Liquid sample inlet systems

It is important to use the correct injector and injection technique for a particular application to obtain maximum performance from a GC system. Most 

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Liquids that are sufficiently volatile to be treated as gases (as in GC) are usually not very polar and have little or no hydrogen bonding between molecules. As molecular mass increases and as polar and hydrogen-bonding forces increase, it becomes increasingly difficult to treat a sample as a liquid with inlet systems such as El and chemical ionization (Cl), which require the sample to be in vapor form. Therefore, there is a transition from volatile to nonvolatile liquids, and different inlet systems may be needed. At this point, LC begins to become important for sample preparation and connection to a mass spectrometer. 

For the purpose of sample introduction, any sample introduction system (also sample inlet system or inlet) suitable for the respective compound can be employed. Hence, direct probes, reservoir inlets, gas chromatographs and even liquid chromatographs can be attached to an El ion source. Which of these inlet systems is to be preferred depends on the type of sample going to be analyzed. Whatever type the inlet system may be, it has to manage the same basic task, i.e., the transfer of the analyte from atmospheric conditions into the high vacuum of the El ion source Table 5.1 provides an overview. 

The interior of a mass spectrometer is under high vacuum (10 torr) in order to minimise the number of collisions undergone by ions and thereby maximise the number of ions reaching the detector. Sample inlet systems must enable the sample to be introduced without loss of the vacuum and must be capable of handling samples as gases, liquids, or solids, either as single components or as multi-component mixtures. 

Figure 13.4 Sample inlet systems (a) Hypodermic needle syringe and heater block for liquids, (b) gas sample introduction.

The three basic elements of the RAPyD-400 — the vacuum system, the inlet system, and the quadrupole analyzer — can be seen in Figure 3.5. The quadrupole analyzer, which itself consists of two separate parts, lies inside the main vacuum chamber. The high vacuum is attained using a turbomolecular pump, which is backed by a dual-stage rotary pump mounted externally to the main system. The sample inlet system is connected to the ion source of the mass spectrometer via a heated molecular beam tube. Around the underside of the ion source is a copper cold finger, which is cooled by liquid nitrogen and used as a sample dump to prevent carryover from one sample to another. 

Sample Inlet System—Means shall be provided for introducing a measured quantity of representative sample into the column. Pressure-sampling devices can be used to inject a small amount of liquid directly into the carrier gas. Introduction can also be accomplished by use of a gas valve to charge the vaporized liquid. 

Sample Inlet System, Liquid—A. liquid sampling valve shall be provided, capable of entrapping a fixed volume of sample at a pressure at least 200 psi (1379 kPa) above the vapor pressure of the sample at valve temperature, and introducing this fixed volume into the carrier gas stream ahead of the analyzing column. The fixed sample volume should not exceed 1.0 pL and should be reproducible such that successive runs agree within 2 % on each component peak area. The liquid sampling valve is mounted exterior of any type heated compartment and thus can operate at latoratory ambient conditions. 

Sample Inlet System, Gas (Instrument Linearity)— Provision is to be made to introduce a gas phase sample into the carrier gas stream ahead of the chromatographic column so that linearity of the instrument can be estimated from response curves. The fixed volume loop in the gas sample valve shall be sized to deliver a total molar volume approximately equal to that delivered by the liquid sample valve in accor nce with 5.1.2. (See Section 6 for further explanation of instrument linearity check procedures.)... 

Sample Inlet System—A liquid sample injection valve is required, capable of reproducibly introducing samples in the O.OS to 0.50-pL liquid volume range. The inlet system should be operated at between 25 and 30 C. The sample inlet system must be connected to the chromatographic column so that loss of chromatographic efficiency is avoided. 

Sample Inlet System—The sample inlet sy em must have variable temperature control capable of operating continuously at a temperature up to the maximum column temperature employed. The sample inlet system must allow a constant volume of liquid sample to be injected by means of a syringe or liquid sampling valve. 

The sample inlet system for a typical mass spectrometer is versatile enough to handle gas, liquids, and solids. The device is usually held at 200°C and 0.02 torr pressure. Accordingly, any solids must have a sufficient vapor pressure under these conditions to allow transport to the ion source as a gas before a spectrum can be recorded. Modern instruments usually incorporate an electron multiplier or channel electron multiplier array as a detecting system. Both these devices work on the principle of electrons released from a material on ion impact. The electrons are... 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

Some solids inlet systems are also suitable for liquids (solutions) if the sample is first evaporated at low temperatures to leave a residual solid analyte, which must then be vaporized at higher temperatures. 

The MC-ICP-MS consists of four main parts 1) a sample introduction system that inlets the sample into the instrument as either a liquid (most common), gas, or solid (e.g., laser ablation), 2) an inductively coupled Ar plasma in which the sample is evaporated, vaporized, atomized, and ionized, 3) an ion transfer mechanism (the mass spectrometer interface) that separates the atmospheric pressure of the plasma from the vacuum of the analyzer, and 4) a mass analyzer that deals with the ion kinetic energy spread and produces a mass spectrum with flat topped peaks suitable for isotope ratio measurements. 

Many manufacturers now offer other sample injection systems compatible with the vacuum lock used for the solids probe. These include small (e.g., 75-ml) heatable batch inlet systems, usually accessible via syringe (gas syringe or GC microliter syringe for liquids), which can be particularly useful as inlets for mass reference compounds. Other probes are designed as flexible, easily removed connections to a gas chromatograph via some form of interface. 

Batch inlet systems are in many ways the most convenient for gases and volatile, thermally stable liquids or solids, provided there is sufficient sample available. They consist of reservoirs, varying in volume from 20 ml to several liters, which are connected to the ion source via a molecular leak, usually a porous ceramic material, or a pinhole in thin gold foil or glass. The leak serves to reduce the pressure from 10 torr in the inlet system to torr in the ion source. Large ballast bulbs... 

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