This section describes some basic considerations and guidelines for analytical conditions to be used during ICP-AES analysis.
The dissolution procedure described in "Flux-Fusion Preparation of Rocks and Sediments" results in an aqueous solution that can be analyzed in a single analytical session for major and trace elements. It represents a 4000× (nominal) dilution of the sample. This dilution factor is appropriate for igneous rocks, sedimentary rocks, and sediments. We suggest working from this 4000× dilution as a baseline. Should there be leg-specific analytical requests, different dilutions can be prepared, as long as the overall goal of generating enough analyte at an appropriate concentration is fulfilled. See additional discussion in "Nebulizers". As always, prepare the standard reference materials used to calibrate the ICP-AES at the same dilution factor and acid concentration as the unknown samples.
As mentioned previously, the most common dilution factor for interstitial waters is a simple 10× dilution in DI. Undiluted seawater may be analyzed, particularly when low concentration elements are to of concern, but salting of the nebulizer must be monitored carefully. Use of the Ar humidifier will improve results for undiluted seawater analysis.
In all circumstances, great care must be taken to avoid nebulizer clogging. Thus, all samples and standards should be filtered prior to analysis. Clogging can result from either a physical blockage of the small capillary inside the nebulizer or from the formation of a precipitate around the ejection orifice. Nebulizer clogging can be easily diagnosed when the nebulizer Ar flow rate decreases simultaneously with an increase in Ar backpressure, which signals inhibition of Ar transport through the nebulizer. When the nebulizer begins to clog, the stability of the analysis greatly decreases and drift becomes insurmountable. The analytical run must be discontinued, the instrument shut down, and the nebulizer cleaned before the run can be resumed. It is very difficult to clean a nebulizer, and it can become tremendously time consuming to have to repeatedly deal with such clogging (see "Basic Maintenance Suggestions").
A great deal of effort goes into selecting the proper nebulizer for a particular application. The most commonly used nebulizer is a concentric nebulizer (commonly referred to as a Meinhard nebulizer although other companies also manufacture them). See the literature on board the ship for a description of how this nebulizer operates. Meinhard and other concentric nebulizers provide excellent stability and signal to noise ratios, although they are more prone to clogging than other nebulizer types. Wide orifice type-C concentric nebulizers are appropriate for use with rocks and sediments as well as with interstitial water samples. For the dilutions described above, these nebulizers work very well, with minimal clogging, as long as proper cleaning procedures are followed.
ODP has also acquired several V-groove nebulizers, which allow for the analysis of samples with very high TDS. This type of nebulizer requires a slightly higher sample aspiration speed (controlled through the peristaltic pump) than a concentric nebulizer and thus consumes more analyte solution; however, the flexibility to easily analyze variable TDS solutions makes this nebulizer particularly well suited for very high TDS operations. It is important to note, however, that signal stability is significantly poorer for V-groove nebulizers, and they should only be used in exceptional circumstances. Testing of type-C concentric and V-groove nebulizers during Leg 187 and at Boston University documents that the concentric nebulization scheme provides a greater signal, better stability, and less noise for the analysis of rocks, sediments, and interstitial waters, at the dilution factors suggested here for routine operation.
After a period of time, flux-fusion solutions may become unstable resulting in the precipitation of major and trace elements or the formation of a gel. These are not always immediately visible (the gel is clear), so solutions must be visually inspected carefully prior to analysis. An unstable solution must be discarded because the gel will strip dissolved trace metals from the analyte.
The stability of a solution is directly proportional to the dilution factor and acid content, and inversely proportional to the SiO2 content. A dilute solution is more stable than a concentrated one, and there is likely to be a noticeable difference in the shelf life of a flux-fusion solution prepared at 100× TDS vs. 1000× TDS. Likewise, a solution prepared in a 10% HNO3 matrix is more stable than one prepared in 1% HNO3. Conversely, a flux-fusion solution resulting from the dissolution of diatomaceous ooze or rhyolite (either of which are enriched in SiO2), is likely to be less stable than a solution prepared from a low-Si basalt or shale.
As with many aspects of analytical geochemistry, a balance exists between these parameters. The dilution factor from the procedure described herein should yield solutions that are stable at least for several days, and perhaps for up to several weeks.
Diluted and undiluted interstitial waters are stable indefinitely, particularly if they are acidified. The most common contributing factor to a solution's demise is evaporation. All solutions must be stored tightly sealed (e.g., with a Parafilm gasket between the cap and the solution) and refrigerated, if possible. Samples should not be analyzed, however, until they have returned to room temperature.
It is necessary to analyze a drift solution multiple times throughout an analytical run (see "Setting Up A Typical Analytical Run"). This solution will be used to identify and correct for instrumental drift. Typical drift observed for pore waters and igneous rock analyses during Leg 189 and the Leg 189 transit is on the order of 1%-2% per hour.
There are two important considerations in selecting a drift solution. (1) The drift solution must be matrix matched to the samples and standards that are being analyzed. If the matrix of the drift solution does not match the matrix of the samples and standards, then the behavior of the ICP-AES throughout the day may differ for the two different matrices, and thus the quantified drift would not accurately reflect variations in the sample analysis. (2) There must be an adequate concentration of target elements in the drift solution so the instrument can accurately quantify the intensity of each element.
It is not necessary to know the exact concentration of each element in the drift solution; however, it is critical that the composition of this solution remain uniform throughout the run. The analyst will not use the element concentrations during data reduction but rather the relative change in response to each element. This indicates the change in the ability of the ICP-AES to consistently measure these concentrations throughout the run.
The best drift monitors are samples that have been previously analyzed and are no longer needed. Thus, such finished solutions should be combined into a larger storage container for use as drift solution in the future. Solutions that have become unstable and thus unsuitable for quantitative analysis can also be added to this drift solution, provided they have been filtered to remove any gel that has formed and that no further gels are forming. Drift solutions are also useful, for example, to supply experimental solutions for nebulizer testing, checking or adjusting instrument parameters, setting up an analytical run, and other diagnostic purposes. Because of the importance of matrix matching, it may be advisable to keep separate drift solutions for carbonate-rich sediments, mid-ocean-ridge basalts (MORBs), shaley-type sediments, arc lavas, etc. There is no need to oversegregate, however, because the dominant matrix component is LiBO2 in all cases. Finally, even in natural samples, if the concentration of any element is too low, the analyst should spike the drift solution with a single element standard of the low element so that the peak is easily detected by the ICP-AES.
Because of the limited availability of interstitial water and its great value, unused sample solutions are likely to be archived or retained by the shipboard chemists. Thus, such solutions should not be used as drift solutions.
An artificial drift solution for interstitial waters can be constructed from filtered surface seawater. Recall that the concentrations need not be exact. To construct such a drift solution, the following equation can be used:
where
For example, most single element standard solutions are supplied at a concentration of 1000 µg/mL (C2). To prepare 1 L of drift solution (V1) containing Fe at a concentration of 10 µmol (C1)(to mimic a 10× dilution of pore water that would have an undiluted concentration of 100 µmol), equation 1 is set up as follows:
Solving the equation for V2 results in 0.6 mL of Fe single-element solution to be added to prepare the 1-L drift solution. In practice, the best way to do this is to take 100 mL of filtered surface seawater, add 900 mL of DI (to yield 1 L), and then spike this solution with 0.6 mL of 1000 µg/mL Fe standard solution. Although this will result in a total volume of 1000.6 mL, the deviation from exact volume is not significant enough to affect the utility of this solution as a drift monitor (which need not be quantitative).
Equation 1 can be applied for each element commonly analyzed by ICP-AES (Li, B, Fe, Mn, Sr, and Ba), and a single multielement drift solution can be prepared. B, Li, and Sr do not need to be added to the artificial drift solution because their concentrations in seawater are high enough to be measured without an additional spike. Spiking with only the small amount of Ba indicated in Table T2 does not cause precipitation of barite (BaSO4).
Table T2 provides the calculated results (from Equation 1) for preparation of a comprehensive artificial interstitial water drift solution. A companion Excel spreadsheet to Table T2, complete with linked Excel formulas and explanatory annotations, has been provided to ODP to assist in the construction of this drift solution.
It is necessary to rinse the sample uptake tubing and torch glassware between each sample and each standard to prevent carryover from the previous sample or standard. Rinsing also helps combat nebulizer clogging. The critical parameter of constraint is that the acid concentration of the rinse should be the same as the acid concentration of the samples and standards. This ensures that the plasma is not exposed to different matrices spaced closely together, which would negatively affect instrument stability throughout the run.
For the LiBO2 matrix, a 10% HNO3 rinse solution works very well. A typical rinse time is ~1.5 min between each sample or standard. This is a minimum time; it is in the analyst's best interest to use an adequate rinse time because the analytical run will be of higher quality and there will be less buildup of deposits on the torch glassware. Longer rinse times result in improvement of (i.e., reduction of) instrumental drift throughout the run.
There is essentially no acid in the interstitial water samples, because the minimal acid amounts added during acidification of the initial squeezed sample is not enough to be of concern. However, rinsing with DI alone does not adequately keep the nebulizer orifice free of precipitating salts. A weak acid rinse (~0.5% HNO3) performs very well and does not present a sufficient matrix difference to adversely affect the analysis. Again, rinse times should be ~1.5 min between each sample and standard.
Wavelength selection is somewhat of an individual choice that commonly varies from analyst to analyst. However, there is a developing consensus regarding the wavelengths best suited for a particular target analyte. The wavelengths provided in Tables T3 and T4 have yielded good results in a variety of studies but should be considered to be suggestions only. If a particular shipboard scientist wishes to select a different wavelength that he or she has had better success with in the analysis of some type of sample, this can easily be accommodated by the JY2000 software. In the ICP-AES literature on board the ship, most of the publications suggest wavelengths appropriate for geological matrices, not only for the elements included here but also for others that a scientist may be interested in analyzing.
Appropriate wavelengths for ICP-AES analysis of rocks and sediments are listed in Table T3.
For interstitial waters, use of spectrophotometric procedures (Gieskes et al., 1991) is preferred for the analysis of P and Si, particularly for those samples with relatively low concentrations. Similarly, ion chromatography (IC) is preferred for the analysis of Ca, Mg, and K (with Na being calculated by charge balance). However, ICP-AES analyses of these elements can provide important confirmation of spectrophotometric or IC results and is also worthwhile if there are difficulties with these other instruments. Experiments conducted during Leg 189 for the major cations provided excellent results, and preliminary work during the Leg 189 transit documented that total S can potentially be analyzed by ICP-AES in interstitial waters as well.
For higher concentration elements (such as the major cations), greater dilution factors may be required so the intensity of the wavelength remains within photomultiplier range. Additionally, to conserve sample, the greatest dilution possible should be used.
In summary, routine ICP-AES analyses can be relatively easily performed on interstitial water samples for Fe, Mn, Sr, Ba, B, and Li, with other elements added or deleted as deemed appropriate by the shipboard scientists and technical staff. These six elements can be determined on a single 10× dilution, based on 0.5 mL of sample and 4.5 mL of DI. Suggested wavelengths appropriate to the analysis of interstitial waters are given in Table T4.
ICP-AES is a comparative analytical technique, in that the instrument response (measured in "counts" units) must be calibrated against standards in which the concentrations of the various elements are known. There are two main calibration methods: (1) calibration against internationally recognized and approved standard reference materials (SRMs), or (2) calibration against synthetically prepared calibration standard solutions that have been constructed to closely mimic the matrix of the unknown samples. Each method has its strengths and weaknesses.
Calibration is best achieved through comparison to SRMs in a similar manner to that used in XRF analysis; however, whereas the XRF can retain a given calibration for several months, the ICP-AES must be calibrated before each analytical run. Selection of appropriate SRMs for calibration is dependent on the anticipated range of concentrations in the samples. Unlike the XRF, which is commonly calibrated on the basis of tens of individual SRMs, for ICP-AES analysis, robust linear calibration can be achieved with five SRMs and a blank. Because LiBO2 is the dominant matrix component, SRMs of differing lithologies can be used in the same calibration. For example, a single calibration composed of shale, andesite, basalt, and obsidian SRMs yields strong linearity for many elements. It is essential that the SRMs used for calibration be prepared with exactly the same TDS and acid concentration as the samples.
Because standard calibrations are required for each run, SRMs tend to be consumed relatively rapidly. In the interest of conservation, if there are enough calibration solutions left over from a previous run, and if these solutions were prepared identically, then it is appropriate to reuse those SRM solutions until they are depleted.
It is common for laboratories to develop their own in-house calibration powders, to ease consumption of the often expensive and analytically valuable SRMs. The ODP laboratory should build a library of calibration standards using available rocks and sediments. After several preparations and analyses of these in-house samples, they can then be used for calibration purposes, as long as at least one independent certified SRM is analyzed as an unknown in each run to verify the calibration.
We recommend calibration using the SRMs abbreviated as DNC-1, BIR-1, BHVO-2, W-2, and BCR-2. Additional well-characterized rocks (although not certified SRMs) used to check calibration results include K1919 and BAS140. For other igneous rock types, the above suite of SRMs can be modified to bracket the anticipated dynamic range of the data set.
Variability in sedimentary compositions precludes use of a single set of SRMs to cover all sediment ranges. The three end-member compositions of sediments are shales, carbonates, and siliceous deposits. At this time, ODP has only a few sedimentary SRMs available. For shales, MAG-1, SCo-1, and other shales such as BCSS-1 or MESS-1, used in combination with high silica basalts will usually suffice. For carbonates, inclusion of NIST-1C (an argillaceous limestone) and other carbonate SRMs is essential. Variations in Ca are widely known to cause matrix effects, so care must be taken to select appropriate SRMs. Depending on the lithology of the targeted unknowns, a calibration using a blank (as a zero point), and two or three carbonate SRMs works moderately well. For siliceous sediments, use of rhyolite, as a high-silica standard, along with shale and basalt SRMs, works adequately.
Calibration standards for interstitial waters must be constructed by the analyst, because an extended suite of seawater SRMs does not exist. Such standards can be prepared using one of the two following similar methods. In the first (less preferred) method, a spiked International Association for the Physical Sciences of the Ocean (IAPSO) solution is used as a master standard from which serial dilutions (approximately matrix matched to seawater by diluting with the synthetic seawater of Gieskes et al. [1991]) are prepared for eventual analysis. This technique has the advantage that the concentrations of the elements in the standards are exactly known, although the matrix match is not ideal. In the rare circumstances where the surface seawater method described below is not appropriate (low salinity due to rain or river input), this IAPSO method can be used to construct standards, although it is somewhat expensive.
The second approach (recommended here) is to use filtered surface seawater as the primary matrix, spiked accordingly to create a master standard solution from which serial dilutions in seawater are prepared. Surface seawater can be assumed to have concentrations of Li = 25 µM, B = 416 µM, and Sr = 90 µM (Millero, 1996), and the analyst can easily verify by ICP-AES that the concentrations of Fe, Mn, and Ba (found in nanomolar concentrations in surface seawater [Millero, 1996]) are essentially zero. If dissolved silica will be measured by ICP-AES, the concentration in the surface water standard can be determined spectrometrically. This technique using surface seawater to construct the intermediate standard has the advantage that the matrix match is very robust but has the disadvantage that the initial concentrations of Li, B, and Sr (given above) may not be precise. However, in practice, this limitation is deemed to be minimal and can be easily verified using standard additions. The IAPSO solution can also be used as an appropriate check of accuracy.
For the preparation of these standards, we have provided ODP with a detailed series of numbered steps and explanations in spreadsheet form. Using the linked Excel formulas throughout the spreadsheet, the analyst can vary a number of parameters to construct different volumes and concentrations as needed.