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Cross-Examination of the Expert

Cross-Examination of the Expert

California DUI Defense: Analytical Chemistry Terms 

The following is a glossary of scientific terms used in analytical chemistry that relate to DUI cases.

This list was compiled by Pennsylvania attorney Justin McShane and is reprinted in California Drunk Driving Law and this website with his permission. It may not be reproduced elsewhere without his express consent.

-ometry: Suffix meaning “to weigh.”

-scopy: Suffix meaning “to look.”

Absorption: The process of one material (absorbate) being retained by another (absorbent).

Abundance: (in mass spectrometry) The intensity (height) of each peak depends on the abundance of that isotope in the sample, and the unique location of the peak on the x-axis indicates the mass-to-charge ratio (m/z) of the isotope.

Accurate: A measure of how closely the results are to the true value. It is characterized by a high standard deviation, but may or may not have a low average deviation from the true (actual) value.

Adsorption: The binding of molecules or particles to a surface must be distinguished from absorption.

Aliquant: A known amount of a homogeneous material, assumed to be taken with negligible sampling error. The term is usually applied to fluids. The term is usually used when the fractional part is not an exact divisor of the whole (e.g., a 15 mL portion is an aliquant of 100 mL). This concept is important in the context of sampling.

Aliquot: A known amount of a homogeneous material, assumed to be taken with negligible sampling error. The term is usually applied to fluids. The term is usually used when the fractional part is an exact divisor of the whole. When a laboratory sample or test sample is “aliquoted” or otherwise subdivided, the portions have been called split samples (e.g., a 10 mL portion is an aliquant of 100 mL). This concept is important in the context of sampling.

Alpha cleavage: The fission of a bond originating at an atom which is adjacent to one assumed to bear the charge.

Analyte: A substance that undergoes analysis. A substance or chemical constituent that is determined in an analytical procedure.

Analytical drift: A non-random change in signal with time.

Anion: An ionic species having a negative charge.

Anode: An electrode through which electric current flows into a polarized electrical device. Electrode at which oxidation takes place.

ASCLD/LAB: Association of Crime Lab Directors/Laboratory Accrediting Board. An ISO 17025 accrediting body that can provide certifications to both government and non-government labs.

ASCLD/LAB International: The registered trademark of ASCLD/LAB’s version and interpretation of ISO 17025. ASLCD/LAB is not a participating or observing member in the Council Committee on Conformity Assessment (ISO committee) that sets ISO standards.

Assay: A set of operations having the object of determining the value of a quantity. In analytical chemistry, this term is synonymous with measurement.

ASTM International: Originally known as the American Society for Testing and Materials and founded over 100 years ago, ASTM International is one of the largest voluntary standards development organizations in the world—a trusted source for technical standards for materials, products, systems, and services. It is a participating or observing member in the Council Committee on Conformity Assessment (ISO committee) that sets ISO standards.

Autodiluter: In the blood testing context, the autodiluter is a volumetric delivery device that is designed to take a precise portion of the blood sample and add into it a very precise amount of internal standard, which, for example, in blood testing for blood alcohol content in DUI cases is most typically n-propanol.

Autosampler: Provides the means to automatically, without human input, introduce a sample into the inlet of the chromatographic device.

Base peak: Refers to the most intense peak in the mass spectrum and is used as the reference peak when normalizing all peaks within the spectrum.

Baseline: Refers to the constant signal produced by the background level of the instrument. It is usually represented after integration as a flat line on the chromatogram.

Baseline drift: Refers to any deviation in the positive or negative as measured along the y-axis on the chromatogram to the expected baseline.

Blank: To be distinguished from a true blank, infra, a blank is a sample that is prepared using the same process that the unknown is to undergo, but does not contain the analyte of interest or any other detectable analyte (excepting purposefully spiked substances, such as the internal standard). It can be used to check for analytical carry-over.

Calibration and bias: The current preferred method to describe the analytical instrument’s precision and accuracy.

Calibration curve: Despite its name, it is not a curve but rather, if well-preformed, a straight line as determined by various measures of linearity. It is the analytical method that results in a plot of calibration data. It is a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration.

Calibrator: The known amount of analyte of interest that is placed within the batch of the run to insure that the analytical instrument is detecting the known within an established tolerance. It is called a calibrator if it is placed before the unknowns are tested and a verifier if placed after the unknowns are tested.

Carrier gas: A gas introduced in order to transport a sample for analytical purposes. In gas chromatography it is the gas which is passed continuously through the column and whose passage promotes the elution of the components of the sample. The carrier gas together with the portions of the sample present in this phase constitutes the mobile phase

Carry-over: A process by which materials are carried into a reaction mixture to which they do not belong. It can be either unidirectional or bidirectional in a series of specimens or assays. The term carry-over effect is used for carry-over from specimen to specimen.

Cathode: Electrode at which reduction takes place.

Cation: An ion or group of ions having a positive charge and characteristically moving toward the negative electrode in electrolysis.

Chemical ionization: A process used in mass spectrometry that uses a lower energy process than electron ionization. The lower energy yields less fragmentation, and usually a simpler spectrum. A typical CI spectra has an easily identifiable molecular ion.

ChemStation: Proprietary software developed by Agilent available in the international stream of commerce that records the raw data of a given analytical configuration and allows the analyst to alter the recorded data.

Chiral compounds: A specific type of isomer that is characterized as a type of molecule that lacks an internal plane of symmetry and has a non-superimposable mirror image.

Chromatogram: The printed graphical representation of the record of the analytical and interpretative process of the test performed. The printed paper graphical record of the instrument process that may or may not be subject to interpretation by the chromatographer, the software or both.

Chromatograph: The actual analytical device. It is not the graphical representation of the test.

Chromatography: From Greek words crwua (chroma), meaning “color,” and crafeiu (graphein), meaning “to write.” It is the collective term for a set of laboratory techniques for the separation of mixtures into its constituent parts. In two words, it can be summed up as “separation science.”

Co-elution: When two or more analytes elute from the column without sufficient resolution.

Collision cell: A method in tandem mass spectrometry (MS-MS) wherein after initial fragmentation occurs in the first mass analyzer purposeful collision of the remaining fragmented ions occurs with the molecules or atoms of some collision gas.

Column: The tube and the stationary phase contained within, through which the mobile phase passes.

Conclusory report: A report that is issued typically by the crime that simply states a conclusion of the testing without data to support the purported result.

Confidence interval: A particular kind of interval estimate of a population parameter. Instead of estimating the parameter by a single value, an interval likely to include the parameter is given. Thus, confidence intervals are used to indicate the reliability of an estimate. How likely the interval is to contain the parameter is determined by the confidence level or confidence coefficient. Increasing the desired confidence level will widen the confidence interval.

Control: The test portion or test solution used for assessment of the performance of an analytical procedure.

Crimp: The physical act of attaching both the septum (the membrane that the needle passes through) and the cap (the metal ring) to the headspace vial. The device used to do so is called a crimp (it is similar in appearance to a walnut cracker).

Dextrorotatory: The property of a molecule that exhibits on a rotating plane polarized light clockwise.

Diode: In electronics, a diode is a two-terminal electronic component that conducts electric current in only one direction.

Duplicate: Two measurements made concurrently and in the same location, or side-by-side. Used to evaluate the precision of the measurement method.

Electrospray mass spectrometry: A method of ionization in mass spectrometry most typically used in liquid chromatography. The liquid containing the analyte(s) of interest is dispersed by electrospray into a fine aerosol. Because the ion formation involves extensive solvent evaporation, the typical solvents for electrospray ionization are prepared by mixing water with volatile organic compounds (e.g., methanol, acetonitrile).

Electron impact mass spectrometry: Electron impact (EI) is the original mass spectrometry (MS) ionization method, and is still probably the most widely used of all ionization methods. In the EI process, the sample of interest is vaporized into the mass spectrometer ion source, where it is impacted by a beam of electrons with sufficient energy to ionize the molecule causing fragmentation.

Elucidation: Literally means to make clear through analysis. Typically, it is used in chemistry to explain the concept of taking raw data and coming to a conclusion. It is most frequently used in spectral interpretation and mass spectrometry.

Elute: Describes the emergence of analytes from the column of a chromatograph.

Equilibrium: A condition in which a chemical reaction is occurring at equal rates in its forward and reverse directions, so that the concentrations of the reacting substances do not change with time.

Error: The result of a measurement minus the true value of the measurand.

External standard: A compound present in a standard sample of known concentration and volume which is analyzed separately from the unknown sample under identical conditions. It is used to facilitate the qualitative identification and/or quantitative determination of the sample components.

EZChrom Elite: Proprietary software developed by Agilent available in the international stream of commerce that records the raw data of a given analytical configuration and allows the analyst to alter the recorded data.

Ferrell: A bushing or metal ring or cap-like item that is designed to keep the column in its place.

Flame ionization detector: A specific type of detector that is used to arrive at a quantitative result. It counts the increase in the number of ions that occurs as hydrocarbon bonds are burned. A polarizing voltage attracts these ions to a collector located near the flame. The current is sensed by an electrometer, converted to a digital form and sent to an output device that gives the peak.

Fronting: May be a presentation of co-elution, supra. It occurs when the A symmetrical hemisphere is greater than the B symmetrical hemisphere in a chromatographic peak.

Gas chromatography: A powerful, yet not perfect, analytical method whose goals are to uniquely separate and identify analytes of interest (qualitative measure) from a mixture and if desired arrive at a unique amount of that unique analyte of interest (quantitative measure). The mobile phase is that of a gaseous matrix as opposed to liquid chromatography whose mobile phase is liquid.

Gaussian: Normal distribution, characterized by a bell-shaped curve and having a mean at the center of the curve with tail widths proportional to the standard deviation of the data about the mean.

Good laboratory practices:
Generally refers to a system of management controls for laboratories and research organizations to ensure the consistency and reliability of results as outlined in the Organization for Economic Co-operation and Development (OECD) Principles of GLP and national regulations. GLP embodies a set of principles that provides a framework within which laboratory studies are planned, performed, monitored, recorded, reported and archived. There is no equivalent to this in forensic science generally and specifically in DUI forensic testing for BAC determinations.

Goldenseal: A consumable used in gas chromatography. It is housed within the injector portion of the instrument. Its primary function is to seal the inlet to help maintain the chromatographic conditions.

Headspace: The gaseous portion over the liquid portion in a closed system. It is also a method of indirect analysis of the gaseous phase relating the gaseous phase measure to the liquid phase measure based upon partition ratio once equilibrium is achieved.

Henry’s law: At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Or stated differently, an equivalent way of stating the law is that the solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid. Headspace gas chromatography requires a closed system (i.e., no pressure leaking from the gaseous phase).

Histogram: A graphical representation, showing a visual impression of the distribution of experimental data.

Hit histogram: A feature in certain software versions of mass spectral data systems wherein the unknown which is reduced to a chromatogram is compared against the knowns in the spectral library displayed in a histogram. Hit histograms indicate how the best matching units of a data set/some data sets are distributed.

Hit list: A panel in some mass spectral software programs that displays match factor scoring, relative match and probability scoring based upon computer-assisted pattern recognition to a loaded spectral library.

Hyphenated technique: A combination of chromatographic and spectral methods to exploit the advantages of both. It is a combination of two or more uncorrelated techniques.

Identity search: A method of search algorithms preformed by the software typically used to perform pattern recognition of a spectral library. An identity search is designed to find exact matches of the compound that produced the submitted spectrum, and therefore presumes that the unknown compound is represented in the reference library. Only experimental variability prevents a perfect match. The “similarity” search is optimized to find similar compounds and is intended for use when a compound cannot be identified by the “identity” search.

Injector: A device by which a liquid or gaseous sample is introduced into the apparatus.

Instrument configuration: The constellation of variables that the instrument itself is set for. Adjusting the instrument configuration will result in the change of the raw data generated. This is to be contrasted with software configuration which does not alter the raw data accumulated.

Integration: The act of altering the raw data. There are a number of different ways one can arrange the data collected. It comes in two forms: manual or software produced.

Internal standard: A compound purposefully added to a sample in known concentration to facilitate the qualitative identification and/or quantitative determination of the sample components.

Invalid: Proven to be not valid; not the same as “not validated” as “not validated” means that it may be valid or it may be invalid.

Ion source: Generally an assembly composed of: (1) an ionization chamber in which a stream of electrons flows from a hot filament across a stream of gas to collector (the potential between filament and collector is usually between 50 and 70 v); and (2) a device for the acceleration of these ions.

Ionization: The physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions.

ISO 17025: A conceptual framework that was adopted by an international committee comprised of people not in the law enforcement community, but rather in the scientific community as a whole, as well as those who are in industry. It does not specifically address the forensic science community, but rather covers any and all laboratories involved in testing and calibration services.

Isomers: Compounds with the same molecular formula but different structural formulas.

Isothermic: A characteristic of a thermodynamic process during which the temperature remains constant.

Levorotatory: The property of a molecule that exhibits on a rotating plane polarized light counterclockwise.

Limit of detection: The lowest amount of analyte in a sample that can be detected, but not necessarily quantified as an exact value. In other words, it is known that the analyte of interest is there, but the analyst cannot tell how much of it is present (i.e., the quantity). LOD is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limit (generally 1%). At the LOD, no quantification can occur. Hyper-technically, the LOD may be expressed as LOD=3.3*SD/S where SD=the standard deviation of the response and S=the slope of the calibration curve.

Limit of linearity: The highest point on a calibration curve that has been demonstrated to be linear in that a data point from a CRM was used to establish the upper most level of linearity. In other words, the LOL is set at the concentration of the highest standard analyzed even though in theory it could be extended beyond this. It is the maximum level where linearity is proven.

Limit of quantification: The limit at which we can reasonably tell the difference between two different values (zero and non-zero). It is a demonstrated point where quantification can begin.

Linear dynamic range: The concentration range over which the analytical working calibration curve has been demonstrated to remain linear. It is only between the lower and upper limits of the reference points used that the demonstrated linear dynamic range exists. Only when unknown values fall between the extremis points along the calibration curve can scientifically valid conclusions concerning quantification be drawn.

Liner: A consumable component of the inlet injector system. It is where flash vaporization occurs if there is a direct injection, not of the headspace. It is also the last point where a collection of alien material such as pieces of septa can be prevented from coming into contact with the column. There are many different types of liners.

Make-up gas: In the context of a flame ionization detector, gas injected into the effluent stream that elutes off the column to serve as a method of diluting the concentration so to not overwhelm the detector and also to insure constant pressure and flow.

Mass analyzer: In mass spectrometry, the method through which the fragmented ions are selected and then transported by magnetic or electrical fields to the detector or caused to be pumped out through the vacuum not to be analyzed by the detector.

Mass Hunter: A type of proprietary software offered by Agilent that can be used in mass spectrometry.

Mass spectrometry: The branch of science dealing with all aspects of mass spectroscopes and the results obtained with these instruments. It measures the mass-to-charge ratio of charged particles. It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules. The MS principle consists of ionizing chemical compounds to generate charged molecules or molecule fragments and measurement of their mass-to-charge ratios.

Mass-to-charge ratio: (m/z) represents the mass of a given particle (Daltons or Da) to the number (z) of electrostatic charges (e) that the particle carries. The term “m/z” is measured in Da/e.

Match factor: One of the features of mass spectrometry software wherein through a process of computer algorithm computer assisted pattern recognition is employed to arrive at the “match fact”. A perfect match is 999.

Matrix: The components of the sample, including the analyte.

Matrix effect: The combined effect of all components of the sample other than the analyte on the measurement of the quantity.

Mean: Arithmetic mean (average) is the sum of a series of observations divided by the number of observations.

Method: A way of doing something, especially a systematic way, that implies an orderly logical arrangement, usually in steps.

Metrology: The scientific study of measurement.

Mobile phase: A fluid which percolates through or along the stationary phase of a column, in a definite direction. It may be a liquid (liquid chromatography) or a gas (gas chromatography). In gas chromatography, the expression carrier gas may be used for the mobile phase.

Molecular ion: An ion formed by the removal from (positive ions) or addition to (negative ions) a molecule of one or more electrons without fragmentation of the molecular structure. The mass of this ion corresponds to the sum of the masses of the most abundant naturally occurring isotopes of the various atoms that make up the molecule (with a correction for the masses of the electron(s) lost or gained).

Neutral loss scan: In mass spectrometry, a scan which determines, in a single experiment, all the parent ion mass-to-charge ratios which react to the loss or gain of a selected neutral mass.

NIST: National Institute for Standards and Technology is the federal technology agency that works with industry to develop and apply technology, measurements and standards

Noise: The always-present random fluctuations occurring in a signal that are inherent in the combination of instrument and method.

Oven: In gas chromatography, the heated enclosure that is designed to be of a low thermal mass to permit more rapid programmed temperature changes, and to provide good oven cooling for work on the columns between analyses. It is where the column can be found.

Partition ratio: The ratio of the concentration of a substance in a single definite form in the extract to its concentration in the same form in the other phase at equilibrium (e.g., for an aqueous form to a gaseous one).

Pattern recognition: The identification of objects, images or data by their shapes, forms, outlines or other attributes, sometimes by automatic means.

Peak-tailing: Can be a presentation of co-elution, defined supra. It occurs when the B symmetrical hemisphere is greater than the A symmetrical hemisphere in a chromatographic peak.

Precise: A measure of how closely the results can be to one another. It is characterized by a low standard deviation, but may or may not have a high average deviation from the true (actual) value.

Protonate: An ion formed by interaction of a molecule with a proton abstracted from an ion, as often occurs in chemical ionization according to the reaction. The symbolism [M + H]+ may also be used to represent the protonated molecule. The widely used term “protonated molecular ion” to describe the MH+ ion is not recommended, since it suggests an associated product of a proton with a molecular ion.

Quadrapole or quadrupole: A type of mass analyzer, infra. An arrangement in which ions with a desired quotient mass/charge are made to describe a stable path under the effect of a static and a high-frequency electric quadrupole field, and are then detected. Ions with a different mass/charge are separated from the detected ions because of their unstable paths and a “pumped out” b the vacuum.

Qualitative measure: Seeks to answer the question, “What is it?” The goal of such measuring is to reporting one unique molecule to the exclusion of all other molecules or possibilities.

Quality assurance: The guarantee that the quality of a product (analytical data set, etc.) is actually what is claimed on the basis of the quality control applied in creating that product. Quality assurance is not synonymous with quality control. Quality assurance is meant to protect against failures of quality control.

Quality control: Procedures which give insight into the precision and accuracy of analysis results. The maintenance and statement of the quality of a product (data set, etc.) specifically that it meets or exceeds some minimum standard based on known, testable criteria.

Quantitative measure: Seeks to answer the question, “How much?” The goal of such measurement is to report one unique amount of the molecule selected to the exclusion of all other molecules or possibilities.

Random error: Varies when a measurement is repeated under the same conditions.

Random sampling error: The failure of sample selection wherein the selection method does not accurately reflect the population. It can lead people to draw incorrect conclusions in research.

Raw data: The data recorded in real time as the analysis occurs that is saved by the integrated computer system. The GC itself does not record the data, but the PC based and linked system does. As the data comes off the instrument itself, the integrated machine captures it into a series of files. The raw data is the pre-interpretation and pre-software manipulation/editing information of the separation and measurement event.

Relative probability: In mass spectrometry, the probability value for a hit is derived assuming that the compound is represented by a spectrum in the libraries searched. It only employs the difference between adjacent hits in the hit list to get the relative probability that any hit in the hit list is correct.

Relative retention time: The ratio of the adjusted or net retention volume (time) or retention factor of a component relative to that of a standard, obtained under identical conditions. It is calculated by dividing the retention time of the peak of interest by the retention time of the main peak or selected peak. Any peak with an RRT <1 elutes before the selected or main peak, and any peak with an RRT >1 elutes after the main or selected peak.

Reliable: Inversely related to random error, it is the consistency of a set of measurements or of a measuring instrument.

Repeatable: Day-in and day-out the test using the same method on the same instrumentation on the same unknown arrives at the same result.

Replicate measurement: Multiple measurements made upon the same sample. Replicate measurements allow statistical treatment of data—this allows, for example, the spread of data and relative standard deviations to be calculated. Replicate measurements also allow some sources of error (especially indeterminate) to be identified.

Reproducible: The ability of a test or experiment to be accurately reproduced, or replicated, by someone else working independently.

Resolution: A characteristic of the separation of two adjacent peaks.

Retention time:
The absolute time taken by a compound to travel from the injector through the column to the detector. This is very dependent on operational conditions, so it is useful only to compare retention times for various components on one instrument for experiments performed with the same conditions. It is not meaningful to compare retention times for different instruments or the same instrument under different conditions.

Salting-out: This can occur if “too much” salt—for example, NaF—is present in the sample. “Too much” NaF means too high a ratio of blood to the inorganic salt (usually in excess of 10%- 20% of total volume). It is not as a measure of an absolute, but the proportion. However, the most comprehensive answer is that when inorganic salts are added (e.g., NaF, sodium chloride) there is a “salting out effect.” By adding these salts to an aqueous or liquid matrix, some analytes such as ETOH can be forced to enter the gaseous phase above that which would happen if no salts were present. If there is not the same amount of inorganic salt added consistently throughout the verifiers, the standards, the calibrators, the n-propanol blanks and the unknowns, then there is a change in the conditions. Doing so results in the change of the validated method. In isothermal static headspace gas chromatography, Henry’s law is violated. Quantification is in jeopardy. As the standards, the calibrators, the n-propanol blanks and the verifiers are made as to one volume and proportion of salt, they have one K value. N-propanol has a different polarity from ETOH, so they have different K values at different temperatures.

Robust: The method and instrument combine to have a low percent false positive and low percent false negative throughout all designed analytical conditions.

Sample: A specimen or small quantity of something.

Sample preparation: An essential stage in the analysis process. It takes place between sample taking and analytical measuring of the prepared sample.

Sample selection: Seeks to answer the question, “How large should a sample be from any given population?” or “What part of the sample should be taken for sampling?”

Sampling:
The physical act of taking a sample.

Scalars: Measurements that have a magnitude but have no direction.

Selected ion monitoring: This term is used to describe the operation of the mass spectrometer in which the intensities of several specific ion beams are recorded rather than the entire mass spectrum.

Selective: Selectivity refers to the extent to which the method can be used to determine particular analytes in mixtures or matrices without interferences from other components of similar behavior. Selectivity gives an indication of how strongly the result is affected by other components in the sample. A selective test may not be a specific test due to cross-reactivity, interference or codetermination.

Sensitive: The quantitative measure is within the demonstrated linear dynamic range in that the unknown amount exceeds both the limit of detection and the limit of quantification but does not exceed the limit of linearity.

Separation matrix: A mix of standards in one sample that is used to prove that the analytical device can indeed separate with sufficient resolution like analytes.

Septum: Septa maintain the leak-free seal and exclude air from the inlet. They are also used in headspace vials to allow for piercing so that a sample may be extracted but not to allow any leak. They come in many different sizes and are made from many different types of material specific to analysis needs. It is responsible, in part, for maintaining the sample flow path from the outside world. It must provide a barrier that is readily penetrated by the injector needle while maintaining internal pressure without contaminating the analysis.

Sigma: Standard deviation.

Similarity search: A method of search algorithms preformed by the software typically used to perform pattern recognition of a spectral library. An identity search is designed to find exact matches of the compound that produced the submitted spectrum and therefore presumes that the unknown compound is represented in the reference library. Only experimental variability prevents a perfect match. The “similarity” search is optimized to find similar compounds and is intended for use when a compound cannot be identified by the “identity” search.

Specific:
A test that reacts only with the analyte of interest and nothing else so that when there is a measure it is unique to that which is examined. It is not simply selective, which is an exhibition of a degree of preference for the substance tested for, but not necessarily unique.

Standard deviation: A statistic that tells us how tightly all the various examples are clustered around the mean in a set of data. When the examples are pretty tightly bunched together and the bell-shaped curve is steep, the standard deviation is small. When the examples are spread apart and the bell curve is relatively flat, that tells us that we have a relatively large standard deviation. Technically, it is defined as the positive square root of the sum of the squares of the deviations between the observations and the mean of the series, divided by one less than the total number in the series. The standard deviation is the positive square root of the variance, a more fundamental statistical quantity.

Static: Pertaining to or characterized by a fixed or stationary condition.

Stationary phase:
One of the two phases forming a chromatographic system (the other is the mobile phase). It may be a solid, a gel or a liquid. If a liquid, it may be distributed on a solid. This solid may or may not contribute to the separation process. The liquid may also be chemically bonded to the solid (bonded phase) or immobilized onto it (immobilized phase). The expression chromatographic bed or sorbent may be used as a general term to denote any of the different forms in which the stationary phase is used.

Systemic error: Remains fixed when the measurement is repeated under the same conditions.

Total ion current chromatogram: Represents the summed intensity across the entire range of masses being detected at every point in the analysis. The range is typically several hundred mass-to-charge units or more. In complex samples, the TIC chromatogram often provides limited information as multiple analytes elute simultaneously, obscuring individual species.

Traceable: The completeness of the information about every step in a process chain.

True blank: A sample that is prepared free of any detectable analytes. It can be used to check for analytical carry-over.

True: The value that characterizes a quantity perfectly in the conditions that exist when that quantity is considered. It is an ideal value, which could be arrived at only if all causes of uncertainty (error) are eliminated.

Type A error: A method of evaluation by statistical analysis of a series of observations.

Type B error: Anything that is not Type A error; a method of evaluation by any means other than statistical analysis of a series of observations.

Type I error: Also known as an “error of the first kind,” an a error, or a “false positive”; the error of rejecting a null hypothesis when it is actually true. Plainly speaking, it occurs when we are observing a difference when in truth there is none, thus indicating a test of poor specificity. An example of this would be if a test shows that a woman is pregnant, when in reality she is not. Type I error can be viewed as the error of excessive credulity.

Type II error: Also known as an “error of the second kind,” a b error, or a “false negative”; the error of failing to reject a null hypothesis when it is in fact not true. In other words, this is the error of failing to observe a difference when in truth there is one, thus indicating a test of poor sensitivity. An example of this would be if a test shows that a woman is not pregnant, when in reality she is. Type II error can be viewed as the error of excessive skepticism.

Uncertainty measurement (UM): A metrological principle that is a combination of both qualitative and quantitative uncertainty in the measurement itself.

Valid: Documented proof that the process undertaken is suitable for its intended use and achieves the intended reported result correctly and uniquely.

Vectors: A measurement that has a magnitude (an amount) and a direction.

Verifiable: The ability to take all information provided and arrive at the same conclusions as the original analyst without the need to independently test.

Verifier: The known amount of analyte of interest that is placed within the batch of the run to insure that the analytical instrument is detecting the known within an established tolerance. It is called a calibrator if it is placed before the unknowns are tested and a verifier if placed after the unknowns are tested.

Volatility: A measure of the tendency of a substance to vaporize. It has also been defined as a measure of how readily a substance vaporizes.

Weighted mean: Similar to a mean, supra, where instead of each of the data points contributing equally to the final average, some data points contribute more than others. This can occur when outliers are eliminated without reason. Unless the weights can be assigned objectively, the use of the weighted mean is not normally recommended.
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