CHAPTER 6
Section II
Preliminary Experiments
II.6.1 Cleaning and Maintenance of FAAS and FAES Instruments
FAAS and FAES units require little cleaning and maintenance. Before and after completing a series of analyses, the uptake tube, nebulizer chamber, and burner head should be aspirated with reagent water containing dilute acid. This will ensure an analyte-free system during analysis. Rarely, the instrument response will stabilize very slowly when a new solution is introduced. Rinsing or soaking the burner in dilute nitric acid for a few hours will eliminate this problem. If the instrument has not been used recently, bacteria and mold colonies may build up in the nebulizer and drain tube and will need to be cleaned to allow proper drainage of the nebulizer. When rich flame conditions (high fuel concentrations) are used or when high sugar-containing samples have been analyzed, carbon deposits on the burner head will need to be removed.
II.6.2 Cleaning and Maintenance of ICP-MS Systems
ICP-AES instruments require minimal attention, but ICP-MS instruments require constant attention to possible contamination sources because of their part-per-trillion detection limits. Many instruments are even housed in a clean room or HEPA hood to avoid dust and atmospheric contamination. Dilution and rinse water, as well as acids, must constantly be monitored for contamination. The most common maintenance in ICP systems is weekly cleaning and conditioning of the cones. Cones are cleaned by removing them and gently scrubbing (using an analytical grade swab) with Calgamite followed by a thorough rinsing with ultra pure DI. The cones should be dried with a heat gun prior to re-installation. Cones are conditioned, after re-installation, by running a mid- to high-range calibration standard (containing all analytes of interest) on the instrument under normal conditions for 20-30 minutes. This allows any equilibrium adsorption of metals to the cones to occur prior to the analysis of samples and will result in more stable and reproducible analyte readings at low concentrations.
Depending on instrument use and the type of samples analyzed, the focusing lenses will need to be cleaned monthly to every six months. Reaction cells usually require cleaning every six months to a year, but frequently break during cleaning and need to be replaced. The cleaning process is similar to the cone cleaning process, but Citranox is used. Quadruples rarely need cleaning but if the pre-mass filters units are contaminated, then the mass filter may need attention. Electron multiplier detectors decrease in sensitivity during the first year of operation but usually last for several years. The rotary vacuum pump oil and filter will need to be replaced approximately every six months.
II.6.3 Ensuring Adequate Rinsing Between Samples and Standards
FAAS and FAES systems are normally rinsed between every sample and standard and blank instrument readings are taken to ensure adequate rinsing. This is automatically preformed by most modern AAS systems by the automatic sampler. Additional rinsing after a series of samples will usually prevent any buildup of analytes in the nebulizer, but sometimes a slow recovery of the blank reading can occur due to absorption in the nebulizer. When this occurs, the entire mixing chamber (the inlet tube, nebulizer, fins, and burner head) must be thoroughly washed with soap and water, and rinsed with DI and dilute acid.
The part-per-trillion detection limits of ICP-MS systems create the need for more thorough rinsing between samples and standards. Usually, the rinsing part of the sample sequence is the most time consuming. As in FAAS and FAES measurements, not only must the inlet system be rinsed, but it must be confirmed by analysis that the instrument is clean.
| Frank's Homepage |
©Dunnivant & Ginsbach, 2008