Flow-cells mass detection

The commonly used detector-cell volumes, though much smaller than required to minimize band broadening in conventional LC, render flow-cell FTIR detection compatible with miniaturized LC systems. In microbore-LC chromatographic peak volumes are several orders of magnitude smaller and peak concentrations are higher than in conventional LC. However, the sample capacity (both in mass and volume) of LC columns decreases in proportion to their cross-sectional area and the advantage of microbore-LC therefore is only partly fulfilled. 

Pretty,J. R., Blubaugh, E. A., and Caruso, J. A. (1993). Detemination of arsenic (III) and selenium (IV) using an on-line anodic stripping voltammetry flow cell with detection by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry. Anal. Chem. 65(23), 3396. 

FTIR in multiply hyphenated systems may be either off-line (with on-line collection of peaks) [666,667] or directly on-line [668,669]. Off-line techniques may be essential for minor components in a mixture, where long analysis times are required for FT-based techniques (NMR, IR), or where careful optimisation of the response is needed. In an early study a prototype configuration comprised SEC, a triple quadrupole mass spectrometer, off-line evaporative FTIR with splitting after UV detection see Scheme 7.12c [667]. Off-line IR spectroscopy (LC Transform ) provides good-quality spectra with no interferences from the mobile phase and the potential for very high sensitivity. Advanced approaches consist of an HPLC system incorporating a UV diode array, FTIR (using an ATR flow-cell to obtain on-flow IR spectra), NMR and ToF-MS. 

Fig. 2.9. Current transient and mass signal responses during oxidation of methanol adsorbate in pure base electrolyte (flow cell procedure). Methanol was adsorbed from a 10 2 M CD3OH + 10 4 M HC104 + 0.1 M NaC104. ad = 356 mV tad = 400 s. Potential step to 975 mV vs. Pd-H for 0.5s to produce C02 (m/e = 44) and hydrogen ions, followed by a step to —574 mV vs. Pd-H to detect HD...

When a fast LC system is connected to a detector, care must be taken to ensure that the detector is well suited for the expected flow ranges and peak widths. Most manufacturers, especially those offering dedicated systems for sub-2-micron particle columns, offer efficient UV detectors. Flow rate is usually not an issue for UV and other flow-through cell-based detection systems. However, flow rate can become limiting for dead-end detectors that alter the column effluent, mainly by eliminating mobile phases such as ELSD, CAD, CLND, and mass spectrometers. 

It is appropriate at this time to discuss some of the limitations associated with LC-NMR. It is more accurate to say the limitations of the NMR spectrometer in an LC-NMR instrument. As compared to MS, NMR is an extremely insensitive technique in terms of mass sensitivity. This is the key feature that limits NMR in its ability to analyze very small quantities of material. The key limiting factor in obtaining NMR data is the amount of material that one is able to elute into an active volume of an NMR flow-probe. The quantity of material transferred from the LC to the NMR flow-cell is dependant on several features. The first being the amount of material one is able to load on an LC column and retain the resolution needed to achieve the desired separation. The second is the volume of the peak of interest. The peak volume of your analyte must be reasonably matched to the volume of the flow-cell. An example would be a separation flowing at lml/min with the peak of interest that elutes for 30 s. This corresponds to a peak volume of 500 pi, which clearly exceeds the volume of the typical flow-cell. This is the crux of the problem in LC-NMR. There is a balance that must be struck between the amount of compound needed to detect a signal in an... 

Thermal and mass flow-through sensors rely on differential measurements owing to the low selectivity of these types of detection. They use two flow-cells arranged in series (Fig. 2.9.B) or parallel (Fig. 2.9.C), each containing a sensitive microelement (a piezoelectric crystal or a thermistor). One of the cells houses the sensitive microzone, whereas the other is empty or accommodates an inert support containing no immobilized reagent (e.g. see [35]). 

Narrow-bore columns of between 1.0 and 2.5 mm ID are available for use in specially designed liquid chromatographs having an extremely low extracolumn dispersion. For a concentration-sensitive detector such as the absorbance detector, the signal is proportional to the instantaneous concentration of the analytes in the flow cell. Peaks elute from narrow-bore columns in much smaller volumes compared to those from standard-bore columns. Consequently, because of the higher analyte concentrations in the flow cell, the use of narrow-bore columns enhances detector sensitivity. The minimum detectable mass is directly proportional to the square of the column radius (107) therefore, in theory, a 2.1-mm-ID column will provide a mass sensitivity about five times greater than that of a 4.6-mm-ID column of the same length. 

In many HPLC detectors the column eluent flows through a cell within which some physicochemical interaction with the solutes takes place. Exceptions to this include mass spectrometric detectors where the eluent has to be vaporised before introduction into the vacuum system, or the evaporative mass detector, where again the eluent is heated and vaporised before undergoing analysis by light scattering. Often a number of flow cell options are offered by detector manufacturers, and this reflects the effect of the detection volume on the detected peak. The total peak variance, a, is the sum of all the variance contributions. 

Many detectors have been developed for use with liquid chromatographs (Table 6-4). Examples include photometric, spectrophotometric, fiuorometric, and electrochemical detectors. A key and integral component of such detectors is the flow cell (Figure 6-18), through which passes the eluate from the chromatographic column. Dissolved analytes are then detected and an electronic signal generated. (Mass spectrometers, which also have been used as LC detectors, are discussed in Chapter 7.)... 

---