Characteristics of a liquid chromatograph/atmospheric pressure ionization mass spectrometer

Abstract

The wide applicability and high sensitivity of a liquid chromatograph/atmospheric pressure Ionization mass spectrometer (LC/API-MS) using an Interface with direct heating nebulizer and vaporizer are reported. This system can produce molecular Ions of many nonvolatile compounds comprising amines, amino acids, peptides, antibiotics, nucleosides, nucleotide, steroids, sugars, lipids, vitamins, and alkaloids, which are difficult or Impossible to handle by gas chromatography/mass spectrometry. The detection limit of this system by selected Ion detection (SID) mode is 5 pg for some compounds such as theophylline and caffeine. In the API method, reduced fragmentation for some types of compounds provides Insufficient structural Information. However, a simple CID method operated at a relatively high pressure proves to be useful for producing fragment Ions which give structural Information.Recently, analysis of mixtures of nonvolatile compounds has become increasingly important. For this reason, a liquid chromatograph/atmospheric pressure ionization mass spectrometer (LC/MS) shows promise as a new analysis method in various fields. Many groups have reported various kinds of LC/MS systems including a thermospray method (1-5), a moving belt method (6, 7), a DLI method (8, 9), a vacuum nebulizing method (10,11), an atmospheric pressure ionization (API) method (12-20), and so on.Basically, an API method has a moderately high sensitivity. However, the sensitivities of the reported liquid chromatograph / atmospheric pressure ionization mass spectrometer (LC/API-MS) systems are not so high (12,(16)(17)(18). In a LC/API-MS system, the sensitivity strongly depends on the size of gas clusters produced by an interface. To obtain very small gas clusters, a nebulizer producing very small droplets is required in the interface. Previously, the authors used a nebulizer with an ultrasonic vibrator for this purpose (14,15). However, the lifetime of the vibrator was not long enough. Consequently, a direct heating nebulizer like a thermospray interface probe (1-3) is used here. This nebulizer is very simple and has a long life. Other problems in LC/API-MS system are formation of cluster ions (19,21,22) and reduced fragmentation resulting from mild ionization (19). These difficulties can be overcome by a collision-induced dissociation (CID) method in a relatively high pressure region of a mass spectrometer (21, 22). This paper reports the detail of this system and demonstrates the high performance and wide applicability of the system. EXPERIMENTAL SECTION Apparatus. A schematic diagram of the interface and the API ion source of the LC/API-MS system is shown in Figure 1. In this system, effluents from a LC are vaporized by the interface. The vaporized sample and solvent molecules are then ionized by corona discharge followed by ion/molecule reactions. The resulting ions are introduced into mass analyzing region of a sector type mass spectrometer through two apertures.LC. A Hitachi 655A high-performance liquid chromatograph (HPLC) with a loop in...

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