Sensitivity curve flame

Sensitivity Sensitivity in flame atomic emission is strongly influenced by the temperature of the excitation source and the composition of the sample matrix. Normally, sensitivity is optimized by aspirating a standard solution and adjusting the flame s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices. 

Table 8.7). Thus, intensity and concentration are directly proportional. However, the intensity of a spectral line is very sensitive to changes in flame temperature because such changes can have a pronounced effect on the small proportion of atoms occupying excited levels compared to those in the ground state (p. 274). Quantitative measurements are made by reference to a previously prepared calibration curve or by the method of standard addition. In either case, the conditions for measurement must be carefully optimized with reference to the choice of emission line, flame temperature, concentration range of samples and linearity of response. Relative precision is of the order of 1-4%. Flame emission measurements are susceptible to interferences from numerous sources which may enhance or depress line intensities. 

The limit of detection is a useful figure which takes into account the stability of the total instrumental system. It may vary from instrument to instrument and even from day to day as, for example, mains-borne noise varies. Thus, for atomic absorption techniques, spectroscopists often also talk about the characteristic concentration (often erroneously referred to as the sensitivity—erroneously as it is the reciprocal of the sensitivity) for 1% absorption, i.e. that concentration of the element which gives rise to 0.0044 absorbance nnits. This can easily be read off the calibration curve. The characteristic concentration is dependent on such factors as the atomization efficiency and flame system, and is independent of noise. Both this figure and the limit of detection give different, but useful, information about instrumental performance. 

The assay was carried out using a Varian gas chromatograph (model 5000 LC) under the following experimental condition. The oven injector and flame ionization detector temperatures were 125°C and 225°C respectively. A Porapak column was used, the eluent was N2 at a flow rate of 30 ml/min and the injected volume 2 pi. Various concentrations of purified methylene chloride in purified methanol were injected (both solvents were distilled to discard any impurity which might interfere with the sensitive assay). Calibration curves were linear in the range 50-500 ppm (the limit of detection was 10 ppm). Methylene chloride detection in the microspheres was performed by dissolving various amounts (20-200 mg) of microspheres in 220 ml of purified methanol prior to the injection. 

Figure 4 shows calibration curves measured as the peak absorbance at 235 nm. This shows good linearity and high sensitivity for all substances. The detection limit is well below 10 ng for all substances and thus competes favourably with flame ionization detector - GC. The retention times were also measured at all concentrations since it was suspected, due to the apparent tailing, that the chromatography might be non-linear. This did not prove to be so. 

Mass Spectrometry. The use of a quadrupole mass spectrometer as a GC detector for nonmethane hydrocarbon analysis has come of age in recent years. Development of capillary columns with low carrier gas flows has greatly facilitated the interfacing of the GC and mass spectrometer (MS). The entire capillary column effluent can be dumped directly into the MS ion source to maximize system sensitivity. GC-MS detection limits are compound-specific but in most cases are similar to those of the flame ionization detector. Quantitation with a mass spectrometer as detector requires individual species calibration curves. However, the NMOC response pattern as represented by a GC-MS total ion chromatogram is usually very similar to the equivalent FID chromatogram. Consequently, the MS detector can... 

Specific detectors are also available for quantification of radiopharmaceuticals. These detectors use a position-sensitive proportional counter. These detectors are sensitive to the beta and gamma nuclides listed in Table 3.5. The detector analog output can also be represented as an analog curve. Various other detection procedures have also been used, such as flame ionization [66], mass spectrometry [27,67], and infrared (IR) [68,69]. 

Figure 3. Schematic of turbulent combustor geometry and optical data acquisition system for vibrational Raman-scattering temperature measurements using SAS intensity ratios. Also shown are sketches of the expected Raman contours viewed by each of the photomultiplier detectors, the temperature calibration curve, and several expected pdf s of temperature at different flame radial positions. The actual SAS temperature calibration curve was calculated theoretically to within a constant factor. This constant, which accounted for the optical and electronic system sensitivities, was determined experimentally by means of SAS measurements made on a premixed laminar flame of known temperature. Measurements of Ne concentration were made also with this apparatus, based on the integrated Stokes vibrational Q-branch intensities. These signals were related to gas densities by calibration against ambient air signals.

Table XIV presents the results of a series of comparisons of calibration curves obtained with single and mixed analyte standards. Both the sensitivity of a particular analyte and the linearity of its calibration curve depend on whether it is analyzed in a single or a mixed analyte matrix and on which flame condition is selected for that matrix.

Neither No. 3 KIRI dynamite, Itemite nor ANFO bums when contacted with a small open flame. Hence, those materials might not bum when partially decomposed in these experiments. The order of the compounds with respect to their relative safety against shock is No. 3 KIRI < Iremite < ANFO. To compare the sensitivities of the materials in Fig. 3.83, one may compare the curves. The further to the right a curve lies, the lower the sensitivity of the compound. 

This method used extensively in flame photometry is based on the principle that the interfering agents are added in equal but large amounts to standards and samples alike, and that all determinations are carried out in their presence. The curves obtained with the anionic depressors (Section 5.2.1) indicate that this approach could be used even in cases where the approximate concentration of the interfering agent in the samples is not known. In practice, however, sensitivity is considerably... 

Table 17.3 lists some representative detection limits of various elements by atomic absorption and flame emission spectrometry. We should distinguish here between the sensitivity and detection limits in atomic absorption. The former term is frequently used in the atomic absorption literature. Sensitivity is defined as the concentration required to give 1% absorption (0.0044 A). It is a measure of the slope of the analytical calibration curve and says nothing of the signal-to-noise ratio (S/N). Detection limit is generally defined as the concentration required to give a signal equal to three times the standard deviation of the baseline (blank)— see Chapter 3. 

The most important diagnostic we have for furnace AAS is the stable slope of the working curve. The characteristic mass, mo. has been defined to represent analyte sensitivity. The mo is measured in terms of the mass of analyte in pg that will produce an integrated A signal. Aj, equal to 0.0044 s. This measure of sensitivity is analogous to the flame AAS term for sensitivity which is the concentration in mg/L that will produce 1% absorption (0,0044 absorbance). The mo values are specific for each analyte and relatively independent of the matrix. Using analytical conditions summarized in Table 1. the characteristic masses are summarized in Table 2. Table 2 also includes detection limits in fig/L. The fig/L detection limits assume a sample aliquot of 100/characteristic mass of about 20% may reflect differences between individual instruments. For an individual instrument, the day-to-day variation should spread less than about 20%. This slope is matrix independent. 

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