SETTING UP A TYPICAL ANALYTICAL RUN

Described here is the process that is usually performed on a daily basis when the ICP-AES is used to perform a series of quantitative analyses. Because of the instrument's ability to provide rapid analysis, it is most efficient to analyze 20-25 samples per run. Along with calibration SRMs, drift solutions, blanks, replicate samples, calibration verifications, and other items, these tasks will fill an analytical day.

Additionally, for reasons relating to good analytical practice, this recommended general number of samples also makes sense--analytical precision is optimal when as many samples from a given site/hole are analyzed at the same time. For interstitial waters, samples from an entire site can be analyzed for a number of different trace elements in one run.

In all situations, it is advisable to analyze samples in random order (i.e., not in order of depth). Although analytical drift is accommodated throughout the run (see "Data Reduction") analyzing samples in random depth order ensures that the final observed downhole trends are not caused artificially. All samples should analyzed at room temperature. Aspirating cold solutions (e.g., interstitial waters from the refrigerator) leads to high relative standard deviations and increased instrument drift.

The following descriptions apply to rocks and sediments as well as to interstitial waters and assumes that the method has been developed (complete with background corrections, selection of analytical mode, and all other parameters), calibration standards are constructed, samples have been diluted and are ready to run, and the sample file has been built. Some of the terminology may be unique to the JY instrument.

1. Warm up for at least 30 minutes. Do not be tempted to rush this. The warm-up period is not only for the generator but also for the glassware, torch box, and other aspects of the sample introduction system. Be sure to aspirate DI during the warm-up period. It is commonly helpful to aspirate a solution of the same matrix as the samples for the last ~5 min of the warm-up period. The plasma should not be run dry for long periods of time (exceeding 3 min). A representative start-up procedure is provided in "Appendix".

2. Perform a zero-order check, if needed. Zero order is the term used to define when the grating within the spectrometer behaves as a mirror, reflecting incoming light rather than refracting it into several wavelengths. A zero-order check physically moves the diffraction grating to its zero position, where all light is reflected. Although the software will check whether zero order is okay or not whenever the instrument is turned on, this internal software check is not a true mechanical test. It is instead a test of whether the electronics have been compromised (i.e., turned on or off, been subjected to a surge, etc.) since the ICP was last used. Therefore, it is advisable to perform a zero-order check (Shift-F2; Center Control) every few days. It is also helpful to record in a logbook at what position (step number) zero order was found, as this parameter is useful for diagnostics, should the need arise.

3. Perform an autosearch. During elemental analysis (e.g., Fe at 238.204 nm), the spectrometer searches for a reference peak (the carbon line at 193.031 nm) and subsequently moves to the chosen wavelength as a function of its distance (in motor steps, which corresponds to nm) from the reference peak. Prior to each analysis, the ICP searches for the reference peak and locates it. The autosearch function calibrates the instrument exactly for each peak location with respect to the reference, so during analysis it can locate the wavelength precisely. The autosearch also quantifies an offset, which is the distance (in nanometers) that the reference peak was found from where it was expected. Typical offsets are on the order of 0.005 nm or less. Perform autosearch repeatedly until the peaks are visually on-line each time, paying attention to the amount of offset (in nanometers). Offsets significantly and consistently greater than ~0.005 nm may indicate something is amiss (in practice this will depend on ODP's exact instrument). We recommend performing several autosearch commands in a series immediately prior to calibration. The first time an element is autosearched, or whenever a method has been changed significantly, be sure to use a single element solution to ensure that the peak found during an autosearch is in fact a peak for the element of interest. Subsequently, a standard or drift solution can be used to Autosearch. Using the Small Window option on the JY2000 software also helps ensure that the peak found is the one of interest.

4. Perform a calibration. If the calibration feature of the software is chosen, the instrument must be calibrated for every analyte of interest prior to each run (see "Constructing a Sample File and Performing Data Reduction" for details on calibration options). The operating conditions of the ICP during the calibration must be identical to analysis conditions, so all manipulation of gas flows, pump speeds, and the like must have taken place prior to this stage. In fact, if any operating conditions have changed, a new Autosearch will need to be performed. Using the autosampler helps ensure that the calibration conditions (e.g., rinse time) will be identical. Note: the JY2000 software does not allow for correction of drift between calibration standards, so the analyst is forced to assume that only minimal drift takes place during this aspect of a run. This assumption has been shown to be relatively robust and its effects minimized during data reduction.

5. Start Run. Initiation of the analytical run must occur as soon as possible after the calibration is completed (usually within 5-10 min). This is of vital importance because the first item in the sample file is a drift solution, which will be used as the master drift to correct the entire run. Provided the sample file is ready, the autosampler is loaded, and all other preparations are completed, starting the run this quickly is not difficult.

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