Detector performance

The requirements of a detector for GC are exacting, and include adequate sensitivity to monitor the eluted sample components present in very low concentrations in the major eluting component, the carrier gas, and a rapid response to the changing concentration of the minor components. There are four criteria to consider  

sensitivity, defined as the smallest concentration of analyte in the detector that produces a signal of at least twice the noise level of the background signal  

response, the magnitude of the signal produced for a unit concentration of analyte in the carrier gas response can also be defined in terms of the amount of an analyte measured as peak area  

response time, that defines the detector s ability to produce a signal which accurately follows the change in analyte concentration, that is, how rapidly the detector can respond to a change in analyte concentration and 

linear range, that defines the range of concentration of analyte over which the detector responds with the same sensitivity. 

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The ultrasonic instrument will be set up according to the test specification in the common way. Connection of the instrument to the ISONIC extends the flaw detector performance instrument to a reliable ultrasonic testing system which provides full documentation of the scan. 

One of the functions of an LC-MS interface is to remove the mobile phase and this results in buffer molecules being deposited in the interface and/or the source of the mass spectrometer with a consequent reduction in detector performance. Methods involving the use of volatile buffers, such as ammonium acetate, are therefore preferred. 

Careful energy cahbration on each detector was done to achieve optimal detection rate. Each SH was temperature cycled (153-293 K). During cycling energy spectra were measured. As a result of the analysis of these spectra, optimal firmware parameters were calculated for each detector and each temperature window. During operation instrument firmware automatically adjusts those parameters depending on temperature and ensures best detector performance. 

With improving detector performance, the smaller can be the sample size and, consequently, the more rapid the sample pretreatment. However, as shown repeatedly in quantitative analysis, small sample sizes (several mg) face homogeneity problems and set a... 

Figure 4. The Bio-Detector performs 8 immunoassays simultaneously and is currently marketed by Smiths Detection (Edgewood, MD).

Radioactive contaminations of individual construction materials, as well as the laboratory environment, were measured and the impact on detector performance was determined by Monte Carlo computations [83], The background sources which were considered are ... 

Another reason for using small electrode dimensions, is to prevent bandbroadening. An expression for the detector performance was derived by Sternberg ... 

Operation of detectors with their associated alarm panels should be checked and calibrated after installation. Detector performance can be impaired in a hostile environment by blockages to the detector (i.e., ice, salt crystals, wind blown particles, water or even fire fighting foam, or by inhibition of the catalysts by airborne contaminants such as compounds of silicon, phosphorus, chlorine or lead. It is essential that detectors and alarm panels be checked and re-calibrated on a routine basis. 

Fortunately, the detector performance is in the same order for commercially available instruments. Still a re-validation of the DL is recommended for methods that investigate trace impurities. A significant change of precision data by a different S/N ratio would be found during the repeatability test. 

In order to eliminate some of these variables in comparing detector performance characteristics, it is usual to use a parameter described as specific detectivity (D ) and defined as ... 

Make sure that the flow cell is clean and free of gas bubbles when performing the detector performance tests. Dirty flow cell and gas bubbles are the main reasons for poor results for detector noise and drift. 

Gas flow through a narrow column may be too low for best detector performance. Therefore, the optimum gas for separation is used in the column and the best gas for detection (called makeup gas) is added between the column and the detector. 

Detectors are used in many different ways. Table 5.1 gives some of the qualitative and quantitative functions of detectors with the corresponding requirements of each. No detector performs all these functions. A particular detector may be used mostly for one purpose during one analysis, and for others during a later analysis. 

There are some detectors (thermal conductivity, for example) which respond to the concentration in the detector, rather than the mass flowrate. Consider such a detector used with a quarter-inch column, where the typical carrier flowrate, Fc, is 1 cm3/sec. Suppose the MDL is measured to be 16 ng/sec. The MDL can be made a factor of two better (8 ng/sec) by going to an eighth inch column (where the flow is typically 0.5 cm3/sec). This is because, when the same mass flowrate of compound is going through the detector as before, there is less carrier gas diluting the sample, so the detector performs with high sensitivity. 

The purpose of the carrier is to transport the sample through the column to the detector. The selection of the proper carrier gas is very important because it affects both column and detector performance. Unfortunately, the carrier gas that gives the optimum column performance is not always ideal for the particular detector. The detector that is employed usually dictates the carrier to be used. For instance, an electron capture detector operating in the pulsed mode requires an argon-methane mixture a thermal conductivity detector works best with hydrogen or helium. The most common carrier gases are listed in Table 6.1. 

Liquid chromatography with electrochemical detection (LCEC) is in widespread use for the trace determination of easily oxidizable and reducible organic compounds. Detection limits at the 0.1-pmol level have been achieved for a number of oxidizable compounds. Due to problems with dissolved oxygen and electrode stability, the practical limit of detection for easily reducible substances is currently about 10-fold less favorable. As with all detectors, such statements of the minimum detectable quantity must be considered only with the proverbial grain of salt. Detector performance varies widely with the analyte and the chromatographic conditions. For example, the use of 100- m-diameter flow systems can bring attomole detection limits within reach, but today this is not a practical reality. 

There are a variety of FPA detectors available that are sensitive in the NIR spectral region. The optimal choice of detectors depends on several factors desired wavelength range, whether the application will be laboratory based or part of a process environment, the sensitivity needed to adequately differentiate sample spectra and price. The figure of merit most often used to describe detector performance is specific detectivity or D, which is the inverse of noise equivalent power (NEP), normalized for detector area and unit bandwidth. NEP is defined as the radiant power that produces a signal-to-dark-current noise ratio of unity. 

The following table gives the properties of common gas chromatographic carrier gases. These properties are those used most often in designing separation and optimizing detector performance. The density values are determined at CPC and 0.101 MPa (760 torr).1 The thermal conductivity values, X, are determined at 48.9°C (120°F).1 The viscosity values are determined at the temperatures listed and at 0.101 MPa (760 torr).1 The heat capacity (constant pressure) values are determined at 15°C and 0.101 MPa (750 torr).2... 

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