Spray-on technique

Layer HPTLC silica gel 60F-254 samples applied by the spray-on technique using a Linomat IV. 

The Linomat-5 is the latest development in a long line of CAMAG sample applicators, using the spray-on technique in qualitative, quantitative, and preparative TLC/ HPTLC (Fig. 5). This applicator sprays samples, preferably in the form of bands of selectable length or as spots. The spray-on technique enables larger sample volumes to be applied to the layer by contact transfer. The sample dosage syringe is selectable, 100 or 500 pL. 

Apply in small increments or if spray-on techniques are used with a slow application rate... 

The first semiautomatic instrument utilizing all advantages of the spray-on technique was the (Lino-mat 5) (Figure 2). Today s latest model is software controlled. Only the y-position of the application is set manually and the user loads the sample into the... 

The Camag Automatic TLC Sampler 4 (ATS 4) is an advanced, fuUy automated, PC-controlled device for sequential application of up to 66 samples from a rack of 2 ml septum-covered vials or 96 samples from well plates (15 05 45 mm height) as spots by contact transfer (0.1-5 xl) or as bands by the spray-on technique (0.5->50 xl) a motor driven dosing syringe sucks up the sample volume and feeds a steel capillary connected to a capillary atomizer. The speed, volume, and X- and Y-position pattern of application are controllable, and a programmable rinse cycle between the applications can eliminate cross-contamination. 

The homogenous application of the MALDI matrix is another important issue, solved by different application techniques for UTLC plates thus far. The simple deposition of a few microliters by syringe next to the analyte spot or the spray application over the entire plate was described [13,15]. Another spray-on technique with the Linomat 4 (CAMAG) was performed to apply the matrix solution... 

Figure 5.3 shows the microprocessor-controlled Camag Linomat IV, which applies samples in the form of narrow bands by a spray-on technique in which nitrogen carrier gas atomizes the sample from a syringe onto the plate, which is moving back and forth under the atomizer. Band length is selectable between 0 ( = spot) and 190 mm. Although sample volumes of 1-100 (xl can be applied by the Linomat, the sample size most used is 5-20 pi for TLC and HPTLC plates. 

Figure 4 Linomat for sample application bandwise by spray-on technique. The dynamic range is 1-490 pL, the practical range for quantitative analysis is 2 to about 50 pL. With a typical band length of 7 mm with a distance of 3 mm to the next, a 20 x 10 cm HPTLC plate accommodates 18 sample tracks per side.

The Linomat (Fig. 4) allows sample application in narrow bands by a spray-on technique. This combines sample transfer with sample concentration. Sample volumes of more than 2 pL up to 100 microliters can be applied onto HI TLC layers. This technique is suitable to dramatically lower the determination limits with respect to the concentration in the solution. The reduction of sample volumes by a factor of 10 from conventional to HPTLC material is not necessary when the spray-on technique is employed. Another benefit of the spray-on technique is the ability to apply different volumes of the same solution instead of identical volumes of different concentrations. This greatly reduces the amount of effort required for preparing standard solutions for multilevel calibration. The spray-on technique also allows unknowns to be simply oversprayed with spiking or standard-addition calibration solutions. 

By employing the Linomat spray-on technique, the gain in precision of quantitative TLC analysis can be expected to be in the range of 30%, i.e. from 1.5% RSD to 1.0%, when chromatographic resolution is not a problem. If samples are complex, dirty, tend to tailing, or are otherwise difficult, the choice between band application or spot application can determine whether or not a meaningful quantitative result can be obtained at all. 

The Automatic TLC Sampler III is the latest version of this kind. It applies samples automatically from a rack of sample vials. The samples are dispensed from a steel capillary, spotwise by contact transfer or bandwise using the spray-on technique. The capillary which delivers the samples moves according to the plotter principle at a lateral speed of 25 cm/s. The sample dispensing speed is selectable between 10 and 1(X)0 nL/s. The application module is controlled by an IBM-compatible PC, normally the same computer that controls densitometric chromatogram evaluation. 

Bandwise application of samples using spray-on technique and modem instrumental applicators, enables application of relatively large volumes of solutions (up to 99 (il). In some instances, it is even necessary to dilute solutions. The opposite situation is seen in the case of spotwise application on HPTLC plates. Recommended volumes for achieving the optimum results should not exceed 0.5 l. To obtain sufficient concentrations, samples may have to be concentrated, but concentrated aqueous solutions usually have relatively high viscosity and surface tension. This prevents the filling and emptying of capillaries caused by capillary forces. Therefore, it is advisable, if possible, to dilute concentrated solutions with methanol. Solid samples should be dissolved in distilled water (pH 5.5), dilute acid or alcohol and water mixtures, and treated as solutions. 

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