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21 Cards in this Set
- Front
- Back
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Classification of Analytical Methods |
- Qualitative analysis (what?): measured property indicates presence of analyte in matrix.
Classical: identification by colors, boiling points, odors.. Instrumental: chromatography, electrophoresis, spectroscopy, electrode potential.. - Quantitative analysis: magnitude of measured property is proportional to concentration of analyte in matrix. Classical: mass or volume (gravimetric, volumetric) Instrumental: measuring property and determining relationship to concentration |
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Instrument Characteristics
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Converting chemical/physical properties into information.
Stimulus -> Analytical Sample -> Response Stimulus: chemical, optical, electrical Response: detectable effect of stimulus (related to quantity of analyte present) |
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Non-electrical Data Domains
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physical (light intensity, pressure)
chemical (pH) scale position (length) number (objects) |
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Electrical Domains
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- Analog domain: continuous in both magnitude and time (current, voltage, charge); susceptible to electrical noise.
- Time domain: frequency, period, pulse width frequency: the number of signals per unit time period: time required for one cycle pulse width: the time between successive LO to HI transition. - Digital |
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Digital Signals
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Digital: easy to store, not susceptible to noise
1. count serial data 2. Binary coding to represent “5” count serial data: 11111, 5 time intervals binary: 101, 3 time intervals, 1x20 + 0x21+1x22 = 5 With 10 time intervals: In count serial data, we can only record numbers 0-10. In binary encoding, we can count up to 210-1 = 1023 by different combinations of Hi or LO in each of 10 time interval. 1023/10 >100 times. 3. Serial vs. parallel signal To use multiple transmission channels instead of a single transmission line to represent three binary digits. Have all the information simultaneously. |
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Detector
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Device that indicates a change in one variable in its environment (eg., pressure, temp, particles).
Can be mechanical, electrical, or chemical. |
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Sensor
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Analytical device capable of monitoring specific chemical species continuously and reversibly.
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Transducer
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Devices that convert information in nonelectrical domains to electrical domains and the converse.
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Signal
Background or Baseline Drift |
Signal: Derived from the output of the difference detector.
Background or Baseline: non-zero output even when there is no difference at the inputs. Drift: background varies slowly with time The analytical signal is the difference between the output amplitude and the expected baseline at the same moment in time. |
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Noise
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Unwanted periodic, random, or almost random time-dependent changes in the output signal.
Measured in the same unit as a signal. Two common measures of noise: Peak-to-Peak Root-Mean-Square (RMS) |
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Signal-to-Noise Ratio
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Measure the difference between the output and
background. Blurred by the presence of noise. Measurability of quantity must account for both signal level and noise level. |
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Performance Characteristics
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Describes a general property of an analytical technique that permits comparisons so that a user can evaluate its applicability in a given situation.
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Performance Characteristics (How questions)
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• How reproducible? – Precision
• How close to true value? – Accuracy • How small a difference can be detected? Sensitivity • What application range? – Dynamic Range • How much interference? – Selectivity |
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Precision
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Mutual agreement of replicate measurements.
Variation arises from random errors. • Standard Deviation and Variance are most common measures of precision. • Repeatability: agreement between replicate measures taken by same analyst on same instrument on the same day. (How good is the analyst?) • Reproducibility: agreement between replicate measures taken by various analysts and various instruments over a long time. (How robust is the technique?) |
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Accuracy
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A measure of how close the measured response is to the true value of the quantity.
- Instrumental: something is wrong with the instrument (batteries low, temperature effects, etc.) - Analyst: judgment errors, reading meter from wrong angle, lack of careful technique. - Method: the method itself is inherently inaccurate, non-ideal chemical behaviour, slow reactions, contaminants, instability of reagents. |
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Sensitivity
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Technique´s ability to detect changes in the signal
property. - Slope of response curve. - Precision of Measurement |
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Dynamic range (DNR)
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The range of concentration between the limit of quantitation and the linearity limit; the range over which the technique is useful.
Worthwhile technique must have dynamic range of at least two orders of magnitude. Some techniques have 5 or six orders of magnitude. |
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Selectivity
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• Each analysis looks for a signal that comes from a
specific analyte. • However, signal always has a contribution from everything present in the sample. • Need to minimize contributions from other species or know their contribution from their selectivity coefficient. |
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Detection Limit (LOD)
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• The smallest amount of analyte that can be reliably detected.
• Depends upon signal-to-noise ratio. • Analysis signal must be larger than the blank signal. How much larger? |
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Quantitation Limit (LOQ)
(How low can you go) |
• The lowest concentration of an analyte that can be
determined with acceptable precision (repeatability) and accuracy under the stated conditions of the test. • Quantitation requires a larger signal-to-noise level. Answers question “How much of the analyte is present?” |
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Linearity Limit
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• At the other end, as analyte concentration increases, every detector finally stops responding linearly. (Amplifier cannot produce a larger output, the balance arm bends or breaks, etc.).
• Point of saturation for an instrument detector so that higher amounts of analyte do not produce a linear response in signal. |
