Nebulizing gas pressure

Factors may be classified as quantitative when they take particular values, e.g. concentration or temperature, or qualitative when their presence or absence is of interest. As mentioned previously, for an LC-MS experiment the factors could include the composition of the mobile phase employed, its pH and flow rate [3], the nature and concentration of any mobile-phase additive, e.g. buffer or ion-pair reagent, the make-up of the solution in which the sample is injected [4], the ionization technique, spray voltage for electrospray, nebulizer temperature for APCI, nebulizing gas pressure, mass spectrometer source temperature, cone voltage in the mass spectrometer source, and the nature and pressure of gas in the collision cell if MS-MS is employed. For quantification, the assessment of results is likely to be on the basis of the selectivity and sensitivity of the analysis, i.e. the chromatographic separation and the maximum production of molecular species or product ions if MS-MS is employed. 

Some electrospray parameters are known to be critical for achieving stable conditions and thereby good quantitative results. These parameters are the sheath liquid composition and flow rate, the nebulizing gas pressure, the applied electrospray voltage and the capillary outlet position. On the other hand, in previous studies, the impact of drying gas flow rate and temperature on stability and sensitivity were demonstrated to be moderate [3, 76]. Most of the quoted parameters are well described in the literature, apart from the capillary position which is disregarded for CE-ESI-... 

An example of a total ion current (TIC) spectrum and the corresponding mass spectrum is shown in Figures 9.5a and b, respectively. The graphs were obtained at lOpLmin-1 flow rate and 6.5 bar argon nebulization gas pressure, 260 L h 1 cone gas flow and 3300 V, 48 V and 15 V for the capillary, cone and extraction voltages, respectively. Over a time frame of 4 min the measured standard deviation of the total ion current was less than 8%, which is comparable with values for chip-MS reported in the literature.29,30... 

The combination of nebulizing and desolvation gases (probe parameters) described above is suited to liquid flows of -0.2 ml/min. Because mass spectrometry is a concentration-dependent technique, reducing solvent flow will increase the residence time of the analyte in the source. The consequent enhancement of absolnte sensitivity can reach 1,000-fold. The potential for snch inCTeases in sensitivity led to the development of instrumentation and techniqnes for LC where solvent flows are 0.3-1.0 til/min, nano-LC. At such flow rates fused silica and stainless steel lines (with tips tapered to 10-20 ti) can be used as the ESI source. The flow rates in nano-LC also permit the reduction of the nebulization gas pressure to -250 Torr (<1 bar), sometimes even to zero. Furthermore, a drying gas is no longer required. When there is no need to separate the components of the sample, small volumes of material (-1 al) can be loaded into a capillary and placed in the ESI housing. In this case no gases are needed and the potential difference between the liquid in the capillary and that in the instrument chamber is sufficient to create an electro-osmotic flow of nebulized droplets from the tip of the capillary. 

MSD system (Agilent, Waldbronn, Germany) were capillary voltage 2.0 kV, skimmer voltage 35 kV, lens -4.3 V, drying gas temperature 300°C, drying gas flow rate 8 L/ min, and nebulizer gas pressure 40 psig. 

Geometric parameters of the source were as follows the distauce betweeu capillaries was set to 5 mm the nebubzation capillary height over the sample was 2 mm nebulization capillary angle was 58° MS inlet height over the sample was 1 mm and nebulizing gas pressure (nitrogen) was 11 atm. 

Capillary IC features a flow rate in the range of 10-30 pF/min, thus requiring modifications and reoptimization of existing electrospray interfaces that are usually optimized for either analytical flow (100 pF/min to several milliliters/minute) or nanoflow (<1 pF/min) rates. The optimization of interface parameters such as probe temperature, nebulizer gas, needle voltage, type of desolvation solvent, and the flow rate plays a critical role in establishing instrument sensitivity. When capillary IC is operated at a flow rate of 10-20 pF/min, a probe temperature of 300 °C, a needle voltage of 3 kV, a nebulizer gas pressure of 65 psi, and the use of... 

It was determined that 30 mg/ml of DHB in acetoni-trile water (50 50, v v, with 0.1% TEA) can form evenly distributed small (0.5-20 gm mean diameter) matrix crystals on the sample surfiice. Several different combinations of matrix solution flow rate, nebulizing gas pressure, and OCN-sample distance were evaluated for matrix deposition and have been reported (12). Briefly, the matrix molecules formed irregular clusters without obvious crystal boundaries when using a high flow rate with a low gas pressure and short sprayer-sample distance. This was anticipated because these conditions would likely produce big droplets with a high solvent content (9). A reduction of the matrix solution flow rate and an increase of gas pressure and sprayer-sample distance resulted in the matrix crystals... 

The nebulizer gas pressure was 35 psi. The capillary voltages were 4000V and 3500V for the positive and negative modes, respectively. SIM mode was used for the determination. The details of the fragmenter voltage and m/z were shown in Table-1. 

Detector MS, PE Sciex API-III-l- triple quadrupole, APCI, positive ion, heated nebulizer interface, nebulizer probe 500°, nebulizer gas nitrogen 0.6 L/min, curtain gas nitrogen at 0.8 L/min, auxiliary gas nitrogen 2.0 L/min, nebulizer gas pressure 70 psi, m/z 455—199... 

Note These observations are equally relevant to (pneumatically assisted) ESI at standard flow rates [88]. Then, the corresponding voltages are just higher by a factor of > 2, which is mostly due to the increased gap between spray capillary and counter electrode to accommodate the larger plume. The optimization of the electrospray for temporal stability by adjustment of liquid flow, nebulizer gas pressure, and spray voltage is therefore necessary for any analytical ESI work. 

Fig. 13.2. Influence of experimental parameters on the intensity of the [M-t-SH] "" ion of me-littin (a) spray impact angle, (b) spray high voltage, (c) nebulizing gas pressure (14.5 psi 1 bar), (d) solvent flow rate. Reprinted from Ref. [12] with permission. John Wiley Sons, Ltd. 2005.

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