Temperature limit detectors

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

There are a number of limitations on the use of extremes of temperature in HPLC. Clicq et al. [91] note that instrumental issues become increasingly limiting as one goes to very high temperatures and flow rates. They suggest that most separations will occur below 90°C where there are less instrumental constraints. As detailed below, column bleed can limit the selection of columns. Highspeed separations require a faster detector response than many systems allow and constrain extra column volume. This is especially true for narrow bore columns and sub-2 jam particles. In many cases, the additional speed gained above the temperature limits of commercial HPLC ovens will not be worth the additional expense and complexity required. For macromolecules, the effect of extreme pressure can also impact retention time as noted by Szabelski et al. [92]. 

Injector temperature, °C Column temperature, C Detector temperature, °C Lower limit of quantitation (ppm)b... 

The primary requirements of a stationary phase are to provide separation of the sample with reasonable column life. Therefore, in addition to having suitable selectivity, the phase should have reasonable chemical and thermal stability. Many catalogs list upper temperature limits for stationary phases, but these should be used only as approximations because the true limit depends upon the type of detector used and the amount of column bleed one can tolerate to get the job done. Even if a phase is stable to 250°C, the column will last much longer if the temperature is limited to 200°C. Excessive temperatures result not only in shorter column life, but also in more rapid fouling of the detector. 

The capture process of the electron capture detector can be very temperature-sensitive. The sensitivity may either increase or decrease with an increase in temperature, depending on the compound involved, as illustrated in Figure 6.24 for three benzene derivatives. Since detector temperature may affect sensitivity it is sometimes possible to improve the analysis by operating at a different temperature. The radioactive source determines the maximum temperature limit for the detector which is listed in Table 6.6. Exact values vary with manufacturer. 

TABLE 6.6 TEMPERATURE LIMITS OF ELECTRON CAPTURE DETECTOR SOURCES... 

At sufficiently high temperatures, the detector also responds to a variety of nitroaro-matics and to (V-nitrosodimethylamine. NOz yields of 0.70 and 0.90 were obtained for 2,4-DNT and TNT, respectively. The detection system exhibits a linear response for all nitrogen-containing compounds studied, and the detection limit for the determination of organic nitrates was found to be 0.05pmol. The relative response factors for aromatic nitro compounds and nitrosamine for the pyrolyser/luminol detector are presented in Table 3. 

DETECTOR MINIMUM DETECTABLE QUANTITY (gsec-1) LINEAR RANGE TEMPERATURE LIMIT (°C) REMARKS... 

This detector is insensitive to temperature change, so it is a favorite when programmed column temperatures are used. The detector temperature limit is about 400 "C. 

The detector cathode consists of a metal foil impregnated with a /S-emitting element, usuaUy tritium or nickel-63. The former isotope gives greater sensitivity than the latter, but it has an upper temperature limit of 220° C because of losses of tritium at high temperatures nickel-63 can be used routinely at temperatures up to 350°C. Also, nickel is easier to clean than the tritium source these radioactive sources inevitably acquire a surface film that decreases the /3-emission intensity and hence the sensitivity. A 30% KOH solution is usually used to clean the sources. 

There are several factors to consider in selecting a stationary phase. General considerations include temperature limits of the stationary phase, column efficiency, and lifetime and detector compatibility. Since nonpolar phases generally give more efficient columns that also exhibit superior lifetimes, it is wise to use the least polar phase that provides satisfactory separation. Phases containing the specific element corresponding with element-selective detectors (e.g., cyanopropyl phases with an NPD detector trifluoro-propyl phases with an BCD detector) should be avoided where possible. These selective detectors will be substantially more sensitive to normal column bleed with such phases. 

The vapor pressure of modifier deposited in small amounts (e.g., monomolecular layer) on the adsorbent surface is essentisilly below the equilibrium value of the SLP vapor pressure for a given temperature. This circumstance accounts in turn for an increase in the upper temperature limit of the modified adsorbent compared to that of the given SLP, and also for a decrease in detector noise due to SLP vapors. This characteristic of modified adsorbents was first pointed out in [4]. Discussion of this problem can be also found in reference [28]. 

Graphite-Vespel (15% 85%) 350°C upper temperature limit general-purpose use in capillary columns recommended for MS or oxygen-sensitive detectors, also compatible with other detectors, such as FID and NPD most reliable leak-free connection proper installation requires a finger-tight turn on the nut, then an additional quarter-turn with a wrench the ferrule hole must match the column o.d. exactly to ensure a leak-free seal not reusable. 

Note 7—Detector tubes have temperature limits from 0 to 40°C (32 to 104 F), and sample gases must remain in that range throughout the test. Cooling probes are available for sample temperatures exceeding 40-C. 

---