Anal. Chem. 1990, 62,612-615
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Suspension Postcolumn Reaction Detection Method for Liquid Chromatography Douglas T. Gjerde* Sarasep, Inc., 1600 Wyatt Drive, Suite 10,Santa Clara, California 95054
James V. Benson Benson Polymeric, Inc., 4255 Garlan Lane, Reno, Nevada 89509
A novel detection method for hlgh-performance llquld chromatography utlllzes the addltlon of a colioldal suspension of solid r e v t polcto the eluant stream. he average particle &e of the l m u b l e WM reagent k -H enough to keep It In -nslon. m e solid reagent may react with the sample to reduce the background signa^ or may react wlth the elulte making It detectable. Slnce the reagent Is solid, It Is “lnvklble” to detectors that repond only to the liquid phase, e.g. conductometric and potentlometrk detectors. TMs paper examlnes the properties of sulfonated polystyrene-type solid reagents and describes their appllcation In conductlvlty detection. Other potentlal uses of the solld phase reagents are dlscussed.
INTRODUCTION The need for better detector selectivity and sensitivity has led to the development of postcolumn reaction (PCR) derivatization techniques. Reagents are added to the eluant stream to react with sample peaks and make them detectable (I).The simplest postcolumn reactions involve only the addition of energy e.g. heat, UV irradiation, or electrons. Similarly, reactants such as color-forming reagents can be added to the stream using a mixing chamber and a reagent pump. Often, the reagent itself causes a background signal. This can be a problem especially when highly concentrated reagent is used. Minute fluctuations in reagent flow can produce high background noise levels. Also, reagent addition to a stream may produce undesirable precipitation reactions. Another PCR detection approach employs a packed bed column reactor containing a solid reagent. However, as the reagent is used, the column is depleted and must be replaced or regenerated. A third method uses a membrane. The membrane allows the continuous passage of selected reagents to and from the eluant stream. But, membranes have limited surface area and can be fragile, and only a few membrane types are available. Reactions that form precipitates may irreversibly damage the membrane. The purpose of this paper is to introduce a new approach to PCR detection for liquid chromatography. A suspension of a particulate reagent is added postcolumn to the eluant stream. The reagent is insoluble, but the average particle size is small, and the reagent remains in suspension. The solid reagent can react with the eluant to reduce the background signal and/or with the sample to produce detectable peaks. A detector such as conductivity responds only to the species dissolved in the liquid phase. The solid phase reagent does not interfere with the detection process. This paper shows the applications of suspended reagent PCR/conductivity detection for ion chromatography. A *Author to whom correspondence should be addressed.
sulfonated polystyrene latex solid reagent is used to treat basic eluants used in the separation of anions. The suspension converts the eluant to low conducting weak acids while converting sample anions to highly conducting acids. Factors affecting the performance of the suspension detection method are discussed.
EXPERIMENTAL SECTION The LC instrument consisted of an Altex llOA eluant pump, Rheodyne 7125 injector equipped with a 20- or 100-pLinjection loop, and Wescan 213A (3.3-cm-I cell constant) conductivity detector. A LDC Perista peristaltic pump was used for pumping the suspension reagent through an Upchurch low void volume tee. After the reagent was introduced to the eluant, the mixture was directed to a serpentine mixer (2)and then directed to the conductivitycell. For most of this work, tubing size at the point of reagent addition and down stream was 0.030 in. i.d. In a few cases, smaller, 0.010 in. i.d. tubing, was used. The Sarasep AN1 column, 250 X 4.6 mm, is manufactured with PEEK tubing and ceramic frits. The suspension reagent was taken from the “fines” leftover from the synthesis of a chromatographic grade cation exchange resin (Benson Polymeric OA850). This material is a fully sulfonated microporous polystyrene with 8% divinylbenzenecross linking. The
