# Annotation of MS-based Metabolomics Data

#### 1 November 2022

#### Package

MetaboAnnotation 1.2.0

Package: MetaboAnnotation
Authors: Michael Witting [aut] (https://orcid.org/0000-0002-1462-4426), Johannes Rainer [aut, cre] (https://orcid.org/0000-0002-6977-7147), Andrea Vicini [aut] (https://orcid.org/0000-0001-9438-6909), Carolin Huber [aut] (https://orcid.org/0000-0002-9355-8948), Nir Shachaf [ctb]
Compiled: Tue Nov 1 21:17:02 2022

# 1 Introduction

The `MetaboAnnotation` package defines high-level user functionality to support and facilitate annotation of MS-based metabolomics data (Rainer et al. 2022).

# 2 Installation

The package can be installed with the `BiocManager` package. To install `BiocManager` use `install.packages("BiocManager")` and, after that, `BiocManager::install("MetaboAnnotation")` to install this package.

# 3 General description

`MetaboAnnotation` provides a set of matching functions that allow comparison (and matching) between query and target entities. These entities can be chemical formulas, numeric values (e.g. m/z or retention times) or fragment spectra. The available matching functions are:

- `matchFormula`: to match chemical formulas.
- `matchSpectra`: to match fragment spectra.
- `matchValues` (formerly `matchMz`): to match numerical values (m/z, masses, retention times etc).

For each of these matching functions parameter objects are available that allow different types or matching algorithms. Refer to the help pages for a detailed listing of these (e.g. `?matchFormula`, `?matchSpectra` or `?matchValues`). As a result, a `Matched` (or `MatchedSpectra`) object is returned which streamlines and simplifies handling of the potential one-to-many (or one-to-none) matching.

# 4 Example use cases

The following sections illustrate example use cases of the functionality provided by the `MetaboAnnotation` package.

    library(MetaboAnnotation)

## 4.1 Matching of m/z values

In this section a simple matching of feature m/z values against theoretical m/z values is performed. This is the lowest level of confidence in metabolite annotation. However, it gives ideas about potential metabolites that can be analyzed in further downstream experiments and analyses.

The following example loads the feature table from a lipidomics experiments and matches the measured m/z values against reference masses from LipidMaps. Below we use a `data.frame` as reference database, but a `CompDb` compound database instance would also be supported.

    ms1_features <- read.table(system.file("extdata", "MS1_example.txt",
                                           package = "MetaboAnnotation"),
                               header = TRUE, sep = "\t")
    head(ms1_features)

    ##     feature_id       mz    rtime
    ## 1 Cluster_0001 102.1281 1.560147
    ## 2 Cluster_0002 102.1279 2.153590
    ## 3 Cluster_0003 102.1281 2.925570
    ## 4 Cluster_0004 102.1281 3.419617
    ## 5 Cluster_0005 102.1270 5.801039
    ## 6 Cluster_0006 102.1230 8.137535

    target_df <- read.table(system.file("extdata", "LipidMaps_CompDB.txt",
                                        package = "MetaboAnnotation"),
                            header = TRUE, sep = "\t")
    head(target_df)

    ##   headgroup        name exactmass    formula chain_type
    ## 1       NAE  NAE 20:4;O  363.2773  C22H37NO3       even
    ## 2       NAT  NAT 20:4;O  427.2392 C22H37NO5S       even
    ## 3       NAE NAE 20:3;O2  381.2879  C22H39NO4       even
    ## 4       NAE    NAE 20:4  347.2824  C22H37NO2       even
    ## 5       NAE    NAE 18:2  323.2824  C20H37NO2       even
    ## 6       NAE    NAE 18:3  321.2668  C20H35NO2       even

For reference (target) compounds we have only the mass available. We need to convert this mass to m/z values in order to match the m/z values from the features (i.e. the query m/z values) against them. For this we need to define the most likely ions/adducts that would be generated from the compounds based on the ionization used in the experiment. We assume the most abundant adducts from the compounds being `"[M+H]+"` and `"[M+Na]+`. We next perform the matching with the `matchValues` function providing the query and target data as well as a parameter object (in our case a `Mass2MzParam`) with the settings for the matching. With the `Mass2MzParam`, the mass or target compounds get first converted to m/z values, based on the defined adducts, and these are then matched against the query m/z values (i.e. the m/z values for the features). To get a full list of supported adducts the `MetaboCoreUtils::adductNames(polarity = "positive")` or `MetaboCoreUtils::adductNames(polarity = "negative")` can be used). Note also, to keep the runtime of this vignette short, we match only the first 100 features.

    parm <- Mass2MzParam(adducts = c("[M+H]+", "[M+Na]+"),
                               tolerance = 0.005, ppm = 0)

    matched_features <- matchValues(ms1_features[1:100, ], target_df, parm)
    matched_features

    ## Object of class Matched
    ## Total number of matches: 55
    ## Number of query objects: 100 (55 matched)
    ## Number of target objects: 57599 (1 matched)

From the tested 100 features 55 were matched against at least one target compound (all matches are against a single compound). The result object (of type `Matched`) contains the full query data frame and target data frames as well as the matching information. We can access the original query data with `query` and the original target data with `target` function:

    head(query(matched_features))

    ##     feature_id       mz    rtime
    ## 1 Cluster_0001 102.1281 1.560147
    ## 2 Cluster_0002 102.1279 2.153590
    ## 3 Cluster_0003 102.1281 2.925570
    ## 4 Cluster_0004 102.1281 3.419617
    ## 5 Cluster_0005 102.1270 5.801039
    ## 6 Cluster_0006 102.1230 8.137535

    head(target(matched_features))

    ##   headgroup        name exactmass    formula chain_type
    ## 1       NAE  NAE 20:4;O  363.2773  C22H37NO3       even
    ## 2       NAT  NAT 20:4;O  427.2392 C22H37NO5S       even
    ## 3       NAE NAE 20:3;O2  381.2879  C22H39NO4       even
    ## 4       NAE    NAE 20:4  347.2824  C22H37NO2       even
    ## 5       NAE    NAE 18:2  323.2824  C20H37NO2       even
    ## 6       NAE    NAE 18:3  321.2668  C20H35NO2       even

Functions `whichQuery` and `whichTarget` can be used to identify the rows in the query and target data that could be matched:

    whichQuery(matched_features)

    ##  [1]  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64
    ## [20]  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83
    ## [39]  84  85  86  87  88  89  90  91  92  93  94  95  96  97  98  99 100

    whichTarget(matched_features)

    ## [1] 3149

The `colnames` function can be used to evaluate which variables/columns are available in the `Matched` object.

    colnames(matched_features)

    ##  [1] "feature_id"        "mz"                "rtime"
    ##  [4] "target_headgroup"  "target_name"       "target_exactmass"
    ##  [7] "target_formula"    "target_chain_type" "adduct"
    ## [10] "score"             "ppm_error"

These are all columns from the `query`, all columns from the `target` (the prefix `"target_"` is added to the original column names in `target`) and information on the matching result (in this case columns `"adduct"`, `"score"` and `"ppm_error"`).

We can extract the full matching table with `matchedData`. This returns a `DataFrame` with all rows in query the corresponding matches in target along with the matching adduct (column `"adduct"`) and the difference in m/z (column `"score"` for absolute differences and `"ppm_error"` for the m/z relative differences). Note that if a row in query matches multiple elements in target, this row will be duplicated in the `DataFrame` returned by `data`. For rows that can not be matched `NA` values are reported.

    matchedData(matched_features)

    ## DataFrame with 100 rows and 11 columns
    ##        feature_id        mz     rtime target_headgroup target_name
    ##       <character> <numeric> <numeric>      <character> <character>
    ## 1   Cluster_00...   102.128   1.56015               NA          NA
    ## 2   Cluster_00...   102.128   2.15359               NA          NA
    ## 3   Cluster_00...   102.128   2.92557               NA          NA
    ## 4   Cluster_00...   102.128   3.41962               NA          NA
    ## 5   Cluster_00...   102.127   5.80104               NA          NA
    ## ...           ...       ...       ...              ...         ...
    ## 96  Cluster_00...   201.113   11.2722               FA  FA 10:2;O2
    ## 97  Cluster_00...   201.113   11.4081               FA  FA 10:2;O2
    ## 98  Cluster_00...   201.113   11.4760               FA  FA 10:2;O2
    ## 99  Cluster_00...   201.114   11.5652               FA  FA 10:2;O2
    ## 100 Cluster_01...   201.114   11.7752               FA  FA 10:2;O2
    ##     target_exactmass target_formula target_chain_type      adduct     score
    ##            <numeric>    <character>       <character> <character> <numeric>
    ## 1                 NA             NA                NA          NA        NA
    ## 2                 NA             NA                NA          NA        NA
    ## 3                 NA             NA                NA          NA        NA
    ## 4                 NA             NA                NA          NA        NA
    ## 5                 NA             NA                NA          NA        NA
    ## ...              ...            ...               ...         ...       ...
    ## 96           200.105       C10H16O4              even      [M+H]+ 0.0007312
    ## 97           200.105       C10H16O4              even      [M+H]+ 0.0005444
    ## 98           200.105       C10H16O4              even      [M+H]+ 0.0005328
    ## 99           200.105       C10H16O4              even      [M+H]+ 0.0014619
    ## 100          200.105       C10H16O4              even      [M+H]+ 0.0020342
    ##     ppm_error
    ##     <numeric>
    ## 1          NA
    ## 2          NA
    ## 3          NA
    ## 4          NA
    ## 5          NA
    ## ...       ...
    ## 96    3.63578
    ## 97    2.70695
    ## 98    2.64927
    ## 99    7.26908
    ## 100  10.11476

Individual columns can be simply extracted with the `$` operator:

    matched_features$target_name

    ##   [1] NA           NA           NA           NA           NA
    ##   [6] NA           NA           NA           NA           NA
    ##  [11] NA           NA           NA           NA           NA
    ##  [16] NA           NA           NA           NA           NA
    ##  [21] NA           NA           NA           NA           NA
    ##  [26] NA           NA           NA           NA           NA
    ##  [31] NA           NA           NA           NA           NA
    ##  [36] NA           NA           NA           NA           NA
    ##  [41] NA           NA           NA           NA           NA
    ##  [46] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [51] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [56] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [61] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [66] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [71] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [76] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [81] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [86] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [91] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"
    ##  [96] "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2" "FA 10:2;O2"

`NA` is reported for query entries for which no match was found. See also the help page for `?Matched` for more details and information. In addition to the matching of query m/z against target exact masses as described above it would also be possible to match directly query m/z against target m/z values by using the `MzParam` instead of the `Mass2MzParam`.

## 4.2 Matching of m/z and retention time values

If expected retention time values were available for the target compounds, an annotation with higher confidence could be performed with `matchValues` and a `Mass2MzRtParam` parameter object. To illustrate this we randomly assign retention times from query features to the target compounds adding also 2 seconds difference. In a real use case the target `data.frame` would contain masses (or m/z values) for standards along with the retention times when ions of these standards were measured on the same LC-MS setup from which the query data derives.

Below we subset our data table with the MS1 features to the first 100 rows (to keep the runtime of the vignette short).

    ms1_subset <- ms1_features[1:100, ]
    head(ms1_subset)

    ##     feature_id       mz    rtime
    ## 1 Cluster_0001 102.1281 1.560147
    ## 2 Cluster_0002 102.1279 2.153590
    ## 3 Cluster_0003 102.1281 2.925570
    ## 4 Cluster_0004 102.1281 3.419617
    ## 5 Cluster_0005 102.1270 5.801039
    ## 6 Cluster_0006 102.1230 8.137535

The table contains thus retention times of the features in a column named `"rtime"`.

Next we randomly assign retention times of the features to compounds in our target data adding a deviation of 2 seconds. As described above, in a real use case retention times are supposed to be determined by measuring the compounds with the same LC-MS setup.

    set.seed(123)
    target_df$rtime <- sample(ms1_subset$rtime,
                              nrow(target_df), replace = TRUE) + 2

We have now retention times available for both the query and the target data and can thus perform a matching based on m/z and retention times. We use the `Mass2MzRtParam` which allows us to specify (as for the `Mass2MzParam`) the expected adducts, the maximal acceptable m/z relative and absolute deviation as well as the maximal acceptable (absolute) difference in retention times. We use the settings from the previous section and allow a difference of 10 seconds in retention times. The retention times are provided in columns named `"rtime"` which is different from the default (`"rt"`). We thus specify the name of the column containing the retention times with parameter `rtColname`.

    parm <- Mass2MzRtParam(adducts = c("[M+H]+", "[M+Na]+"),
                           tolerance = 0.005, ppm = 0,
                           toleranceRt = 10)
    matched_features <- matchValues(ms1_subset, target_df, param = parm,
                                    rtColname = "rtime")
    matched_features

    ## Object of class Matched
    ## Total number of matches: 31
    ## Number of query objects: 100 (31 matched)
    ## Number of target objects: 57599 (1 matched)

Less features were matched based on m/z and retention times.

    matchedData(matched_features)[whichQuery(matched_features), ]

    ## DataFrame with 31 rows and 13 columns
    ##        feature_id        mz     rtime target_headgroup target_name
    ##       <character> <numeric> <numeric>      <character> <character>
    ## 1   Cluster_00...   201.113   5.87206               FA  FA 10:2;O2
    ## 2   Cluster_00...   201.113   5.93346               FA  FA 10:2;O2
    ## 3   Cluster_00...   201.113   6.03653               FA  FA 10:2;O2
    ## 4   Cluster_00...   201.114   6.16709               FA  FA 10:2;O2
    ## 5   Cluster_00...   201.113   6.31781               FA  FA 10:2;O2
    ## ...           ...       ...       ...              ...         ...
    ## 27  Cluster_00...   201.113   11.2722               FA  FA 10:2;O2
    ## 28  Cluster_00...   201.113   11.4081               FA  FA 10:2;O2
    ## 29  Cluster_00...   201.113   11.4760               FA  FA 10:2;O2
    ## 30  Cluster_00...   201.114   11.5652               FA  FA 10:2;O2
    ## 31  Cluster_01...   201.114   11.7752               FA  FA 10:2;O2
    ##     target_exactmass target_formula target_chain_type target_rtime      adduct
    ##            <numeric>    <character>       <character>    <numeric> <character>
    ## 1            200.105       C10H16O4              even      15.8624      [M+H]+
    ## 2            200.105       C10H16O4              even      15.8624      [M+H]+
    ## 3            200.105       C10H16O4              even      15.8624      [M+H]+
    ## 4            200.105       C10H16O4              even      15.8624      [M+H]+
    ## 5            200.105       C10H16O4              even      15.8624      [M+H]+
    ## ...              ...            ...               ...          ...         ...
    ## 27           200.105       C10H16O4              even      15.8624      [M+H]+
    ## 28           200.105       C10H16O4              even      15.8624      [M+H]+
    ## 29           200.105       C10H16O4              even      15.8624      [M+H]+
    ## 30           200.105       C10H16O4              even      15.8624      [M+H]+
    ## 31           200.105       C10H16O4              even      15.8624      [M+H]+
    ##         score ppm_error  score_rt
    ##     <numeric> <numeric> <numeric>
    ## 1   0.0004538   2.25645  -9.99030
    ## 2   0.0004407   2.19131  -9.92890
    ## 3   0.0005655   2.81186  -9.82583
    ## 4   0.0015560   7.73698  -9.69527
    ## 5   0.0006845   3.40357  -9.54455
    ## ...       ...       ...       ...
    ## 27  0.0007312   3.63578  -4.59014
    ## 28  0.0005444   2.70695  -4.45431
    ## 29  0.0005328   2.64927  -4.38634
    ## 30  0.0014619   7.26908  -4.29719
    ## 31  0.0020342  10.11476  -4.08719

## 4.3 Matching of `SummarizedExperiment` or `QFeatures` objects

Results from LC-MS preprocessing (e.g. by the xcms package) or generally metabolomics results might be best represented and bundled as `SummarizedExperiment` or `QFeatures` objects (from the same-named Bioconductor packages). A `XCMSnExp` preprocessing result from `xcms` can for example be converted to a `SummarizedExperiment` using the `quantify` method from the `xcms` package. The feature definitions (i.e. their m/z and retention time values) will then be stored in the object’s `rowData` while the assay (the numerical matrix) will contain the feature abundances across all samples. Such `SummarizedExperiment` objects can be simply passed as `query` objects to the `matchValues` method. To illustrate this, we create below a simple `SummarizedExperiment` using the `ms1_features` data frame from the example above as `rowData` and adding a `matrix` with random values as assay.

    library(SummarizedExperiment)

    ## Loading required package: MatrixGenerics

    ## Loading required package: matrixStats

    ##
    ## Attaching package: 'MatrixGenerics'

    ## The following objects are masked from 'package:matrixStats':
    ##
    ##     colAlls, colAnyNAs, colAnys, colAvgsPerRowSet, colCollapse,
    ##     colCounts, colCummaxs, colCummins, colCumprods, colCumsums,
    ##     colDiffs, colIQRDiffs, colIQRs, colLogSumExps, colMadDiffs,
    ##     colMads, colMaxs, colMeans2, colMedians, colMins, colOrderStats,
    ##     colProds, colQuantiles, colRanges, colRanks, colSdDiffs, colSds,
    ##     colSums2, colTabulates, colVarDiffs, colVars, colWeightedMads,
    ##     colWeightedMeans, colWeightedMedians, colWeightedSds,
    ##     colWeightedVars, rowAlls, rowAnyNAs, rowAnys, rowAvgsPerColSet,
    ##     rowCollapse, rowCounts, rowCummaxs, rowCummins, rowCumprods,
    ##     rowCumsums, rowDiffs, rowIQRDiffs, rowIQRs, rowLogSumExps,
    ##     rowMadDiffs, rowMads, rowMaxs, rowMeans2, rowMedians, rowMins,
    ##     rowOrderStats, rowProds, rowQuantiles, rowRanges, rowRanks,
    ##     rowSdDiffs, rowSds, rowSums2, rowTabulates, rowVarDiffs, rowVars,
    ##     rowWeightedMads, rowWeightedMeans, rowWeightedMedians,
    ##     rowWeightedSds, rowWeightedVars

    ## Loading required package: GenomicRanges

    ## Loading required package: IRanges

    ##
    ## Attaching package: 'IRanges'

    ## The following object is masked from 'package:grDevices':
    ##
    ##     windows

    ## Loading required package: GenomeInfoDb

    ## Loading required package: Biobase

    ## Welcome to Bioconductor
    ##
    ##     Vignettes contain introductory material; view with
    ##     'browseVignettes()'. To cite Bioconductor, see
    ##     'citation("Biobase")', and for packages 'citation("pkgname")'.

    ##
    ## Attaching package: 'Biobase'

    ## The following object is masked from 'package:MatrixGenerics':
    ##
    ##     rowMedians

    ## The following objects are masked from 'package:matrixStats':
    ##
    ##     anyMissing, rowMedians

    ## The following object is masked from 'package:AnnotationHub':
    ##
    ##     cache

    se <- SummarizedExperiment(
        assays = matrix(nrow = nrow(ms1_features), ncol = 4),
        rowData = ms1_features)

We can now use the same `matchValues` call as before to perform the matching. Matching will be performed on the object’s `rowData`, i.e. each row/element of the `SummarizedExperiment` will be matched against the target using e.g. m/z values available in columns of the object’s `rowData`:

    parm <- Mass2MzParam(adducts = c("[M+H]+", "[M+Na]+"),
                         tolerance = 0.005, ppm = 0)
    matched_features <- matchValues(se, target_df, param = parm)
    matched_features

    ## Object of class Matched
    ## Total number of matches: 9173
    ## Number of query objects: 2842 (1969 matched)
    ## Number of target objects: 57599 (3296 matched)

As `query`, the result contains the full `SummarizedExperiment`, but `colnames` and `matchedData` will access the respective information from the `rowData` of this `SummarizedExperiment`:

    colnames(matched_features)

    ##  [1] "feature_id"        "mz"                "rtime"
    ##  [4] "target_headgroup"  "target_name"       "target_exactmass"
    ##  [7] "target_formula"    "target_chain_type" "target_rtime"
    ## [10] "adduct"            "score"             "ppm_error"

    matchedData(matched_features)

    ## DataFrame with 10046 rows and 12 columns
    ##          feature_id        mz     rtime target_headgroup   target_name
    ##         <character> <numeric> <numeric>      <character>   <character>
    ## 1     Cluster_00...   102.128   1.56015               NA            NA
    ## 2     Cluster_00...   102.128   2.15359               NA            NA
    ## 3     Cluster_00...   102.128   2.92557               NA            NA
    ## 4     Cluster_00...   102.128   3.41962               NA            NA
    ## 5     Cluster_00...   102.127   5.80104               NA            NA
    ## ...             ...       ...       ...              ...           ...
    ## 10042 Cluster_28...   957.771   20.2705               TG    TG 54:2;O3
    ## 10043 Cluster_28...   960.791   20.8865           HexCer HexCer 52:...
    ## 10044 Cluster_28...   961.361   13.0214               NA            NA
    ## 10045 Cluster_28...   970.873   22.0981             ACer ACer 60:1;...
    ## 10046 Cluster_28...   972.734   15.6914          Hex2Cer Hex2Cer 42...
    ##       target_exactmass target_formula target_chain_type target_rtime
    ##              <numeric>    <character>       <character>    <numeric>
    ## 1                   NA             NA                NA           NA
    ## 2                   NA             NA                NA           NA
    ## 3                   NA             NA                NA           NA
    ## 4                   NA             NA                NA           NA
    ## 5                   NA             NA                NA           NA
    ## ...                ...            ...               ...          ...
    ## 10042          934.784      C57H106O9              even      15.9950
    ## 10043          959.779     C58H105NO9              even      10.5076
    ## 10044               NA             NA                NA           NA
    ## 10045          947.888     C60H117NO6              even       4.2806
    ## 10046          971.727  C54H101NO1...              even      19.7329
    ##            adduct      score ppm_error
    ##       <character>  <numeric> <numeric>
    ## 1              NA         NA        NA
    ## 2              NA         NA        NA
    ## 3              NA         NA        NA
    ## 4              NA         NA        NA
    ## 5              NA         NA        NA
    ## ...           ...        ...       ...
    ## 10042     [M+Na]+ -0.0021897  2.286241
    ## 10043      [M+H]+  0.0045398  4.725089
    ## 10044          NA         NA        NA
    ## 10045     [M+Na]+ -0.0045054  4.640545
    ## 10046      [M+H]+ -0.0004240  0.435885

Subsetting the result object, to e.g. just matched elements will also subset the `SummarizedExperiment`.

    matched_sub <- matched_features[whichQuery(matched_features)]
    query(matched_sub)

    ## class: SummarizedExperiment
    ## dim: 1969 4
    ## metadata(0):
    ## assays(1): ''
    ## rownames: NULL
    ## rowData names(3): feature_id mz rtime
    ## colnames: NULL
    ## colData names(0):

A `QFeatures` object is essentially a container for several `SummarizedExperiment` objects which rows (features) are related with each other. Such an object could thus for example contain the full feature data from an LC-MS experiment as one assay and a compounded feature data in which data from ions of the same compound are aggregated as an additional assay. Below we create such an object using our `SummarizedExperiment` as an assay of name `"features"`. For now we don’t add any additional assay to that `QFeatures`, thus, the object contains only this single data set.

    library(QFeatures)

    ## Loading required package: MultiAssayExperiment

    ##
    ## Attaching package: 'QFeatures'

    ## The following object is masked from 'package:MultiAssayExperiment':
    ##
    ##     longFormat

    ## The following object is masked from 'package:AnnotationHub':
    ##
    ##     display

    ## The following object is masked from 'package:base':
    ##
    ##     sweep

    qf <- QFeatures(list(features = se))
    qf

    ## An instance of class QFeatures containing 1 assays:
    ##  [1] features: SummarizedExperiment with 2842 rows and 4 columns

`matchValues` supports also matching of `QFeatures` objects but the user needs to define the assay which should be used for the matching with the `queryAssay` parameter.

    matched_qf <- matchValues(qf, target_df, param = parm, queryAssay = "features")
    matched_qf

    ## Object of class Matched
    ## Total number of matches: 9173
    ## Number of query objects: 2842 (1969 matched)
    ## Number of target objects: 57599 (3296 matched)

`colnames` and `matchedData` allow to access the `rowData` of the `SummarizedExperiment` stored in the `QFeatures`’ `"features"` assay:

    colnames(matched_qf)

    ##  [1] "feature_id"        "mz"                "rtime"
    ##  [4] "target_headgroup"  "target_name"       "target_exactmass"
    ##  [7] "target_formula"    "target_chain_type" "target_rtime"
    ## [10] "adduct"            "score"             "ppm_error"

    matchedData(matched_qf)

    ## DataFrame with 10046 rows and 12 columns
    ##          feature_id        mz     rtime target_headgroup   target_name
    ##         <character> <numeric> <numeric>      <character>   <character>
    ## 1     Cluster_00...   102.128   1.56015               NA            NA
    ## 2     Cluster_00...   102.128   2.15359               NA            NA
    ## 3     Cluster_00...   102.128   2.92557               NA            NA
    ## 4     Cluster_00...   102.128   3.41962               NA            NA
    ## 5     Cluster_00...   102.127   5.80104               NA            NA
    ## ...             ...       ...       ...              ...           ...
    ## 10042 Cluster_28...   957.771   20.2705               TG    TG 54:2;O3
    ## 10043 Cluster_28...   960.791   20.8865           HexCer HexCer 52:...
    ## 10044 Cluster_28...   961.361   13.0214               NA            NA
    ## 10045 Cluster_28...   970.873   22.0981             ACer ACer 60:1;...
    ## 10046 Cluster_28...   972.734   15.6914          Hex2Cer Hex2Cer 42...
    ##       target_exactmass target_formula target_chain_type target_rtime
    ##              <numeric>    <character>       <character>    <numeric>
    ## 1                   NA             NA                NA           NA
    ## 2                   NA             NA                NA           NA
    ## 3                   NA             NA                NA           NA
    ## 4                   NA             NA                NA           NA
    ## 5                   NA             NA                NA           NA
    ## ...                ...            ...               ...          ...
    ## 10042          934.784      C57H106O9              even      15.9950
    ## 10043          959.779     C58H105NO9              even      10.5076
    ## 10044               NA             NA                NA           NA
    ## 10045          947.888     C60H117NO6              even       4.2806
    ## 10046          971.727  C54H101NO1...              even      19.7329
    ##            adduct      score ppm_error
    ##       <character>  <numeric> <numeric>
    ## 1              NA         NA        NA
    ## 2              NA         NA        NA
    ## 3              NA         NA        NA
    ## 4              NA         NA        NA
    ## 5              NA         NA        NA
    ## ...           ...        ...       ...
    ## 10042     [M+Na]+ -0.0021897  2.286241
    ## 10043      [M+H]+  0.0045398  4.725089
    ## 10044          NA         NA        NA
    ## 10045     [M+Na]+ -0.0045054  4.640545
    ## 10046      [M+H]+ -0.0004240  0.435885

## 4.4 Matching of MS/MS spectra

In this section we match experimental MS/MS spectra against reference spectra. This can also be performed with functions from the Spectra package (see SpectraTutorials, but the functions and concepts used here are more suitable to the end user as they simplify the handling of the spectra matching results.

Below we load spectra from a file from a reversed-phase (DDA) LC-MS/MS run of the Agilent Pesticide mix. With `filterMsLevel` we subset the data set to only MS2 spectra. To reduce processing time of the example we further subset the `Spectra` to a small set of selected MS2 spectra. In addition we assign feature identifiers to each spectrum (again, for this example these are arbitrary IDs, but in a real data analysis such identifiers could indicate to which LC-MS feature these spectra belong).

    library(Spectra)
    library(msdata)
    fl <- system.file("TripleTOF-SWATH", "PestMix1_DDA.mzML", package = "msdata")
    pest_ms2 <- filterMsLevel(Spectra(fl), 2L)
    ## subset to selected spectra.
    pest_ms2 <- pest_ms2[c(808, 809, 945:955)]
    ## assign arbitrary *feature IDs* to each spectrum.
    pest_ms2$feature_id <- c("FT001", "FT001", "FT002", "FT003", "FT003", "FT003",
                             "FT004", "FT004", "FT004", "FT005", "FT005", "FT006",
                             "FT006")
    ## assign also *spectra IDs* to each
    pest_ms2$spectrum_id <- paste0("sp_", seq_along(pest_ms2))
    pest_ms2

    ## MSn data (Spectra) with 13 spectra in a MsBackendMzR backend:
    ##       msLevel     rtime scanIndex
    ##     <integer> <numeric> <integer>
    ## 1           2   361.651      2853
    ## 2           2   361.741      2854
    ## 3           2   377.609      3030
    ## 4           2   377.699      3031
    ## 5           2   378.120      3033
    ## ...       ...       ...       ...
    ## 9           2   378.959      3039
    ## 10          2   379.379      3041
    ## 11          2   380.059      3045
    ## 12          2   380.609      3048
    ## 13          2   381.029      3050
    ##  ... 35 more variables/columns.
    ##
    ## file(s):
    ## PestMix1_DDA.mzML
    ## Processing:
    ##  Filter: select MS level(s) 2 [Tue Nov  1 21:17:46 2022]

This `Spectra` should now represent MS2 spectra associated with LC-MS features from an untargeted LC-MS/MS experiment that we would like to annotate by matching them against a spectral reference library.

We thus load below a `Spectra` object that represents MS2 data from a very small subset of MassBank release 2021.03. This small `Spectra` object is provided within this package but it would be possible to use any other `Spectra` object with reference fragment spectra instead (see also the SpectraTutorials workshop). As an alternative, it would also be possible to use a `CompDb` object representing a compound annotation database (defined in the CompoundDb package) with parameter `target`. See the `matchSpectra` help page or section Query against multiple reference databases below for more details and options to retrieve such annotation resources from Bioconductor’s AnnotationHub.

    load(system.file("extdata", "minimb.RData", package = "MetaboAnnotation"))
    minimb

    ## MSn data (Spectra) with 100 spectra in a MsBackendDataFrame backend:
    ##       msLevel     rtime scanIndex
    ##     <integer> <numeric> <integer>
    ## 1           2        NA        NA
    ## 2           2        NA        NA
    ## 3           2        NA        NA
    ## 4           2        NA        NA
    ## 5           2        NA        NA
    ## ...       ...       ...       ...
    ## 96         NA        NA        NA
    ## 97          2        NA        NA
    ## 98          2        NA        NA
    ## 99          2        NA        NA
    ## 100         2        NA        NA
    ##  ... 42 more variables/columns.
    ## Processing:
    ##  Filter: select spectra with polarity 1 [Wed Mar 31 10:06:28 2021]
    ##  Switch backend from MsBackendMassbankSql to MsBackendDataFrame [Wed Mar 31 10:07:59 2021]

We can now use the `matchSpectra` function to match each of our experimental query spectra against the target (reference) spectra. Settings for this matching can be defined with a dedicated param object. We use below the `CompareSpectraParam` that uses the `compareSpectra` function from the `Spectra` package to calculate similarities between each query spectrum and all target spectra. `CompareSpectraParam` allows to set all individual settings for the `compareSpectra` call with parameters `MAPFUN`, `ppm`, `tolerance` and `FUN` (see the help on `compareSpectra` in the Spectra package for more details). In addition, we can pre-filter the target spectra for each individual query spectrum to speed-up the calculations. By setting `requirePrecursor = TRUE` we compare below each query spectrum only to target spectra with matching precursor m/z (accepting a deviation defined by parameters `ppm` and `tolerance`). By default, `matchSpectra` with `CompareSpectraParam` considers spectra with a similarity score higher than 0.7 as matching and these are thus reported.

    mtches <- matchSpectra(pest_ms2, minimb,
                           param = CompareSpectraParam(requirePrecursor = TRUE,
                                                       ppm = 10))
    mtches

    ## Object of class MatchedSpectra
    ## Total number of matches: 16
    ## Number of query objects: 13 (5 matched)
    ## Number of target objects: 100 (11 matched)

The results are reported as a `MatchedSpectra` object which represents the matching results for all query spectra. This type of object contains all query spectra, all target spectra, the matching information and the parameter object with the settings of the matching. The object can be subsetted to e.g. matching results for a specific query spectrum:

    mtches[1]

    ## Object of class MatchedSpectra
    ## Total number of matches: 0
    ## Number of query objects: 1 (0 matched)
    ## Number of target objects: 100 (0 matched)

In this case, for the first query spectrum, no match was found among the target spectra. Below we subset the `MatchedSpectra` to results for the second query spectrum:

    mtches[2]

    ## Object of class MatchedSpectra
    ## Total number of matches: 4
    ## Number of query objects: 1 (1 matched)
    ## Number of target objects: 100 (4 matched)

The second query spectrum could be matched to 4 target spectra. The matching between query and target spectra can be n:m, i.e. each query spectrum can match no or multiple target spectra and each target spectrum can be matched to none, one or multiple query spectra.

Data (spectra variables of either the query and/or the target spectra) can be extracted from the result object with the `spectraData` function or with `$` (similar to a `Spectra` object). The `spectraVariables` function can be used to list all available spectra variables in the result object:

    spectraVariables(mtches)

    ##  [1] "msLevel"                        "rtime"
    ##  [3] "acquisitionNum"                 "scanIndex"
    ##  [5] "dataStorage"                    "dataOrigin"
    ##  [7] "centroided"                     "smoothed"
    ##  [9] "polarity"                       "precScanNum"
    ## [11] "precursorMz"                    "precursorIntensity"
    ## [13] "precursorCharge"                "collisionEnergy"
    ## [15] "isolationWindowLowerMz"         "isolationWindowTargetMz"
    ## [17] "isolationWindowUpperMz"         "peaksCount"
    ## [19] "totIonCurrent"                  "basePeakMZ"
    ## [21] "basePeakIntensity"              "ionisationEnergy"
    ## [23] "lowMZ"                          "highMZ"
    ## [25] "mergedScan"                     "mergedResultScanNum"
    ## [27] "mergedResultStartScanNum"       "mergedResultEndScanNum"
    ## [29] "injectionTime"                  "filterString"
    ## [31] "spectrumId"                     "ionMobilityDriftTime"
    ## [33] "scanWindowLowerLimit"           "scanWindowUpperLimit"
    ## [35] "feature_id"                     "spectrum_id"
    ## [37] "target_msLevel"                 "target_rtime"
    ## [39] "target_acquisitionNum"          "target_scanIndex"
    ## [41] "target_dataStorage"             "target_dataOrigin"
    ## [43] "target_centroided"              "target_smoothed"
    ## [45] "target_polarity"                "target_precScanNum"
    ## [47] "target_precursorMz"             "target_precursorIntensity"
    ## [49] "target_precursorCharge"         "target_collisionEnergy"
    ## [51] "target_isolationWindowLowerMz"  "target_isolationWindowTargetMz"
    ## [53] "target_isolationWindowUpperMz"  "target_spectrum_id"
    ## [55] "target_spectrum_name"           "target_date"
    ## [57] "target_authors"                 "target_license"
    ## [59] "target_copyright"               "target_publication"
    ## [61] "target_splash"                  "target_compound_id"
    ## [63] "target_adduct"                  "target_ionization"
    ## [65] "target_ionization_voltage"      "target_fragmentation_mode"
    ## [67] "target_collision_energy_text"   "target_instrument"
    ## [69] "target_instrument_type"         "target_formula"
    ## [71] "target_exactmass"               "target_smiles"
    ## [73] "target_inchi"                   "target_inchikey"
    ## [75] "target_cas"                     "target_pubchem"
    ## [77] "target_synonym"                 "target_precursor_mz_text"
    ## [79] "target_compound_name"           "score"

This lists the spectra variables from both the query and the target spectra, with the prefix `"target_"` being used for spectra variable names of the target spectra. Spectra variable `"score"` contains the similarity score.

We could thus use `$target_compound_name` to extract the compound name of the matching target spectra for the second query spectrum:

    mtches[2]$target_compound_name

    ## [1] "Azaconazole" "Azaconazole" "Azaconazole" "Azaconazole"

The same information can also be extracted on the full `MatchedSpectra`. Below we use `$spectrum_id` to extract the query spectra identifiers we added above from the full result object.

    mtches$spectrum_id

    ##  [1] "sp_1"  "sp_2"  "sp_2"  "sp_2"  "sp_2"  "sp_3"  "sp_4"  "sp_4"  "sp_5"
    ## [10] "sp_6"  "sp_6"  "sp_6"  "sp_7"  "sp_8"  "sp_8"  "sp_8"  "sp_8"  "sp_8"
    ## [19] "sp_9"  "sp_9"  "sp_10" "sp_11" "sp_12" "sp_13"

Because of the n:m mapping between query and target spectra, the number of values returned by `$` (or `spectraData`) can be larger than the total number of query spectra. Also in the example above, some of the spectra IDs are present more than once in the result returned by `$spectrum_id`. The respective spectra could be matched to more than one target spectrum (based on our settings) and hence their IDs are reported multiple times. Both `spectraData` and `$` for `MatchedSpectra` use a left join strategy to report/return values: a value (row) is reported for each query spectrum (even if it does not match any target spectrum) with eventually duplicated values (rows) if the query spectrum matches more than one target spectrum (each value for a query spectrum is repeated as many times as it matches target spectra). To illustrate this we use below the `spectraData` function to extract specific data from our result object, i.e. the spectrum and feature IDs for the query spectra we defined above, the MS2 spectra similarity score, and the target spectra’s ID and compound name.

    mtches_df <- spectraData(mtches, columns = c("spectrum_id", "feature_id",
                                                 "score", "target_spectrum_id",
                                                 "target_compound_name"))
    as.data.frame(mtches_df)

    ##    spectrum_id feature_id     score target_spectrum_id    target_compound_name
    ## 1         sp_1      FT001        NA               <NA>                    <NA>
    ## 2         sp_2      FT001 0.7869556           LU056604             Azaconazole
    ## 3         sp_2      FT001 0.8855473           LU056603             Azaconazole
    ## 4         sp_2      FT001 0.7234894           LU056602             Azaconazole
    ## 5         sp_2      FT001 0.7219942           LU056605             Azaconazole
    ## 6         sp_3      FT002        NA               <NA>                    <NA>
    ## 7         sp_4      FT003 0.7769746           KW108103 triphenylphosphineoxide
    ## 8         sp_4      FT003 0.7577286           KW108102 triphenylphosphineoxide
    ## 9         sp_5      FT003        NA               <NA>                    <NA>
    ## 10        sp_6      FT003 0.7433718           SM839501            Dimethachlor
    ## 11        sp_6      FT003 0.7019807           EA070705            Dimethachlor
    ## 12        sp_6      FT003 0.7081274           EA070711            Dimethachlor
    ## 13        sp_7      FT004        NA               <NA>                    <NA>
    ## 14        sp_8      FT004 0.7320465           SM839501            Dimethachlor
    ## 15        sp_8      FT004 0.8106258           EA070705            Dimethachlor
    ## 16        sp_8      FT004 0.7290458           EA070710            Dimethachlor
    ## 17        sp_8      FT004 0.8168876           EA070711            Dimethachlor
    ## 18        sp_8      FT004 0.7247800           EA070704            Dimethachlor
    ## 19        sp_9      FT004 0.7412586           KW108103 triphenylphosphineoxide
    ## 20        sp_9      FT004 0.7198787           KW108102 triphenylphosphineoxide
    ## 21       sp_10      FT005        NA               <NA>                    <NA>
    ## 22       sp_11      FT005        NA               <NA>                    <NA>
    ## 23       sp_12      FT006        NA               <NA>                    <NA>
    ## 24       sp_13      FT006        NA               <NA>                    <NA>

Using the `plotSpectraMirror` function we can visualize the matching results for one query spectrum. Note also that an interactive, `shiny`-based, validation of matching results is available with the `validateMatchedSpectra` function. Below we call this function to show all matches for the second spectrum.

    plotSpectraMirror(mtches[2])

Not unexpectedly, the peak intensities of query and target spectra are on different scales. While this was no problem for the similarity calculation (the normalized dot-product which is used by default is independent of the absolute peak values) it is not ideal for visualization. Thus, we apply below a simple scaling function to both the query and target spectra and plot the spectra again afterwards (see the help for `addProcessing` in the `Spectra` package for more details on spectra data manipulations). This function will replace the absolute spectra intensities with intensities relative to the maximum intensity of each spectrum.

    scale_int <- function(x) {
        x[, "intensity"] <- x[, "intensity"] / max(x[, "intensity"], na.rm = TRUE)
        x
    }
    mtches <- addProcessing(mtches, scale_int)
    plotSpectraMirror(mtches[2])

The query spectrum seems to nicely match the identified target spectra. Below we extract the compound name of the target spectra for this second query spectrum.

    mtches[2]$target_compound_name

    ## [1] "Azaconazole" "Azaconazole" "Azaconazole" "Azaconazole"

As alternative to the `CompareSpectraParam` we could also use the `MatchForwardReverseParam` with `matchSpectra`. This has the same settings and performs the same spectra similarity search than `CompareSpectraParam`, but reports in addition (similar to MS-DIAL) to the (forward) similarity score also the reverse spectra similarity score as well as the presence ratio for matching spectra. While the default forward score is calculated considering all peaks from the query and the target spectrum (the peak mapping is performed using an outer join strategy), the reverse score is calculated only on peaks that are present in the target spectrum and the matching peaks from the query spectrum (the peak mapping is performed using a right join strategy). The presence ratio is the ratio between the number of mapped peaks between the query and the target spectrum and the total number of peaks in the target spectrum. These values are available as spectra variables `"reverse_score"` and `"presence_ratio"` in the result object). Below we perform the same spectra matching as above, but using the `MatchForwardReverseParam`.

    mp <- MatchForwardReverseParam(requirePrecursor = TRUE, ppm = 10)
    mtches <- matchSpectra(pest_ms2, minimb, param = mp)
    mtches

    ## Object of class MatchedSpectra
    ## Total number of matches: 16
    ## Number of query objects: 13 (5 matched)
    ## Number of target objects: 100 (11 matched)

Below we extract the query and target spectra IDs, the compound name and all scores.

    as.data.frame(
        spectraData(mtches, c("spectrum_id", "target_spectrum_id",
                              "target_compound_name", "score", "reverse_score",
                              "presence_ratio")))

    ##    spectrum_id target_spectrum_id    target_compound_name     score
    ## 1         sp_1               <NA>                    <NA>        NA
    ## 2         sp_2           LU056604             Azaconazole 0.7869556
    ## 3         sp_2           LU056603             Azaconazole 0.8855473
    ## 4         sp_2           LU056602             Azaconazole 0.7234894
    ## 5         sp_2           LU056605             Azaconazole 0.7219942
    ## 6         sp_3               <NA>                    <NA>        NA
    ## 7         sp_4           KW108103 triphenylphosphineoxide 0.7769746
    ## 8         sp_4           KW108102 triphenylphosphineoxide 0.7577286
    ## 9         sp_5               <NA>                    <NA>        NA
    ## 10        sp_6           SM839501            Dimethachlor 0.7433718
    ## 11        sp_6           EA070705            Dimethachlor 0.7019807
    ## 12        sp_6           EA070711            Dimethachlor 0.7081274
    ## 13        sp_7               <NA>                    <NA>        NA
    ## 14        sp_8           SM839501            Dimethachlor 0.7320465
    ## 15        sp_8           EA070705            Dimethachlor 0.8106258
    ## 16        sp_8           EA070710            Dimethachlor 0.7290458
    ## 17        sp_8           EA070711            Dimethachlor 0.8168876
    ## 18        sp_8           EA070704            Dimethachlor 0.7247800
    ## 19        sp_9           KW108103 triphenylphosphineoxide 0.7412586
    ## 20        sp_9           KW108102 triphenylphosphineoxide 0.7198787
    ## 21       sp_10               <NA>                    <NA>        NA
    ## 22       sp_11               <NA>                    <NA>        NA
    ## 23       sp_12               <NA>                    <NA>        NA
    ## 24       sp_13               <NA>                    <NA>        NA
    ##    reverse_score presence_ratio
    ## 1             NA             NA
    ## 2      0.8764394      0.5833333
    ## 3      0.9239592      0.6250000
    ## 4      0.7573541      0.6250000
    ## 5      0.9519647      0.4285714
    ## 6             NA             NA
    ## 7      0.9025051      0.7500000
    ## 8      0.9164348      0.5000000
    ## 9             NA             NA
    ## 10     0.8915201      0.5000000
    ## 11     0.8687003      0.3333333
    ## 12     0.8687472      0.3703704
    ## 13            NA             NA
    ## 14     0.8444402      0.5000000
    ## 15     0.9267965      0.5000000
    ## 16     0.8765496      0.7500000
    ## 17     0.9236674      0.4814815
    ## 18     0.8714208      0.8571429
    ## 19     0.8743130      0.7500000
    ## 20     0.8937751      0.5000000
    ## 21            NA             NA
    ## 22            NA             NA
    ## 23            NA             NA
    ## 24            NA             NA

In these examples we matched query spectra only to target spectra if their precursor m/z is ~ equal and reported only matches with a similarity higher than 0.7. `CompareSpectraParam`, through its parameter `THRESHFUN` would however also allow other types of analyses. We could for example also report the best matching target spectrum for each query spectrum, independently of whether the similarity score is higher than a certain threshold. Below we perform such an analysis defining a `THRESHFUN` that selects always the best match.

    select_top_match <- function(x) {
        which.max(x)
    }
    csp <- CompareSpectraParam(ppm = 10, requirePrecursor = FALSE,
                               THRESHFUN = select_top_match)
    mtches <- matchSpectra(pest_ms2, minimb, param = csp)
    res <- spectraData(mtches, columns = c("spectrum_id", "target_spectrum_id",
                                           "target_compound_name", "score"))
    as.data.frame(res)

    ##    spectrum_id target_spectrum_id                   target_compound_name
    ## 1         sp_1           SM839603                             Flufenacet
    ## 2         sp_2           LU056603                            Azaconazole
    ## 3         sp_3           SM839501                           Dimethachlor
    ## 4         sp_4           KW108103                triphenylphosphineoxide
    ## 5         sp_5           LU100202        2,2'-(Tetradecylimino)diethanol
    ## 6         sp_6           SM839501                           Dimethachlor
    ## 7         sp_7           RP005503              Glycoursodeoxycholic acid
    ## 8         sp_8           EA070711                           Dimethachlor
    ## 9         sp_9           KW108103                triphenylphosphineoxide
    ## 10       sp_10           JP006901                  1-PHENYLETHYL ACETATE
    ## 11       sp_11           EA070711                           Dimethachlor
    ## 12       sp_12           EA070705                           Dimethachlor
    ## 13       sp_13           LU101704 2-Ethylhexyl 4-(dimethylamino)benzoate
    ##           score
    ## 1  0.000000e+00
    ## 2  8.855473e-01
    ## 3  6.313687e-01
    ## 4  7.769746e-01
    ## 5  1.772117e-05
    ## 6  7.433718e-01
    ## 7  1.906998e-03
    ## 8  8.168876e-01
    ## 9  7.412586e-01
    ## 10 4.085289e-04
    ## 11 4.323403e-01
    ## 12 3.469648e-03
    ## 13 7.612480e-06

Note that this whole example would work on any `Spectra` object with MS2 spectra. Such objects could also be extracted from an `xcms`-based LC-MS/MS data analysis with the `chromPeaksSpectra` or `featureSpectra` functions from the xcms package. Also,

Matches can be also further validated using an interactive Shiny app by calling `validateMatchedSpectra` on the `MatchedSpectra` object. Individual matches can be set to TRUE or FALSE in this app. By closing the app via the Save & Close button a filtered `MatchedSpectra` is returned, containing only matches manually validated.

### 4.4.1 Query against multiple reference databases

Getting access to reference spectra can sometimes be a little cumbersome since it might involve lookup and download of specific resources or eventual conversion of these into a format suitable for import. `MetaboAnnotation` provides compound annotation sources to simplify this process. These annotation source objects represent references (links) to annotation resources and can be used in the `matchSpectra` call to define the targed/reference spectra. The annotation source object takes then care, upon request, of retrieving the annotation data or connecting to the annotation resources.

Also, compound annotation sources can be combined to allow matching query spectra against multiple reference libraries in a single call.

At present `MetaboAnnotation` supports the following types of compound annotation sources (i.e. objects extending `CompAnnotationSource`):

- Annotation resources that provide their data as a `CompDb` database (defined by the CompoundDb) package. These are supported by the `CompDbSource` class.

- Annotation resources for which a dedicated `MsBackend` backend is available hence supporting to access the data via a `Spectra` object. These are supported by the `SpectraDbSource` class.

Various helper functions, specific for the annotation resource, are available to create such annotation source objects:

- `CompDbSource`: creates a compound annotation source object from the provided `CompDb` SQLite data base file. This function can be used to integrate an existing (locally available) `CompDb` annotation database into an annotation workflow.

- `MassBankSource`: creates a annotation source object for a specific MassBank release. The desired release can be specified with the `release` parameter (e.g. `release = "2021.03"` or `release = "2022.06"`). The function will then download the respective annotation database from Bioconductor’s AnnotationHub.

In the example below we create a annotation source for MassBank release 2022.06. This call will lookup the requested version in Biocondutor’s (online) `AnnotationHub` and download the data. Subsequent requests for the same annotation resource will load the locally cached version instead. Upcoming MassBank database releases will be added to `AnnotationHub` after their official release and all previous releases will be available as well.

    mbank <- MassBankSource("2022.06")
    mbank

    ## Object of class CompDbSource
    ## Metadata information:
    ##   - source: MassBank
    ##   - url: https://massbank.eu/MassBank/
    ##   - source_version: 2022.06
    ##   - source_date: 2022-06-21
    ##   - organism: NA
    ##   - db_creation_date: Tue Aug 30 06:51:39 2022
    ##   - supporting_package: CompoundDb
    ##   - supporting_object: CompDb

We can now use that annotation source object in the `matchSpectra` call to compare the experimental spectra from the previous examples against that release of MassBank.

    res <- matchSpectra(
        pest_ms2, mbank,
        param = CompareSpectraParam(requirePrecursor = TRUE, ppm = 10))

    ## Loading required package: CompoundDb
    ## Loading required package: AnnotationFilter
    ## Loading required package: S4Vectors
    ## Loading required package: stats4
    ## Loading required package: BiocGenerics
    ##
    ## Attaching package: 'BiocGenerics'
    ##
    ## The following objects are masked from 'package:stats':
    ##
    ##     IQR, mad, sd, var, xtabs
    ##
    ## The following objects are masked from 'package:base':
    ##
    ##     Filter, Find, Map, Position, Reduce, anyDuplicated, aperm, append,
    ##     as.data.frame, basename, cbind, colnames, dirname, do.call,
    ##     duplicated, eval, evalq, get, grep, grepl, intersect, is.unsorted,
    ##     lapply, mapply, match, mget, order, paste, pmax, pmax.int, pmin,
    ##     pmin.int, rank, rbind, rownames, sapply, setdiff, sort, table,
    ##     tapply, union, unique, unsplit, which.max, which.min
    ##
    ##
    ## Attaching package: 'S4Vectors'
    ##
    ## The following objects are masked from 'package:base':
    ##
    ##     I, expand.grid, unname

    res

    ## Object of class MatchedSpectra
    ## Total number of matches: 14
    ## Number of query objects: 13 (6 matched)
    ## Number of target objects: 10 (10 matched)

The result object contains only the matching fragment spectra from the reference database.

    target(res)

    ## MSn data (Spectra) with 10 spectra in a MsBackendDataFrame backend:
    ##      msLevel     rtime scanIndex
    ##    <integer> <numeric> <integer>
    ## 1         NA        NA        NA
    ## 2         NA        NA        NA
    ## 3         NA        NA        NA
    ## 4         NA        NA        NA
    ## 5         NA        NA        NA
    ## 6         NA        NA        NA
    ## 7         NA        NA        NA
    ## 8         NA        NA        NA
    ## 9         NA        NA        NA
    ## 10        NA        NA        NA
    ##  ... 17 more variables/columns.
    ## Processing:
    ##  Switch backend from MsBackendCompDb to MsBackendDataFrame [Tue Nov  1 21:20:53 2022]

And the names of the compounds with matching fragment spectra.

    matchedData(res)$target_name

    ## NULL

# Session information

    ## R version 4.2.1 (2022-06-23 ucrt)
    ## Platform: x86_64-w64-mingw32/x64 (64-bit)
    ## Running under: Windows Server x64 (build 20348)
    ##
    ## Matrix products: default
    ##
    ## locale:
    ## [1] LC_COLLATE=C
    ## [2] LC_CTYPE=English_United States.utf8
    ## [3] LC_MONETARY=English_United States.utf8
    ## [4] LC_NUMERIC=C
    ## [5] LC_TIME=English_United States.utf8
    ##
    ## attached base packages:
    ## [1] stats4    stats     graphics  grDevices utils     datasets  methods
    ## [8] base
    ##
    ## other attached packages:
    ##  [1] msdata_0.37.0               QFeatures_1.8.0
    ##  [3] MultiAssayExperiment_1.24.0 SummarizedExperiment_1.28.0
    ##  [5] Biobase_2.58.0              GenomicRanges_1.50.0
    ##  [7] GenomeInfoDb_1.34.0         IRanges_2.32.0
    ##  [9] MatrixGenerics_1.10.0       matrixStats_0.62.0
    ## [11] Spectra_1.8.0               ProtGenerics_1.30.0
    ## [13] BiocParallel_1.32.0         S4Vectors_0.36.0
    ## [15] MetaboAnnotation_1.2.0      AnnotationHub_3.6.0
    ## [17] BiocFileCache_2.6.0         dbplyr_2.2.1
    ## [19] BiocGenerics_0.44.0         BiocStyle_2.26.0
    ##
    ## loaded via a namespace (and not attached):
    ##   [1] colorspace_2.0-3              rjson_0.2.21
    ##   [3] ellipsis_0.3.2                MetaboCoreUtils_1.6.0
    ##   [5] XVector_0.38.0                base64enc_0.1-3
    ##   [7] fs_1.5.2                      clue_0.3-62
    ##   [9] mzR_2.32.0                    DT_0.26
    ##  [11] bit64_4.0.5                   interactiveDisplayBase_1.36.0
    ##  [13] AnnotationDbi_1.60.0          fansi_1.0.3
    ##  [15] xml2_1.3.3                    codetools_0.2-18
    ##  [17] ncdf4_1.19                    cachem_1.0.6
    ##  [19] knitr_1.40                    jsonlite_1.8.3
    ##  [21] cluster_2.1.4                 png_0.1-7
    ##  [23] shiny_1.7.3                   BiocManager_1.30.19
    ##  [25] compiler_4.2.1                httr_1.4.4
    ##  [27] assertthat_0.2.1              Matrix_1.5-1
    ##  [29] fastmap_1.1.0                 lazyeval_0.2.2
    ##  [31] cli_3.4.1                     later_1.3.0
    ##  [33] htmltools_0.5.3               tools_4.2.1
    ##  [35] igraph_1.3.5                  gtable_0.3.1
    ##  [37] glue_1.6.2                    GenomeInfoDbData_1.2.9
    ##  [39] dplyr_1.0.10                  rsvg_2.3.2
    ##  [41] rappdirs_0.3.3                Rcpp_1.0.9
    ##  [43] jquerylib_0.1.4               vctrs_0.5.0
    ##  [45] Biostrings_2.66.0             xfun_0.34
    ##  [47] stringr_1.4.1                 mime_0.12
    ##  [49] lifecycle_1.0.3               zlibbioc_1.44.0
    ##  [51] MASS_7.3-58.1                 scales_1.2.1
    ##  [53] CompoundDb_1.2.0              promises_1.2.0.1
    ##  [55] parallel_4.2.1                AnnotationFilter_1.22.0
    ##  [57] yaml_2.3.6                    curl_4.3.3
    ##  [59] memoise_2.0.1                 gridExtra_2.3
    ##  [61] ggplot2_3.3.6                 sass_0.4.2
    ##  [63] stringi_1.7.8                 RSQLite_2.2.18
    ##  [65] highr_0.9                     BiocVersion_3.16.0
    ##  [67] filelock_1.0.2                rlang_1.0.6
    ##  [69] pkgconfig_2.0.3               bitops_1.0-7
    ##  [71] evaluate_0.17                 lattice_0.20-45
    ##  [73] purrr_0.3.5                   ChemmineR_3.50.0
    ##  [75] htmlwidgets_1.5.4             bit_4.0.4
    ##  [77] tidyselect_1.2.0              magrittr_2.0.3
    ##  [79] bookdown_0.29                 R6_2.5.1
    ##  [81] magick_2.7.3                  snow_0.4-4
    ##  [83] generics_0.1.3                DelayedArray_0.24.0
    ##  [85] DBI_1.1.3                     withr_2.5.0
    ##  [87] pillar_1.8.1                  MsCoreUtils_1.10.0
    ##  [89] KEGGREST_1.38.0               RCurl_1.98-1.9
    ##  [91] tibble_3.1.8                  crayon_1.5.2
    ##  [93] utf8_1.2.2                    rmarkdown_2.17
    ##  [95] grid_4.2.1                    blob_1.2.3
    ##  [97] digest_0.6.30                 xtable_1.8-4
    ##  [99] httpuv_1.6.6                  munsell_0.5.0
    ## [101] bslib_0.4.0

# References

Rainer, Johannes, Andrea Vicini, Liesa Salzer, Jan Stanstrup, Josep M. Badia, Steffen Neumann, Michael A. Stravs, et al. 2022. “A Modular and Expandable Ecosystem for Metabolomics Data Annotation in R.” Metabolites 12 (2): 173. https://doi.org/10.3390/metabo12020173.