Detector response measurement

To determine the correction factors, an internal standard is generally added to each of the mixtures and the detector response measured relative to the internal standard. If the ratio of a monosaccharide to the internal standard decreases after hydrolysis of a test mixture of monosaccharides, decomposition has occurred and a recovery factor should be determined. The internal standard must either be resistant to decomposition during hydrolysis or should be added after hydrolysis it is usually better to add the internal standard after hydrolysis, to be on the safe side. The internal standard must not appear in the samples and must be resolved from other components in the sample, as with any internal standard. 

Essential features of an automated method are the specificity, ie, the assay should be free from interference by other semm or urine constituents, and the sensitivity, ie, the detector response for typical sample concentration of the species measured should be large enough compared to the noise level to ensure assay precision. Also important are the speed, ie, the reaction should occur within a convenient time interval (for fast analysis rates), and adequate range, the result for most samples should fall within the allowable range of the assay. 

Oxygen Transport. The most widely used methods for measuring oxygen transport are based upon the Ox-Tran instmment (Modem Controls, Inc.). Several models exist, but they all work on the same principle. The most common apphcation is to measure the permeabihty of a film sample. Typically, oxygen is introduced on one side of the film, and nitrogen gas sweeps the other side of the film to a coulometric detector. The detector measures the rate that oxygen comes through the film. The detector response at steady state can easily be converted to At (eq. 1). Simple... 

Water Transport. Two methods of measuring water-vapor transmission rates (WVTR) ate commonly used. The newer method uses a Permatran-W (Modem Controls, Inc.). In this method a film sample is clamped over a saturated salt solution, which generates the desired humidity. Dry air sweeps past the other side of the film and past an infrared detector, which measures the water concentration in the gas. For a caUbrated flow rate of air, the rate of water addition can be calculated from the observed concentration in the sweep gas. From the steady-state rate, the WVTR can be calculated. In principle, the diffusion coefficient could be deterrnined by the method outlined in the previous section. However, only the steady-state region of the response is serviceable. Many different salt solutions can be used to make measurements at selected humidity differences however, in practice,... 

Photometric Moisture Analysis TTis analyzer reqiiires a light source, a filter wheel rotated by a synchronous motor, a sample cell, a detector to measure the light transmitted, and associated electronics. Water has two absorption bands in the near infrared region at 1400 and 1900 nm. This analyzer can measure moisture in liquid or gaseous samples at levels from 5 ppm up to 100 percent, depending on other chemical species in the sample. Response time is less than 1 s, and samples can be run up to 300°C and 400 psig. 

Procedure. Inject 1 fiL of the sample solution and obtain a chromatogram. Under the above conditions the compounds are separated in about 3 minutes, the elution sequence being (1) aspirin (2) phenacetin (3) caffeine. Measure peak areas with an integrator and normalise the peak area for each compound (i.e. express each peak area as a percentage of the total peak area). Compare these results with the known composition of the mixture discrepancies arise because of different detector response to the same amount of each substance. 

To obtain an accurate quantitative analysis of the composition of a mixture, a knowledge of the response of the detector to each component is required. If the detector response is not the same for each component, then the areas under the peaks clearly cannot be used as a direct measure of the proportion of the components in the mixture. The experiment described illustrates the use of an internal normalisation method for the quantitative analysis of a mixture of the following three components ethyl acetate (ethanoate), octane, and ethyl n-propyl ketone (hexan-3-one). 

If the detector response differs, make up by weight a 1 1 mixture of each of the separate components (I, II, and III) with compound (IV). Inject a 0.1 pL sample of each mixture, measure the corresponding peak area, and hence deduce the factors which will correct the peak areas of components (I), (II), and (III) with respect to the internal standard (IV). 

Delta function response - Over most of the wavelengths of interest, optical and infrared detectors produce one photoelectron for every detected photon, which provides a one-to-one correspondence between detected photons and photoelectrons. This means that the detector response is exactly linear to the intensity incident on the detector - an attribute that allows astronomers to precisely remove sky background and electronic bias to accurately measure the intensity of the astronomical object. 

Secondly, the intensity of response for a certain compound from one type of detector is not necessarily the same as that obtained from the other detector. This should not be unexpected, since the two detectors are measuring quite different properties of the analyte, in this case UV absorption at a particular wavelength and how readily it is ionized and fragmented under the conditions employed. These properties are urn-elated. 

To enable qnantitative measurements to be made, the analyst requires the ability to determine the areas or heights of the detector responses of analyte(s) and any internal standard that may be present and then, from these figures, to derive the amount(s) of analyte(s) present in the unknown sample. The software provided with the mass spectrometer allows this to be done with a high degree of automation if the analyst so desires. 

The buffer concentration also directly affects the size of droplets produced - the higher the buffer concentration, then the smaller they are, and this is desirable. The buffer concentration, however, has an effect on the ionization efficiency and at high buffer concentrations (>10 M) the relationship between detector response and analyte concentration is not linear. As indicated earlier in Figure 2.6, this situation must be avoided for precise quantitative measurements. 

The FDA requests that the method exhibit sufficient sensitivity to measure accurately the residue of interest after fortification of the control matrix at half the tolerance concentration. Minimally, the detector response at the tolerance should be at least 10 times the average background response. 

Assay sensitivity is defined here as the concentration of analyte that inhibits the observed absorbance by 50% or the IC50. The lower limit of detection (LLD) is the lowest analyte concentration that elicits a detector response significantly different from the detector response in the absence of analyte. In some cases, the LLD is defined as three standard deviations from the mean of the zero analyte control. In other cases, the LLD is defined empirically by determining the lowest concentration of analyte that can be measured with a given degree of accuracy. Readers are referred to Grotjan and Keel for a simplified explanation and to Rodbard for the complete mathematics on the determination of LLD. 

Potentiometry is the measurement of the potential at an electrode or membrane electrode, so the detector response is in units of volts. The potentio-metric response tends to be slow, so potentiometry is used infrequently in analysis.47 One example is the use of a polymeric membrane impregnated with ionophores for the selective detection of potassium, sodium, ammonium, and calcium 48 In process chromatography, potentiometry may be used to monitor selected ions or pH as these values change over the course of the gradient. 

Bulk property detectors function by measuring some bulk physical property of the mobile phase, e.g., thermal conductivity or refractive index. As a bulk property is being measured, the detector responses are very susceptible to changes in the mobile phase composition or temperature these devices cannot be used for gradient elution in LC. They are also very sensitive to the operating conditions of the chromatograph (pressure, flow-rate) [31]. Detectors such as TCD, while approaching universality in detection, suffer from limited sensitivity and inability to characterise eluate species. 

OD Detectors are the classical proportional counters that are used in laboratory goniometers for decades. Because every point of reciprocal space is measured with the same cell, the detector response is uniform by definition. 

Such detectors reveal their built-in intelligence if the user interrupts a measurement. Thereafter the detector response memory may contain nonsensical data, and the following measurement may be corrupted. 

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