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Some project-specific or regulatory-specific requirements or data quality objectives may require Tentatively Identified Compound (TIC) data to be reported for volatile and/or semi-volatile analyses using gas chromatography mass spectrometry (GC/MS) or liquid chromatography mass spectrometry (LC/MS) analytical techniques.  

 

What are TICs?

 

For GC/MS and LC/MS analytical methods, qualitative identification of each target compound is based upon a comparison and match of the retention time and mass spectrum of the compound in the sample to the retention time and mass spectrum of the compound in the calibration standards run on the instrument. Additionally, the concentration of target compounds in samples is determined by the relative instrument response of the compound in the sample compared to the response of the compound in the associated calibration standards.

 

TICs are compounds contained in a sample that are not target compounds or spike compounds added to a sample (e.g., internal standards, deuterated monitoring compounds, or surrogates). Identification of TICs is based upon search of the TIC’s mass spectrum against a mass spectral library available in the instrument’s software as TICs are not contained in the calibration standards. Because TICs are not contained in the calibration standards, the calculation of the concentration of a TIC is an estimate. TICs are often qualified by data validators as “NJ”, with the definition in the National Functional Guidelines for Organic Data Review as “The analyte has been ‘tentatively identified’ or ‘presumptively’ as present and the associated numerical value is the estimated concentration in the sample.”

 

How to Handle TICs in EQuIS

 

Target compound and TIC data are stored in the DT_RESULT table in EQuIS. The main differences between reporting target compound results and TIC results in EQuIS relates to population of the following fields:

•RESULT_TYPE_CODE

•CHEMICAL_NAME

•CAS_RN

•TIC_RETENTION_TIME

 

To easily discern TIC data in EQuIS, create a record in the RT_RESULT_TYPE table with a value of “TIC” in the RESULT_TYPE_CODE field. All TIC data uploaded to EQuIS will have the DT_RESULT.RESULT_TYPE_CODE value set to “TIC”.

 

Additionally, for each TIC, a retention time will be reported from the laboratory. The retention time value is stored in the DT_RESULT. TIC_RETENTION_TIME field. For standard EQuIS formats, the TIC_RETENTION_TIME field is an 8-character text field that is expecting a time pattern of hh:mm:ss.

 

TIC data can present a challenge when it comes to reporting a chemical name and a CAS_RN as TICs often do not have a mass spectral library search match to a known compound. As a best practice for naming TICs, apply a cascade approach. The goal is to name the TIC to the extent possible.

•Identify TIC to analyte name (if possible).

•If cannot identify analyte, identify compound class (e.g., isomer or functional group)

•If cannot identify analyte name or class, then mark as unknown.

 

Where the exact CAS_RN cannot be determined for a TIC, use the TIC name to populate the CAS_RN field. In a sample that contains multiple TICs with the same name (e.g., Unknown), append the name with a number to provide uniqueness for each CAS_RN (e.g., Unknown1). The CAS_RN field in EQuIS is constrained to 15 characters so abbreviations may be required.

 

For unknown TICs, CAS_RN values can be added to the RT_ANALYTE table based on the class of compounds. The following table provides example nomenclature for naming unknown TICs, assuming a limit of 10 TICs in each category.

 

PAH = Polycyclic aromatic hydrocarbon

VOA = Volatile organic analyte

SV = Semi-volatile compound

 

When using the class of compounds to create a CAS_RN for unknown TICs, a TIC with the same CAS_RN in 2 different samples should not be treated as the same compound. For example, Unknown01 in sample 1 may not be the same compound as Unknown01 in sample 2. To determine if Unknown01 is the same in both samples, and assuming that sample 1 and sample 2 were analyzed on the same analytical instrumentation within the same general timeframe (e.g., 12-hour mass spectral tune period), an evaluation and comparison of the mass spectra and retention times of each TIC would need to be performed.

 

As an alternative to making use of the TIC name to populate the CAS_RN field, the TIC’s retention time may be used in the CAS_RN field to identify individual TICs as long as the retention time of every TIC in a sample for a given method of analysis is unique. This alternative still requires population of the RT_ANALYTE table with each CAS_RN value and will likely result in a larger number of valid values to be entered in the RT_ANALYTE table than if the TIC naming approach is used.

 

TIC data maybe uploaded with standard EQuIS formats (EQEDD, EZEDD, and EFWEDD) or any custom format configured to support TICs.