Spot test (lichen)
A spot test in
Three common spot tests use either 10% aqueous
Tests
Four spot tests are used most commonly to help with lichen identification.[3]
K test
The reagent for the K test is an aqueous solution of potassium hydroxide (KOH) (10–25%), or, in the absence of KOH, a 10% aqueous solution of sodium hydroxide (NaOH, lye), which provides nearly identical results.[4] A 10% solution of KOH will retain its effectiveness for about 6 months to a year.[5] The test depends on salt formation and requires the presence of at least one acidic functional group in the molecule. Lichen compounds that contain a quinone as part of their structure will produce a dark red to violet colour. Example compounds include the pigments that are anthraquinones, naphthoquinones, and terphenylquinones. Yellow to red colours are produced with the K test and some depsides (including atranorin and thamnolic acid), and many β-orcinol depsidones. In contrast, xanthones, pulvinic acid derivatives, and usnic acid do not have any reaction.[4]
Some common and widely distributed lichens that have lichen products with a positive reaction to K include
C test
This test uses a saturated solution of
Colours typically observed with the C test are red and orange-rose. Chemicals causing a red reaction include anziaic acid, erythrin, and lecanoric acid, while those resulting in orange-red include gyrophoric acid.[8] Rarely, an emerald-green colour is produced, caused by reaction with dihydroxy dibenzofurans,[9] such as the chemical strepsilin.[8] Another rare colour produced by this test is yellow, which is observed with Cladonia portentosa as a result of the dibenzofuran usnic acid.[10]
Some common and widely distributed lichens that have lichen products with a positive reaction to C include Lecanora expallens, which is C+ (orange) because of the xanthone thiophanic acid, and Diploschistes muscorum, which is C+ (red) because of the didepside diploschistesic acid.[10]
PD test
This is also known as the P test. It uses a 1–5%
- R−CHO + H2N−C6H4−NH2 → R−CH=N−C6H4−NH2 + H2O
Products of this reaction are yellow to red in colour. Most β-orcinol depsidones and some β-orcinol depsides will react positively.[11] The PD test, known for its high specificity towards substances that yield K+ yellow or red reactions, has largely replaced the simpler yet less conclusive K test.[12] PD is poisonous both as a powder and a solution, and surfaces that come in contact with it (including skin) will discolour.[13]
Some common and widely distributed lichens that have lichen products with a positive reaction to P include
KC test
This spot test may be performed by wetting the
Hypogymnia tubulosa is a lichen that is KC+ (orange-pink) because of the depsidone physodic acid; Cetrelia olivetorum is KC+ (pink-red) due to the depsidone alectoronic acid.[10]
Less common tests
There are several spot tests that are infrequently used due to their limited applicability, but may be useful in situations where particular lichen metabolites need to be detected, or to distinguish between certain species when other tests are negative.
- A 10% solution of barium hydroxide (Ba(OH)2) gives a violet colour when tested with diploschistesic acid, a chemical found in some Diploschistes species.
- A saturated solution of barium peroxide (BaO2), when tested with olivetoric acid, will turn a yellow colour that becomes green after a few minutes.
- A 1% (weight per volume) solution of ferric chloride (FeCl3) in ethanol produces several possible colours when tested with compounds that have phenolic groups.[17]
- The N test uses a 35% solution of nitric acid, which can be used to distinguish species of Melanelia from brown species of Xanthoparmelia.[5]
- The S test uses a snow lichens, Stereocaulon alpinum and S. groenlandicum without having to resort to more laborious chemical analysis.[18]
- The copper wire; halogenated compounds cause a temporary deep green flame colour.[18]
Performing spot tests
Spot tests are performed by placing a small amount of the desired reagent on the portion of the lichen to be tested. Often, both the
In a variation of this technique, suggested by the Swedish chemist Johan Santesson,[20] a piece of filter paper is used to try to make the colour reaction more readily observable. The lichen fragment is pressed on the paper, and lichen substances are extracted with 10–20 drops of acetone. After evaporating the acetone, the lichen substances are left on the paper in a ring around the lichen fragment. The filter paper can then be spot tested in the usual way.[21] In cases where the results of a spot test on the thallus are uncertain, it is possible to squash a thin section of the tissue on a microscope slide in a minimal amount of water and reagent under a cover slip. A colour change is visible under a low-power microscope objective, or when the slide placed against a white background.[8] This technique is useful when testing lichens with dark pigments, such as Bryoria.[5]
Spot tests may be used individually or in combination. The results of a spot tests are typically represented with a short code that includes, in order, (1) a letter indicating the reagent used, (2) a "+" or "−" sign indicating a colour change or lack of colour change, respectively, and (3) a letter or word indicating the colour observed. In addition, care should be taken to indicate which part of the lichen was tested. For example, "Cortex K+ orange, C−, P−" means the cortex of the test specimen turned orange with application of KOH and did not change under bleach or para-phenylenediamine. Similarly, "Medulla K−, KC+R" would indicate the medulla of the lichen was insensitive to application of KOH, but application of KOH followed immediately by bleach caused the medulla to turn red.[12]
Occasionally, it takes some time for the colour reaction to develop. For example, in certain Cladonia species, the PD reaction with fumarprotocetraric acid can take up to half a minute.[13] In contrast, the reactions with C and KC are usually fleeting and occur within a second of applying the reagent, so a colour change can easily be missed. There are several possible reasons that an anticipated test result does not occur. Causes include old and chemically inactive reagents, and low concentrations of lichen substances in the sample. If the colour of the thallus is dark, a colour change might be obscured, and other techniques are more appropriate, like the filter paper technique.[8]
Other tests
It may sometimes be useful to perform other diagnostic measures in addition to spot tests. For example, some lichen metabolites
More advanced analytical techniques, such as thin-layer chromatography, high-performance liquid chromatography, and mass spectrometry may also be useful in initially characterising the chemical composition of lichens or when spot tests are unrevealing.[23]
History
Finnish lichenologist
See also
References
- ^ .
- ^ Truong, Camille; Clerc, Philippe (2003). "The Parmelia borreri group (lichenized ascomycetes) in Switzerland". Botanica Helvetica. 113 (1): 49–61.
- ISBN 0-87071-394-9.
- ^ a b Ahmadjian & Hale 1973, p. 636.
- ^ OCLC 862053107.
- ^ Orange, James & White 2001, p. 15.
- ^ Ahmadjian & Hale 1973, p. 635.
- ^ a b c d e f Walker, F.J.; James, P.W. (May 1980). "A revised guide to microchemical techniques for the identification of lichen products". Bulletin of the British Lichen Society. 46 (Supplement): 13–29.
- ^ ISBN 978-81-322-2180-7.
- ^ a b c Orange, James & White 2001, p. 16.
- ^ a b Ahmadjian & Hale 1973, pp. 636–637.
- ^ ISBN 978-0-7131-2456-9.
- ^ a b Dahl & Krog 1973, p. 23.
- ^ Orange, James & White 2001, p. 17.
- ^ Ahmadjian & Hale 1973, p. 637.
- ISBN 978-0300082494.
- ^ Orange, James & White 2001, p. 9.
- ^ .
- ^ a b Dahl & Krog 1973, p. 24.
- .
- ^ Ahmadjian & Hale 1973, p. 634.
- doi:10.1139/b91-099.
- ^ "Arizona State University Lichen Herbarium: Lichen TLC". nhc.asu.edu. Retrieved 18 September 2016.
- ^ JSTOR 3244891.
- ^ Nylander, W. (1866). "Circa novum in studio lichenum criterium chemicum" [A new chemical criterion in the study of lichen]. Flora (in Latin). 49: 198–201.
- ^ Nylander, W. (1866). "Quaedam addenda ad nova criteria chemica in studio lichenum" [New criteria to be added to the chemical study of lichens]. Flora (in Latin). 49: 233–234.
- ^ Asahina, Y. (1934). "Über die Reaktion vom Flechten-Thallus" [About the response from the lichen thallus]. Acta Phytochimica (in German). 8: 47–64.
- ^ Asahina, Y. (1936). "Mikrochemischer Nachweis der Flechtenstoffe (I)" [Microchemical detection of lichen substances (I)]. The Journal of Japanese Botany (in German). 12: 516–525.
- JSTOR 40597010.
Cited literature
- ISBN 978-0-12-044950-7.
- ISBN 9788200022626.
- ISBN 978-0-9540418-0-9.