High Performance Liquid Chromatography (HPLC) / Kromatografi Cair Kinerja Tinggi (KCKT)

Introduction to High Performance Liquid Chromatography (HPLC)

High Performance Liquid Chromatography, commonly referred to as HPLC or Kromatografi Cair Kinerja Tinggi (KCKT) in Indonesia, represents a sophisticated, high-level development of standard column chromatography. Unlike traditional gravity-fed systems, HPLC utilizes high-pressure pumps to force solvents through the column at pressures reaching up to $400\,atm$ (approximately $5878\,psi$ ) or higher, significantly accelerating the separation process. The material packed within the column consists of extremely small particles, which provide a significantly larger surface area than conventional materials. This increased surface area is critical as it facilitates superior and more efficient separation of various components within a complex mixture. Furthermore, the modern HPLC system is designed to be highly automated and extremely sensitive, incorporating advanced detection methods that can identify even trace amounts of analytes.

The evolution of this technique has been heavily influenced by advancements in material technology and electronics. Initially, the acronym HPLC stood for High Pressure Liquid Chromatography, but as the technology matured and shifted focus toward efficiency and resolution, the name was refined to High Performance Liquid Chromatography. It fundamentally serves as a modern liquid chromatography technique that operates under high-pressure elution systems, often standardized at approximately $5000\,psi$ .

Core Principles and Success Factors

The fundamental key to the success of HPLC lies in the column packing, also known as the stationary phase. The material must consist of small-diameter particles, typically less than $40\,\mu m$ . To maintain a consistent flow through these tightly packed particles, the eluent is pumped under high pressure at a flow rate ranging from $1\,mL/min$ to $10\,mL/min$ . As the analytes exit the column, they are immediately detected by highly sensitive detectors.

Advanced column technology has introduced the concept of the "Bonded Phase," where the stationary phase is chemically bonded to the support material. This allows for specific modifications of the sorbent to facilitate specialized techniques such as Reverse Phase (RP) chromatography and Ion Exchange Chromatography (KPI/Kromatografi Pertukaran Ion). These modifications allow the system to be tailored to the specific chemical properties of the target analytes.

Column Theory and Resolution Optimization

In HPLC theory, the efficiency of a column is represented by the number of theoretical plates ( $N$ ). This value is influenced by several factors: the band width ( $W$ ), the particle size, the distribution of the particle size, and the nature of the particles themselves (whether they are fully porous or pellicular). A smaller band width ( $W$ ) results in a larger value for $N$ , making $N$ a primary benchmark for assessing the quality and efficiency of a column. High-efficiency columns are essential for complex and difficult analyses that require high resolution.

To achieve high resolution ( $Rs$ ), three primary parameters must be optimized:

The Capacity Factor ( $k'$ ): The goal is to maintain a value between $1$ and $5$ by selecting an appropriate eluent.
The Selectivity Factor ( $\alpha$ ): Optimization is achieved by changing the eluent while maintaining the same elution strength, or by adding additives to a third eluent, such as changing the buffer, adding salts, or introducing ion pairs.
The Efficiency ( $N$ ): This can be increased by reducing the flow rate of the eluent, utilizing longer columns, or employing smaller particle sizes within the column.

General Separation Methods in HPLC

HPLC methods are broadly categorized into Normal Phase and Reverse Phase (RP-HPLC) based on the polarity of the stationary and mobile phases.

Normal Phase HPLC involves a polar stationary phase (such as very small silica particles) and a non-polar mobile phase (such as hexane). The columns typically have an internal diameter ( $i.d.$ ) of less than $4.6\,mm$ and a length between $150\,mm$ and $250\,mm$ . In this setup, polar compounds in the mixture interact more strongly with the polar silica and adhere to the column longer. Consequently, non-polar compounds pass through the column more quickly and elute first.

Reverse Phase HPLC (RP-HPLC) is the more common method, utilizing a non-polar stationary phase and a polar mobile phase. Here, the silica is modified to be non-polar by attaching long hydrocarbon chains, such as $C8$ or $C18$ , to its surface. The mobile phase is typically a polar solvent like water or an alcohol-water mixture (e.g., methanol). In RP-HPLC, polar molecules move faster through the column and elute first, while non-polar molecules are retained longer by the non-polar stationary phase.

Functional Components of the HPLC System

A standard KCKT/HPLC system is composed of several critical components and accessories:

Solvent Reservoir / Gradient Controller: This stores the mobile phase. Essential requirements include using Millipore filters (pore size approximately $5\,\mu m$ ) and degassing the solvent to remove dissolved gases.
High-Pressure Pump: This component pushes the mobile phase through the column. Modern systems may use dual pumps and programmed eluent systems.
Sample Introduction / Injector: This is the entry point for the sample, typically introduced via a syringe. Auto-injectors are also available for batch processing.
Column: Often described as the "heart" of the HPLC system, this is where the actual separation of the sample components occurs. Column temperature controllers may be used to maintain stability.
Detector: This follows the column to detect the components as they elute. Common types include UV-Vis, RI, and Fluorescence detectors.
Data Output / Microcomputer: This unit displays the results, utilizing control and data processing software for recorder functions and data analysis.

Pump Specifications and Types

Pumps in HPLC must meet stringent requirements to ensure analytical precision. They must be constructed from materials resistant to various eluents, handle pressures between $500\,psi$ and $5000\,psi$ , and be pulse-free (or equipped with a pulse dampener). They should maintain a flow rate capacity of around $3\,mL/min$ with a flow rate reproducibility of less than $1\%$ .

There are two primary types of pumps:

Reciprocating Pumps (Pompa Bolak-Balik): These are the most widely used. They allow for an unlimited solvent volume and are suitable for gradient elution. However, they naturally produce a pulsed flow that requires dampening.
Syringe Pumps (Pompa Jenis Penyuntik): These have a limited solvent capacity (typically $250-500\,mL$ ). They are pulse-free and are most often used for micro-bore columns.

Column Specifications and Column Quality Parameters

Columns are classified as either analytical or preparative. Analytical columns often measure $15\,cm \times 3.9\,cm$ (stainless steel) or $10\,cm \times 8\,cm$ (plastic, such as Radial-Pak), with particle sizes of $5\,\mu m$ or $10\,\mu m$ . Preparative columns are larger for sample purification; micro-preparative columns ( $15\,cm \times 19\,mm$ or $21.5\,mm$ ) handle $10\,mg$ to $1.0\,g$ of sample, while macro-preparative columns ( $30\,cm \times 57\,mm$ ) handle $1.0\,g$ to $1.0\,kg$ . Preparative particles are usually greater than $20\,\mu m$ .

Examples of column quality from Merck and ODS characteristics include:

Merck Si-300: Particle size ( $dp$ ) of $10\,\mu m$ , pore volume ( $Vp$ ) of $0.79\,ml/g$ .
Merck RP-18: $dp$ of $4$ , $5$ , or $10\,\mu m$ , Carbon content ( $\%C$ ) of $21.4$ , and bonded phase total of $3.9$ .
ODS LC-18: Carbon content $10.76$ , $N = 73 \times 10^{-3}$ , asymmetry at $10\% = 1.03$ , backpressure of $950\,psi$ , and $5\,\mu m$ spherical particles.
ODS C-18: Carbon content $15.28$ , $N = 80 \times 10^{-3}$ , asymmetry $1.29$ , pressure $1800\,psi$ , and $5\,\mu m$ spherical particles.

Detection Mechanisms and Systems

The choice of detector depends on the chemical nature of the analytes. The UV Detector is the most common for organic compounds because many absorb UV light at specific wavelengths. In this system, UV light is shone through the eluate; the detector on the opposite side measures how much light is absorbed, providing a direct reading of the concentration. Other detector types include:

Refractive Index (RI) Detector
Fluorescence Detector
Electrochemical Detector
Infrared (IR) Detector
LC-MS (Mass Spectrometry)
LC-NMR (Nuclear Magnetic Resonance)

Functional Advantages and Applications

HPLC offers numerous advantages, including automated procedures, high separation power, and the requirement of very small sample volumes. It is a fast, sensitive, accurate, precise, and reproducible method suitable for both analytical and preparative work. It can analyze liquid, volatile, non-volatile, thermally stable, and thermally unstable organic and inorganic samples. This versatility makes it indispensable in pharmaceutical, chemical, biochemical, and medical fields.

Classification of HPLC and Interaction Mechanisms

HPLC is classified based on the interaction between the solute, stationary phase, and eluent:

Separation by Size: Size-Exclusion Chromatography.
Separation by Charge: Ion Exchange Chromatography (KPI) and Ion-Pair Chromatography (KIP).
Separation by Hydrophobicity: Utilizes modified silica gel with polar groups (CN, $NH_2$ ) or non-polar groups ( $C8$ , $C18$ ). Mechanisms include partition between the hydrocarbon layer and mobile phase, partition between the mobile and modified stationary phase, or adsorption onto the hydrocarbon surface.

Solvent and Stationary Phase Selection

A wide range of solvents can be used in HPLC, ranging from non-polar to highly polar. Common solvents include N-hexane, Cyclohexane, Tetrachloromethane, Methylbenzene, Trichloromethane, Dichloromethane, THF, Propanone, Acetonitrile, Iso-propanol, Ethanol, Methanol, Ethanoic Acid, and Water.

Stationary phases are divided into:

Bonded Phases: ODS (Octadecylsilica/ $C18$ ), Octylsilica, Propylsilica, Aminopropyl, Sulfonic Acid, and Quaternary Amine.
Polymer Phases: Cross-linked styrene or DVB (divinylbenzene), primarily used for Exclusion or Ion Exchange chromatography.

Chromatographic Analysis Examples

Specific chromatogram examples demonstrate the system's precision. For instance, analyzing milk extracts via HPLC used a silica gel stationary phase ( $100 \times 2\,mm \, i.d.$ , $3\,\mu m$ particle size) and a mobile phase of $0.25\%$ octanol in hexane. With a flow rate of $0.4\,mL/min$ and detection at $325\,nm$ , the system could distinguish between various isomers including 11-cis, 13-cis, 9-cis, and all-trans-retinol. Another example shows the separation of lutein, $\alpha$ -carotene, and $\beta$ -carotene using a Chrom-Sep ChromSpher PAH column ( $100 \times 3\,mm \, i.d.$ , $5\,\mu m$ particles) with a mobile phase of acetonitrile:methanol:dichloromethane ( $80:14:6\,v/v$ ) at a flow rate of $0.7\,mL/min$ .