Detection dipping

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

Detection Dipping into silver nitrate reagent followed by exposure to UV light dark brown spots on a white background, sensitivity 25-100 ng. 

Figure 10 Separation of DL-phenylalanine. Plate HPTLC precoated plate CHIR. Eluent metha-nol/water/acetonitrile 50/50/30 (v/v/v). Migration distance 10 cm. Chamber normal chamber with saturation. Compounds 1, D-phenylalanine 2, L-phenylalanine (all 0.01%). Application volume 5 pi. Detection dipped in 0.5% ninhydrin in ethanol/glacial acetic acid 98/2 (v/v), heating to 120°C for 5 min in situ evaluation with TLC7HPTLC scanner (Camag) UV 520 nm.

FIGURE 3.2 Densitograms obtained in the quantitative determination of TLC-separated enantiomers of ferf-leucine (a) L-terf-leucine, (b) L-fe/t-leucine+0.1%D-tert-leucine, (c) L-tert-leucine + 1 % D-tert-leucine, and (d) external reference standard. Layer, Chiralplate mobile phase, methanol/water (10 80) detection, dip in 0.3% ninhydrin solution in acetone quantification, scanning at 520 nm. (Reprinted from Guenther, K. and Moeller, K., in Handbook of Thin Layer Chromatography, 3rd edn., Sherma, J. and Fried, B., Eds., Marcel Dekker, Inc., New York, NY, 2003, pp. 471-533. With permission.)... 

In a similar procedure, the atomizer test, which depends on the behavior of an advancing rather than a receding contact angle, a fine mist of water is apphed to the metal surface and the spreading of water is observed. On a clean surface, water spreads to a uniform film. With oleic acid as the test soil, the atomizer test can detect the presence of 10 mg of soil per cm, less than a monomolecular layer (115). For steel that is to be electroplated, the copper dip test is often employed. Steel is dipped into a cupric salt solution and the eveimess of the resulting metallic copper deposit is noted. 

Acidic and basic substances can be detected using pH indicators. Indicators changing color in the acid region are primarily employed. They are apphed to the chromatogram by dipping or spraying with 0.01 to 1% solutions. The pH is... 

Treatment of the solvent-free chromatogram with iodine vapor or by dipping in or spraying with iodine solution (0.5 — 1%) is a rapid and economical universal method of detecting lipophilic substances. Molecular iodine is enriched in the chromatogram zones and colors them brown. 

Rather than dipping the chromatogram in acid solution it is preferable to heat it to 100°C for 2—5 min (fume cupboard ) in order to evaporate the ammonia and turn the background yellow. By this means it is possible to increase the sensitivity of detection for some of cations e.g. Sr and Ba. However, these zones fade after some time, so that it is necessary to quantify the chromatogram immediately after heating. 

After the dipped or sprayed chromatogram has been dried in a stream of cold air long-wave UV light (2 = 365 nm) reveals fluorescent yellow zones (flavonoids). Sterigmatocystine, which can be detected without derivatization on account of its red intrinsic fluorescence (detection limit 0.5 pg), also fluoresces pale yellow after being heated to 80°C [9] or 100°C [13] for 10 min on the other hand, citrinine, zearalenone and vomitoxin fluoresce blue. 

Detection and result The chromatogram was freed from mobile phase and dipped for 1 s in solution I and after drying for 1 min in a stream of cold air it was dipped in a solution of liquid paraffin — -hexane (1 + 2) in order to stabilize and increase the intensity of fluorescence by a factor of 1.5—2.5. The derivatives which were pale yellow in daylight after drying fluoresce pale blue to turquoise in long-wave... 

Note The dipping solution, which can also be used as a spray solution, can be employed with silica gel, kieselguhr, cellulose, RP, NH2 and CN phases. Sugars (exceptions include, for example, fructose, melezitose and raffinose) yield brilliantly colored zones on an almost colorless background when the spray solution is employed. Aldohexoses appear brown, aldopentoses bright red and hexuronic acids orange in color [3]. The detection limit differs for different substances it ranges from 10 ng (pyridoxal) over 100 ng (cinnamaldehyde) up to 2 pg (citral). 

For the detection of ampicillin it is necessary to add acetic acid to the dipping or spray solution. Ampicillin then yields pale zones on a blue background [2]. 

Detection and result The chromatograms had to be freed from mobile phase before they were immersed otherwise a blue background was produced. After it had been dipped the chromatogram was dried in a stream of cold air. Zones appearing on an initially pale background were first brown and then turned blue. The background, however, darkened so much that after 5 min it was scarcely possible to discern the zones. Table 1 lists some hRf values. 

Detection and result The chromatogram was freed from mobile phase and immersed for 3 s in the dipping solution and heated to 125°C for 5 — 10 min. The carboxylic acids terephthalic acid (tiRi 5), succinic acid (fiRi 50 — 55), phthalic acid (hRf 55 — 60), suberic acid (tiRi 60 — 65), sebacic acid (fiRi 65 — 70), benzoic acid (tiRi 75 — 80) and salicylic acid (hRf 80 — 85) yielded brown zones on a light brown background. The detection limit was 2 pg acid per chromatogram zone. 

Note If a dipping solution was employed for detection whose concentration was reduced to 1/10th that given above the acids appeared as white zones on a light brown background. 

Note The dipping solution can also be employed as spray reagent. The detection limits per chromatogram zone are reported to be 1 — 5 pg substance [1] for the oxyacids of halogens and ca. 10 pg substance for reducing sugars [4]. 

Detection and result The chromatogram was freed from mobile phase and immersed for 1 s in the dipping solution and then heated to 100 °C for 10 min. 

Note The dipping reagent, which can also be applied as a spray reagent, can be employed on cellulose and silica gel layers. A 3% solution of 2,2 -bipyridine in 40% thioglycolic acid can be employed as a specific spray reagent for the detection of iron (red coloration) [7],... 

Note Note that the diazotization of primary aromatic amines can also be achieved by placing the chromatogram for 3 — 5 min in a twin-trough chamber containing nitrous fumes (fume cupboard ). The fumes are produced in the empty trough of the chamber by addition of 25% hydrochloric acid to a 20% sodium nitrite solution [2, 4], iV-(l-Naphthyl)ethylenediamine can be replaced in the reagent by a- or -naphthol [10, 14], but this reduces the sensitivity of detection [2]. Spray solutions Ila and lib can also be used as dipping solutions. 

Detection and resnit The chromatogram was freed from mobile phase in a stream of warm air or in the drying cupboard (10 min 100°C), immersed in dipping... 

Detection and result The chromatogram was dried in a current of warm air and either immersed in reagent solution I for 1 s or placed for 15 min in a twin-trough chamber in whose second trough 5 ml of dipping solution I had been placed ca. 10 min previously. If the chromatogram was derivatized by dipping it had to be dried for ca. 1 min in a stream of hot air and allowed to cool to room temperature. 

Detection and result The chromatogram was freed from mobile phase in a stream of warm air, then immersed in dipping solution I for 1 s, dried for 90 s in a stream of warm air and after cooUng immersed in dipping solution II for 1 s. After drying briefly in a stream of warm air it was heated to 110°C for 2 min and treated with hydrochloric acid vapor for 15 min in a twin-trough chamber (37% hydrochloric acid in the vacant trough). 

Note If the spray solution or a nonbasic dipping solution [2] is employed for detection then it is advisable to spray again with a 10% aqueous sodimn carbonate solution. The necessary basicity can also often be achieved by placing the treated chromatogram in a twin-trough chamber whose second trough contains 5 ml ammonia solution (25%). 

Occasionally, instead of dipping solution I it may be advisable to employ the following alternative dipping solution la mixed with citrate buffer in order to increase the detection sensitivity (e.g. with nitrazepam) [1]. 

Detection and result The chromatogram was freed from mobile phase (NH3 ) in a stream of cold air for 45 min. It was then immersed in the dipping solution for 1 s and heated to 100°C for ca. 20 min. The chromatograms could be further treated with ammonia vapor if a colored background was found to be troublesome. The pale red zones then became bright red spots and the background frequently... 

Note The detection is not affected if the dipping solution exhibits a slight opalescent turbidity. Fast blue salt BB [18] or fast blue salt RR [18,19] can be employed in the reagent in place of fast blue salt B. It is occasionally preferable not to apply spray solutions I and II separately but to work directly with a 0.1% solution of fast blue salt B in caustic soda solution (c=l—2mol/l) [13, 15] or in 0.5% methanolic caustic potash [3]. 

A dipping solution consisting of 0.2% fast blue salt B in hydrochloric acid (c = 0.5 mol/1, immersion time 30 s) has been reported for the detection of resorcinol homologues [1]. 

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