Chromatography and mass spectrometry are the two laboratory methods used most to find out exactly what a sample contains. The chromatograph separates a sample into its parts. The mass spectrometer identifies each part by weighing its molecules. Together they are the core of quality control in drug making. The most common kind of chromatograph in a drug laboratory is the high-performance liquid chromatograph, or HPLC.
How does the chromatograph separate a sample? The sample is dissolved in a liquid. A pump pushes the liquid through a narrow steel tube packed with fine particles, called the column. Different substances stick to the particles by different amounts. So they come out of the column at different times. A detector at the end records each substance as a peak on a graph. The time at which a peak appears tells you which substance it is. The size of the peak tells you how much there is.
The mass spectrometer goes one step further. It turns the molecules coming out of the column into charged particles, called ions. Then it measures the mass of each ion very precisely. Since each substance has its own mass, an unknown peak can be identified.
Here is a typical case. A quality-control laboratory at a tablet plant near Ahmedabad tests a batch of paracetamol 500 mg tablets. An analyst dissolves the powder of crushed tablets and injects it into the HPLC. The main peak appears at 4.2 minutes. The analyst compares its size with a reference standard, a sample of known purity. Each tablet contains 99.1% of the stated amount, within the limit of 95% to 105%. A small second peak is the impurity 4-aminophenol, at 0.04%, below its limit. A third, unknown peak appears. The mass spectrometer measures its mass and identifies it, so that the laboratory can decide whether it matters.
The instruments run on firmware, the software built into the hardware that controls the pumps, the heaters and the detector. On a computer beside them, the chromatography data system (CDS) runs each test and stores every result. Each result then goes to the laboratory's LIMS, often through lab instrument integration. If a result fails its limit, the failure is investigated in the quality management system. All of this must follow GxP rules, and the instruments themselves must be proven to work correctly through computer system validation.