Cell blank

Type in the data for the standard curve in the A and B spreadsheet columns. Use the A column for the concentrations and the B column for the corresponding instrument readout values. For the unknowns and control, type in the instrument readout values in the B column cells, but leave the concentration cells blank. When finished, the A and B columns should appear as in Table 6.1, in which there are four standards with concentrations of 1,2, 3, and 4 ppm, two unknowns, and one control. 

Figure 1 A cellular automata grid showing occupied cells of different states A and B, and unoccupied cells (blank).

FIGURE 6.3 Comparison of MTT tetrazolium and tritiated thymidine incorporation assays to measure effects of hGM-CSF on proliferation of TF-1 cells. A no-cell blank absorbance of 0.065 was subtracted from all MTT values before plotting. Similar ED50 values are shown for both assays. (Source Modified from Promega Corporation Technical Bulletin 112. CellTiter 96 Non-Radio active Cell Proliferation Assay.)... 

Figure 5 shows the result of using the Williams and Clapper transformation when analysing glucose. The parameters P and Dr are obtained by means of a 3-point calibration, one of which corresponds to the cell blank. 

This may be a very appreciable fraction of the total extinction in surface waters and should be determined on the surface and 10-m samples of each cast. Measure at progressively greater depths until the value becomes approximately constant. This value (generally less than 0.01 below 25 m in offshore waters) is then roughly equal to the cell-to-cell blank (Sect. G, 1 above) and may often be slightly negative. 

Carry out the method exactly as described in Section F, paragraphs 2-9 inclusive, using redistilled water in place of the sample. The reagent blank corrected for cell-to-cell blank should not exceed 0.100. If the blank exceeds this amount either the molybdate reagent or the iso-butanol is suspect. The reagent blank should be determined for each batch of samples measured. 

If turbidity is suspected (it should not exceed an extinction of 0.05 on a 10-cm cell) fill a clean dry 10-cm cell with the sample (before adding the mixed reagent), measure the extinction at 8850 A, and empty the spectrophotometer cell back into the flask. Drain the cell but do not wash it. The retention of about 0.5 ml of sample in the cell will cause little error. Determine all turbidity blank corrections before adding mixed reagent to the samples. Deduct any cell-to-cell blank from the turbidity blank before adding this value to the reagent blank (see the following). 

Place 8 ml of perchloric acid and 2-3 drops of potassium iodide solution into a conical flask. Evaporate, without coverglass, until solid separates and only a few tenths of a milliliter of perchloric acid remains. Cool somewhat, add 5 ml of dilute ammonia solution, and evaporate but do not bake to absolute dryness. Leave 1-2 ml of liquid or the formation of polyphosphate will be excessive. Continue exactly as described in Section F, paragraphs 5-7, and correct the resulting extinction for any cell-to-cell blank. 

Carry out the method exactly as described in Section F paragraphs 1-4 using the 1-, 5-, or 10-cm cells that are appropriate. Add the concentrated ammonium chloride solution to 100 ml of redistilled water in a clean Erlenmeyer flask and use a column previously flushed with at least 50 ml of dilute ammonium chloride solution just before use. The blank extinction corrected by any cell-to-cell blank should not exceed about 0.1 using a 10-cm cell. 

As the precise values of comparatively small extinctions have to be measured, corrections for all optical inequalities become important. Fill both spectrophotometer cells with 90% acetone and find the cell-to-cell blank of the sample cell against the reference cell at all wavelengths used in the method. Correct all extinction values by this cell-to-cell blank which may amount to 0.01 or more. 

The extinction at 7500 A is corrected for any cell-to-cell blank at this wavelength and the resulting extinction (Ej,) is multiplied by a factor / to give the turbidity blank extinction to be used with spectrophotometer readings at other wavelengths. 

Total blank correction = cell-to-cell blank + (/ X Ej) where f has the values shown below ... 

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