (MDL), the practical quantitation limit (PQL), and the limit of quantitation (LOQ). Even when the same terminology is used, there can be differences in the LOD according to nuances of what definition is used and what type of noise contributes to the measurement and calibration.[7]

The figure below illustrates the relationship between the blank, the limit of detection (LOD), and the limit of quantitation (LOQ) by showing the probability density function for normally distributed measurements at the blank, at the LOD defined as 3 × standard deviation of the blank, and at the LOQ defined as 10 × standard deviation of the blank. (The identical spread along Abscissa of these two functions is problematic.) For a signal at the LOD, the alpha error (probability of false positive) is small (1%). However, the beta error (probability of a false negative) is 50% for a sample that has a concentration at the LOD (red line). This means a sample could contain an impurity at the LOD, but there is a 50% chance that a measurement would give a result less than the LOD. At the LOQ (blue line), there is minimal chance of a false negative.

Template:Wide image

Instrument detection limit

Most analytical instruments produce a signal even when a blank (matrix without analyte) is analyzed. This signal is referred to as the noise level. The instrument detection limit (IDL) is the analyte concentration that is required to produce a signal greater than three times the standard deviation of the noise level. This may be practically measured by analyzing 8 or more standards at the estimated IDL then calculating the standard deviation from the measured concentrations of those standards.

The detection limit (according to IUPAC) is the smallest concentration, or the smallest absolute amount, of analyte that has a signal statistically significantly larger than the signal arising from the repeated measurements of a reagent blank.

Mathematically, the analyte's signal at the detection limit () is given by:

where, is the mean value of the signal for a reagent blank measured multiple times, and is the known standard deviation for the reagent blank's signal.

Other approaches for defining the detection limit have also been developed. In atomic absorption spectrometry usually the detection limit is determined for a certain element by analyzing a diluted solution of this element and recording the corresponding absorbance at a given wavelength. The measurement is repeated 10 times. The 3σ of the recorded absorbance signal can be considered as the detection limit for the specific element under the experimental conditions: selected wavelength, type of flame or graphite oven, chemical matrix, presence of interfering substances, instrument... .

Method detection limit

Often there is more to the analytical method than just performing a reaction or submitting the analyte to direct analysis. Many analytical methods developed in the laboratory, especially these involving the use of a delicate scientific instrument, require a sample preparation, or a pretreatment of the samples prior to being analysed. For example, it might be necessary to heat a sample that is to be analyzed for a particular metal with the addition of acid first (digestion process). The sample may also be diluted or concentrated prior to analysis by means of a given instrument. Additional steps in an analysis method add additional opportunities for errors. Since detection limits are defined in terms of errors, this will naturally increase the measured detection limit. This "global" detection limit (including all the steps of the analysis method) is called the method detection limit (MDL). The practical way for determining the MDL is to analyze seven samples of concentration near the expected limit of detection. The standard deviation is then determined. The one-sided Student's t-distribution is determined and multiplied versus the determined standard deviation. For seven samples (with six degrees of freedom) the t value for a 99% confidence level is 3.14. Rather than performing the complete analysis of seven identical samples, if the Instrument Detection Limit is known, the MDL may be estimated by multiplying the Instrument Detection Limit, or Lower Level of Detection, by the dilution prior to analyzing the sample solution with the instrument. This estimation, however, ignores any uncertainty that arises from performing the sample preparation and will therefore probably underestimate the true MDL.

Limit of each model

The issue of limit of detection, or limit of quantification, is encountered in all scientific disciplines. This explains the variety of definitions and the diversity of juridiction specific solutions developed to address preferences. In the simplest cases as in nuclear and chemical measurements, definitions and approaches have probably received the clearer and the simplest solutions. In biochemical tests and in biological experiments depending on many more intricate factors, the situation involving false positive and false negative responses is more delicate to handle. In many other disciplines such as geochemistry, seismology, astronomy, dendrochronology, climatology, life sciences in general, and in many other fields impossible to enumerate extensively, the problem is wider and deals with signal extraction out of a background of noise. It involves complex statistical analysis procedures and therefore it also depends on the models used,[5] the hypotheses and the simplifications or approximations to be made to handle and manage uncertainties. When the data resolution is poor and different signals overlap, different deconvolution procedures are applied to extract parameters. The use of different phenomenological, mathematical and statistical models may also complicate the exact mathematical definition of limit of detection and how it is calculated. This explains why it is not easy to come to a general consensus, if any, about the precise mathematical definition of the expression of limit of detection. However, one thing is clear: it always requires a sufficient number of data (or accumulated data) and a rigorous statistical analysis to render better signification statistically.

Limit of quantification

The limit of quantification (LoQ, or LOQ) is the lowest value of a signal (or concentration, activity, response...) that can be quantified with acceptable precision and accuracy.

The LoQ is the limit at which the difference between two distinct signals / values can be discerned with a reasonable certainty, i.e., when the signal is statistically different from the background. The LoQ may be drastically different between laboratories, so another detection limit is commonly used that is referred to as the Practical Quantification Limit (PQL).

See also

References

  1. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "detection limit".
  2. ↑ "Guidelines for Data Acquisition and Data Quality Evaluation in Environmental Chemistry". Analytical Chemistry 52 (14): 2242–49. 1980. doi:10.1021/ac50064a004. 
  3. ↑ Saah AJ, Hoover DR (1998). "[Sensitivity and specificity revisited: significance of the terms in analytic and diagnostic language."]. Ann Dermatol Venereol 125 (4): 291–4. PMID 9747274. https://pubmed.ncbi.nlm.nih.gov/9747274. 
  4. ↑ 4.0 4.1 "Limit of blank, limit of detection and limit of quantitation". The Clinical Biochemist. Reviews 29 Suppl 1 (1): S49–S52. August 2008. PMC 2556583. PMID 18852857. https://www.ncbi.nlm.nih.gov/pmc/articles/2556583. 
  5. ↑ 5.0 5.1 "R: "Detection" limit for each model" (in English). search.r-project.org. https://search.r-project.org/CRAN/refmans/bioOED/html/calculate_limit.html. 
  6. ↑ "Signal enhancement on gold nanoparticle-based lateral flow tests using cellulose nanofibers". Biosensors & Bioelectronics 141: 111407. September 2019. doi:10.1016/j.bios.2019.111407. PMID 31207571. http://ddd.uab.cat/record/218082. 
  7. ↑ Long, Gary L.; Winefordner, J. D., "Limit of detection: a closer look at the IUPAC definition", Anal. Chem. 55 (7): 712A–724A, doi:10.1021/ac00258a724 

Further reading

  • "Limits for qualitative detection and quantitative determination. Application to radiochemistry". Analytical Chemistry 40 (3): 586–593. 1968. doi:10.1021/ac60259a007. ISSN 0003-2700. 

External links

Template:BranchesofChemistry Template:Authority control

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