Sample injection spectroscopy

Atomic absorption spectroscopy is an alternative to the colorimetric method. Arsine is stiU generated but is purged into a heated open-end tube furnace or an argon—hydrogen flame for atomi2ation of the arsenic and measurement. Arsenic can also be measured by direct sample injection into the graphite furnace. The detection limit with the air—acetylene flame is too high to be useful for most water analysis. 

In flame spectroscopy, the residence time of analyte in the optical path is < 1 s as it rises through the flame. A graphite furnace confines the atomized sample in the optical path for several seconds, thereby affording higher sensitivity. Whereas 1—2 mL is the minimum volume of solution necessary for flame analysis, as little as 1 pL is adequate for a furnace. Precision is rarely better than 5-10% with manual sample injection, but automated injection improves reproducibility to —1%. 

FIGURE 10.9. Schematic drawing of the apparatus for the centrifugal liquid membrane resonance Raman microprobe spectroscopy (a) and the centrifugal liquid membrane (CLM) cell with a sample injection hole at the bottom (b). 

Fig. 1. Schematic representation of vacuum furnace closed-cycle helium refrigeration system used for metal vapor microsolution optical spectroscopy, as well as conventional metal vapor-matrix isolation experiments. (A) NaCl or Suprasil optical window, horizontal configuration (B) stainless steel vacuum shroud (C) NaCl or Suprasil optical viewing ports (D) cajon-rubber septum, liquid or solution injection port (E) gas deposition ports (F) vacuum furnace quartz crystal microbalance assembly. With the optical window in a fixed horizontal configuration, liquid or solution sample injection onto the window at any desired temperature in the range 12-300 K is performed in position 1A, metal deposition is conducted in position IB, and optical spectra are recorded in position 1C see Procedure).

Online analysis Online sample processing techniques such as flow injection provide advantages such as reliability, sample economy, ease of automation, measurement standardization, high speed, optional sample dilution, and the ability to derivatize the analyte so as to suit the analyzer/detector. These procedures facilitate the online monitoring of fermentation substrate materials, respiratory gases, and biomass. The modifications to flow injection analysis for accurate discontinuous flow operation include sequential injection analysis and bead injection spectroscopy. The most recent invention in online techniques is the introduction of the Lab-on-a-Valve, which opens the way to development of a novel type of microflow analytical system monitored by UV-visible spectrophotometry using fiber optics. This system is an ideal tool for fermentation monitoring. 

Flow injection on renewable surfaces techniques (FI-RST [12,13]) are suitable for micro miniaturization, since their key component, the jet ring cell (Fig. 3), has in its present form a circular detection area of 800 microns and a depth of 50 to 1000 microns. This device, originally designed for FI microscopy of live adherent cells [13], has found its application in fluorescence based immunoassays and reflectance based UV-VIS spectroscopy. The sensing layer in the JR cell comprises several thousand 35 micron polymer beads, which serve as reactive surfaces for protein and reagent adsorption. The cell is a part of a sequential injection system, which allows, besides conventional FI operations (sample injection, carrier pumping, reagent addition) also injection of a well... 

Figure 9 Liquid chromatography-RRS analysis of azo dyes. The spectra shown are for an 80-fjl. sample injection of 80 ppm eriochrome blue SE (EBSE). Spectra are collected with a 1-sec integration time at 4-sec intervals. (Reprinted with permission from CK Chong, CK Mann, TJ Vickers. Resonance Raman spectroscopic detection system for liquid chromatography. Appl Spec-trosc 46 249-254, 1992. Copyright 1992 Society for Applied Spectroscopy.)...

As in tic, another method to vaUdate a chiral separation is to collect the individual peaks and subject them to some type of optical spectroscopy, such as, circular dichroism or optical rotary dispersion. Enantiomers have mirror image spectra (eg, the negative maxima for one enantiomer corresponds to the positive maxima for the other enantiomer). One problem with this approach is that the analytes are diluted in the mobile phase. Thus, the sample must be injected several times. The individual peaks must be collected and subsequently concentrated to obtain adequate concentrations for spectral analysis. 

A promising technique is cavity ringdown laser absorption spectroscopy (307), in which the rate of decay of laser pulses injected into an optical cavity containing the sample is measured. Absorption sensitivities of 5 x 10 have been measured on a ]ls time scale. AppHcations from the uv to the ir... 

Advantage was taken of these solubility differences in refining mixtures of the chlorinated dibenzodioxins. Digestion with boiling chloroform was effective in removing trichlorodibenzodioxin while recrystallization from anisole reduced the penta-substituted isomer content. In a typical purification (Table II) these two procedures were alternated through four cycles. The assays were made by mass spectroscopy using the batch injection method to introduce the sample into the spectrometer. X-ray studies 14) confirmed the structure. 

Systems have been developed by some of the major spectrometer manufacturers to deal specifically with this type of application. These systems are designed with automation very much a priority. Typically, an integrated robot adds a predetermined volume of solvent to each of the wells and then injects the resultant solution into a flow line that transfers it into the spectrometer s probe, which is of course fitted with a flow cell. Spectroscopy can then be performed without the time constraints of the HPLC-NMR system and the sample returned to the well on the plate where it came from, or into a fresh one if required. 

As Beer s law in absorption spectroscopy has a path length dependence, the observe volume, Vobs, or active volume of an NMR probe is an important determinant of the sensitivity of NMR measurements. The observe volume is the fraction of the total sample volume, Vtot. that returns a signal when a sample is inserted in an NMR tube or is injected into a flow system. The relationship between chromatographic peak shape, peak volume and flow rate, and sensitivity in hyphenated NMR measurements is complex and is discussed in greater detail in Section 7.2. For the purpose of this discussion, the sample is assumed to be present at a uniform concentration in a sample volume, Vtot. The probe observe factor, /o, is calculated as shown in the following equation ... 

The philosophies for automation have been described in the foregoing sections. However, to solve an analytical problem there may well be more than one approach that offers potential. The Hterature abounds with methods that have been automated by flow-injection and by continuous-flow methodologies. Also, very often a procedure which involves several stages prior to the actual measurement can be configured by combining two of the approaches. An example of this is the automated Quinizarium system described by Tucker et al. [46]. This was a continuous extraction followed by a hatch extraction which is finally completed by a batch measurement on a discrete sample for quantification and measurement. Whereas sample preparation is almost always required, there is no doubt in my mind that the best approach to this area of activity is to avoid it totally. The application of near infra-red spectroscopy is an example of this strategy. 

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