Detection in SFC

Detection in SFC can be achieved in the condensed phase using optical detectors similar to those used in liquid chromatography or in the gas phase using detectors similar to those used in gas chromatography. Spectroscopic detectors, such as mass spectrometry and Fourier transform infrared spectroscopy, are relatively easily interfaced to SFC compared to the problems observed with liquid mobile phases (see Chapter 9). The range of available detectors for SFC is considered one of its strengths. 

With recent emphasis shifting strongly to packed columns it is not surprising to notice a shift in detector development from GC- to LC-like detectors. Several authors [36,46,236,238,264] have discussed detection in SFC. 

Principles and Characteristics Fourier-transform infrared detection in SFC is attractive because it can offer structural information about the analytes [372]. The coupling was introduced in 1983 [373]. Various approaches have been advanced ... 

The nature of a supercritical fluid enables both gas and liquid chromatographic detectors to be used in SFC. Flame ionization (FID), nitrogen phosphorus (NPD), flame photometric (FPD) GC detectors (p. 100 etseq.) and UV and fluorescence HPLC monitors are all compatible with a supercritical fluid mobile phase and can be adapted to operate at the required pressures (up to several hundred bar). A very wide range of solute types can therefore be detected in SFC. In addition the coupled or hyphenated techniques of SFC-MS and SFC-FT-IR are attractive possibilities (cf. GC-MS and GC-IR, p. 114 el seq.). 

M. Lafosse, Evaporative light scattering detection in SFC, Chromatogr. Princ. Pract. 201-218 (1999). 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

SFC-NMR is available from 200 to 800 MHz, and is suitable for all common NMR-detected nuclei. SFC/SFE-NMR requires dedicated probe-heads for high pressure (up to 350 bar) and elevated temperature (up to 100 °C). SFC-NMR is carried out with conventional packed columns, using modifier, pressure and temperature gradients. The resolution of 1H NMR spectra obtained in SFE-NMR and SFC-NMR coupling under continuous-flow conditions approaches that of conventionally recorded NMR spectra. However, due to the supercritical measuring conditions, the 111 spin-lattice relaxation times 7) are doubled. 

In supercritical fluid chromatography (SFC) the mobile phase is a supercritical fluid, such as carbon dioxide [15]. A supercritical fluid can be created either by heating a gas above its critical temperature or compressing a liquid above its critical pressure. Generally, an SFC system typically has chromatographic equipment similar to a HPLC, but uses GC columns. Both GC and LC detectors are used, thus allowing analysis of samples that cannot be vaporized for analysis by GC, yet cannot be detected with the usual LC detectors, to be both separated and detected using SFC. SFC is also in other... 

Wall [23] has discussed recent developments including timed-split injection, extraction and detection systems in SFC. 

FIGURE 4.8 Chiral selectivity as a function of amount and type of alcohol modifiers in the mobile phase in SFC. Conditions Al-(2-heptyl)-p-tolylamide enantiomers as the analyte 250 mm X 4.6 mm ID column, 10 xm silica particles coated with celhtlose trihenxoate carhon dioxide and various types and amounts of modifiers = methanol, o = ethanol, A = 1-butanol, x = 2-propanol, and = 2-butanol 25°C flow rate 4.5 mL/min at 0°C UV detection at 229 nm average column pressure 140 bar. (Reprinted from Macaudiere, R et al. 1989. J. Chromatogr. Sci. 27 383-394. With permission.)... 

In this chapter we attempt to present an organized approach to method development in SFC, with most of our emphasis on step 6, the optimization of the separation. Although a detailed discussion of sample characterization (step 1) and system selection (steps 2-4) is beyond our present scope, we have included for the novice a brief summary of the choices available for columns, stationary phases, mobile phases, sample introduction, and detection at the beginning of this chapter. We assume the reader is familiar with the basics of SFC if not, numerous reviews and monographs are available (1-5). 

In SFC, the mobile phase is delivered by a high-pressure pump. The sample is usually injected as a solution by means of a high-pressure injection valve. The column may either be a packed column, comparable to a high-performance liquid chromatographic (HPLC) column, or an open capillary column, comparable to a capillary gas chromatographic (GC) column, but with somewhat smaller internal diameters (50-100 / m). Detection is performed either on-line, (i.e., UV-VIS) or after the expansion of the fluid [i.e., flame ionization detection (FID) or mass spectrometry... 

Many detectors have been used to detect pesticides and herbicides in SFC. Among these detectors, the flame ionization detector (FID) is most commonly used for detection of a wide range of pesticides and herbicides, with a detection limit ranging from 1 ppm (for carbonfuran) to 80 ppm (for Karmex, Harmony, Glean, and Oust herbicides). The UV detector has frequently been used for the detection of compounds with chromophores. The detection limit was as low as 10 ppt when solid-phase extraction (SPE) was on-line coupled to SFC. The mass spectrometric detector (MSD) has also been used in many applications as a universal detector. The MSD detection limit reached 10 ppb with on-line SFE (supercritical fluid extraction)-SFC. Selective detection of chlorinated pesticides and herbicides has been achieved by an electron-capture detector (ECD). The limit of detection for triazole fungicide metabolite was reported to be 35 ppb. Other detectors used for detection of pesticides and herbicides include thermoionic, infrared, photometric, and atomic emission detectors. 

A flame ionization detector and MSD were also used for detection of PCBs in SFC. Capillary columns packed with aminosilane-bonded silica and open tubular columns coated with polysiloxane were employed for PCB separation in these works. 

The most popular detector for PAHs is the UV detector. The detection limit was 0.2-2.5 ppb for 16 PAHs. A diode-array detector was also used for PAHs in SFC, and the detection limit was reported to be as low as 0.4 ppb. Other detections used for PAHs include mass spetrometric, thermoionic, infrared, photoionization, sulfur chemiluminescence, and fluorescence detectors. 

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