Scope_and_Field_of_Application_and_Accounting_Essay

Scope And Field Of Application Accounting Essay While doing the measurements or assessing the performance of the measurement procedure, quality assurance and control measures are important to ensure that the measurement process is stable and in control. Such measures include qualified staff, proper maintenance of equipment and reagent. Introduction Good analytical results are necessary to get more reliable decisions. A key element of good results is comparability. Comparability is shown to a standard set of measurement units and scales. The most of chemical measurement results are provided with the internationally accepted system of unit’s i.e. SI. Traceability is not a new concept in chemical analysis but before the invention of instrumental techniques titration and gravitation played a vital role in chemical laboratory. With the development of the technology the requirement for traceability has come to end. Method development has an equation explaining how to calculate measurement from the measured quantities and specifies under which condition the equation holds good. Validation demonstrates that this equation and set of conditions is sufficiently complete for the purpose in hand. This is achieved by calibration using appropriate measurement standard. Validation explains the equation and a set of specified condition which are required. Establishing traceability makes the values of the measured quantities and the values of the Specified conditions are related to appropriate standards. Two distinct types of measurement may be described: physical and chemical. The steps to achieving traceability for each are shown below: Physical Measurements Data is collected (thickness, weight, temperature, etc.) using … Equipment (callipers, balance, thermometer, etc.) calibrated by an … accredited calibration laboratory that … provide a certificate showing that the measurements are traceable to … a national or international reference standard. Chemical Measurements concentration, mass Data is collected using … Analytical equipment calibrated using … Laboratory standards prepared from … A reference material whose properties are traceable to … A national or international standard or.. validated through inter laboratory comparisons, use of CRM, consensus standards, or use of Specified methods. Key elements in establishing traceability are as follows The acceptable uncertainty and the measured are specified. A suitable method is chosen to estimating the value of a measurement procedure with associated calculation, an equation and measurement conditions The calculation and measurement conditions can affect the result or the value assigned to standard by demonstrating through validation. The relative importance of each influence quantity is identified. Reference standards is correctly chosen and applied. The uncertainty is estimated. Principles of traceability Methods, Measurand and Results Measurand:- it is a quantity of measurement like mass, volume and concentration. The quantity of measurement is clearly and unambiguously defined. For example, volume is defined for a specific temperature and concentration of a particular analyse and chemical species. Some misbrands are defined based on there methods used. For example, ‘extractable lead’ requires the specification of extraction conditions. Measurand defined by using a method are called empirical and rational measurand are explained without reference to a specific method. Measurement methods: –it is a procedure that provides estimates of measurands. Methods are developed and documented to get reliable estimates to incorporate all necessary controls and corrections. Results are values of measurands and measurement of an appropriate method. It is an estimation of measurands. It has properties like uncertainty, accuracy and traceability. Measurement scales, standards and units To get Meaningful comparisons between measurement results than the results are expressed in the same units. It is achieved by giving measurement results as multiples of a given unit. For example, a mass of 2.1 kilograms has a mass equal to 2.1 times the mass of the international kilogram. The mass of the international kilogram is the unit of mass. In order to express one mass as a multiple of another, the two have to be compared. It is difficult to compare all masses with the international kilogram. So we can compare by reference standards, which are inturn calibrated against other standards. So it forms a chain of comparisons to primary unit. Providing access to primary units of measurement by using reference standards is the principal function of traceability; without it, there is no meaningful measurement. A measurement scale is an agreed method of units of measurement having an origin (a zero point).Mass, length and concentration are expressed in linear measurement scale with zero at the origin and it is called as ratio scale. For example pH is expressed in Logarithmic scale with a reference point of hydrogen ion activity of 1. When two results are described using same measurement scale they are expressed by same units and the same origin. Calibration Calibration is the fundamental process in establishing traceability. By calibration the traceability to appropriate reference standards is achieved in practice. As Calibration is described by the VIM it is the process of establishing the relationship between values of a given measuring instrument or system, and the values provided by measurement standards. Calibration is applied to parts of a measurement system. Generally the Instruments are calibrated in isolation, and then used in a larger measurement system. Due to this we should use a pure chemical as the calibration material, though it is added to a matrix similar to the samples expected to reduce matrix effects. Instruments like balances and thermometers are calibrated less frequently because their results do not vary much in the medium term where as instruments like GC or ICP equipment tend to vary more and are calibrated more frequently while performing the same run. Due to this the reference standard values appear in the calculation of the result (perhaps indirectly) and it is therefore the result is traceable to these reference values. In some cases, calibration standards are taken through out the measurement process. For example, a matrix reference material and test results are analysed at the same time And used to correct the results, by a known amount of material (a ‘spike’) may be used to estimate and correct the actual analyse recovery during a run. Clearly, if the procedures use the reference material value or the amount of ‘spike’ added must appear in the calculation for the result through an intermediate ‘recovery factor’ and the results are traceable according to the value used. During method development and validation, it should be followed that a fixed correction should be applied to all future measurements, based on observations of a particular reference material which is not used for regular, day to day calibration. So the value appears in the calculation of each result, it is meaningful to speak of traceability to the value in question. Effects on measurement results Combination of one or more determinations of any measurement give a Result under specified conditions. For example, analysis of a soil sample contain contaminants, involves the quantitative determination of the mass of soil taken, and the concentration of analyte in a measured volume of solution of the sample. These parameters are qualified by the conditions of measurement. Mass determined strictly by weighing in vacuo at a specific gravitational field, volume taken as ‘volume at 20C’ and extraction conditions like complete extraction or partial extraction given in terms of time, solvent, and temperature. The mass, concentration and volume vary from one measurement to the next, due to different sized samples and they are the measured values of the ‘variables’ in the calculation of the final result. The extraction and other conditions are taken near to their nominal values and they do not need change as they are fixed conditions, and are not generally included in the calculation. For a specified measurement method, if the fixed conditions are changed, than the results are changed. For example, if extraction conditions are changed from those specified in the method, the result will be wrong. It follows both the fixed conditions required for the measurement and measured values obtained, are put into the calculation of the result, affecting the analytical result. If there is wrong in fixed conditions or measured quantities, then also the results become wrong. These measured values and specified conditions, are the ‘Influence quantities’ for the measurement and all have influence on the result so all measured values and specified condition must be controlled. Controlling fixed conditions To get the same reading for a measurement, we can get by using same measuring instrument. Consider the soil analysis example with one simple physical aspect of the measurement, extraction time. The two analysts use the same clock to determine the extraction time. If it is followed, it is possible to conclude that all the results are traceable to the time given by the clock and the clock provides the reference standard of time. If the same clock is used for every measurement it results in a consistent set of conditions, and extraction timing will not cause in different results. But in practice it is not possible to use the same clock by different person at different places. It is achieved by comparing all the clocks with the master clock so that all the results are traceable to their own clock interval and also to their master clock. This traceability is a single reference standard, the master clock in the example generates consistent measurement in the different laboratories. This leads to one key principle; Traceability to common reference standards allows laboratories to obtain the same set of fixed conditions required for measurements. Controlling variables with calibration standards To use a consistent measurement scale, it is achieved by using the same calibration standards for successive measurements. The following short discussion develops this concept. Consider two laboratories, A and B, carrying measurements of samples of the same type (see Figure 1). Each equipment is calibrated using a reference standard with a known nominal concentration (x1 and x2 respectively). From a calibration equation we get the respective results of y1 and y2 including the respective values of x. In each case, the result y is a function of the reference value x. The reference value x provides the units of measurement. Here, y1 is traceable to x1, and y2 to x2.The question arise is the relationship between y1 and y2. Figure 1 Test sample Result y1 RM x1 y1=f1(x1) Test sample Result y2 RM x2 y2=f2(x2) A B Here the two reference standards are both calibrated against some common reference, so comparison becomes meaningful (Figure 2). Here both results are obtained from the same value (x0) and have the same units of measurement and direct comparison of the values y1 and y2 is now not only possible but also meaningful. Here traceability does not make the results identical for different samples. But traceability through calibration permits meaningful comparison by ensuring ‘consistency’ of measurement units. Figure 2 Test sample Result y1 RM x1 Test sample Result y2 RM x2 y1=g1(x0) y2=g2(x0) CRM x0 From this discussion two principles are concluded:- When a result is calculated from a reference value, it is traceable to that value. Traceability to common references allows meaningful comparisons between results Common references allow arbitrary definition From above figure 2 it is possible to derive a direct mathematical relationship between y1 and y2 in which the value of x0 is eliminated. For example, in case of linear responses, the ratio y1/y2 does not contain x0 of fig1. If traceability to a common reference is assured, the value of the common reference can be defined arbitrarily without affecting the relationship between end results. Role of method development Method development produces a standard operating procedure, having a set of instructions for carrying out a measurement, a set of measurement conditions and equation from which the result is calculated using the values of the measured parameters. It also provides an equation, having consistent results provided that the specified conditions are correctly set and stable. The results will be consistent if all the parameters are traceable to stable reference. However, this expectation is based on some assumptions like specifically, linearity of response, freedom from overall bias, and absence of other significant effects. If these assumptions are incorrect due to the presence of unsuspected effects, results will be unreliable and often incorrect. Role of method validation Method validation is a mechanism used to test these crucial assumptions. It answers the question by making experimental tests of the assumptions and performing measurements on appropriate reference materials, or by comparison with the results of independent methods. When an effect is discovered, the method is modified and subjected to further development and validation. Such a modification has three basic forms: Eliminates the effect (example by changing digestion conditions to eliminate precipitation in elemental analysis) Reduction of variation caused by adding or reducing a control range. For example, it is necessary to specify a particular operating temperature or range of temperatures to reduce variation. Correction of effect by including it in the calculation of the result. 3.10 Traceability and measurement uncertainty Uncertainty arises to some extent or entirely from the calculation of the result. When a reference value is uncertain than it leads to the uncertainty in the result. ii) Uncertainty in results arises due to combination of all the uncertainties in reference values and arising from the measurement procedure. To estimate the uncertainty of a particular result, the analyst should view the uncertainty arising due to measurement procedure and the uncertainty associated with their reference values. Traceability: The International Definition According to International Vocabulary of Basic and General Terms in Metrology (VIM): Traceability is defined as Property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. International System of Quantities and Units (SI) The measurements should be expressed in agreed measurement units. The appropriate system of units in chemical measurement is the "System International"(SI). It forms a coherent system which is used universally in science and widely in trade. SI defines base units for mass (kilogram, kg), length (metre, m), time (second, s), Thermodynamic temperature (Kelvin, K) , electric current (amp, A), luminous intensity (candela, cd) and amount of substance (mole, mol). It also defines derived units in terms of the base units. Table 1 Quantities and units in chemical measurement Quantity Units Concentration of a specified chemical entity: mol/kg; mol/cm3; g/cm3; mass or Volume ratio Mole fraction: mol/mol Purity of a chemical substance: mol/kg; mass ratio pH pH :(negative logarithm of hydrogen ion activity) enzyme activity :katal (mol s-1) (SI unit), U (mol/min Establishing Traceability Essential activities in establishing traceability A set of activities which are necessary to establish traceability in a working laboratory are as follows. i) The measured and the acceptable uncertainty are specified. ii) Selection of suitable method of estimating the value like measurement procedure involving calculation, an equation, and measurement conditions. iii) Make demonstration of validation having calculation and measurement conditions which include all the influence quantity, affecting the result, or the value assigned to a standard. iv) The relative importance of each influence quantity is identified. v) Appropriate reference standards is applied and chosen. vi) The uncertainty is estimated 6.2 Specifying the measurand and required uncertainty To have a meaningful measurement it requires unambiguous specification of the measurand, or quantity to be measured. These are: 1. Identity of the analyte. Chemical measurement quantifies the molecular or elemental species. For example, different isotopes, Isotope mixtures, isotopomers, enantiomers, or crystalline form need to be distinguished. 2.Implied measurement conditions. The analytical results are measured under conditions close to normal ambient temperature, pressure and humidity. 3. Recovery correction. It is important to mention the quantity of interest is a material recovered from a substrate or it is the total amount present. The former is not corrected for analytical recovery but the latter needs a recovery correction. Validation: Validation of a single laboratory has the following activities for the reasons given below. 1. Selectivity and specificity are assed, to ensure that the method response to a particular species of interest but not to other similar species. 2. A certified reference material checks whether the method is not significantly biased by comparison to traceable values which are obtained independently. 3. Precision studies with time interval and set of conditions will provide another test for presence of significant unsuspected effects. 4. A linearity check to confirm that the units given is calculated and quoted as a simple ratio. Intercomparisons between analysts and different laboratories or with other methods will provide possible mistakes in the method. The second important consideration in validation studies is references used to control, calibrate and test the method during validation are traceable. It is important that the validation studies are directly relevant to laboratory results. Validation plays a key role in establishing traceability. It is not an optional activity. While adopting a standard method which is validated and thoroughly tested has some level of validation remains necessary. Importance of different influence quantities - The importance of different influence quantities is necessary in deciding the degree of control or calibration. Establishment of specific calibration for every quantity is not always necessary. -In general the quantitative effects on measurement results are detected by the importance of different influence quantities .Quantities with large and immediate effect on the results are important. at trace level, physical measurements like time, mass and volume are easily controlled and measured compared to chemical effect. - The uncertainty associated with a particular effect is small compared to the overall uncertainty than control is not required. The method development is carried under restricted environmental conditions and laboratories are generally operated according to the conditions mentioned in method development. Choosing and applying appropriate reference standards Calibration along with validated methods is key to traceability. In practice the calibrated and certified reference standards are not always available so it is necessary to develop appropriate measurement standards. However there are different types of measurement standard for chemical measurement and there are different circumstances for their use. Choice of the Reference References play a vital role in traceability so it is crucial. The following points consider the choice of reference for: 1. Physical measurements of an analytical work 2. Identity is conformed. 3. Certified reference standards used for calibration. 4. Other materials used for calibration 5. Reference data used for calibration. 6. Certified matrix reference materials used for validation studies. . Reference materials for method development, validation and verification. Reference materials particularly matrix reference materials, play a vital role in method development, validation and verification so it is strongly recommended. Matrix effects factors like concentration range can be more important than the uncertainty of the certified value. The factors to consider include: ·Measurand (analyte) ·Measurement range (concentration) ·Matrix match and potential interferences ·Sample size ·Homogeneity and stability ·Measurement uncertainty ·Certification procedures (measurement and statistical) Assessing the traceability of commercial reference materials While choosing the reference materials the following factors should be considered. a) Conforming the production of the reference materials to quality standards such as ISOGuides 34, ISO 17025 and conformation should be demonstrated through third party assessment. b) Both producer and material are track recorded. c) Conforming availability of a certificate and report to ISO Guide 31. d) The validity of certification, uncertainty data and conformance of key procedures with ISO Guide 35. Reporting Traceability Evidence of traceability is reported only for calibration certificates and it is mandatory under ISO 17025. The details of traceability are not given on test reports. When it is necessary to report evidence of traceability results, the report include: 1. The identity of calibration standards used 2. The identity of references used to control the conditions of measurement is significant. Conclusion The document shows discussions of principles underlying the establishment of traceability for a method used by a calibration, measurement or testing laboratory. 1. Method development develops an optimised procedure for getting an acceptable quantity of the measurand along with the calculation and a set of measurement conditions. 2. Validation demonstrates that this calculation and set of conditions are necessarily required. 3. If the condition is ready the laboratory establishes traceability or control for each value in the equation and for each of the specified conditions. 4. Traceability, established by calibration using standard measurements, is important for the critical values in the measurement and if critical values are less, than it is recognised that the required control may be less affected. . 10 Bibliography 1 International vocabulary of basic and general terms in metrology. ISO, Geneva (1993) 2 EURACHEM/CITAC Guide: Guide to Quality in Analytical Chemistry (2002). Available from the Eurachem secretariat, http://www.eurachem.org/, or CITAC at http://www.citac.cc/ 3 The Fitness for Purpose of Analytical Methods: A Laboratory Guide to Method Validation and Related Topics. Eurachem (1998). Available from http://www.eurachem.org/ 4 A Williams, S L R Ellison, M Roesslein (eds); EURACHEM/CITAC Guide: Quantifying Uncertainty in Analytical Measurement, 2nd Edition (2000). Available from the Eurachem secretariat (Europe), from LGC, Queens road, Teddington, England (UK) or http://www.eurachem.org/ 5 ILAC Guidelines for the Competence of Reference Material Producers, ILAC G12, (2000) (see www.ILAC.org) 6 EEE/RM/062: The selection and use of reference materials; A basic guide for laboratories and accreditation bodies (2002). Available from the Eurachem secretariat or website (http://www.eurachem.org/). Read more: http://www.ukessays.com/essays/accounting/scope-and-field-of-application-accounting-essay.php#ixzz2Q8JoWZgQ