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Glossary Aliquot Analyte Bell curve Blank Bulk sample (or sample) Determinate error Gaussian distribution GLP Half-life A portion of the "bulk sample". The particular element or compound which is to be quantified. See Gaussian Distribution. A sample which has been prepared that contains no analyte. The material provided for analysis. A ‘systematic’ error. Systematic errors can have a variety of origins: for example, a single measurement might be wrong because the instrument baseline was not zeroed between samples, or the balance used for weighing materials might have caused all weights to be too high. Systematic errors can have other causes; a stock solution of the wrong concentration might have been used, or there may be a limitation in the background theory, such as the assumption that the ideal gas law, pV=nRT, is exact. Errors can also arise through repeated error by the chemist. For example, most pipettes are not of the "blow-out" type; the liquid in these pipettes must be allowed to drain, but a small, and reproducible, amount of liquid remains in the tip. If you blow out this liquid each time you will introduce an error to the concentration of every solution. Often also called a normal distribution, or a Bell curve. A Gaussian distribution will arise if the experimental errors are truly random. When that is the case, the readings will cluster around the mean value and the curve will be symmetrical. Deviation from a symmetrical shape indicates the presence of systematic error. Good Lab Practice is a set of controls and specifications that define how an experimental procedure is to be carried out. If followed, the procedure ensures that the results of the experiment can be relied upon, within specified error limits. Normal analytical experiments should be carried out according to the principles of GLP. The time required for the concentration of a species to fall by 1/2 ; similar to, but not identical to the lifetime. Indeterminate error Knurl Lifetime Method spike Method blank Multistandard QC QC sample Sample Sample spike Spike An indeterminate error is a ‘random’ error. These appear due to random variations in preparation and measurement of samples. In most types of experiment the distribution of such errors is generally well described by a "normal" or "Gaussian" distribution. The calculation and use of certain statistical measures, such as standard deviations, depends upon random errors forming a Gaussian distribution. To provide with ridges, to assist the grasp, as in the edge of a flat knob, or coin; to mill. The lifetime of a species whose concentration is changing is the time required for the concentration to fall to 1/e of its initial value. Lifetimes in chemistry vary over a large range, from around 10-16 s for some very short-lived atomic states to billions of years for elements that are weakly radioactive. See also half-life. A spike sample made up using the same sample preparation procedure that you used for your ‘unknown’ sample (i.e., all the steps required by the method are carried out when preparing the spike). The method spike contains all the required reagents, plus a known quantity of analyte. A method blank is made up using the same sample preparation procedure used for your ‘unknown’ sample i.e., all the steps required by the method, but it only contains the necessary reagents. Most analytical standards contain just a single analyte. However, in some analytical procedures more than one analyte is measured, and it is then common to use multistandards. A multistandard contains several analytes in a single solution, so a set of multistandards allows one to prepare several calibration curves. Multistandards may be prepared by making a single stock solution containing all analytes, then diluting this to make a series of more dilute solutions, in each of which the ratio of different analytes will be the same as the ratio in the stock. Alternatively, each multistandard may be prepared individually, so that the ratio between the concentrations of different analytes in the stock can vary from one solution to the next. Quality Control. Quality control refers to any actions taken to ensure that analytical results are accurate and precise. Often summarized by the GLP. The Quality Control sample is a sample of defined composition, whose purpose is to allow you to confirm that the instrument, standards, method, and analyst are all working correctly. The sample contains an aliquot of the bulk sample, plus all reagents etc. It is standard practice to prepare samples in duplicate or triplicate. The term "sample" also refers to any portion of the bulk sample, blank or spike that is being prepared for analysis The sample spike is an aliquot of an ‘unknown’ sample, to which has been added a known amount of analyte; this sample is then carried through the same sample preparation procedure as the ‘unknown’ sample (i.e., all the steps required by the method). It therefore contains a known quantity of sample (which itself is of unknown concentration…) and an additional known quantity of analyte, plus all reagents etc. The amount of material added should be sufficiently small that the change in the matrix of the sample is minimized. Spiking is the enrichment of a sample or blank with a known quantity of the analyte under investigation. The amount added is usually chosen to be from 50-100% of the expected sample analyte concentration in order to ensure that, Spike recovery when measured, the spiked solution will lie comfortably within the range defined by the standards. The spike recovery is the percentage of the added analyte that is ‘recovered’, i.e., measured during the analytical procedure. Since the measurement will unavoidably contain some error, there is uncertainty in the percentage, and this may lead to a "recovery" in excess of 100%. Split/splitless injection Capillary columns are the most widely used type of GC column. Chromatographic material coats the inner surface of the column; since the column is very narrow and the coating extremely thin, the amount of material is tiny; consequently, only a very small amount of sample can be injected without overloading the column. In "split" injection the sample is injected into, and evaporates within, a small heated volume at the head of the column. One exit from this volume leads straight into the column, while a larger exit leads to waste. The sample is thereby split into two portions, and only a small fraction is fed into the column. If the sample is extremely dilute, detection limits may be an issue, so the entire sample may be fed into the column using "splitless" injection. [Figure from sge.com.] Standard A standard is a sample of accurately-known composition, generally containing a single analyte in the same matrix as the unknown samples. Although the solvent used for the standard is typically the same as that in which the unknown analyte is dissolved, the method by which a standard is prepared is often quite different to that used to prepare the unknowns; consequently one often needs separate blanks to correct the signal from the standards and from unknown samples. See also multistandards. The standard blank is prepared using the same reagents as those required for the standards, but need not (though it may be) prepared in the same way as the method blanks. Standard blank