Detectors suppressed conductivity

Figure 10 Separation of monochloroacetate, dichloroacetate, and trichloroacetate on a sulfonated poly(styrene-divinyl benzene) column with suppressed conductivity detection. Column 2% cross-linked sulfonated poly(styrene-divinyl benzene) capacity 0.02 meq/g. Flow rate 64 ml/hr. Eluant 15 mM sodium phenate. Suppressor 0.28 x 25 cm Dowex 50W X8 column (200-400 mesh). Detector Chromatronix conductivity cell connected to a Dow conductivity meter. (Reprinted with permission from Small, H., Stevens, T. S., and Bauman, W. C., Anal. Chem., 47,1801,1975. 1975 Analytical Chemistry.)...

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

Although AS and AES can be detected at a low UV wavelength, sensitivity is lacking and a more suitable detection was achieved using indirect photometric detection, post-column colour formation reactions, or a pre-column derivatisation, suppressed conductivity detection and refractive index detection [1,42,43]. A comparison of detection limits for the determination of these anionic surfactants shows that photometric and conductivity detectors are better (picomole or nanogram range) than refractive index or fluorometry detectors by about a factor of 1000 [40],... 

Ion chromatography is a convenient method for simultaneous determination of alkali, alkaline earth and ammonium ion concentrations in solution. The identity of the analytes is set by the retention times and quantation is carried out by a detector. Conductivity is frequently used, after chemical suppression of the eluate, by which chloride counterions were exchanged by hydroxide. UVV spectrophotometric measurement of hydroxide ions at 200 nm was proposed as an alternative method to suppressed conductivity. Both methods... 

Avicel RC and CL are water-dispersible, colloidal, microcrystalline cellulose products made for use in liquid preparations. Avicel RC and CL are coprocessed mixtures of microcrystalline cellulose and sodium carboxymethylcellulose. The amount of NaCMC can be determined using the IC method. About 10 g of Nasonex and about 25 mg of NaCMC NF are separately refluxed with 30 mL of glacial acetic acid for 2 h. The refluxed mixture is transferred to a 100-mL volumetric flask and diluted to volume with purified water, and mixed. The ion chromatograph (IC) was equipped with a suppressed conductivity detector, a 4-mm CSRS suppressor, current at 50 mA, a 250 mm x 4-mm Ion CS 12A column and a 50 mm x 4 mm Ion CG 12A guard column. The mobile phase is 0.13% methanesulfonic acid in water with a flow rate of 1 mL/min. Equal... 

Figure 2.10 Separation of anions in a carbonated apple juice using suppressed ion chromatography. Chromatography conditions column, AS11 with AG11 guard detector, CD20 conductivity detector with the ASRS self-regenerating suppressor in the recycle mode. (Courtesy of Dionex Corporation.)...

Because the sensitivity of the detector decreases with decreasing analyte ionization, the pH of the mobile phase should be chosen to maximize solute dissociation. For example, anions with pKa values above 7 are not detectable by conductivity detection. However, conductivity detection is often the preferred method for organic acids with carboxylate, sulfonate, or phospho-nate functional groups, since the pKa values are below 5. For cations, most aliphatic amines have pKa values around 10 and are readily detected by conductivity detection. The pKa values of aromatic amines, however, are in the range 2 to 7, which is too low to be detected by suppressed conductivity. Sensitivity by nonsuppressed conductivity is also poor, so these amines are monitored by UV absorption or pulsed amperometric detection. 

Ultraviolet absorbance detection is the most prevalent type of detection in CE, and UV detectors operate in both the direct and indirect modes. Laser-induced fluorescence detection is often used for high-sensitivity work. Conductivity detection, suppressed conductivity detection, and mass spec-... 

Detection limits depend on several factors in addition to the conductivity system used. These include the sample volume, the goodness of temperature control and the inherent sensitivity of the conductivity detector used. With a direct injection of a 50 iL sample, the detection limits for chloride, nitrate and sulfate in drinking water have been estimated to be around 10 ppb using suppressed conductivity [15], With the same volume of sample the detection limits using non-suppressed conductivity are probably around 100 to 200 ppb. However, considerably lower limits of detection are possible with non-suppressed conductivity of a carboxylic acid eluent is used. Table 6.12 gives detection limits for a 100 pL water sample. 

With modem columns and dilute solutions of a strong acid as the eluent, cations may be separated and detected with excellent sensitivity by direct conductivity as well as by suppressed conductivity [2]. The basis for direct conductivity detection is that the highly conductive (equivalent conductance = 350 S cm equi r ) in the eluent is partially replaced by a cation of lower conductance when a sample zone passes through the detector. For example, the equivalent conductance of Li+, Na+, and K+ is 39,50 and 74 S cm e-quiv", respectively. The decrease in conductance on an equivalent basis can be calculated as follows Background + NO3" = 350 + 71 = 421 (S cm equitr ). Sample peaks Li+ +... 

Conductivity detection is the most popular for ion chromatography. Although UV detection is often overlooked, it can be quite powerful. Amperometric detection, for example, offers selectivity and sensitivity, in many cases unsurpassed. The optimum eluent separation pH may not be the optimum pH for detection. An anion may be separated but not detected. This is especially true for some weak acid anions and suppressed conductivity detection. Chapter 4 discusses the use of different detectors for IC. 

Recently, Karmarkar reported an impressive dual IC-flow injection analysis (FIA) method for the sequential determination of anionic (nitrate and phosphate) and cationic (ammonium) nutrients in wastewater samples. The dual system was based upon the use of an anion exchange column (Lachat QS-A5) and two detectors, one suppressed conductivity detector using a Lachat Instruments QE-Al small suppressor cartridge, which is regenerated between samples, and a second visible absorbance detector. Upon injection of the sample the conductivity detector was switched off line and the nonretained ammonium was passed through the analytical column and detected by the visible absorbance detector, following an on hne colorimetric reaction. The conductivity detector was then immediately switched on hne to detect the retained nutrient anions. The method reported detection limits for phosphate of 0.006 mg/1 phosphate. 

For suppressed conductivity detection, the end of the separation column is connected to a tubular ion exchange membrane suppressor surrounded by a reservoir of regenerant solution [512,513]. The electrodes for conductivity detection are located in a separate capillary downstream of the suppressor. The high voltage electrode for the separation is located in the regenerant reservoir. In this way, the detector is decoupled from the electric field for the separation, and the electroosmotic flow generated in the separation column is used to drive the electrolyte solution through the suppressor and detector. The function of the suppressor (see section 5.7.4.1) is to neutralize electrolyte solution ions. 

Two publications from the mid-1990s indicated that perchlorate could be analyzed at microgram per liter concentrations using capillary electrophoresis separation (Avdalovic et al, 1993 Nann and Pretsch, 1994). The initial paper describes a custom-built suppressed conductivity detector that was capable of minimum detection limits... 

Suppressed conductivity. UV detector (A = 19CL205 nm) is suitable to confirm the conductivity results only. 

But of all of the many choices for detector, the conductivity detector remains the most popular. Conductivity detection is universal, rugged and sensitive. Adding a suppressor (discussed in Chapter 4 and elsewhere) dramatically improves the sensitivity for the majority of anions. The only exception to this is weak base anions which become low conducting in a suppressor. But the gain in detection for most anions and cations is so impressive that suppressed conductivity is the most popular form of detection, which means that a suppressor is placed after the exit of the separation column and before the conductivity cell. Membrane suppressors are marketed by Dionex. As described above, several column suppressor patents have expired and there are several companies offering this type of suppressor. Nonsuppressed conductivity, where the exit end of the column is connected directly to the conductivity cell, is effective for measuring many types of cations and weak acid anions. 

This acetonitrile, acetone, and water eluent is ideal for this detector because it is completely volatile. Ammonium acetate may also be a useful eluent because of its high volatility. This eluent can provide both anions and cations as eluent ions. Sodium (or potassium or Hthium) carbonate bicarbonate eluents can be suppressed and then sent through an ELSD. This eluent operates at a high pH so can be used to separate on an anion exchanger weak acid anions such as borate or several organic acids. While these anions cannot be detected by suppressed conductivity detection (because of low conductance) it may be possible to adjust detector conditions to detect many of these weak acid anions by ELSD. 

Conductivity, direct absorbance or a differential refractometer are the most common forms of detection for lEC, PAD and ELSD. A pulsed amperometric detector (PAD) or, more recently, an evaporative light-scattering detector (ELSD) is appropriate for detection of carbohydrates. Both non-suppressed and suppressed conductivity have been used extensively. The need to incorporate a low concentration of a strong acid into the eluent has been an impediment to direct conductivity detection. 

Detectors. In ion chromatography, suppressed conductivity detection is still the most frequently used technique. Indirect UV detection (Section 12.2.6.3) can also he a powerful tool for the detection of ions, although with respect to sensitivity. conductivity detection (detection ptower 10-100 ng/g) cannot be surpassed. In amino acid analysis, UV and fluorescence detection (after post- or precolumn derivatization) is often applied. 

US EPA Method 326.0 allows the combination of suppressed conductivity detection and postcolumn derivatization for UV absorbance detection, as schematically depicted in Figure 8.45. A volume of sample (225 pL) is introduced into an ion chromatography system. After separation and eluent suppression, standard anions and oxyhalide ions are measured using conductivity detection. The suppressed effluent from the conductivity detector is then combined with an acidic solution of potassium iodide (0.26 mol/L) containing a catalytic amoimt of ammonium molybdate tetrahydrate (43 pmol/L). The mixture is heated in the knitted reaction coil to 80 °C to facilitate complete reaction. The formed triiodide is measured by its UV absorption at 352 nm. Because KI is photosensitive, the KI/Mo PCR solution develops a light yellow color with time, even when stored under helium. Therefore, the PCR reservoir has to be protected from light by covering it with aluminum foil and should be pressimzed with helium. Under... 

---