Compound Data Subsetting By Threshold

compound-data-subsetting-by-threshold

Summary

This skill filters a compound list from GC-MS data by applying quantitative quality thresholds (e.g., Match.Factor ≥65, ≥80, ≥90) to retain only compounds meeting specified matching criteria. It is essential for reducing false positives and focusing analysis on high-confidence chemical identifications.

When to use

Use this skill when you have a GC-MS dataset with a Match.Factor column (or equivalent quality metric) and need to evaluate how many unique compounds are retained at different confidence thresholds, or when you must subset the compound list to a user-defined quality level before downstream cheminformatics analysis.

When NOT to use

• Input data lacks a Match.Factor or equivalent quality/confidence column.
• Analysis goal is to retain ALL compounds regardless of quality score (no filtering needed).
• Compound identifications are already validated by orthogonal methods and do not require further quality-based subsetting.

Inputs

• GC-MS data CSV file with columns: Compound.Name, Match.Factor (and optionally Component.RT, Base.Peak.MZ, Component.Area, File.Name)
• Threshold specification(s) (numeric values, e.g., 65, 80, 90)

Outputs

• Filtered unique compound name vector(s) for each threshold
• Summary table (rows = threshold levels; columns = threshold value, unique compound count)
• CSV file containing the comparison table

How to apply

Load the GC-MS CSV file (with required columns including 'Compound.Name' and 'Match.Factor') into R. Apply independent logical filters at each threshold level (e.g., Match.Factor >= 65, >= 80, >= 90), extracting the unique compound names that meet each condition. Count unique compounds retained under each threshold to quantify the trade-off between stringency and dataset size. The rationale is that higher Match.Factor thresholds reduce chemical identifications to high-confidence matches, but may exclude valid compounds; comparing counts across thresholds helps users choose an appropriate cutoff for their analysis goals. Construct a summary table documenting threshold value and compound count for each condition, and export as CSV for reporting.

Related tools

• R (Primary language for loading CSV, applying logical filters, counting unique compounds, and constructing summary table)
• uafR (R package providing spreadOut() function to prepare raw GC-MS CSV for intelligent sorting and downstream filtering operations) — https://github.com/castratton/uafR
• Agilent Unknowns Analysis (Recommended software to generate the GC-MS data in the correct default format with required column names (Compound.Name, Match.Factor))

Examples

query_chemicals_mf65 <- input_dat$Compound.Name[input_dat$Match.Factor >= 65]; query_chemicals_mf80 <- input_dat$Compound.Name[input_dat$Match.Factor >= 80]; query_chemicals_mf90 <- input_dat$Compound.Name[input_dat$Match.Factor >= 90]; summary_table <- data.frame(Threshold = c('>=65', '>=80', '>=90'), Compound_Count = c(length(unique(query_chemicals_mf65)), length(unique(query_chemicals_mf80)), length(unique(query_chemicals_mf90)))); write.csv(summary_table, 'threshold_comparison.csv', row.names = FALSE)

Evaluation signals

• Verify that the output compound list contains only unique Compound.Name values (no duplicates).
• Check that compound counts are monotonically non-increasing as threshold increases (e.g., count at ≥90 ≤ count at ≥80 ≤ count at ≥65).
• Confirm that all compounds in the ≥90 subset also appear in the ≥80 and ≥65 subsets (proper nesting).
• Validate that the summary table has exactly 3 rows (one per threshold) and 2 columns (threshold value and compound count).
• Spot-check a sample of retained compounds to confirm their Match.Factor values meet or exceed the declared threshold.

Limitations

• The skill assumes Match.Factor is the only quality metric; compounds with high Match.Factor may still represent false positives if other metrics (e.g., retention time, peak area) are anomalous.
• Optimal threshold values (65, 80, 90) are dataset- and application-dependent; no universal cutoff is recommended by the paper.
• The skill does not account for isotope patterns, fragmentation consistency, or database cross-validation, which may be necessary for rigorous compound identification.
• If the input CSV has missing or invalid Match.Factor values, they will be excluded silently; no imputation or handling is specified.

Evidence

• [other] Match.Factor filtering can be applied to standard_data.csv at multiple threshold levels (≥65, ≥80, ≥90) to subset the compound list based on matching quality criteria.: "Match.Factor filtering can be applied to standard_data.csv at multiple threshold levels (≥65, ≥80, ≥90) to subset the compound list"
• [other] Load standard_data.csv into R and extract the Compound.Name and Match.Factor columns. Apply three independent filters: Match.Factor >= 65, Match.Factor >= 80, and Match.Factor >= 90, retaining unique compound names for each filter.: "Load standard_data.csv into R and extract the Compound.Name and Match.Factor columns. Apply three independent filters: Match.Factor >= 65, Match.Factor >= 80, and Match.Factor >= 90"
• [readme] The input .CSV file has strict column name/input data requirements. The column names MUST include: 'Component.RT', 'Component.Area', 'Base.Peak.MZ', 'File.Name', 'Compound.Name', and 'Match.Factor' in no particular order.: "The column names MUST include: 'Component.RT', 'Component.Area', 'Base.Peak.MZ', 'File.Name', 'Compound.Name', and 'Match.Factor'"
• [readme] query_chemicals = input_dat$Compound.Name[input_dat$Match.Factor > 80]: "query_chemicals = input_dat$Compound.Name[input_dat$Match.Factor > 80]"
• [readme] spreadOut() prepares the read in .CSV for intelligent sorting (using retention times and published masses): "spreadOut() prepares the read in .CSV for intelligent sorting (using retention times and published masses)"