Standard calibration curve methods

Standard Calibration Curve Method. Samples of metal free blank oils of viscosities 20 and 75 (Cat. No. 254 C 20 and 254 C 75), respectively, were spiked ... 

A seventh spectrophotometric method for the quantitative determination of Pb + levels in blood gives a linear internal standards calibration curve for which... 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

Protein concentration can be determined using a method introduced by Bradford,4 which utilises Pierce reagent 23200 (Piece Chemical Company, Rockford, IL, USA) in combination with an acidic Coomassie Brilliant Blue G-250 solution to absorb at 595 nm when the reagent binds to the protein. A 20 mg/1 bovine serum albumin (Piece Chemical Company, Rockford, IL, USA) solution will be used to prepare a standard calibration curve for determination of protein concentration. The sample for analysis of SCP is initially homogenised or vibrated in a sonic system to break down the cell walls. 

Hasegawa et al. [76] measured miconazole serum concentration by a high performance liquid chromatographic method. The authors assessed whether the internal standard method produced an intra-assay error and found that the method gave more precise and more reproducible results compared to the absorption calibration curve method. With 0.5 pg/mL of miconazole, the coefficient of variation produced by that method was 3.41%, whereas that of the absorption calibration curve method was 5.20%. The concentration of absorptions calibration curve method showed higher values than the internal standard method. This indicated that the internal standard method was far more precise in measuring the miconazole serum concentrations than the absorption calibration curve method. 

Yamamoto et al. [6] studied preservation of arsenic- and antimony-bearing samples of seawater. One-half of the sample (201) was acidified to pH 1 with hydrochloric acid immediately after sampling, and the remaining half was kept without acidification. In order to clarify the effect of acidification on storage, measurements were made over a period of a month after sampling. Results are given in Table 1.1. In this study, a standard addition method and calibration curve method were used for comparison and it was proven that the two gave the same results for the analyses of seawater. 

In general, the calibration curve method is suitable for all samples where the test substance is not bound in complexes or when it can be liberated from complexes by suitable sample pretreatment. Otherwise, the compositions of the samples and of the standard solutions must be as similar as possible to obtain results with acceptable accuracy. In view of the ISE potential drift, the calibration must be repeated often (at least twice a day). As mentioned above, the precision of the determination is not particularly high with a common precision of the potential measureihent at a laboratory temperature of 1 mV the relative error is 4% for univalent and 8% for divalent ions [58], However, this often suffices for practical analytical purposes. An advantage is that the same precision... 

The inocula totalled 1 ml in each case, the mixed culture having 0.5 ml inocula of each of the two organisms. For each experiment, a bottle was taken of each culture and the sulfide and dry weight measured. Sulfide was determined by the method of Pachmayr as described by Triiper and Schlegel (11). The concentration of sulfide was measured by absorbance using a Beckman DB spectrophotometer at 670 nm, and a standardized calibration curve. 

Cadmium in acidified aqueous solution may be analyzed at trace levels by various instrumental techniques such as flame and furnace atomic absorption, and ICP emission spectrophotometry. Cadmium in solid matrices is extracted into aqueous phase by digestion with nitric acid prior to analysis. A much lower detection level may be obtained by ICP-mass spectrometry. Other instrumental techniques to analyze this metal include neutron activation analysis and anodic stripping voltammetry. Cadmium also may be measured in aqueous matrices by colorimetry. Cadmium ions react with dithizone to form a pink-red color that can be extracted with chloroform. The absorbance of the solution is measured by a spectrophotometer and the concentration is determined from a standard calibration curve (APHA, AWWA and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington, DC American Public Health Association). The metal in the solid phase may be determined nondestructively by x-ray fluorescence or diffraction techniques. 

Iodine in water also may be determined by the Leucocrystal violet colorimetric method. An aqueous sample is treated with mercuric chloride followed by Leucocrystal violet reagent [4,4 ,4 —methylidynetris(N,N-dimethylani-hne)] in the pH range 3.5 to 4.0. A violet color is produced. The absorbance or transmittance is measured at 592 nm by a spectrophotometer or filter photometer. Iodine concentration is calculated from a standard calibration curve. 

Amounts of injected compounds are proportional to the peak area. BA concentrations are calculated by comparison to standard calibration curves. This method does not need an internal standard for quantitative analysis. 

The laboratory quality control program has several components documentation of standard operating procedures for all analytical methods, periodic determination of method detection levels for the analytes, preparation of standard calibration curves and daily check of calibration standards, analysis of reagent blank, instrument performance check, determination of precision and accuracy of analysis, and preparation of control charts. Determination of precision and accuracy of analysis and method detection limits are described under separate subheadings in the following sections. The other components of the quality control plan are briefly discussed below. 

The intensity of color of the ferric thiocyanate formed is proportional to the concentration of thiocyanate ion in the sample. The absorbance or transmittance is measured at 460 nm using a spectrophotometer or a filter photometer. The concentration of SCN- in the sample is determined from a standard calibration curve. The detection range of this method is 0.1 to 2.0 mg SCN-/L. Dilute the samples if the concentration exceeds this range. 

Alternatively, air drawn through a PTFE membrane followed by sodium bisulfite solution in impingers impinger solution treated with chromotropic acid and H2S04 color developed due to formation of a derivative of formaldehyde absorbance measured by a spectrophotometer at 580 nm a standard calibration curve prepared from formaldehyde standard solutions for quantitation (NIOSH Method 3500,1989) recommended air flow 500 mL/min sample volume 50 L. 

Air drawn through Palmes tube with three triethanolamine (TEA)-treated screens analyte converted into nitrite ion (NO, ) NO, treated with an aqueous solution of a reagent mixture containing sulfanilamide, H3P04, and V-1 -naphthy 1-ethylenediamine dihydrochloride color develops absorbance measured at 540 nm by a spectrophotometer, concentration determined from a standard calibration curve made from NaN02 (NIOSH Method 6700, 1984). 

The correlation coefficient (y) is a measure of linear relationship between two sets of data. It can attain a value which may vary between 0 and 1. A value of+1 (or-1, when the slope is negative) indicates the maximum possible linearity on the other hand, a zero y indicates there is absolutely no link between the data. In environmental analysis, especially in spectrophotometric methods, y is calculated to determine the linearity of the standard calibration curve, y may be calculated from one of the following equations. 

A simplified version of the method of standard additions can be used when a longer series of samples with the same matrix, volume, and sample mass is to be analyzed. Then the standard addition curve is shifted to pass through zero and hence becomes a matrix-matched standard calibration curve. 

Calibration is carried out using standard calibration curves. The simplicity, repeatability, and low cost of the method have allowed its use for routine determination of trihalomethanes in tap water. SOME has also been compared with solid phase microextraction (SPME), purge and trap (P T), and direct aqueous injection (DAI) [10]. This technique offers accuracy comparable with that obtained using P T and DAI. With respect to conventional LEE, the SDME method is more accurate. In contrast to DAI and P T, it requires no special equipment. SDME has been used for extraction of chlorophenols [II], pesticides [12, 13], warfare agents [14], and butanone derivatives [15], and for control of food products [16]. The low costs of the SDME method (typical GC syringe and 2-3 pL of solvent), simplicity, and short extraction time (approximately 15 min) make it particularly suitable for preliminary analyses of organic pollutants in water samples. It can also be an effective alternative to SPME, as it does not require the use of expensive instrumentation. 

In order to minimize the possibility of errors as a result of variable injection volumes, the internal standard (IS) method should be used. It involves the addition of a compound, the IS, which is not already present in the sample. This is normally a substance that elutes at a position near the sample component of interest and should be well resolved and readily detected under the given chromatographic conditions. The IS is added in constant amount to solutions that contain varying amounts of the analytical standard. Calibration curves are then constructed by plotting the ratio of either the areas or peak heights of the two peaks (analyte/IS) versus amount or concentration of analyte. The amount or concentration of each chromatographed sample can then be obtained by interpolation of the calibration curve. The calibration plot should be a straight line with an intercept of zero. However, non-linear standard curves may result when... 

The second consideration for quantitative analysis is that if the elements are present in trace quantities the problems of multiple sample preparation by ashing increase. A vicious circle is endured when the quantity and percentage recovery of the element cannot be determined until the quantity of the element present is known for dilution limits and range of standard calibration curve required. If the quantity is known, the percentage recovery may be determined by calibration curve or by standard addition. The percentage recovery results should be 100% 2% to allowing for errors and loss. Drawbacks to this method are ... 

Interferences due to matrix effects can be detected by comparing the slopes of the curves for the spike sample and the pure standard solutions. In the absence of interferences both slopes should be parallel. In effect, the method is equivalent to preparing the standard calibration curve with exact matrix matching. To apply this... 

All samples are analysed for Cd, Pb, Cr against standard calibration curves prepared from 0.0, 0.5, 2.5, 5.0 and lO.Oppm of each metal in 0.25 M HNO3. The ultrasonic nebuliser is used for the determination of Cd, Pb and Cr while the continuous cold vapour trap method is used for the determination of Hg. The recovery of each metal is determined for each metal. The Hg forms the vapour ion of the metal in solution after reduction with SnCl2(Sn2+ + Hg2+ > Sn4+ + Hg°) and the metallic mercury is swept to the plasma torch by the argon gas. This method is sensitive for Hg and has the advantage that it removes the analyte from the main solution and has very low limits of detection. 

To evaluate the influence of given metals in oil products it is fortunate that analytical methods are available that are sensitive, accurate and informative. Analytical methods associated with oils involve step-by-step procedures from sample preparation to suitable solutions for measurements against certified standard calibration curves. The sample preparation step is very important in the analysis, and the method used is decided by the concentration of metal present and whether or not it is soluble in the oil or present as particulates. Preparation by simple dilution may not be sufficient for very low concentrations of elements as the dilution may inhibit the ability to measure low levels due to non-detection, precipitation or settling out of the metals of interest. 

Standard Calibration Curve with Internal Standard Method. The above experiments were repeated using samples prepared in Table 5.7 and include yttrium (Y) as an internal standard. The Y internal standard (obtained dissolved in oil available from MBH Conostan) was added to each sample and standard at a concentration equal to the highest concentration of metal, i.e. 10.0 ppm. The internal standard can be used to quantify each of the metals in each solution. 

Analysis Using Method of Standard Addition. Sometimes it is not possible to overcome interference effects using standard calibration curves or internal standard methods of analysis. However, a third method involving the standard addition may be used to achieve reproducible results. Under these conditions all solutions will have the same matrix composition, so influence of the matrix will be the same. It must be emphasised that this method only corrects for the slope of the calibration curve i.e. element measurements and not for effects of sample plasma noise, shifts, etc. The oil... 

Conclusion to Study of Non-Destructive Methods of Metal Analysis of Oil Products. The results in Table 5.9 show accurate results for analysis of metal spiked low viscosity (Conostan 20 blend) oil when analysed against a standard calibration curve in solvents kerosene, decalin and tetralin, respectively. The scatter of results for the six measurements of each sample is acceptable. The results for higher viscosity (Conostan 75 blend) oil gave consistently lower values, which illustrates the effect of viscosity on the nebulisation efficiency. 

The dilution methods of sample preparation of petroleum fractions for metal analysis are limited to the concentration of metals in each fraction. The dilution method is applicable for routine monitoring crude and lubricating oils, providing the concentration of metals is quantifiable using standard calibration curve, internal standard or standard addition method. The method is unsuitable for low concentration of metals especially as those can build up accumulatively, causing poisoning in all catalytic fractionation plants. The low concentration of toxic metals may be undetected by dilution methods and may escape monitoring if a more sensitive method is not used. 

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