Sam ple Appl ication Procedures

3. Sam ple Appl ication Procedures

Before the sample solutions can be appl ied to a TLC plate, an appropriate plate m u st be selected, washed, activated , and marked . These procedures are described in detail in each material analysis protocol .

The application of a solution to a plate involves the fi lling of the m icropi pette with the solution and the transfer of the solution to the TLC plate. The spot diameter must be as small as possible since the separation zones te nd to i ncrease as the chromatogram develops. The su rface of the stationary phase should not be p ricked with the capi l lary micropi pette; disturbances to the su rface will affect the s hape of the separation zones. Gloves should be worn throughout the sample

application procedure to a void contamination of the sample or the TL C plate. T h e samples are appl ied according t o a scheme that alternates between

reference and sample solutions to m i n i m ize systematic errors that might arise from the appl ication of the solutions.

1 ) Place the p i pette i nto the holder

The micropi pette holder is a glass tube with a d ropper bulb attached to one end and a flexible fitti ng at the other. C heck to see that the d ropper bulb is attached near the end of the glass tube. I n se rt the m icropi pette into

Sample Application for Thin-Layer Chromatography

the opening of the flexible fitti ng so that a small portion of the m i crop i pette (about 3 mm) can be seen inside the holder.

2) F i l l the p ipette with the sample solution

I n sert the free end of the m icropi pette i nto the sam ple solution . This is someti m es difficult. depending on the size of the vial i n which the sample is stored . It can be faci litated by carefu lly ti pping the vial so that the solu­ tion flows along one wall of the vial . The m icro pi pette should fi ll rapidly by capi l lary action. The men i scus of the solution can be seen traveling u p t h e micropi pette.

The vial can be tilted so that the solution is more easily reached.

Solution as it fills the pipette.

3 ) Apply the sample solution to the TLC plate

The solution is applied to the su rface i n small increments (approxi m ately

0 . 5 IJI in size). Transfer the sample solution to the TLC plate by placing the free end of the m icropi pette in contact with the surface of the TLC

Striegel . Hill

plate at the tick mark of a lane. Hold the micropi pette vertically and gently touch it to the su rface of the TLC plate.

It is sometimes difficult to start the flow of the sample solution from the micropi pette. If the flow does not start naturally, the sol ution can be gently

forced from the micropi pette. The d ro pper bulb contains a small hole. Cover the hole and very grad ually apply pressure to the bulb. This should start the flow of the solution.

After a portion of the solution is applied , the spot is rapidly d ried using a heat gun. The heat gun is set on a low, cool setti ng and is gently fan ned back and forth across the spot. Care is taken to keep the micropi pette away from

the heat gun, as the sample solution could evaporate from the end of the m icropi pette. After the spot is dried, another portion i s appl ied. This process

contin ues unti l the micropi pette is em pty.

Sample Application for Thin-Layer Chromatography

1 47

The sol utions can be applied to the plate using the general application scheme below. I ntersperse the u n k nown and reference solutions to mini m ize

systematic error i n the analysis.

Scheme for Sample Application

R1, R2, etc. = reference solutions U1, U2, etc. sample solutions

Name, TLC #, Date

R1 R2 U1 R3 R4 U2 RS R6 U3 R7 RS

Protocol I

Evaluation of a TLC Plate

Sum mary G uidelines for the evaluation of a TLC plate

Supplies:

Lamp (tungsten or fluorescent bulb)

U ltraviolet lamp Accurate ruler with metric (mm) markings Calculator

Scope

A com plete eval uation of a chromatogram aids in the docu mentation and i nter­ pretation of the analysis. The eval uation begi n s when the plate is removed from the developing cham ber. The location and i rregularities of the solvent front are marked . The plate is observed u nder visible light, and then u nder short­ wave and long-wave ultraviolet light. After all visi ble and fluorescent areas are

noted , the chromatographic plate is sprayed with a detection reagent for further visual ization . Agai n , the plate is observed u nder visible and u ltraviolet

light. For each spot, the location (i . e . , d istance from the origi n), color, and

i nte nsity are recorded. N ext, Rf values are calcu lated . This protocol descri bes the basic methodology for eval uati ng a chromatogram. For co m puter-aided eval u ­ atio n , see Protocol L.

Striegel . H i l l

Procedures

1 ) Mark the location and shape of the solvent front

Upon removal of the TLC plate from the development chamber, observe the location and shape of the solvent front of the plate . Using a pencil, lightly m ark the front. Measure the d istance between the o rigin and the solvent front (ds) and record this val ue in the written documentation.

2) Note any visible spots or areas under visible l ight Visually examine the TLC plate , noting any colored spots or areas. If spots are seen, note the hue and value of the spot in the written docu men­ tation (see Protocol E) . For instance, a spot may be yellow-orange (hue) and med ium-dark (val ue) .

3) Note any fl uorescent spots or areas under fluorescent l ight Hold or prop a fluorescent lamp over the TLC plate about 2 i nches from the su rface. Be sure to wear UV protective goggles during this exami nation to prevent poss ible eye damage . N ote any fluorescent spots.

4) Spray and heat the TLC plate Spray the TLC plate with the specified detection reagent and heat for the

req u i red length of ti m e . Follow the i n structions given in the individ ual protocols regard ing the use of the detection reagent.

5) Observe the TLC plate under visible l ight and note any changes After detection with the appropriate reagent and heati ng, the plate is again studied under visible light. Faint d iscoloration or distinct spots m ay be seen on the plate. These are noted in the written docu mentatio n . They can

be marked on the plate by circl ing with a penci l . If the plate is to be photographed , circl ing the spots is not reco m mended, as marki ngs on the plate should be kept to a m i n i m u m .

6) Observe t h e T L C p l ate under fluorescent l ight a n d mark t h e spots The plate is agai n placed under an ultraviolet lamp and closely inspected . The n u m ber, color, and location of spots in each lane are noted . The spots can

be marked if photographing of the plate is not planned . Otherwise, photog­ raphy of the plate precedes any marking.

7) Measure the d i stance from the center of each spot to the baseline of the TLC plate Once the plate has been examined and photographed , the d istance of each spot from the baseline is measured with an accu rate metric ruler with milli meter m arkings. It i s useful to circle the spots before measuring the locatio n , particularly if they are fluorescent spots .

There are two ways of determ ining the center of the spot. One way is to lay the ruler perpendicular to the baseline, along the lane, and esti mate the

center of the spot by eye. The second way is to measure the d istance to the front edge of the spot (d1) and the distance to the back edge of the

spot (d2) . The center d istance (de) is calculated from the average of the two distances:

d, + d2

Evaluation of a TLC Plate

1 51

This second m ethod of determ ining the d istance traveled by the spot m ay be more accurate, but it req u i res more time to process the i nformation.

D istances for each spot within a lane are tabu lated i n the written documentatio n .

8) Calcu late t h e R, value f o r each spot on t h e TLC p l ate Once the d i stances are tabu lated, the distance to the solvent front i s measured, ds. The Rf value i s calculated a s follows:

where de is the d istance that the spot traveled from the origin (see step 7 above) , and ds i s the d i stance traveled by the solvent front (see step 1 above).

9) Compare the u n known sample to the reference materials that were chromatographed on the same TLC plate Exam ine the chromatographic patterns of each u n known and reference material . Com pare the R, values of each material . The R, val ues should match within about 3 % for a positive identification . Next, look at the color and

i ntensity of each separation zone for both the un known and the ref- erence chromatograms. The final identification should take i nto consider­ ation the Rf val ue, color, and i ntensity of each separatio n zone. It is possible that the un known sample may be a m i xture of components. If this is the case, each separation zone must be carefully assigned to a component of the m i xture .

1 0) Document the T L C plate

Following the visual exami nation of the TLC plate, written documentation is recorded following Protocol E. The plate is then photographed under normal or ultraviolet l ight (see Protocols F and G).

Protocol J