The Photoionization Detector PID

Compound class selective (for organic compounds with more easily ionizable 7r-electrons, especially aromatic compounds) nondestructive mass-flow detector slightly more sensitive than FID for many compounds it detects, but with less dynamic range than the FID. 

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In this manner, a nearly universal and very nonselective detector is created that is a compromise between widespread response and high selectivity. For example, the photoionization detector (PID) can detect part-per-billion levels of benzene but cannot detect methane. Conversely, the flame ionization detector (FID) can detect part-per-billion levels of methane but does not detect chlorinated compounds like CCl very effectively. By combining the filament and electrochemical sensor, all of these chemicals can be detected but only at part-per-million levels and above. Because most chemical vapors have toxic exposure limits above 1 ppm (a few such as hydrazines have limits below 1 ppm), this sensitivity is adequate for the initial applications. Several cases of electrochemical sensors being used at the sub-part-per-million level have been reported (3, 16). The filament and electrochemical sensor form the basic gas sensor required for detecting a wide variety of chemicals in air, but with little or no selectivity. The next step is to use an array of such sensors in a variety of ways (modes) to obtain the information required to perform the qualitative analysis of an unknown airborne chemical. 

When a compound absorbs the energy of a photon of light it becomes ionized and gives up an electron. This is the basis for the photoionization detector (PID). The capillary column effluent passes into a chamber containing an ultraviolet (UV) lamp and a pair of electrodes. As the UV lamp ionizes the compound, the ionization current is measured. 

Detection systems for GC are chosen for their sensitivity and selectivity for a particular class of VOCs. Detectors for GC include FID, the BCD, the photoionization detector (PID), the pulsed discharge detector (PDD), and the reduction gas detector (RGD). A variety of mass spectrometers can also be interfaced with a GC for confirmation of molecular structure and quantitation. Singlewavelength ultraviolet-visible detectors (190 to 600 nm) and diode array detectors are used to detect carbonyls as their 2,4-dinitrophenylhydrazone derivatives. The absorption maxima for aliphatic carbonyls, aromatic carbonyls, and dicarbonyls are near 360 nm, 385 to 390 nm, and 415 to 430 nm, respectively. Formic, acetic, and pyruvic acid are detected by ion conductivity. 

For these classes of substances, other methods have been developed, such as the coupling of ESI with an electrochemical cell [20-31], the coordination ion-spray [31-46], or the dissociative electron-capture ionization [37-41]. The APPI or the dopant-assisted (DA) APPI presented by Syage et al. [42, 43] and Robb et al. [44,45], respectively, are relatively new methods for photoionization (PI) of nonpolar substances by means of vacuum ultraviolet (VUV) radiation. Both techniques are based on photoionization, which is also used in ion mobility mass spectrometry [46-49] and in the photoionization detector (PID) [50- 52]. 

Two of the more recently developed detectors, namely, the Hall electrolytic conductivity detector (ELCD) and the photoionization detector (PID) are recommended by the EPA for the analysis of volatile and semivolatile halogenated organic compounds and low molar mass aromatics. Chemical emission based detectors, such as the thermal energy analyzer (TEA) for its determination of... 

The photoionization detector (PID) is a nondestructive and concentration-sensitive detector, which is universal/selective for organic compounds, depending on their ionization potential. In the PID, molecular ions are formed using high-energy photons from a sealed light source. The molecular ions move toward a cathode, where they are neutralized. The current needed for neutralization is a measure of the amount (concentration) of a compound. The detection limit is about lOpgCs and the linearity is 10 . Main area of use is measurement of trace levels of aromatic compounds of environmental or health concerns. It can be rather easily incorporated in portable GCs because of no flame and is also rather simple and robust. 

The photoionization detector (PID) nses UV radiation as a means of ionizing the analyte exiting the chromatographic colnmn. It was introduced by Lovelock... 

Until the last few years (as of 2003), the most popular detector used in the gas chromatographic detection of ignitable liquid residues in fire debris has been the FID. It offers adequate sensitivity and because of the complex chromatogram that is generated with FID (pattern recognition), it has been used by most laboratories for class identification of ignitable liquid products. The photoionization detector (PID) and the thermionic ionization detector (TID) have both found applications in the analysis of ignitable liquid residues however, both are seldom used in this application. 

Methods Samples were collected for the period of three consecutive days for indoors and outdoors, at each microenvironment. Sampling duration was for 8 h from 10 00 am to 6 00 pm during all the seasons. TVOC levels were measured using a portable data logging Ion Science PhoCheck+ photoionization detector (PID) equipped with 10.6eV ultra-violet lamp4. The sampling instrument was placed... 

The analytes separated on GC column are determined by a halogen-specific detector, such as an electrolytic conductivity detector (ELCD) or a microcoulo-metric detector. An ECD, FID, quadrupole mass selective detector, or ion trap detector (ITD) may also be used. A photoionization detector (PID) may also be used to determine unsaturated halogenated hydrocarbons such as chlorobenzene or trichloroethylene. Among the detectors, ELCD, PID, and ECD give a lower level of detection than FID or MS. The detector operating conditions for ELCD are listed below ... 

Before a well is sampled, as a health and safety measure, we may record organic vapor contents in the wellhead with a field photoionization detector (PID). Chemical odors emanating from the well may overpower the sampler and warrant the use of appropriate PPE. 

Volatile aromatic and chlorinated compounds are usually analyzed with the photoionization detector/electrolytic conductivity detector (PID/ELCD) combination in EPA Method 8021. In this method, the PID detects aromatic compounds, typically the volatile constituents of petroleum fuels (BTEX) and oxygenated additives, and the ELCD detects chlorinated solvents. Both detectors are considered to be selective for the target analytes of EPA Method 8021. But are they sufficiently selective for making unambiguous decisions on the presence and the concentrations of these analytes ... 

Flame ionization detectors (FIDs) and photoionization detectors (PIDs) can be used for the detection of hydrocarbons. Both detectors have been utilized for combustibles monitoring in portable and fixed installation designs. The FID actually burns the sample in an H2 flame. A charged electrical field is positioned across the flame, and utilizing the ions in the flame can conduct a current. When most combustible materials are introduced into the flame, they produce ions in their combustion products, and these are detected by the increased flow of current across the electric field (flame). 

Another example12 is the separation of atmospheric hydrocarbons on an FID and a photoionization detector (PID), which is more sensitive for unsaturated hydrocarbons (Figure 6.7). The identities of the peaks are given in Table 1 along with the PID/FID response ratios. It can be seen that the ratios can be used as additional information in assigning peak identities. In fact, Figure 6.8 shows that the hydrocarbon type (saturates, olefins, or aromatics) can be assigned from the detector ratio in many cases. 

There are a number of other GC detectors commercially available. Photoionization detectors (PID) are primarily used for the selective, low-level detection of the compounds which have double or triple bonds or an aromatic moiety in their structures. Electrolytic conductivity... 

The methods most commonly used to detect the major hydrocarbon components of gasoline in environmental samples include GC/FID, GC/MS, HRGC/FID, HRGC/MS, and HRGC/photoionization detector (PID)/FID. GC combined with photoionization-ion mobility spectrometry (PI-IMS) has been used and is selective to aromatic hydrocarbons. See Table 6-2 for a summary of the analytical methods used to determine gasoline in environmental samples. Several of the gasoline components have been discussed in detail in their individual toxicological profiles (e.g., benzene, toluene, xylene, cyclohexane, ethane, ethylene, and lead), which should be constituted for more information on analytical methods. 

The photoionization detector can be used to detect essentially the same types of compounds as an FID detector as well as many inorganic compounds but at levels 10-50 times lower. The PID can detect S-containing compounds about 20 times lower than an FPD. Its LDR is 10 ... 

Residual solvent in rotogravure-printed brochures has given rise to complaints from postal workers (Jensen et al., 1996b). The emission decay [SER (t)] of toluene was measured, e.g. using a photoionization detector (PID) in combination with a FLEC positioned on the front page of the brochures. These measurements correlated well with personal exposure measurements in a 41-m test chamber during simulated mail handling. 

At the temperatures and pressures generally used in gas chromatography the common carrier gases employed behave as perfect insulators. In the absence of conduction by the gas molecules themselves, the increased conductivity due to the presence of very few charged species is easily measured, providing the low sample detection limits characteristic of ionization based detectors [259]. Examples of ionization detectors in current use include the flame ionization detector (FID), thermionic ionization detector (TID), photoionization detector (PID), the electron-capture detector (ECD), and the helium ionization detector (HID). Each detector employs a different method of ion production, but in all cases the quantitative basis of detector operation corresponds to the fluctuations of an ion current in the presence of organic vapors. 

While much of the surveyed research exhibits promising vapor-phase sensing performance, many of the technologies remain experimental and bound to a laboratory setting. Most of the commercial gas sensors available today utilize older, more mature technologies such as electrochemical cells, catalytic beads, photoionization detectors (PID), SAW, metal oxide semiconductors (MOS), and QCM. The dearth of viable organic solid-state vapor-phase chemosensors indicates that there is much work still to be done (in terms of material stability, selectivity, etc.) before commercialization becomes commonplace for organic sensors. 

Several measurement techniques exist for the determination of short-term concentrations. Depending on the exposure situation, there is a choice of easily usable, directly indicating sampHng devices (see Section 6.9.3.1), e.g., substance-unspecific analytical instruments Hke photoionization detectors (PID), flame ionization detectors (FID), infrared analyzers (see Section G.9.3.4), and specific sampling techniques based on adsorption on a substrate and thermodesorption (Section 6.9.4). 

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