Protocol for freshwater and saltwater stable isotope analysis on Picarro

Figure 1: Typical layout of workspace during the preparation of a run with sample list, labels, sample vials, GC vials, pipette tips, pipette, sample tray, and waste area (from left to right).

Project preparations

1. Customer contacts FARLAB member with a price inquiry, measurement capability.
2. The customer fills out the registration form to provide contact and sample information.
3. For water isotope analyses, HS or PTM approve the analysis request,
4. Register project in FARLAB database (http://echo.geo.uib.no:8085),
5. Prefill and rename sample list (File:water_isotopes_sample_list_template.xlsx.zip) and send it to the customer.

Sample handling

1. Receive completed sample list and samples from customer
2. Label samples with name of customer and FARLAB project ID, and place in fridge
3. Include FARLAB IDs in sample list and check provided information
4. Print out sample list.
5. Print labels for small vials using Excel form: File:Mal merkelapper_xxxx-xx-HS.xlsx.zip

Preparing runs/batches

1. Prepare sample containers and corresponding labelled 1.5 mL GC vials (see Fig. 1) such that chances for mix-up are minimised
2. When samples do not need to be filtered, use 1000 uL pipette to transfer 1ml of water to the GC vial. Change pipette after each sample. For clean samples (snow) it can be acceptable to reuse tips after complete drying. Vials should be filled at max below neck level. Tighten vial caps only slightly to prevent damage of the septum.
3. When filtering of samples is required (groundwater, runoff, visible contaminations), use a syringe to pull liquid, place single-use Nylon filter on syringe tip, and transfer water to vial through the filter. Discard filter after use.
4. Include two calibration standards and one drift standard in each run. One vial of the drift will be with the calibration standards, and an additional one for every 7-9 samples included in the run.
5. Prepare standards from bottles in FARLAB fridge with 0.5 to 1.0 mL of standard. The range of standards depends on the samples. A too close proximity of the standards will give more uncertainty during calibration. The drift standard should be either DI2 (precipitation, runoff) or BERM (sea water analysis). Turn shake glass bottles with standards lightly to mix in any condensate on the bottle walls before pipetting. After each use, consider to refill bottles to marking from standard water tanks in the gas room, using the nozzle on each tank.
6. After vials have warmed to room temperature (15 min), briefly open the cap to allow for pressure equilibration in the vial. This prevents humidity variations during injections.

Running analysis for a batch

Create sample_description_<FARLAB_ID>_sample_description_runxx.csv

The sample description is a simple text file in csv format with columns indicated below, and using comma for separation. It is most convenient to create such files with OpenOffice and to export as csv. The first indicator is the name of the sample provided by the customer, indicator2 is the FARLAB ID. In case of standards, the name is the name of the lab standard, and the indicator is composed of standard_<date of filling vial>. This allows to control the reuse of vials. Example:

tray, vial, indicator1, indicator2
1,1, GLW, standard_20200201
1,2, DI2, standard_20200201
1,3, EVAP2, standard_20200201
1,4, RTG_01, 2020-01-HS-001
1,5, RTG_02, 2020-01-HS-002

The sample description is loaded with the coordinator button "load sample description" at any time during the run (see below).

Log run details in instrument logfile (Picarro-L2140i-HKDS2038/9.txt) using Notepad++

Each analyzer has an electronic log file that is placed in the directory C:\IsotopeData. The name of the file starts with the instrument type, serial number and ends in "log", for example Picarro-L2140i-HKDS2039-log.txt

Every new log entry starts with information of date (YYYY-MM-DD) and a two-letter identifier of the author in brackets, followed by the time of the entry in UTC time. When several significant entries are made on the same day with a time gap in-between, only the time needs to be stated. For example:

2019-02-01 (HS)
12:00 UTC

Upon preparation and starting of a run, the following details are noted:

• Short description of samples, FARLAB project
• Job method (FARLAB_freshwater/saltwater with sample volume, rinses)
• Mode (High prec double wet peak, 17O, High throughput,...)
• Number of samples
• Sample description file name
• Coordinator file name
• Sequence of samples. This is a summary listing the sequence of samples and standards in which they are sampled. This sequence can be complete, but can also be limited to providing information on the number of injections for standards, and a bulk sequence (e.g. 10 injection for vial 4 - 20, 16 injections for vial 3,2,1)

Upon completion of the run, the result (success/failure) and possible comments should be noted. Comments include change of septum, cleaning and change of syringe, and other relevant notes. Example:

31.08.2018 (HS)
running samples for 2818-21 Poo the Bear
Job method:  FARLAB_freshwater_1dot80 (sample volume 1.8 uL, 3 pre rinse only between vials, 1 fill stroke)
Mode: High Prec double wet peak
Filename: HKDS2038_IsoWater_20180831_095233.csv
Number of samples: 20 vials
Sample description file: PTM/2018_21_Poo_180828_21-40.csv
Filename: HKDS2039_IsoWater_20180621_112436.csv
Sequence:
DI(23.05.18),SEAII,EVAP ,Sea old(11.09.17),
2018-21-001,002,003,004,005
Sea old
006,007,008,009,010
Sea old
011,012,013,014.015
Sea old
016,017,018,019,020
DI,SEAII,EVAP,Sea old

1. 1. Test injection peaks 3x and adjust syringe volume
   2. Start run
   3. Monitor run from Remote Desktop/lab
   4. Check for humidity variations outside 15’000-25’000 ppmv
   5. Clean syringe, exhange septum if needed
   6. Recap/store or discard sample vials
   7. Update sample list
2. Calibrate runs with FLIIMP
3. Upload sample results to FARLAB database
4. Repack and return/archive/discard samples

Choosing the range of standards

Depending on the known or expected range of the samples, the calibration standards need to be chosen. The calibration standards should bracket the range of samples as close as possible. If the samples are processed in several batches from different sources (groundwater/snow/rain), the calibration standards may be adjusted for each run.

Typical combinations are:

• EVAP/VATS for mid-latitude precipitation samples
• DI/GSM1 or DI/VATS for snow
• EVAP/VATS for lake or stream water

DI2 or SEAII should always be included as the long-term drift standard of the laboratory. The sequence of the standards should be such that the jump to the first sample is minimised. For example, when measuring snow samples, the sequence would be EVAP2-DI2-GLW-Sample1 etc.

Duplicate samples and standards

Memory is a challenge for all liquid measurements. Standards are measured with 12 injections, with 2 preceeding double-peak wet flushes, requiring in total 16 injections. Samples in contrast are commonly run with 6 effective injections, i.e. 10 injections with a double-peak wet flush. The isotope composition between the samples of one run typically varies less than the jump from the last standard and the first sample. Therefore, sample 1 is duplicated in the run, and discarded in the analysis.

The drift standard is introduced after 10 samples in the run sequence, and measured as if it were a sample. Memory correction routines in FLIIMP will minimize memory from previous injections, and thus no duplication of sample vials is required. The resulting setup of the run is shown in Fig. 2.