Base line absorbance

A base-line method was used by three authors [22,23,24]. In all cases a correlation between the ultraviolet method and HPLC was observed, in those cases where the concentration was considerably higher than the detection limit. The detection limit was 0.27 mg per kg [22,23] or 0.3 to 1.7 mg per kg [24]. The latter range was found in an interlaboratory study (see also Section 7.5.3). The base-line method used differed from the original method for determination of natamycin in pharmaceuticals [1, page 547]. In the latter the base-line absorbance was calculated from the absorbances at the minima at 295 and 311 nm and the maximum at 303 nm (Figure 4-A). 

The base line absorbance was plotted against the concentration of Santonox R in mg/100 ml. 

Santonox R is extracted from a finely ground sample with boiling cyclohexane. The extract is then shaken with aqueous sodium hydroxide. Santonox R forms a sodium salt in the alkali layer. This layer is then examined on an ultraviolet spectrophotometer. The base line absorbance is calculated and referred to a previously prepared calibrated graph relating weight of Santonox R to base line absorbance. lonol does not interfere in the determination... 

Procedure Dissolve accurately 22.5 mg of /ram-clomiphene citrate and 52.5 mg of cis-clomiphene citrate (approx. 1 2.3) into 10 ml of DW in a clean 50 ml separating funnel. Add to it 1 ml solution of sodium hydroxide (5% w/v in DW). In the alkaline medium the base is liberated which is extracted successively with 3 portions of solvent ether (10 ml each). The combined ethereal layer is washed with two portions of DW (10 ml each). The resulting ethereal fraction is dried over anhydrous sodium sulphate, filter, evaporate to diyness carefully over an electric water-bath and dissolve the residue in 1 ml of CS2. Now, record the absorption curve in a 0.2 mm cell over the range 12.50 to 14.00 pm. Calculate the absorbance for the peaks at 13.16 and 13.51 pm respectively by employing the base-line method (see section 3. l. B in this chapter) between the minima at 12.66 and 13.89 pm. 

Detection Limit It may be defined as the concentration (meg ml-1) of an element that gives rise in the shifting of absorbance signal to an amount which equals to the peak-to-peak noise of the base-line. 

The typical trace obtained with this instrument (Fig, 7) can be used to explain the operation of the system. The absorbance recorded by the spectrophotometer with both Scrubs turned in is shown as a function of time (the right-hand ordinate is converted to ppm ozone by using an extinction coefficient (22)), Scrub 1 provides a base line for the absorbance (no ozone). At 2 min. Scrub 1 is removed and the absorption rises to a level calculated to be 250-260 ppm of ozone, A 30 second lag occurs due to the volume of the system. The reaction vessel is added to the flow route at U min, and at 8 min Scrub 2 is removed. 

The result of our effort to develop the best possible detector for MES is as follows. Our detector has a resolution of approximately 2 KeV (fwhm) at 15 KeV as shown in Figure 6. There is virtually no deterioration in performance over a period of several months. The overall efficiency of the detector when used for MES with 14-KeV y-radiation is such that a 0.001-inch thick sample of stainless steel type 302 (natural isotopic abundance) gives a spectrum with the peak height some 400% of the base line, Figure 7. (For comparison, when we started we were quite content with 50%.) With our 10-mc Co-57 source, the data acquisition rate in the peak is approximately 500 counts/min. This means that in a matter of a minute or less one obtains a recognizable spectrum. As a bonus, the observance of 6-KeV x-rays yields an effect of approximately 50% of the baseline. To accomplish this, we interpose a plastic filter between the source and the sample to absorb most of the 6-KeV radiation from the source (which does not contribute to the effect but is elastically... 

In comparison with an azo dye adsorbed on anhydrous barium sulfate (31), the K/S values of Figure 5 are remarkably large. In part this is because of the high extinction coefficient of the Pseudocyanine s /-band (cf., Figure 1) and in part to the non-zero base line in Figure 5A. It was experimentally convenient to use an 0.2 absorbance filter in the sample beam of the spectrophotometer in the absence of this filter—it had no influence on the final results—the K/S value at saturation coverage would... 

The absorbance measured with a base line at 3300 cm-l or 3 70 cm-l. The calibration curves were linear in the concentration range between 0-5 to 2 mg/ml and the results were in good agreement with those determined colorimetrically. The determination was not interferred with the coexistance of a limited amount of 1 0 possible drug constituents e.g., phenacetin and pyrabital (1 1 ). 

Accurate measurement of photon flux is essential in order to calculate quantum efficiency (whether based on absorbed photons flux or the flux of photons impinging on the surface). The response of commercial sensors is wavelength dependence, hence, the readout, which is in energy flux units (W cm ), is usually calibrated according to either the 365 nm line of mercury or the 254 nm line, depending on the sensor. For more details on radiation sources see de Lasa et al. (2005). A different method for measuring photon flux is actinometry. This method was very popular in the past, however, very few researchers still use it due to its complexity and the time it consumes. 

Figure 8-19. Idealized hyperbolic relationship between the photosynthetic photon flux incident on the upper leaf surface and the net C02 uptake rate for a C3 plant. The intercept on the ordinate (y-axis) indicates the net COz flux by respiration in the dark (-1 pmol m-2 s 1), the intercept on the dashed line indicates the light compensation point (a PPF of 15 pmol m 2s l), the essentially linear initial slope (37co2 ppf) indicates the quantum yield (Eq. 4.16) for photosynthesis [(5 - 0 pmol m 2 s l)/(115 -15 pmol m-2 s l) = 0.05 mol C02/mol PPF], and the maximum Jco2reached asymptotically at high PPF indicates the light-saturated net C02 uptake rate (about 12 (xrnol m-2 s l often designated AmaK or Amax). Here the quantum yield is based on incident photons, but more appropriately it should be based on absorbed photons.

Spectrophotometric Measurements. Spectrophotometric measurements were made with a Cary Model 14 recording spectrophotometer. Absorption spectra of solutions were obtained in silica cells. Absorption spectra of the crystalline salts were obtained using mixtures of the materials with petrolatum between glass or silica plates using the Cary Model 1417200 source. Blanks for the solid spectra were CaCOa mulls in petrolatum plus aqueous starch solution if necessary to produce a flat base line. The reference was adjusted so the base line was flat in the 520 to 600 m/x region where the U(VI) acetate complexes do not absorb. Slit widths for spectra of solids were typically <0.1 mm. 

The intensity of the incident radiation (/q) and the intensity of the transmitted radiation (/) can be measured by the base-line method illustrated in Fig. 10. In the FTIR spectrometer, the computer is capable of converting the transmittance into absorbance, subtracting the background or the solvent interference from the sample spectrum, generating a calibration curve, and calculating the unknown concentration. 

For cheese extracts this method is not suitable due to the interference of the matrix. However, it works well if a part of the spectrum at higher wavelengths is used for the calculation. This alternative base-line method uses the straight line between the absorbance at 311 and 329 nm as die base line. The net absorbance may be taken from the UV spectrum as the difference between the absorbance at the maximum at 317 nm and the base-line (Figure 4-B). It may also be calculated from ... 

Since the magnitude of the absorption varies from one solvent to another and from one wavelength to another, base-line shifts are usually observed during gradient elution. These base-line shifts are often linear with concentration of the second solvent. The size of such shifts depends on the extent of the compositional change and on the absorbance characteristics of the mobile phase, the optical path-length and the detector sensitivity. 

Calibration. Adjust the base line to zero absorbance while aspirating 0.6N hydrochloric acid. Aspirate each standard cadmium solution in turn and record the reading. Plot the readings obtained against cadmium concentration. 

Application of the Zeeman effeci to atomic absorption instruments is based on the differing response of the two types of absorption lines to polarized radiation, The 7T line absorbs only that radiation that is plane. polarized in a direction parallel to the external magnetic held the it lines, in contrast, absorb only ni-diation polarized at W to the field. 

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