Novel NIST Hybrid Search for Unknown Identifications

Extending EI and MS/MS Library Searching for Unknown Identification

Introduction

The NIST Hybrid Search is a powerful approach for identifying compounds that are not present as exact matches in a searched mass spectral library. Rather than relying only on direct peak-to-peak spectral matching, Hybrid Search combines two complementary comparisons:

matching of common fragment ions; and
matching of neutral losses after accounting for the mass difference between the unknown and the library compound.

This combined approach can reveal a closely related compound—or “nearest neighbor”—that may not appear among the best results from a conventional identity or similarity search. Hybrid Search can be applied to both EI and MS/MS tandem mass spectra, extending the practical coverage of NIST, commercial, crowd-sourced, and user-created libraries.

How Hybrid Search Works

The mass difference between the unknown and a library candidate is reported as the DeltaMass. This value frequently represents a recognizable structural modification, such as the addition, removal, or replacement of an atom or functional group.

Hybrid Search works best when:

a structurally similar compound is present in the library;
the structural difference is confined primarily to one region of the molecule; and
the modification does not substantially alter the overall fragmentation behavior.

For EI Hybrid Search, a reliable nominal molecular mass must be supplied or estimated. For MS/MS Hybrid Search, the precursor mass provides the corresponding reference point.

Many fragment ions remain unchanged because they originate from portions of the two molecules that are structurally identical. Other ions are shifted by 18 mass units because they retain the modified region. Hybrid Search combines both types of information, producing a strong match between the related compounds even though they are not identical.

The Excel DeltaMass Table

Interpreting the DeltaMass value is often the key step in converting a Hybrid Search result into a useful structural proposal. I created an Excel DeltaMass Table after reviewing more than 50,000 EI library spectra and observing recurring mass differences among related compounds.

The table lists common structural changes associated with specific DeltaMass values. Although it was originally developed through extensive use of EI Hybrid Search, it is also useful for interpreting MS/MS Hybrid Search results.

Accurate-mass measurements can be especially valuable when several elemental or structural changes have the same nominal DeltaMass. Isotope patterns, fragment accurate masses, precursor or molecular mass, and chemical context should therefore be used whenever available.

An odd nominal DeltaMass can also provide a useful clue that the number of nitrogen atoms differs by an odd number, analogous to the nitrogen rule used in EI mass spectral interpretation.

DeltaMass Tables

Excel DeltaMass Table
PDF DeltaMass Table

Practical Interpretation

A Hybrid Search result should be treated as a structural clue rather than stand-alone confirmation. The most reliable identifications are obtained by combining:

the best Hybrid Search candidates;
results from conventional identity or similarity searches;
the observed DeltaMass;
direct and shifted fragment-ion matches;
accurate-mass and isotope information;
the proposed molecular or precursor mass;
sample history and analytical context; and
complementary structure and database searches.

In many cases, the Hybrid Search suggests a related library compound while the DeltaMass indicates the modification needed to arrive at the unknown structure. The analyst must then determine whether that proposed change is chemically and spectrally reasonable.

Novel NIST Hybrid Search for Unknown Identifications

Extending EI and MS/MS Library Searching for Unknown Identification

Introduction

How Hybrid Search Works

The Excel DeltaMass Table

DeltaMass Tables

Practical Interpretation

Short Demonstration

Detailed Videos

Associated Handouts

Supporting Software Courses

NIST Publications Hybrid Search